U.S. patent application number 09/741358 was filed with the patent office on 2003-10-23 for semiconductor optical devices.
Invention is credited to Cho, Si Hyung, Dautremont-Smith, William Crossley, Huang, Sun-Yuan, Joyner, Charles H., Leibenguth, Ronald Eugene, Ougazzaden, Abdallah, Reynolds, Claude Lewis JR..
Application Number | 20030198267 09/741358 |
Document ID | / |
Family ID | 28792483 |
Filed Date | 2003-10-23 |
United States Patent
Application |
20030198267 |
Kind Code |
A1 |
Cho, Si Hyung ; et
al. |
October 23, 2003 |
SEMICONDUCTOR OPTICAL DEVICES
Abstract
The invention is a semiconductor optical device and method of
fabrication where the device includes an active region with an
active layer having a first index of refraction, and a blocking
region having a second, lower index of refraction. A semiconductor
layer having an index of refraction higher than the blocking region
formed over both the active and blocking regions so that the layer
is in closer proximity to the active layer in areas not covered by
the blocking region so as to decrease the difference between the
effective index of refraction in the active region and the
effective refractive index of the blocking region. Such devices are
particularly useful for pumping optical amplifiers since greater
power can be achieved while maintaining single mode emission.
Inventors: |
Cho, Si Hyung; (Silver
Spring, MD) ; Dautremont-Smith, William Crossley;
(Orefield, PA) ; Huang, Sun-Yuan; (Union City,
CA) ; Joyner, Charles H.; (Red Bank, NJ) ;
Leibenguth, Ronald Eugene; (Palmerton, PA) ;
Ougazzaden, Abdallah; (Breiningsville, PA) ;
Reynolds, Claude Lewis JR.; (Sinking Spring, PA) |
Correspondence
Address: |
SEAN FITZGERALD, ESQ.
ATER WYNNE LLP
222 SW COLUMBIA, SUITE 1800
PORTLAND
OR
97201
US
|
Family ID: |
28792483 |
Appl. No.: |
09/741358 |
Filed: |
December 20, 2000 |
Current U.S.
Class: |
372/46.01 |
Current CPC
Class: |
H01S 5/2059 20130101;
H01S 5/2218 20130101; H01S 5/3434 20130101; H01S 5/2275 20130101;
B82Y 20/00 20130101; H01S 5/227 20130101 |
Class at
Publication: |
372/46 |
International
Class: |
H01S 005/00 |
Claims
What is claimed is:
1. A semiconductor optical device comprising: an active region
including an active layer having a first index of refraction; a
blocking region having a second, lower index of refraction wherein
a fundamental mode is generated in the active layer when a bias is
applied to the device, the mode experiencing effective refractive
indices in the active and blocking regions; and a semiconductor
layer formed over both the active and blocking regions so that the
layer is in closer proximity to the active layer in areas not
covered by the blocking region so as to decrease the difference
between the effective refractive index of the active region and the
effective refractive index of the blocking region.
2. The device according to claim 1 wherein the layer comprises
InGaAsP.
3. The device according to claim 1 wherein the thickness of the
semiconductor layer is within the range 100 to 400 angstroms.
4. The device according to claim 1 wherein the effective index of
refraction difference is within the range 0.005 to 0.02.
5. The device according to claim 1 wherein the active region has a
width which is at least 4 .mu.m and emits light having a single
lateral mode.
6. The device according to claim 5 wherein the light has a facet
power of at least 400 mW.
7. The device according to claim 1 wherein the active region
comprises a multi-quantum well active layer.
8. The device according to claim 1 wherein the device is a
laser.
9. The device according to claim 1 wherein the device includes at
least two separate confinement layers above the active layer, at
least one of said layers in contact with the semiconductor
layer.
10. Apparatus comprising: an optical amplifier; and a semiconductor
pump laser coupled to the amplifier, the laser including: an active
region with an active layer having a first index of refraction; a
blocking region having a second, lower index of refraction wherein
a fundamental mode is generated in the active layer when a bias is
applied to the device, the mode experiencing effective refraction
indices in the active and blocking regions; and a semiconductor
layer formed over both the active and blocking regions so that the
layer is in closer proximity to the active layer in areas not
covered by the blocking region so as to decrease the difference
between the effective refractive index of the active region and the
effective refractive index of the blocking region.
11. The apparatus according to claim 10 wherein the layer comprises
InGaAsP.
12. The apparatus according to claim 10 wherein the thickness of
the semiconductor layer is within the range 100 to 400
angstroms.
13. The apparatus according to claim 10 wherein the index of
refraction difference is within the range 0.0005 to 0.02.
14. The apparatus according to claim 10 wherein the active region
has a width which is at least 4 .mu.m and emits light having a
single mode.
15. The apparatus according to claim 14 wherein the light has a
facet power of at least 400 mW.
16. The apparatus according to claim 10 wherein the active region
comprises a multi-quantum well active layer.
17. The apparatus according to claim 10 wherein the laser is an
InP-based laser.
18. The apparatus according to claim 10 wherein the laser includes
at least two separate confinement layers above the active layer, at
least one of said layers in contact with the semiconductor
layer.
19. A method of forming a semiconductor optical device comprising
the steps of: forming an active region including an active layer
having a first index of refraction; forming a blocking region
having a second, lower index of refraction so that a fundamental
mode will be generated in the active layer when a bias is supplied
to the device, the mode experiencing effective refractive indices
in the active and blocking regions; and forming a semiconductor
layer having an index of refraction greater than the blocking
region over both the active and blocking regions so that the layer
is in closer proximity to the active layer in areas not covered by
the blocking region so as to decrease the difference between the
effective refractive index of the active region and the effective
refractive index of the blocking region.
20. The method according to claim 19 wherein the semiconductor
layer comprises InGaAsP.
21. The method according to claim 19 wherein the semiconductor
layer is formed by MOCVD.
22. The method according to claim 19 wherein the semiconductor
layer is formed over a window etched in a blocking layer.
23. The method according to claim 19 wherein the index of
refraction difference is within the range 0.0005 to 0.02.
24. The method according to claim 19 wherein the thickness of the
semiconductor layer is within the range 100 to 400 angstroms.
25. The method according to claim 19 wherein at least two separate
confinement layers are formed above the active layer, at least one
of said separate confinement layers in contact with the
semiconductor layer in an area where the semiconductor layer is in
closer proximity to the active layer.
Description
FIELD OF THE INVENTION
[0001] This invention relates to semiconductor optical devices such
as lasers and optical amplifiers.
BACKGROUND OF THE INVENTION
[0002] Optical Networks are currently of great interest primarily
due to their ability to carry a large amount of information.
Particularly significant are Dense Wavelength Division Multiplexing
(DWDM) Systems which carry several wavelengths in a single optical
fiber. Important components of such systems are Erbium-Doped Fiber
Amplifiers (EDFAs) and Raman fiber amplifiers, which allow
long-haul transmission of the signal light without regeneration.
These amplifiers operate by pumping of charge carriers in the fiber
with pump lasers to cause amplification of the signal light. The
amplification is a strong function of the pump power, particularly
for systems carrying several channels. For example, in a typical
80-channel system, two 980 nm pump lasers and four 1480 nm pump
lasers, each operating at 150 mW, are employed for an EDFA. In
systems using Raman amplifiers, four or more pump lasers at
multiple wavelengths are typically used.
[0003] It is desirable, therefore, to increase the power generated
by a pump laser in order to simplify the system. This can be done,
for example, by increasing the length of the laser chip, but this
tends to increase internal optical losses. The width of the active
region of the laser could also be increased, but this typically
produces an unwanted additional transverse mode. Another
possibility is to use a Master Oscillator Power Amplifier
structure, but this introduces complexity in fabrication and
difficulty in coupling the pump light into a single mode fiber.
(See, e.g., Cho, et al, "1.9-W Quasi-CW from a
Near-Diffraction-Limited 1.55-micron InGaAsP-InP Tapered Laser,"
IEEE Photonics Technology Letters, vol. 10, No. 8, pp1091-1093
(August 1998)).
[0004] It is desirable, therefore, to increase the power of a laser
emitting light in a single mode. It is also desirable to increase
the power of semiconductor optical amplifiers.
SUMMARY OF THE INVENTION
[0005] The invention in one aspect is a semiconductor optical
device which includes an active region with an active layer having
a first index of refraction, and a blocking region having a second,
lower index of refraction. A fundamental mode is generated in the
active layer when a bias is applied to the device, the mode
experiencing effective refractive indices in the active and
blocking regions. A semiconductor layer having an index of
refraction higher than the blocking region is formed over both the
active and blocking regions so that the layer is in closer
proximity to the active layer in areas not covered by the blocking
region so as to decrease the difference between the effective
refractive index of the active region and the effective refractive
index of the blocking region.
[0006] In accordance with another aspect, the invention is
apparatus which includes an optical amplifier, and a semiconductor
pump laser coupled to the amplifier. The laser includes an active
region with an active layer having a first index of refraction, and
a blocking region having a second, lower index of refraction. A
fundamental mode is generated in the active layer when a bias is
applied to the device, the mode experiencing effective refractive
indices in the active and blocking regions. A semiconductor layer
having an index of refraction higher than the blocking region is
formed over both the active and blocking regions so that the layer
is in closer proximity to the active layer in areas not covered by
the blocking region so as to decrease the difference between the
effective refractive index of the active region and the effective
refractive index of the blocking region.
[0007] In accordance with a further aspect, the invention is a
method of forming a semiconductor optical device including the
steps of forming an active layer having a first index of
refraction, and forming a blocking region having a second, lower
index of refraction. A fundamental mode is generated in the active
layer when a bias is applied to the device, the mode experiencing
effective refractive indices in the active and blocking regions so
that a semiconductor layer having an index of refraction between
that of the blocking region and the active layer is formed over
both the active and blocking regions so that the layer is in closer
proximity to the active layer in areas not covered by the blocking
region so as to decrease the difference between the effective
refractive index of the active region and the effective refractive
index of the blocking region.
BRIEF DESCRIPTION OF THE FIGURES
[0008] These and other features of the invention are delineated in
detail in the description to follow. In the drawing:
[0009] FIG. 1 is a cross sectional view of a semiconductor laser
incorporating features of the invention in accordance with one
embodiment;
[0010] FIG. 2 is an illustration of calculated light output as a
function of drive current for the laser of FIG. 1 compared to a
prior art laser;
[0011] FIG. 3 is a schematic block diagram of a portion of an
optical network incorporating features of the invention in
accordance with one embodiment; and
[0012] FIGS. 4-6 are cross sectional views of a laser in various
stages of fabrication in accordance with one embodiment of the
method aspect of the invention.
[0013] It will be appreciated that, for purposes of illustration,
these figures are not necessarily drawn to scale.
DETAILED DESCRIPTION
[0014] FIG. 1 illustrates a typical semiconductor laser, 10, which
utilizes the present invention in accordance with one embodiment.
The particular structure shown is an overgrown etched channel
laser, but it will be appreciated that other semiconductor lasers
may also incorporate features of the invention. (For information
regarding etched channel lasers, see, U.S. Pat. No. 5,539,762
issued to Belenky et al.)
[0015] The laser structure is formed on a semiconductor substrate,
11, which in this example comprises n-type InP covered by an n-type
buffer layer (not separately shown). Formed on the substrate,
usually by Metal Organic Chemical Vapor Deposition (MOCVD), is a
first layer, 12, which in this example comprises InGaAsP with a
thickness of approximately 300 angstroms. Formed on the cladding
layer, again by MOCVD is a second layer, 13, preferably comprising
InGaAsP with a thickness of approximately 100 angstroms. The
layers, 12 and 13, together form a stepped bandgap Separate
Confinement Layer (SCL) as known in the art.
[0016] Formed on the layer, 13, again preferably by MOCVD, is the
active layer, 14, of the laser. In this example, the active layer
was a multi-quantum well design including multiple well layers of
InGaAsP separated by barrier layers comprising InGaAsP of a
different higher bandgap composition. It will be appreciated that
this composition is only illustrative, and the invention may be
used with any type of semiconductor active layer. In this example,
the active well thickness is approximately 40 angstroms, and the
overall active layer thickness is approximately 500 angstroms.
[0017] A first layer, 15, of a stepped separate confinement layer
(SCL) is formed on the active layer, 14, again preferably by MOCVD.
In this example, the layer, 15, is an InGaAsP layer with a
thickness of approximately 100 angstroms. Formed on the SCL is an
electron stopper layer, 16, preferably comprising InAlAs with a
thickness of approximately 200 angstroms. (For more details on
electron stopper layers, see U.S. Pat. No. 5,539,762 issued to
Belenky et al cited previously.) The layer, 16, is also preferably
formed by MOCVD. The second SCL layer, 17, of the stepped SCL is
formed on the stopper layer, again by MOCVD. This layer is also
InGaAsP but with a slightly different composition from layer 15.
The inclusion of this additional SCL layer is preferred since it
aids in adjustment of the effective index of refraction as
discussed below. Generally, layer 17 will have a thickness of 300
angstroms, and an index of refraction similar to that of layer
12.
[0018] A blocking layer of n-InP, 18, was formed over essentially
the entire surface of the layer, 17, again by MOCVD, and then
etched by standard photolithography to form a window, 21, in order
to expose the portion of layer, 17, and the underlying layers which
will comprise the active region of the device. The width of the
etched window, 21, at the surface of the layer 17 helps define the
width of the active region, which is illustrated by dashed lines 19
and 20. (Dashed lines 19 and 20 represent the extent of current
spreading in the structure when a bias is applied and therefore
define the width of the active region. The width, w, is the width
of the active region in the active layer, 14.) The blocking layer
in this example is n-type InP with a thickness of approximately
0.60 microns, but it will be understood that other materials which
block an electrical bias to areas outside the active region may be
utilized. Preferably, the material is a single crystal epitaxial
insulator which is lattice matched to the underlying semiconductor.
It is also preferred that it be selectively etched with respect to
underlying layer 17. For reasons to be discussed, a large active
region width, w, is possible, and in this example is approximately
6.2 microns.
[0019] It should be understood that the "active layer" is the layer
(14) in which light is generated by recombination of free carriers
therein. The "active region" is the region, including layers 12-17,
wherein applied current is confined to produce this recombination,
i.e., the region defined by dashed lines 19 and 20. The "blocking
region" is the region outside the active region, i.e. outside of
the dashed lines, 19 and 20, and includes layers 12-18. The
"blocking layer" is the layer, 18, formed over the layers 12-17,
which blocks current to the blocking region. In this particular
example, only layer 18 provides the blocking function. However,
there are other structures where blocking layers are formed
adjacent to the active region.
[0020] A semiconductor layer, 22, is formed over the blocking
layer, 18, and the exposed area of the SCL layer, 17, in the
window, 21. In this example, the layer, 22, comprises InGaAsP with
a band gap of approximately 1.13 microns and is essentially the
same composition as layer 17. While InGaAsP would be preferred for
an InP based laser, other materials such as GaAlAs would be more
suitable for a GaAs based laser. The layer was deposited by MOCVD
to a thickness of approximately 300 angstroms. Other techniques
could be used, and the thickness will generally be in the range 100
to 400 angstroms. The index of refraction of the layer, 22, is
preferably slightly greater than that of the blocking layer, 18,
for reasons to be discussed. The layer, 22, should also preferably
be lattice matched to InP.
[0021] It should be understood that the "effective refractive
index" is the refractive index experienced by the fundamental mode
generated in the active region within the active layer, 14, as
illustrated by the shaded portion in FIG. 1. While not being bound
by any theory, it is believed that it is the difference between
this effective index of the active region and the effective
refractive index of the blocking region which is adjusted by the
presence of the layer 22. As known in the art, the effective
refractive index is the average refractive index, weighted by the
local mode intensity integrated over the layers that the mode
overlaps (layers 12-18, and 22). It is desired to reduce by a small
amount the difference between this effective refractive index of
the active region and the blocking region so that no transverse
modes are supported even while the width, w, is increased. It is
believed that the difference between the effective refractive index
of the active region and the effective refractive index of the
blocking region is most desirably in the range 0.005-0.02. In this
example, the difference was approximately 0.01, which was produced
by the layer, 22, having a refractive index of 3.28, and the
blocking layer, 18, and the cladding layer, 23, both have a
refractive index of 3.17. Of course, it will be appreciated that
the effective refractive index will also be a function of the
thickness of the layers 12-17, and 22, and it is well within the
skill of the artisan to provide an appropriate index for layer 22
with different layer thickness.
[0022] It will be noted further that adjustment of the effective
refractive index is enabled by the fact that layer 22 is closer to
active layer 14 in the area over the active region than in the area
over the blocking region. Thus, layer 22 will have little or no
effect on the effective refractive index of the blocking
region.
[0023] The device is completed by deposition of a cladding layer,
23, on the layer, 22, again by MOCVD, a cap layer, 24, deposited on
the cladding layer, 23, and the deposition of electrodes (not
shown) on the cap layer and the bottom of the substrate. In this
example, the cladding layer, 23, is p-type InP with a thickness of
about 2 .mu.m, and the cap layer, 24, is p+ type lattice-matched
InGaAs with a thickness of about 0.1 .mu.m.
[0024] In this particular example, the index of refraction
difference allows an expansion of the active region width from a
standard 2.4 microns in a capped mesa buried heterostructure (CMBH)
to 6.2 microns in this example while still producing single mode
emission. Preferably, the active region width is at least 4 .mu.m.
FIG. 2 illustrates calculated power v. drive current curves for the
conventional CMBH laser (curve 30) and the BRW laser illustrated in
FIG. 1. It is apparent that significantly greater power is produced
by the laser according to the invention. (For information on a
capped mesa buried heterostructure laser, see, e.g., Zilko, et al
"Growth and Characterization of High Yield, High Power, High Speed
InP/InGaAsP Capped Mesa Buried Heterostructure Distributed Feedback
Lasers'" IEEE J. of Quantum Electronics, vol. 25, No. 10, pp.
2091-2095 (October 1989).)
[0025] FIG. 3 illustrates a portion of an optical network which may
utilize the present invention. Multiple sources of signal light,
40, are coupled to a wavelength division multiplexer (WDM), 41,
through an optical fiber, 42. Typically, the sources are a
semiconductor lasers which deliver light of several wavelengths
around 1.5 microns. A pump laser, 43, which can be the device of
FIG. 1, is also coupled to the WDM, through a single mode fiber,
44. As previously discussed, one of the advantages of the present
invention is the ability to couple high power light, advantageously
greater than 200 mW, into a single mode fiber. The pump light is
typically about 0.98 microns or 1.48 microns in wavelength. The
pump and source light are combined through the WDM, 41, into an
optical amplifier, which in this example is an Erbium Doped Fiber
Amplifier (EDFA) so that the 1.5 .mu.m signal light is amplified
for further transmission.
[0026] FIGS. 4-6 illustrate various stages of fabrication of the
device of FIG. 1. Layers 12-17 (shown as a single layer for
purposes of simplicity) have already been deposited by known
techniques. Blocking layer, 18, has been deposited by known
techniques over essentially the entire surface of layer 17. A mask,
50, has been formed over layer 18, by depositing a layer such as
silicon dioxide on essentially the entire top surface of the layer
and then photolithographically forming a window, 51, to expose the
area of the underlying semiconductor which will comprise the active
region.
[0027] As illustrated in FIG. 5, the exposed portion of the
blocking layer, 18, is etched down to the layer 17 by standard
selective chemical etching. The layer, 22, is then deposited over
the blocking layer, 18, and the exposed portion of the layer, 17,
as illustrated in FIG. 6. Deposition is typically done by Metal
Organic Chemical Vapor Deposition (MOCVD). The thickness, t, of the
layer at the bottom of the window tends to be thicker than on the
sides of the window. As previously discussed, the thickness, t, is
preferably 100-400 angstroms (10-40 nm). The proportions of the
components of layer 22, in this case InGaAsP, are chosen according
to known techniques to produce the desired refractive index
difference previously discussed. Layers, 23, and 24, are
successively deposited over the layer 22 by standard techniques as
shown in FIG. 1.
* * * * *