U.S. patent application number 09/893003 was filed with the patent office on 2002-01-10 for photonic crystal multilayer substrate and manufacturing method thereof.
This patent application is currently assigned to NEC CORPORATION. Invention is credited to Yamada, Hirohito.
Application Number | 20020004307 09/893003 |
Document ID | / |
Family ID | 18701501 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020004307 |
Kind Code |
A1 |
Yamada, Hirohito |
January 10, 2002 |
Photonic crystal multilayer substrate and manufacturing method
thereof
Abstract
A lightwave circuit, by which the degree of integration can be
further improved by employing a multilayered structure of wiring,
is disclosed. The disclosed photonic crystal multilayer substrate
has portions, each portion having a slab-waveguide type photonic
crystal structure, multilayered in the direction of the thickness
of the substrate. In the photonic crystal structure, a photonic
crystal layer is disposed between cladding layers; the photonic
crystal layer is made of a photonic crystal having a two or three
dimensional periodically modulated structure with respect to the
effective refractive index in the order of optical wavelengths; and
each cladding layer is made of a material whose effective
refractive index differs from the effective refractive index of the
photonic crystal layer.
Inventors: |
Yamada, Hirohito; (Tokyo,
JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3202
US
|
Assignee: |
NEC CORPORATION
|
Family ID: |
18701501 |
Appl. No.: |
09/893003 |
Filed: |
June 28, 2001 |
Current U.S.
Class: |
438/691 ;
438/689; 438/692 |
Current CPC
Class: |
G02B 6/42 20130101; B82Y
20/00 20130101; G02B 6/1225 20130101 |
Class at
Publication: |
438/691 ;
438/689; 438/692 |
International
Class: |
H01L 021/461; C30B
001/00; H01L 021/302 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2000 |
JP |
P2000-204198 |
Claims
What is claimed is:
1. A photonic crystal multilayer substrate having portions, each
portion having a slab-waveguide type photonic crystal structure,
multilayered in the direction of the thickness of the substrate,
wherein in the slab-waveguide type photonic crystal structure: a
photonic crystal layer is disposed between cladding layers; the
photonic crystal layer is made of a photonic crystal having a two
or three dimensional periodically modulated structure with respect
to the effective refractive index in the order of optical
wavelengths; and each cladding layer is made of a material whose
effective refractive index differs from the effective refractive
index of the photonic crystal layer.
2. A photonic crystal multilayer substrate as claimed in claim 1,
wherein the effective refractive index of the material of each
cladding layer is smaller than an effective refractive index of the
photonic crystal layer.
3. A photonic crystal multilayer substrate as claimed in claim 2,
wherein a multilayered lightwave circuit is formed by forming
optical devices in the photonic crystal layers.
4. A photonic crystal multilayer substrate as claimed in claim 1,
wherein each cladding layer has a multilayered film in which two or
more kinds of materials having different effective refractive
indexes are alternately layered.
5. A photonic crystal multilayer substrate as claimed in claim 1,
wherein each cladding layer is made of a photonic crystal having a
two or three dimensional periodically modulated structure with
respect to the effective refractive index.
6. A photonic crystal multilayer substrate as claimed in claim 4,
wherein the multilayered film for forming the cladding layer has
optical reflective characteristics in which the reflectance with
respect to a lightwave having a specific wavelength is larger than
the reflectance with respect to other lightwaves.
7. A photonic crystal multilayer substrate as claimed in claim 5,
wherein the photonic crystal for forming the cladding layer has
optical reflective characteristics in which the reflectance with
respect to a lightwave having a specific wavelength is larger than
the reflectance with respect to other lightwaves.
8. A photonic crystal multilayer substrate as claimed in claim 6,
wherein in the optical reflective characteristics of the
multilayered film, the reflectance with respect to a lightwave
introduced into an optical device formed in the multilayer
substrate is larger than the reflectance with respect to other
lightwaves.
9. A photonic crystal multilayer substrate as claimed in claim 7,
wherein in the optical reflective characteristics of the photonic
crystal, the reflectance with respect to a lightwave introduced
into an optical device formed in the multilayer substrate is larger
than the reflectance with respect to other lightwaves.
10. A photonic crystal multilayer substrate as claimed in any one
of claims 3 to 9, including: a base layer on which the portions
having the slab-waveguide type photonic crystal structure are
formed; optical devices formed in at least two layers among the
base layer and the photonic crystal layers; and a mechanism for
transmitting and receiving an optical signal between the optical
devices of each layer.
11. A photonic crystal multilayer substrate as claimed in claim 10,
wherein: an optical device is formed in one of the base layer and
the photonic crystal layers, and an optical waveguide is formed in
the circuit plane of the optical device; and the mechanism for
transmitting and receiving an optical signal between the optical
devices is a mode converter for converting a lightwave in a manner
such that a lightwave, which is transmitted along the optical
waveguide, is radiated in the direction significantly perpendicular
to the circuit plane, or a lightwave incident on the circuit plane
in the direction significantly perpendicular to the circuit plane
is transmitted along the optical waveguide.
12. A photonic crystal multilayer substrate as claimed in claim 11,
wherein the mode converter has an optical resonance mechanism,
formed at an end portion of the optical waveguide in the circuit
plane of the optical device, by which resonance for a lightwave
with a certain wavelength transmitted through the optical waveguide
occurs, and the lightwave transmitted through the optical waveguide
is radiated in the direction significantly perpendicular to the
circuit plane.
13. A photonic crystal multilayer substrate as claimed in claim 12,
wherein the optical resonance mechanism has a hole having a shape
and a size by which resonance with a lightwave transmitted through
the optical waveguide occurs, and a resonant portion which
surrounds the hole.
14. A photonic crystal multilayer substrate as claimed in claim 10,
wherein: an optical device is formed in one of the base layer and
the photonic crystal layers, and a first optical waveguide is
formed in the circuit plane of the optical device; a second optical
waveguide is formed in one of the remaining base layer and photonic
crystal layers and the second optical waveguide is close to an end
portion of the first optical waveguide; and the mechanism for
transmitting and receiving an optical signal between the optical
devices inputs a lightwave from the first optical waveguide to the
second optical waveguide, and is a mode converter for converting a
lightwave in a manner such that a lightwave, which is transmitted
along the first optical waveguide, leaks at the end portion of the
first optical waveguide, and the leaked lightwave is input into the
second optical waveguide.
15. A photonic crystal multilayer substrate as claimed in claim 14,
wherein the mode converter includes the end portion of the first
optical waveguide and the second optical waveguide, where the end
portion has a tapered shape in which the thickness gradually
decreases towards a head point.
16. A method of manufacturing a photonic crystal multilayer
substrate, comprising the steps of: forming a cladding layer on a
first substrate; forming a photonic crystal layer on the cladding
layer, where the photonic crystal layer is used for forming a
lightwave circuit; producing a first wafer by forming a lightwave
circuit in the photonic crystal layer, where the lightwave circuit
is assigned to a first layer of the photonic crystal multilayer
substrate; producing a second wafer by forming a cladding layer on
a second substrate and forming a photonic crystal layer on this
cladding layer; producing a composite wafer by putting the first
and second wafers together in a manner such that the photonic
crystal layer of the first wafer adheres with the cladding layer of
the second wafer; removing the substrate portion of the second
wafer from the composite wafer; forming a lightwave circuit in the
photonic crystal layer which is exposed after the removal of the
substrate portion of the second wafer, where this lightwave circuit
is assigned to a second layer of the photonic crystal multilayer
substrate; and forming a multilayered lightwave circuit in the
photonic crystal layers by repeating the above steps.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a photonic crystal
multilayer substrate, and in particular, to a structure of a micro
lightwave circuit using a photonic crystal, a relevant multilayered
structure, and a structure for realizing interlayered optical
wiring, and to manufacturing methods thereof.
[0003] 2. Description of the Related Art
[0004] Recently, photonic crystals functioning with two or three
dimensional periodic structures with respect to the effective
refractive index in the order of optical wavelengths have become
the focus of attention. The size of the existing lightwave circuits
(or optical circuits) may be reduced using such a photonic crystal
by three or more orders of magnitude; therefore, application to
micro lightwave circuits in optical communication or the like is
anticipated.
[0005] In a photonic crystal, a photonic band gap for prohibiting
the transmission of lightwaves with specific wavelength can be
generated. If a linear defect is introduced in a photonic crystal
having such a photonic band gap, lightwaves can be completely
confined in the linear defect, and additionally, this photonic
crystal can be used as an optical waveguide in which light is
transmitted along the linear defect.
[0006] Such a photonic crystal optical waveguide may include a
sharply bent portion, thereby improving the flexibility of the
design of the pattern of the relevant lightwave circuit, and
decreasing the size of the lightwave circuit.
[0007] In recent tests, various optical devices such as optical
waveguides and the like were formed in a photonic crystal, thereby
forming micro lightwave circuits.
[0008] However, the conventional photonic crystal lightwave
circuits have some problems.
[0009] First, in the concept of the conventional lightwave
circuits, the circuit is formed in a single plane. Therefore, even
if the flexibility of the circuit pattern can be improved by
employing sharply bent portions of optical wiring in the circuit by
using a photonic crystal, the possible degree of integration is
considerably limited.
[0010] Therefore, similarly to the multilayered structures in
electronic integrated circuits on Si substrates, multilayered
structures of optical wiring in lightwave circuits have been
examined so as to improve the degree of integration. However,
multilayered structures of optical wiring (i.e., optical
waveguides) could not be easily realized, in comparison with the
case of the multilayered structures in electronic integrated
circuits.
[0011] The reason for this is that in optical wiring (i.e., optical
waveguides), the mechanism for confining lightwaves in the optical
waveguide is not as powerful in comparison with the confinement of
electric current in electric wiring. Therefore, when two optical
waveguides are positioned close to each other, interference (i.e.,
crosstalk) is generated between them. In addition, optical
confinement of lightwaves is also insufficient at sharply bent
portions; thus, lightwave signals may leak at such bent portions of
the optical waveguide.
[0012] Furthermore, the scale of the lightwave circuit which can be
formed in a single layer obviously has a limit; therefore,
multilayered structures of the lightwave circuit have been strongly
required. However, no concrete multilayered structure applied to
the lightwave circuit and relevant manufacturing method have yet
been proposed.
SUMMARY OF THE INVENTION
[0013] In consideration of the above circumstances, an object of
the present invention is to solve the above-explained problems
relating to the conventional lightwave circuits and to provide a
lightwave circuit by which the degree of integration can be further
improved by employing a multilayered structure of wiring, similar
to that in electronic integrated circuits.
[0014] Therefore, the present invention provides a photonic crystal
multilayer substrate having portions, each portion having a
slab-waveguide type photonic crystal structure, multilayered in the
direction of the thickness of the substrate, wherein in the
slab-waveguide type photonic crystal structure:
[0015] a photonic crystal layer is disposed between cladding
layers;
[0016] the photonic crystal layer is made of a photonic crystal
having a two or three dimensional periodically modulated structure
with respect to the effective refractive index in the order of
optical wavelengths; and
[0017] each cladding layer is made of a material whose effective
refractive index differs from the effective refractive index of the
photonic crystal layer.
[0018] Preferably, the effective refractive index of the material
of each cladding layer is smaller than an effective refractive
index of the photonic crystal layer. For example, each cladding
layer is made of a one or two dimensional photonic crystal, so that
a slab-type optical waveguide can be formed, and such slab-type
optical waveguides can be multiply layered, thereby considerably
improving the degree of integration of optical integrated
circuits.
[0019] In the above structure, a multilayered lightwave circuit can
be formed by forming optical devices, such as optical waveguides,
optical coupling-splitting circuits, optical-wavelength filters,
light emitting elements, light receiving elements, or the like, in
the photonic crystal layers.
[0020] In the above structure, each cladding layer may have a
multilayered film in which two or more kinds of materials having
different effective refractive indexes are alternately layered.
[0021] In addition, each cladding layer may be made of a photonic
crystal having a two or three dimensional periodically modulated
structure with respect to the effective refractive index.
[0022] Preferably, the multilayered film or the photonic crystal
for forming the cladding layer has high optical reflectivity for
the wavelengths at which the lightwave circuit is operated.
[0023] The above structure may include a base layer on which the
portions having the slab-waveguide type photonic crystal structure
are formed;
[0024] optical devices formed in at least two layers among the base
layer and the photonic crystal layers; and
[0025] a mechanism for transmitting and receiving an optical signal
between the optical devices of each layer.
[0026] It is possible that:
[0027] an optical device is formed in one of the base layer and the
photonic crystal layers, and an optical waveguide is formed in the
circuit plane of the optical device; and
[0028] the mechanism for transmitting and receiving an optical
signal between the optical devices is a mode converter for
converting a lightwave in a manner such that a lightwave, which is
transmitted along the optical waveguide, is radiated in the
direction significantly perpendicular to the circuit plane, or a
lightwave incident on the circuit plane in the direction
significantly perpendicular to the circuit plane is transmitted
along the optical waveguide.
[0029] The mode converter may have an optical resonance mechanism,
formed at an end portion of the optical waveguide in the circuit
plane of the optical device, by which resonance for a lightwave
with a certain wavelength transmitted through the optical waveguide
occurs, and the lightwave transmitted through the optical waveguide
is radiated in the direction significantly perpendicular to the
circuit plane.
[0030] The optical resonance mechanism may have a hole having a
shape and a size by which resonance with a lightwave transmitted
through the optical waveguide occurs, and a resonant portion which
surrounds the hole.
[0031] It is also possible that:
[0032] an optical device is formed in one of the base layer and the
photonic crystal layers, and a first optical waveguide is formed in
the circuit plane of the optical device;
[0033] a second optical waveguide is formed in one of the remaining
base layer and photonic crystal layers and the second optical
waveguide is close to an end portion of the first optical
waveguide; and
[0034] the mechanism for transmitting and receiving an optical
signal between the optical devices inputs a lightwave from the
first optical waveguide to the second optical waveguide, and is a
mode converter for converting a lightwave in a manner such that a
lightwave, which is transmitted along the first optical waveguide,
leaks at the end portion of the first optical waveguide, and the
leaked lightwave is input into the second optical waveguide.
[0035] In this case, the mode converter may include the end portion
of the first optical waveguide and the second optical waveguide,
where the end portion has a tapered shape in which the thickness
gradually decreases towards a head point.
[0036] According to the mechanism for transmitting and receiving an
optical signal between the optical devices, optical signals can be
transmitted between different layers of the photonic crystal
multilayer substrate.
[0037] The present invention also provides a method of
manufacturing a photonic crystal multilayer substrate, comprising
the steps of:
[0038] forming a cladding layer on a first substrate;
[0039] forming a photonic crystal layer on the cladding layer,
where the photonic crystal layer is used for forming a lightwave
circuit;
[0040] producing a first wafer by forming a lightwave circuit in
the photonic crystal layer, where the lightwave circuit is assigned
to a first layer of the photonic crystal multilayer substrate;
[0041] producing a second wafer by forming a cladding layer on a
second substrate and forming a photonic crystal layer on this
cladding layer;
[0042] producing a composite wafer by putting the first and second
wafers together in a manner such that the photonic crystal layer of
the first wafer adheres with the cladding layer of the second
wafer;
[0043] removing the substrate portion of the second wafer from the
composite wafer;
[0044] forming a lightwave circuit in the photonic crystal layer
which is exposed after the removal of the substrate portion of the
second wafer, where this lightwave circuit is assigned to a second
layer of the photonic crystal multilayer substrate; and
[0045] forming a multilayered lightwave circuit in the photonic
crystal layers by repeating the above steps.
[0046] According to this method, a photonic crystal multilayer
substrate having a desired structure including multiple layers can
be manufactured.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a perspective view showing an embodiment of the
photonic crystal multilayer substrate according to the present
invention.
[0048] FIGS. 2A to 2D show cross-sectional structures for
explaining the process of manufacturing a multilayered optical
integrated circuit according to the present invention.
[0049] FIGS. 3A to 3C are diagrams showing the structure of a
photonic crystal multilayer substrate having cladding layers made
of a three-dimensional photonic crystal, as another embodiment
according to the present invention: FIG. 3A is a perspective view,
FIG. 3B is a partial sectional view showing an example of the
structure of the cladding layer made of a one-dimensional photonic
crystal, and FIG. 3C is a partial sectional view showing another
example of the structure of the cladding layer made of a
two-dimensional photonic crystal.
[0050] FIG. 4 is a perspective view showing an example of the
structure of a mode converter provided in a photonic crystal
multilayer substrate according to the present invention.
[0051] FIGS. 5A and 5B show an embodiment of the photonic crystal
multilayer substrate of the present invention, where the photonic
crystal multilayer substrate comprises a mode converter used
together with the VCSEL: FIG. 5A is a cross sectional view of a
portion of the photonic crystal multilayer substrate, while FIG. 5B
is a perspective view of the photonic crystal multilayer
substrate.
[0052] FIGS. 6A and 6B show an embodiment of the photonic crystal
multilayer substrate of the present invention, which comprises a
planar-type semiconductor laser and a mode converter: FIG. 6A is a
cross sectional view of a portion of the photonic crystal
multilayer substrate, while FIG. 6B is a perspective view of the
photonic crystal multilayer substrate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0053] Hereinafter, embodiments according to the present invention
will be explained in detail with reference to the drawings in order
to clearly show the above and other objects, distinctive features,
and effects of the present invention.
[0054] Below, the structure and manufacturing method of a
slab-waveguide type photonic crystal multilayer substrate as an
embodiment of the present invention will be explained. In addition,
a method of forming a multilayered lightwave circuit or a
multilayered optical wiring in the multilayer substrate will also
be explained.
[0055] FIG. 1 is a perspective view showing the slab-waveguide type
photonic crystal multilayer substrate of the present embodiment.
FIG. 1 shows a two-layered photonic crystal multilayer substrate as
an example; however, the present invention can similarly be applied
to a structure with three or more layers.
[0056] In the photonic crystal multilayer substrate of FIG. 1, the
following layers are formed in turn from the bottom: (i) a first
SiO.sub.2 cladding layer 2a on a substrate 1 made of Si, (ii) a
first photonic crystal layer 3, made of Si, in which a lightwave
circuit for the first layer is formed, (iii) a second SiO.sub.2
cladding layer 2b, and (iv) a second photonic crystal layer 4, made
of Si, in which a lightwave circuit for the second layer is
formed.
[0057] The photonic crystal layer 3 including a lightwave circuit
in FIG. 1 has a slab waveguide structure in the direction of its
thickness. Due to the structure, lightwaves can be confined in the
photonic crystal layer 3 between the cladding layers 2a and 2b,
thereby suppressing interference (i.e., crosstalk) between the
lightwave circuits of each photonic crystal layer.
[0058] In addition, in the photonic crystal multilayer substrate,
the slab waveguide structure whose core is the second photonic
crystal layer 4 and the slab waveguide structure whose core is the
first photonic crystal layer 3, these slab waveguides being
adjacent to each other, use a common cladding layer, that is, the
second SiO.sub.2 cladding layer.
[0059] In order to form a slab waveguide structure, preferably, the
effective refractive indexes of the cladding layers 2a and 2b are
smaller than those of the photonic crystal layers 3 and 4.
[0060] That is, preferably, the effective refractive index of the
material of which the cladding layers 2a and 2b are made (i.e., the
refractive index of SiO.sub.2:1.5), is smaller than the effective
refractive index of the photonic crystal layers 3 and 4 which
function as cores (i.e., almost the volume average of the
refractive indexes of Si and air).
[0061] Below, the method of manufacturing the photonic crystal
multilayer substrate in the present embodiment (as shown in FIG. 1)
will be explained with reference to FIGS. 2A to 2D. More
specifically, FIGS. 2A to 2D show cross-sectional structures for
explaining the process of manufacturing a multilayered optical
integrated circuit in the present embodiment.
[0062] First, as shown in FIG. 2A, an SOI (silicon on insulator)
wafer is made in which an SiO.sub.2 cladding layer 6 having a
thickness of approximately 2 .mu.m is formed on an Si substrate 5
(corresponding to the first substrate of the present invention),
and an Si layer having a thickness of approximately 1 .mu.m is
further formed on the SiO.sub.2 cladding layer 6.
[0063] Thereafter, a photonic crystal layer 7 is formed by
providing holes which are regularly and periodically arranged on
the above-described Si layer, and a lightwave circuit for the first
layer is formed in the photonic crystal layer 7, so that the first
wafer is manufactured.
[0064] In order to form the above-described lightwave circuit, the
following methods may be used: (i) in the first method, when the
top Si layer is processed to form the photonic crystal layer 7, a
circuit pattern is formed, in advance, in a mask used for
crystal-pattern processing, and (ii) in the second method, a
uniform photonic crystal layer 7 is first formed, and then a
circuit pattern is formed using an etching or embedding method, or
the like.
[0065] In the next step, as shown in FIG. 2B, the second wafer is
manufactured, which includes (i) an SiO.sub.2 layer 9 having a
thickness of approximately 0.2 .mu.m formed on an Si substrate 8
(corresponding to the second substrate of the present invention),
and (ii) an Si layer 10 having a thickness of approximately 1 .mu.m
formed thereon, and (iii) an SiO.sub.2 layer 11 having a thickness
of approximately 2 .mu.m formed thereon.
[0066] The composite wafer is formed by putting the second wafer on
the first wafer (that is, both wafers are adhered to each other).
Here, the photonic crystal layer 7 of the first wafer and the
SiO.sub.2 layer 11 of the second wafer are made to face each
other.
[0067] The adherent condition between the first and second wafers
can be realized using an already-established technique such as
thermocompression bonding.
[0068] In the next step, the Si substrate 8 and the SiO.sub.2 layer
9 having a thickness of approximately 0.2 .mu.m, both belonging to
the composite wafer, are removed by using the etching, as shown in
FIG. 2C, so as to expose the Si layer 10.
[0069] In the next step, as shown in FIG. 2D, the exposed Si layer
10 is processed so as to form a photonic crystal layer 12 by using
the above-explained method (of forming a photonic crystal layer
(7)), so that a lightwave circuit for the second layer is formed in
the photonic crystal layer 12. The lightwave circuit can also be
formed by using the above-explained method (of forming a lightwave
circuit).
[0070] By repeating the process from the step as shown by FIG. 2B
to the step as shown by FIG. 2D, lightwave circuits can be multiply
layered on an Si substrate.
[0071] Here, the optical interference between adjacent lightwave
circuits can be suppressed to a certain degree because in the
present slab waveguide structure, a photonic crystal layer is
positioned between cladding layers. However, in order to completely
suppress optical interference, each cladding layer between the
circuits is preferably made of a photonic crystal having a photonic
band gap.
[0072] FIGS. 3A to 3C are diagrams showing the structure of a
photonic crystal multilayer substrate as another embodiment
according to the present invention. FIG. 3A is a perspective view
showing the photonic crystal multilayer substrate of the present
embodiment, FIG. 3B is a partial sectional view showing an example
of the structure of the cladding layer, and FIG. 3C is a partial
sectional view showing another example of the structure of the
cladding layer.
[0073] As shown in FIG. 3A, the photonic crystal multilayer
substrate of the present embodiment includes the following layers
formed in turn on a substrate: (i) a cladding layer 14a, (ii) a
photonic crystal layer 15a, (iii) a cladding layer 14b, (iv) a
photonic crystal layer 15b, (v) a cladding layer 14c, (vi) a
photonic crystal layer 15c, and (vii) a cladding layer 14d. In
addition, the photonic crystal multilayer substrate of the present
embodiment is formed using the above-explained method of
manufacturing a photonic crystal multilayer substrate.
[0074] In the present embodiment, preferably, the photonic crystal
for forming the cladding layers 14a to 14d has a three-dimensional
photonic crystal structure having a complete band gap in the
wavelength range of lightwaves introduced into the relevant
lightwave circuit. However, a one-dimensional photonic crystal 16
(i.e., a multilayered film) as shown in FIG. 3B, or a
two-dimensional photonic crystal 17 as shown in FIG. 3C may be
used.
[0075] If the one-dimensional photonic crystal (i.e., multilayer
film) 16 is used, the thickness of the film is determined under the
condition that the condition of the Bragg reflection is satisfied
with respect to light which is incident on the multilayer film in
the direction significantly perpendicular to the film.
[0076] Below, each expression "perpendicular to" basically means
"significantly perpendicular to".
[0077] In the above-explained embodiments, Si and SiO.sub.2 are
used as materials deposited on an Si substrate, that is, Si and
SiO.sub.2 are respectively used for forming a photonic crystal
layer and a cladding layer. However, a similar structure can be
applied when a layer made of an AlGaAs material is formed on a GaAs
substrate, or when a layer made of an InGaAsP material is formed on
an InP substrate.
[0078] Next, a method for realizing transmission of an optical
signal between lightwave circuits formed in each photonic crystal
layer of a photonic crystal multilayer substrate will be explained
with reference to FIGS. 4 to 6B.
[0079] According to the present invention, a multilayered photonic
crystal substrate can be realized by employing the structure as
shown in FIG. 1 and by using the manufacturing method explained
referring to FIGS. 2A to 2D. In addition, a structure having a
mechanism for transmitting (and receiving) an optical signal
between lightwave circuits of each photonic crystal layer, or a
structure having a mechanism for transmitting (and receiving) an
optical signal between layers in which lightwave circuits are
multilayered is possible.
[0080] FIG. 4 shows an example of the structure of a mode converter
for performing optical signal transmission or optical connection
between layers. FIG. 4 is a perspective view showing an enlarged
structure of the mode converter formed in the photonic crystal
layer 3 of the photonic crystal multilayer substrate in FIG. 1.
[0081] The photonic crystal layer 3 between the cladding layers 2a
and 2b in the mode converter shown in FIG. 4 has (i) a hole 19a
having a shape and a size by which resonance occurs for the
wavelength of a lightwave (i.e., optical signal) 18a transmitted
through an optical waveguide 18, and (ii) a resonant portion
(no-hole portion) 19 which surrounds the hole 19a. This resonant
portion 19 is connected to an end of an optical waveguide 18a in
the same plane.
[0082] According to the converting function of the mode converter,
a lightwave 18 transmitted along the optical waveguide 18 in the
circuit plane of the photonic crystal layer 3 is output in the
direction perpendicular to the circuit plane, or a lightwave
incident on the circuit plane in the direction perpendicular to the
circuit plane is transmitted along the optical waveguide 18 in the
circuit plane.
[0083] In the present mode converter, the resonance wavelength can
be changed depending on the size and shape of the hole 19a provided
at the center of the resonant portion 19. Therefore, the size and
shape of the hole 19a are determined according to the wavelength of
the lightwave 18a introduced into the optical waveguide 18, thereby
realizing the optical signal transmission (and reception) suitable
for the employed lightwave 18a.
[0084] In addition, in optical wiring formed in each of different
photonic crystal layers of a photonic crystal multilayer substrate,
if two mode converters, as explained above, are provided at
positions between which optical signal transmission (i.e., optical
connection) is desired, in a manner such that the mode converters
face each other, then the optical signal transmission between
layers is realized.
[0085] The transmission (and reception) of an optical signal is
performed according to the function (or operation) explained
below.
[0086] For example, a lightwave transmitted along the optical
waveguide in the circuit plane of the lightwave circuit formed in
the first photonic crystal layer is output in the direction
perpendicular to the circuit plane by using the first mode
converter, so as to obtain the converted lightwave which is
incident on the second photonic crystal layer in the direction
perpendicular to the layer.
[0087] In the next step, the above lightwave incident on the second
photonic crystal layer in the direction perpendicular to the
circuit plane of the second photonic crystal layer is introduced
into the optical waveguide of the second photonic crystal layer, by
using the second mode converter which faces the first mode
converter.
[0088] In this way, the lightwave transmitted in the circuit plane
of the first photonic crystal layer can be guided into the circuit
plane of the second photonic crystal layer.
[0089] The mode converters which operate as explained above can be
used accompanied with a vertical cavity surface emitting laser
(VCSEL). FIGS. 5A and 5B show an embodiment of the photonic crystal
multilayer substrate of the present invention, where the photonic
crystal multilayer substrate comprises a mode converter used
together with the VCSEL. FIG. 5A is a cross sectional view of a
portion of the photonic crystal multilayer substrate, while FIG. 5B
is a perspective view of the photonic crystal multilayer
substrate.
[0090] In FIG. 5A, the photonic crystal multilayer substrate of the
present embodiment has the following layers formed in turn on an
InP device-forming layer 21 (corresponding to the base layer of the
present invention) which includes a VCSEL 20: (i) an SiO.sub.2
cladding layer 22a, (ii) a photonic crystal layer 23 in which a
lightwave circuit including a mode converter 24 is formed, and
(iii) an SiO.sub.2 cladding layer 22b.
[0091] In the photonic crystal multilayer substrate as shown in
FIGS. 5A and 5B, light emitted from the VCSEL 20 is incident on the
mode converter 24 in the direction perpendicular to the circuit
plane, and the mode converter 24 operates so as to introduce the
incident lightwave into the optical waveguide 18 formed in the
circuit plane.
[0092] Instead of the VCSEL 20, a plane-input type photodetector
may be provided in the substrate 21. In this case, according to the
mode converter 24, a lightwave transmitted along the optical
waveguide 18 is output in the direction perpendicular to the
relevant plane and can also be introduced into the plane-input type
photodetector.
[0093] In addition, the mode converter may have a structure for
introducing light emitted from a planar-type semiconductor laser or
a light emitting element into an optical waveguide in the relevant
circuit, or for introducing light output from an optical waveguide
into a device such as a planar-type light detecting element or the
like.
[0094] Below, the above-described structure and function will be
explained in detail, with reference to FIGS. 6A and 6B. FIGS. 6A
and 6B show an embodiment of the photonic crystal multilayer
substrate of the present invention, which comprises a mechanism for
introducing a lightwave from a planar-type optical device into an
optical waveguide in the circuit plane of a photonic crystal layer.
FIG. 6A is a cross sectional view of a portion of the photonic
crystal multilayer substrate, while FIG. 6B is a perspective view
of the photonic crystal multilayer substrate.
[0095] The photonic crystal multilayer substrate of the present
embodiment as shown in FIGS. 6A and 6B has the following layers
formed in turn on an InP device-forming layer 26 (corresponding to
the base layer of the present invention) which includes a
planar-type semiconductor laser 25: (i) an SiO.sub.2 cladding layer
27a, (ii) a photonic crystal layer 28 in which a mode converter 29
and a lightwave circuit are formed, and (iii) an SiO.sub.2 cladding
layer 27b.
[0096] Here, the mode converter 29 is positioned in the photonic
crystal layer 28 in a manner such that the mode converter 29 faces
the head portion 25a of the planar-type semiconductor laser 25.
[0097] In the photonic crystal multilayer substrate as shown in
FIGS. 6A and 6B, portions of two waveguides are optically coupled
with each other, thereby transmitting and receiving lightwaves
between different layers.
[0098] The head portion 25a of the planar-type semiconductor laser
25 shown in FIG. 6A has a tapered optical waveguide whose thickness
gradually decreases towards the head point. Such a tapered optical
waveguide has characteristics in which lightwaves leak out and are
emitted from the waveguide. Therefore, a lightwave is emitted from
the planar-type semiconductor laser 25 in a direction almost
perpendicular to the taper face 25b of the head portion 25a.
[0099] Here, in the photonic crystal layer 28, an optical waveguide
is formed in a manner such that the optical waveguide faces the
taper face 25b of the head portion 25a. Therefore, the leaked light
from the planar-type semiconductor laser 25 is introduced into the
optical waveguide and is converted into a lightwave transmitted in
the circuit plane of the photonic crystal layer 28.
[0100] According to the above-explained function, light output from
the planar-type semiconductor laser can be input into the optical
waveguide.
[0101] The present invention is not limited to the above-explained
embodiments, and any modification is possible within the scope and
spirit of the present invention.
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