U.S. patent application number 10/083674 was filed with the patent office on 2003-03-06 for integrated planar composite coupling structures for bi-directional light beam transformation between a small mode size waveguide and a large mode size waveguide.
This patent application is currently assigned to PHOSISTOR TECHNOLOGIES, INC.. Invention is credited to Ho, Seng-Tiong, Zhou, Yan.
Application Number | 20030044118 10/083674 |
Document ID | / |
Family ID | 26769580 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030044118 |
Kind Code |
A1 |
Zhou, Yan ; et al. |
March 6, 2003 |
Integrated planar composite coupling structures for bi-directional
light beam transformation between a small mode size waveguide and a
large mode size waveguide
Abstract
Composite optical waveguide structures or mode transformers and
their methods of fabrication and integration are disclosed, wherein
the structures or mode transformers are capable of bi-directional
light beam transformation between a small mode size waveguide and a
large mode size wavguide. One aspect of the present invention is
directed to an optical mode transformer comprising a waveguide core
having a high refractive index contrast between the waveguide core
and the cladding, the optical mode transformer being configured
such that the waveguide core has a taper wherein a thickness of the
waveguide core tapers down to a critical thickness value, the
critical thickness value being defined as a thickness value below
which a significant portion of the energy of a light beam
penetrates into the cladding layers surrounding the taper structure
thereby enlarging the small mode size. This primary tapered core
structure may be present in either a vertical or horizontal
direction and may be combined with further up taper or down taper
structures in the directions transverse to the primary taper
direction. Another aspect of the present invention is directed to a
non-cylindrical graduated refractive index (GRIN) lens structure.
The non-cylindrical GRIN structure has a graded refractive index
having a maximum value at its core and a minimum value at its outer
edges. The grading of the refractive index is provided in a either
the vertical or horizontal directions and may have either a fixed
refractive index or a graded refractive index in the transverse
directions. Yet another aspect of the present invention is directed
to composite optical mode transformers that are combinations of the
taper waveguide structures and the non-cylindrical graduated
refractive index structures. Yet another aspect of the present
invention is the further integration of the mode transformers with
V-grooves for multiple input/output fibers and alignment platform
for multiple input/output photonic chips or devices.
Inventors: |
Zhou, Yan; (Pleasanton,
CA) ; Ho, Seng-Tiong; (Wheeling, IL) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
PHOSISTOR TECHNOLOGIES,
INC.
Pleasanton
CA
94588
|
Family ID: |
26769580 |
Appl. No.: |
10/083674 |
Filed: |
October 19, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60242213 |
Oct 20, 2000 |
|
|
|
Current U.S.
Class: |
385/43 ; 385/129;
385/28 |
Current CPC
Class: |
G02B 2006/12176
20130101; G02B 6/1228 20130101; G02B 6/14 20130101; G02B 2006/12188
20130101; G02B 6/4206 20130101 |
Class at
Publication: |
385/43 ; 385/129;
385/28 |
International
Class: |
G02B 006/26; G02B
006/10 |
Claims
What is claimed is:
1. An optical mode transformer comprising: a substrate; a lower
waveguide cladding layer disposed on the substrate, the lower
waveguide cladding layer having a first refractive index and an
upper surface; a waveguide core disposed on the upper surface of
the lower waveguide cladding layer, the waveguide core having a
long axis, the waveguide core having a second refractive index, the
ratio of the second refractive index to the first refractive index
being at least about 1.3, the waveguide core further having a first
end optically coupled to a small beam port, a second end defining
an intermediate beam port, and an upper surface; side waveguide
cladding disposed on the upper surface of the lower waveguide
cladding layer adjacent to both sides of the waveguide core, the
side waveguide cladding having a third refractive index, the ratio
of the second refractive index to the third refractive index being
at least about 1.3, the side waveguide cladding further having an
upper surface; and an upper waveguide cladding layer disposed on
the upper surface of the waveguide core and the upper surface of
the side waveguide cladding, the upper waveguide cladding having a
fourth refractive index, the ratio of the second refractive index
to the fourth refractive index being at least about 1.3; the
optical mode transformer being configured such that the waveguide
core has a vertical taper wherein a thickness of the waveguide core
in a dimension normal to the substrate surface decreases along the
long axis from a first thickness value at a first point near the
small beam port to a second thickness value at a second point near
the intermediate beam port, the second thickness value being
smaller than a critical thickness value, the critical thickness
value being defined as a thickness value below which a significant
portion of the energy of a light beam having a small mode size
received at the small beam port and propagating in the waveguide
core penetrates into at least one of the upper waveguide cladding
layer and the lower waveguide cladding layer, thereby enlarging the
small mode size.
2. An optical mode transformer for guiding a light beam and
transforming a mode size of the light beam, the optical mode
transformer having a small beam port at a small-beam end and a
large beam port at a large-beam end, the optical mode transformer
comprising: a substrate; a lower waveguide cladding layer disposed
on the substrate, the lower waveguide cladding layer having a first
refractive index and an upper surface; a waveguide core disposed on
the upper surface of the lower waveguide cladding layer, the
waveguide core having a long axis and having a cross section in a
plane normal to the long axis, the waveguide core having a second
refractive index, the ratio of the second refractive index to the
first refractive index being at least about 1.3, the waveguide core
further having a first end optically coupled to the small beam
port, a second end defining an intermediate beam port, and an upper
surface; side waveguide cladding disposed on the upper surface of
the lower waveguide cladding and adjacent to the waveguide core,
the side waveguide cladding having a third refractive index, the
ratio of the second refractive index to the third refractive index
being at least about 1.3, the side waveguide cladding further
having an upper surface; and an upper waveguide cladding disposed
on the upper surface of the waveguide core and the upper surface of
the side waveguide cladding, said upper waveguide cladding having a
fourth refractive index, the ratio of the second refractive index
to the fourth refractive index being at least about 1.3; the
optical mode transformer being configured such that: in a first
region along the long axis between the small beam port and a
transition point, the waveguide core cross section has a thickness
in a dimension normal to the substrate surface that is
substantially constant and equal to a first thickness value; in a
second region along the long axis between the transition point and
the intermediate beam port, the waveguide core cross section has a
thickness that changes along the long axis from the first thickness
value to a second thickness value smaller than the first thickness
value, the second thickness value being smaller than a critical
thickness value, the critical thickness value being defined as a
thickness value below which a significant portion of the energy of
a light beam having a small mode size received at the small beam
port and propagating in the waveguide core penetrates into at least
one of the upper waveguide cladding layer and the lower waveguide
cladding layer, thereby enlarging the small mode size; and in a
third region along the long axis between the intermediate beam port
and the large beam port, the waveguide core cross section has a
thickness that is substantially constant and approximately equal to
the second thickness value.
3. The optical mode transformer of claims 1 or 2, wherein the
second thickness value is substantially equal to zero.
4. The optical mode transformer of claims 1 or 2, wherein the
thickness of the waveguide core changes substantially uniformly
from the first thickness value to the second thickness value.
5. The optical mode transformer of claims 1 or 2, wherein the
substrate comprises silicon.
6. The optical mode transformer of claims 1 or 2, wherein the
waveguide core comprises silicon.
7. The optical mode transformer of claims 1 or 2, wherein the lower
waveguide cladding comprises silicon dioxide.
8. The optical mode transformer of claims 1 or 2, wherein each of
the side waveguide cladding and the upper waveguide cladding
comprises a mixture of silicon dioxide and titanium dioxide.
9. The optical mode transformer of claim 2, wherein a light beam
having an initial mode size enters the waveguide core from the
small beam port and exits at the large beam port having a final
mode size larger in a dimension normal to the substrate surface
than the initial mode size.
10. The optical mode transformer of claim 2, wherein a light beam
having an initial mode size enters at the large beam port and exits
the waveguide core at the small beam port having a final mode size
smaller in a dimension normal to the substrate surface than the
initial mode size.
11. The optical mode transformer of claims 1 or 2, wherein the
waveguide core further has a lateral taper along the direction of
light propagation, wherein a width of the waveguide core in a
dimension parallel to the substrate surface and transverse to the
direction of light propagation increases from a first width to a
second width, the second width being substantially equal to a
desired large mode size of a light beam.
12. The optical mode transformer of claims 1 or 2, wherein the
waveguide core further has a lateral taper along the direction of
light propagation, wherein a width of the waveguide core decreases
from a first width value to a second width value, the second width
value being smaller than a critical width value, the critical width
value being defined as a width value below which a significant
portion of the energy of a light beam having a small mode size
received at the small beam port and propagating in the waveguide
core penetrates into the side waveguide cladding, thereby enlarging
the small mode size.
13. An optical mode transformer comprising: a substrate; a lower
waveguide cladding layer disposed on the substrate and having a
first refractive index and an upper surface; a waveguide core
disposed on the upper surface of the lower waveguide cladding
layer, the waveguide core having a long axis, the waveguide core
having a second refractive index, the ratio of the second
refractive index to the first refractive index being at least about
1.3, the waveguide core further having a first end optically
coupled to a small beam port, a second end defining an intermediate
beam port, and an upper surface; side waveguide cladding disposed
on the upper surface of the lower waveguide cladding layer adjacent
to both sides of the waveguide core, the side waveguide cladding
having a third refractive index, the ratio of the second refractive
index to third refractive index being at least about 1.3, the side
waveguide cladding further having an upper surface; and an upper
waveguide cladding layer disposed on the upper surface of the
waveguide core and the upper surface of the side waveguide
cladding, the upper waveguide cladding having a fourth refractive
index, the ratio of the second refractive index to the fourth
refractive index being at least about 1.3; the optical mode
transformer being configured such that the waveguide core has a
lateral taper wherein a width of the waveguide core in a dimension
parallel to the substrate surface and transverse to the long axis
decreases along the direction of light propagation from a first
width value at a first point near the small beam port to a second
width value at a second point near the intermediate beam port, the
second width value being smaller than a critical width value, the
critical width value being defined as a width value below which a
significant portion of the energy of a light beam having a small
mode size received at the small beam port and propagating in the
waveguide core penetrates into the side waveguide cladding, thereby
enlarging the small mode size.
14. An optical mode transformer comprising: a substrate; a lower
waveguide cladding layer disposed on the substrate and having a
first refractive index and an upper surface; a waveguide core
disposed on the upper surface of the lower waveguide cladding
layer, the waveguide core having a long axis and having a cross
section in a plane normal to the long axis, the waveguide core
having a second refractive index, the ratio of the second
refractive index to the first refractive index being at least about
1.3, the waveguide core further having a first end optically
coupled to a small beam port, a second end defining an intermediate
beam port, and an upper surface; side waveguide cladding disposed
on the upper surface of the lower waveguide cladding and adjacent
to the waveguide core, the side waveguide cladding having a third
refractive index, the ratio of the second refractive index to the
third refractive index being at least about 1.3, the side waveguide
cladding further having an upper surface; and an upper waveguide
cladding disposed on the upper surface of the waveguide core and
the upper surface of the side waveguide cladding, said upper
waveguide cladding having a fourth refractive index, the ratio of
the second refractive index to the fourth refractive index being at
least about 1.3; the optical mode transformer being configured such
that: in a first region along the long axis between the small beam
port and a transition point, the waveguide core cross section has a
width that is substantially constant and equal to a first width
value; in a second region along the long axis between the
transition point and the intermediate beam port, the waveguide core
cross section has a width in a dimension parallel to the substrate
surface and transverse to the long axis that changes along the long
axis from the first width value to a second width value smaller
than the first width value, the second width value being smaller
than a critical width value, the critical width value being defined
as a width value below which a significant portion of the energy of
a light beam having a small mode size received at the small beam
port and propagating in the waveguide core penetrates into the side
waveguide cladding, thereby enlarging the small mode size; and in a
third region along the long axis between the intermediate beam port
and a large beam port, the waveguide core cross section has a width
that is substantially constant and approximately equal to the
second width value.
15. The optical mode transformer of claims 13 or 14, wherein the
second width value is substantially equal to zero.
16. The optical mode transformer of claims 13 or 14, wherein the
width of the waveguide core changes substantially uniformly from
the first width value to the second width value.
17. The optical mode transformer of claims 13 or 14, wherein the
substrate comprises silicon.
18. The optical mode transformer of claims 13 or 14, wherein the
waveguide core comprises silicon.
19. The optical mode transformer of claims 13 or 14, wherein the
lower waveguide cladding comprises silicon dioxide.
20. The optical mode transformer of claims 13 or 14, wherein each
of the side waveguide cladding and the upper waveguide cladding
comprises a mixture of silicon dioxide and titanium dioxide.
21. The optical mode transformer of claim 14, wherein a light beam
having an initial mode size enters the waveguide core form the
small beam port and exits at the large beam port, having a final
mode size larger in a dimension parallel to the substrate surface
and transverse to the direction of light propagation than the
initial mode size.
22. The optical mode transformer of claim 14, wherein a light beam
having an initial mode size enters the waveguide core from the
large beam port and exits at the small beam port having a final
mode size larger in a dimension parallel to the substrate surface
and transverse to the direction of light propagation than the
initial mode input beam.
23. The optical mode transformer of claims 13 or 14, wherein the
waveguide core further has a vertical taper along the direction of
light propagation, wherein a thickness of the waveguide core in a
dimension normal to the substrate surface increases from a first
thickness to a second thickness, wherein the second thickness is
approximately equal to a desired large mode size of a light
beam.
24. The optical mode transformer of claims 13 or 14, wherein the
waveguide core has a vertical taper along the direction of light
propagation, wherein a thickness of the waveguide core in a
dimension normal to the substrate surface decreases from a first
thickness value to a second thickness value, the second thickness
value being smaller than a critical thickness value, the critical
thickness value being defined as a thickness value below which a
significant portion of the energy of a light beam having a small
mode size received at the small beam port and propagating in the
waveguide core penetrates into at least one of the upper waveguide
cladding layer and the lower waveguide cladding layer, thereby
enlarging the small mode size.
25. An optical mode transformer comprising: a substrate; a lower
waveguide cladding layer disposed on the substrate and having a
first refractive index, an upper surface and side surfaces; a
waveguide core disposed on the upper surface of the lower waveguide
cladding layer, the waveguide core having a long axis, the
waveguide core having a second refractive index, the ratio of the
second refractive index to the first refractive index being at
least about 1.3, the waveguide core further having a first end
optically coupled to a small beam port, a second end defining an
intermediate beam port, and an upper surface; side waveguide
cladding disposed on the side surfaces of the lower waveguide
cladding layer adjacent to both sides of the waveguide core, the
side waveguide cladding having a third refractive index, the ratio
of the second refractive index to the third refractive index being
at least about 1.3, the side waveguide cladding further having an
upper surface; and an upper waveguide cladding layer disposed on
the upper surface of the waveguide core and the upper surface of
the side waveguide cladding, the upper waveguide cladding having a
fourth refractive index, the ratio of the second refractive index
to the fourth refractive index being at least about 1.3; the
optical mode transformer being configured such that the waveguide
core has a lateral taper wherein a width of the waveguide core in a
dimension parallel to the substrate surface and transverse to the
long axis increases along the direction of light propagation from a
first width value at a first point near the small beam port to a
second width value at a second point near the intermediate beam
port, the second width value being substantially equal to a desired
value that defines a large mode size of a light beam.
26. An optical mode transformer comprising: a substrate; a lower
waveguide cladding layer disposed on the substrate and having a
first refractive index, an upper surface and side surfaces; a
waveguide core disposed on the upper surface of the lower waveguide
cladding layer, the waveguide core having a long axis and having a
cross section in a plane normal to the long axis, the waveguide
core having a second refractive index, the ratio of the second
refractive index to the first refractive index being at least about
1.3, the waveguide core further having a first end optically
coupled to a small beam port, a second end defining an intermediate
beam port, and an upper surface; side waveguide cladding disposed
on the side surfaces of the lower waveguide cladding and adjacent
to the waveguide core, the side waveguide cladding having a third
refractive index, the ratio of the second refractive index to the
third refractive index being at least about 1.3, the side waveguide
cladding further having an upper surface; and an upper waveguide
cladding disposed on the upper surface of the waveguide core and
the upper surface of the side waveguide cladding, said upper
waveguide cladding having a fourth refractive index, the ratio of
the second refractive index to the fourth refractive index being at
least about 1.3; the optical mode transformer being configured such
that: in a first region along the direction of light propagation
between the small beam port and a transition point, the waveguide
core cross section has a width in a dimension parallel to the
substrate surface and transverse to the long axis that is
substantially constant and equal to a first width value; in a
second region along the direction of light propagation between the
transition point and the intermediate beam port, the waveguide core
cross section has a width that changes along the long axis from the
first width value to a second width value larger than the first
width value, the second width value being substantially equal to a
desired value that defines a large mode size of a light beam; and
in a third region along the direction of light propagation between
the intermediate beam port and a large beam port, the waveguide
core cross section has a width that is approximately constant and
equal to the second width value.
27. The optical mode transformer of claims or, wherein the second
width value is substantially equal to a mode size of an optical
fiber.
28. The optical mode transformer of claims or, wherein the width of
the waveguide core changes substantially uniformly from the first
width value to the second width value.
29. The optical mode transformer of claims 25 or 26, wherein the
substrate comprises silicon.
30. The optical mode transformer of claims 25 or 26, wherein the
waveguide core comprises silicon.
31. The optical mode transformer of claims 25 or 26, wherein the
lower waveguide cladding comprises silicon dioxide.
32. The optical mode transformer of claims 25 or 26, wherein each
of the side waveguide cladding and the upper waveguide cladding
comprises a mixture of silicon dioxide and titanium dioxide.
33. The optical mode transformer of claim 26, wherein a light beam
having an initial mode size enters the waveguide core from the
small beam port and exits at the large beam port having a final
mode size larger in a dimension parallel to the substrate surface
and transverse to the direction of light propagation than the
initial mode size.
34. The optical mode transformer of claim 26, wherein a light beam
having an initial mode size enters the waveguide core at the large
beam port and exits at the small beam port having a final mode size
smaller in a dimension parallel to the substrate surface and
transverse to the direction of light propagation than initial mode
size.
35. The optical mode transformer of claims 25 or 26, wherein the
waveguide core further has a vertical taper along the direction of
light propagation, wherein a thickness of the waveguide core in a
dimension normal to the substrate surface to increase from a first
thickness value to a second thickness value, wherein the second
thickness value is approximately equal to a desired large mode size
of a light beam.
36. The optical mode transformer of claims 25 or 26, wherein the
waveguide core has a vertical taper along the direction of light
propagation, wherein a thickness of the waveguide core in a
dimension normal to the substrate surface to decrease from a first
thickness value to a second thickness value, the second thickness
being smaller than a critical thickness value, the critical
thickness value being defined as a thickness value below which a
significant portion of the energy of a light beam having a small
mode size received at the small beam port and propagating in the
waveguide core penetrates into at least one of the upper waveguide
cladding layer and the lower waveguide cladding layer, thereby
enlarging the small mode size.
37. An optical waveguide comprising: a substrate having a substrate
surface; a lower waveguide cladding disposed on the substrate
surface; a non-cylindrical waveguide core aligned in axial
direction disposed on the lower waveguide cladding; said waveguide
core having a center and an outer border; and an upper waveguide
cladding disposed on the waveguide core, the waveguide core having
a refractive index having a value that is graded in the
y-coordinate and gradually decreases from a maximum effective
refractive index at the center of the core to a minimum effective
refractive index at the outer border of said waveguide core, the
y-coordinate representing a distance from the substrate
surface.
38. The optical waveguide of claim 37, wherein said waveguide core
has a refractive index that is graded in the x-coordinate and
gradually decreases from a maximum effective refractive index at
the center of the core to a minimum effective refractive index at
the outer border of said waveguide core, the x-coordinate
representing a direction transverse to said y-coordinate and
perpendicular to said axial direction.
39. The optical waveguide of claim 37, wherein said waveguide core
has a refractive index that is constant in the x-coordinate, the
x-coordinate representing a direction transverse to said
y-coordinate and perpendicular to said axial direction.
40. An optical waveguide comprising: a substrate having a substrate
surface; a lower waveguide cladding disposed on the substrate
surface; a non-cylindrical waveguide core aligned in an axial
direction disposed on the lower waveguide cladding; said waveguide
core having a center and an outer border; an upper waveguide
cladding disposed on the waveguide core, the waveguide core having
a first refractive index distribution that is graded in a first
direction normal to the substrate surface and that gradually
decreases from a maximum effective refractive index at the center
of the core to a first minimum effective refractive index at the
outer border of said waveguide core, and wherein said waveguide
core has a second refractive index distribution that is graded in a
second direction transverse to said first direction perpendicular
to said axial direction and that gradually decreases from said
maximum effective refractive index at the center of the core to a
second minimum effective refractive index at the outer border of
said waveguide core.
41. An optical waveguide comprising: a substrate having a substrate
surface; a lower waveguide cladding disposed on the substrate
surface; a non-cylindrical waveguide core aligned in an axial
direction disposed on the lower waveguide cladding; said waveguide
core having a center and an outer border; an upper waveguide
cladding disposed on the waveguide core, the waveguide core having
a refractive index having a value that is graded in the
y-coordinate and gradually decreases from a maximum effective
refractive index at the center of the core to a minimum effective
refractive index at the outer border of said waveguide core, the
y-coordinate representing a distance from the substrate surface,
and wherein said waveguide core has a refractive index that is
constant in the x-coordinate, the x-coordinate representing a
direction transverse to said y-coordinate and perpendicular to said
axial direction.
42. An optical waveguide comprising: a substrate having a substrate
surface; a lower waveguide cladding disposed on the substrate
surface; a non-cylindrical waveguide core aligned in axial
direction disposed on the lower waveguide cladding; said waveguide
core having a center and an outer border; and an upper waveguide
cladding disposed on the waveguide core, the waveguide core having
a refractive index having a value that is constant in the
y-coordinate, the y-coordinate representing a distance from the
substrate surface.
43. The optical waveguide of claim 42, wherein said waveguide core
has a refractive index that is graded in the x-coordinate and
gradually decreases from a maximum effective refractive index at
the center of the core to a minimum effective refractive index at
the outer border of said waveguide core, the x-coordinate
representing a direction transverse to said y-coordinate and
perpendicular to said axial direction.
44. The optical waveguide of claim 42, wherein said waveguide core
has a refractive index that is constant in the x-coordinate, the
x-coordinate representing a direction transverse to said
y-coordinate and perpendicular to said axial direction.
45. An optical waveguide comprising: a substrate having a substrate
surface; a lower waveguide cladding disposed on the substrate
surface; a non-cylindrical waveguide core aligned in axial
direction disposed on the lower waveguide cladding; said waveguide
core having a center and an outer border; and an upper waveguide
cladding disposed on the waveguide core, the waveguide core having
a refractive index having a value that is constant in the
y-coordinate, the y-coordinate representing a distance from the
substrate surface, and wherein said waveguide core has a refractive
index that is graded in the x-coordinate and gradually decreases
from a maximum effective refractive index at the center of the core
to a minimum effective refractive index at the outer border of said
waveguide core, the x-coordinate representing a direction
transverse to said y-coordinate and perpendicular to said axial
direction.
46. An optical waveguide comprising: a substrate having a substrate
surface; a lower waveguide cladding disposed on the substrate
surface; a non-cylindrical waveguide core aligned in axial
direction disposed on the lower waveguide cladding; said waveguide
core having a center and an outer border; and an upper waveguide
cladding disposed on the waveguide core, the waveguide core having
a refractive index having a value that is constant in the
y-coordinate, the y-coordinate representing a distance from the
substrate surface, and wherein said waveguide core has a refractive
index that is constant in the x-coordinate, the x-coordinate
representing a direction transverse to said y-coordinate and
perpendicular to said axial direction.
47. An optical mode transformer comprising: a substrate having a
substrate surface; a lower waveguide cladding disposed on the
substrate surface, the lower waveguide cladding having a refractive
index distribution that varies according to a first function of a
y-coordinate, the y-coordinate representing a distance from the
substrate surface, the first function having a maximum value and a
minimum value, the lower waveguide cladding further having an upper
surface; a waveguide core disposed on the upper surface of the
lower waveguide cladding, the waveguide core having a core
refractive index, the ratio of the core refractive index to the
maximum value of the first function being at least about 1.3, the
waveguide core further having a first end located substantially at
a small beam port, a second end defining an intermediate beam port,
and an upper surface; and an upper waveguide cladding disposed on
the upper surface of the waveguide core and on the upper surface of
the lower waveguide cladding, the upper waveguide cladding having a
refractive index distribution that varies as a second function of a
y-coordinate, the second function having a maximum value and a
minimum value, the ratio of the core refractive index to the
maximum value of the second function being at least about 1.3; the
optical mode transformer being configured such that the waveguide
core has a vertical taper along the long axis, the vertical taper
being a changing thickness of the waveguide core in a dimension
normal to the substrate surface, wherein the thickness decreases
along the long axis from a first thickness value at a first point
near the small beam port to a second thickness value at a second
point near the intermediate beam port, the second thickness value
being less than a critical thickness value, the critical thickness
value being defined as a thickness value below which a significant
portion of the energy of a light beam having a small mode size
received at the small beam port and propagating in the waveguide
core penetrates into at least one of the upper waveguide cladding
layer and the lower waveguide cladding layer, thereby enlarging the
small mode size.
48. The optical mode transformer of claim 47, further comprising a
low refractive index buffer layer between said waveguide core and
said lower waveguide cladding.
49. The optical mode transformer of claim 47, further comprising a
low refractive index buffer layer between said waveguide core and
said upper waveguide cladding.
50. An optical mode transformer comprising: a substrate having a
substrate surface; a lower waveguide cladding disposed on the
substrate surface, the lower waveguide cladding having a refractive
index distribution that varies according to a first function of a
y-coordinate, the y-coordinate representing a distance from the
substrate surface, the first function having a maximum value and a
minimum value, the lower waveguide cladding further having an upper
surface; a waveguide core disposed on the upper surface of the
lower waveguide cladding, the waveguide core having a core
refractive index, the ratio of the core refractive index to the
maximum value of the first function being at least about 1.3, the
waveguide core further having a first end located substantially at
a small beam port, a second end defining an intermediate beam port,
and an upper surface; and an upper waveguide cladding disposed on
the upper surface of the waveguide core and on the upper surface of
the lower waveguide cladding, the upper waveguide cladding having a
refractive index distribution that varies as a second function of a
y-coordinate, the second function having a maximum value and a
minimum value, the ratio of the core refractive index to the
maximum value of the second function being at least about 1.3; the
optical waveguide being configured such that: in a first region
along the long axis between the small beam port and a transition
point, the waveguide core cross section has a thickness in a
dimension normal to the substrate surface that is substantially
constant and equal to a first thickness value; in a second region
along the long axis between the transition point and the
intermediate beam port, the waveguide core cross section has a
thickness that changes along the long axis from the first thickness
value to a second thickness value smaller than the first thickness
value, the second thickness being small enough to cause a small
mode size of a small light beam received at the small beam port to
propagate into at least one of the upper waveguide cladding layer
and the lower waveguide cladding layer, thereby enlarging the small
mode size; and in a third region along the long axis between the
intermediate beam port and the large beam port, the waveguide core
cross section has a thickness that is substantially constant and
approximately equal to the second thickness value.
51. The optical mode transformer of claim 50, further comprising a
low refractive index buffer layer between said waveguide core and
said lower waveguide cladding.
52. The optical mode transformer of claim 50, further comprising a
low refractive index buffer layer between said waveguide core and
said lower waveguide cladding.
53. The optical mode transformer of claims 47 or 50, wherein: the
lower waveguide cladding comprises a first plurality of lower
cladding layers substantially parallel to the substrate, wherein
each lower cladding layer has a layer-specific refractive index
that is a function of the y-coordinate; and the upper waveguide
cladding comprises a second plurality of upper cladding layers,
wherein each upper cladding layer has a layer-specific refractive
index that is a function of the y coordinate.
54. The optical mode transformer of claims 47 or 50, wherein, for
any value of the y-coordinate: the effective layer-specific
refractive index of each of the first plurality of lower cladding
layers is higher than the layer-specific refractive index of the
lower cladding layer below; and the effective layer-specific
refractive index of each of the second plurality of upper cladding
layers is lower than the layer-specific refractive index of the
upper cladding layer below.
55. The optical mode transformer of claims 47 or 50, wherein the
layer-specific refractive index of each of the first plurality of
lower cladding layers forms a first refractive-index distribution
that is symmetric with a second refractive-index distribution
formed by the layer-specific refractive index of each of the second
plurality of upper cladding layers.
56. The optical mode transformer of claims 47 or 50, wherein the
first and second distributions together comprise a substantially
parabolic distribution.
57. The optical mode transformer of claims 47 or 50, wherein the
first function has a substantially parabolic dependence on the
y-coordinate and the second function has a substantially parabolic
dependence on the y-coordinate.
58. The optical mode transformer of claims 47 or 50, wherein: the
difference between the maximum value of the first function and the
minimum value of the first function is not less than about 0.02;
and the difference between the maximum value of the second function
and the minimum value of the second function is not less than about
0.02.
59. The optical mode transformer of claims 47 or 50, wherein: the
first function is a constant function of the y-coordinate; and the
upper waveguide cladding comprises a plurality of upper cladding
layers substantially parallel to the substrate, wherein each upper
cladding layer has a layer-specific refractive index that is a
function of the x coordinate.
60. The optical mode transformer of claims 47 or 50, wherein for
any value of the y-coordinate: the first function is a stepwise
function of the x-coordinate having substantially a first value in
a first range of x-coordinate values, substantially a second value
in a second range of x-coordinate values, and substantially the
first value in a third range of x-coordinate values, wherein the
second value is higher than the first value and wherein the
waveguide core is located at a position having x-coordinate values
within the second range, wherein the x-coordinate is in the axial
direction parallel to the surface of said optical mode transformer
and perpendicular to the propagation direction of the light through
said optical mode transformer.
61. The optical mode transformer of claims 47 or 50, wherein the
waveguide core further has a lateral taper along the direction of
light propagation, the lateral taper causing a width of the
waveguide core in a dimension parallel to the substrate surface and
transverse to the direction of light propagation to increase from a
first width value to a second width value, the second width value
being substantially equal to a desired large mode size of a light
beam.
62. The optical mode transformer of claims 47 or 50, wherein the
waveguide core has a lateral taper along the direction of light
propagation, the lateral taper causing a width of the waveguide
core to decrease from a first width value to a second width value,
the second width value being smaller than a critical width value,
the critical width value being defined as a width value below which
a significant portion of the energy of a light beam having a small
mode size received at the small beam port and propagating in the
waveguide core penetrates into the waveguide cladding, thereby
enlarging the small mode size.
63. The optical mode transformer of claims 47 or 50, wherein: a
light beam having a small mode size enters the optical waveguide at
the small beam port, the small mode size being substantially equal
to a mode size of a semiconductor optical device; the light beam is
modified to have an intermediate mode size as it passes through the
second region; and the light beam is further modified to have a
large mode size as it passes through the third region, the large
mode size being substantially equal to a mode size of an optical
fiber.
64. The optical mode transformer of claims 47 or 50, wherein: a
light beam having a large mode size enters the optical waveguide at
the large beam port, the large mode size being substantially equal
to a mode size of an optical fiber; the light beam is modified to
have an intermediate mode size as it passes through the third
region; and the light beam is further modified to have a small mode
size as it passes through the second and first regions to the small
beam port, the small mode size being substantially equal to a mode
size of a semiconductor optical device.
65. The optical mode transformer of claims 47 or 50, wherein: a
recess is formed in the substrate near the small beam port, the
recess being configured for mounting of a semiconductor optical
device in alignment with the small beam port; and a groove is
formed in the substrate near the large beam port, the groove being
configured to hold an optical fiber in alignment with the large
beam port.
66. The optical mode transformer of claim 65, wherein a
semiconductor optical device is mounted in the recess.
67. The optical mode transformer of claim 65, wherein an optical
fiber is mounted in the groove.
68. The optical mode transformer of claims 47 or 50, wherein: the
first function and the second function are chosen such that the
upper and lower cladding provide a lens function in the third
region, whereby a light beam propagating from the small beam port
to the large beam port is caused to be enlarged and collimated.
69. The optical mode transformer of claim 50, wherein: the first
function and the second function are chosen such that the upper and
lower cladding provide a lens function in the third region, whereby
a light beam propagating from the large beam port to the small beam
port is caused to be reduced and focused onto the intermediate beam
port.
70. An optical mode transformer comprising: a substrate having a
substrate surface; a lower waveguide cladding disposed on the
substrate surface, the lower waveguide cladding having a vertical
refractive index having a vertical value that varies according to
first substantially stepwise function of a y-coordinate, the
y-coordinate representing a distance from the substrate surface,
the first function having a maximum value and a minimum value, and
the lower waveguide cladding having a horizontal refractive index
having a horizontal value that varies according to first
substantially stepwise function of an x-coordinate, the
x-coordinate representing a position in a dimension parallel to the
substrate surface and transverse to the long axis, the first
function having a maximum value and a minimum value, the lower
waveguide cladding further having an upper surface; a waveguide
core disposed on the upper surface of the lower waveguide cladding,
the waveguide core having a core refractive index, the ratio of the
core refractive index to the maximum value of the first function
being at least about 1.3, the waveguide core further having a first
end located substantially at the small beam port, a second end
defining an intermediate beam port, and an upper surface; and an
upper waveguide cladding disposed on the upper surface of the
waveguide core and on the upper surface of the lower waveguide
cladding, the upper waveguide cladding having a refractive index
having a value that varies as a second function of the y-coordinate
and of the x-coordinate, the second function having a maximum value
and a minimum value, the ratio of the core refractive index to the
maximum value of the second function being at least about 1.3; the
optical mode transformer being configured such that the waveguide
core has a vertical taper along the long axis, the vertical taper
being a changing thickness of the waveguide core in a dimension
normal to the substrate surface, wherein the thickness decreases
along the long axis from a first thickness value at a first point
near the small beam port to a second thickness value at a second
point near the intermediate beam port, the second thickness value
being less than a critical thickness value, the critical thickness
value being defined as a thickness value below which a significant
portion of the energy of a light beam having a small mode size
received at the small beam port and propagating in the waveguide
core penetrates into at least one of the upper waveguide cladding
layer and the lower waveguide cladding layer, thereby enlarging the
small mode size.
71. An optical mode transformer comprising: a substrate having a
substrate surface; a lower waveguide cladding disposed on the
substrate surface, the lower waveguide cladding having a vertical
refractive index distribution that varies according to first
substantially stepwise function of a y-coordinate, the y-coordinate
representing a distance from the substrate surface, the first
function having a maximum value and a minimum value, and the lower
waveguide cladding having a horizontal refractive index
distribution that varies according to first substantially stepwise
function of an x-coordinate, the x-coordinate representing a
position in a dimension parallel to the substrate surface and
transverse to the long axis, the first function having a maximum
value and a minimum value, the lower waveguide cladding further
having an upper surface; a waveguide core disposed on the upper
surface of the lower waveguide cladding, the waveguide core having
a core refractive index, the ratio of the core refractive index to
the maximum value of the first function being at least about 1.3,
the waveguide core further having a first end located substantially
at a small beam port, a second end defining an intermediate beam
port, and an upper surface; and an upper waveguide cladding
disposed on the upper surface of the waveguide core and on the
upper surface of the lower waveguide cladding, the upper waveguide
cladding having a refractive index distribution that varies as a
second function of the y-coordinate and of the x-coordinate, the
second function having a maximum value and a minimum value, the
ratio of the core refractive index to the maximum value of the
second function being at least about 1.3; the optical mode
transformer being configured such that: in a first region along the
long axis between the small beam port and a transition point, the
waveguide core cross section has a thickness in a dimension normal
to the substrate surface that is substantially constant and equal
to a first thickness value; in a second region along the long axis
between the transition point and the intermediate beam port, the
waveguide core cross section has a thickness that changes along the
long axis from the first thickness value to a second thickness
value smaller than the first thickness value, the second thickness
being small enough to cause a small mode size of a small light beam
received at the small beam port to propagate into at least one of
the upper waveguide cladding layer and the lower waveguide cladding
layer, thereby enlarging the small mode size; and in a third region
along the long axis between the intermediate beam port and a large
beam port, the waveguide core cross section has a thickness that is
approximately constant and equal to the second thickness value.
72. A method of fabricating an optical waveguide having a tapered
waveguide core using a silicon-on-insulator wafer having a silicon
substrate layer, an insulator layer, and a silicon upper layer, the
method comprising: depositing a photoresist layer on the silicon
upper layer of the silicon-on-insulator wafer; applying a mask to
the photoresist, the mask having a gray-scale mask pattern that
provides an exposure level that varies with position along a length
of the mask; exposing the photoresist and mask to radiation from a
radiation source; performing a selective etching procedure that
etches the photoresist and the silicon upper layer and does not
substantially etch the insulator, the presence of the photoresist
during the selective etching procedure causing a vertically tapered
shape to be formed in the silicon upper layer; and depositing an
upper cladding layer over the top and sides of the vertically
tapered shape.
73. The method of claim 72, wherein the radiation source comprises
an ultraviolet radiation source.
74. The method of claim 72, wherein the radiation source comprises
an e-beam source.
75. The method of claim 72, wherein the gray-scale mask pattern has
a tapered shape, the gray-scale mask pattern being wide at an end
at which the gray scale is darkest and narrow at an opposite
end.
76. The method of claim 72, wherein the gray-scale mask pattern has
a tapered shape, the gray-scale mask pattern being narrow at an end
at which the gray scale is darkest and wide at an opposite end.
77. A method of fabricating an optical waveguide using a
silica-on-silicon wafer having a silicon substrate layer and a
silica layer, the optical waveguide having a step refractive index
distribution in both a vertical and a lateral dimension, the method
comprising: depositing a dielectric waveguiding film on the silica
layer; depositing a photoresist layer of said dielectric
waveguiding film; applying a photomask to the dielectric
waveguiding film, the photomask having a stripe defined therein;
exposing the photomask to ultraviolet radiation; and dry-etching a
stripe in the dielectric waveguiding film using a photolithographic
process.
78. A method of fabricating an optical waveguide on a silicon
substrate, the optical waveguide having a step refractive index
distribution in both a vertical and a lateral dimension, the method
comprising the steps of: providing a silica-on-silicon wafer having
a silicon substrate layer and a silica layer; depositing a
photosensitive dielectric waveguiding film on the silica layer;
applying a photomask to the photosensitive dielectric waveguiding
film, said photomask having a stripe defined therein; and exposing
the photosensitive dielectric waveguiding film to ultraviolet
radiation, thereby causing an index of refraction of the
waveguiding film to be selectively increased in an area
corresponding to the stripe defined in the photomask.
79. The method of claim 78, wherein the photosensitive dielectric
waveguiding film comprises a silica-based glass film with germanium
or lead incorporated therein.
80. The method of claim 78, further comprising depositing a
cladding material over the dielectric waveguiding film.
81. The method of claim 78, further comprising depositing a
cladding material over the photosensitive dielectric waveguiding
film.
82. The method of claim 81, wherein exposing the photosensitive
dielectric waveguiding film to ultraviolet radiation is performed
after depositing a cladding material over the photosensitive
dielectric waveguiding film, and wherein the cladding material
comprises a material that does not substantially absorb ultraviolet
radiation.
83. A method of fabricating an optical waveguide on a silicon
substrate, the optical waveguide having a graded refractive index
distribution in a vertical dimension and a step refractive index
distribution in a lateral dimension, the method comprising:
providing a silica-on-silicon wafer having a silicon substrate
layer and a silica layer; successively depositing a first plurality
of thin layers of dielectric material on the silica layer, wherein
each of the first plurality of thin layers of dielectric material
has an effective refractive index larger than a refractive index of
the preceding layer; successively depositing a second plurality of
thin layers of dielectric material, wherein each of the second
plurality of thin layers of dielectric material has an effective
refractive index smaller than a refractive index of the preceding
layer; depositing a photoresist layer on said second plurality of
thin layers of dielectric material; applying a photomask to the
uppermost layer of dielectric material, the photomask having a
stripe defined therein; exposing the photomask to ultraviolet
radiation; and dry-etching a stripe in the dielectric material
using a photolithographic process, thereby forming a waveguide
channel.
84. A method of fabricating an optical waveguide on a silicon
substrate, the optical waveguide having a graded refractive index
distribution in a vertical dimension and a step refractive index
distribution in a lateral dimension, the method comprising:
providing a silica-on-silicon wafer having a silicon substrate
layer and a silica layer; successively depositing a first plurality
of thin layers of photosensitive dielectric material on the silica
layer, wherein each of the first plurality of thin layers of
photosensitive dielectric material has an effective refractive
index larger than a refractive index of the preceding layer;
successively depositing a second plurality of thin layers of
dielectric material, wherein each of the second plurality of thin
layers of dielectric material has an effective refractive index
smaller than a refractive index of the preceding layer; applying a
photomask to the uppermost layer of photosensitive dielectric
waveguiding film, said photomask having a stripe defined therein;
and exposing the layers of photosensitive dielectric waveguiding
film to ultraviolet radiation, thereby causing the index of
refraction of each of the layers of photosensitive dielectric
waveguiding film to be selectively increased in an area
corresponding to the stripe defined in the photomask.
85. The method of claim 83, further comprising depositing a
cladding material over the uppermost layer of dielectric
waveguiding film.
86. The method of claim 84, further comprising depositing a
cladding material over the uppermost layer of photosensitive
dielectric waveguiding film.
87. A method of fabricating an optical waveguide on a silicon
substrate, the optical waveguide having a graded refractive index
distribution in a vertical dimension and a graded refractive index
distribution in a lateral dimension, the method comprising:
providing a silica-on-silicon wafer having a silicon substrate
layer and a silica layer; successively depositing a first plurality
of thin layers of photosensitive dielectric material on the silica
layer, wherein each of the first plurality of thin layers of
photosensitive dielectric material has an effective refractive
index larger than a refractive index of the preceding layer;
successively depositing a second plurality of thin layers of
dielectric material, wherein each of the second plurality of thin
layers of dielectric material has an effective refractive index
smaller than a refractive index of the preceding layer; applying a
photomask to the uppermost layer of photosensitive dielectric
waveguiding film, said photomask having a grayscale pattern defined
therein; and exposing the layers of photosensitive dielectric
waveguiding film to ultraviolet radiation, thereby causing the
index of refraction of each of the layers of photosensitive
dielectric waveguiding film to be selectively increased in
proportion to the grayscale pattern defined in the photomask and
producing a graded index of refraction along the lateral dimension
of each of the layers of photosensitive dielectric waveguiding
film.
88. A method of fabricating an optical waveguide on a silicon
substrate, the optical waveguide having a tapered
high-refractive-index waveguide core and a cladding having a graded
refractive index distribution in a vertical dimension and a step
refractive index distribution in a lateral dimension, the method
comprising: providing a silica-on-silicon wafer having a silicon
substrate layer and a silica layer; successively depositing a first
plurality of thin layers of photosensitive dielectric material on
the silica layer, wherein each of the first plurality of thin
layers of photosensitive dielectric material has an effective
refractive index larger than a refractive index of the preceding
layer; bonding a silicon layer on the uppermost layer of the first
plurality of thin layers of photosensitive dielectric material;
depositing a photoresist layer on the silicon upper layer; applying
a first photomask to the photoresist, the first photomask having a
gray-scale mask pattern that provides an exposure level that varies
with position along a length of the first photomask; exposing the
photoresist and photomask to radiation from a radiation source;
performing a selective etching procedure that etches the
photoresist and the silicon upper layer and does not substantially
etch the insulator, the selective etching procedure causing a
vertically tapered shape to be formed in the silicon upper layer;
successively depositing a second plurality of thin layers of
dielectric material, wherein each of the second plurality of thin
layers of dielectric material has an effective refractive index
smaller than a refractive index of the preceding layer; applying a
second photomask to the uppermost layer of photosensitive
dielectric waveguiding film, said second photomask having a stripe
defined therein; and exposing the layers of photosensitive
dielectric waveguiding film to ultraviolet radiation, thereby
causing the index of refraction of each of the layers of
photosensitive dielectric waveguiding film to be selectively
increased in an area corresponding to the stripe defined in the
second photomask.
89. The optical mode transformer of claim 1, wherein the second end
is optically coupled to a large beam port, and wherein a light beam
having an initial mode size enters the waveguide core from the
small beam port and exits at the large beam port having a final
mode size larger in a dimension normal to the substrate surface
than the initial mode size.
90. The optical mode transformer of claim 1, wherein the second end
is optically coupled to a large beam port, and wherein a light beam
having an initial mode size enters at the large beam port and exits
the waveguide core at the small beam port having a final mode size
smaller in a dimension normal to the substrate surface than the
initial mode size.
91. The optical mode transformer of claim 13, wherein the second
end is optically coupled to a large beam port, and wherein a light
beam having an initial mode size enters the waveguide core form the
small beam port and exits at the large beam port, having a final
mode size larger in a dimension parallel to the substrate surface
and transverse to the direction of light propagation than the
initial mode size.
92. The optical mode transformer of claim 13, wherein the second
end is optically coupled to a large beam port, and wherein a light
beam having an initial mode size enters the waveguide core from the
large beam port and exits at the small beam port having a final
mode size larger in a dimension parallel to the substrate surface
and transverse to the direction of light propagation than the
initial mode input beam.
93. The optical mode transformer of claim 25, wherein the second
end is optically coupled to a large beam port, and wherein a light
beam having an initial mode size enters the waveguide core from the
small beam port and exits at the large beam port having a final
mode size larger in a dimension parallel to the substrate surface
and transverse to the direction of light propagation than the
initial mode size.
94. The optical mode transformer of claim 25, wherein the second
end is optically coupled to a large beam port, and wherein a light
beam having an initial mode size enters the waveguide core at the
large beam port and exits t at the small beam port having a final
mode size smaller in a dimension parallel to the substrate surface
and transverse to the direction of light propagation than initial
mode size.
95. The optical mode transformer of claims 1, 2, 13, 14, 36, 47 or
70, wherein the critical thickness value is defined as
approximately .lambda..sub.0/(4{square root}{square root over
(n.sub.c.sup.2-n.sub.cl.s- up.2)}), where .lambda..sub.0 is the
wavelength of the light beam in a vacuum, where n.sub.c is the
refractive index of the waveguide core, and wherein n.sub.cl is the
index of refraction of the waveguide cladding.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority from co-pending U.S.
Provisional Patent Application No. 60/242,213, filed Oct. 20, 2000,
entitled MULTIPORT INTEGRATED COUPLER FOR BI-DIRECTIONAL LIGHT BEAM
TRANSFORMATION BETWEEN A SMALL SIZE WAVEGUIDE AND A LARGE SIZE
WAVEGUIDE, which is hereby incorporated by reference, as if set
forth in full in this document, for all purposes.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates, in general, to optical
structures that enable optical beam transformation between a
large-mode-size waveguide and a small-mode-size waveguide, and
methods of making the same. In particular, the present invention
relates to methods for transforming the optical mode between a
photonic device and one or more optical fibers. The present
invention also relates, in particular, to the integrated
fabrication of such structures on a module platform or the photonic
device, their connections with one or more input/output optical
fibers.
[0004] 2. Description of the Related Art
[0005] The current strong demand for bandwidth over the Internet
has resulted in great demand for photonic device components in
optical communications and data or information processing. These
device components include fiber optics, non-linear crystal optics,
and integrated optics in such material systems as dielectrics,
polymers, optical crystals, and semiconductors (also called
electro-optic or optoelectronic systems).
[0006] Optical-crystal and dielectric-material-based discrete
optical components such as LiNbO.sub.3-based-modulators, glass
ion-exchange-based optical power splitters and
flame-hydrolysis-deposited silica-on-silicon
multiplexers/demultiplexers, can play certain roles, but their
sizes are generally large and their functions are limited. Hence,
in the long run, it is very unlikely that they can compete with
waveguide based photonic devices such as Photonic Integrated
Circuits (PICs), which can be made very small and also
multifunctional with high packing density similar to today's large
scale integration of microelectronic circuits.
[0007] Waveguides are used for inputting and out putting light
energy for such photonic devices to optical fibers. The
input/output waveguides in a photonic device are typically made up
of dielectric or semiconductor materials. A photonic device may
contain one or more such input/output waveguides. Unless otherwise
specified, such input/output optical waveguides will be referred to
as device waveguides below.
[0008] In spite of the promise of waveguide based photonic devices,
in general, and photonic integrated circuits, in particular,
however, several challenges remain. One challenge is at the optical
interface. Light must be efficiently coupled, with high precision
and stability, between drastically dissimilar components and
materials, in a cost effective, manufacturable way. There are
several issues that need to be addressed with respect to this
challenge, including the following:
[0009] (1) The drastically different spot or mode profile in terms
of size and symmetry between a fiber and a photonic device
waveguide.
[0010] (2) The difficulty in the alignment of a fiber and a
photonic device waveguide, as well as of any other intermediate
component such as a ball lens.
[0011] (3) The difficulty of coupling multiple fibers to a photonic
device with mmultiple device waveguides efficiently in a cost
effective way.
[0012] Prior-art efforts addressing each of these challenges are
summarized below.
[0013] (1) Prior Art in Dealing with the Mode-size Conversion
Issue
[0014] With regard to mode-size conversion, in order to ensure
single-mode operation (as required for high-speed, large-capacity
optical signal manipulation), the dimension of a device waveguide
is typically one order of magnitude less than that of a silica
fiber waveguide. The result is a substantial mode-field mismatch
between these two waveguides. As shown in FIG. 1(a) and (b), for
optimal performance, the mode profile of a single-mode optical
fiber 110 is circular, and its size is generally about 5 to 10
.mu.m in diameter, whereas the mode profile of a photonic device
waveguide 120 is elliptical and its dimension is typically less
than 1 to 3 .mu.m--as small as 0.2 .mu.m for high-density photonic
integrated circuits.
[0015] Various methods are currently used for transforming the
optical modes between an optical fiber and a device waveguide.
These methods are broadly summarized below.
[0016] Method 1--Butt-coupling method
[0017] The simplest coupling arrangement is a direct butt-joining
between a fiber and a semiconductor laser (or other semiconductor
waveguide) as shown in FIG. 2. Since light is only required to
couple in one direction--i.e., from the laser 210 to the fiber
220--one can adjust the gap distance 230 to allow the divergent
cone of light 240 to expand and roughly match the size of a fiber
core 250. One problem with this approach is the relatively low
coupling efficiency caused by the large divergence angle and the
fact that a fiber can only capture and guide a narrower cone of
light within a small capturing angle. As a result, the typical
coupling efficiency for a direct butt-joining is less than 5-30%
depending on the size of the device waveguide. In spite of the low
coupling efficiency, this technique is being explored by NEC of
Japan (among others) for low-cost mass packaging of transceivers
because this technique requires the fewest of components, which
minimizes component cost. (Kenji Yamauchi et al., "Automated mass
production line for optical module using passive alignment
technique," 50th Electronic Components and Technology Conference,
May 21-24, 2000, Las Vegas, Nev., USA).
[0018] Method 2--Lensed fiber or microlens method
[0019] A method improved over the direct butt-joining technique is
to make the fiber end into a lens 310 (lensed fiber) as shown in
FIG. 3 so that more light can be captured by the fiber. (Kazuhiko
Kurata, "Mass production techniques for optical modules," 48th
Electronic Components and Technology Conference, May 27-28, 1998,
Seattle, Wash., USA). Another improved method uses a separate lens
410 placed in the gap 420 as shown in FIG. 4. Various lenses have
been used, including glass ball lenses and GRIN (graded refractive
index) rod lenses, as well as aspheric injection molded plastic
lenses. (Keith Anderson, "Design and manufacturability issues of a
co-packaged DFB/MZ module," 49th Electronic Components and
Technology Conference, Jun. 2-4, 1999, San Diego, Calif., USA).
With these lenses, the coupling efficiency is increased to 50% to
70% for device waveguide mode about 2 .mu.m in size.
[0020] Method 3--Cylindrical lenses method
[0021] Besides geometric discontinuities between a device waveguide
and an optical fiber, the imperfect coupling efficiency results
also in part from the elliptical shape of the light cone emerging
from a typical device waveguide such as that from a Fabry-Perot
cavity semiconductor laser, which causes a non-perfect match with
the circular mode pattern of the fiber. A method to correct for
such elliptical or astigmatic beam shape is shown in FIG. 5, which
illustrates the use of a combination of two perpendicular
cylindrical lenses 510 and 520 of different focusing powers along
the vertical lens (510) and lateral lens (520) directions, which
can circularize the elliptical beam and theoretically increase the
coupling efficiency to about 85% for a typical semiconductor laser
with a mode size of about 1 .mu.m (vertical) by 3 .mu.m
(horizontal). (Sun-Yuan Huang et al., "High coupling optical design
for laser diodes with large aspect ratio," 49th Electronic
Components and Technology Conference, Jun. 2-4, 1999, San Diego,
Calif., USA).
[0022] Method 4--Cylindrical lensed fiber method
[0023] A cylindrical lensed fiber (CLF) has also been used. (Soon
Jang "Automation manufacturing systems technology for
opto-electronic device packaging" 50th Electronic Components and
Technology Conference, May 21-24, 2000, Las Vegas, Nev., USA).
Although the coupling efficiency with the use of a CLF can be high
(.about.90%), the cost is also high because a CLF is not easy to
make, and achieving high coupling efficiency also requires
difficult labour-intensive alignment, as a practical matter.
[0024] Method 5--Laterally tapered rectangular waveguide on top of
a large rectangular waveguide method
[0025] Another approach to mode-size conversion is to place a
laterally tapered rectangular waveguide on a large mode size
rectangular waveguide, where light coupling between the top and the
bottom waveguide occurs as a result of the top lateral taper. This
method can serve the function of mode-size conversion in both the
vertical and horizontal directions, but it is not well accepted in
practice due to the difficulty in integrating such a structure with
a device waveguide and also the cost of manufacturing such a
structure. FIG. 6 shows such a polymer based waveguide structure
610 inserted between a semiconductor laser 620 and a fiber 630. (D.
J. Goodwill et al., "Polymer tapered waveguides and flip-chip
solder bonding as compatible technologies for efficient OEIC
coupling," 47th Electronic Components and Technology Conference
(ECTC), May 18-21, 1997, San Jose, Calif., USA). One difficulty in
this approach is the integration of such a tapered waveguide 610
made of polymer with a laser 620 made of semiconductor material due
to the large difference in their coefficients of thermal expansion
and mechanical stabilities. In the case where such a structure is
made of the same semiconductor material as that of the
semiconductor laser, it would require the epitaxial growth of a
large bottom waveguide layer and the cost will be high.
[0026] Method 6--Vertically tapered down rectangular waveguide
method
[0027] To enable easy integration, vertically tapered down
semiconductor waveguide spot-size converters that squeeze the
guided optical mode into the cladding have been integrated with
semiconductor lasers. (Aaron E. Bond et al., "High speed packaged
electroabsorption modulators for optical communications" 50th
Electronic Components and Technology Conference, May 21-24, 2000,
Las Vegas, Nev., USA; Y. Inaba et al., "Multiquantum-well lasers
with tapered active stripe for direct coupling to single mode
fiber" IEEE Photonics Technology Letters, Vol. 6, pp. 722, 1997; M.
Kitamura, "Method of making a tapered thickness waveguide
integrated semiconductor laser," U.S. Pat. No. 5,792,674, issued
Aug. 11, 1998; Jeon et al., "Laser diode device having a
substantially circular light output beam and a method of forming a
tapered section in a semiconductor device to provide for a
reproducible mode profile of the output beam," U.S. Pat. No.
6,052,397, issued Apr. 18, 2000). Although this method can enlarge
the optical mode in the vertical direction, Problems associated
with such structures include the required length of the tapered
down structure that will lead to additional light propagation loss
and the additional expense of III-V semiconductor materials.
[0028] 2) Costs of Photonic Device Module Connection with Optical
Fibers [Problem #2]
[0029] While the above-mentioned methods may be employed to
transfer optical energy somewhat efficiently between an optical
fiber and a device waveguide of about 2 .mu.m in size, the
approaches of these methods are costly. Typically, an enclosure is
used to house the device, the discrete mode-transferring element
(e.g. a ball lens), and the optical fiber, thereby forming a
packaged module. To align the device waveguide to the fiber and the
mode transferring module, most photonic device manufacturers are
still performing manual alignment under a microscope because of the
very disparate nature of the components, their high price and low
product volumes. Such a process is not well suited to high-volume,
low-cost production.
[0030] Existing techniques for fixing a fiber (and lens) in
position with respect to a rectangular semiconductor waveguide
include epoxy curing, soldering, mechanical fixture, and laser
welding. In order to reduce the need for manual placement/alignment
and fixing in the packaging process, efforts have been focused on
automating the fixing process. For example, Newport, JDS-Uniphase
and NEC are developing automatic parts-handling and assembling
procedures using machine vision combined with micro-stages or
micro-robots to achieve sub-micron precision (Soon Jang,
"Automation manufacturing systems technology for opto-electronic
device packaging," 50th Electronic Components and Technology
Conference, May 21-24, 2000, Las Vegas, Nev., USA; Peter Mueller
and Bernd Valk, "Automated fiber attachment for 980 nm pump
module," 50th Electronic Components and Technology Conference, May
21-24, 2000, Las Vegas, Nev., USA; Kazuhiko Kurata, "Mass
production techniques for optical modules," 48th Electronic
Components and Technology Conference, May 27-28, 1998, Seattle,
Wash., USA).
[0031] At the same time, the concept of a silicon optical bench
(SiOB) on which V-grooves are wet-etched to guide the mounting or
placement of photonic components including fibers, lenses, and even
semiconductor chips has been well accepted; SiOBs are disclosed in
several U.S. patents (e.g., Murphy, "Fiber-waveguide self alignment
coupler," U.S. Pat. No. 4,639,074, issued Jan. 27, 1987; Albares et
al,. "Optical fiber-to-channel waveguide coupler," U.S. Pat. No.
4,930,854, issued Jun. 5, 1990; Benzoni et al., "Single in-line
optical package," U.S. Pat. No. 5,337,398, issued Aug. 9, 1994;
Francis et al., "Waveguide coupler," U.S. Pat. No. 5,552,092,
issued Sep. 3, 1996; Harpin et al., "Assembly of an optical
component and an optical waveguide," U.S. Pat. No. 5,881,190,
issued Mar. 9, 1999; Roff, "Package for an optoelectronic device,"
U.S. Pat. No. 5,9:37,124, issued Aug. 10, 1999). It is very likely
that high precision automation will be combined with silicon
V-groove technology to produce fiber-pigtailed or fiber-connectable
photonic devices. The V-groove technology, however, still needs
some alignment procedure.
[0032] In spite of the above-mentioned approaches, the current
packaging cost is still very high. For example, about 70 to 80% of
the total cost of any fiber-pigtailed III-V optoelectronic module
such as an optical transceiver is due to its packaging. Moreover,
most of the prior art has been aimed at solving the
semiconductor-laser-to-fiber coupling problem, which is
one-directional. For future dense wavelength division multiplexing
(DWDM) optical communication systems, bi-directional multi-port
devices like M.times.N switches will be in large demand, and the
prior art is not able to provide an adequate solution.
[0033] (3) Difficulty of Current Methods for Multiple Fiber
Connections [Problem #3]
[0034] The current methods are somewhat adequate for large photonic
device with one input/output waveguide, they generally become
difficult when more than one input/output and fibers are involved.
This is because the yield for the alignment procedures referred to
above rapidly decreases as the number of input and output fibers
increases. This yield reduction can seriously limit the application
of such coupling and packaging techniques to high-density photonic
integrated circuits, for which tens to hundreds of input and output
fibers are expected to be connected to a single photonic chip.
[0035] The main criteria needed for optical mode transferring
methods and devices to achieve a cost-effective and efficient
optical energy transfer between a device waveguide and one or more
optical fibers can be more specifically described as follows:
[0036] (i) The methods and devices should be able to achieve mode
size transformation from about 10 .mu.m down to about 2 .mu.m (for
.lambda.=1.55 .mu.m) or in the reverse direction for typical
waveguide devices.
[0037] (ii) The methods and devices should be able to achieve mode
size transfer from about 10 .mu.m to below 1 .mu.m (for
.lambda.-1.55 .infin.m) or in the reverse direction for more
challenging waveguide devices such as high-density semiconductor
photonic integrated circuit.
[0038] (iii) The methods and devices should be capable of achieving
self-alignment between the photonic device and the optical fibers
or other intermediate beam transferring elements. Self-alignment
lends itself to low-cost manufacturing. It also allows
cost-effective coupling between a photonic device and more than one
optical fibers.
[0039] (iv) The methods and devices should have low optical
reflection and absorption losses between the photonic device and
the optical fibers.
[0040] (v) The methods and devices should have the flexibility of
transferring the vertical and lateral mode size separately. This
allows it to correct for beam astigmatism in the device waveguide
mode.
[0041] (vi) The methods and devices should have high yield and low
fabrication costs.
[0042] The current mode transformation methods can not adequately
achieve the majority of criterias (i)-(vi). For example, the ball
lens method can achieve (i) and (iv) but not (ii), (iii), (v) and
(vi)
[0043] What is still needed in the field, therefore, are devices
and methods for transferring the mode size between photonic device
and one or more optical fibers that satisfy some or a majority of
critirias (i) to (vi) above.
[0044] The present invention described herein overcomes the various
difficulties encountered by the previous methods by the use of new
optical structures referred to as integrated composite coupling
structures (ICCS). The mode transformation device or mode
transformer is referred to as an Integrated Composite Mode
Transformer (ICMT). With the use of the new optical structures
according to the present invention, disadvantages associated with
prior methods are addressed.
BRIEF SUMMARY OF THE INVENTION
[0045] The integrated composite coupling structures of the present
invention provide an integrated approach to optical mode
transformation. The integrated approach allows fabrication of a
large number of couplers using established processes used
frequently in electronics industries and photonic integrated
circuit industries, thereby resulting in lower fabrication cost.
The composite optical structures allow the beam to be transformed
differently in the vertical and lateral directions.
[0046] An embodiment of the present invention provides a planar
optical structure that can transform the vertical mode size between
a photonic device and an optical fiber. The size of the optical
structure is small relative to the optical fiber diameter, which
reduces alignment sensitivity. In one aspect of the present
invention, the vertical mode transformation is achieved via the use
of a micro vertical graded refractive index (.mu.-VGRIN) structure
that is capable of beam size transformation down to below
.lambda./1.5 (or 1 .mu.m for .lambda.=1.5 .mu.m). Moreover, the
.mu.-VGRIN structure can be fabricated according to the present
invention using established process technology such as
Ion-Assisted-Deposition with low cost and low optical loss.
[0047] In another aspect of the present invention, a composite
structure is formed by combining a .mu.-VGRIN structure with a
horizontal taper structure to achieve separate transformation of
the horizontal and vertical beam mode sizes, thereby allowing 2-D
beam transformation capable of astigmatic beam size correction. The
composite structure can include a cascaded or concurrent
geometry.
[0048] In yet another aspect of the present invention, an
integrated composite coupling structure or its composite variants
is further integrated to an alignment V-groove structure for fiber
on one side and a alignment platform for photonic chip on the other
side to achieve self-alignment between the photonic device, the
optical fiber and the coupling structure (or its composite
variants).
[0049] In yet another aspect of the present invention, a .mu.-VGRIN
structure is combined with a micro-lateral graded refractive index
(.mu.-LGRIN) structure to achieve separate transformation of the
vertical and lateral beamsizes. The .mu.-LGRIN structure can be
fabricated with low cost and large quantity using UV-imprinting
process used in the photonic industry. The composite .mu.-VGRIN and
.mu.-LGRIN structure can include a cascaded or concurrent
geometry.
[0050] In yet another aspect of the present invention, a
high-refractive-index-contrast vertical sharp taper (HRC-VST) and
dielectric structure is used for which the relative refractive
index of the vertical taper material is substantially higher than
that of the dielectric material. The high index contrast allows
beam transformation down to about .lambda./15 (or 0.1 .mu.m for
.lambda.=1.5 .mu.m) when the taper is made up of silicon and the
dielectric is made up of glass. The HRC-VST can be fabricated
according to the present invention using established processes in
the electronics and photonics industries with low costs.
[0051] In yet another aspect of the present invention, a composite
structure is formed by combining an HRC-VST structure with either a
high-refractive-index-contrast laterally gradual taper (HRC-LGT) or
a .mu.-LGRIN structure to achieve separate transformation of the
vertical and lateral beam sizes thereby allowing 2-D
transformations capable of astigmatic beam size correction. The
composite structure can include a cascaded or concurrent
geometry.
[0052] In yet another aspect of the present invention, a HRC-VST
structure or a composite variant is further integrated to a
V-groove structure for fiber on one side and an alignment platform
for photonic chip on the other side to achieve self-alignment
between the photonic device, the optical fiber and the HRC-VST
structure (or its composite variants).
[0053] In yet another aspect of the present invention, the sharp
taper is in the lateral/horizontal direction, resulting in a
high-refractive-index-contrast lateral sharp taper (HRC-LST) which
provide beam transformation in the lateral/horizontal
direction.
[0054] In yet another aspect of the present invention, a plurality
of .mu.-GRIN structures, sharp taper structures, gradual taper
structures and their composite variants are further integrated to a
V-groove structure for fiber on one side and an alignment platform
for photonic chip on the other side to achieve self-aligned beam
transformation between a photonic chip device and more than one
optical fibers.
[0055] In yet another aspect of the present invention, a .mu.-GRIN
structure or a sharp taper structure, or one of their composite
variants is fabricated directly on a photonic device chip.
[0056] A better understanding of the features and advantages of the
present invention will be obtained by reference to the following
detailed description that sets forth illustrative embodiments, in
which the principles of the invention are utilized, and the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] FIG. 1A illustrates the mode profile of a typical
single-mode fiber.
[0058] FIG. 1B illustrates the mode profile of a typical single
mode III-V compound semiconductor channel waveguide.
[0059] FIG. 2 shows a prior-art system in which light is
transmitted directly from a semiconductor laser to a single mode
optical fiber.
[0060] FIG. 3 shows a prior-art system in which light is
transmitted from a semiconductor laser into a lensed optical
fiber.
[0061] FIG. 4 shows a prior-art system in which light is
transmitted via a lens from a semiconductor laser into an optical
fiber.
[0062] FIG. 5 shows side and top views of a prior-art system in
which light is transmitted via two perpendicular cylindrical lenses
of different focusing powers from a semiconductor laser into an
optical fiber.
[0063] FIG. 6 shows a prior-art system in which light is
transmitted via a tapered polymer waveguide from a semiconductor
laser into an optical fiber.
[0064] FIG. 7 shows an optical interface between two media,
illustrating the principles of reflection, refraction, the critical
angle, and total internal reflection.
[0065] FIG. 8 illustrates a principle of light guiding in a
step-index optical fiber or waveguide.
[0066] FIG. 9 illustrates a principle of light guiding in a GRIN
optical fiber or waveguide.
[0067] FIG. 10 illustrates a principle of approximating a
continuous graded refractive index change by using a series of
small refractive index steps.
[0068] FIG. 11A illustrates a planar silicon waveguide film.
[0069] FIGS. 11B-G show a profile of the propagating mode of a
silicon waveguide film with a thickness of 0.4 micron, 0.3 micron,
0.2 micron, 0.1 micron, 0.05 micron, and 0.01 micron,
respectively.
[0070] FIG. 12 shows the mode size of the propagating mode of a
silicon waveguide as a function of the waveguide core
thickness.
[0071] FIG. 13 is a side view of a vertically down-tapered basic
mode transformation module according to the present invention.
[0072] FIG. 14 is a result of a computer simulation of light
propagation in the module of FIG. 13.
[0073] FIGS. 15A-C illustrates exemplary fabrication steps for the
module of FIG. 13.
[0074] FIG. 16 is a top view of a horizontally/laterally
down-tapered basic mode transformation module according to the
present invention.
[0075] FIG. 17 is a result of a computer simulation of light
propagation in the module of FIG. 16.
[0076] FIGS. 18A-D illustrates exemplary fabrication steps for the
module of FIG. 16.
[0077] FIG. 19 is a top view of a horizontally/laterally up-tapered
basic mode transformation module according to the present
invention.
[0078] FIG. 20 is a result of a computer simulation of light
propagation in the module of FIG. 19.
[0079] FIGS. 21A-E illustrates exemplary fabrication steps for the
module of FIG. 19.
[0080] FIGS. 22A-B are, respectively, a top and a side view of a
vertically down-tapered, horizontally/laterally up-tapered basic
mode transformation module according to the present invention.
[0081] FIG. 23 is a result of a computer simulation of light
propagation in the module of FIG. 22.
[0082] FIGS. 24A-F illustrates exemplary fabrication steps for the
module of FIG. 22.
[0083] FIG. 25 is a perspective view of a glass/polymer waveguide
module with a step refractive index in the vertical and
horizontal/lateral direction according to the present
invention.
[0084] FIG. 26 is a result of a computer simulation of light
propagation in the module of FIG. 25.
[0085] FIGS. 27A-E illustrates exemplary fabrication steps for the
module of FIG. 25.
[0086] FIG. 28 is a perspective view of a glass/polymer waveguide
module with a step refractive index in the horizontal/lateral
direction and a graded refractive index in the vertical direction
according to the present invention.
[0087] FIG. 29 is a result of a computer simulation of light
propagation in the module of FIG. 28.
[0088] FIGS. 30A-D illustrates exemplary fabrication steps for the
module of FIG. 28.
[0089] FIG. 31 is a perspective view of a glass/polymer waveguide
module with a graded refractive index in both the
horizontal/lateral direction and the vertical direction according
to the present invention.
[0090] FIG. 32 is a result of a computer simulation of light
propagation in the module of FIG. 31.
[0091] FIGS. 33A-B illustrates exemplary fabrication steps for the
module of FIG. 31.
[0092] FIGS. 34A-B are, respectively, a side view and a top view of
a super mode transformation coupler with a vertically down-tapered,
horizontally/laterally up-tapered high-index waveguide core
embedded in a glass/polymer waveguide with a graded refractive
index in the vertical direction and a step refractive index in the
horizontal/lateral direction according to the present
invention.
[0093] FIG. 35 is a result of a computer simulation of light
propagation in the super mode transformation coupler of FIG.
34.
[0094] FIGS. 36A-N illustrate exemplary fabrication steps for the
super mode transformation coupler of FIG. 34.
[0095] FIGS. 37A-B are, respectively, a side view and a top view of
a super mode transformation coupler with a vertically down-tapered,
horizontally/laterally down-tapered high-index waveguide core
embedded in a glass/polymer waveguide with a graded refractive
index in both the horizontal/lateral and the vertical direction
according to the present invention.
[0096] FIG. 38 is a result of a computer simulation of light
propagation in the super mode transformation coupler of FIG.
37.
[0097] FIGS. 39A-N illustrate exemplary fabrication steps for the
super mode transformation coupler of FIG. 37.
[0098] FIG. 40A is a side view of a super mode transformation
coupler with a vertically down-tapered high-index waveguide core
embedded in a glass/polymer waveguide with a non-symmetric graded
refractive index in the vertical direction and a step refractive
index in the horizontal/lateral direction according to the present
invention.
[0099] FIG. 40B is a top view of the super mode transformation
coupler of FIG. 40A, in which the high-index waveguide core is
horizontally up-tapered.
[0100] FIG. 40C is an alternative top view of the super mode
transformation coupler of FIG. 40A, in which the high-index
waveguide core is horizontally down-tapered.
[0101] FIG. 41 is a result of a computer simulation of light
propagation in the super mode transformation coupler of FIG.
40A.
[0102] FIGS. 42A-N illustrate exemplary fabrication steps for the
super mode transformation couplers of FIG. 40.
[0103] FIG. 43 is a result of a computer simulation of light
propagation in a variation of the super mode transformation coupler
of FIG. 40A.
[0104] FIGS. 44A-C illustrate exemplary fabrication steps for a
variation of the super mode transformation coupler of FIG. 40A.
[0105] FIGS. 45A-C illustrate exemplary fabrication steps for
mounting an optical fiber and a semiconductor optical device
coupled by a super mode transformation coupler according to the
present invention.
[0106] FIG. 46 illustrates a device in which a number of
semiconductor optical devices are coupled to a number of optical
fibers using super mode transformation couplers according to the
present invention.
[0107] FIG. 47 is a schematic illustration of a photonic breadboard
on which various integrated photonic chips are mounted and
interconnected via coupler modules according to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0108] Contents of Detailed Description
[0109] I. General Background and Terminology
[0110] II. General Introduction
[0111] III. Exemplary devices and embodiments
[0112] (1) Exemplary device 1: a high-refractive-index-contrast
vertical sharp-down-taper (HRC-VSDT) ICMT device
[0113] (2) Exemplary device 2: a high-refractive-index-contrast
lateral sharp-down-taper (HRC-LSDT) ICMT device
[0114] (3) Exemplary device 3: a
high-refractive-index-contrast-lateral gradual-up-taper (HRC-LGUT)
ICMT device
[0115] (4) Exemplary device 4: a vertical sharp-down-taper and
lateral gradual-up-taper (VSDT.times.LGUT) ICMT device
[0116] (5) Exemplary device 5: a lateral-step-refractive-index and
vertical-step-refractive-index (LSRIN.times.VSRIN) ICMT device
[0117] (6) Exemplary device 6:
[0118] (A) a composite-lateral-step-refractive-index and
vertical-graded-refractive index (LSRIN.times.VGRIN) ICMT
device
[0119] (B) a composite-lateral-graded-refractive-index and
vertical-graded-refractive index (LGRIN.times.VGRIN) ICMT
device
[0120] (7) Exemplary device 7: a
vertical-sharp-down-taper-and-lateral-gra-
dual-up-taper-cascaded-with-a-vertical-graded-refractive-index-and-lateral-
-step-refractive-index (VSDT.times.LGUT+VGRIN.times.LSRIN) ICMT
device
[0121] (8) Exemplary device 8: a
vertical-sharp-down-taper-and-lateral-sha-
rp-down-taper-embedded-in-a-symmetric-vertical-graded-refractive-index-and-
-lateral-graded-refractive-index
(VSDT.times.LSDT+VGRIN.times.LGRIN) ICMT device
[0122] (9) Exemplary device 9: a
vertical-sharp-down-taper-cascaded-with-a-
-nonsymmetric-vertical-graded-refractive-index-(VSDT+NSVGRIN) ICMT
device
[0123] (10) Variations of exemplary devices and integration of ICMT
with v-grooves for fiber alignments platform for photonic chips
[0124] IV. Applications
[0125] Description of the Specific Exemplary Embodiments
[0126] 1. General background and Terminology
[0127] Described herein are various exemplary processes and
embodiments of the integrated composite mode transformer (ICMT)
structures of the present invention. The structures can, for
example, be used to provide efficient beam transformation between a
photonic waveguide device and one or more optical fibers, wherein
the transformation can correct for astigmatic beam sizes in the
waveguides of the photonic devices. The ICMT structures can further
be placed on a platform for self-alignment with the photonic device
and one or more optical fibers.
[0128] To be consistent throughout the present specification and
for clear understanding of the present invention, the following
background and terminological definitions are hereby provided for
terms used therein:
[0129] a) Refractive Index, Optical Wavelength, Law of Reflection,
Snell's Law of Refraction, Critical Angle
[0130] As is well known, light is an electromagnetic wave
oscillating at very high frequency. When light travels in an
electrically nonconductive medium such as glass, its speed will be
reduced as compared to traveling in a vacuum and the ratio of the
phase velocity of light in a vacuum (c) to that in a medium (v) is
the refractive index n of the medium, 1 n = c v .
[0131] Due to the fact that the frequency of the wave will not
change as light travels from vacuum into a medium, the optical
wavelength in a medium, .lambda., is thus reduced to
.lambda.=.lambda..sub.0/n, where .lambda..sub.0 is the wavelength
in a vacuum.
[0132] Light can be regarded as a ray traveling in a straight line
within a medium of the same refractive index if the size of the
medium (such as a lens) is much greater than the wavelength. At an
optical interface 710 of two different refractive indices n.sub.i
and n.sub.t, a light ray will be reflected back into the first
medium 720 (n.sub.i) and also refracted/transmitted into the second
medium 730 (n.sub.t) as shown in FIG. 7. Law of reflection tells us
that the angle of incidence (.theta..sub.i) equals the angle of
reflection (.theta..sub.r) and Snell's law of refraction says that
the retraction angle (.theta..sub.t) is related to the incidence
angle (.theta..sub.i) through the equation
n.sub.i sin .theta..sub.i=n.sub.t sin .theta..sub.t.
[0133] However, if n.sub.i>n.sub.t, as .theta..sub.i increases
to a particular value called the critical angle, .theta.c will
reach 90.degree., and afterwards, the incident light ray will be
totally reflected 740. Thus, as shown in FIG. 8, light guiding in a
step refractive index optical waveguide or fiber can be explained
by successive total internal reflections 810.
[0134] (b) Optical Waveguide, Planar Waveguide, Channel
Waveguide
[0135] From the above, we see that an optical waveguide is made up
of a material with a high refractive index, surrounded by materials
with a lower refractive index. The optical energy that is guided
lies primarily within the layer with high refractive index. The
layer with high refractive index is called the waveguide core while
the surrounding layers with low refractive indices are called the
waveguide cladding. If the waveguide is in the form of a
two-dimensional layered structure, it is known to those skilled in
the art as a planar waveguide for which optical energy will be
confined to guide along a plane. If the waveguide core is in the
form of a cylindrical or rectangular or other bar type shape, it is
known to those skilled in the art as a channel waveguide. While the
present application is primarily directed towards channel
waveguides, planar waveguides are often used to demonstrate the
basic idea of the principles involved because of their
simplicity.
[0136] (c) Graded Refractive Index and Related Waveguide
Structures, Parabolic Distribution Periodic Focusing
[0137] As shown in FIG. 9, in a graded refractive index (GRIN)
optical waveguide or fiber 910, the waveguiding material has a
refractive index that decreases continuously from the central axis.
In this case, light rays travel through the waveguide or fiber in a
fashion as shown in FIG. 9. Such a GRIN waveguide relies on
refraction and not reflection, except possibly at the cladding
layer.
[0138] A parabolic refractive index distribution will cause the
light rays to bend towards the axis and to get refocused
periodically. As shown in FIG. 10, the behavior can be simply
explained by modeling the continuous change as a series of small
step changes 1010. Note that at each step or interface, the bending
of the ray follows the law of refraction. As the ray travels from
the axis 1020 to the low refractive index region, at each step or
interface, it will bend further towards the horizontal direction
until the incident angle exceeds the critical angle in which case,
the ray will be totally internally reflected 1030. Now the ray is
traveling from low to high refractive index medium and hence, the
law of refraction tells us that at each boundary or interface, the
ray will bend towards the axis until it crosses the axis.
Afterward, it will repeat the cycle. Note that, in addition to
guiding light, the GRIN waveguide also functions as a lens to focus
or expand/collimate a beam of light if the waveguide is cut into
the right length.
[0139] (d) Wave Behavior and Critical Wave-behavior Dimension
(CWBD) for Waveguide Taper
[0140] When the size of the guiding region is no longer much larger
than the wavelength but rather smaller than the wavelength, the ray
concept can no longer give an accurate picture of how light
propagates, and one must resort to wave theory to better describe
light propagation. As an example, consider a semiconductor taper
embedded in a glass medium. There exists a critical wave-behavior
dimension (CWBD) for the waveguide taper of
CWBD=.lambda..sub.0/(4{square root}{square root over
(n.sub.c.sup.2-n.sub.cl.sup.2)}) (where .lambda..sub.0is the
wavelength in vacuum, n.sub.c is the refractive index of the
tapering waveguide core and n.sub.cl is the refractive index of the
waveguide cladding), below which light will penetrate substantially
into the cladding medium while being guided. FIG. 11b to 11f show
the electric field profile of the propagating mode for a silicon
waveguide 1110 (FIG. 11a) with a thickness equal to 0.4 .mu.m, 0.3
.mu.m, 0.2 .mu.m, 0.1 .mu.m, 0.05 .mu.m, and 0.01 .mu.m,
respectively. In FIG. 11, the x-axis indicates space position
across the waveguide in .mu.m, the y-axis indicates normalized
electric field of the lowest transverse electric ("TE") mode. The
wavelength of light is 1.5 .mu.m, the refractive index of the
cladding 1120 is assumed to be 1.5 and the refractive index of the
silicon waveguide core 1110 is assumed to be 3.5. FIG. 12 plots the
mode size (given by the full-width at half maximum of the mode
intensity) as a function of the waveguide core thickness t. From
FIG. 12, as the Si waveguide thickness decreases, the mode size
first decreases and then starts to become larger at approximately
t=1.5/(4{square root}{square root over
(3.5.sup.2-1.5.sup.2)}).apprxeq.0.1 .mu.m.
[0141] (c) Propagating Refractive Index of Waveguide, Waveguide End
Facet Reflectance, Sharp-tip Tapered Waveguide, Sharp Taper and
Gradual Taper
[0142] In an optical waveguide, it is useful to define the
propagating refractive index of the waveguide as n.sub.g=n.sub.c
sin(.theta.i). This propagating refractive index takes a value
between the refractive index of the core n.sub.c and the refractive
index of the cladding n.sub.cl. As the waveguide gets thinner,
.theta..sub.i approaches the critical angle, and n.sub.g approaches
the refractive index of the cladding n.sub.cl. In fact, most of the
optical energy will reside in the cladding when t is very small. If
this waveguide (thinner than the critical wave-behavior dimension
CWBD) is abruptly terminated at an end facet, beyond which is
filled by the cladding material, the reflectance (percentage of
reflected optical power with respective to the incident optical
power) for a guided wave hitting the end facet is given
approximately by the end facet reflectance of 2 R = ( n g - n cl n
g + n cl ) 2 .
[0143] Hence, such end-facet reflectance can be reduced to zero
provided n.sub.g approaches n.sub.cl, which will be the case if the
waveguide is very thin (i.e. all is transmitted).
[0144] A waveguide with a gradually decreasing or increasing
thickness along the direction of propagation is called a taper
waveguide. A taper waveguide can in some way be seen as composed of
many waveguide sections with decreasing or increasing thickness.
From the above discussion, we see that a waveguide taper that
tapers down to a near-zero thickness will first reduces the mode
size and when the mode hits a region with a thickness smaller than
the critical thickness, the mode size will starts to be enlarged.
Furthermore, since the thickness of the waveguide approaches zero
at the taper end, we can expect the optical power reflection at the
taper end is to be negligible. Thus, a waveguide taper that tapers
down to near zero thickness (a sharp-tip tapered waveguide) can be
used to enlarge the mode size of a guided wave and also reduce the
end facet reflection to a negligible value at the same time. Such a
sharp-tip tapered waveguide will be referred to as "sharp taper" as
opposed to "gradual taper"; a gradual taper will not taper down to
reach the CWBD or reach a sharp tip. A gradual taper, for example,
will slowly reduce the mode size and is typically used as a mode
reducer. Furthermore, a gradual down taper will have reflection at
the end face and will typically use anti-reflection techniques to
reduce the end facet reflection.
[0145] The foregoing description serves as a general background
providing a context within which embodiments of the present
invention will be described below and are not meant to restrict any
of the embodiment of the present invention.
[0146] II. General Introduction
[0147] When a submicron semiconductor channel waveguide is to be
connected to a single mode optical fiber with a typical dimension
of about 10 .mu.m, one critical need is the expansion or
enlargement of the mode size from the semiconductor channel
waveguide to match that of the optical fiber, or the reverse
process.
[0148] According to the embodiments of present invention, several
different mode transformation and coupling structures can be cost
effectively fabricated in either a single structure format or a
composite structure format forming various mode transformation
modules.
[0149] As discussed above, the integrated composite mode
transformer (ICMT) structures include at least three types of basic
mode transformation structures, namely:
[0150] (I). The sharp-taper structure (ST) includes a sharp
tapering in either the vertical direction (VST) or the lateral
direction (LST). These structures may be referred to below simply
as ST, VST, and LST, respectively, with the
high-refractive-index-contrast assumed. The sharp taper can further
be distinguished as down or up depending on the direction of
assumed mode propagation. For the purpose of nomenclature
convenience only, the mode propagating direction below is taken to
be from the photonic chip to the optical fiber. Those skilled in
the art will readily be able to generalize it to other mode
propagating configurations.
[0151] (II). The gradual-taper structure (GT), includes gradual
tapering in either the vertical direction (VGT) or the lateral
direction (LGT). These structures may be referred to below simply
as GT, VGT, and LGT respectively. The gradual taper can further be
distinguished as down or up and the propagating direction issue is
as described in (I).
[0152] (III). The micro graded refractive index structure
(.mu.-GRIN) includes a graded refractive index distribution in
either the vertical (.mu.-VGRIN) or lateral (.mu.-LGRIN)
directions. These structures may be referred to below simply as
GRIN, VGRIN, or LGRIN. The GRIN structure can further be
distinguished as symmetric or non-symmetric depending on whether
the graded index has a profile symmetric or asymmetric with respect
to the optical axis of propagation for the input wave.
[0153] Further, the sharp taper (ST) and the gradual taper (GT) may
be in the form of high refractive index contrast structures and
will be referred to as HRC-ST and HRC-GT, respectively. The high
refractive index contrast enables much shorter tapering lengths to
be used while maintaining large transformation of mode sizes.
[0154] A brief description of some of these basic module types are
provided below.
[0155] (I). HRC-ST (High-Refractive-Index-Contrast, Sharp Taper
Structure)
[0156] In this structure type, a high-refractive-index waveguide
layer with vertical or lateral tapering to a sharp tip provides the
mode size transformation. For the case of vertical tapering, the
high-index waveguide layer is fabricated on top of a lower cladding
layer. The upper and side cladding material may be either identical
to or different from the lower cladding; hence the cladding index
can be either symmetric or asymmetric. One novel feature of
embodiments of this structure is the high contrast of the
refractive index of the down taper waveguide core relative to the
refractive index of the cladding, which confines propagating light
to a very small mode size. For example, for the case where the
material of the waveguide core is Si; the refractive index can be
as high as about 3.5. The cladding material can be glass of various
compositions that can have a range of refractive index values from
about 1.45 to about 2.5. The sharp point of the taper must reach a
dimension smaller than the critical wave-behavior dimension of
CWBD=.theta..sub.0/(4{square root}{square root over
(n.sub.c.sup.2-n.sub.cl.sup.2)}) introduced above. In that case,
transformation of the mode size from as small as .lambda./15,
(submicron for .lambda.=1.5 .mu.m) to a few .lambda. (a few microns
for .lambda.=1.5 .mu.m) can be typically achieved.
[0157] (II). HRC-GT (High-Refractive-Index-Contrast, Gradual Taper
Structure)
[0158] In this structure type, a gradual taper provides the mode
size transformation in either the lateral or the vertical
direction. The mode size transformation is due to the fact that
gradual mode size will follow the waveguide size if the waveguide
changes size gradually. One aspect of the embodiments of this
structure is the high contrast of the refractive index of the taper
waveguide core relative to the refractive index of the cladding,
which allows it to achieve large changes in the mode size within a
short propagation distance.
[0159] (III). .mu.-GRIN (Micro-Graded-Refractive-Index
Structure)
[0160] In this structure type, a graded refractive index layer with
either a vertical or lateral refractive index gradiant provides the
mode size transformation. For the case of a vertical graded
refractive index structure, a stepwise refractive index
distribution (e.g., the core has a first constant index, and the
cladding has a second constant index, lower than the first) or a
graded refractive index distribution (index varies throughout the
waveguide, in one of the embodiments, being highest near the
center) is present in the vertical direction. One feature of the
embodiments of this structure is the smallness of the structure,
for which the GRIN structure is less than about 50 .mu.m. This
thickness is small relative to the diameter of the optical
fiber.
[0161] The above three basic beam-transforming structure types
together with other waveguide structures, are combined in various
composite ways in the vertical and lateral directions, to form
different basic beam-transforming modules. The composite structures
allows a basic module to transform the beam size in either the
vertical or lateral direction, or both the vertical and lateral
directions.
[0162] Furthermore, two or more basic modules can be cascaded
spatially to form a combined module. The combined module can be
used to either increase the degree of mode transformation over that
of a basic module, or to have one basic module perform the vertical
mode size transformation and another basic module perform the
lateral mode size transformation.
[0163] Described below are specific embodiments of various mode
transforming devices based on the integrated composite mode
transformer (ICMT) structures of the present invention. The various
basic modules as well as the combined modules will be described. In
the sequence of presentation below, this includes:
[0164] (1) A downward VST or vertical sharp down taper (VSDT) to
enlarge the vertical mode size of an optical beam from a photonic
chip to an optical fiber (e.g., FIGS. 13-15).
[0165] (2) A downward LST or lateral sharp down taper (LSDT) to
enlarge the lateral mode size of an optical beam from a photonic
chip to an optical fiber (e.g., FIGS. 16-18).
[0166] (3) An upward LGT or lateral gradual up taper (LGUT) to
enlarge the latral mode size of an optical beam from a photonic
chip to an optical fiber (e.g., FIGS. 19-21).
[0167] (4) A composite VSDT and LGUT structure forming a
(VSDT.times.LGUT) module to enlarge the mode size vertically and
laterally for an optical beam from a photonic chip to an optical
fiber (e.g., FIGS. 22-24).
[0168] (5) A step-refactive-index (SRIN) waveguide module that
provides simple wave propagation without mode size transformation.
This SRIN module is used as a basic module to combine with other
modules (e.g., FIGS. 25-27). In the case illustrated in FIGS.
25-27, a vertical step index waveguide (VSRIN) and a lateral step
index waveguide (LSRIN) structures are combined to provide simple
mode waveguiding in both the vertical and the lateral
directions.
[0169] (6) A composite vertical graded refractive index (VGRIN) and
LSRIN structure forming a (VGRIN.times.LSRIN) module to enlarge the
vertical mode size for an optical beam from a photonic chip to an
optical fiber (e.g., FIGS. 28-30).
[0170] (7) A composite vertical graded refractive index (VGRIN) and
laterial graded refractive index (LGRIN) structure forming a
(VGRIN.times.LGRIN) module to enlarge the mode size vertically and
laterally for an optical beam from a photonic chip to an optical
fiber (e.g., FIGS. 31-33).
[0171] (8) A combined module including a cascaded (VSDT.times.LGUT)
module and (VGRIN.times.LSRIN) module, namely a
(VSDT.times.LGUT)+(VGRIN.times.L- SRIN) combined module, to
increase the enlargement of the mode size vertically and laterally
for an optical beam from a photonic chip to an optical fiber (e.g.,
FIGS. 34-36).
[0172] (9) A combined module made up of a cascade of
(VSDT.times.LSDT) module and (VGRIN.times.LGRIN) module, namely a
(VSDT.times.LSDT)+(VGRIN.- times.LGRIN) combined module, to
increase the enlargement of the mode size vertically and laterally
for an optical beam from a photonic chip to an optical fiber (e.g.,
FIGS. 37-39).
[0173] (10) A combined module made up of either a cascade of
(VSDT.times.LGUT) module and (VGRIN.times.LSRIN) module with
asymmetric GRIN structure, namely a
(VSDT.times.LGUT)+(VGRIN.times.LSRIN) combined asymmetric GRIN
module, or a cascade of (VSDT.times.LSDT) module and a
(VGRIN.times.LGRIN) module with asymmetric GRIN structure, namely a
(VSDT.times.LSDT)+(VGRIN.times.LGRIN) combined asymmetric GRIN
module, to increase the enlargement of the mode size vertically and
laterally for an optical beam from a photonic chip to an optical
fiber (e.g., FIGS. 40-43).
[0174] (11) A waveguide bonding process useful for the fabrication
of the vertical sharp taper structures (e.g., FIG. 44)
[0175] (12) The integration of the ICMT structures with a V-groove
structure for fiber on one side and an alignment platform for
photonic chip on the other side. Such an integrated structure
allows self-alignment of the ICMT with a photonic chip and an
optical fiber (e.g., FIGS. 45-46).
[0176] While the exemplary devices (1)-(12) typically refer to
transformation of an optical beam between a photonic chip and
optical fibers, it is not meant to limit the applications,
embodiments, or scopes of the exemplary devices. It should be clear
to those skilled in the art that these exemplary devices can be
more generally used to transform optical beams from any type of
small core waveguide to any type of large core waveguide or vice
versa.
[0177] III. Exemplary Deviced and Embodiments
[0178] Beam transforming devices based on ICMT according to
embodiments of the present invention are described in detail
below.
[0179] The devices will be described with respect to transmission
of a single light beam with wavelength .lambda.. It should be
understood that the terms light beam, optical beam, laser beam,
etc., are used interchangeably. Moreover, while descriptions of the
devices refer to a single light beam, there may be more than one
light beam propagating in the device, the light beams may be made
up of light with many wavelengths, the light beam may be
continuous-wave light or pulsed light, and the light beam may have
various beam sizes. Thus the nature of the light beam is used only
for illustrative purposes and is not meant to limit the scope of
the invention.
[0180] Unless otherwise stated, the exemplary dimensions below will
be specified with respect to an exemplary optical wavelength of 1.5
.mu.m. Those skilled in the art will know that the exemplary
dimensions will scale proportionally to the wavelength used which
can range from ultra-violet (e.g., on the order of 0.1 .mu.m) to
far infrared (e.g., on the order of 10 .mu.m).
[0181] The various device embodiments described herein are useful
for transforming the mode size of an input beam having a dimension
on the order of from about 0.2.mu.m or even less to a beam
dimension on the order of about 10 .mu.m to 50 .mu.m or more, and
vice versa, with appropriate changes to the various device
parameters disclosed. Thus one skilled in the art should understand
that the various device embodiment parameters (e.g., length and
width dimensions) disclosed herein are exemplary and may be varied
according to the desired application.
[0182] (1) Exemplary Device 1: A High-refractive-index-contrast
Vertical Sharp-down-taper (HRC-VSDT) ICMT Device
[0183] FIG. 13 illustrates a first general embodiment of an ICMT
device 1300 employing a vertical down-tapering beam enlarger with
high refractive-index contrast between the enlarger core region and
its surrounding cladding region. The device performs as a one
dimensional beam-size enlarging element in the vertical direction
for a propagating optical beam and can, for example, enlarge an
optical beam from a semiconductor waveguide with a beam size as
small as .lambda./7.5 (or 0.2 .mu.m for .lambda.=1.5 .mu.m) to a
large optical beam such as one with a beam size more than five
times larger. The device is not limited to use as a beam enlarger
but can also function as a beam reducer when the optical beam
propagates in the reverse direction. Furthermore, the device can be
used in conjunction with another ICMT module such as a graded
refractive index waveguide module (described below) to further
enlarge and collimate the beam for direct coupling into an optical
fiber and vice versa. It should be understood that these exemplary
applications of device 1300 are intended to illustrate the uses for
device 1300 and are not intended to limit the applications of other
embodiments of device 1300. The device can be referred to as a
"high-refractive-index-contrast -vertical-sharp-down-taper
(HRC-VSDT) ICMT".
[0184] HRC-VSDT ICMT 1300 preferably includes a tapering-down
waveguiding core region occupied by Waveguide Core WC 1345.
Waveguide Core WC 1345 is surrounded by Upper Waveguide Cladding
UWCL 1350 above and Lower Waveguide Cladding LWCL 1310 below. The
Waveguide Core WC 1345 preferably includes a small-beam
input/output port SB-PT 1346, a straight waveguiding core region
SWC 1347, a tapering-down waveguiding core region TDWC 1348, a
straight radiation core region SRC 1351, and a large-beam
output/input port LB-PT 1349. The straight waveguiding core region
SWC 1347 has a length l.sub.SWC and a thickness t.sub.SWC. The
tapering down waveguiding core region TDWC 1348 has a length
l.sub.TDWC, a thickness t.sub.TDWCSB at the small-beam input/output
side and a thickness t.sub.TDWCLB at the large-beam input/output
side. The straight radiation core region SRC 1351 has a length
l.sub.SRC and a thickness t.sub.SRC. The total length of the
waveguide core is given by l.sub.WC=l.sub.SWC+l.sub.TDWC+l.sub.SRC.
The thickness of the lower waveguide cladding LWCL 1310 is
t.sub.LWCL. The thickness of the upper waveguide cladding UWCL 1350
at the small-beam input/output side is t.sub.UWCLSB and at the
large-beam input-output side is t.sub.UWCLLB. The length of the
upper cladding l.sub.UWCl and the length of the lower cladding
l.sub.LWCL are about equal to the total length of the waveguide
core l.sub.WC. The refractive index of the Waveguide Core WC 1345
is n.sub.WC. The refractive index of the Upper Waveguide Cladding
LWCL 1350 is n.sub.UWCL. The refractive index of the Lower
Waveguide Cladding LWCL 1310 is n.sub.LWCL. Lower waveguide
cladding LWCL 1310 is formed on a substrate 1315 as will be
described in more detail below. The length of the straight
waveguide core l.sub.SWC and the straight radiation core l.sub.SRC
are typically not very critical to the operation of the device and
can be zero in some applications (i.e., with these sections
absent).
[0185] For choice of refractive index, there are three options for
operation for the ICMT device 1300, namely the
small-refractive-index-con- trast option, the
medium-refractive-index-contrast option and the
large-refractive-index-contrast option. For the
small-refractive-index contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1345 and that of
the Waveguide Cladding 1350 or 1310, i.e. n.sub.WC/n.sub.UWCL or
n.sub.WC/n.sub.LWCL, is assumed to be larger than 1.0 but smaller
than about 1.3. That is 1.3.gtoreq.n.sub.WC/n.sub.UWCL>1.0 or
1.3.gtoreq.n.sub.WC/n.sub.LWCL&g- t;1.0. For the
medium-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1345 and that of
the Waveguide Cladding 1350 or 1310, i.e. n.sub.WC/n.sub.UWCL or
n.sub.WC/n.sub.LWCL, is assumed to be larger than about 1.3 but
smaller than about 1.5. That is
1.5.gtoreq.n.sub.WC/n.sub.UWCL.gtoreq.1.3 or
1.5.gtoreq.n.sub.WC/n.sub.LWCL.gtoreq.1.3. For the
large-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1345 and that of
the Waveguide Cladding 1350 or 1310, i.e. n.sub.WC/n.sub.UWCL or
n.sub.WC/n.sub.LWCL, is assumed to be larger than about 1.5. That
is n.sub.WC/n.sub.UWCL.gtoreq.1.5 or
n.sub.WC/n.sub.LWCL.gtoreq.1.5. A short tapering length can be
achieved if the relative refractive index contrast is high--in the
region of either the medium-refractive-index contrast or the
large-refractive-index contrast. The medium and large refractive
index contrast will be referred to as the high-refractive index
case.
[0186] In an exemplary device, the input/output port SB-PT 1346 is
configured to receive/transmit a light beam 1341 typically having
wavelength .lambda. with a beam diameter d.sub.SB, and the
output/input port LB-PT 1349 is configured to transmit/receive a
light beam 1342 typically having wavelength .lambda. with a beam
diameter d.sub.LB.
[0187] (i) An Exemplary Device for High-Refractive-Index-Contrast
Case
[0188] In an exemplary embodiment of device 1300 with high
refractive-index contrast, the Waveguide Core WC 1345 is made up of
silicon (Si) with a refractive index of n.sub.WC=3.5, the Upper
Waveguide Cladding 1350 is made up of silica-titania
(SiO.sub.2-TiO.sub.2) material mixture with a mixture composition
to achieve a refractive index of n.sub.UWCL=1.7 or alternatively
Si.sub.3N.sub.4 with a refractive index of about 1.7 can be used.
The Lower Waveguide Cladding 1310 is made up of silicon dioxide
(SiO.sub.2) with a refractive index of n.sub.LWCL=1.5. The
thicknesses of the waveguide core are t.sub.SWC=0.3 .mu.m,
t.sub.TDWCSB=0.3 .mu.m, and t.sub.TDWCLB=t.sub.SRC=0, by "0" it is
meant that t.sub.TDWCLB=t.sub.SRC<<.lambda.. The thicknesses
of the waveguide claddings are t.sub.UWCLSB=5.0 .mu.m,
t.sub.UWCLLB=5.3 .mu.m, and t.sub.LWCL=0.6 .mu.m. The lengths of
the waveguide core are l.sub.SWC=10 .mu.m, l.sub.TDWC=30 .mu.m,
l.sub.SRC=10 .mu.m, and l.sub.WC =50 .mu.m. The lengths of the
waveguide claddings are l.sub.UWCL=l.sub.LWCL=l.sub.WC=50 .mu.m. It
should be appreciated by one skilled in the art that all parameter
values used in this and other exemplary embodiments are approximate
and that the actual values can vary significantly. It should also
be appreciated by one skilled in the art that other materials for
the core and cladding of the various exemplary embodiments may be
used.
[0189] (ii) General Operation of the Device.
[0190] FIG. 14 shows the results of a computer simulation of the
spatial distribution of the electric field strength for light input
at .lambda.=1.5 .mu.m after propagating into waveguide 1347, using
the above exemplary parameters. The mode size at the input end was
approximately 0.3 .mu.m. After propagating for about 35 .mu.m, it
begins to expand at the point where the waveguide thickness (at 5
.mu.m distance from the tip) is tapered down to about 0.05 microns,
which is near the tip of the tapering down region. The mode then
radiates at an angle from that point to a larger mode size. The
mode size reaches a size of about 5 .mu.m at 10 .mu.m away from the
tip of the taper. The compact mode transfer device thus expands the
mode from about 0.3 to about 5 .mu.m over a distance of about 50
.mu.m. This is a much smaller distance compared with mode expansion
distances provided by other prior art devices. Thus such ICMT
devices according to the present invention provide a clear
advantage for coupling fibers to a photonic chip relative to prior
art devices.
[0191] (iii) Device Fabrication Procedures
[0192] An exemplary procedure for fabricating a HRC-VSDT-ICMT such
as device 1300 will now be described with reference to FIGS. 15A-C.
This procedure is given for the purpose of illustration and not
limitation as other similar procedures can be used to achieve the
same fabrication results, and other materials systems or device
structures can be utilized to fabricate devices with the same
functional capabilities.
[0193] The HRC-VSDT-ICMT type structure is fabricated according to
an embodiment by starting with a Silicon-On-Insulator (SOI) wafer.
For a SOI wafer, a high refractive index silicon (Si) layer 1305
having a thickness of t.sub.SWC is already made or bonded on top of
a low refractive index layer of SiO.sub.2 1310 with a thickness of
t.sub.LWCL. The SiO.sub.2 layer 1310 is typically either deposited
on a Si substrate 1315 or thermally oxidized on the Si substrate or
thermally oxidized in the Si substrate after an oxygen ion
implantation process. A fabrication procedure according to one
embodiment is now described below.
[0194] A photoresist layer 1320 is deposited (e.g. spin-coated) on
the silicon layer 1305, as shown in FIG. 15A. A gray-scale mask
1325 is used as a mask to expose the photoresist 1320 under UV
light 1330. The gray-scale mask pattern is designed so that it
provides a graduated exposure level with an exposure dosage that
varies from a small value to a large value that will somewhat
result in a linear photoresist taper after exposure and development
across a length of approximately 30 .mu.m. The photoresist is then
developed. The shape of the photoresist after exposure and
development has a vertical down tapered shape that corresponds to
the variation in the exposure dosage, as shown by photoresist
pattern 1335 in FIG. 15B.
[0195] A dry plasma etching procedure 1340 with selectivity of
1:0.3:.about.0 between photoresist and Si and SiO.sub.2 is used to
etch down the Si layer vertically. Such a process is accomplished
using a reactive ion etching system, or an inductively coupled
plasma system, or another equivalent dry plasma system. The
exemplary processing gases are SF.sub.6/O.sub.2/Cl.sub.2, as
F-reactants and/or their neutrals etch Si, whereas O.sub.2 etch
photoresist and S-Cl reactants and/or their neutrals inhibit the
etching of SiO.sub.2. Exemplary process parameters using a reactive
ion etching system are: a mixture of 30 sccm:20 sccm:20 sccm of
SF.sub.6:O.sub.2:Cl.sub.2, with an RF power of 350 W, and a process
pressure of 25 mTorr. This etching process transfers the down
tapered pattern of the photoresist to the high-refractive-index Si
layer and forms the vertically tapered down Si section 1345. It
should be noted that the ease of this transfer process is dependent
on the thickness tswc of the top waveguiding Si layer. Typically
the starting thickness for a tapered down Si waveguide is in the
range of 0.2 to 0.5 .mu.m. Considering that the photoresist layer
has a typical thickness of about 1.0 .mu.m and that the etching
process typically etches Si at a rate that is not drastically
different from that for the photoresist, such a direct pattern
transfer can be achieved. As the etching process can be made to
etch SiO.sub.2 at a lower rate, the interface between the top Si
layer 1305 and the lower cladding SiO.sub.2 1310 can be used as a
natural stop during the dry etching process. It should be
understood that the above process parameters are presented for
purposes of illustrating a useful embodiment of the fabrication
method and are not intended as a limitation on the device. For
example, a variety of dry etching parameters can be used depending
on the photoresist type and the quality of the SOI wafer. As is
known to those skilled in the art, the end result of a dry etched
structure can be achieved via various combinations of the etching
parameters.
[0196] To make the glass upper cladding layer 1350, flame
hydrolysis deposition, chemical vapor deposition, sputtering,
Ion-Assisted-Deposition or sol-gel spin coating of dielectric
material, can generally be employed to deposit the upper cladding
layer 1350, shown in FIG. 15C. The TiO.sub.2-SiO.sub.2 material
required can, in particular, be achieved using a sol-gel mixture of
TiO.sub.2 and SiO.sub.2 precursors as is well known to those
skilled in the art. The surrounding cladding, including the bottom,
top and side cladding medium, can all have various refractive
indices and also a spatial variation or distribution of the
refractive index value (as will be discussed below), and the actual
value that can be selected for this purpose can cover a wide range,
for instance 1.4 to 2.5.
[0197] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1300 and are not intended to limit other
embodiments of any exemplary device, or the device 1300. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0198] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the various embodiments of the present
invention, including the side surfaces, the top and bottom
surfaces, and the input/output surfaces.
[0199] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0200] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the devices.
[0201] It should be understood to those skilled in the art that the
device 1300 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0202] (2) Exemplary Device 2: A
High-refractive-index-contrast-lateral sharp-down-taper (HRC-LSDT)
ICMT Device
[0203] FIG. 16 illustrates a second general embodiment of an ICMT
device 1400 employing a lateral down-tapering beam enlarger with
high refractive-index contrast between the enlarger core region and
its surrounding cladding region. The device performs as a one
dimensional beam-size enlarging element in the lateral direction
for a propagating optical beam and can, for example, enlarge an
optical beam from a semiconductor waveguide with a beam size as
small as .lambda./7.5 (or 0.2 .mu.m for .lambda.=1.5 .mu.m) to a
large optical beam such as one with a beam size close to that of an
optical fiber. The device is not limited to use as a beam enlarger
but can also function as a beam reducer when the optical beam
propagates in the reverse direction. Furthermore, the device can be
used in conjunction with another ICMT module such as a
graded-refractive-index waveguide module to further enlarge and
collimate the beam for direct coupling into an optical fiber. It
should be understood that these exemplary applications of device
1400 are intended to illustrate the uses for device 1400 and are
not intended to limit the application of other embodiments of
device 1400 to these examples. The device can be referred to as a
"high-refractive-index-contrast-lateral-sh- arp-down-taper
(HRC-LSDT) ICMT".
[0204] The present HRC-LSDT ICMT preferably includes a
tapering-down waveguiding core region occupied by Waveguide Core WC
1450, Waveguide Core WC 1450 is surrounded on both sides by
Waveguide Cladding WCL 1460. Waveguide Core 1450 preferably
includes a small-beam input/output port SB-PT 1451, a straight
waveguiding core region SWC 1452, a tapering-down waveguiding core
region TDWC 1453, a straight radiation core region SRC 1454, and a
large-beam input/output port LB-PT 1455. The straight waveguiding
core region SWC 1452 has a length l.sub.SWC and a width of
w.sub.SWC. The tapering down waveguiding core region TDWC 1453 has
a length of l.sub.TDWC, a width of w.sub.TDWCSB at the small-beam
input/output side, and a width w.sub.TDWCLB at the large-beam
input/output side. The straight radiation core region SRC 1454 has
a length of l.sub.SRC and a width of w.sub.SRC. The total length of
the waveguide core is given by
l.sub.WC=l.sub.SWC+l.sub.TDWC+l.sub.SRC. The width of the waveguide
cladding WCL 1460 on both sides of the waveguide core WC 1450 is
w.sub.WCLSB at the small beam input/output side, and is w.sub.WCLLB
at the large beam side. The length of the waveguide cladding WCL
1460 l.sub.WCL is about equal to the total length of the waveguide
core l.sub.WC. The refractive index of the Waveguide Core WC 1450
is n.sub.WC. The refractive index of the Waveguide Cladding WCL
1460 is w.sub.WCL. The length of the straight waveguide core lswc
and the straight radiation core l.sub.SRC are typically not very
critical to the operation of the device and can be zero in some
applications (i.e. with these sections absent).
[0205] For choice of refractive index, there are three options for
the operation of the ICMT device 1400, namely the
small-refractive-index-cont- rast option, the
medium-refractive-index-contrast option and the
large-refractive-index-contrast option. For the
small-refractive-index-co- ntrast option, the refractive index
ratio between the refractive index of the Waveguide Core 1450 and
that of the Waveguide Cladding 1460 i.e. n.sub.WC/n.sub.WCL, is
assumed to be larger than 1.0 but smaller than 1.3. That is,
1.3.gtoreq.n.sub.WC/n.sub.WCL.gtoreq.1.0. For the
medium-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1450 and that of
the Waveguide Cladding 1460 i.e. n.sub.WC/n.sub.WCL, is assumed to
be larger than 1.3 but smaller than 1.5. That is,
1.5.gtoreq.n.sub.WC/n.sub.WCL.gto- req.1.3. For the
large-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1450 and that of
the Waveguide Cladding 1460 i.e. n.sub.WC/n.sub.WCL, is assumed to
be larger than 1.5. That is, n.sub.WC/n.sub.WCL.gtoreq.1.5. A short
tapering length can be achieved if the relative refractive index
contrast is HIGH--in the region of either the
medium-refractive-index contrast or the large-refractive-index
contrast. The medium and large refractive index contrast will be
referred to as the high-refractive index case.
[0206] In an exemplary device, the input/output port SB-PT 1451 is
configured to receive/transmit a light beam 1461 typically having
wavelength .lambda. with a beam width W.sub.SB, and the
input/output port LB-PT 1455 is configured to receive/transmit a
light beam 1465 typically having wavelength .lambda. with a beam
width w.sub.LB.
[0207] (i) An Exemplary Device for High-Refractive-Index-Contrast
Case
[0208] In an exemplary embodiment of device 1400 in the
high-refractive-index-contrast operation region, the Waveguide Core
WC 1450 is made up of silicon (Si) with a refractive index of
n.sub.WC=3.5, the Waveguide Cladding WCL 1460 is made up of
silica-titania (SiO.sub.2-TiO.sub.2) material mixture with a
mixture composition to achieve a refractive index of n.sub.UWCL=1.7
or alternatively Si.sub.3N.sub.4 with a refractive index of about
1.7 can be used. The widths of the waveguide core are w.sub.SWC=0.3
.mu.m, w.sub.TDWCSB=0.3 .mu.m, and w.sub.TDWCLB=w.sub.SRC=0 .mu.m.
The widths of the waveguide claddings are w.sub.WCLSB=4.85 .mu.m
and w.sub.WCLLB=5.0 .mu.m. The lengths of the waveguide core
regions are l.sub.SWC=0 .mu.m, l.sub.TDWC=30 .mu.m, l.sub.SRC=26
.mu.m, and l.sub.WC=56 .mu.m. The length of the waveguide cladding
is l.sub.WCL=56 .mu.m. It should be appreciated by one skilled in
the art that all parameter values used in this and other exemplary
embodiments are approximate and that the actual values can vary
significantly.
[0209] (ii) General Operation of the Device
[0210] FIG. 17 shows the results of a computer simulation of the
spatial distribution of the electric field strength for the light
input at .lambda.=1.5 .mu.m after propagating into waveguide 1452.
The mode size at the input end was 0.3 .mu.m. After propagating for
25 .mu.m, it begins to expand at the point where the waveguide
thickness (at 5 .mu.m from the tip) is tapered down to 0.05
microns, which is near the tip of the tapering down region. The
mode then radiates at an angle from that point to a larger mode
size. The mode size reaches a size of about 10 .mu.m at 10 .mu.m
away from the tip of the taper.
[0211] (iii) Device Fabrication Procedures
[0212] An exemplary procedure for fabricating HRC-LSDT-ICMT device
1400 will now be described with reference to FIGS. 18A-D. This
procedure is given for the purpose of illustration and not
limitation, as there are other procedures that can be used to
achieve the same fabrication results and other materials systems or
device structures that can be utilized to fabricate devices with
the same functional capabilities.
[0213] The HRC-LSDT ICMT can be fabricated by starting with a
Silicon-On-Insulator (SOI) wafer. For a SOI wafer, a high
refractive index silicon (Si) layer 1410 having a thickness of
t.sub.1 is already made or bonded on top of a low refractive index
layer of SiO.sub.2 1415 with a thickness of t.sub.2. The SiO.sub.2
layer 1415 is typically either deposited on a Si substrate 1420 or
thermally oxidized on the Si substrate or thermally oxidized in the
Si substrate after an oxygen ion implantation process. A
fabrication procedure according to one embodiment is described
below:
[0214] A UV or e-beam resist layer 1405 is first spin-coated on the
Si layer 1410, as shown in FIG. 18B. A mask 1425 with a laterally
tapered down/narrow mask pattern 1430 as shown in FIG. 18A is used
as a mask to expose the photoresist 1405 under UV light or e-beam
1435. The photoresist is then developed. The shape of the
photoresist after exposure and development has a lateral
down-tapered shape as shown by photoresist pattern 1440 in FIG. 18B
and FIG. 18C.
[0215] A dry plasma etching procedure 1445 that selectively etches
Si but not the photoresist 1440 or the SiO.sub.2 1415 is used. Such
a process can be accomplished using a reactive ion etching system,
or an inductively coupled plasma system, or any dry plasma system.
The exemplary processing gases are SF.sub.6/Cl.sub.2, as
F-reactants and/or their neutrals etch Si, and S-Cl reactants
and/or their neutrals inhibit the etching of SiO.sub.2. Exemplary
process parameters using a reactive ion etching system are: a
mixture of approximately 30 sccm:20 sccm of SF.sub.6:Cl.sub.2, with
an RF power of 200 W, and a process pressure of about 25 mTorr.
This etching process transfers the laterally down tapered pattern
of the photoresist 1440 to the high-refractive-index Si layer and
forms the laterally tapered down Si section 1450. It should be
noted that this transfer process is performed with the starting
thickness for the Si waveguide layer in the range of 0.2 to 0.5
.mu.m, and the photoresist layer has a typical thickness of about
1.0 .mu.m. As the etching process can be made to etch SiO.sub.2 at
a lower rate, the interface between the top Si layer 1410 and the
lower cladding SiO.sub.2 1415 can be used as a natural stop during
the dry etching process. A top view of the result of the process is
shown in FIG. 18D. It should be understood that the above process
parameters are presented for purposes of illustrating a useful
embodiment of the fabrication method and are not intended to limit
other embodiments of the method. For example, a variety of dry
etching parameters can be used depending on the photoresist type
and the quality of the SOI wafer. As is known to those skilled in
the art, the end result of a dry etched structure can be achieved
via various combinations of the etching parameters. It should, also
be noted that due to the limitation in the smallest feature size of
UV based photolithography, e-beam lithography might be necessary or
at least it might be required to form the tip part of the
taper.
[0216] To make the glass cladding 1460, flame hydrolysis
deposition, chemical vapor deposition, sputtering,
Ion-Assisted-Deposition, or sol-gel spin coating of dielectric
material, is employed to deposit the cladding regions 1460 shown in
FIG. 16. The TiO.sub.2-SiO.sub.2 material required can, in
particular, be achieved using a sol-gel mixture of TiO.sub.2 and
SiO.sub.2 precursors as is well known to those skilled in the art.
Note that the surrounding cladding, including the bottom, top and
side cladding medium, can all have various fixed or spatially
varying or distributed refractive indices and also a spatial
variation or distribution of the refractive index value (as
described below), and the actual value that can be selected for
this purpose can cover a wide range of 1.4 to 2.5.
[0217] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1400 and are not intended to limit other
embodiments of any exemplary device, or the device 1400. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0218] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the present invention, including the side
surfaces, the top and bottom surfaces, and the input/output
surfaces
[0219] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0220] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the devices.
[0221] It should be understood to those skilled in the art that the
device 1400 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0222] (3) Exemplary Device 3: A
High-refractive-index-contrast-lateral gradual-up-taper (HRC-LGUT)
ICMT Device
[0223] FIG. 19 illustrates a third general embodiment of an ICMT
device 1500 employing lateral tapering up beam enlarger with high
refractive index contrast between the enlarger core region and its
surrounding cladding region. The device performs as a one
dimensional beam-size enlarging element in the lateral direction
for a propagating optical beam and can for example enlarge an
optical beam from a semiconductor waveguide with a beam size as
small as .lambda./7.5 (or 0.2.mu.m for .lambda.=1.5 .mu.m) to a
large optical beam such as one with a beam size close to that of an
optical fiber. The device is not limited to use as a beam enlarger
but can also function as a beam reducer when the optical beam
propagates in the reverse direction. Furthermore, the device can be
used in conjunction with another ICMT module such as a step-index
waveguide module as will be described below to further collimate
the beam for direct coupling into an optical fiber. It should be
understood that these exemplary applications of device 1500 are
intended to illustrate the uses for device 1500 and are not
intended to limit the application of other embodiments of device
1500. The device can be referred to as a
"high-refractive-index-contrast-lateral-gradual-up-taper (HRC-LGUT)
ICMT", and the tapering up is in the lateral direction.
[0224] The present HRC-LGUT ICMT preferably includes a tapering-up
waveguiding core region occupied by Waveguide Core WC 1550.
Waveguide Core WC 1550 is surrounded on both sides by Waveguide
Cladding WCL 1560. Waveguide Core 1550 preferably includes a
small-beam input/output port SB-PT 1551, a straight waveguiding
core region SWC 1552, a tapering-up waveguiding core region TUWC
1553, a straight radiation core region SRC 1554, and a large-beam
input/output port LB-PT 1555. The straight waveguiding core region
SWC 1552 has a length l.sub.SWC and a width of w.sub.SWC. The
tapering up waveguiding core region TUWC 1553 has a length of
l.sub.TUWC, a width of w.sub.TUWCSB at the small-beam input/output
side, and a width w.sub.TUWCLB at the large-beam input/output side.
The straight radiation core region SRC 1554 has a length of
l.sub.SRC and a width of w.sub.SRC. The total length of the
waveguide core is given by l.sub.WC=l.sub.SWC+l.sub.TUWC+l.sub.SRC.
The width of the waveguide cladding WCL 1560 on both sides of the
waveguide core WC 1550 is w.sub.WCLSB at the small beam
input/output side, and is w.sub.WCLLB at the large beam side. The
length of the waveguide cladding WCL 1560 l.sub.WCL is about equal
to the total length of the waveguide core l.sub.WC. The refractive
index of the Waveguide Core WC 1550 is n.sub.WC. The refractive
index of the Waveguide Cladding WCL 1560 is n.sub.WCL. The length
of the straight waveguide core l.sub.SWC and the straight radiation
core l.sub.SRC are typically not very critical to the operation of
the device and can be zero in some applications (i.e. with these
sections absent).
[0225] There are three options for the refractive index contrast of
the ICMT device 1500, namely the small-refractive-index-contrast
option, the medium-refractive-index-contrast option and the
large-refractive-index-co- ntrast option. For the
small-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1550 and that of
the Waveguide Cladding 1560 i.e. n.sub.WC/n.sub.WCL, is assumed to
be larger than 1.0 but smaller than about 1.3. That is
1.3.gtoreq.n.sub.WC/n.sub.WCL.gtoreq.1.0. For the
medium-refractive-index- -contrast option, the refractive index
ratio between the refractive index of the Waveguide Core 1550 and
that of the Waveguide Cladding 1560 i.e. n.sub.WC/n.sub.WCL, is
assumed to be larger than about 1.3 but smaller than about 1.5.
That is 1.5.gtoreq.n.sub.WC/n.sub.WCL.gtoreq.1.3. For the
large-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1550 and that of
the Waveguide Cladding 1560 i.e. n.sub.WC/n.sub.WCL, is assumed to
be larger than about 1.5. That is n.sub.WC/n.sub.WCL.gtoreq.1.5. A
short tapering length can be achieved if the relative refractive
index contrast is HIGH--in the region of either the
medium-refractive-index contrast or the large-refractive-index
contrast. In an exemplary device, the input/output port SB-PT 1551
is configured to receive/transmit a light beam 1561 typically
having wavelength .lambda. with a beam width w.sub.SB, and the
input/output port LB-PT 1555 is configured to receive/transmit a
light beam 1565 typically having wavelength .lambda. with a beam
width w.sub.LB. The medium and large refractive index contrast will
be referred to as the high-refractive index case.
[0226] (i) An Exemplary Device for High-Refractive-Index-Contrast
Case
[0227] In an exemplary embodiment of device 1500 with
high-refractive-index-contrast, the Waveguide Core WC 1550 is made
up of silicon (Si) with a refractive index of n.sub.WC=3.5, the
Waveguide Cladding WCL 1560 is made up of silica-titania
(SiO.sub.2-TiO.sub.2) material mixture with a mixture composition
to achieve a refractive index of nUWCL=1.7 or alternatively
Si.sub.3N.sub.4 with a refractive index of about 1.7 can be used. T
with a refractive index of nWCL=1.7. The widths of the waveguide
core are w.sub.SWC=0.3 .mu.m, w.sub.TUWCSB=0.3 .mu.m, and
w.sub.TUWCLB=w.sub.SRC=10 .mu.m. The widths of the waveguide
claddings are w.sub.WCLSB=6.85 .mu.m and w.sub.WCLLB=2 .mu.m. The
lengths of the waveguide core regions are l.sub.SWC=10 .mu.m,
l.sub.TUWC=100 .mu.m, l.sub.SRC=10 .mu.m, and l.sub.WC=120 .mu.m.
The length of the waveguide cladding is l.sub.WCL=120 .mu.m. It
should be appreciated by one skilled in the art that all parameter
values used in this and other exemplary embodiments are approximate
and that the actual values can vary significantly.
[0228] (ii) General Operation of the Device
[0229] FIG. 20 shows the results of a computer simulation of the
spatial distribution of the electric field strength for the light
input at .lambda.=1.5 .mu.m after propagating into waveguide 1552.
The mode size at the input end was 0.3 .mu.m. After propagating for
10 .mu.m, it begins to expand at the point where the waveguide core
tapers up. The mode then enlarges following the up-tapering
waveguide core section to a larger mode size. The mode size reaches
a size of about 10 .mu.m at the end of the up-taper.
[0230] (iii) Device Fabrication Procedures
[0231] An exemplary procedure for fabricating an LGUT-ICMT device
1500 will now be described with reference to FIGS. 21A-E. This
procedure is given for the purpose of illustration and not
limitation, as there are other procedures that can be used to
achieve the same fabrication results and other materials systems or
device structures that can be utilized to fabricate devices with
the same functional capabilities.
[0232] It should be noted that the fabrication steps here are very
similar to those described for the laterally tapered down case. In
fact, the present structure is even easier to fabricate because
e-beam lithography is not required and hence the cost is even
lower. The HRC-LGUT ICMT type structure can be fabricated by
starting with a Silicon-On-Insulator (SOI) wafer that can be
fabricated via a commercial process known to those skilled in the
art. For a SOI wafer, a high refractive index silicon (Si) layer
1510 having a thickness of t.sub.1 is already made or bonded on top
of a low refractive index layer of SiO.sub.2 1515 with a thickness
of t.sub.2, as shown in FIG. 21A. The SiO.sub.2 layer 1515 is
typically either deposited on a Si substrate 1520 or thermally
oxidized on the Si substrate or thermally oxidized in the Si
substrate after an oxygen ion implantation process. The fabrication
procedure according to one embodiment is now described below.
[0233] A photoresist layer 1505 is first spin-coated on the Si
layer 1510, as shown in FIG. 21A. A mask 1525 with a laterally
tapered up mask pattern 1530, shown in FIG. 21B, is used as a mask
to expose the photoresist 1505 under UV light 1535. The photoresist
is then developed. The shape of the photoresist after exposure and
development has a lateral up tapered shape as shown by photoresist
pattern 1540 in FIG. 21D.
[0234] A dry plasma etching procedure 1545 that selectively etches
Si but not the photoresist 1540 or the SiO.sub.2 1515 can be used,
as shown in FIG. 21C. Such an etch process is accomplished using a
reactive ion etching system, or an inductively coupled plasma
system, or any dry plasma system. The exemplary processing gases
are SF.sub.6/Cl.sub.2, as F-reactants and/or their neutrals etch
Si, and S-Cl reactants and/or their neutrals inhibit the etching of
SiO.sub.2. Exemplary process parameters using a reactive ion
etching system are: a mixture of 30 sccm:20 sccm of
SF.sub.6:Cl.sub.2, with an RF power of 200 W, and a process
pressure of 25 mTorr. This etching process transfers the laterally
up tapered pattern of the photoresist 1540 to the
high-refractive-index Si layer and forms the laterally up-tapered
Si section 1550. It should be noted that this transfer process is
possible because the starting thickness of the Si waveguide layer
is in the range of 0.2 to 0.5 .mu.m, and the photoresist layer has
a typical thickness of about 1.0 .mu.m. As the etching process can
be made to etch SiO.sub.2 at a lower rate, the interface between
the top Si layer 1510 and the lower cladding SiO.sub.2 1515 can be
used as a natural stop during the dry etching process. It should be
understood that the above process parameters are presented for
purposes of illustrating a useful embodiment of the fabrication
method and are not intended to limit other embodiments of the
method. For example, a variety of dry etching parameters can be
used depending on the photoresist type and the quality of the SOI
wafer. As is known to those skilled in the art, the end result of a
dry etched structure can be achieved via various combinations of
the etching parameters.
[0235] To make the glass cladding 1560, flame hydrolysis
deposition, chemical vapor deposition, sputtering,
Ion-Assisted-Deposition, or sol-gel spin coating of dielectric
material, can generally be employed to deposit the cladding regions
1560 shown in FIG. 19. The TiO.sub.2-SiO.sub.2 material required
can, in particular, be achieved using a sol-gel mixture of
TiO.sub.2 and SiO.sub.2 precursors as is well known to those
skilled in the art. Note that the surrounding cladding, including
the bottom, top and side cladding medium, can all have various
refractive indices and also a spatial variation or distribution of
the refractive index value (as described below), and the actual
value that can be selected for this purpose can cover a wide range,
for instance 1.4 to 2.5.
[0236] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1500 and are not intended to limit other
embodiments of any exemplary device, or the device 1500. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0237] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the present invention, including the side
surfaces, the top and bottom surfaces, and the input/output
surfaces.
[0238] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0239] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the devices.
[0240] It should be understood to those skilled in the art that the
device 1500 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0241] (4) Exemplary Device 4: A Vertical Sharp-down-taper and
lateral Gradual-up-taper (HRC-VSDT.times.LGUT) ICMT Device
[0242] FIGS. 22A-B illustrates a fourth general embodiment of an
ICMT device 1600 including a lateral gradual up-tapered and
vertical sharp down-tapered beam transformer with high refractive
index contrast between the enlarger core region and its surrounding
cladding region. The device can perform as a two dimensional
beam-size enlarging element in both the lateral and the vertical
directions for a propagating optical beam and can, for example,
enlarge an optical beam from a semiconductor waveguide with a beam
size as small as .lambda./7.5 (or 0.2 .mu.m for .lambda.=1.5 .mu.m)
to a large optical beam such as one with a beam size more than five
times larger. The device is not limited to use as a beam enlarger
but can also function as a beam reducer when the optical beam
propagates in the reverse direction. Furthermore, the device can be
used other ICMT modules such as a lateral step-index waveguide
module and a vertical graded index waveguide module as will be
discussed below to further collimate the beam for direct coupling
into an optical fiber. It should be understood that these exemplary
applications of device 1600 are intended to illustrate the uses for
device 1600 and are not intended to limit the applications of other
embodiments of device 1600. The device can be referred to as a
"high-refractive-index-contrast-vertical sharp-down-taper and
lateral gradual-up-taper ICMT" (HRC-VSDT.times.LGUT ICMT).
[0243] The present HRC-VSDT.times.LGUT ICMT preferably includes a
laterally up-tapered and vertically down-tapered waveguiding core
region occupied by Waveguide Core WC 1650. When viewed from the
top, Waveguide Core WC 1650 is surrounded on both sides by Side
Waveguide Cladding SWCL 1670. Waveguide Core 1650 preferably
includes a lateral small-beam input/output port LSB-PT 1671, a
laterally straight waveguiding core region LSWC 1672, a laterally
tapering-up waveguiding core region LTUWC 1673, a laterally
straight radiation core region LSRC 1674, and a lateral large-beam
input/output port LLB-PT 1675. The laterally straight waveguiding
core region LSWC 1672 has a length l.sub.LSWC and a width of
w.sub.LSWC. The laterally tapering up waveguiding core region LTUWC
1673 has a length of l.sub.LTUWC, a lateral width of w.sub.LTUWCSB
at the small-beam input/output side, and a width w.sub.LTUWCLB at
the large-beam input/output side. The laterally straight radiation
core region LSRC 1674 has a length of l.sub.LSRC and a width of
w.sub.LSRC. The total length of the waveguide core when viewed from
the top is given by l.sub.WC=l.sub.LSWC+l.sub.LTUWC+l.sub.LSRC. The
width of the side waveguide cladding SWCL 1670 on both sides of the
waveguide core WC 1650 is w.sub.SWCLSB at the small beam
input/output side, and is w.sub.SWCLLB at the large beam side. The
length of the side waveguide cladding SWCL 1670 l.sub.SWCL is about
equal to the total length of the waveguide core l.sub.WC. The
length of the straight waveguide core l.sub.LSWC and the straight
radiation core l.sub.LSRC. are typically not very critical to the
operation of the device and can be zero in some applications (i.e.,
with these sections absent).
[0244] When viewed from the side, Waveguide Core WC 1650 is
surrounded at the bottom by Lower Waveguide Cladding LWCL 1615, and
at the top by Upper Waveguide Cladding UWCL 1690. Waveguide Core
1650 preferably includes a vertical small-beam input/output port
VSB-PT 1691, a vertically straight waveguiding core region VSWC
1692, a vertically tapering-down waveguiding core region VTDWC
1693, a vertically straight radiation core region VSRC 1694, and a
vertical large-beam input/output port VLB-PT 1695. The vertically
straight waveguiding core region VSWC 1692 has a length l.sub.VSWC
and a thickness of t.sub.VSWC. The vertically tapering down
waveguiding core region VTDWC 1693 has a length of l.sub.VTDWC, a
vertical thickness of t.sub.VTDWCSB at the small-beam input/output
side, and a vertical thickness of t.sub.VTDWCLB at the large-beam
input/output side. The vertically straight radiation core region
VSRC 1694 has a length of l.sub.VSRC and a thickness of t.sub.VSRC.
The total length of the waveguide core when viewed from the side is
given by l.sub.WC=l.sub.VSWC+l.sub.VTDWC+l.sub.VSRC. The thickness
of the lower waveguide cladding LWCL 1615 is t.sub.LWCL. The
thickness of the upper waveguide cladding UWCL 1690 at the
small-beam input/output side is t.sub.UWCLSB and at the large-beam
input-output side is t.sub.UWCLLB. The length of the upper cladding
l.sub.UWCL and the length of the lower cladding l.sub.LWCL are
about equal to the total length of the waveguide core l.sub.WC. The
length of the straight waveguide core l.sub.LSWC and the straight
radiation core l.sub.LSRC are typically not very critical to the
operation of the device and can be zero in some applications (i.e.,
with these sections absent).
[0245] The refractive index of the Waveguide Core WC 1650 is
n.sub.WC. The refractive index of the Side Waveguide Cladding SWCL
1670 is n.sub.SWCL. The refractive index of the Lower Waveguide
Cladding LWCL 1615 is n.sub.LWCL. The refractive index of the Upper
Waveguide Cladding UWCL 1690 is n.sub.UWCL.
[0246] There are three options for the refractive index of the ICMT
device 1600, namely the small-refractive-index-contrast option, the
medium-refractive-index-contrast option and the
large-refractive-index-co- ntrast option. For the
small-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1650 and that of
the Waveguide Claddings 1615/1690/1670 i.e. n.sub.WC/n.sub.LWCL,
n.sub.WC/n.sub.UWCL, n.sub.WC/n.sub.SWCL, is assumed to be larger
than 1.0 but smaller than about 1.3. That is
1.3.gtoreq.n.sub.WC/n.sub.LWCL, n.sub.WC/n.sub.UWCL,
n.sub.WC/n.sub.SWCL, .gtoreq.1.0. For the
medium-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1650 and that of
the Claddings 1615/1690/1670 i.e. n.sub.WC/n.sub.LWCL,
n.sub.WC/n.sub.UWCL, n.sub.WC/n.sub.SWCL, is assumed to be larger
than 1.3 but smaller than about 1.5. That is
1.5.gtoreq.n.sub.WC/n.sub.LWCL, n.sub.WC/n.sub.UWCL,
n.sub.WC/n.sub.SWCL, .gtoreq.1.3. For the
large-refractive-index-contrast option, the refractive index ratio
between the refractive index of the Waveguide Core 1650 and that of
the Claddings 1615/1690/1670 i.e. n.sub.WC/n.sub.LWCL,
n.sub.WC/n.sub.UWCL, n.sub.WC/n.sub.SWCL, is assumed to be larger
than about 1.5. That is n.sub.WC/n.sub.LWCL,
n.sub.WC/.sub.n.sub.UWCL, n.sub.WC/n.sub.SWCL, .gtoreq.1.5. A short
tapering length can be achieved if the relative refractive index
contrast is high--in the region of either the
medium-refractive-index contrast or the large-refractive-index
contrast. The medium and large refractive index contrast will be
referred to as the high-refractive index case.
[0247] In an exemplary device, the input/output port LSB-PT/VSB-PT
1671/1691 is configured to receive/transmit a light beam 1678/1698
typically having wavelength .lambda. with a lateral beam width
w.sub.LSB, and a vertical beam width w.sub.VSB. The input/output
port LLB-PT/VLB-PT 1675/1695 is configured to receive/transmit a
light beam 1679/1699 typically having wavelength .lambda. with a
lateral beam width of w.sub.LLB, and a vertical beam width of
w.sub.VLB.
[0248] (i) An Exemplary Device
[0249] In an exemplary embodiment of device 1600, the Waveguide
Core WC 1650 is made up of silicon (Si) with a refractive index of
n.sub.WC=3.5, the Lower Waveguide Cladding LWCL 1615 is made up of
silica (SiO.sub.2) with a refractive index of n.sub.LWCL=1.5, and
the Upper and Side Waveguide Claddings UWCL/SWCL are made of
silica-titania (SiO.sub.2-TiO2) material mixture with a mixture
composition to achieve a refractive index of n.sub.UWCL=1.7 or
alternatively Si.sub.3N.sub.4 with a refractive index of about 1.7
can be used.
[0250] When viewed from the top, the lateral widths of the
waveguide core are w.sub.LSWC=0.3 .mu.m, w.sub.LTUWCSB=0.3 .mu.m,
and w.sub.LTUWCLB=w.sub.LSRC=10 .mu.m. The widths of the side
waveguide claddings are w.sub.SWCLSB=4.85 .mu.m and w.sub.SWCLLB=0
.mu.m. The lengths of the waveguide core regions are l.sub.LSWC=10
.mu.m, l.sub.LTUWC=100 .mu.m, l.sub.LSRC=10 .mu.m, and l.sub.WC=120
.mu.m. The length of the side waveguide cladding is l.sub.SWCL=120
.mu.m.
[0251] When viewed from the side, the vertical thicknesses of the
waveguide core are t.sub.VSWC=0.3 .mu.m, t.sub.VTDWCSB=0.3 .mu.m,
and t.sub.VTDWCLB=t.sub.VSRC=0. The thicknesses of the upper and
lower waveguide claddings are t.sub.UWCLSB=9.7 .mu.m,
t.sub.UWCLLB=10 .mu.m, and t.sub.LWCL=0.6 .mu.m. The lengths of the
waveguide core regions are l.sub.VSWC=80 .mu.m, l.sub.VTDWC=30
.mu.m, l.sub.VSRC=0 .mu.m, and l.sub.WC=110 .mu.m. The lengths of
the upper and lower waveguide claddings are
l.sub.UWCL=l.sub.LWCL=l.sub.WC=120 .mu.m. It should be appreciated
by one skilled in the art that all parameter values used in this
and other exemplary embodiments are approximate and that the actual
values can vary significantly.
[0252] (ii) General Operation of the Device
[0253] FIG. 23 shows the result of a computer simulation of the
spatial distribution of the electric field strength in the vertical
and lateral directions for the light input at .lambda.=1.5 .mu.m
after propagating into waveguide 1672/1692. The mode size at the
input end was 0.3 .mu.m in both the vertical and lateral
directions.
[0254] When viewed from the side, the light beam begins to expand
at the point where the waveguide thickness (at 5 .mu.m distance
from the tip) is tapered down to 0.05 microns, which is near the
tip of the tapering down region. The mode then radiate at an angle
from that point to a larger mode size. The mode size reaches a size
of about 5 .mu.m at 10 .mu.m away from the tip of the taper.
[0255] When viewed from the top, the light beam begins to expand at
the point where the waveguide core tapers up laterally. The mode
then enlarges following the lateral up-tapering waveguide core
section 1673 to a larger mode size. The mode size reaches a size of
about 10 .mu.m at the end of the up-taper.
[0256] (iii) Device Fabrication Procedures
[0257] An exemplary procedure for fabricating the
HRC-VSDT.times.LGUT ICMT device 1600 will now be described with
reference to FIGS. 24A-F. This procedure is given for the purpose
of illustration and not limitation, as there are other procedures
that can be used to achieve the same fabrication results and other
materials systems or device structures that can be utilized to
fabricate devices with the same functional capabilities.
[0258] The HRC-VSDT.times.LGUT ICMT is fabricated by starting with
a Silicon-On-Insulator (SOI) wafer. For a SOI wafer, a high
refractive index silicon (Si) layer 1610 having a thickness of
t.sub.1 is already made or bonded on top of a low refractive index
layer of SiO.sub.2 1615 with a thickness of t.sub.2, as illustrated
in FIG. 24A. The SiO.sub.2 layer 1615 is typically either deposited
on a Si substrate 1620 before wafer bonding or thermally oxidized
on the Si substrate before wafer bonding or thermally oxidized in
the Si substrate after an oxygen ion implantation process. The
fabrication procedure according to one embodiment is now described
below:
[0259] A photoresist layer 1605 is first spin-coated on the Si
layer 1610, as shown in FIG. 24A. A mask 1625 with a pattern 1630
as shown in FIG. 24B is used to expose the photoresist 1605 under
UV light 1635. The mask pattern 1630 is gray scaled in the
longitudinal direction and laterally tapered up in the lateral
direction. The gray-scale mask pattern is designed so that it gives
a graduated exposure level with exposure dosage that varies from a
small value to a large value across a length of 30 .mu.m. The
photoresist is then developed. The shape of the photoresist after
exposure and development is a vertical down tapered shape that
corresponds to the variation in the exposure dosage and a laterally
tapered up shape as shown by photoresist pattern 1640 in FIG. 24C
and FIG. 24D.
[0260] A dry plasma etching procedure 1645 with selectivity of
1:0.3:.about.0 between photoresist and Si and SiO.sub.2 is used to
etch down the Si layer vertically. Such a process is accomplished
using a reactive ion etching system, or an inductively coupled
plasma system, or any dry plasma system. The exemplary processing
gases are SF.sub.6/O.sub.2/Cl.sub.2, as F-reactants and/or their
neutrals etch Si, whereas O.sub.2 etches photoresist and S-Cl
reactants and/or their neutrals inhibit the etching of SiO.sub.2.
Exemplary process parameters using a reactive ion etching system
are: a mixture of 30 sccm:20 sccm:20 sccm of
SF.sub.6:O.sub.2:Cl.sub.2, with an RF power of 350 W, and a process
pressure of 25 mTorr. This etching process transfers the laterally
up tapered and vertically down tapered pattern of the photoresist
1640 to the high-refractive-index Si layer and forms the laterally
tapered up and vertically tapered down Si section 1650. The
resulting structure is shown in FIG. 24E and 24F. It should be
noted that the easiness of this transfer process is dependent on
the thickness t.sub.1 of the top waveguiding Si layer. Typically
the starting thickness for such a Si waveguide should be in the
range of 0.2 to 0.5 .mu.m. Considering that the photoresist layer
has a typical thickness of about 1.0 .mu.m and that the etching
process typically etches Si at a rate that is not drastically
different from that for the photoresist, such a direct pattern
transfer can be achieved. As the etching process can be made to
etch SiO.sub.2 at a much slower rate, the interface between the top
Si layer 1610 and the lower cladding SiO.sub.2 1615 can be used as
a natural stop during the dry etching process. It should be
understood that the above process parameters are presented for
purposes of illustrating a useful embodiment of the fabrication
method and are not intended to limit other embodiments of the
method. For example, a variety of dry etching parameters can be
used depending on the photoresist type and the quality of the SOI
wafer. As is known to those skilled in the art, the end result of a
dry etched structure can be achieved via various combinations of
the etching parameters.
[0261] To make the glass cladding, flame hydrolysis deposition,
chemical vapor deposition, sputtering, Ion-Assisted-Deposition, or
sol-gel spin coating of dielectric material, can generally be
employed to deposit the top and side cladding regions 1690/1670
shown in FIGS. 22A-B. The TiO.sub.2-SiO.sub.2 material required
can, in particular, be achieved using a sol-gel mixture of
TiO.sub.2 and SiO.sub.2 precursors as is well known to those
skilled in the art. Note that the surrounding cladding, including
the bottom, top and side cladding medium, can all have various
refractive indices and also a spatial variation or distribution of
the refractive index value (described below), and the actual value
that can be selected for this purpose can cover a wide range, e.g.,
1.4 to 2.5.
[0262] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1600 and are not intended to limit other
embodiments of any exemplary device, or the device 1600. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0263] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the present invention, including the side
surfaces, the top and bottom surfaces, and the input/output
surfaces.
[0264] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0265] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the devices.
[0266] It should be understood to those skilled in the art that the
device 1600 can be fabricated on adifferent substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0267] (5) Exemplary Device 5: A Lateral-step-refractive-index and
Vertical-step-refractive-index (LSRIN.times.VSRIN) ICMT Device
[0268] FIG. 25 illustrates a fifth general embodiment of an ICMT
device 1700 including a vertical and lateral step refractive index
distribution to form a step index channel waveguide with refractive
index difference between the core region and its surrounding
cladding region. The device can perform as a two dimensional
beam-size collimating element in both the lateral and the vertical
directions for a propagating optical beam and can, for example,
confine an optical beam that has already been expanded or enlarged
from a small semiconductor waveguide using devices such as any of
exemplary devices 1-4. The device is not limited to use as a beam
collimator but can also function as a waveguide to direct a light
beam to a beam reducer when the optical beam propagates in the
reverse direction. Furthermore, the device can be used for direct
light beam coupling into an optical fiber. It should be understood
that these exemplary applications of device 1700 are intended to
illustrate the uses for device 1700 and are not intended to limit
the applications of other embodiments of device 1700. The device
can be referred to as a
"low-refractive-index-contrast-vertical-step-refractive-index and
lateral-step-refractive-index (LRC-LSRIN.times.VSRIN) ICMT".
[0269] The present LRC-LSRIN.times.VSRIN ICMT preferably includes a
waveguiding core region occupied by Waveguide Core WC 1730.
Waveguide Core WC 1730 is surrounded at the bottom by Lower
Waveguide Cladding LWCL 1710, on the top by Upper Waveguide
Cladding UWCL 1745 and on both sides by Side Waveguide Cladding
SWCL 1720. Waveguide Core 1730 preferably includes a front beam
input/output port FB-PT 1731, a straight waveguiding core region
SWC 1730, and a back beam input/output port BB-PT 1732. The
straight waveguiding core region SWC 1730 has a length l.sub.SWC, a
width of w.sub.SWC and a thickness of t.sub.SWC. Lower Waveguide
Cladding LWCL 1710 has a length of l.sub.LWCL, a width of
w.sub.LWCL and a thickness of t.sub.LWCL. Upper Waveguide Cladding
UWCL 1745 has a length of l.sub.UWCL, a width of w.sub.UWCL and a
thickness of t.sub.UWCL. The two side waveguide claddings SWCL 1720
have a length of l.sub.SWCL, a width of w.sub.SWCL and a thickness
of t.sub.SWCL. The lengths of the waveguide claddings
LWCL/UWCL/SWCL 1710/1745/1720, l.sub.LWCL, l.sub.UWCL, and
l.sub.SWCL are about equal to the length of the waveguide core
l.sub.WC. The refractive index of the Waveguide Core WC 1730 is
n.sub.WC. The refractive index of the Lower Waveguide Cladding LWCL
1710 is n.sub.LWCL. The refractive index of the Upper Waveguide
Cladding UWCL 1710 is n.sub.UWCL. The refractive index of the Side
Waveguide Cladding SWCL 1710 is n.sub.SWCL.
[0270] In an exemplary device, the front beam input/output port
FB-PT 1731 is configured to receive/transmit a light beam typically
having wavelength .lambda. with a beam size that roughly equals the
size of an optical fiber, and the back beam input/output port LB-PT
1732 is also configured to receive/transmit a light beam typically
having wavelength .lambda. with a beam size that roughly equals the
size of an optical fiber.
[0271] (i) An Exemplary Device
[0272] In an exemplary embodiment, the Waveguide Core WC 1730 is
made up of lead-titania-silica with a refractive index of
n.sub.WC=1.71, or Si.sub.3N.sub.4 with a refractive index of about
1.7. The Lower Waveguide Cladding LWCL 1710 is made up of silica
with a refractive index of n.sub.LWCL=1.5. The Upper Waveguide
Cladding UWCL is made up of silica-titania with a refractive index
of n.sub.UWCL=1.7, or Si.sub.3N.sub.4 with a refractive index of
about 1.7. The Side Waveguide Cladding SWCL is made up of lead
silica-titania with a refractive index of n.sub.SWCL =1.7, or
Si.sub.3N.sub.4 with a refractive index of about 1.7. The size of
the waveguide core is l.sub.WC =50 .mu.m, w.sub.WC=10 .mu.m and
t.sub.WC=10 .mu.m. The size of the lower waveguide cladding is
l.sub.LWCL=50 .mu.m, w.sub.LWCL=20 .mu.m and t.sub.LWCL=1 .mu.m.
The size of the upper waveguide cladding is l.sub.UWCL=50 .mu.m,
w.sub.UWCL=20 .mu.m and t.sub.UWCL=1 .mu.m. The size of the two
side waveguide claddings is l.sub.SWCL=50 .mu.m, w.sub.SWCL=5 .mu.m
and t.sub.SWCL=10 .mu.m. It should be appreciated by one skilled in
the art that all parameter values used in this and other exemplary
embodiments are approximate and that the actual values can vary
significantly.
[0273] (ii) General Operation of the Device
[0274] FIG. 26 shows the result of a computer simulation of the
spatial distribution of the electric field strength for the light
input at .lambda.=1.5 .mu.m after propagating into waveguide 1730.
The mode size at the input end is 10 .mu.m. The waveguide confines
the mode and guides its propagation to the other port. The mode
size remains at about 10 .mu.m at the other end of the
waveguide.
[0275] (iii) Device Fabrication Procedures
[0276] An exemplary procedure for fabricating the
LRC-LSRIN.times.VSRIN ICMT device 1700 will be described with
reference to FIGS. 27A-E. This procedure is given for the purpose
of illustration and not limitation, as there are other procedures
that can be used to achieve the same fabrication results and other
materials systems or device structures that can be utilized to
fabricate devices with the same functional capabilities.
[0277] The LRC-LSRIN.times.VSRIN ICMT can be fabricated by starting
with a Silica-On-Silicon (SOS) wafer that can be fabricated via a
commercial process known to those skilled in the art, as shown in
FIG. 27A. For a SOS wafer, a low refractive index layer of
SiO.sub.2 1710 with a thickness of t.sub.LWCL is already made on
the Si substrate (not shown). The SiO.sub.2 layer 1710 is typically
either deposited on a Si substrate or thermally oxidized on the Si
substrate. There are at least two possible ways to fabricate the
LRC-LSRIN.times.VSRIN ICMT.
[0278] Method 1: As shown in FIG. 27A, a dielectric or glass
waveguiding film 1705 is deposited on the SiO.sub.2 layer 1710,
which acts as a lower waveguide cladding. Depending on the
dielectric material, an appropriate film deposition method can be
used including evaporation, sputtering, Ion-Assisted-Deposition,
chemical vapor deposition, flame hydrolysis, and spin or dip
coating. As is well known to those skilled in the art, a common way
to make a lateral step refractive index distribution is to dry etch
a stripe 1706 in the deposited film. This can be easily achieved by
photolithography. A photoresist layer 1702 can be deposited and
UV-exposed through a conventional stripe mask. After photoresist
development, a photoresist stripe pattern 1703 is generated, as
shown in FIG. 27B. Dry etching can then be used to transfer the
photoresist stripe pattern to the dielectric or glass waveguide
film 1705 to form a stripe pattern 1706, shown in FIG. 27C. While
air can be used as the top and side cladding, generally speaking, a
surrounding cladding material is preferred and such a cladding
1715, which acts both as the upper waveguide cladding and the side
waveguide cladding, can always be deposited, as shown in FIG.
27C.
[0279] Method 2: Another approach to make a step refractive index
distribution in the lateral direction is to first deposit a
photosensitive waveguide film 1720 on the SiO.sub.2 1710, as shown
in FIG. 27D. In the case of silica-based glass, typically, Ge or Pb
can be incorporated to make the glass film photosensitive. The
incorporation of Ge or Pb for making a glass film photosensitive
are described in a copending U.S. patent application Ser. No.
09/884,691 having the same inventors herein, entitled: "Method for
Forming a Refractive-Index-Patter- ned Film for Use in Optical
Device Manufacturing," the disclosure of which is hereby
incorporated by reference herein in its entirety. After the
deposition of such a film 1720, UV photo-imprinting 1725 through a
conventional photomask 1735 can be used to define a refractive
index increased stripe 1730. If an upper cladding is preferred, a
film 1745 with a refractive index lower than that of the
photoimprinted stripe can then be deposited, as shown in FIG. 27E.
Alternatively, such a film can even be deposited before the UV
photoimprinting as long as this upper cladding film does not
substantially absorb the UV light. An obvious advantage of the
photoimprinting approach is that the fabrication of a buried
channel waveguide is significantly simplified as it does not
involve etching, and hence the fabrication cost is also
significantly lowered.
[0280] Either of the two methods produces a buried channel
waveguide that has a step refractive index distribution in both the
vertical and the horizontal/lateral direction.
[0281] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1700 and are not intended to limit other
embodiments of any exemplary device, or the device 1700. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0282] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the present invention, including the side
surfaces, the top and bottom surfaces, and the input/output
surfaces.
[0283] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0284] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the devices.
[0285] It should be understood to those skilled in the art that the
device 1700 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the protonic devices by sharing the same
substrate as the photonic device.
[0286] (6) Exemplary Device 6 (A) A
Composite-lateral-step-refractive-inde- x and
Vertical-graded-refractive Index (LSRIN.times.VGRIN) ICMT
Device
[0287] FIG. 28 illustrates a sixth general embodiment of an ICMT
device 1800 including a vertically graded and laterally step
refractive index distribution to form a channel waveguide. The
device 1800 can perform as a two dimensional beam-size collimating
element in both the lateral and the vertical directions for a
propagating optical beam and can, for example, confine and
collimate an optical beam that has already been expanded or
enlarged from a small semiconductor waveguide using a device such
as any of exemplary devices 1-4. The device is not limited to use
as a beam collimator but can also function as a waveguide to direct
a light beam to a beam reducer when the optical beam propagates in
the reverse direction. Furthermore, the device can be used for
direct light beam coupling into an optical fiber. It should be
understood that these exemplary applications of device 1800 are
intended to illustrate the uses for device 1800 and are not
intended to limit the applications of other embodiments of device
1800. The device can be referred to as a
"composite-lateral-step-refractive-index and
vertical-graded-refractive index (LSRIN.times.VGRIN) ICMT".
[0288] LSRIN.times.VGRIN ICMT 1800 preferably includes a
waveguiding core region occupied by Waveguide Core WC 1835.
Waveguide Core WC 1835 is surrounded at the bottom by Lower
Waveguide Cladding LWCL 1810, on the top by Upper Waveguide
Cladding UWCL 1840 and on both sides by Side Waveguide Cladding
SWCL 1825. Waveguide Core 1835 preferably includes a front beam
input/output port FB-PT 1831, a straight waveguiding core region
SWC 1835, and a back beam input/output port BB-PT 1832. The
straight waveguiding core region SWC 1835 has a length l.sub.SWC, a
width of w.sub.SWC and a thickness of t.sub.SWC. The Lower
Waveguide Cladding LWCL 1810 has a length of l.sub.LWCL, a width of
w.sub.LWCL and a thickness of t.sub.LWCL. The Upper Waveguide
Cladding UWCL 1840 has a length of l.sub.UWCL, a width of
w.sub.UWCL and a thickness of t.sub.UWCL. The two side waveguide
claddings SWCL 1825 have a length of l.sub.SWCL, a width of
w.sub.SWCL and a thickness of t.sub.SWCL. The lengths of the
waveguide claddings LWCL/UWCL/SWCL 1810/1840/1825, l.sub.LWCL,
l.sub.UWCL, and l.sub.SWCL are about equal to the length of the
waveguide core l.sub.WC. The refractive index of the Waveguide Core
WC 1835 is not a constant. It is graded in the vertical direction
and varies from the center of the core to the outer border of the
core; the variation is represented by n.sub.WC(y) with y being the
vertical coordinate. In the horizontal or lateral direction, the
refractive index has a step profile. In other words, for a
particular vertical coordinate y.sub.o, the refractive index is a
constant n.sub.WC(y.sub.0) within the core region and drops to the
refractive index value of the side cladding n.sub.SWCL(y.sub.0) at
the two side borders. The refractive index of the Lower Waveguide
Cladding LWCL 1810 is n.sub.LWCL. The refractive index of the Upper
Waveguide Cladding UWCL 1840 is n.sub.UWCL. The refractive index of
the Side Waveguide Cladding SWCL 1825 is n.sub.SWCL(y), which means
it is y-coordinate dependent.
[0289] In an exemplary device, the front beam input/output port
FB-PT 1831 is configured to receive/transmit a light beam typically
having wavelength .lambda. with a beam size that is already fully
enlarged in the lateral direction and partially enlarged to an
intermediate size in the vertical direction by a
high-refractive-contrast tapered waveguide, and the back beam
input/output port LB-PT 1832 is configured to receive/transmit a
light beam typically having wavelength .lambda. with a beam size
that roughly equals the size of an optical fiber.
[0290] (i) An Exemplary Device
[0291] In an exemplary embodiment, the Waveguide Core WC 1835 is
made up of lead-titania-silica with a refractive index distribution
that approximates a parabolic distribution as given in Table I
below.
1TABLE I Refractive index distribution Refractive index of
Refractive index of y coordinate (.mu.m) WC SWCL -5.0 to -4.6 1.610
1.600 -4.6 to -4.2 1.624 1.614 -4.2 to -3.8 1.636 1.626 -3.8 to
-3.4 1.648 1.638 -3.4 to -3.0 1.658 1.648 -3.0 to -2.6 1.668 1.658
-2.6 to -2.2 1.676 1.666 -2.2 to -1.8 1.683 1.673 -1.8 to -1.4
1.688 1.678 -1.4 to -1.0 1.693 1.683 -1.0 to -0.6 1.696 1.686 -0.6
to -0.2 1.699 1.689 -0.2 to 0.2 1.700 1.690 0.2 to 0.6 1.699 1.689
0.6 to 1.0 1.696 1.686 1.0 to 1.4 1.693 1.683 1.4 to 1.8 1.688
1.678 1.8 to 2.2 1.683 1.673 2.2 to 2.6 1.676 1.666 2.6 to 3.0
1.668 1.658 3.0 to 3.4 1.658 1.648 3.4 to 3.8 1.648 1.638 3.8 to
4.2 1.636 1.626 4.2 to 4.6 1.624 1.614 4.6 to 5.0 1.610 1.600
[0292] The Lower Waveguide Cladding LWCL 1810 is made up of silica
with a refractive index of n.sub.LWCL=1.5. The Upper Waveguide
Cladding UWCL is made up of silica-titania with a refractive index
of n.sub.UWCL=1.6. The Side Waveguide Cladding SWCL is made up of
lead silica-titania with a refractive index of distribution given
by Table I. The size of the waveguide core is l.sub.WC=50 .mu.m,
w.sub.WC=10 .mu.m and t.sub.WC=10 .mu.m. The size of the lower
waveguide cladding is l.sub.LWCL=50 .mu.m, w.sub.LWCL=30 .mu.m and
t.sub.LWCL=10 .mu.m. The size of the upper waveguide cladding is
l.sub.UWCL=50 .mu.m, w.sub.UWCL=30 .mu.m and t.sub.UWCL=10 .mu.m.
The size of the two side waveguide claddings is l.sub.SWCL=50
.mu.m, w.sub.SWCL=10 .mu.m and t.sub.SWCL=10 .mu.m. It should be
appreciated by one skilled in the art that all parameter values
used in this and other exemplary embodiments are approximate and
that the actual values can vary significantly.
[0293] (ii) General Operation of the Device
[0294] FIG. 29 shows the results of a computer simulation of the
spatial distribution of the electric field strength for the light
input at .lambda.=1.5 .mu.m after propagating into waveguide 1835.
The mode size at the input end is 10 .mu.m in both the lateral and
vertical directions. In the lateral direction, the waveguide
confines the mode and guides its propagation to the other port. In
the vertical direction, in addition to the guiding of the light
beam, the waveguide also functions as a lens in the sense that the
beam size is changed periodically.
[0295] (iii) Device Fabrication Procedures
[0296] An exemplary procedure for fabricating LSRIN.times.VGRIN
ICMT device 1800 will now be described with reference to FIGS.
30A-D. This procedure is given for the purpose of illustration and
not limitation, as there are other procedures that can be used to
achieve the same fabrication results and other materials systems or
device structures that can be utilized to fabricate devices with
the same functional capabilities.
[0297] The LSRIN.times.VGRIN ICMT can be fabricated by starting
with a Silica-On-Silicon (SOS) wafer that can be fabricated via a
commercial process known to those skilled in the art. For a SOS
wafer, as shown in FIG. 30A, a low refractive index layer of
SiO.sub.2 1810 with a thickness of t.sub.LWCL is already made on
the Si substrate (not shown). There are at least two ways to
fabricate the LSRIN.times.VGRIN ICMT structure and these are now
described by fabrication methods 1-2 below:
[0298] Method 1: As shown in FIG. 30A, a graded index distribution
in the vertical direction can be achieved by depositing multiple
sufficiently thin layers of different material compositions 1805 on
a lower cladding layer 1810. In such a case, a continuous
distribution of the refractive index can be approximated by a
series of small effective refractive index steps with each thin
layer having a different refractive index value. Depending on the
property of the dielectric material, an appropriate film deposition
method can be used. These methods include evaporation, flame
hydrolysis, sputtering, Ion-Assisted-Deposition, chemical vapor
deposition, and others. An exemplary method is sol-gel spin or dip
coating which offers a possibility to vary the material composition
of each thin layer easily. As a natural extension, a channel
waveguide with a graded refractive index distribution in the
vertical direction and a step refractive index distribution in the
horizontal/lateral direction can be fabricated by etching a stripe
1815 in the film 1805; the result of etching is shown in FIG. 30B.
If a surrounding cladding is preferred, a subsequent deposition of
a cladding material 1820 can always be performed.
[0299] Method 2: Another approach to make a step refractive index
distribution in the lateral direction is to first deposit a
photosensitive vertically graded refractive index film 1825 on the
SiO.sub.2 1810, as shown in FIG. 30C. In the case of silica-based
glass, typically, Ge or Pb can be incorporated to make the glass
film photosensitive. After the deposition of such a film 1825, UV
photo-imprinting through a conventional channel photomask 1830 can
be used to induce a nearly step refractive index distribution in
the lateral direction to form the channel waveguide 1835.
[0300] If an upper cladding is preferred, a film 1840 with a
refractive index lower than that of the photoimprinted stripe can
then be deposited as shown in FIG. 30D. Alternatively, such a film
can even be deposited before the UV photoimprinting as long as this
upper cladding film does not substantially absorb the UV light. An
obvious advantage of the photoimprinting approach is that the
fabrication of a buried channel waveguide is significantly
simplified as it does not involve etching and hence the fabrication
cost is also significantly lowered.
[0301] Either method produces a buried channel waveguide that has a
step refractive index distribution in the horizontal/lateral
direction and a graded refractive index distribution in the
vertical direction.
[0302] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1800 and are not intended to limit other
embodiments of any exemplary device, or the device 1800. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0303] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the present invention, including the side
surfaces, the top and bottom surfaces, and the input/output
surfaces of the present invention
[0304] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0305] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bidirectional
nature of the said devices.
[0306] It should be understood to those skilled in the art that the
device 1800 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0307] (B) A Composite-lateral-graded-refractive-index and
Vertical-graded-refractive index (LGRIN.times.VGRIN) ICMT
Device
[0308] FIG. 31 illustrates an alternative embodiment of an ICMT
device 1850 including a vertical as well as a lateral graded
refractive index distribution to form a channel waveguide. The
device 1850 can perform as a two dimensional beam-size collimating
element in both the lateral and the vertical directions for a
propagating optical beam and can, for example, confine and
collimate an optical beam that has already been partially expanded
or enlarged from a small semiconductor waveguide via a tapered
structure such as any of exemplary devices 1-4. The device is not
limited to use as a beam collimator but can also function as a
waveguide to direct a light beam to a beam reducer when the optical
beam propagates in the reverse direction. Furthermore, the device
can be used for direct light beam coupling into an optical fiber.
It should be understood that these exemplary applications of device
1850 are intended to illustrate the uses for device 1850 and are
not intended to limit the applications of other embodiments of
device 1850. The device can be referred to as a
"(composite-lateral-graded-refractive-index and
vertical-graded-refractiv- e index" (LGRIN.times.VGRIN) ICMT.
[0309] LGRIN.times.VGRIN ICMT 1850 preferably includes a
waveguiding core region occupied by Waveguide Core WC 1885.
Waveguide Core WC 1885 is surrounded at the bottom by Lower
Waveguide Cladding LWCL 1860, on the top by Upper Waveguide
Cladding UWCL 1890 and on both sides by Side Waveguide Cladding
SWCL 1875. Waveguide Core 1885 preferably includes a front beam
input/output port FB-PT 1881, a straight waveguiding core region
SWC 1885, and a back beam input/output port BB-PT 1882. The
straight waveguiding core region SWC 1885 has a length l.sub.SWC, a
width of w.sub.SWC and a thickness of t.sub.SWC. The Lower
Waveguide Cladding LWCL 1860 has a length of l.sub.LWCL, a width of
w.sub.LWCL and a thickness of t.sub.LWCL. The Upper Waveguide
Cladding UWCL 1890 has a length of l.sub.UWCL, a width of
w.sub.UWCL and a thickness of t.sub.UWCL. The two side waveguide
claddings SWCL 1875 have a length of l.sub.SWCL, a width of
w.sub.SWCL and a thickness of t.sub.SWCL. The lengths of the
waveguide claddings LWCL/UWCL/SWCL 1860/1890/1875, l.sub.LWCL,
l.sub.UWCL, and l.sub.SWCL are about equal to the length of the
waveguide core l.sub.WC. The refractive index of the Waveguide Core
WC 1885 is not a constant. In both the vertical and the lateral
directions, it is graded and varies from the center of the core to
the outer borders of the core. The variation can be represented in
the vertical direction by n.sub.WC(y) with y being the vertical
coordinate and in the lateral direction by n.sub.WC(x) with x being
the lateral coordinate. The refractive index of the Lower Waveguide
Cladding LWCL 1860 is n.sub.LWCL. The refractive index of the Upper
Waveguide Cladding UWCL 1890 is n.sub.UWCL. The refractive index of
the Side Waveguide Cladding SWCL 1875 is n.sub.SWCL(y), which means
it can be y coordinate dependent.
[0310] In an exemplary device, the front beam input/output port
FB-PT 1881 is configured to receive/transmit a light beam typically
having wavelength .lambda. with a beam size that is already
enlarged to an intermediate size by a high-refractive-contrast
tapered waveguide, and the back beam input/output port LB-PT 1882
is configured to receive/transmit a light beam typically having
wavelength .lambda. with a beam size that roughly equals the size
of an optical fiber.
[0311] (i) An Exemplary Device
[0312] In an exemplary embodiment, the Waveguide Core WC 1885 is
made up of lead-titania-silica with a refractive index distribution
that approximates a parabolic distribution in both the vertical and
the lateral directions as given by the equation 3 n WC ( x , y ) =
1.61 + 0.09 [ 1 - ( x 2 + y 2 ( 5 m ) 2 ) ] .
[0313] The Lower Waveguide Cladding LWCL 1860 is made up of silica
with a refractive index of n.sub.LWCL=1.5. The Upper Waveguide
Cladding UWCL is made up of silica-titania with a refractive index
of n.sub.UWCL=1.6. The Side Waveguide Cladding SWCL is made up of
lead silica-titania with a refractive index distribution given by
Table I (above). The size of the waveguide core is l.sub.WC=50
.mu.m, w.sub.WC=10 .mu.m and t.sub.WC=10 .mu.m. The size of the
lower waveguide cladding is l.sub.LWCL=50 .mu.m, w.sub.LWCL=30
.mu.m and t.sub.LWCL=10 .mu.m. The size of the upper waveguide
cladding is l.sub.UWCL=50 .mu.m, w.sub.UWCL=30 .mu.m and
t.sub.UWCL=10 .mu.m. The size of the two side waveguide claddings
is l.sub.SWCL=50 .mu.m, w.sub.SWCL=10 .mu.m and t.sub.SWCL=10
.mu.m. It should be appreciated by one skilled in the art that all
parameter values used in this and other exemplary embodiments are
approximate and that the actual values can vary significantly.
[0314] (ii) General Operation of the Device
[0315] FIG. 32 shows the results of a computer simulation of the
spatial distribution of the electric field strength for the light
input at .lambda.=1.5 .mu.m after propagating into waveguide 1885.
The mode size at the input end is 10 .mu.m in both the lateral and
vertical directions. In both the vertical and the lateral
directions, in addition to the guiding of the light beam, the
waveguide also functions as a lens in the sense that the beam size
get focused and collimated periodically.
[0316] (iii) Device Fabrication Procedures
[0317] An exemplary procedure for fabricating LGRIN.times.VGRIN
ICMT device 1850 will now be described with reference to FIGS.
33A-B. This procedure is given for the purpose of illustration and
not limitation, as there are other procedures that can be used to
achieve the same fabrication results and other materials systems or
device structures that can be utilized to fabricate devices with
the same functional capabilities.
[0318] The LGRIN.times.VGRIN ICMT can be fabricated by starting
with a Silica-On-Silicon (SOS) wafer that can be fabricated via a
commercial process known to those skilled in the art. For a SOS
wafer, as illustrated in FIG. 33A, a low refractive index layer of
SiO.sub.2 1860 with a thickness of t.sub.LWCL is already made on
the Si substrate (not shown). The fabrication of the
LGRIN.times.VGRIN ICMT according to one embodiment below.
[0319] As shown in FIG. 33A, a graded refractive index distribution
in the vertical direction can be achieved by depositing a multiple
sufficiently thin layers of photosensitive materials with different
compositions 1855 on a lower cladding layer 1860. In such a case, a
continuous distribution of the refractive index can be approximated
by a series of small effective refractive index steps with each
thin layer having a different refractive index value. Depending on
the property of the dielectric material, an appropriate film
deposition method can be used. These methods include evaporation,
flame hydrolysis, sputtering, Ion-Assisted-Deposition, chemical
vapor deposition, and others. An exemplary method is sol-gel spin
or dip coating which offers a possibility to vary the material
composition of each thin layer easily. In the case of silica-based
glass, the photosensitivity of the glass material can be enabled by
incorporating Ge or Pb into the glass film as set forth above.
[0320] As for the graded refractive index distribution in the
lateral direction, it can be created by using a gray scale mask
1880 to optically imprint a graded refractive index change in the
film because the refractive index change can be made to depend on
the dosage of the photoimprinting which can be controlled by the
gray scale of the gray mask. If an upper cladding 1890 is
preferred, such a film can then be deposited on top of the
photosensitive film, as shown in FIG. 33B. Alternatively, such an
upper cladding film 1890 can be deposited before the UV
photoimprinting as long as it does not substantially absorb the
photoimprinting light. The approaches described here are obviously
advantageous because of their simplicity in terms of fabrication as
set forth above. Alternatively, a vertical refractive index
variation can be achieved by varying the amount of photosensitive
materials such as the Ge or Pb content in the vertical direction.
In this case, the UV photoimprinting described above will result in
vertical refractive index variation in addition to the lateral
refractive index variation.
[0321] The resulting device is a buried channel waveguide that has
a graded index distribution in the both the vertical and the
lateral directions.
[0322] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1850 and are not intended to limit other
embodiments of any exemplary device, or the device 1850. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0323] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the present invention, including the side
surfaces, the top and bottom surfaces, and the input/output
surfaces.of the present invention
[0324] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0325] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the said devices.
[0326] It should be understood to those skilled in the art that the
device 1850 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0327] (7) Exemplary Device 7: A
Vertical-sharp-down-taper-and-lateral-gra-
dual-up-taper-cascaded-with-a-vertical-graded-refractive-index-and-lateral-
-step-refractive-index (VSDT.times.LGUT+VGRIN.times.LSRIN) ICMT
Device
[0328] FIGS. 34A-B illustrates a seventh general embodiment of a
combined module ICMT device 1900 including a
vertical-sharp-down-taper and lateral-gradual-up-taper waveguide
core embedded in and connected to a large channel waveguide with a
nearly symmetric vertical graded refractive index and lateral step
refractive index distribution. It is in fact one possible
combination of a taper waveguide integrated with a lower
refractive-index-contrast large-size channel waveguide. The device
1900 performs as a two dimensional beam-size enlargement and
collimating element in both the lateral and the vertical directions
for a propagating optical beam, and can, for example, enlarge and
collimate an optical beam from a small semiconductor waveguide to a
large beam size such as one with a beam size close to that of an
optical fiber. The device is not limited to use as a beam
enlarger/collimator but can also function as a beam reducer when
the optical beam propagates in the reverse direction. Furthermore,
the device is used for direct light beam coupling into an optical
fiber. It should be understood that these exemplary applications of
device 1900 are intended to illustrate the uses for device 1900 and
are not intended to limit the applications of other embodiments of
device 1900 to these examples. The device can be referred to as a
"vertical-sharp-down-taper-and-lateral-gradual-up-taper-cascaded-with-a-v-
ertical-graded-refractive-index-and-lateral-step-refractive-index
(VSDT.times.LGUT+VGRIN.times.LSRIN) ICMT.
[0329] The present VSDT.times.LGUT+VGRIN.times.LSRIN ICMT 1900
preferably includes a Waveguiding Core region occupied by Waveguide
Core WC 1945. This Waveguide Core WC 1945 is embedded in an optical
medium 1915/1950 that acts as the cladding for the Waveguide Core
WC 1945 wherever the Waveguide Core WC 1945 exists, but at the same
time the same optical medium 1915/1950 acts as a Lower Refractive
index Contrast Waveguiding Core region. This Lower Refractive index
Contrast Waveguiding Core region 1915/1950 is occupied by Waveguide
Core LRCWC 1915/1950, and is further surrounded by an even lower
refractive index cladding region 1905/1955/1976.
[0330] Waveguide Core WC 1945 is surrounded at the bottom by Lower
Graded Waveguide Cladding LGWCL 1915, on the top by Upper Graded
Waveguide Cladding UGWCL 1950 and on both sides by Side Stratified
Waveguide Cladding SSWCL 1970, which is basically a combination of
the Lower Graded Waveguide Cladding LGWCL 1915 and the Upper Graded
Waveguide Cladding UGWCL 1950.
[0331] When viewed from the side, the Waveguide Core WC 1945
preferably includes a small beam input/output port SB-PT 1971a, a
straight waveguiding core region SWC 1972a, a vertically tapered
down region VTD 1973a, and an intermediate beam output/input port
region 1974a. The straight waveguiding core region SWCa 1972a has a
length l.sub.HRCSWCa, and a thickness of t.sub.HRCSWCa. The
vertically tapered down waveguiding core region VTDWC 1973a has a
length of l.sub.HRCVTDWCa, a vertical thickness of
t.sub.HRCVTDWCaSB at the small-beam input/output side, and a
vertical thickness of t.sub.HRCVTDCaLB at the large-beam
input/output side.
[0332] When viewed from the top, Waveguide Core 1945 preferably
includes a small beam input/output port SB-PT 1971b, a straight
waveguiding core region SWC 1972b, a laterally tapered up region
LTU 1973b, a wider straight waveguiding core region WSWC 1975 and a
large beam output/input port region 1974b. The straight high
refractive index contrast waveguiding core region SWCb 1972b has a
length l.sub.HRCSWCb, and a width of w.sub.HRCSWCb. The laterally
tapered up waveguiding core region LTUWC 1973b has a length of
l.sub.HRCLTUWCb, a width of w.sub.HRCLTUWCbSB at the small-beam
input/output side, and a width of w.sub.HRCLTUCbLB at the
large-beam input/output side. The wider straight waveguide core
region WSWC 1975 has a length of l.sub.HRCWSWCb and a width of
w.sub.HRCWSWCb=w.sub.HRCLTUCbLB.
[0333] It should be understood that the straight waveguiding core
regions 1972a and 1972b (based on whether the structure is viewed
from the side or from the top), may have the same or a different
length. Similarly, the vertically tapered down region 1973a (when
viewed from the top) and the laterally tapered up region 1973b
(when viewed from the side) may have the same or a different
length. In other words, the vertical and lateral beam size
transformation may be achieved independently or at the same time or
with one slightly earlier than the other. Furthermore, the taper
regions do not need to be symmetric with respect to the central
axis and also the tapering up or down slope do not need to be
straight and may be of any curve shape.
[0334] Lower-Refractive-index-Contrast-Waveguide Core LRCWC
1915/1950 preferably surrounds and embeds the Waveguide Core WC
1945. When viewed from the side, the Lower Refractive index
Contrast Waveguide Core LRCWC 1915/1950 is sandwiched at the bottom
by a Lower Waveguide Cladding LWCL 1905, and on the top by an Upper
Waveguide Cladding UWCL 1955. As the
Lower-Refractive-index-Contrast-Waveguide Core LRCWC 1915/1950 has
a graded refractive index distribution along the vertical
direction, it can take the intermediate size beam launched at the
tip region 1974a from the Waveguide Core WC 1945 and further expand
the beam; after the beam has traveled a certain distance equivalent
to the focal length of the graded refractive index lens structure,
the beam will be collimated and reach the back large beam
output/input port LB-PT 1977, from where another single mode
waveguide such as a single mode optical fiber will continue to
guide the light beam. When viewed from the top, the LRCWC 1970
(i.e. 1915/1950) is surrounded on both sides by Side Waveguide
Cladding SWCL 1976, (with lower refractive index when compared to
the Waveguide Core LRCWC 1970). The Lower Refractive index Contrast
Waveguide Core LRCWC 1970 has a length of l.sub.LRCWC, a width of
w.sub.LRCWC and a thickness of t.sub.LRCWC. The Lower Waveguide
Cladding LWCL 1905 has a length of l.sub.LWCL, a width of
w.sub.LWCL and a thickness of t.sub.LWCL. The Upper Waveguide
Cladding UWCL 1955 has a length of l.sub.UWCL, a width of
w.sub.UWCL and a thickness of t.sub.UWCL. The two side waveguide
claddings SWCL 1976 have a length of l.sub.SWCL, a width of
w.sub.SWCL and a thickness of t.sub.SWCL. The lengths of the
waveguide claddings LWCL/UWCL/SWCL 1905/1955/1976, l.sub.LWCL,
l.sub.UWCL, and l.sub.SWCL are about equal to the length of the
lower refractive index contrast waveguide core l.sub.LRCWC.
[0335] The refractive index of the Waveguide Core WC 1945 is
n.sub.HRCWC. The refractive index of the Lower Refractive index
Contrast Waveguide Core LRCWC 1915/1950 is n.sub.LRCWC(y), which
means it is y coordinate dependent, with y being the vertical
coordinate. The refractive index of the Lower Waveguide Cladding
LWCL 1905 is n.sub.LWCL. The refractive index of the Upper
Waveguide Cladding UWCL 1955 is n.sub.UWCL. The refractive index of
the Side Waveguide Cladding SWCL 1976 is n.sub.SWCL(y), which means
it can also be y coordinate dependent. It should be understood that
the Side Waveguide Cladding SWCL 1976 can have either a uniform or
non-uniform refractive index distribution. In other words, the
refractive index of the Side Waveguide Cladding may or may not be a
constant. In the latter case, it can be graded in the vertical
direction. In device 1900, as in exemplary device 6, the refractive
index of the Waveguide Core LRCWC 1915/1950 is not a constant. It
is graded in the vertical direction and varies from the center
plane of the core to the top and bottom border of the core. The
variation can be represented by n.sub.LRCWC(y) with y being the
vertical coordinate. In the horizontal or lateral direction, the
refractive index may have a step profile. In other words, for a
given vertical coordinate y.sub.o, the refractive index is a
constant, n.sub.LRCWC(y.sub.0), within the core region and drops at
the two side borders to the refractive index value of the side
cladding, n.sub.SWCL(y.sub.0).
[0336] In an exemplary device, the front beam input/output port
FB-PT 1971a/1971b is configured to receive/transmit a light beam
typically having wavelength .lambda. with a very small beam size.
The mode size of the straight section of the Waveguide Core WC 1945
can be designed to match the mode size of a preceding very small
size waveguide. The laterally tapering up section of the WC 1945
will enlarge the beam fully in the lateral direction and the
vertical tapering down section will partially enlarged the beam in
the vertical direction to an intermediate size. The vertically
graded Lower Refractive index Contrast Waveguide Core LRCWC is
configured to take the vertically intermediate size beam launched
at region 1974a and further expand and collimate it. The back beam
input/output port LB-PT 1977 is configured to receive/transmit a
light beam typically having wavelength .lambda. with a beam size
that roughly equals the size of an optical fiber.
[0337] (i) An Exemplary Device
[0338] In an exemplary embodiment, the High Refractive index
Contrast Waveguide Core WC 1945 is made up of silicon with a
refractive index of n.sub.HRCWC3.5, the Low Refractive index
Contrast Waveguide Core LRCWC 1915/1950 is made up of
lead-titania-silica material mixture with a mixture composition to
achieve a refractive index of a refractive index distribution that
approximates a parabolic distribution as given in Table II
below.
2TABLE II Refractive index distribution y coordinate Refractive
index of Refractive index of (.mu.m) LRCWC SWCL -5.0 to -4.6 1.610
1.600 -4.6 to -4.2 1.624 1.614 -4.2 to -3.8 1.636 1.626 -3.8 to
-3.4 1.648 1.638 -3.4 to -3.0 1.658 1.648 -3.0 to -2.6 1.668 1.658
-2.6 to -2.2 1.676 1.666 -2.2 to -1.8 1.683 1.673 -1.8 to -1.4
1.688 1.678 -1.4 to -1.0 1.693 1.683 -1.0 to -0.6 1.696 1.686 -0.6
to -0.2 1.699 1.689 -0.2 to 0.2 1.700 1.690 0.2 to 0.6 1.699 1.689
0.6 to 1.0 1.696 1.686 1.0 to 1.4 1.693 1.683 1.4 to 1.8 1.688
1.678 1.8 to 2.2 1.683 1.673 2.2 to 2.6 1.676 1.666 2.6 to 3.0
1.668 1.658 3.0 to 3.4 1.658 1.648 3.4 to 3.8 1.648 1.638 3.8 to
4.2 1.636 1.626 4.2 to 4.6 1.624 1.614 4.6 to 5.0 1.610 1.600
[0339] The Lower Waveguide Cladding LWCL 1905 is made up of
silica-titania with a refractive index of n.sub.LWCL=1.5. The Upper
Waveguide Cladding UWCL is made up of silica-titania with a
refractive index of n.sub.UWCL=1.5. The Side Waveguide Cladding
SWCL is made up of lead silica-titania with a refractive index
distribution also given in Table II.
[0340] The dimensions of the Waveguide Core WC 1945 are as follows:
l.sub.HRCSWCa=110 .mu.m, t.sub.HRCSWCa=0.3 .mu.m,
l.sub.HRCVTDWCa=30 .mu.m, t.sub.HRCVTDWCaSB=0.3 .mu.m,
t.sub.HRCVTDCaLB=0 .mu.m, l.sub.HRCSWCb10 .mu.m, w.sub.HRCSWCb.=0.3
.mu.m, l.sub.HRCLTUWCb=100 .mu.m, w.sub.HRCLTUWCbSB=0.3 .mu.m,
w.sub.HRCLTUCbLB=10 .mu.m, l.sub.HRCWSWCb=30 .mu.m and
w.sub.HRCWSWCb=10 .mu.m. The dimensions of the Lower Refractive
index Contrast Waveguide Core LRCWC 1915/1950 are as follows:
l.sub.LRCWC=170 .mu.m, w.sub.LRCWC=10 .mu.m, t.sub.LRCWC=10 .mu.m.
The Lower Waveguide Cladding LWCL 1905 has a length of
l.sub.LWCL=170 .mu.m, a width of w.sub.LWCL=30 .mu.m and a
thickness of t.sub.LWCL=2 .mu.m. The Upper Waveguide Cladding UWCL
1955 has a length of l.sub.UWCL=170 .mu.m, a width of w.sub.UWCL=30
.mu.m and a thickness of t.sub.UWCL=2 .mu.m. The two side waveguide
claddings SWCL 1976 have a length of l.sub.SWCL=170 .mu.m, a width
of w.sub.SWCL=10 .mu.m and a thickness of t.sub.SWCL=10 .mu.m. It
should be appreciated by one skilled in the art that all parameter
values used in this and other exemplary embodiments are approximate
and that the actual values can vary significantly.
[0341] (ii) General Operation of the Device
[0342] FIG. 35 shows the results of a computer simulation of the
spatial distribution of the electric field strength for the light
input at .lambda.=1.5 .mu.m after being launched from the left into
the small light beam input/output port 1971a/1971b. The mode size
at the input end is 0.3 .mu.m in both the lateral and vertical
directions. In the vertical direction, as the tapering down section
only lies towards the right for the last 30 .mu.m and the cascading
beam expansion action occurs on both the left and the right part of
the WC taper tip, the computer simulation is thus zoomed into the
last 60 .mu.m of the coupler structure. In the lateral direction,
the beam expansion is entirely enabled by the tapering up section
1973b, which has a length of 100 .mu.m, hence the computer
simulation is zoomed mainly in this section. As can be seen from
FIG. 35, the combined module coupler structure can transform a very
small beam of about 0.3 .mu.m in size to a large size beam of about
10 .mu.m in both the vertical and the horizontal directions.
[0343] (iii) Device Fabrication Procedures
[0344] An exemplary procedure for fabricating a
HRC-VSDT.times.LGUT+VGRIN.- times.LSRIN ICMT device 1900 will now
be described with reference to FIGS. 36A-N. This procedure is given
for the purpose of illustration and not limitation, as there are
other procedures that can be used to achieve the same fabrication
results and other materials systems or device structures that can
be utilized to fabricate devices with the same functional
capabilities.
[0345] The HRC-VSDT.times.LGUT+VGRIN.times.LSRIN ICMT 1900 may be
fabricated by starting with a Silica-On-Silicon (SOS) wafer shown
in FIG. 36A, which may be fabricated via a commercial process known
to those skilled in the art. For a SOS wafer, a low refractive
index layer of SiO.sub.2 1905 with a thickness of t.sub.LWCL is
already made on a Si substrate 1910. The fabrication of the
HRC-VSDT.times.LGUT+VGRIN.times.LS- RIN ICMT structure according to
one embodiment now described below.
[0346] As shown in FIGS. 36B-C, a graded index distribution in the
vertical direction can be achieved by depositing multiple
sufficiently thin layers of photosensitive materials with different
compositions 1915 on a lower cladding layer 1905. In such a case, a
continuous distribution of the refractive index can be approximated
by a series of small refractive index steps with each thin layer
having a different refractive index value. Depending on the
property of the dielectric material, an appropriate film deposition
method can be used. These methods include evaporation, flame
hydrolysis, sputtering, Ion-Assisted-Deposition, chemical vapor
deposition, and others. An exemplary method is sol-gel spin or dip
coating which offers a possibility to vary the material composition
of each thin layer easily. In the case of silica-based glass, the
photosensitivity of the glass material can be enabled by
incorporating Ge or Pb into the glass film. It should be understood
that any optically transparent dielectric material in the spectrum
region of interest to optical communication can be used for the
deposition; examples include lead silica, geimania-silica,
titania-silica, silicon oxynitride, silicon nitride, polysilicon,
silicon-rich-silica, silicon carbide, polymer and a combination of
different materials. The parameter details of one exemplary
embodiment of the design for the GRIN layers has already been shown
in Table II. Note that the refractive index distribution does not
have to follow the parabolic profile and may be of any profile. It
should also be understood that other film deposition techniques
such as flame hydrolysis, sputtering, Ion-Assisted-Deposition and
chemical vapor deposition may also be used to deposit the bottom
half of the GRIN dielectric waveguide.
[0347] As shown in FIG. 36D, a thin silicon layer 1920, which is to
be made into the high refractive index contrast waveguide core WC
1945, may be defined in another piece of bare Si wafer using, for
example, ion implantation. A thin silicon layer 1920 is formed on
an ion implanted layer 1925 that is sitting on top of a Si
substrate 1930. This ion-implanted wafer may then be flipped over
and wafer-bonded to the GRIN dielectric-coated SiO.sub.2-Si wafer,
as shown in FIG. 36E. The top thick Si part 1930 and the
ion-implanted layer 1925 may then be removed using a lift-off
technique such as rapid thermal annealing and/or wafer thinning.
The result after removing the ion-implanted layer is shown in FIG.
36F. This technique may be modified if, for instance, a
non-symmetric vertical-GRIN waveguide is desired, as will be
described below.
[0348] To form a vertically down-tapered and horizontally/laterally
up-tapered high-index core, the fabrication steps described with
regard to Exemplary device 4 may be used. In short, a photoresist
layer is first spin-coated on the Si waveguide layer 1920. A mask
pattern 1935 with a gray-scaled transparency along the longitudinal
direction, and a horizontal/lateral up taper, shown in FIG. 36G,
can be used together with UV exposure and photoresist development
to make a vertically tapered down and horizontally/laterally
tapered up photoresist pattern 1940, shown in FIG. 36H. The lateral
and vertical tapers may be made independent from each other,
although in FIG. 36H, they have been put together to save space and
also to illustrate the principle. Followed by dry etching, as
indicated in FIG. 36I, the vertically down tapered and
horizontally/laterally up tapered photoresist pattern 1940 is
transferred to the high refractive index Si layer to form the
vertically down-tapered and horizontally/laterally up-tapered Si
section 1945, as shown in FIGS. 36J-K. As was previously noted, the
interface between the top Si layer and the SiO.sub.2 based layer
may be used as a natural stop during the dry etching process. It
should also be noted that a shadow mask based dry etching process
or a diffusion-limited wet etching process could also be used to
form the Si taper as well.
[0349] To form the top half of the vertically GRIN dielectric
waveguide part, an effective refractive-index-decreasing dielectric
region 1950 is deposited, as shown in FIG. 36L. Preferably
photosensitive sol-gel silica is spin-coated in almost the same way
as for the bottom half of the dielectric waveguide except that the
order of the layers is now reversed. It should again be understood
that the parabolic refractive index distribution cited here is only
one example and, as is well known to those skilled in the art,
various other refractive index distributions may be used. On the
very top, a relatively thick (say 2 .mu.m) silica layer 1955 can be
deposited to act as an upper cladding.
[0350] Note that the sol-gel technique has an advantage in that the
spin-coated film will change shape from conformal coating to
planarized coating. The initial layer thickness may not be so even
and uniform. However, due to the fact that the Si taper 1945 is
generally only about 0.2.about.0.5 .mu.m high, after a few spin-on
layers, the following layers should be flat and uniform as
suggested in FIG. 36L. With a top cladding 1955, a buried GRIN
planar waveguide 1915/1950 is thus formed with the Si taper 1945 in
the center of the GRIN waveguide core.
[0351] To form a dielectric channel waveguide with a step
refractive index distribution in the lateral direction to confine
light propagation, dry etching can be used. However, UV imprinting
is preferred since the deposited GRIN dielectric film can be made
photosensitive. A step channel mask 1960, as shown in FIG. 36M, can
be used to form a single mode step refractive index channel
waveguide 1965 to confine light in the horizontal/lateral
direction. When viewed from the top, as shown in FIG. 36N, the
result is a tapered up Si taper 1945 integrated with a step index
channel waveguide 1965.
[0352] It should be understood that these dimensions and exemplary
lengths are presented for the purposes of illustrating a useful
embodiment of the device 1900 and are not intended to limit other
embodiments of any exemplary device, or the device 1900. A variety
of dimensions and sizes can be used, depending on the application
desired, as well as the fabrication materials, processes and
technologies that are employed.
[0353] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the various embodiments of the present
invention, including the side surfaces, the top and bottom
surfaces, and the input/output surfaces, of the present
invention
[0354] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0355] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the said devices.
[0356] It should be understood to those skilled in the art that the
device 1900 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0357] (8) Exemplary Device 8: A
Vertical-sharp-down-taper-and-lateral-sha-
rp-down-taper-cascaded-with-a-symmetric-vertical-graded-refractive-index-a-
nd-lateral-graded-refractive index
(VSDT.times.LSDT+VGRIN.times.LGRIN) ICMT Device
[0358] FIGS. 37A-B illustrates an eighth general embodiment of a
combined module ICMT device 2000 including a vertically and
laterally down-tapered waveguide core embedded in and connected to
a large channel waveguide with a nearly symmetric vertically and
laterally graded refractive index distribution. It is in fact a
second possible combination of a taper integrated with a lower
-refractive-index-contrast large-size channel waveguide. The device
2000 can perform as a two dimensional beam-size enlargement and
collimating element in both the lateral and the vertical directions
for a propagating optical beam, and can in particular enlarge and
collimate an optical beam from a small semiconductor waveguide to
enable the beam to match with a single mode optical fiber. The
device is not limited to use as a beam enlarger/collimator but can
also function as a beam reducer when the optical beam propagates in
the reverse direction. Furthermore, the device can be used for
direct light beam coupling into an optical fiber. It should be
understood that these exemplary applications of device 2000 are
intended to illustrate the uses for device 2000 and are not
intended to limit the applications of other embodiments of device
2000. The device can be referred to as a "vertical-sharp-down-taper
and lateral-sharp-down-taper-cascaded-with-a-s-
ymmetric-vertical-graded-refractive-index-and-lateral-graded-refractive-in-
dex (VSDT.times.LSDT+VGRIN.times.LGRIN) ICMT".
[0359] VSDT.times.LSDT+VGRIN.times.LGRIN ICMT 2000 preferably
includes a Waveguiding Core region occupied by Waveguide Core WC
2045. This Waveguide Core WC 2045 is embedded in an optical medium
2015/2050 that acts as the cladding for the Waveguide Core WC 2045
wherever the Waveguide Core WC 2045 exists, but at the same time,
the same optical medium 2015/2050 acts as a Lower Refractive index
Contrast Waveguiding Core region. This Lower Refractive index
Contrast Waveguiding Core region is occupied by Waveguide Core
LRCWC 2015/2050, and is further surrounded by an even lower
refractive index cladding region 2005/2055/2076.
[0360] Waveguide Core WC 2045 is surrounded at the bottom by Lower
Graded Waveguide Cladding LGWCL 2015, on the top by Upper Graded
Waveguide Cladding UGWCL 2050 and on both sides by Side Stratified
Waveguide Cladding SSWCL 2070, which is basically a combination of
the Lower Graded Waveguide Cladding LGWCL 2015 and the Upper Graded
Waveguide Cladding UGWCL 2050.
[0361] When viewed from the side, the Waveguide Core WC 2045
preferably includes a small beam input/output port SB-PT 2071a, a
high refractive index contrast straight waveguiding core region SWC
2072a, a high refractive index contrast vertically tapered down
region VTD 2073a, and an intermediate beam output/input port region
2074a. The straight high refractive index contrast waveguiding core
region SWCa 2072a has a length l.sub.HRCSWCa, and a thickness of
t.sub.HRCSWCa. The high refractive index contrast vertically
tapered down waveguiding core region VTDWC 2073a has a length of
l.sub.HRCVTDWCa, a vertical thickness of t.sub.HRCVTDWCaSB at the
small-beam input/output side, and a vertical thickness of
t.sub.HRCVTDCaLB at the large-beam input/output side.
[0362] When viewed from the top, the Waveguide Core 2045 preferably
includes a small beam input/output port SB-PT 2071b, a straight
waveguiding core region SWC 2072b, a laterally tapered down region
LTD 2073b, and an intermediate beam output/input port region 2074b.
The straight waveguiding core region SWCb 1972b has a length
l.sub.HRCSWCb, and a width of w.sub.HRCSWCb. The laterally tapered
down waveguiding core region LTDWC 1973b has a length of
l.sub.HRCLTDWCb, a width of w.sub.HRCLTDWCbSB at the small-beam
input/output side, and a width of w.sub.HRCLTDCbLB at the
large-beam input/output side.
[0363] It should be understood that the straight waveguiding core
regions 2072a and 2072b (based on whether the structure is viewed
from the side or from the top), may have the same or a different
length. Similarly, the vertically tapered down region 2073a (when
viewed from the side) and the laterally tapered down region 2073b
(when viewed from the side) may have the same or a different
length, provided that the tip for both tapering down geometries
ends at the same point in space. Furthermore, the taper regions do
not need to be symmetric with respect to the central axis and also
the tapering down slopes do not need to be straight and may be of
any curve shape.
[0364] Lower-Refractive-index-Contrast-Waveguide Core LRCWC
2015/2050 preferably surrounds and embeds the Waveguide Core WC
2045. When viewed from the side, the Lower Refractive index
Contrast Waveguide Core LRCWC 2015/2050 is sandwiched at the bottom
by a Lower Waveguide Cladding LWCL 2005, and on the top by an Upper
Waveguide Cladding UWCL 2055. As the
Lower-Refractive-index-Contrast-Waveguide Core LRCWC 2015/2050 has
a graded refractive index distribution along both the vertical and
lateral directions, it can take the intermediate size beam launched
at the tip region 2074a/2074b from the Waveguide Core WC 2045 and
further expand the beam. After the beam has traveled a certain
distance equivalent to the focal length of the graded refractive
index lens structure, the beam will be collimated and reach the
back large beam output/input port LB-PT 2077, from where another
single-mode waveguide such as a single-mode optical fiber will
continue to guide the light beam. The Lower Refractive index
Contrast Waveguide Core LRCWC 2070 has a length of l.sub.LRCWC, a
width of w.sub.LRCWC and a thickness of t.sub.LRCWC. The Lower
Waveguide Cladding LWCL 2005 has a length of l.sub.LWCL, a width of
w.sub.LWCL and a thickness of t.sub.LWCL. The Upper Waveguide
Cladding UWCL 2055 has a length of l.sub.UWCL, a width of
w.sub.UWCL and a thickness of t.sub.UWCL. The two side waveguide
claddings SWCL 2076 have a length of l.sub.SWCL, a width of
w.sub.SWCL and a thickness of t.sub.SWCL. The lengths of the
waveguide claddings LWCL/UWCL/SWCL 2005/2055/2076, l.sub.LWCL,
l.sub.UWCL, and l.sub.SWCL are about equal to the length of the low
refractive index contrast waveguide core l.sub.LRCWC.
[0365] The refractive index of the Waveguide Core WC 2045 is
n.sub.HRCWC. The refractive index of the Lower Refractive index
Contrast Waveguide Core LRCWC 2015/2050 is n.sub.LRCWC(x, y), which
means it is x and y coordinate dependent, with x being the lateral
coordinate and y being the vertical coordinate. The refractive
index of the Lower Waveguide Cladding LWCL 2005 is n.sub.LWCL. The
refractive index of the Upper Waveguide Cladding UWCL 2055 is
n.sub.UWCL. The refractive index of the Side Waveguide Cladding
SWCL 2076 is n.sub.SWCL(y), which means it may also be y-coordinate
dependent. It should be understood that the Side Waveguide Cladding
SWCL 2076 may have either a uniform or non-uniform refractive index
distribution. In other words, the refractive index of the Side
Waveguide Cladding may or may not be a constant. In the latter
case, it may be graded in the vertical direction. In a preferred
embodiment, the refractive index of the Waveguide Core LRCWC
2015/2050 is not a constant. It is graded in the vertical direction
and varies from the center plane of the core to the top and bottom
border of the core. The variation can be represented by
n.sub.LRCWC(y) with y being the vertical coordinate. In the lateral
direction, the refractive index of the Lower Refractive index
Contrast Waveguide Core LRCWC 2070 also have a graded profile.
However, for a given vertical coordinate y.sub.o, the refractive
index n.sub.LRCWC(x, y.sub.0) is graded within the core region and
drops to n.sub.SWCL(y.sub.0), the refractive index value of the
side cladding at the two side borders.
[0366] In an exemplary device, the front beam input/output port
FB-PT 2071a/2071b is configured to receive/transmit a light beam
typically having wavelength .lambda. with a very small beam size.
The mode size of the straight section of the Waveguide Core WC 2045
can be designed to match the mode size of a preceding
very-small-size waveguide. The laterally and vertically tapering
down section 2073a/2073b will partially enlarge the beam in both
the vertical and the lateral directions to an intermediate size.
The laterally and vertically graded Lower Refractive index Contrast
Waveguide Core LRCWC is configured to take the intermediate size
beam launched at region 2074a/2074b and further expand and
collimate it. The back beam input/output port LB-PT 2077 is
configured to receive/transmit a light beam typically having
wavelength .lambda. with a beam size that roughly equals the size
of an optical fiber.
[0367] (i) An Exemplary Device
[0368] In an exemplary embodiment, the Waveguide Core WC 2045 is
made up of silicon with a refractive index of n.sub.HRCWC=3.5, the
Lower Refractive index Contrast Waveguide Core LRCWC 2015/2050 is
made up of lead-titania-silica material mixture with mixture
composite designed to give a refractive index distribution that
approximates a parabolic distribution in both the vertical and the
lateral directions as given by the equation: 4 n LRCWC ( x , y ) =
1.61 + 0.09 [ 1 - ( x 2 + y 2 ( 5 m ) 2 ) ] .
[0369] The Lower Waveguide Cladding LWCL 2005 is made up of
silica-titania with a refractive index of n.sub.LWCL=1.5. The Upper
Waveguide Cladding UWCL is made up of silica-titania with a
refractive index of n.sub.UWCL=1.5. The Side Waveguide Cladding
SWCL is made up of lead silica-titania with a refractive index
distribution given in Table II.
[0370] The dimensions of the Waveguide Core WC 2045 are as follows:
l.sub.HRCSWCa=10 .mu.m, t.sub.HRCSWCa=0.3 .mu.m, l.sub.HRCVTDWCa=30
.mu.m, t.sub.HRCVTDDWCaSB=0.3 .mu.m, t.sub.HRCVTDCaLB=0 .mu.m,
l.sub.HRCSWCb=10 .mu.m, w.sub.HRCSWCb.=0.3 .mu.m,
l.sub.HRCLTDWCb=30 .mu.m, w.sub.HRCLTDWCbSB=0.3 .mu.m,
w.sub.HRCLTDCbLB=0 .mu.m. The dimensions of the Lower Refractive
index Contrast Waveguide Core LRCWC 2015/2050 are as follows:
l.sub.LRCWC=70 .mu.m, w.sub.LRCWC=10 .mu.m, t.sub.LRCWC=10 .mu.m.
The Lower Waveguide Cladding LWCL 2005 has a length of
l.sub.LWCL=70 .mu.m, a width of w.sub.LWCL=30 .mu.m and a thickness
of t.sub.LWCL=2 .mu.m. The Upper Waveguide Cladding UWCL 2055 has a
length of l.sub.UWCL=70 .mu.m, a width of w.sub.UWCL=30 .mu.m and a
thickness of t.sub.UWCL=2 .mu.m. The two side waveguide claddings
SWCL 2076 have a length of l.sub.SWCL=70 .mu.m, a width of
w.sub.SWCL=10 .mu.m and a thickness of t.sub.SWCL=10 .mu.m. It
should be appreciated by one skilled in the art that all parameter
values used in this and other exemplary embodiments are approximate
and that the actual values can vary significantly.
[0371] (ii) General Operation of the Device
[0372] FIG. 38 shows the results of a computer simulation of the
spatial distribution of the electric field strength for the light
input at .lambda.-1.5 .mu.m after being launched from the left into
the small light beam input/output port 2071a/2071b. The mode size
at the input end is 0.3 .mu.m in both the lateral and vertical
directions. As the tapering down section only lies towards the
right for the last 30 .mu.m in both the vertical and the lateral
directions and the cascading beam expansion action occurs on both
the left and the right part of the WC taper tip, the computer
simulation is thus zoomed into the last 60 .mu.m of the coupler
structure. Although the computer simulation is only 2 dimensional,
it can be applied to both the vertical and the lateral directions
because the structures are very similar in this case. As can be
seen from FIG. 38, the whole combined module coupler structure can
transform a very small beam of 0.3 .mu.m in size to a large size
beam of 10 .mu.m in both the vertical and the horizontal
directions.
[0373] (iii) Device Fabrication Procedures
[0374] An exemplary procedure for fabricating a
VSDT.times.LSDT+VGRIN.time- s.LGRIN ICMT device 2000 will now be
described with reference to FIGS. 39A-N. This procedure is given
for the purpose of illustration and not limitation, as there are
other procedures that can be used to achieve the same fabrication
results and other materials systems or device structures that can
be utilized to fabricate devices with the same functional
capabilities.
[0375] The VSDT.times.LSDT+VGRIN.times.LGRIN ICMT may be fabricated
by starting with a Silica-On-Silicon (SOS) wafer, shown in FIG.
39A, which may be fabricated via a commercial process known to
those skilled in the art. For an SOS wafer, a low refractive index
layer of SiO.sub.2 2005 with a thickness of t.sub.LWC is already
made on a Si substrate 2010. The fabrication of the
VSDT.times.LSDT+VGRIN.times.LGRIN ICMT structure according to one
embodiment is now described below.
[0376] As shown in FIGS. 39B-C, a graded refractive index
distribution in the vertical direction may be achieved by
depositing multiple sufficiently thin layers of photosensitive
materials with different compositions 2015 on a lower cladding
layer 2005. In such a case, a continuous distribution of the
refractive index can be approximated by a series of small effective
refractive index steps with each thin layer having a different
refractive index value. Depending on the property of the dielectric
material, an appropriate film deposition method can be used. These
methods include evaporation, flame hydrolysis, sputtering,
Ion-Assisted-Deposition, chemical vapor deposition, and others. An
exemplary method is sol-gel spin or dip coating which offers the
ability to vary the material composition of each thin layer easily.
In the case of silica-based glass, the photosensitivity of the
glass material may be enabled by incorporating Ge or Pb into the
glass film. It should be understood that any optically transparent
dielectric material in the spectrum region of interest to optical
communication may be used for the deposition; examples include lead
silica, germania-silica, titania-silica, silicon oxynitride,
silicon nitride, polysilicon, silicon-rich-silica, silicon carbide,
polymer and a combination of different materials. The parameter
details of one preferred embodiment of the design for the GRIN
layers has already been shown in Table II. Note that the refractive
index distribution does not have to follow the parabolic profile
and may be of any profile. It should also be understood that other
film deposition techniques such as flame hydrolysis, sputtering,
Ion-Assisted-Deposition and chemical vapor deposition may also be
used to deposit the bottom half of the GRIN dielectric
waveguide.
[0377] As shown in FIG. 39D, a thin silicon layer 2020, which is to
be made into the high index contrast waveguide core, may be defined
in another piece of bare Si wafer using, for example, ion
implantation. This will form a thin silicon layer 2020 on an ion
implanted layer 2025 that is sitting on top of a Si substrate 2030.
This ion-implanted wafer may be flipped over and wafer-bonded to
the GRIN dielectric-coated SiO.sub.2-Si wafer, as shown in FIG.
39E. The top thick Si part 2030 and the ion-implanted layer 2025
may then be removed using a lift-off technique such as rapid
thermal annealing and/or wafer thinning. The result is shown in
FIG. 39F. This technique may be modified if, for instance, a
non-symmetric vertical-GRIN waveguide is desired, as will be
described below.
[0378] To form a vertically as well as horizontally/laterally
tapered down section, the fabrication steps are similar to those
described above for exemplary device 4. A photoresist layer is
first spin-coated on the Si waveguide layer 2020. A mask pattern
2035, shown in FIG. 39G, with a gray scaled transparency along the
longitudinal direction and meanwhile a horizontal/lateral
down/narrow taper can be used together with UV exposure and
photoresist development to make a vertically as well as
horizontally/laterally down tapered photoresist pattern 2040, as
shown in FIG. 39H. Followed by dry etching, as shown in FIG. 39I,
the vertically as well as horizontally/laterally down tapered
photoresist pattern 2040 can be transferred to the high refractive
index Si layer and form the vertically and horizontally/laterally
tapered down/narrow Si section 2045, as shown in FIGS. 39J-K. It
should again be noted that the interface between the top Si layer
and the glass-based cladding material can be used as a natural stop
during the dry etching process.
[0379] To form the top half of the vertically GRIN glass/polymer
waveguide, a refractive-index-decreasing dielectric region 2050 is
deposited as shown in FIG. 39L. Preferably photosensitive sol-gel
silica is spin-coated in almost the same way as for the bottom half
of the glass/polymer waveguide except that the order of the layers
is now reversed. It should again be understood that the parabolic
refractive index distribution cited here is only one example and,
as is well known to those skilled in the art, other refractive
index distributions may be used. On the very top, a relatively
thick (say 3 .mu.m) silica layer 2055 may be deposited to act as an
upper cladding.
[0380] It should be noted that the sol-gel technique has an
advantage in that the spin-coated film will change shape from
conformal coating to planarized coating. The initial layer
thickness may not be so even and uniform. However, due to the fact
that the Si taper 2045 is generally only about 0.2.about.0.5 .mu.m
high, after a few spin-on layers, the following layers should be
flat and uniform. With a top cladding 2055, a buried GRIN planar
waveguide 2015/2050 can thus be formed with the Si taper 2045 in
the center of the GRIN waveguide core.
[0381] As has been described before, to form a dielectric channel
waveguide with a GRIN distribution in the horizontal/lateral
direction to confine light propagation, UV imprinting may be used
since the deposited GRIN glass/polymer film may be made
photosensitive. A GRIN channel mask 2060, as shown in FIG. 39M can
be used to form a single mode GRIN channel waveguide 2065 to
confine light in the horizontal/lateral direction, resulting in the
structure shown in FIG. 39N.
[0382] It should be understood that the above dimensions and
exemplary lengths are presented for the purposes of illustrating a
useful embodiment of the device 2000 and are not intended to limit
other embodiments of any exemplary device, or the device 2000. A
variety of dimensions and sizes can be used, depending on the
application desired, as well as the fabrication materials,
processes and technologies that are employed.
[0383] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the various embodiments of the present
invention, including the side surfaces, the top and bottom
surfaces, and the input/output surfaces.
[0384] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0385] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the devices.
[0386] It should be understood to those skilled in the art that the
device 2000 can be fabricated on a different substrate other than
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0387] (9) Exemplary Device 9: A
Vertical-sharp-down-taper-cascaded-with-a-
-nonsymmetric-vertical-graded-refractive-index-(VSDT+NSVGRIN)
Device
[0388] FIGS. 40A-C illustrates a ninth general embodiment of a
combined module ICMT device 2100 involving a vertically
down-tapered waveguide core cascaded with and connected to a large
channel waveguide with a non-symmetric vertically graded refractive
index distribution. In the lateral direction, the high refractive
index contrast taper can be either gradually tapered up, as shown
in FIG. 40B, or sharply tapered down, as shown in FIG. 40C. In the
former case, the large channel waveguide has a step index profile
in the lateral direction. In the latter case, the large channel
waveguide has a graded index profile. Device 2100 differs from
exemplary device 7 or 8 in that the vertical GRIN waveguide in
device 2100 is non-symmetric.
[0389] The device 2100 can perform as a two-dimensional beam-size
enlargement and collimating element in both the lateral and the
vertical directions for a propagating optical beam, and can in
particular enlarge and collimate an optical beam from a small
semiconductor waveguide to enable the beam to match with a single
mode optical fiber. The device is not limited to use as a beam
enlarger/collimator but can also function as a beam reducer when
the optical beam propagates in the reverse direction. Furthermore,
the device can be used for direct light beam coupling into an
optical fiber. It should be understood that these exemplary
applications of device 2100 are intended to illustrate the uses for
device 2100 and are not intended to limit the applications of other
exemplary embodiments of device 2100 to these examples. The device
can be referred to as a
"vertical-sharp-down-taper-cascaded-with-a-nonsymmetric--
vertical-graded-refractive-index (VSDT+NSVGRIN) ICMT".
[0390] The present VSDT+NSVGRIN ICMT preferably includes a
Waveguiding Core region occupied by Waveguide Core WC 2145/2150.
This Waveguide Core WC 21415/2150 is sandwiched at the bottom by a
Lower Waveguide Cladding LWC 2110 and on the top as well as at both
sides by a vertically graded refractive medium 2155 that acts as
the top and side cladding for the Waveguide Core WC 2145/2150
wherever the Waveguide Core WC 2145/2150 exists. The same graded
refractive index medium 2155 acts as a Lower Refractive Index
Contrast Waveguiding Core region. This Lower Refractive index
Contrast Waveguiding Core region is occupied by Waveguide Core
LRCWC 2155, and is further surrounded at the top by an even lower
refractive index Upper Waveguide Cladding 2160.
[0391] When viewed from the side, the Waveguide Core WC 2145/2150
preferably includes a small beam input/output port SB-PT 2171a, a
high refractive index contrast straight waveguiding core region SWC
2172a, a high refractive index contrast vertically down-tapered
region VTD 2173a, and an intermediate beam output/input port region
2174a. The straight high refractive index contrast waveguiding core
region SWCa 2172a has a length l.sub.HRCSWCa, and a thickness of
t.sub.HRCSWCa. The high refractive index contrast vertically
tapered down waveguiding core region VTDWC 2173a has a length of
l.sub.HRCVTDWCa, a vertical thickness of t.sub.HRCVTDWCaSB at the
small-beam input/output side, and a vertical thickness of
t.sub.HRCVTDCaLB at the large-beam input/output side.
[0392] In the laterally up-tapering case, when viewed from the top,
the Waveguide Core 2145 preferably includes a small beam
input/output port SB-PT 2171b, a straight waveguiding core region
SWC 2172b, a laterally up-tapered region LTU 2173b, a wider
straight waveguiding core region WSWC 2175 and a large beam
output/input port region 2174b. The straight waveguiding core
region SWCb 2172b has a length l.sub.HRCSWCb, and a width of
w.sub.HRCSWCb. The laterally up-tapered waveguiding core region
LTUWC 2173b has a length of l.sub.HRCLTUWCb, a width of
w.sub.HRCLTUWCbSB at the small-beam input/output side, and a width
of w.sub.HRCLTUCbLB at the large-beam input/output side. The wider
straight waveguide core region WSWC 2175 has a length of
l.sub.HRCWSWCb and a width of w.sub.HRCWSWCb=w.sub.HRCLTUCbLB.
[0393] In the laterally down-tapering case, when viewed from the
top, the Waveguide Core 2150 preferably includes a small beam
input/output port SB-PT 2171c, a straight waveguiding core region
SWC 2172c, a laterally down-tapered region LTD 2173c, and an
intermediate beam output/input port region 2174c. The straight
waveguiding core region SWCc 2172c has a length l.sub.HRCSWCc, and
a width of w.sub.HRCSWCc. The laterally tapered down waveguiding
core region LTDWC 2173c has a length of l.sub.HRCLTDWCc, a width of
w.sub.HRCLTDWCcSB at the small-beam input/output side, and a width
of w.sub.HRCLTDCcLB at the large-beam input/output side.
[0394] It should be understood that the straight waveguiding core
regions 2171a and 2171b/c (based on whether the structure is viewed
from the side or from the top), may have the same or a different
length. Similarly, the vertically down-tapered region 2172a (when
viewed from the side) and the laterally up/down-tapered region
2172b/c (when viewed from the side) may have the same or a
different length, provided that in the case where the waveguide
core is both laterally and vertically down-tapered, the tip for
both down-tapering geometries ends at the same point in space.
Furthermore, the taper regions do not need to be symmetric with
respect to the central axis and also the down-tapering slope (or
slopes) do(es) not need to be straight and may be of any curve
shape.
[0395] Lower-Refractive-index-Contrast-Waveguide Core LRCWC 2155
surrounds and embeds the Waveguide Core WC 2145/2150 either
partially or entirely. When viewed from the side, the Lower
Refractive index Contrast Waveguide Core LRCWC 2155 is sandwiched
at the bottom by a Lower Waveguide Cladding LWCL 2110, and on the
top by an Upper Waveguide Cladding UWCL 2160. As the
Lower-Refractive-index-Contrast-Waveguide Core LRCWC 2155 has a
graded refractive index distribution in the vertical direction, it
can take a vertically intermediate size beam launched at the tip
region 2174a from the Waveguide Core WC 2145/2150 and further
expand the beam. After the beam has traveled a certain distance
equivalent to the focal length of the graded refractive index
structure, the beam will be collimated and reach the back large
beam output/input port LB-PT 2177, from where another single mode
waveguide such as a single mode optical fiber will continue to
guide the light beam. The Lower Refractive index Contrast Waveguide
Core LRCWC 2155 has a length of l.sub.LRCWC, a width of W.sub.LRCWC
and a thickness of t.sub.LRCWC. The Lower Waveguide Cladding LWCL
2110 has a length of l.sub.LWCL, a width of w.sub.LWCL and a
thickness of t.sub.LWCL. The Upper Waveguide Cladding UWCL 2160 has
a length of l.sub.UWCL, a width of w.sub.UWCL and a thickness of
t.sub.UWCL. The side waveguide claddings SWCL 2176b/c have a length
of l.sub.SWCL, a width of w.sub.SWCL and a thickness of t.sub.SWCL.
The lengths of the waveguide claddings LWCL/UWCL/SWCL
2005/2055/2076, l.sub.LWCL, l.sub.UWCL, and l.sub.SWCL are about
equal to the length of the low refractive index contrast waveguide
core l.sub.LRCWC.
[0396] The refractive index of the Waveguide Core WC 2145/2150 is
n.sub.HRCWC. The refractive index of the Lower Refractive index
Contrast Waveguide Core LRCWC 2155 is n.sub.LRCWC(x, y), which
means that it may be x- and y-coordinate dependent, with x being
the lateral coordinate and y being the vertical coordinate. The
refractive index of the Lower Waveguide Cladding LWCL 2110 is
n.sub.LWCL. The refractive index of the Upper Waveguide Cladding
UWCL 2160 is n.sub.UWCL. The refractive index of the Side Waveguide
Cladding SWCL 2176b/c is n.sub.SWCL(y), which means it may also be
y coordinate dependent. It should be understood that the Side
Waveguide Cladding SWCL 2176b/c may have either a uniform or
non-uniform refractive index distribution. In other words, the
refractive index of the Side Waveguide Cladding may or may not be a
constant. In the latter case, it may be graded in the vertical
direction. Similar to exemplary device 6, the refractive index of
the Lower Refractive index Contrast Waveguide Core LRCWC 2155 is
preferably not a constant. It is graded in the vertical direction
and varies from the bottom to the top. The variation can be
represented by n.sub.LRCWC(y), with y being the vertical
coordinate. In the lateral direction, the refractive index of the
Lower Refractive index Contrast Waveguide Core LRCWC 2155 may have
either a step or a graded profile. In the step profile case, for a
fixed y value, the refractive index n.sub.LRCWC(x, y) has a fixed
higher value within the core region and a lower value outside the
core region. For the graded profile case, for a given y coordinate
y.sub.0, the refractive index n.sub.LRCWC(x, y.sub.0) is graded
within the core region and drops to n.sub.SWCL(y.sub.0), the
refractive index value of the side cladding, at the two side
borders.
[0397] In an exemplary device, the front beam input/output port
FB-PT 2171a/b/c is configured to receive/transmit a light beam
typically having wavelength .lambda. with a very small beam size.
The mode size of the straight section 2172a/b/c of the Waveguide
Core WC 2145/2150 is preferably designed to match the mode size of
a preceding very small size waveguide. The vertically down-tapering
section 2173a will partially enlarge the beam in the vertical
direction to an intermediate size. The vertically graded Lower
Refractive index Contrast Waveguide Core LRCWC 2155 is configured
to take the intermediate size beam launched at region 2174a and
further expand and collimate it. Note that in device 2100, only the
top half of a symmetric GRIN waveguide is fabricated on top of the
Waveguide Core taper. The consequence is that the refractive index
distribution is no longer symmetric in the vertical direction (Note
that in the lateral direction across a channel waveguide, the
refractive index distribution can still be made symmetric using,
e.g., UV imprinting as has been explained). The coupler is
functional because the situation is equivalent to using half of a
lens with the Si taper located on the central axis of the lens. The
lateral beam enlargement for both the laterally up tapering and
down tapering cases is substantially the same as has been described
for exemplary devices 7 and 8 and will thus not be repeated. The
back beam input/output port LB-PT 2177 is configured to
receive/transmit a light beam typically having wavelength .lambda.
with a beam size that roughly equals the size of an optical
fiber.
[0398] (i) An Exemplary Device
[0399] In an exemplary embodiment, the Waveguide Core WC 2145/2150
is made up of silicon with a refractive index of n.sub.HRCWC=3.5.
The Lower Refractive index Contrast Waveguide Core LRCWC 2155 is
made up of lead-titania-silica material mixture with mixture
composite designed to give a refractive index distribution that
approximates half of a parabolic distribution in the vertical
direction as governed by the following equation and detailed in
Table III. 5 n LRCWC ( y ) = 1.65 + 0.3 [ 1 - ( y 2 ( 10 m ) 2 ) ]
, y 0
3TABLE III The refractive index profile of a high focusing power 10
.mu.m height half GRIN waveguide coordinate y (.mu.m) LRCWC
refractive index n SWCL refractive index n 0.0-0.4 1.950 1.940
0.4-0.8 1.948 1.938 0.8-1.2 1.946 1.936 1.2-1.6 1.942 1.932 1.6-2.0
1.938 1.928 2.0-2.4 1.933 1.923 2.4-2.8 1.926 1.916 2.8-3.2 1.919
1.909 3.2-3.6 1.911 1.901 3.6-4.0 1.902 1.892 4.0-4.4 1.892 1.882
4.4-4.8 1.881 1.871 4.8-5.2 1.869 1.859 5.2-5.6 1.856 1.846 5.6-6.0
1.842 1.832 6.0-6.4 1.827 1.817 6.4-6.8 1.811 1.801 6.8-7.2 1.794
1.784 7.2-7.6 1.777 1.767 7.6-8.0 1.758 1.748 8.0-8.4 1.738 1.728
8.4-8.8 1.718 1.708 8.8-9.2 1.696 1.686 9.2-9.6 1.674 1.664
9.6-10.0 1.650 1.640 >10.0 1.470 1.470
[0400] The Lower Waveguide Cladding LWCL 2110 is made up of silica
with a refractive index of n.sub.LWCL=1.47. The Upper Waveguide
Cladding UWCL is made up of silica with a refractive index of
n.sub.UWCL=1.47. The Side Waveguide Cladding SWCL is made up of
lead silica-titania with a refractive index of distribution given
in Table III.
[0401] For the case of a laterally up-tapered Waveguide Core WC
2145, the dimensions of the WC are as follows: l.sub.HRCSWCa=110
.mu.m, t.sub.HRCSWCa=0.3 .mu.m, l.sub.HRCVTDWCa=30 .mu.m,
t.sub.HRCVTDWCaSB=0.3 .mu.m, t.sub.HRCVTDCaLB=0 .mu.m,
l.sub.HRCSWCb=10 .mu.m, w.sub.HRCSWCb.=0.3 .mu.m,
l.sub.HRCLTUWCb=100 .mu.m, w.sub.HRCLTUWCbSB=0.3 .mu.m,
w.sub.HRCLTUCbLB=10 .mu.m, l.sub.HRCWSWCb=30 .mu.m and
w.sub.HRCWSWCb=10 .mu.m. The dimensions of the Lower Refractive
index Contrast Waveguide Core LRCWC 2155 are as follows:
l.sub.LRCWC=170 .mu.m, w.sub.LRCWC=10 .mu.m, t.sub.LRCWC=10 .mu.m.
The Lower Waveguide Cladding LWCL 2110 has a length of
l.sub.LWCL=170 .mu.m, a width of w.sub.LWCL=30 .mu.m and a
thickness of t.sub.LWCL=2 .mu.m. The Upper Waveguide Cladding UWCL
2160 has a length of l.sub.UWCL=170 .mu.m, a width of w.sub.UWCL=30
.mu.m and a thickness of t.sub.UWCL=2 .mu.m. The two side waveguide
claddings SWCL 2176b have a length of l.sub.SWCL=170 .mu.m, a width
of w.sub.SWCL=10 .mu.m and a thickness of t.sub.SWCL=10 .mu.m.
[0402] For the case of a laterally down-tapered Waveguide Core WC
2150, the dimensions of the WC are as follows: l.sub.HRCSWCa=10
.mu.m, t.sub.HRCSWCa=0.3 .mu.m, l.sub.HRCVTDWCa=30 .mu.m,
t.sub.HRCVTDWCaSB=0.3 .mu.m, t.sub.HRCVTDCaLB=0 .mu.m,
l.sub.HRCSWCb=10 .mu.m, w.sub.HRCSWCb.=0.3 .mu.m,
l.sub.HRCLTDWCb=30 .mu.m, w.sub.HRCLTDWCbSB=0.3 .mu.m,
w.sub.HRCLTDCbLB=0 .mu.m. The dimensions of the Lower Refractive
index Contrast Waveguide Core LRCWC 2155 are as follows:
l.sub.LRCWC=70 .mu.m, w.sub.LRCWC=10 .mu.m, t.sub.LRCWC=10 .mu.m.
The Lower Waveguide Cladding LWCL 2110 has a length of
l.sub.LWCL=70 .mu.m, a width of w.sub.LWCL=30 .mu.m and a thickness
of t.sub.LWCL=2 .mu.m. The Upper Waveguide Cladding UWCL 2160 has a
length of l.sub.UWCL=70 .mu.m, a width of w.sub.UWCL=30 .mu.m and a
thickness of t.sub.UWCL=2 .mu.m. The two side waveguide claddings
SWCL 2176c have a length of l.sub.SWCL=70 .mu.m, a width of
w.sub.SWCL=10 .mu.m and a thickness of t.sub.SWCL=10 .mu.m. It
should be appreciated by one skilled in the art that all parameter
values used in this and other exemplary embodiments are approximate
and that the actual values can vary significantly.
[0403] (ii) General Operation of the Device
[0404] FIG. 41 shows the results of a computer simulation of the
spatial distribution of the electric field strength in the vertical
direction for light input from either direction at .lambda.=1.5
.mu.m. The upper graph shows the behavior when light is input into
the small light beam input/output port 2171a; The mode size at
input/output port 2171a is 0.3 .mu.m in the vertical direction. As
the tapering down section only lies towards the right for the last
30 .mu.m and the cascading beam expansion action occurs on both the
left and the right part of the vertical WC taper tip, the computer
simulation is thus zoomed into the last 60 .mu.m of the coupler
structure. The lower graph is similar. It shows the behavior when
light is input to the large light beam output/input port 2177.
Comparing the two graphs demonstrates that the device has good
coupling efficiency in both directions.
[0405] The beam expansion in the lateral direction for the
laterally tapered up and laterally tapered down cases are similar
to what has been described for exemplary devices 7 and 8
respectively; these cases are not shown in FIG. 41.
[0406] As can be seen from FIG. 41, the combined module coupler
structure can transform a very small beam of about 0.3 .mu.m in
size to a large size beam of about 10 .mu.m in both the vertical
and the horizontal directions. Very efficient bidirectional
coupling is thus made possible by the supercoupler.
[0407] (iii) Device Fabrication Procedures
[0408] An exemplary procedure for fabricating a VSDT+NSVGRIN ICMT
device 2100 will now be described with reference to FIGS. 42A-N.
This procedure is given for the purpose of illustration and not
limitation, as there are other procedures that can be used to
achieve the same fabrication results and other materials systems or
device structures that can be utilized to fabricate devices with
the same functional capabilities.
[0409] The VSDT+NSVGRIN ICMT may be fabricated by starting with a
Silicon-On-Insulator (SOI) wafer, as shown in FIG. 42A, which may
be fabricated via a commercial process known to those skilled in
the art. For an SOI wafer, a silicon layer 2105 is already made on
an insulating SiO.sub.2 2110 which is on top on a silicon substrate
2115. The fabrication of the VSDT+NSVGRIN ICMT structure according
to one embodiment is now described.
[0410] A Si taper may be fabricated by spin-coating a photoresist
layer 2120 on the Si waveguide layer 2105, as shown in FIG. 42A. A
mask pattern with a gray scale transparency along the longitudinal
direction and also a laterally tapered up pattern 2125, as shown in
FIG. 42B, or a laterally tapered down pattern 2130, as shown in
FIG. 42C, can be used together with UV exposure and photoresist
development to make a tapered photoresist pattern that is
vertically tapered down and laterally tapered up (pattern 2135 in
FIG. 42D) or down (pattern 2140 in FIG. 42E). Followed by dry
etching, indicated in FIG. 42F, the tapered photoresist pattern
2135 or 2140 may be transferred to the high refractive index Si
layer and form the corresponding Si section 2145 or 2150, shown in
FIGS. 42G-I. The interface between the top Si layer and the lower
SiO.sub.2 layer may be used as a natural stop during the dry
etching process. A shadow mask based dry etching or a
diffusion-limited wet etching may also be used to form the Si
taper.
[0411] To form the top half of the vertically GRIN waveguide 2155,
multiple layers of effective refractive-index-decreasing
dielectrics may be deposited on top of the Si taper, as shown in
FIG. 42J. Preferably photosensitive silica is spin-coated so that
confinement of light in the horizontal/lateral direction may be
easily achieved using UV imprinting. On the very top, a relatively
thick (say 2 .mu.m) silica layer 2160 may be deposited to act as an
upper cladding.
[0412] As has already been discussed, to form a dielectric channel
waveguide with either a GRIN or step refractive index distribution
in the lateral direction to confine light propagation, UV
imprinting may be used as the deposited vertically GRIN film can be
made photosensitive. In this respect, a step channel mask 2165, as
shown in FIG. 42K, or a GRIN channel mask 2170, as shown in FIG.
42L, may be used to form a single-mode step channel waveguide 2185,
as shown in FIG. 42M, or GRIN channel glass waveguide 2190, as
shown in FIG. 42N, to confine light in the horizontal/lateral
direction.
[0413] One skilled in the art will recognize that high efficiency
coupling from the buried GRIN channel waveguide to a single mode
optical fiber or vice versa is not a problem as the mode size is
already designed to match that of a single mode fiber. The chief
consideration is the location for this joining. Preferably, the
fiber is made to butt-join the GRIN channel waveguide at a fully
expanded/collimated location rather than a focused location. The
fiber is also preferably located at the first fully expanded
location from the tip of the Si taper, in order to reduce light
propagation losses in the glass channel waveguide.
[0414] It should be understood that the above dimensions and
exemplary lengths are presented for the purposes of illustrating a
useful embodiment of the device 2100 and are not intended to limit
other embodiments of any exemplary device, or the device 2100. A
variety of dimensions and sizes can be used, depending on the
application desired, as well as the fabrication materials,
processes and technologies that are employed.
[0415] Also, it should be understood that the shapes of the
waveguides or the taper (for example the shapes as defined by the
surfaces dividing the cladding regions and the core regions) do not
generally have to be linear or in the form of straight lines and
planar surfaces. Curved shapes and different waveguide dimensions
may be utilized as long as they achieve the same functions such as
waveguiding or optical mode size transformation with similar
topological connections. This applies for all the surfaces of the
waveguide structures of the various embodiments of the present
invention, including the side surfaces, the top and bottom
surfaces, and the input/output surfaces.
[0416] In addition, it should be understood that the substrate is
used to mechanically support the waveguide structures, and can be
made up of irregular shapes, or structures, or materials as long as
it serves the function of providing mechanical support for the
waveguide structures.
[0417] Furthermore, it should be understood that the output ports
can also be used as input ports and the input ports can be used as
output ports. This is due to the reciprocal nature of light
propagation in passive optical devices and hence the bi-directional
nature of the devices.
[0418] It should be understood to those skilled in the art that the
device 2100 can be fabricated on a different substrate other than a
silicon substrate. In particular, it can be fabricated directly on
InP or GaAs substrates used for making semiconductor photonic
devices or integrated circuits and may be fabricated directly at
the input/output ports of the photonic devices by sharing the same
substrate as the photonic device.
[0419] (10) Variations of Exemplary Devices and Integration of ICMT
with V-(Grooves for Fiber Alignments Platform for Photonic
Chips
[0420] In the above mentioned exemplary combined module ICMTs that
involve connecting a vertical sharp-down-taper waveguide core with
a lower-refractive-index-contrast vertically graded waveguide, only
two cases of placing the taper in the GRIN waveguide have been
discussed, namely at the center of a symmetric GRIN waveguide or at
the bottom of a half-GRIN waveguide. One skilled in the art will
recognize that the GRIN waveguide need not to be restricted to
these two cases. For example, the GRIN waveguide may have a
refractive index distribution that is similar to the bottom half of
a symmetric GRIN profile, and the taper can then be fabricated near
the top of the GRIN waveguide. As another example, the GRIN
waveguide can be three quarters of a symmetric GRIN profile; as
long as the taper is fabricated near the high-refractive-index
region of the GRIN profile, cascaded light beam expansion or
reduction can be achieved. In fact, the GRIN waveguide may have an
arbitrary profile, and the taper may be placed within a relatively
large tolerance around the high-index region of the GRIN profile.
FIG. 43 shows the structure and simulation results of light
coupling from either side to the other. Note that compared to
device 2100 of FIG. 40A, the Si taper 2210 is now shifted upward
into the GRIN region 2220 and is buried above the lower cladding
SiO2 layer 2230. It can be seen that the while the taper to the
half GRIN waveguide coupling is basically not changed as compared
to the previous case, there is a slight increase in the reverse
direction light coupling efficiency.
[0421] In terms of device fabrication, a silicon waveguide layer
may be bonded on top of an X-SiO.sub.2-Si structure illustrated in
FIG. 44A. X may be a dielectric thin film 2310 with a refractive
index equal to the GRIN waveguide core. SiO.sub.2 layer 2320 is a
lower cladding layer for the GRIN glass/polymer waveguide, and Si
layer 2330 is the substrate. The structure can be easily achieved
by, for example, sputtering a glass film 2310 on the SiO2-Si wafer
2320/2330. An ion-implanted silicon wafer with a structure of Si
layer 2340 on ion-implanted layer 2350 on Si 2360, shown in FIG.
44B, can be flipped over and bonded with the X-SiO2-Si wafer. Si
layer 2360 is lifted off as discussed previously, leaving the
structure shown in FIG. 44C. The rest of the fabrication steps are
similar to those described above for exemplary device 9, beginning
with fabrication of a tapered waveguide core in Si layer 2340.
[0422] With respect to the various combined module ICMTs, at least
three basic vertical structure configurations may be manufactured.
While the symmetric GRIN waveguide structure offers the best
coupling efficiency, it is the most challenging structure to
fabricate as there is a need to bond an ion implanted Si wafer to a
GRIN dielectric coated SiO.sub.2-Si wafer. However, it should be
understood that the sol-gel spin-coating approach described above
is only one exemplary way to fabricate such a structure;
modifications of the fabrication process can be made that may
simplify the fabrication process. For example, instead of using the
sol-gel technique described above, any other thin film deposition
technique including flame hydrolysis, sputtering, Ion-Assisted
Deposition, evaporation and chemical vapor deposition may be used
to deposit the graded refractive index layers. As a resullt, the
wafer-bonding can be relatively easily done as long as the bottom
half of the GRIN waveguide is of high quality.
[0423] One preferred technique for simplifying fabrication is to
first deposit a relatively thin dielectric film having a refractive
index about equal to that of the GRIN waveguide core onto a SiO2-Si
wafer before wafer bonding to an ion implanted Si wafer. For
example, sputtering is a well-established process for depositing
such films; as a sputtered film is relatively thin, it will be less
challenging to wafer-bond an ion implanted Si wafer to a sputtered
dielectric-SiO.sub.2-Si wafer.
[0424] Another approach is to use a commercially available
Si-SiO.sub.2-Si or SOI wafer, make the top Si waveguide layer into
a taper and spin-coat a half GRIN waveguide. Although the resulting
light-coupling efficiency is slightly lower as compared to couplers
with a full GRIN waveguide, the commercial availability of SOI
wafers makes this a simple and easy option in terms of
fabrication.
[0425] In terms of photonic chip mounting and optical fiber
alignment with the couplers of the present invention, there are
various options. FIGS. 45A-C illustrates one process. After the
formation of channel waveguides in the vertically GRIN glass
waveguide layer 2410 using, for example, UV imprinting,
photolithography, as illustrated in FIG. 45A, can be used to define
the Si V-groove wet etching opening 2420. Selective dry etching of
the GRIN glass waveguide layer 2410 and the insulating SiO.sub.2
layer 2430 can then be employed to define a vertical wall 2440 in
the GRIN glass waveguide layer, and Si V-grooves 2450 can be wet
etched through the dry etched V-groove openings, as shown in FIG.
45B. To fabricate a photonic chip recess, another photolithography
process can be carried out to define the recess/well opening 2460.
Dry etching can be carried out to etch the recess/well to a precise
depth. FIG. 45C illustrates a section of the completed device.
Metal contact pads 2470 as well as electrical conduction paths may
be made by metal evaporation. Photonic chip 2480 may be mounted and
soldered in the recess/well with slight heating to ensure good
adhesion and electrical contact. The silicon wafer with the
photonic chips attached and the Si V-grooves already made can be
diced into small pieces. Subsequently, optical fiber arrays 2490
can be mounted in the Si V-grooves and fixed.
[0426] The process is readily adaptable to multiport devices, as
illustrated in FIG. 46, where photonic chip 2480 may comprise one
or more semiconductor optical devices connected by waveguides 2482
to multiport integrated couplers of the present invention 2484.
These devices, in turn, couple to optical fibers 2490.
[0427] While the exemplary devices described above use a silicon
substrate and a silicon based high refractive index taper, it
should be understood that these materials are only meant to
illustrate exemplary cases. The embodiments of the present
invention also include the use of other suitable materials for the
substrate and the high index waveguide core or taper, wherein these
suitable materials include compound semiconductor based material
such as gallium arsenide, indium phosphide and gallium nitride, or
an optical crystal based material such as lithium niobate, lithium
tantalate and barium titanate, or a dielectric material such high
index glasses.
[0428] IV. Applications
[0429] The applications of the mode transformation couplers of the
present invention are numerous as the technology addresses the
bottleneck of the present photonic integrated circuit (PIC)
technologies. Couplers are used in the pigtailing and packaging of
almost all semiconductor and optical crystal based photonic
devices, especially those with multi-function and multiple ports.
One application is in optical communication, where a coupler can be
used for the packaging of all kinds of semiconductor and optical
crystal based devices including semiconductor lasers, modulators,
switches, mulitplexers/demultiplexers, amplifiers, power splitters
and so on. Presently, there are a number of technologies that are
producing these photonic devices, such as III-V-based OEICs,
Si-based optical or photonic MEMS, SOI- and SiGe-based integrated
optical systems. All these will need mode conversion couplers to
link to each other and to the outside world.
[0430] Another application is the concept of a photonic breadboard
on which different integrated photonic chips are mounted and
interconnected to one another via couplers of the present invention
as shown in FIG. 47. Such an optical breadboard can be used to
construct a system and test its function or performance before a
fully integrated system chip is fabricated.
[0431] Note that a future trend of photonics is in the integration
of multiple functional components on the same chip with multiple
input and output ports to be connected to fiber arrays. In addition
to optical communication, these chips will basically do the work
that today is done by microelectronics chips, but at a much faster
speed than their electronic counterparts. It can thus be foreseen
that the cost of each component will drastically drop through
photonic integration, as has happened for semiconductor-based
electronics. The couplers of the present invention together with
the associated packaging technology provides a significant
reduction in the overall cost of such a multi-port photonic chip.
The application areas of these photonic chips are potentially very
wide, encompassing, for example, processors, computers, sensors,
etc.
[0432] The foregoing description has provided exemplary embodiments
of multiport integrated couplers and processes for fabricating
these exemplary devices; these examples are intended to illustrate
and not to limit the scope of the invention. One skilled in the art
will recognize that various modifications are possible. For
example, the waveguides are described with reference to coupling
with particular optical devices such as semiconductor optical
devices and optical fibers. One skilled in the art will recognize
that the utility of the waveguides according to the present
invention is not limited to the particular devices mentioned
herein; indeed, the waveguides may be used with any optical device,
and the dimensions of the waveguides may be varied for optimal
matching to the optical device. Therefore, the scope of the present
invention should be determined by the following claims, including
their full range of equivalents.
* * * * *