U.S. patent application number 09/916286 was filed with the patent office on 2002-10-31 for optoelectronic waveguiding device and optical modules.
Invention is credited to Sato, Hiroshi, Shinoda, Kazunori, Taike, Akira, Tsuji, Shinji.
Application Number | 20020158266 09/916286 |
Document ID | / |
Family ID | 18977743 |
Filed Date | 2002-10-31 |
United States Patent
Application |
20020158266 |
Kind Code |
A1 |
Sato, Hiroshi ; et
al. |
October 31, 2002 |
Optoelectronic waveguiding device and optical modules
Abstract
An optical transmission device suitable for a high-speed and
large-capacity optical transmission system. An optoelectronic
waveguiding device including an optical waveguide layer and
cladding layers each having a larger band gap than that of the
optical waveguide are deposited above and beneath the optical
waveguide layer formed on a semiconductor substrate. The waveguide
and cladding layers comprise optical waveguides each having a MQW
structure in a direction of a light propagation axis of the optical
waveguide layer. Among these optical waveguides, there exists first
and second optical waveguides, whose layer structures may be
mutually different. The optoelectronic waveguiding device maybe
characterized in that an optical waveguide made of a bulk crystal
exists in a connection part between the MQW structure waveguides
each having a different layer structure. The specific connected
optoelectronic waveguiding device elements may include
semiconductor lasers, modulators and/or amplifiers.
Inventors: |
Sato, Hiroshi; (Kokubunji,
JP) ; Taike, Akira; (Kokubunji, JP) ; Shinoda,
Kazunori; (Tokorozawa, JP) ; Tsuji, Shinji;
(Hidaka, JP) |
Correspondence
Address: |
REED SMITH HAZEL& THOMAS LLP
3110 Fairview Park Drive
Suite 1400
Falls Church
VA
22042
US
|
Family ID: |
18977743 |
Appl. No.: |
09/916286 |
Filed: |
July 30, 2001 |
Current U.S.
Class: |
257/98 ;
385/131 |
Current CPC
Class: |
G02B 6/4274 20130101;
G02B 6/12007 20130101; G02B 6/4215 20130101; G02B 6/1228 20130101;
G02B 6/4265 20130101; H01S 5/0265 20130101; G02B 2006/12078
20130101; H01S 5/12 20130101 |
Class at
Publication: |
257/98 ;
385/131 |
International
Class: |
H01L 033/00; G02B
006/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2001 |
JP |
2001-129178 |
Claims
What is claimed is:
1. An optoelectronic waveguiding device, comprising: at least two
optoelectronic device elements, each having an optical waveguide;
and an optical waveguide made of a bulk crystal, wherein each of
said optical waveguides in said at least two optoelectronic device
elements are connected to each other with said optical waveguide
made of a bulk crystal.
2. An optoelectronic waveguiding device according to claim 1,
wherein at least two of the optoelectronic device elements are
built into a single semiconductor substrate.
3. An optoelectronic waveguiding device according to claim 2,
wherein at least a first of said at least two optoelectronic device
elements is a semiconductor laser part and at least a second of
said at least two optoelectronic device elements is an optical
modulator part.
4. An optoelectronic waveguiding device according to claim 3,
further comprising: a plurality of pairs, each pair comprising one
of said semiconductor laser parts and one of said optical modulator
parts, each of said plurality of pairs formed on a single
substrate.
5. An optoelectronic waveguiding device according to claim 1,
wherein the optical waveguides of said at least two optoelectronic
device elements each include a multiple quantum well (MQW)
structure existing in a direction of a light propagation axis
thereof.
6. An optoelectronic waveguiding device according to claim 5,
wherein said at least two optoelectronic device elements are
further comprised of: cladding layers each placed above or beneath
the optical waveguides of said at least two optoelectronic device
elements, wherein said cladding layers are comprised of a material
having a refractive index lower than that of the optical waveguides
of said at least two optoelectronic device elements.
7. An optoelectronic waveguiding device according to claim 6,
wherein said optical waveguide made of a bulk crystal comprises a
semiconductor bulk crystal having a refractive index higher than
that of said cladding layers.
8. An optoelectronic waveguiding device according to claim 5,
wherein said at least two optoelectronic device elements including
an MQW structure have mutually different functions.
9. An optoelectronic waveguiding device according to claim 8,
wherein said two optoelectronic waveguiding device elements having
mutually different functions are a semiconductor laser part and an
optical modulator part.
10. An optoelectronic waveguiding device according to claim 5,
wherein said at least two optoelectronic device elements each
having an MQW structure include a semiconductor laser part, an
optical modulator part, and a semiconductor optical amplifier part;
wherein at least either the layer structures or constituent
materials of the MQW structures are mutually different, and further
wherein said optical waveguide made of a bulk crystal exists at
connection points between said optical modulator part and said
semiconductor laser part and between said optical modulator part
and said semiconductor optical amplifier part.
11. An optoelectronic waveguiding device according to claim 9,
wherein the MQW layer of the laser part has a different thickness
than the MQW layer of the modulator part, further wherein said bulk
crystal optical waveguide is tapered to interconnect the different
thicknesses.
12. An optoelectronic waveguiding device according to claim 5,
wherein said at least two optoelectronic device elements including
an MQW structure include a plurality of laser parts and a modulator
part, said device further comprising: an optical multiplexer
including a, MQW structure; wherein said optical waveguide made of
a bulk crystal is adapted to connect said plurality of laser parts
to said modulator part through said multiplexer, further wherein
light emitted from one of the semiconductor laser parts is
multiplexed by said optical multiplexer and made to enter said
optical modulator,
13. An optoelectronic waveguiding device according to claim 1,
wherein said at least two optical waveguides of the optoelectronic
device elements are a MQW structure made of an InGaAlAs system
material and a MQW structure made of an InGaAsP system material,
and said bulk crystal optical waveguide is made of a bulk crystal
that is selected from a group consisting of InGaAsP system,
InGaAlAs system, and InAlAs system materials.
14. An optoelectronic waveguiding device according to claim 3,
wherein a layered structure that constitutes said optical modulator
part comprises a MQW structure made of an InGaAlAs system material,
a layered structure that constitutes said semiconductor laser part
comprises a MQW structure made of an InGaAsP system material, and
said bulk crystal optical waveguide is made of a bulk crystal that
is selected from a group consisting of InGaAsP system, InGaAlAs
system, and InAlAs system materials.
15. An optoelectronic waveguiding device according to claim 3,
wherein both said optical modulator part and said semiconductor
laser part have an MQW structure made of an InGaAsP system
material; wherein one of a thickness of the quantum well, a
thickness of the barrier layer, and the number of cycles of the
quantum wells which constitute the MQW structure of said optical
modulator part is different from a counterpart of the MQW structure
of the semiconductor laser part; and further wherein the optical
waveguide made of a bulk crystal that is selected from a group
consisting of InGaAsP system, InGaAlAs system, and InAlAs system
materials exists at a connection part between said optical
modulator part and said semiconductor laser part.
16. An optoelectronic waveguiding device according to claim 3,
wherein both said optical modulator part and said semiconductor
laser part have an MQW structure made of an InGaAlAs system
material; wherein one of a thickness of the quantum well, a
thickness of the barrier layer, and the number of cycles of the
quantum wells which constitute the MQW structure of said optical
modulator part is different from a counterpart of the MQW structure
of the semiconductor laser part; and further wherein the optical
waveguide made of a bulk crystal that is selected from a group
consisting of InGaAsP system, InGaAlAs system, and InAlAs system
materials exists at a connection part between said optical
modulator part and said semiconductor laser part.
17. An optical module, comprising: an optical fiber; and a
semiconductor optoelectronic waveguiding device comprising a
plurality of device element structures each having at least a
semiconductor laser part and an optical modulator part arranged
side by side in a direction parallel to a light propagation axis in
said semiconductor optoelectronic waveguiding device, wherein said
device element structures arranged side by side are capable of
emitting light having the same wavelength or mutually different
wavelengths, both said optical modulator part and said
semiconductor laser part of each device element structure having
MQW structures, wherein layer structures or constituent materials
of these MQW structures of each device element structure are
mutually different, and further wherein an optical waveguide made
of a bulk crystal exists at a connection part of said optical
modulator part and said semiconductor laser part of each device
element structure.
18. An optical module according to claim 17, wherein said plurality
of device element structures includes a plurality of semiconductor
laser parts and an optical modulator part, said device further
comprising: an optical multiplexer including an optical waveguide
capable of multiplexing light emitted from one of the semiconductor
laser parts arranged side by side and subsequently transmitting the
light to said optical modulator part; wherein said semiconductor
optoelectronic waveguiding device is capable of emitting single
longitudinal mode light, wherein said semiconductor laser parts and
said optical modulator parts being formed on the same semiconductor
substrate, and further wherein the optical waveguide made of a bulk
crystal exists at least in a portion of the optical waveguide in
the optical multiplexer.
19. A method for manufacturing an optoelectronic waveguiding
device, comprising the steps of: forming a first optoelectronic
device element on a semiconductor substrate; applying a first
resist layer to said first optoelectronic device element; etching
said first optoelectronic device element; forming a second
optoelectronic device element in said etched area on said
semiconductor substrate; applying a second resist layer; etching
said second resist layer to remove a crystal defect formed between
said first and second optoelectronic elements; and forming a
waveguide from a bulk crystal in said etched crystal defect area,
wherein said waveguide from a bulk crystal optically connects said
first and second optoelectronic elements.
20. The method of claim 19, wherein said first and second
optoelectronic elements include MQW structures.
Description
PRIORITY TO FOREIGN APPLICATIONS
[0001] This application claims priority to Japanese Patent
Application No. P2001-129178.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to integrated semiconductor
optical devices, and more specifically, the present invention
relates to semiconductor optoelectronic waveguiding devices
including an optical waveguide comprising a plurality of multiple
quantum wells and to optical modules utilizing such devices.
[0004] 2. Description of the Background
[0005] With the recent increase in the use of information and
communication services, optical communication systems that support
such services with higher speeds and increased capacity are
desired. For example, optical transmission apparatuses for optical
communication of a trunk line in which a plurality of communication
lines are bundled should support high-speed (10 Gbit/s class) and
long-distance transmission. Consequently, a semiconductor laser
capable of operating at a transmission speed as high as at least 10
Gbit/s is preferred as the light source to be built into such
optical transmission apparatuses.
[0006] A promising light source that enables high-speed,
long-distance transmission whose speed is equal to at least 10
Gbit/s is a laser device manufactured by integrating an EA (electro
absorption) type optical modulator and a DFB (distributed feedback)
semiconductor laser on a single substrate ("EA/DFB laser"). Because
the operating principles and availability of the EA/DFB laser are
known, a detailed explanation is omitted and only the basic
structure, advantages, and potential disadvantages thereof will now
be described.
[0007] In the EA/DFB laser, areas of the DFB laser and of the EA
modulator are monolithically formed on the same semiconductor
substrate. In general, a multiple quantum well (MQW) structure of
the laser and that of the modulator are formed to have different
materials, compositions, layer thicknesses, and other properties so
that an energy gap of the MQW layer of the modulator part is larger
than that of the MQW layer of the laser part. Typical methods
whereby the MQW structures having mutually different energy gaps
are formed on the same semiconductor substrate include (1) a
selective area growth method and (2) a butt-joint formation
method.
[0008] Each of these methods has known advantages and
disadvantages. In order to make full use of the EA/DFB laser in
high-speed optical communication systems (such as a transmission
system whose transmission speed is equal to at least 10 Gbit/s), it
is preferable to design both the EA modulator and the DFB laser to
have an optimal structure independently so that each of these can
exploit its own full potential.
[0009] In the case where the EA/DFB laser structure is formed by
the selective area growth method, the modulator and the laser may
be formed by single crystal growth; this method has the advantage
of a simple manufacturing process. However, the materials, the
compositions, and the total numbers of layers of the laser part and
of the modulator are inevitably the same, and the method has little
or no room for independent optimization of these device
elements.
[0010] On the other hand, using the butt-joint formation method,
the laser part and the modulator can be formed by independent
processes. Therefore, the modulator and the laser may be optimized
independently in terms of their materials, compositions, layer
thicknesses, the numbers of layers, and other properties. For 10
Gbit/s-and-higher high-speed and long-distance transmission, where
both the modulator and the laser part preferably each achieve high
end characteristics respectively, integration of the modulator and
the laser by the butt-joint method appears promising.
SUMMARY OF THE INVENTION
[0011] When the butt-joint method is adopted, a laser having the
optical modulator integral therewith is generally formed through
the following processes. (1) First, a device element structure of
the DFB laser is formed on a semiconductor substrate. (2) The DFB
laser area is protected with a dielectric film such as silicon
oxide or silicon nitride. (3) Next, using the above-mentioned
dielectric film as a mask, the MQW layer in the DFB laser area is
selectively etched away to expose the semiconductor substrate. (4)
On the exposed semiconductor substrate, a MQW structure that is
desired for use as the EA modulator and that will act as an
absorption layer ("EA-MQW") is re-grown.
[0012] In the growth process of the EA-MQW, because a feed rate of
growth species exceeds the normal feed rate (by a factor ranging
from zero to a few tens of .mu.m) in the vicinity of the dielectric
mask covering the DFB laser area, the well layers and barrier
layers of the MQW layer become thicker and the absorption edge of
the MQW layer moves toward a longer wavelength. Additionally, it is
known that the crystal quality of the EA-MQW layer in this area
decreases. Details of this phenomenon have been reported, for
example, in the conference proceeding of IEEE lasers and
electro-optics society, 9th annual meeting, WY2, p. 189.
[0013] In the above-mentioned example, a crystal defect whose
absorption edge has moved to a longer wavelength and whose crystal
quality has deteriorated easily absorbs light that propagates from
the DFB laser part to the EA modulator. Therefore, problems may be
caused such as (a) a reduction in the optical output of the device
and (b) the generation of unnecessary carriers in the EA modulator
area. Similar problems were referred to, for example, in Japanese
Patent Application Laid-Open No. 8335745. These potential problems
are preferably addressed when the butt-joint formation is used.
[0014] In at least one preferred embodiment, the present invention
provides a semiconductor optoelectronic waveguiding device with as
small an optical loss as possible in the optical waveguide and that
can respond to high-speed modulation that is desired for high-speed
transmission. The present invention may also provide a
semiconductor optoelectronic waveguiding device that is
manufactured by integrating device elements of the optical
modulator and the semiconductor laser, each of which is optimized
in terms of several characteristics thereof.
[0015] In at least preferred embodiment, the basic structure of the
present invention comprises an optoelectronic waveguiding device
having at least two optoelectronic device element parts each
comprising an optical waveguide. The optical waveguides possessed
by the above-mentioned at least two optoelectronic device element
parts are preferably connected to each other with an optical
waveguide such that at least a core portion thereof is made of a
bulk crystal. In this case, it may be convenient to form the
above-mentioned at least two optoelectronic device element parts on
a single substrate, for example, a semiconductor substrate.
[0016] Examples of the above-mentioned at least two optoelectronic
device element parts may include: a semiconductor laser part; an
optical modulator part; and an optical amplifier part. Further,
optical device elements other than these enumerated device elements
may be used as the need arises. Moreover, in accordance with
particular embodiments of the optoelectronic waveguiding device of
the present invention, a configuration in which a plurality of
pairs each having two of these optical device elements are arranged
side by side may be adopted.
[0017] To achieve the maximum performance of the EA/DFB laser, the
present invention preferably comprises a structure such that the
crystal defect between the DFB laser and the EA modulator is
removed and a waveguide whose optical absorption is extremely low
is inserted into this area. Consequently, the basic structure
according to at least one embodiment of the present invention
includes an optical waveguide of a bulk crystal formed at a
connection part between the MQW of the laser part and the MQW of
the modulator part. The form according to the present invention may
be a structure whose quantum effect is extremely small by using the
above-mentioned bulk crystal area as the optical waveguide.
[0018] The MQW of the above-mentioned laser part to be connected to
the MQW of the above-mentioned modulator part is preferably formed
by a first butt-joint formation. The bulk crystal waveguide is then
formed by a second butt-joint formation so as to have a desirable
refractive index distribution.
[0019] It should be noted that it is preferable to establish an
optical connection between the optical modulator and the laser part
with the optical waveguide in the present invention.
[0020] Thus, by adopting the optical waveguide made of a
newly-formed bulk crystal, the present invention preferably
realizes the one or more of the following: (1) the optical
absorption can be reduced as low as possible by suppressing the
shifting of the absorption edge that may occur in the MQW structure
and (2) a further decrease in the optical absorption and
improvement of the reliability of the device may be achieved by not
providing a crystal defect of poor crystallinity in the device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] For the present invention to be clearly understood and
readily practiced, the present invention will be described in
conjunction with the following figures, wherein like reference
characters designate the same or similar elements, which figures
are incorporated into and constitute a part of the specification,
wherein:
[0022] FIG. 1 is a perspective view of a device according to a
first embodiment of the present invention;
[0023] FIG. 2 is a cross section of a device according to the first
embodiment of the present invention, taken along a plane parallel
to the travelling direction of light therein;
[0024] FIG. 3 is a cross section of a device according to the first
embodiment of the present invention, taken along a plane
intersecting the travelling direction of light therein;
[0025] FIG. 4 shows cross sections of a device according to the
first embodiment of the present invention, illustrating a
manufacturing process in a processing order;
[0026] FIG. 5 is a characteristic diagram illustrating the shift of
the absorption edge in a crystal defect of poor crystallinity;
[0027] FIG. 6 is a diagram in which cumulative distributions of
slope efficiency for the present invention and for the conventional
example are shown for comparison;
[0028] FIG. 7 is a perspective view of a device according to a
second embodiment of the present invention;
[0029] FIG. 8 is a cross section of a device according to the
second embodiment of the present invention, taken along a plane
parallel to the travelling direction of light therein;
[0030] FIG. 9 is a cross section of a device according to the
second embodiment of the present invention, taken along a plane
intersecting the travelling direction of light therein;
[0031] FIG. 10 shows cross sections of a device according to the
second embodiment of the present invention, illustrating the
manufacturing process in a processing order;
[0032] FIG. 11 is a cross section of a device according to a third
embodiment of the present invention, taken along a plane parallel
to the travelling direction of light therein;
[0033] FIG. 12 is a view showing an example of a mask pattern to be
used to form a bulk crystal waveguide of a device according to the
third embodiment of the present invention shown in FIG. 11;
[0034] FIG. 13 is a cross section of a device according to a fourth
embodiment of the present invention, taken along a plane parallel
to the travelling direction of light therein;
[0035] FIG. 14 is a view showing an example of a mask pattern to be
used to form a bulk crystal waveguide of a device according to the
fourth embodiment of the present invention shown in FIG. 13;
[0036] FIG. 15 is a top view of an embodiment that has an array of
light emitting device element parts;
[0037] FIG. 16 is a cross section of an embodiment that has an
array of the light emitting device element parts, taken along a
plane parallel to the travelling direction of light therein;
[0038] FIG. 17 is a top view of an embodiment that has an array of
light emitting device element parts and an optical multiplexer;
[0039] FIG. 18 is a cross section of an embodiment that has an
array of light emitting device element parts and an optical
multiplexer, taken along a plane parallel to the travelling
direction of light therein;
[0040] FIG. 19 is a top view showing an optical module that uses
the optoelectronic waveguiding device according to the present
invention;
[0041] FIG. 20 illustrates the relation between radiation loss in
the device and the distance between the laser part and the optical
modulator;
[0042] FIG. 21 is a cross section showing a basic concept of the
present invention; and
[0043] FIG. 22 is a cross section showing a comparative example
where the same single crystal is used to bury a gap between the
laser part and the optical modulator as well as the laser part and
the modulator.
DETAILED DESCRIPTION OF THE INVENTION
[0044] It is to be understood that the figures and descriptions of
the present invention have been simplified to illustrate elements
that are relevant for a clear understanding of the present
invention, while eliminating, for purposes of clarity, other
elements that may be well known. Those of ordinary skill in the art
will recognize that other elements are desirable and/or required in
order to implement the present invention. However, because such
elements are well known in the art, and because they do not
facilitate a better understanding of the present invention, a
discussion of such elements is not provided herein. The detailed
description will be provided hereinbelow with reference to the
attached drawings.
[0045] Before the description of a concrete manner in which the
present invention may be carried out, the effects that characterize
the basic structure of the invention and a comparison between the
structure of the optoelectronic waveguiding device according to the
invention and that of the conventional optoelectronic waveguiding
device will be further described in detail.
[0046] In preferred embodiments of the present invention, the
above-mentioned crystal defect is removed, and hence, the
absorption loss of light in the device is significantly reduced.
Because the bulk crystal waveguide has no MQW structure, there may
occur little shift of the bad-gap energy (absorption edge) in the
vicinity of the mask pattern edge in this butt-joint formation
process. Therefore, the absorption arising from the bulk crystal
waveguide that is introduced into the structure is decreased
significantly as compared to that of the original MQW
structure.
[0047] By replacing the crystal in the crystal defect with a
newly-grown crystal, the present invention may be differentiated
from the methods in the above-mentioned Japanese Patent Application
Laid-Open No. 8-335745. Note that, in the above-mentioned Japanese
Patent Application Laid-Open No. 8-335745, the absorption edge of
the crystal defect is shifted toward a shorter wavelength by ion
implantation to achieve reduction in the absorption loss. In this
case, although it is possible that the absorption edge is shifted,
the crystal defect itself cannot be removed.
[0048] Contrary to this, in the present invention, because the
crystal defect is preferably replaced with the optical waveguide of
the bulk crystal, not only can the absorption edge be shifted from
the absorption edge of the crystal having the crystal defect to a
desired absorption edge, but also the crystal defect itself may be
removed. By the removal of the crystal defect in the optical
waveguide, additional effects such as an improvement in reliability
of the device may also be expected. That is, the service life of an
apparatus into which the present device is built may be improved.
Further, according to the invention, it may also be possible to
decrease the optical absorption in the optical waveguide as
compared to the above-mentioned prior art.
[0049] Alternatively, if it is desired only to remove the
above-mentioned crystal defect, it is also possible that the
crystal defect is removed and subsequently the area is buried with
the same material as the cladding layer.
[0050] Actually, a structure such that a gap between the DFB laser
and the EA modulator is buried with InP can be found, for example,
in the Japanese Patent Application Laid-Open No. 7-193210. However,
in this example, since the optical waveguide is broken between the
DFB laser and the EA modulator, the light suffers radiation loss at
the gap. The magnitude of the radiation loss is shown in FIG.
20.
[0051] FIG. 20 is a diagram showing a relation between the
magnitude of the radiation loss and the distance between the
optical modulator and the laser part. The horizontal axis denotes
the distance between the optical modulator and the laser part; the
vertical axis denotes the radiation loss in the device. In FIG. 20,
the case where InP exists between the modulator and the laser part
is marked with "buried with InP" and the case of the present
invention is marked with "present invention." In the case of
"buried with InP," it is shown that the radiation loss increases
exponentially with an increase in distance between the modulator
and the laser. When the EA modulator and the laser part are
separated by at least a few tens of .mu.m, there occurs a radiation
loss equal to at least approximately 1 dB, and the loss may become
a large obstacle to an increase of the optical output from the
device.
[0052] Contrary to this, with a structure such that an area where
the crystal defect was removed is buried with a bulk crystal whose
refractive index is preferable for guiding light, as is the case of
the present invention, little optical radiation loss is generated
between the laser part and the modulator. A schematic cross section
of such a structure is shown in FIG. 21. FIG. 21 details a DFB
laser part 501, tan optical modulator part 502, and an area of the
optical waveguide of the bulk crystal formed by new crystal growth
503. As the laser part 501, a first optical confinement layer 514,
an active layer 515, a second optical confinement layer 516, and a
grating 517 are preferably formed on a semiconductor substrate 500.
For the optical modulator part 502, a first optical confinement
layer 511, an optical waveguide part 512, and a second optical
confinement layer 513 are preferably formed. Thereafter, a second
cladding layer 518 is formed over these two parts 501, 502.
[0053] FIG. 22 is a cross section showing an example where an area
of the crystal defect that existed between the optical modulator
502 and the laser part 501 is buried with a second cladding layer
518 by crystal growth. In this example, in a similar manner to the
above-mentioned example of FIG. 21, the laser part includes a first
light confinement layer 514, an active layer area 515, a second
optical confinement layer 516, and a grating 517 are formed on the
semiconductor substrate 500. The optical modulator includes a first
optical confinement layer 511, an optical waveguide part 512, and a
second optical confinement layer 513 formed on the substrate 500.
Further, a second cladding layer 518 is again formed over these
parts 501, 502. It should be noted in this example that a gap
between the optical modulator 502 and the laser part 501 is buried
with the second cladding layer 518.
[0054] Here, FIG. 21 and FIG. 22 are schematic, conceptual
diagrams, and there may be many structural variants that can be
carried out in manufacturing actual devices according to these
diagrams.
[0055] According to the present invention, the loss may be reduced
by at least an order of magnitude as compared to the structure
illustrated in FIG. 22 where the gap between the modulator and the
laser is buried with InP, and a structure suitable to increase the
optical output of a device may be provided. This improvement may be
more fully understood with respect to the comparison of
characteristics shown in FIG. 20 and described above.
[0056] Therefore, it is preferred in the present invention to
establish an optical connection between the optical modulator and
the laser part with the optical waveguide of a bulk crystal. If the
optical modulator and the laser part, which are placed with a gap,
are connected using the same material as the cladding layer, one or
more of the above desired effects may not be obtainable.
[0057] The present invention may be effective for semiconductor
optoelectronic waveguiding devices, especially for semiconductor
optoelectronic waveguiding devices that use compound semiconductor
materials. Typical examples of semiconductor materials to be used
for said semiconductor optoelectronic waveguiding devices include
InGaAlAs, InGaAsP, and similar compounds. The MQW structure may be
composed of one of these enumerated materials. In this case, one
material selected from a group consisting of InGaAsP, InGaAlAs, and
InAlAs is preferable for the bulk part for connecting these MQW
structures. Generally speaking, InP is used as the substrate for
various device applications.
[0058] Further, this invention may be effective for optoelectronic
waveguiding devices that use compound semiconductor materials
containing N. In other words, the optoelectronic waveguiding device
may use an InGaNAs system compound semiconductor material as a
material for the MQW structure formed on a GaAs substrate.
[0059] First Exemplary Embodiment
[0060] A first exemplary embodiment of the present invention as
applied to a semiconductor laser device into which a 1.55 .mu.m
band EA modulator is integrated ("EA modulator integrated
semiconductor laser device") will now be described. The feedback of
light in the present laser may be achieved by distributed feedback
(DFB).
[0061] FIG. 1 is a perspective view of a first exemplary embodiment
according to the present invention, and FIG. 2 is a cross section
of the optical waveguide part taken along a plane parallel to the
travelling direction of light therein. FIG. 3 is a cross section of
the device taken along a plane intersecting the travelling
direction of light therein, and FIG. 4 illustrates cross sections
of the device during various stages of the manufacturing
process.
[0062] As shown in FIG. 1, the device is comprised of two device
elements, a laser part 26 and a modulator 27. A laser electrode 23
and a modulator electrode 24 were formed independently. Between the
laser part 26 and the modulator 27, a groove 21 is preferably
formed for electrically isolating both device elements 26, 27. An
optical waveguide part 31 of the device is formed as a stripe shape
with a buried hetero (BH) structure as is generally known in the
art. In this example, the sides of the stripe optical waveguide in
the buried hetero structure are preferably buried with an Fe-doped
high-resistance InP 32 (see FIG. 3).
[0063] The cross section of a layered structure in the present
embodiment is shown in FIG. 2. In order to optimize the device
characteristics, each of the laser part 26 and the EA modulator
part 27 is of an optimal structure independently. Therefore, each
element has a different layered structure. However, the substrate
is preferably common to both device elements and is a compound
semiconductor substrate made of, for example, n-type InP
[0064] The laser part 26 preferably comprises an n-type InGaAsP
optical confinement layer 2, a strained MQW layer 3, and a p-type
InGaAsP optical confinement layer 4. The MQW layer, that will act
as the active layer region, may consist of seven pairs (cycles) of
a 6 nm thick well layer and a 10 nm thick barrier layer which are
deposited sequentially, with the intention of achieving sufficient
characteristics as a laser. A grating layer 5 made of an InGaAsP
material exists on top of these layers. The active layer region and
a structure of the grating layer may be formed so that the emission
wavelength of the DFB laser at room temperature (25 degree C.) is
approximately 1550 nm.
[0065] The optical confinement layers provided to sandwich the MQW
layer are layers for enhancing the optical confinement of the
above-mentioned MQW layer. The optical waveguiding function is
effected by sandwiching the core region with cladding layers whose
refractive indexes are lower than that of the core region. That is,
the optical waveguiding function is achieved by a layered structure
of cladding layer/MQW layer/cladding layer.
[0066] More concretely, the optical confinement layers sandwiching
the MQW layer are provided to further enhance the optical
confinement in the MQW layer. Therefore, the refractive indexes of
the cladding layers are set to be lower than the refractive indexes
of the above-mentioned optical confinement layers. Thus, the
optical waveguiding function is preferably enhanced by the layered
structure of the cladding layer/optical confinement layer/MQW
layer/optical confinement layer/cladding layer. A form may also be
adopted in which the cladding layer of the substrate side is the
substrate itself. It may also be possible to provide the cladding
layer of the substrate side independently on the semiconductor
substrate.
[0067] The polarity of the grating layer 5 may be either n type or
p type. In the case of p type polarity, the DFB laser preferably
becomes a refractive index coupling type where only the refractive
index varies periodically in the propagation direction of light.
Alternatively, the n-type grating polarity gives rise to a gain
coupling type DFB laser. The reason being that the grating
functions as a periodically varying current-blocking layer, as is
well known in the art. Therefore, not only the refractive index but
also the gain in the active layer suffers periodic changes. In the
present embodiment, a grating that was uniformly formed over the
whole area of the DFB laser is described. However, there may also
be provided a so-called "phase-shifted grating" where a period
(phase) of the grating is shifted in a partial area thereof with
respect to the remainder.
[0068] On the other hand, the EA modulator area 27 is preferably
comprised of an n-type InGaAsP optical confinement layer 11, an
undoped optical absorption layer 12, and an undoped InGaAsP optical
confinement layer 13. For improved performance of the EA modulator,
the optical absorption layer 12 may be formed of an InGaAsP system
strained multiple quantum well (strained MQW) structure. The
thickness of a quantum well is preferably set at approximately 7
nm, and the thickness of a barrier layer is set at approximately 5
nm. These layers may be deposited for 10 cycles. T barrier layer in
the modulator part is preferably thinner than its counterpart in
the laser part because the drift of the carriers is facilitated in
the absorption layer to improve the modulation characteristics.
[0069] To remove the crystal defect that develops in the connection
part between the laser and the modulator, an InGaAsP optical
waveguide layer may be formed in this area by butt-joint formation.
An exemplary method of forming the waveguides of the modulator part
and the butt-joint waveguide layer will now be described with
reference to FIG. 4.
[0070] initially, to form the laser structure, an n-type InGaAsP
optical confinement layer 2, a strained MQW layer 3, and a p-type
InGaAsP optical confinement layer 4 are preferably deposited on an
n-type InP substrate 1.
[0071] On top of these layers, a layered structure including the
grating layer 5 made of an InGaAsP system material is thereafter
formed (FIG. 4A).
[0072] Silicon nitride ("SiN") is coated on the semiconductor wafer
having the above layered structure and is shaped to be a protection
mask 51 on the laser part. Using this SiN mask 51 that covers the
laser part area, a grating layer 5 and the active layer region in
the other area are etched away, as shown in FIG. 4B. The etching
proceeds through the n-type InGaAsP layer 2 and is selectively
stopped at the n-type InP substrate 1. The etching process may be
selected from: a dry etching process such as reactive ion etching
(RIE); a selective wet etching process using a solution whose main
constituent is phosphoric acid or sulfuric acid; and a combination
of both of these processes.
[0073] On the n-type InP substrate 1 that was exposed by the
above-mentioned selective etching, an absorption layer area of the
EA modulator is formed by a first butt-joint process (FIG. 4C). The
absorption layer area is preferably formed by sequentially
depositing an n-type InGaAsP optical confinement layer 11, a MQW
optical absorption layer 12, a p-type optical confinement layer 13,
and a p-type InP cladding layer 15.
[0074] The strained-MQW layer 12 may be comprised of ten cycles of
a 7 nm thick quantum well layer and a 5 nm thick barrier layer. The
absorption edge thereof is designed to be approximately 1490 nm. To
achieve improved modulation characteristics, crystal compositions
of the quantum well and of the barrier well are preferably chosen
so that the former is given with compression strain whereas the
latter is given with tensile strain.
[0075] During this first butt-joint process, when the MQW layer of
the EA part is formed, a crystal defect 17 with poor crystallinity
is also formed in the vicinity of the protection mask 51 in the
laser part. In the crystal defect area 17, crystallinity is bad and
the absorption edge of the MQW has shifted to the longer wavelength
side.
[0076] FIG. 5 shows an example of a shift amount of the absorption
edge of the EA part when the EA part is grown with the butt-joint
method. The horizontal axis denotes the distance from the edge of
the protection mask in the laser part, and the vertical axis
denotes the shift amount of the absorption edge of the EA modulator
part. In this example, the shift of the absorption edge is caused
in the area adjacent to the protection mask in the laser part
within the range of at least approximately 100 .mu.m from a mask
edge, and the wavelength shift of the absorption edge is larger as
the distance from the protection mask in the laser part decreases.
At the closest proximity to the mask, the absorption edge has moved
to the longer wavelength side by as much as about 30 nm.
[0077] In order to remove this crystal defect 17, a mask 52 that
has an opening only in the vicinity of the crystal defect (FIG. 4D)
is formed, and the crystal defect is removed for a length of about
50 .mu.m. Also in this process, in a similar manner to the first
butt-joint process, the etching is selectively stopped at the
surface of the n-type InP substrate 1. Subsequent to this etching,
an optical waveguide layer 14 preferably made of undoped InGaAsP
and an undoped InP layer 18 are deposited thereon (FIG. 4E).
[0078] After the optical waveguide structure is formed according to
the above-mentioned procedure, the p-type InP cladding layer 15 and
a p-type InGaAs layer 16 may be formed. An exemplary process for
growing these crystals uses a metal-organic chemical vapor
deposition (MOCVD) method. Further, the p-type InGaAs layer 16 may
be formed to obtain an ohmic contact when the electrode is to be
formed. An exemplary process for growing the above-mentioned
crystal uses a metal organic vapor phase epitaxy (MOVPE) method. It
should be noted in the cross section of FIG. 3 that numerals for
the p-type InP cladding layer 15 and the p-type InGaAs layer 16 on
the layer 13 are omitted.
[0079] Following the above-mentioned crystal growth process, a
buried hetero (BH) structure is preferably formed by a process of
forming a mesa stripe through a normal dry etching process and a
further process of regrowing a burying layer by the MOVPE method.
Here, the BH structure is a structure where both sides of an
optical waveguide, as viewed from the travelling direction of
light, are buried with a material capable of confining the light.
The confining material normally is characterized by a high
resistance. For the burying layer in this example, a
high-resistance InP 32 was used and Fe was doped therein to
increase its resistivity.
[0080] FIG. 3 is a cross section of the BH laser part taken along a
plane intersecting the travelling direction of light therein. FIG.
3 may facilitate a better understanding of the BH structure.
Following this process, the wafer surface is insulated with silicon
oxide ("SiO.sub.2") 22 and the p-side electrode 24 and an nside
electrode 25 are then formed. Moreover, on a front facet and on a
rear facet of the device, a low-reflection coating 40 and a
high-reflection coating 41 are formed, respectively (see e.g., FIG.
2).
[0081] To aid in the understanding of the effect on a device
according to the present invention, a distribution of optical
output efficiency per unit current (slope efficiency) obtained by
evaluating the optical output versus current characteristics of
exemplary devices is shown in FIG. 6. The horizontal axis of FIG. 6
denotes the slope efficiency, and the vertical axis denotes the
cumulative distribution for the measured samples. FIG. 6 also shows
the cumulative distribution of slope efficiency for devices that
the present invention was not applied to (conventional device) and
hence have crystal defects at the connection parts between the
laser part and the modulator.
[0082] In FIG. 6, a group of solid squares designates samples
according to the present invention, and a group of open circles
designates samples of conventional devices. About 95 percent of the
conventional devices are distributed below a slope efficiency of
approximately 0.15 W/A. By contrast, less than about 25% of the
devices according to the present invention have a slope efficiency
of 0.15 W/A or less. Further, focusing on the center value of the
distribution corresponding to 50 percent of the probability
distribution, the devices according to the present invention
improved the center value (almost equivalent to the average value)
of the slope efficiency by a factor of about 1.3 (2 dB) over the
conventional devices. One reason for this changes is that because
the devices of the present invention have no crystal defect in the
optical waveguides, the absorption loss in the waveguide is reduced
to an extremely small value as compared to the conventional
butt-joint method.
[0083] Second Exemplary Embodiment
[0084] FIG. 7 though FIG. 9 show devices according to a second
exemplary embodiment of the present invention. FIG. 7 is a
perspective view of a device of the second embodiment, and FIG. 8
is a cross section of the waveguide part of the device taken along
a plane parallel to the travelling direction of light therein. FIG.
9 is a cross section of the device taken along a plane intersecting
the travelling direction of light therein, and FIG. 10 shows cross
sections illustrating an exemplary manufacturing process of the
device.
[0085] This embodiment is preferably characterized in that the EA
modulator part 27 is comprised of an InGaAlAs system MQW. The laser
part 26 is formed with an InGaAsP system MQW in a similar manner to
the first exemplary embodiment. There is a groove 21 that separates
these MQW structures. As best seen in the perspective view of FIG.
7, the present exemplary embodiment has a basic configuration
similar to that described with respect to the first embodiment. A
detailed explanation for the similar features in the figures is
omitted.
[0086] It is known that an EA modulator that uses an InGaAlAs
system material has improved modulation efficiency and may achieve
a larger extinction ratio than one that uses an InGaAsP system MQW.
The extinction ratio is a parameter which shows the ratio of the
light outputs for ON and for OFF of the optical signal. In general,
larger the extinction ratio are preferred for the transmission of
light signals. Therefore, for an optical transmission system that
requires a larger extinction ratio, the laser having the EA
modulator made of the InGaAlAs system MQW integral therewith, as
described in this embodiment, may be suitable. Hereafter, an
exemplary manufacturing process for the laser will be described
with reference to FIG. 10.
[0087] The growth temperature of the InGaAlAs system material to be
used for the EA modulator part in this embodiment is higher than
the growth temperature of the InGaAsP system material to be used
for the laser part. Therefore, if the MQW of the laser part is
formed first, that MQW will be exposed to an elevated temperature
above the growth temperature of the laser. When the laser part
undergoes such a thermal hysteresis, minute defects in the crystals
that constitute the MQW of the laser part move easily to easily
effectuate a degradation of the crystals, and these defects cause a
deterioration of the device characteristics. To circumvent this
potential problem, the InGaAlAs-system MQW that requires the high
temperature is preferably grown first in the present embodiment of
the invention.
[0088] The optical absorption layer preferably comprises: an n-type
InGaAlAs optical confinement layer 112; an MQW layer 113; and an
undoped InGaAlAs optical confinement layer 114 formed on an n-type
InP substrate 111. The MQW layer 113 may be manufactured by
depositing a quantum well layer (thickness approximately 7 nm) and
a barrier layer (thickness approximately 5 nm), both of which
consist of the InGaAlAs system material, for ten cycles. On this
optical absorption layer, a cladding layer of a p-type InP 115 is
grown to a thickness of approximately 0.2 .mu.m.
[0089] The semiconductor wafer that has this layered structure is
clad with SiN, which is formed to a mask 116 that serves to protect
the modulator part (FIG. 10A). Using this SiN mask 116, the p-InP
cladding layer and the optical absorption layer are etched away
(FIG. 10B). The etching is allowed to proceed through the n-type
InGaAlAs 112 and is selectively stopped at the n-type InP substrate
111. The etching may be either of: dry etching, for example,
reactive ion etching (RIE); selective wet etching that uses a
solution whose main constituent is phosphoric acid or sulfuric
acid; or a combination of both of these processes.
[0090] On the n-InP substrate 111 that has been exposed by the
etching, an InGaAsP system MQW structure that will act as a laser
is formed by a first butt-joint process. The MQW structure is
manufactured, in a similar manner to the first embodiment, by
depositing an n-type InGaAsP optical confinement layer 121, an
undoped active layer 122, an undoped InGaAsP optical confinement
layer 123, and additional layers thereon. Then, a grating layer 124
made of an InGaAsP system material and a p-type InP cladding layer
125 are preferably formed (FIG. 10C).
[0091] Next, a crystal defect 126 with poor crystallinity that was
formed at the connection part between the modulator and the laser
is removed using a SiN mask 130 (FIG. 10D). In the removal process,
in a similar manner to the first butt-joint process, dry etching
such as the reactive ion etching (RIE) and selective wet etching
using a solution whose main constituent is phosphoric acid or
sulfuric acid are preferably used together. It should be noted that
the etching is selectively stopped at the n-type InP substrate 111.
After the InP substrate 111 is exposed, an undoped bulk InGaAsP
waveguide layer 131 and an undoped InP 132 are deposited thereon as
a second butt-joint process (FIG. 10E).
[0092] After the two butt-joint processes described above, a
grating structure 141 is formed in the laser area by a conventional
method (FIG. 10F) on which a p-type InP cladding layer 151 and a
p-type InGaAs layer 152 are deposited (FIG. 10G).
[0093] Thereafter, a laser structure is constructed through the
same process as the first embodiment. As details are the same as
described with respect to the first embodiment, further explanation
is omitted. FIG. 9 is a cross section of the device taken along a
plane intersecting the travelling direction of light therein. In
FIG. 9, reference numerals for the p-type InP cladding layer 151
and for the p-type InGaAs layer 152 on the layer 114 are
omitted.
[0094] Exemplary optoelectronic waveguiding devices according to
this embodiment are evaluated for optical output versus current
characteristics at an operating temperature of 25 degree C. The
results of the tests show that an average threshold current was
approximately 8 mA, and the average slope efficiency was 0.17 W/A.
An example of the cumulative distribution of slope efficiency was
shown in FIG. 6. From FIG. 6, it can be understood that the present
invention preferably improves the slope efficiency as compared to
conventional devices. Further, exemplary optoelectronic waveguiding
devices according to this embodiment are tested in an optical
transmission experiment using a single mode optical fiber of a
length of 40-80 km, and sufficient transmission characteristics are
achieved.
[0095] Moreover, in an acceleration test for the reliability, an
estimated life exceeding 200,000 hours is successfully secured.
[0096] Third Exemplary Embodiment
[0097] As a third exemplary embodiment of the present invention, as
applied to a 1.55 .mu.m band EA modulator integrated distributed
feedback semiconductor laser, examples in which the bulk crystal
waveguide provided at the connection part between the EA modulator
and the laser has a mode-conversion function are shown in FIG. 11
and FIG. 13. These address the potential problem of mode
mismatching between the waveguides in cases where the EA modulator
part and the laser part have different waveguide layer thicknesses,
and therefore, the guided mode size of light in the respective
areas differ from each other.
[0098] The exemplary embodiment illustrated in FIG. 11 is an
example in which the length of the modulator is shortened with the
intention of improving the operating frequency band thereof. That
is, by shortening the modulator length of the modulator, the
electric capacitance can be reduced, and the modulation bandwidth
of the device can be improved. In this example, the thicknesses of
several layers that constitute the optical modulator are set to be
larger than those of the corresponding layers of the laser part.
FIG. 11 is a cross section of the device taken along a plane
parallel to the travelling direction of light therein, and FIG. 12
is a top view showing locations and widths of patterns of the
protection mask that are used to manufacture respective parts in
this example.
[0099] In an example shown in FIG. 13, the thicknesses of layers
that constitute the optical modulator are set to be smaller than
those of the corresponding layers of the laser part. FIG. 13 is a
cross section of the device taken along a plane parallel to the
travelling direction of light therein, and FIG. 14 is a top view
showing locations and widths of patterns of a protection mask that
are used to manufacture respective parts of this example.
[0100] First, the example of FIG. 11 will be described. In this
structure, to achieve a sufficient extinction ratio with a short
modulator, the number of the quantum well layers of the EA
modulation part was increased, and the optical confinement
coefficient in the absorption layer was made larger. Accordingly,
the thickness of a waveguide 204 in an area near the EA modulator
part is larger than that in an area near the laser part. Also in
this embodiment, the modulator part is preferably comprised of the
MQW structure of an InGaAlAs system material to realize desired
modulation characteristics of the modulator. An exemplary
manufacturing process for this embodiment will now be
described.
[0101] According to the same procedure that is executed in the
second embodiment, an optical absorption layer comprises an n-type
InGaAlAs optical confinement layer 201, an MQW layer 202, and an
undoped InGaAlAs optical confinement layer 203 formed on an n-type
InP substrate 111. The quantum well layer acting as a light
absorbing layer is repeatedly deposited to approximately 14 cycles
which is larger than that of the second embodiment. Accordingly,
the absorption layer is thicker than that of the second embodiment
by approximately 50 nm.
[0102] On this optical absorption layer, a cladding layer of p-type
InP is grown to a thickness of about 0.2 .mu.m. On the
semiconductor wafer having this layered structure, a SiN mask for
protecting the modulator part is formed using the same technique as
the second embodiment. Using this SiN mask, the p-InP cladding
layer and the optical absorption layer are etched away. The etching
is allowed to proceed through the n-type InGaAlAs 201 and is
selectively stopped at the n-type InP substrate 111. The etching
may be either of: dry etching, for example, reactive ion etching
(RIE); selective wet etching that uses a solution whose main
constituent is phosphoric acid or sulfuric acid; or a combination
of these processes.
[0103] Using the above procedure, the surface of the n-type InP
substrate 111 is exposed, and an active layer region of the laser
is formed on this surface. The structure of the active layer region
is the same as in the first and second embodiments. The n-type
InGaAsP optical confinement layer 2, the active layer 3, the
undoped InGaAsP optical confinement layer 4 are formed, On these
layers, the grating layer 5 made of an InGaAsP system material and
a p-type InP cladding layer are formed.
[0104] Subsequently, in a similar manner to the second embodiment,
the crystal defect that was formed at the connection part between
the optical modulator and the laser part is replaced with a bulk
InGaAsP waveguide 204 by a second butt-joint process. In this
embodiment, the mask pattern to be used in this second butt-joint
process is such that the width of a mask stripe pattern on the
optical modulator side is set to be larger than that on the laser
side. By the selective use of stripe pattern widths in the process
of bulk crystal growth of a compound semiconductor material, the
bulk semiconductor layers can be tailored to be thicker near the
modulator than near the laser. Thus, as illustrated in FIG. 11, the
waveguide made of the bulk compound semiconductor can be
manufactured with a tapered structure.
[0105] Also in the process of removing the MQW structure using the
above-mentioned mask, in a similar manner to the first butt-joint
process, it is recommended that the dry etching such as the
reactive ion etching (RIE) and the selective wet etching that uses
a solution whose main constituent is phosphoric acid or sulfuric
acid are used together. The etching is selectively stopped at the
n-type InP substrate 111. After the n-InP substrate 111 is exposed,
the undoped bulk InGaAsP waveguide layer 112 and an undoped InP are
preferably deposited as a second butt-joint process.
[0106] As described above, in the second butt-joint growth process,
if the width of a mask 211 on the EA modulator side is set to be
larger than the width of a mask 212 for protecting the laser part,
the well-known effect of the selective area growth occurs. That is,
since the quantity of the growth species supplied becomes large
near the EA modulator side, the crystal that is grown there in the
second butt-joint process becomes thicker than that near the laser
side. Accordingly, the InGaAsP waveguide layer 204 tends to be
thicker closer to the modulator side than at a point near the laser
side.
[0107] As a result, the thickness of the waveguide can be varied in
a taper manner so that the thickness of the waveguide near the
laser side equals the thickness of the waveguide of the laser and
the waveguide near the EA modulator side equals the thickness of
the absorption layer. Thus, any discontinuity of the waveguide
thickness may be eliminated both at the connection part between the
laser part and the InGaAsP bulk crystal waveguide and at the
connection part between the optical modulator and the InGaAsP bulk
crystal waveguide. Therefore, the propagation loss of light between
the waveguides is decreased.
[0108] After the second butt-joint process is completed according
to the above-mentioned procedure, a grating structure is formed in
the laser area by a conventional technique, and a p-type InP
cladding layer 205 and a p-type InGaAs layer 206 are deposited
thereon. Subsequently, the laser structure is formed through the
same process that is described with respect to the first
embodiment. Previously described details are again omitted.
[0109] Moreover, the above-mentioned description is for the case
where the waveguide thickness of the EA modulator portion is larger
than that of the laser part, but the present invention can be
applied to a converse case (shown in FIG. 13) where the waveguide
thickness of the EA modulator part is smaller than that of the
laser part. In that case, an optical waveguide 251 of a bulk
crystal that is provided between the laser part and the modulator
is such that its thickness at the connection part to the laser part
is thick and becomes thinner nearer to the modulator.
[0110] In the case where the optical waveguide 251 with such a
taper shape is formed, a mask pattern as shown in FIG. 14 may be
used in the second butt-joint process. That is, the width of a SiN
mask 261 (stripe pattern) for protecting the laser side is to be
larger than the width of a SiN mask 262 (stripe pattern) for
protecting the EA modulator part. The reason that the taper
waveguide is formed with such a mask pattern is the same as
description of FIG. 12.
[0111] Note that in the embodiments shown in FIG. 11 and in FIG.
13, the device manufacturing process is basically the same as the
first embodiment excluding alteration of the mask pattern.
[0112] Fourth Exemplary Embodiment
[0113] FIG. 15 and FIG. 16 are examples of the present invention as
applied to side-by-side arrangements of the device elements
described in the first and/or second embodiments to compose an
array of optoelectronic device elements. FIG. 15 is a top view of
the device according to this embodiment, and FIG. 16 is a cross
section of the device taken along a plane parallel to the
travelling direction of light therein. Note that each figure shows
only characteristic portions of the device, and portions common to
the device according to the present invention (such as the
electrodes) are omitted for clarity.
[0114] A device with a plurality of modulator-integrated lasers
arranged side by side, as in this example, provides a plurality of
optical signals to be used with a plurality of transmission
channels. Therefore the present invention can provide light sources
suitable to a further high-capacity optical transmission system
such that the transmission capacity is proportional to a product of
the transmission speed and the number of channels.
[0115] FIG. 15 shows a top view of the device with four pieces of
DFB lasers each acting as a light emitting part arranged side by
side, from a first channel laser 301 to a fourth channel laser 304
mounted on its surface. Further, four modulators, from a first
channel modulator 311 to a fourth channel modulator 314, are also
formed in such a way that each modulator corresponds to one of the
DFB lasers. FIG. 16 shows a cross-sectional structure of the
optical waveguide of the side-by-side arranged channel device. Each
of the four channels has a laser area 321 and a modulator area 322,
and a bulk crystal waveguide 323 is formed between these two areas.
The formation process for these waveguide structures is preferably
the same as that described in the first embodiment.
[0116] In this case, there can be expected a decrease in power
consumption of the device and of an optical module with the device
incorporated therein. This expectation stems from the fact (already
discussed in the first exemplary embodiment) that the application
of the present invention increases the optical output of each
channel by about 2 dB. This increase in output means that a lower
amount of electric power is necessary to obtain the same quantity
of optical output as a conventional device. In the optoelectronic
waveguiding device in which a plurality of device elements are
arranged side by side as shown in FIG. 15, power consumption of the
entire device is a product of power consumption of each device
element and the number of channels. Therefore, the decrease in
power consumption for the entire device is a product of the
decrease in power consumption for each channel and the number of
channels. That is, the device has a greater decrease in power
consumption then a single channel device.
[0117] Fifth Exemplary Embodiment
[0118] Moreover, an embodiment shown in FIG. 17 is an example of
the device such that light emitted from one of a plurality of
lasers arranged side by side is multiplexed to a single optical
waveguide by a multiplexer and then modulated by a single optical
modulator. FIG. 17 includes a laser part 451, an optical modulator
part 452, and an optical multiplexer part 411, all of which are
connected to one another with optical waveguides. Four lasers for
four channels, from a first channel laser 401 to a fourth channel
laser 404, are arranged side by side. Light generated by any of the
four channel lasers is preferably made to enter a single optical
modulator 412 through the optical multiplexer 411, and is modulated
thereby.
[0119] With a device as depicted in FIG. 17, it is possible that a
desired wavelength (i.e., one of four wavelengths) is obtained from
a single device. In this embodiment, emission wavelengths of the
four channel lasers, from the first channel laser 401 to the fourth
channel laser 404, are preferably set at different wavelengths with
a spacing of approximately 2 nm. Specifically, a device with lasers
at frequencies of 1550 nm, 1552 nm, 1554 nm, and 1556 nm,
respectively was formed. Adoption of such a side-by-side structure
makes it possible to obtain light having a desired wavelength by
injecting current to a channel having the desired wavelength
according to need. Note that in this embodiment the number of
channels and wavelength separation between the channels are set at
4 and 2 nm, respectively, but by setting the number of channels and
wavelength separation according to need, the number of selectable
wavelengths and a wavelength region can be set in an almost
unlimited variety of desired values.
[0120] FIG. 18 is an irregular section of the optoelectronic
waveguiding device taken along a non-straight line that passes each
optical waveguide layers. In order that the laser part 451 and the
EA modulator part 452 possess their optimal characteristics,
respectively, both device elements preferably have MQW structures.
As details of their layered structures are the same as in the first
embodiment, further explanation is omitted.
[0121] Between the lasers and the EA modulator, namely at a
position where the multiplexer is provided, an optical waveguide
layer 453 is formed of a bulk crystal having a refractive index
distribution desirable as the optical waveguide according to the
present invention. In the case where a multiplexer is provided, as
in the present embodiment, because the length of the whole device
becomes longer, it may be important to limit the optical absorption
per unit length in the path. If the light is absorbed in the
optical waveguide, optical output emitted from the device becomes
small. As a result, after the device is built into an optical
transmission apparatus, there may occur a failure that results in
deterioration of S/N characteristics of the transmission apparatus.
The optical multiplexer itself may be of a conventional structure,
and many types of device elements may achieve a sufficient
effect.
[0122] To reduce optical loss in an optical waveguide, the
structure according to the present invention where a bulk crystal
waveguide is provided between the laser part and the EA modulator
is preferably suitable. As already described, since the bulk
crystal waveguide has an absorption edge that is far away from the
guided wave wavelength as compared to the MQW waveguide, it is
possible to control the waveguide absorption per unit length to be
low value. Further, although in the present exemplary embodiment
the number of channels is set at 4, if the present invention is
applied to a device whose number of channels is other than four,
the effect of reducing the loss when the light passes through the
multiplexer is the same.
[0123] A conceptual top view of a mounting form including an
optoelectronic waveguiding device according to this embodiment is
shown mounted on an optical module in FIG. 19. An optoelectronic
waveguiding device 603 is placed on a mount board 602 together with
an optical lens 604, and the prepared mount board 602 is then
placed in a package 605 and a fiber 601 is connected thereto.
Current feed to the device may be accomplished from an electrode
pad through a wire in a similar manner to conventional techniques.
In FIG. 19, there is shown a feed pad 611 for EA modulator; an
n-electrode pad 612 for semiconductor laser; p-electrode feed pads
621, 622, 623, and 624 for DFB laser; and a feed wire 631.
[0124] As described in detail in the foregoing, according to the
present invention, a modulator-integrated laser that emits large
optical output and enables high-speed optical communication can
preferably be provided. With the use of the optical module and/or
optical transmission apparatus into which the device according to
the present invention was built, a high-speed optical transmission
system capable of operating at low power consumption can be
provided.
[0125] Some additional exemplary forms according to the present
invention will now be enumerated and briefly described.
[0126] (1) An optoelectronic waveguiding device in which a layered
structure consisting of an optical waveguide layer having a
refractive index desirable for light guiding and cladding layers
each of which is made of a material having a refractive index lower
than that of the optical waveguide layer that exist above or
beneath the optical waveguide is formed on a semiconductor
substrate. The device is characterized in that (a) a plurality of
optical waveguide layers each having the MQW structure exist in a
direction of a light propagation axis of the optical waveguide
layer each have layer structures or constituent materials that are
mutually different, and (b) an optical waveguide made of a
semiconductor bulk crystal that bears a small quantum effect and
has a refractive index higher than those of the cladding layers
exists at the connection part between these MQW structure
waveguides.
[0127] (2) An optoelectronic waveguiding device formed by
depositing a first MQW structure on a semiconductor substrate,
removing a part of the first MQW layer and subsequently forming a
second MQW structure in that area. Further, an optical waveguide
layer made of a semiconductor bulk crystal material that bears a
small quantum effect and has a refractive index higher than those
of the cladding layers is deposited thereon.
[0128] (3) An optoelectronic waveguiding device, characterized in
that a plurality of optoelectronic waveguiding device elements each
having a different function are formed in a direction of the light
propagation axis on the same semiconductor substrate. In these
optoelectronic waveguiding device elements, cladding layers are
each made of a material having a refractive index lower than that
of the optical waveguide layer being deposited above and beneath
the optical waveguide layer, and some or all of the optoelectronic
waveguiding device elements have the MQW structures. At the
connection part between the optoelectronic waveguiding device
elements, an optical waveguide exists that is formed by depositing
a semiconductor bulk crystal bearing an extremely small quantum
effect.
[0129] (4) An optoelectronic waveguiding device that has at least a
semiconductor laser part and an optical modulator in a portion
thereof, characterized in that both the optical modulator and the
semiconductor laser part have the MQW structures, layer structures
or constituent materials of these MQW structures are mutually
different, and an optical waveguide made of a bulk crystal exists
at the connection part between the optical modulator and the
semiconductor laser part.
[0130] (5) An optoelectronic waveguiding device, characterized in
that optoelectronic waveguiding device elements each of which is
one optoelectronic waveguiding device element selected from a group
consisting of at least a semiconductor laser, an optical modulator,
and a semiconductor optical amplifier are formed on the same
semiconductor substrate. These optoelectronic waveguiding device
elements are arranged in a direction of the light propagation axis
with at least the optical modulator and the semiconductor laser
part having the MQW structures. Layer structures or constituent
materials of these MQW structures are mutually different. An
optical waveguide that is formed by depositing a semiconductor bulk
crystal bearing an extremely small quantum effect exists at the
connection parts between two of the optical modulator, the
semiconductor laser, and the semiconductor amplifier.
[0131] (6) An optoelectronic waveguiding device, characterized in
that (a) a plurality of device element structures each comprising
at least a semiconductor laser and an optical modulator are formed
side by side in a direction parallel to the light propagation axis
on the same semiconductor substrate, each device element structure
capable of emitting light of the same or a mutually different
wavelength. Both the optical modulator and the semiconductor part
having the MQW structures, respectively, and layer structures or
constituent materials of these MQW structures being mutually
different. An optical waveguide made of a bulk crystal may exist at
the connection part between the modulator and the semiconductor
laser part.
[0132] (7) An optical module that has at least a semiconductor
optoelectronic waveguiding device and an optical fiber as
constituents in a portion thereof, characterized in that said
semiconductor optoelectronic waveguiding device is a semiconductor
optoelectronic waveguiding device such that a plurality of device
element structures each comprising at least a semiconductor laser
and an optical modulator are formed side by side in a direction
parallel to the light propagation axis. Each device element
structure being capable of emitting light of the same or a mutually
different wavelength.
[0133] The optical modulators and the semiconductor laser parts all
having the MQW structures, and layer structures or constituent
materials of these MQW structures of each device element structure
are mutually different. An optical waveguide made of a bulk crystal
exists at the connection part between the modulator and the
semiconductor laser of each device element structure.
[0134] (8) An optoelectronic waveguiding device, characterized in
that a plurality of device element structures each having at least
a semiconductor laser are formed side by side in a direction
parallel to the light propagation axis on the same semiconductor
substrate and at least an optical multiplexer and an optical
modulator are arranged in the optical waveguide running from the
semiconductor lasers up to a light facet of the device. The
semiconductor lasers, the optical multiplexer, and the optical
modulator being configured so that light emitted from the lasers
arranged side by side is multiplexed to one optical waveguide and
subsequently made to enter the optical modulator.
[0135] (9) An optical module comprising at least an optical fiber
and a semiconductor optoelectronic waveguiding device capable of
emitting single longitudinal mode light in a portion thereof,
characterized in that (a) a plurality of device element structures
each having at least a semiconductor laser are formed side by side
in a direction parallel to the light propagation axis on the same
semiconductor substrate, each device element structure in a
side-by-side arrangement being capable of emitting light of the
same or a mutually different wavelength, (b) at least an optical
multiplexer and an optical modulator are arranged in the optical
waveguides running from the semiconductor lasers up to a light
emitting facet of the device, the optical multiplexer and the
optical modulator being configured in such a way that light emitted
from any of the lasers in a side-by-side arrangement is multiplexed
by the optical multiplexer and then made to enter the optical
modulator, and at least modulator part having the MQW structure,
and (c) an optical waveguide made of a bulk crystal exists at least
in a portion of each waveguide in the optical multiplexer, hence
being capable of emitting light having one of plural wavelengths to
the outside of the module.
[0136] (10) An optoelectronic waveguiding device, characterized in
that the MQW structure made of an InGaAlAs system material and the
MQW structure made of an InGaAsP system material exist on the light
propagation axis of the optoelectronic waveguiding device, and an
optical waveguide made of one crystal selected from among InGaAsP
system, InGaAlAs system, and InAlAs system bulk crystals exist at
the connection part between these two MQW structures.
[0137] (11) An optoelectronic waveguiding device that has at least
a semiconductor laser and an optical modulator (EA modulator) in a
portion thereof, characterized in that a layered structure that
constitutes the EA modulator comprises the MQW structure made of an
InGaAlAs system material, a layered structure that constitutes the
semiconductor laser comprises the MQW structure made of an InGaAsP
system material, and an optical waveguide made of one material
selected from among InGaAsP system, InGaAlAs system, and InAlAs
system bulk crystals exists at the connection part between the
modulator and the semiconductor laser.
[0138] (12) An optoelectronic waveguiding device that has at least
a semiconductor laser and an optical modulator (EA modulator) in a
portion thereof, characterized in that each of the EA modulator and
the semiconductor laser has the MQW structure made of an InGaAsP
system material, at least one of a thickness of the quantum well, a
thickness of the barrier layer, and the number of cycles of the
quantum wells which constitute the MQW structure of the modulator
being different from a counterpart of the MQW structure of the
laser part, and an optical waveguide made of one material selected
from among InGaAsP system, InGaAlAs system, and InAlAs system bulk
crystals exists at the connection part between the modulator and
the semiconductor laser.
[0139] (13) An optoelectronic waveguiding device that has at least
a semiconductor laser and an optical modulator (EA modulator) in a
portion thereof, characterized in that each of the EA modulator and
the semiconductor laser has a MQW structure made of an InGaAlAs
system material, one of a thickness of the quantum well, a
thickness of barrier layer, and the number of cycles of the quantum
well which constitute the MQW structure of the modulator is
different from a counterpart of the MQW structure of the laser
part, and an optical waveguide made of one material selected from
among InGaAsP system, InGaAlAs system, and InAlAs system bulk
crystals exists at the connection part between the modulator and
the semiconductor laser part.
[0140] Nothing in the above description or provided examples is
meant to limit the present invention to any specific materials,
geometry, or orientation of elements. Many part/orientation
substitutions are contemplated within the scope of the present
invention and will be apparent to those skilled in the art. The
embodiments described herein were presented by way of example only
and should not be used to limit the scope of the invention.
[0141] Although the invention has been described in terms of
particular embodiments in an application, one of ordinary skill in
the art, in light of the teachings herein, can generate additional
embodiments and modifications without departing from the spirit of,
or exceeding the scope of, the claimed invention. Accordingly, it
is understood that the drawings and the descriptions herein are
proffered by way of example only to facilitate comprehension of the
invention and should not be construed to limit the scope
thereof.
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