U.S. patent application number 09/896789 was filed with the patent office on 2002-10-17 for fiber devices using grin fiber lenses.
Invention is credited to Reed, William Alfred, Schnitzer, Mark J..
Application Number | 20020150333 09/896789 |
Document ID | / |
Family ID | 46204183 |
Filed Date | 2002-10-17 |
United States Patent
Application |
20020150333 |
Kind Code |
A1 |
Reed, William Alfred ; et
al. |
October 17, 2002 |
Fiber devices using grin fiber lenses
Abstract
A mode converter includes first and second optical waveguides
and a GRIN fiber lens. The GRIN fiber lens is attached to both the
first and the second waveguides.
Inventors: |
Reed, William Alfred;
(Summit, NJ) ; Schnitzer, Mark J.; (Summit,
NJ) |
Correspondence
Address: |
Docket Administrator (Room 3J-219)
Lucent Technologies Inc.
101 Crawfords Corner Road
Holmdel
NJ
07733
US
|
Family ID: |
46204183 |
Appl. No.: |
09/896789 |
Filed: |
June 29, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60269586 |
Feb 17, 2001 |
|
|
|
60292013 |
May 19, 2001 |
|
|
|
Current U.S.
Class: |
385/34 ; 385/16;
385/28 |
Current CPC
Class: |
G02B 6/32 20130101; G02B
6/3582 20130101; C03B 2203/26 20130101; G02B 6/14 20130101; G02B
6/264 20130101; A61B 5/0084 20130101; C03B 2201/32 20130101; G02B
6/3556 20130101; G02B 6/262 20130101; G02B 6/3512 20130101; C03B
2201/12 20130101; G02B 6/3594 20130101; G02B 6/3548 20130101; C03B
2201/28 20130101; C03B 37/01861 20130101; C03B 2201/31 20130101;
G02B 6/3552 20130101 |
Class at
Publication: |
385/34 ; 385/28;
385/16 |
International
Class: |
G02B 006/32; G02B
006/26 |
Claims
What is claimed is:
1. An apparatus for mode converting, comprising: first and second
optical waveguides; and a GRIN fiber lens attached to both the
first and the second waveguides.
2. The apparatus of claim 1, wherein the waveguides are fused or
glued to the GRIN fiber lens.
3. The apparatus of claim 2, wherein the first and second
waveguides are first and second optical fibers, respectively.
4. The apparatus of claim 3, wherein the first fiber has
propagation modes with different sizes than the second fiber.
5. The apparatus of claim 3, wherein the lens has a magnification,
the magnification times the size of a fundamental propagation mode
of the first fiber being about equal to the size of a fundamental
propagation mode of the second fiber.
6. The apparatus of claim 3, wherein the first and second fibers
have cores with different diameters.
7. The apparatus of claim 3, wherein each fiber has a core and a
cladding; and a discontinuity in refractive index across an
interface between the core and cladding, the discontinuities being
different across the interfaces of the first and second fibers.
8. The apparatus of claim 3, wherein the GRIN fiber lens comprises
a series of connected GRIN fiber lenses; the first GRIN fiber lens
of the series being attached to the first fiber and the last GRIN
fiber lens of the series being attached to the second fiber.
9. The apparatus of claim 1, wherein the GRIN fiber lens has a core
with a graded refractive index profile, the profile having a radial
second derivative whose average magnitude is less than about
2.4.times.10.sup.-5 microns.sup.-2 in the core.
10. An apparatus, comprising: first, second, and third optical
fibers; first, second, and third GRIN fiber lenses attached to the
first, second, and third optical fibers, respectively; and an
optical element configured to optically couple the first, second,
and third optical fibers.
11. The apparatus of claim 10, wherein free ends of the first,
second, and third GRIN fiber lenses have separations of less than
about 1 millimeter.
12. The apparatus of claim 10, further comprising: a MEM device;
and wherein the optical element is a moveable reflector whose
position or orientation is controlled by the MEM device.
13. The apparatus of claim 10, wherein the optical element includes
one of an optical circulator, a polarization-selective splitter,
and a wavelength-selective reflector.
14. The apparatus of claim 10, wherein a free surface of one of the
GRIN fiber lenses is cleaved at an angle of less than 8 degrees
from a plane whose normal is the lens' axis.
15. An optical apparatus, comprising: an array of at least three
optical fibers having attached GRIN fiber lenses; and an optical
device configured to direct light between selected ones of the GRIN
fiber lenses of the array and another optical waveguide.
16. The apparatus of claim 15, wherein the other waveguide is an
optical fiber having an attached fiber GRIN lens.
17. The apparatus of claim 15, wherein a free surface of one of the
GRIN fiber lenses is cleaved at an angle of less than 8 degrees
from a plane whose normal is the lens' axis.
18. The apparatus of claim 16, wherein the optical device is
configured to present an acceptance window for light from one of
the fibers, the window having a diameter not greater than the
diameter of one of the optical fibers.
19. The apparatus of claim 15, wherein free ends of the GRIN fiber
lenses have separations of less than about 1 millimeter.
20. The apparatus of claim 15, wherein the optical device includes
one of a reflector, a polarization-sensitive splitter, and a
wavelength-sensitive reflector.
21. The apparatus of claim 16, further comprising: a second array
of optical fibers having attached GRIN fiber lenses, the waveguide
being one of the fibers in the second array; and wherein the
optical device includes a plurality of elements capable of routing
light from the fibers of the first array to the fibers of the
second array.
22. The apparatus of claim 21, wherein at least one of the elements
is one of a polarization-sensitive splitter and a
wavelength-sensitive reflector.
23. The apparatus of claim 21, wherein free ends of the GRIN fiber
lenses of the array have separations of less than about 1
millimeter.
24. The apparatus of claim 21, wherein a free surface of one of the
GRIN fiber lenses of the array is cleaved at an angle of less than
8 degrees from a plane whose normal is the lens' axis.
25. The apparatus of claim 21, wherein the optical device is
configured to present an acceptance window for light from one of
the fibers, the window having a diameter not greater than the
diameter of one of the optical fibers.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/269,586, filed Feb. 17, 2001, and of U.S.
Provisional Application No. 60/292,013, filed May 19, 2001.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to optical devices and graded
refractive index lenses.
[0004] 2. Discussion of the Related Art
[0005] A graded refractive index (GRIN) lens has a refractive index
whose value varies with radial distance from the axis of the lens.
The non-trivial variation in refractive index causes light
refraction and gives the GRIN lens focussing capabilities that are
similar to those of an ordinary lens. Therefore, many optical
devices employ GRIN or ordinary lenses interchangeably.
[0006] Optical devices use lenses to focus, collimate, or expand
light beams. FIG. 1 shows a fiber device 10 in which a GRIN fiber
lens 11 is fused to a terminal end 12 of an optical fiber 13. The
GRIN fiber lens 11 expands the light beam emitted by the optical
fiber 13. The GRIN fiber lens 11 improves the optical coupling
between optical fiber 13 and fiber device 15 as compared to the
coupling that would otherwise exist between the fiber 13 and device
15 due to diffraction. The GRIN fiber lens 11 reduces diffraction
losses when the optical fiber 13 is optically coupled to another
optical fiber.
[0007] Since the diameter of a light beam varies along the axis of
a GRIN lens, the diameter variations provide a measure of the lens'
length. The length over which the variations in the beam diameter
make two complete cycles is known as the pitch of the lens.
Typically, lengths of GRIN lens are referred to in multiples of the
pitch length, e.g., 1/2 pitch or 1/4 pitch.
BRIEF SUMMARY OF THE INVENTION
[0008] One apparatus embodying principles of the inventions is a
mode converter that reduces losses when waveguides with different
propagation modes are end-coupled. In optical fibers, the forms of
the propagation modes depend on the radial dependence of the
refractive index.
[0009] The mode converter includes first and second optical
waveguides and a GRIN fiber lens. The GRIN fiber lens is attached
to both the first and the second waveguides.
[0010] Another apparatus embodying principles of the inventions
end-couples to at least three optical fibers through an optical
element. Rather than lenses with curved refractive surfaces, the
apparatus uses fiber GRIN lenses attached to the fiber ends for
collimation/focusing of light.
[0011] Another device embodying principles of the inventions
includes a fiber array an optical element, and a waveguide. In this
device, GRIN fiber lenses are attached to the fibers of the array.
The optical element directs light between ones of the fibers and
the waveguide
BRIEF DESCRIPTION OF THE FIGURES
[0012] FIG. 1 is a cross-sectional view of a fiber device that uses
a conventional GRIN fiber lens to end-couple two optical
fibers;
[0013] FIG. 2 is a cross-sectional view of a fiber device in which
an optical fiber is fused to an embodiment of a GRIN fiber
lens;
[0014] FIG. 3A shows radial profiles of germanium dopant densities
in a conventional GRIN fiber lens and a new GRIN fiber lens;
[0015] FIG. 3B shows radial profiles of refractive indexes for the
GRIN fiber lenses of FIG. 3A;
[0016] FIGS. 4A and 4B illustrate beam collimation in fiber devices
with new and conventional GRIN fiber lenses, respectively;
[0017] FIG. 5 is a flow chart for a method of fabricating the fiber
device of FIG. 2.
[0018] FIG. 6A is a cross-sectional view of a mode converter;
[0019] FIG. 6B is a cross-sectional view of a mode converter that
uses a compound GRIN fiber lens;
[0020] FIG. 7A is a top view of a 1.times.2 micro-optical
router;
[0021] FIG. 7B is a top view of another topology for a 1.times.2
micro-optical router;
[0022] FIG. 7C is a top view of a device that optically couples
three optical fibers;
[0023] FIG. 8 is a cross-sectional view of a 1.times.N
micro-optical router;
[0024] FIG. 9 is a top view of an N.times.M micro-optical router;
and
[0025] FIG. 10 is a cross-sectional view of an optical fiber with
an in-line optical device.
[0026] In the Figures, like reference numbers refer to functionally
similar features.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0027] 1. Grin Fiber Lenses
[0028] FIG. 2 shows an optical fiber device 16 in which an optical
fiber 17 is end-coupled to a GRIN fiber lens 18, e.g., fused or
glued to the fiber 17. The GRIN fiber lens 18 and optical fiber 17
are co-axial and have similar or equal outer diameters whose values
are in the range of about 100 microns (.mu.m) to about 135 .mu.m,
e.g., 125 .mu.m. The GRIN fiber lens 18 collimates a light beam 19
emitted from the end of the optical fiber 17 thereby decreasing the
numerical aperture below that of a bare optical fiber. The GRIN
fiber lens 18 is also able to focus an incident light beam into the
end 20 of the optical fiber 17.
[0029] Exemplary optical fibers 17 include single-mode and
multi-mode fibers.
[0030] Exemplary GRIN fiber lenses 18 have refractive indexes whose
radial profiles differ significantly from those of conventional
GRIN fiber lenses. The new radial profiles enable decreased
numerical apertures and increased Rayleigh ranges for fiber device
16 as compared to values of the same quantities in conventional
fiber device 10 of FIG. 1. The decreased numerical aperture implies
that an appropriate length GRIN fiber lens 18 would cause less
diffraction and a lower power density in emitted light beam 19 than
in the light beam 14 emitted by conventional fiber device 10. The
increased Rayleigh range implies that emitted beam 19 is better
collimated than the beam 14. The improved properties of the emitted
beam 19 facilitate transverse alignments required to end-couple the
fiber device 16 to another fiber device (not shown).
[0031] In some embodiments of fiber device 16, GRIN fiber lens 18
has an end face 21 that is angle cleaved to reduce back reflections
of light into optical fiber 17. In particular, a normal vector to
the end-face 21 is preferably cleaved at an angle
1.degree.-2.degree. or less with respect to the axis of the GRIN
fiber lens 18. This cleave angle is smaller than a typical cleave
angle of about 8.degree. used to lower reflections from its end
face back into the optical fiber (not shown). The beam expansion
provided by the GRIN fiber lens 18 lowers the amount of angle
cleave needed to produce an equivalent reduction in back
reflections into the fiber 17.
[0032] The new GRIN fiber lens 18 has a circular core 22 and an
annular cladding 24 that surrounds the core 22. In the core 22, the
refractive index varies with the radial distance from the axis of
the GRIN fiber lens 18. In the cladding 24, the refractive index is
constant and has a lower value than in the core 22. The GRIN fiber
lens has an outer diameter of about 125 .mu.m. The outer diameter
is the same as that of conventional GRIN fiber lens 11 shown in
FIG. 1. But, the new and conventional GRIN fiber lenses 11, 18 have
different radial refractive index profiles due to differences in
density distributions of dopant atoms in their cores. Exemplary
dopants include germanium (Ge), aluminum (Al), phosphorus (P), and
fluorine (F).
[0033] FIG. 3A shows radial profiles 26 and 27 of Ge-dopant
densities in conventional GRIN fiber lens 11 and new GRIN fiber
lens 18, respectively. In the core 22 of the new GRIN fiber lens
18, the Ge-dopant density has a radial profile that is largest on
the central axis and curved concave downwards. The profile does not
have an axial density dip, i.e., unlike some conventional GRIN
fiber lenses (not shown). The curvature of the radial profile of
the Ge-dopant has a smaller average magnitude in the core 22 of the
new GRIN fiber lens 18 than in the core of conventional GRIN fiber
lens 1. In the claddings of both the new and conventional GRIN
fiber lenses 18, 11, the Ge-dopant densities are lower than in the
fiber cores and are constant with respect to radial distance from
the fiber axes.
[0034] The boundaries between core and cladding, i.e., at radial
distances of R.sub.c and R.sub.c', are characterized by abrupt
changes in the Ge-dopant densities and/or radial gradients of the
densities. The core diameter is larger in the new GRIN fiber lens
18 than in conventional GRIN fiber lens 11, i.e.,
R.sub.c'>R.sub.c. Increasing the core diameter increases the
Rayleigh range of fiber device 16 when a GRIN fiber lens 18 of
appropriate length is used therein. Exemplary embodiments of the
GRIN fiber lens 18 have an outer diameter of about 125 .mu.m. and a
core 22 with a diameter of about 85 .mu.m, preferably 100 .mu.m or
more, and more preferably 105 .mu.m or more. In some GRIN fiber
lenses 18, cladding is absent so that the core has a diameter of
about 125 .mu.m.
[0035] FIG. 3B shows refractive index profiles 28 and 29 that
correspond to the Ge-dopant density profiles 26 and 27 of GRIN
fiber lenses 11 and 18, respectively. The radial profiles 28, 29
are concave down in the core 22.
[0036] The radial profiles 28, 29 also show that the new GRIN fiber
lens 18 has a refractive index whose radial profile has a
significantly more gentle variation than in the conventional GRIN
fiber lens 11. A parameter "g" measures the radial curvature of the
refractive index profile in the core of a GRIN fiber lens. In
particular, the parameter g is defined as: 1 g = - 1 n 0 2 P ( r )
r 2 r = 0
[0037] Here, "r" is radial distance for the axis of the GRIN fiber
lens, no is the value of the refractive index on the axis of the
GRIN fiber lens, and P(r) is the value of the refractive index at
the distance "r" from the axis of the fiber lens.
[0038] The GRIN fiber lens 18 has a refractive index profile that
has a gentler radial variation over the lens' core. Refractive
index profiles of the GRIN fiber lens 18 typically, have radial
curvatures that are smaller in magnitude than those disclosed in
Table 1 of "Analysis and Evaluation of Graded-Index Fiber-Lenses",
Journal of Lightwave Technology, Vol. LT-5, No. 9 (September 1987),
pages 1156-1164, by W. L. Emkey et al (EMKEY), which is
incorporated by reference herein in its entirety. Typically,
magnitudes of the radial curvature of refractive index profile for
embodiments of the GRIN fiber lens 18 are, at least, twice as small
as values for the same quantity that are disclosed in EMKEY.
Exemplary GRIN fiber lens 18 have a "g" that is less than 1.7
.times.10.sup.-6 .mu.m.sup.-2, preferable less than about
0.9.times.10.sup.-6 .mu.m.sup.-2 and more preferably less than
about 5.0.times.10.sup.-7 .mu.m.sup.-2. For 125 .mu.m--diameter
GRIN fiber lenses 18, values of "g" are selected from the range
1.7.times.10.sup.-6 .mu.m.sup.-2 to 5.0.times.10.sup.-7
.mu.m.sup.-2 and preferably in the range 0.9 .times.10.sup.-6
.mu.m.sup.-2 to 6.0.times.10.sup.-7 .mu.m.sup.-2 to provide good
beam collimation.
[0039] Exemplary GRIN fiber lens 18 have core index profiles that
vary approximately quadratically in the distance from the lens
axis. But, other embodiments of the GRIN fiber lens 18 have
non-quadratic index profiles.
[0040] Referring again to FIG. 2, the new GRIN fiber lens 18 has a
wider core 22 than the conventional GRIN fiber lens 11. The wider
core 22 and the smaller value of the parameter "g" enable the new
GRIN fiber lens 18 of appropriate length to produce a beam with a
wider cross section and a lower energy density when used as a beam
collimator.
[0041] FIGS. 4A and 4B show light beams 31, 32 emitted by new and
conventional fiber devices 16', 10' of the types shown in FIGS. 1
and 2. The fiber devices 16', 10' have GRIN fiber lenses 18', 11'
with equal pitches, e.g., {fraction (5/16)} pitch, but different
refractive index profiles. The new profile in the lens 18'
significantly increases the Rayleigh range, RR, of the fiber device
16' above the Rayleigh range, RR', of the conventional device 10'.
The increased Rayleigh range results from a more gradual beam
expansion in the GRIN fiber lens 18' as compared to the beam
expansion in the conventional GRIN fiber lens 11. In particular,
FIGS. 4A and 4B show that making the radial curvature in refractive
index of a GRIN fiber lens smaller than in conventional GRIN fiber
lenses significantly reduces the divergence of the emitted beam for
a given pitch.
[0042] The Rayleigh range determines the distance range over which
an optical device can couple to a fiber device without substantial
losses. The larger Rayleigh range in the new fiber device 16' makes
a larger set of distances available for end-coupling to such a
device than are available for the conventional fiber device
10'.
[0043] GRIN lenses of equal pitch ordinarily have equal products of
g.sup.1/2 times the lens-length. Since the new GRIN fiber lenses 18
have smaller g-values, the new GRIN fiber lenses 18 are ordinarily
longer than conventional GRIN fiber lenses 11 of equal pitch. The
longer lengths make the new GRIN fiber lenses 18 easier to handle,
align, and fuse to optical fibers than the conventional GRIN fiber
lenses 11. The increased lengths also reduce collimation errors
associated with cleaving errors that occur during production of the
new GRIN fiber lenses 18.
[0044] FIG. 5 is a flow chart for a method 100 of fabricating a
GRIN fiber lens of doped silica-glass through modified chemical
vapor deposition (MCVD). MCVD construction of optical fibers is
described in U.S. Pat. Nos. 4,909,816 and 4,217,027, which are
incorporated herein by reference in their entirety. The fabrication
method 100 includes forming an improved GRIN preform and then,
using the improved GRIN preform to make the GRIN fiber lenses,
e.g., GRIN fiber lenses 18 of FIG. 2.
[0045] To form the GRIN preform, layers of silica-glass are
deposited inside a silica-glass cladding tube by MCVD (step 102).
During the MCVD, a time-varying partial pressure of dopant gases is
bled into the gas mixture used to deposit silica-glass on the
inside of the cladding tube. Exemplary dopants include Ge, Al, P,
and F. Introduction of one or more of these dopants into the
silica-glass changes the refractive index of the glass. The partial
pressure of dopant gas is varied during the MCVD process to produce
a non-trivial radial profile of dopant atoms in the final
silica-glass preform.
[0046] The radial profile in dopant atoms produces a selected
radially graded refractive index in the final preform. Exemplary
profiles for the dopant density and the refractive index have
profiles with concave downward or negative radial curvature. Often,
the index profile varies as the square of the distance from the
preform's axis in the core of the preform, e.g., profiles 27, 29 of
FIGS. 3A and 3B. Other radial profiles may be obtained by suitably
altering the time-variation of the partial pressure of dopant atoms
during the MCVD. Non-quadratic profiles in GRIN fibers are capable
of reshaping of light beams therein as is known to those of skill
in the art.
[0047] The method 100 includes using the tube produced by the
internal deposition to form the rod-like preform. To form the
rod-like preform, heat is applied to partially collapse the tube of
doped silica-glass (step 104). In one embodiment, the heating
includes making repeated passes of the tube through a hot zone of a
furnace. The heating is stopped prior to totally blocking the axial
channel in the tube with glass.
[0048] After partially collapsing the tube, a silica-glass etchant
mixture is passed through the axial channel to remove several
layers of glass from the axis of the tube (step 106). An exemplary
gaseous etchant mixture includes C.sub.2F.sub.7, O.sub.2, and
Cl.sub.2. Other gaseous etchant mixtures include HF. The removed
layers have lower dopant concentrations than adjacent outer layers
of silica-glass, because dopants vaporize and are lost through the
tube's axial canal during the heating used to collapse the tube. If
these layers with lower dopant densities were not removed, the
final preform would have an axial dip in dopant density and a
corresponding axial dip in refractive index. The axial dip in
refractive index interfered the operation of some conventional GRIN
fiber lenses.
[0049] After the etching removal of several central layers of
glass, the tube is externally heated to finish its collapse to a
rod-like preform of doped silica glass (step 108).
[0050] After cooling the preform, etchants are applied to the outer
surface to remove a selected thickness of cladding tube from the
outside of the preform (step 110). Removing a portion of the
cladding tube enables subsequent drawing of glass fibers with less
or no cladding, e.g., see profiles 27 and 29 in FIGS. 3A and 3B.
These thin-clad or non-clad fibers are advantageous for GRIN fiber
lenses, because such fibers enable an optical beam to expand over a
larger portion of the cross section of the final GRIN fiber.
Spreading the beam over a larger cross section decreases the
associated numerical aperture and decreases power densities so that
defects on the end surface of the lens or on the target of the
emitted beam are less likely to cause component damage.
[0051] Fabrication of GRIN fiber lenses also includes using a
standard fiber drawing furnace to draw GRIN fiber from the
graded-index preform (step 112). After cooling, one end of the
drawn GRIN fiber is fused to one end of a standard fiber, i.e., a
fiber with a non-graded index core (step 114). To fuse the GRIN and
standard fibers, the ends of the two fibers are heated with an
electrical arc or a tungsten filament in an argon environment while
the ends are appropriately aligned and positioned adjacent each
other.
[0052] Finally, the GRIN fiber is cleaved to produce an optical
lens with a desired length (step 116). The final attached GRIN
fiber lenses has a pitch of 1/4, 1/2, or any other desired length
and is fused to the fiber on which it functions as a beam
collimator and expander.
[0053] To reduce reflections from the face of the final fiber
device back into the fiber, the cleaving is often performed along a
direction that is not perpendicular to the axis of the GRIN fiber.
In a non-GRIN optical fiber, cleaving the fiber's end face at an 8
degree angle with respect to a direction perpendicular to the
fiber's axis significantly reduces back reflections. For a GRIN
fiber lens, this cleaving angle can be reduced to less than 8
degrees from a direction perpendicular to the lens axis to achieve
the same reduction in back reflections into an attached optical
fiber, e.g., a preferred cleave angle is about 0.5-2 degrees.
[0054] The method 100 produces GRIN fiber lenses, e.g. lens 18 of
FIG. 2, that have lower refractive powers per unit length than
conventional GRIN fiber lenses, e.g., lens 11 of FIG. 1. Thus, the
new GRIN fiber lenses are significantly longer than conventional
GRIN fiber lenses having the same optical power. The longer lenses
collimate light better and are easier to manipulate during device
construction. Exemplary GRIN fiber lenses with low radial dopant
gradients have full pitch lengths of about 2, 3, or 4-20 mm.
[0055] The GRIN fiber lens 18 of FIG. 2 can also be made by vapor
axial deposition (VAD), outer vapor deposition (OVD), and sol-gel
processes that are known to those of skill in the art. Such
processes are also able to avoid creating an axial dip in
refractive index in the final GRIN fiber lens.
[0056] 2. Fiber Devices That Use Grin Fiber Lenses
[0057] Various embodiments provide optical fiber devices that are
described below. The various devices described can use either
conventional GRIN fiber lenses, e.g., lens 11 of FIG. 1, or new
GRIN fiber lenses, e.g., lens 18 of FIG. 2.
[0058] FIG. 6A shows a mode converter 40 that couples a pair of
optical fibers 36, 38 having different fundamental or higher
propagating modes. In some embodiments, the optical fibers 36, 38
have cores of different diameters or have refractive index jumps of
different sizes across core-cladding boundaries. In the mode
converter 40, GRIN fiber lens 43 is attached to the ends of the
optical fibers 36, 38. In exemplary mode converters 40, the GRIN
fiber lens 43 is either fused directly to the optical fibers 36, 38
or joined to the fibers 36, 38 by a glue layer (not shown) whose
thickness is not greater than the width of the cores of fibers 36,
38.
[0059] Since optical fibers 36, 38 have different core diameters
and/or refractive index jumps, the fibers 36, 38 have propagating
modes, e.g., fundamental modes, with different sizes. Herein, the
size of a propagating mode is defined as the mode's full-diameter
between half-maximum amplitude values. Due to the different sizes
of the propagating modes, coupling the optical fibers 36, 38
directly would produce a significant coupling loss of optical
energy, i.e., a splice loss.
[0060] To reduce splice losses, GRIN fiber lens 43 is positioned
between optical fibers 36, 38 and is selected to expand the
narrower propagating mode of optical fiber 36 to have a larger
diameter that equals that of the propagating mode of the optical
fiber 38. Designing the GRIN fiber lens 43 to produce the
appropriate size conversion entails selecting an appropriate lens
length. One of skill in the art would know how to select the length
of GRIN fiber lens 43 based the amount of magnification needed to
convert the size of the propagating mode of one fiber 36 into that
of the propagating mode of the other fiber 38.
[0061] In other embodiments, the mode converter 34 couples a
waveguide other than an optical fiber to optical fiber 38.
[0062] FIG. 6B shows a specific embodiment 34' of the mode
converter 34 of FIG. 6A. In the mode converter 34', GRIN fiber lens
43' is a compound lens made of a sequence of GRIN fiber lens
elements 43A, 43B. The first element 43A is fused directly to the
end of optical fiber 36, and the last element 43B is fused directly
to the end of optical fiber 38. Exemplary GRIN elements 43A and 43B
are fused together and have different refractive index profiles and
lengths. The lengths and index profiles of the two lens elements
43A, 43B are selected to better optically couple the fibers 36, 38.
In some embodiments, the first GRIN element 43A expands the light
beam emitted by fiber 36, and the second element 43B focuses the
beam waist to the size of the propagating mode in the fiber 38.
[0063] FIG. 7A shows a 1.times.2 micro-optical router 46. The
router 46 includes an input optical fiber 48, output optical fibers
50, 52, and a movable reflector 54 for directing light from the
input fiber 48 to a selected one of the output fibers 50, 52. The
terminal ends of the optical fibers 48, 50, 52 are fused to GRIN
fiber lenses 49, 49', 49", e.g., identical GRIN fiber lenses. The
GRIN fiber lens 49 functions to collimate or focus the emitted
light beam from fiber 48. The GRIN fiber lenses 49', 49" function
to collect light and couple the collected light into the associated
optical fibers 50, 52. The output optical fibers 50, 52 are located
so that the waist of the beam emitted by the input optical fiber 48
is at the midpoint of the optical path between the input and output
optical fibers 48, 50, 52. The reflecting surface of reflector 54
is located at the beam waist to within about a Rayleigh range when
positioned to reflect light to the output optical fiber 50.
[0064] To select a routing, reflector 54 is moved in or out of the
path of the light beam emitted by optical fiber 48. The reflector
54 is fixed to a micro-electro-mechanical (MEM) device 56 that
moves the reflector 54 in and out of the beam's optical path in
response to electrical signals applied to the MEM device 56.
[0065] The GRIN fiber lenses 49, 49', 49" improve beam collimation
and collection so that terminal ends 58, 60, 62 can be separated by
distances that are large enough to enable insertion and removal of
reflector 54 in routing region 64. In embodiments of router 46
based on the new GRIN fiber lenses 18 of FIGS. 2, 3A-3B, and 4A,
better beam collimation enables distances between terminal ends 58,
60, 62 to be as large as about 9 mm. For these large inter-fiber
distances, the GRIN fiber lenses 49, 49', 49" reduce optical
coupling losses to less than about 0.5 decibels (dB) and preferably
to less than about 0.2 dB-0.05 dB. However, larger inter-fiber
spaces involve more serious fiber device alignment issues.
[0066] In some embodiments, the micro-router 46 has an overall
size, S, that is much smaller than the overall size of an analogous
router in which the GRIN fiber lenses 49, 49', 49" are replaced by
conventional lenses with curved refractive surfaces. The lenses
with curved refractive surfaces have larger diameters than the GRIN
fiber lenses 49, 49', 49". The larger lens diameters require
positioning the ends of the input and output fibers at larger
separations in such a router than in the micro-router 46. The
lenses with curved refractive surfaces also typically produce
larger diameter collimated beams in the routing region than the
fused GRIN fiber lenses 49 of micro-router 46. The larger beam
diameters necessitate a larger reflective surface on the routering
reflector of the router whose lenses have curved refractive
surfaces than would be needed on the reflector 56 of the
micro-router 46.
[0067] In some embodiments of micro-router 46, the distance, S,
characteristic of separations between GRIN lenses 49, 49', 49' has
a value in the range of about 1-3 times the fiber diameter to about
1-3 times the Rayleigh range, e.g., less than about 1 mm. In these
embodiments, the small size of the region 64 between the lenses 49,
49', 49" is achieved in part, because diameters of the attached
GRIN fiber lenses 49, 49', 49" are small and in part, because the
reflective surface on reflector 54 has a small beam acceptance
window. The acceptance window for reflecting the input beam can be
less than the fiber diameter, because the GRIN fiber lens 49
produces a beam waist that is smaller than the diameter of fiber
48. Both the small diameter GRIN fiber lenses 49, 49', 49" and the
smallness of reflector 54 enable the router 46 to be much smaller
than routers that use lenses with curved refractive surfaces.
[0068] FIG. 7B shows an alternate embodiment 46' of the router 46
shown in FIG. 7A. In router 46', the fibers 48, 50, 52 are adjacent
and located in a linear array 68. A single rotatable reflector 56',
e.g., a MEMS controlled reflector, selectively routes light from
the fiber 48 to either the fiber 50 or the fiber 52. In some
embodiments, the axes the fibers 50 and 52 are slightly tilted with
respect to the axis of the fiber 48 to insure that light from the
reflector 56' parallel to the axis of the fibers 50, 52.
[0069] Arranging the fibers 48, 50, 52 in array 68 makes the width
of the router 46' roughly equal to the width, W, of the array 68.
The small diameters and fine collimation of GRIN fiber lenses 49,
49', 49" enable packing the fibers 48, 50, 52 closely in the array
68. Thus, embodiments of the router 46 can have a width, W, that is
much smaller than the width of a similar-form router in which
lenses with curved refractive surfaces replace the GRIN fiber
lenses 49, 49', 49".
[0070] FIG. 7C shows an embodiment of an optical device 46" that
couples three optical fibers 48, 50, 52 based on light
polarization, light wavelength, or relative fiber position. The
optical fibers 48, 50, 52 have attached GRIN fiber lenses 49, 49',
49" that collimate and collect light. The device 46" includes an
optical element 54' that transmits light between the optical fibers
48, 50, 52, e.g., in a manner that depends on polarization or
wavelength. In various embodiments, optical device 54' includes a
polarizing beamsplitter, a grating, an optical circulator, or a
wavelength selective reflector such as a Bragg grating.
[0071] FIG. 8 shows a 1.times.N micro-optical router 70 that
includes an input optical fiber 72, an output array 73 of N output
optical fibers 74.sub.1-74.sub.N, and a reflector 76. The optical
fibers 72, 74.sub.1-74.sub.N are single-mode fibers to which
terminal GRIN fiber lenses 77.sub.0-77.sub.N have been fused. The
light beam 78 from the input optical fiber 72 intersects the
reflector 76 near the waist of the beam 78, i.e., within 1/2 a
Rayleigh range.
[0072] Exemplary reflectors 76 include mirrors that move or rotate
and diffraction gratings that reflect light in a wavelength
dependent manner. For example, the router may be a spectrally
sensitive demultiplexer for a wavelength division multiplexed
network.
[0073] The GRIN fiber lenses 77.sub.0-77.sub.N expand and collimate
the light beam 78 of the input optical fiber 72 and focus the light
beam 78 into the output optical fibers 74.sub.1-74.sub.N. Due to
the GRIN fiber lenses 77.sub.0-77.sub.N, the output array 73 of
optical fibers 74.sub.1-74.sub.N and input optical fiber 72 can be
separated by an optical path that is long enough to enable
insertion of bulk reflector 76 into the path without significant
coupling losses. For the router 70 coupling losses are typically
less than about 0.5 dB-0.2 dB and preferably less than about 0.1
dB.
[0074] In micro-optical router 70, GRIN fiber lens 77.sub.0 focuses
the beam from fiber 72 onto a reflective acceptance window on the
reflector 76. Perpendicular to direction D, the diameter of the
acceptance window is less than the fiber diameter. Also, the use of
the GRIN fiber lenses 77.sub.0-77.sub.N enables an increased fiber
packing density in the array 73 without interference between light
beams reflected towards different ones of the fibers
74.sub.1-74.sub.N. Finally, the use of GRIN fiber lens 77.sub.0
enables the acceptance window and overall size of reflector 76 to
be smaller than that of the reflector that would otherwise be
needed in a router using lenses curved refractive surfaces (not
shown). Thus, using the GRIN fiber lenses 77.sub.0-77.sub.N enables
greater miniaturization in micro-router 70 than in a fiber router
based on lenses with curved refractive surfaces.
[0075] Other embodiments use the GRIN fiber lens 18 of FIG. 2 to
construct Nxl routers (not shown) by methods that would be obvious
to one of skill in the art in light of the above-disclosure. For
example, a 2.times.1 router can be constructed by exchanging
designations of input and output for fibers 48, 50, 52 in 1.times.2
micro-router 46 of FIG. 7A.
[0076] FIG. 9 is a top view of an N.times.M optical router 80. The
router 90 includes an array 81 of N input optical fibers,
82.sub.1-82.sub.N, and an array 83 of M output optical fibers,
84.sub.1-84.sub.M. The fibers 82.sub.1-82.sub.N, 84.sub.1-84.sub.M
have GRIN fiber lenses 85.sub.1-85.sub.N, 86.sub.1-86.sub.M fused
to terminal ends thereof. The GRIN fiber lenses 85.sub.1-85.sub.N,
86.sub.1-86.sub.M provide beam collimation and collection functions
analogous those previously described in relation to GRIN fiber
lenses 49, 49', 49" of FIG. 7A. Between the input and output fibers
82.sub.1-82.sub.N, 84.sub.1-84.sub.M are banks 87.sub.F, 87.sub.R
of fixed and routing reflectors, 88.sub.F1-88.sub.FN,
89.sub.R1-89.sub.RN. Exemplary reflectors 89.sub.R189.sub.RN
include wavelength-selective reflectors, e.g., gratings, and
wavelength insensitive reflectors. Properly aligning the reflectors
88.sub.R1-88.sub.RN routes light from individual ones of the input
fibers 82.sub.1-82.sub.N to selected ones of the output fibers,
84.sub.1-84.sub.M. The reflectors 88.sub.R1-88.sub.RN are operated
by MEMs devices 89.sub.1-89.sub.N and have acceptance windows for
input beams whose diameters are smaller than the inter-fiber
spacing, IFS, of array 81.
[0077] By using attached GRIN fiber lenses 85.sub.1-85.sub.N,
86.sub.1-86.sub.M the fiber packing densities in the arrays 81, 83
can be increased above fiber packing densities of an N.times.M
fiber router in which lenses with curved refractive surfaces (not
shown) replace the GRIN fiber lenses 85.sub.1-85.sub.N,
86.sub.1-86.sub.M of FIG. 9. Similarly, sizes of reflective
surfaces of reflectors 88.sub.F1-88.sub.FN, 89.sub.R1-89.sub.RN in
the router 80 are smaller than sizes of reflective surfaces of
reflectors in routers based on lenses with curved refractive
surfaces, because the beam diameters produced by the GRIN fiber
lenses 85.sub.1-85.sub.N are small. Both effects enable the new
N.times.M to be smaller than an N.times.M router based on lenses
with curved refractive surfaces.
[0078] FIG. 10 shows a micro-optical device 90 that is located
in-line between ends 91, 93 of optical fibers 92, 94. Exemplary
micro-optical devices 90 include wavelength-sensitive add/drop
modules, polarizers, polarization rotators, one-way optical
isolators, and controllable optical attenuators. The ends 91, 93 of
the optical fibers 92, 94 are fused to GRIN fiber lenses 96, 98.
The GRIN fiber lens 96 collimates light emitted by the optical
fiber 92. The GRIN fiber lens 98 focuses received light into the
optical fiber 94. The micro-optical device 90 has an approximate
thickness, d, that is not greater than the Rayleigh range
associated with the GRIN fiber lenses 96, 98. For such a thickness,
the GRIN fiber lenses 96, 98 reduce diffraction-related coupling
losses.
[0079] Other embodiments of the invention will be apparent to those
skilled in the art in light of the specification, drawings, and
claims of this application.
* * * * *