U.S. patent number RE31,806 [Application Number 06/473,304] was granted by the patent office on 1985-01-15 for monolithic multi-emitting laser device.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Robert D. Brunham, Donald R. Scifres, William Streifer.
United States Patent |
RE31,806 |
Scifres , et al. |
January 15, 1985 |
Monolithic multi-emitting laser device
Abstract
A monolithic laser device produces a plurality of spatially
displaced emitting cavities in an active layer of a semiconductor
body acting as a waveguide for light wave propagation under lasing
conditions. Various means are disclosed to deflect and directly
couple a portion of the optical wave propagation into one or more
different spatially displaced emitting cavities to improve
coherence and reduce beam divergence.
Inventors: |
Scifres; Donald R. (Los Altos,
CA), Streifer; William (Palo Alto, CA), Brunham; Robert
D. (Palo Alto, CA) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
27044087 |
Appl.
No.: |
06/473,304 |
Filed: |
March 8, 1983 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
956307 |
Oct 30, 1978 |
04255717 |
Mar 10, 1981 |
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Current U.S.
Class: |
372/50.121;
372/44.01; 372/45.01; 372/46.01 |
Current CPC
Class: |
H01S
5/4068 (20130101) |
Current International
Class: |
H01S
5/40 (20060101); H01S 5/00 (20060101); H01S
003/19 () |
Field of
Search: |
;372/50,45,46,24
;357/17 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Pankove, "Integrated Phase Coherent Laser Array", RCA Technical
Notes, RCATN No. 760 Apr. 1968, (2 pages)..
|
Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Carothers, Jr.; W. Douglas
Claims
We claim:
1. In a monolithic laser device wherein one or more layers of
semiconductor material are fabricated on a substrate, one of said
layers forming .[.an.]. active .[.layer.]. .Iadd.means .Iaddend.for
light wave propagation and generation under lasing conditions,
means for forward biasing said active .[.layer.]. .Iadd.means
.Iaddend.to produce a plurality of adjacent .Iadd., linear
.Iaddend.light propagating and emitting portions therein, light
guiding regions in said device .Iadd.intermediate of the ends of
and .Iaddend.nonlinear relative to said emitting portions whereby
.[.said.]. .Iadd.the .Iaddend.light waves produced in one portion
of said active .[.layer.]. .Iadd.means .Iaddend.are deflected and
coupled into one or more adjacent emitting portions of said active
.[.layer.]. .Iadd.means .Iaddend., said regions provided by a
refractive index change with which .[.said.]. .Iadd.the
.Iaddend.light wave interacts while .[.said light wave is.]. within
said regions and wherein said refractive index change is provided
by the injected charge distribution determined by current confining
means.
2. In a monolithic laser device wherein one or more layers of
semiconductor material are fabricated on a substrate, one of said
layers forming .[.an.]. active .[.layer.]. .Iadd.means .Iaddend.for
light wave propagation and generation under lasing conditions,
means for forward biasing said active .[.layer.]. .Iadd.means
.Iaddend., means in .Iadd.and intermediate of the ends of
.Iaddend.said device whereby .[.said.]. .Iadd.the .Iaddend.light
waves produced in one portion of said active .[.layer.].
.Iadd.means .Iaddend.are deflected into one or more adjacent
emitting portions of said active .[.layer.]. .Iadd.means .Iaddend.,
said deflection means provided by a refractive index change with
which .[.said.]. .Iadd.the .Iaddend.light wave interacts while
.[.said light wave is.]. within said deflection means and wherein
said refractive index change is provided by an impurity
profile.
3. The device of claim 2 wherein said impurity profile defines a
plurality of .Iadd.linear .Iaddend.optical cavities, said cavities
being coupled to one or more adjacent cavities by an
interconnecting cavity formed by said profile.
4. The device of claim 3 wherein said optical cavities are
angularly disposed relative to each other.
5. The device of claim 3 wherein said optical cavities are
parallel.
6. The device of claim 3 wherein said optical cavities are
unequally spaced relative to each other.
7. The device of claim 3 wherein some of said optical cavities are
unequally spaced.
8. The device of claim 3 wherein all of said optical cavities are
equally spaced relative to each other.
9. In a monolithic laser device wherein one or more layers of
semiconductor material are fabricated on a substrate, one of said
layers forming .[.an.]. active .[.layer.]. .Iadd.means .Iaddend.for
light wave propagation and generation under lasing conditions,
means for forward biasing said active .[.layer.]. .Iadd.means
.Iaddend., means in .Iadd.and intermediate of the ends of
.Iaddend.said device whereby .[.said.]. .Iadd.the .Iaddend.light
waves produced in one portion of said active .[.layer.].
.Iadd.means .Iaddend.are deflected into one or more adjacent
emitting portions of said active .[.layer.]. .Iadd.means .Iaddend.,
said deflection means provided by a refractive index change with
which .[.said.]. .Iadd.the .Iaddend.light wave interacts while
.[.said light wave is.]. within said deflection means and wherein
said deflection means is a plurality of current confining channels,
each of said current confining channels being coupled to an
adjacent channel by an interconnecting current confining
channel.
10. The device of claim 9 wherein said current confining channels
are contact stripes.
11. The device of claim 9 wherein said current confining channels
are angularly disposed relative to each other.
12. The device of claim 9 wherein said current confining channels
are parallel.
13. The device of claim 9 wherein said current confining channels
are unequally spaced relative to each other.
14. The device of claim 9 wherein some of said current confining
channels are unequally spaced.
15. The device of claim 9 wherein all of said current confining
channels are equally spaced relative to each other.
16. In a monolithic laser device wherein one or more layers of
semiconductor material are fabricated on a substrate, one of said
layers forming .[.an.]. active .[.layer.]. .Iadd.means .Iaddend.for
light wave propagation and generation under lasing conditions,
means for forward biasing said active .[.layer.]. .Iadd.means
.Iaddend., means in .Iadd.and intermediate of the ends of
.Iaddend.said device whereby .[.said.]. .Iadd.the .Iaddend.light
waves produced in one portion of said active .[.layer.].
.Iadd.means .Iaddend.are deflected into one or more adjacent
emitting portions of said active .[.layer.]. .Iadd.means .Iaddend.,
said deflection means provided by a refractive index change with
which .[.said.]. .Iadd.the .Iaddend.light wave interacts while
.[.said light wave is.]. within said deflection means and wherein
said refractive index change is provided by a material composition
change.
17. The device of claim 16 wherein the material composition change
defines a plurality of .Iadd.linear .Iaddend.optical cavities, said
cavities being coupled to one or more adjacent cavities by an
interconnecting cavity formed by said material composition change.
.[.18. The device of claim 17 wherein said optical cavities are
angularly disposed relative to each
other..]. 19. The device of claim 17 wherein said optical cavities
are
parallel. 20. The device of claim 17 wherein said optical cavities
are
unequally spaced relative to each other. 21. The device of claim
17
wherein some of said optical cavities are unequally spaced. 22. The
device of claim 17 wherein all of said optical cavities are equally
spaced
relative to each other. 23. In a monolithic laser device wherein
one or more layers of semiconductor material are fabricated on a
substrate, one of said layers forming .[.an.]. active .[.layer.].
.Iadd.means .Iaddend.for light wave propagation and generation
under lasing conditions, means for forward biasing said active
.[.layer.]. .Iadd.means .Iaddend., means in .Iadd.and intermediate
of the ends of .Iaddend.said device whereby .[.said.]. .Iadd.the
.Iaddend.light waves produced in one portion of said active
.[.layer.]. .Iadd.means .Iaddend.are deflected into one or more
adjacent emitting portions of said active .[.layer.]. .Iadd.means
.Iaddend., said deflection means provided by a refractive index
change with which .[.said.]. .Iadd.the .Iaddend.light wave
interacts while .[.said light wave is.]. within said deflection
means and wherein said refractive index change is provided by a
material thickness change.
. The device of claim 23 wherein a plurality of interconnecting
channels are provided in said substrate to provide said material
thickness change.
5. The device of claim 23 wherein the material thickness change
defines a plurality of .Iadd.linear .Iaddend.optical cavities, said
cavities being coupled to one or more adjacent cavities by an
interconnecting cavity formed by said material thickness change.
.[.26. The device of claim 25 wherein said optical cavities are
angularly disposed relative to each
other..]. 27. The device of claim 25 wherein said optical cavities
are
parallel. 28. The device of claim 25 wherein said optical cavities
are
unequally spaced relative to each other. 29. The device of claim
25
wherein some of said optical cavities are unequally spaced. 30. The
device of claim 25 wherein all of said optical cavities are equally
spaced
relative to each other. 31. In a monolithic laser device wherein
one or more layers of semiconductor material are fabricated on a
substrate, one of said layers forming .[.an.]. active .[.layer.].
.Iadd.means .Iaddend.for light wave propagation and generation
under lasing conditions, current confining means for forward
biasing selected portions of said active .[.layer.]. .Iadd.means
.Iaddend.to produce two or more .Iadd.linear and spatially disposed
.Iaddend.lasing and .Iadd.light .Iaddend.emitting cavities in said
active .[.layer.]. .Iadd.means .Iaddend., and a light coupling
region between said .Iadd.linear light emitting .Iaddend.cavities
to deflect a portion of the light wave
propagation in one cavity to one or more adjacent emitting
cavities. 32. In a monolithic laser device wherein one or more
layers of semiconductor material are fabricated on a substrate, one
of said layers forming .[.an.]. active .[.layer.]. .Iadd.means
.Iaddend.for light wave propagation and generation under lasing
conditions, a plurality of current confining means for forward
biasing selected portions of said active .[.layer.]. .Iadd.means
.Iaddend.to produce a plurality of .Iadd.linear light
.Iaddend.emitting cavities therein, and light deflecting means
.Iadd.intermediate of the ends of said cavities and
.Iaddend.coupling each of said current confining means to one or
more adjacent current confining means wherein said current
confining means comprises a plurality of parallel contact stripes
on the surface thereof, each of said stripes being coupled to an
adjacent stripe by an interconnecting stripe means.
The device of claim 32 wherein said interconnecting stripe means
are
curved stripe sections. 34. The device of claim 32 wherein said
interconnecting stripe means are transversely disposed stripe
sections.
The device of claim 34 wherein said sections are geometrically
staggered relative to each other along the length of said device.
36. The device of claim 32 wherein said interconnecting stripe
means are
criss-crossing stripe sections. 37. The device of claim 32 wherein
said interconnecting stripe means comprises a single wide contact
stripe transversely disposed relative to said parallel
.[.contract.]. .Iadd.contact .Iaddend.stripes, said transversely
disposed stripe being
several times wider than said parallel contact stripes. 38. The
device of claim 37 wherein said transversely disposed stripe is
perpendicular to
said parallel contact stripes. 39. The device of claim 32 wherein
said interconnecting stripe means comprises a y-shape stripe
configuration connecting the ends of a pair of said parallel
contact stripes at one end
of said device. 40. The device of claim 32 wherein said contact
stripes are disposed at an angle relative to the longitudinal axis
of said device.
1. The device of claim 32 wherein said current confining means
comprises a plurality of contact stripes on the surface thereof,
said stripes being angularly disposed relative to each other and
said interconnecting stripe
means connecting the ends of adjacently disposed contact stripes.
42. In a monolithic laser device wherein one or more layers of
semiconductor materials are fabricated on a substrate, one of said
layers forming .[.an.]. active .[.layer.]. .Iadd.means .Iaddend.for
light wave generation and propagation, means for forward biasing
said active .[.layer.]. .Iadd.means .Iaddend.to produce a light
wave in at least one linear light propagating region of said active
.[.layer.]. .Iadd.means .Iaddend.and an adjacent light guiding
region .Iadd.intermediate of the ends of and .Iaddend.coupled to
said one region to deflect and couple a portion of said light wave
in said one region into other such regions formed in said active
.[.layer.]. .Iadd.means .Iaddend.which are linear and spatially
displaced from said one region. 43. In a monolithic laser device
wherein one or more layers of semiconductor materials are
fabricated on a substrate, one of said layers forming .[.an.].
active .[.layer.]. .Iadd.means .Iaddend.for light wave generation
and propagation, means for forward biasing said active .[.layer.].
.Iadd.means .Iaddend.to produce a light wave in at least one region
of said active .[.layer.]. .Iadd.means .Iaddend.and means to
deflect and couple a portion of said light wave in said one region
into other regions of said active .[.layer.]. .Iadd.means
.Iaddend.spatially displaced from said one region and wherein said
first and second mentioned means include means to confine current
to selected portions of said active layer to produce multiple
.Iadd.linear light
.Iaddend.emitting cavities. 44. In a monolithic laser device
wherein laser device wherein one or more layers of semiconductor
materials are fabricated on a substrate, one of said layers forming
.[.an.]. active .[.layer.]. .Iadd.medium .Iaddend.for light wave
generation and propagation, means for forward biasing said active
.[.layer.]. .Iadd.medium .Iaddend.to produce a light wave in at
least one region of said active .[.layer.]. .Iadd.medium
.Iaddend.and means to deflect and couple a portion of said light
wave in said one region into other regions of said active
.[.layer.]. .Iadd.medium .Iaddend.spatially displaced from said one
region and wherein said first mentioned means comprises a plurality
of current confining channels, said second mentioned means
comprises a plurality of interconnecting current confining channels
.Iadd.intermediate of the ends of the ends of said regions.
.Iaddend..
The device of claim 44 wherein said current confining channels
comprises a plurality of spaced stripes, said interconnecting
current confining channels comprise a plurality of interconnecting
stripes, at least one such interconnecting stripe being disposed
between adjacent
stripes. 46. In a monolithic laser device wherein one or more
layers of semiconductor materials are fabricated on a substrate,
one of said layers forming .[.an.]. active .[.layer.]. .Iadd.medium
.Iaddend.for light wave generation and propagation.Iadd.,
.Iaddend.means for forward biasing said active .[.layer.].
.Iadd.medium .Iaddend.to produce a light wave in at least one
region of said active .[.layer.]. .Iadd.medium .Iaddend.and means
to deflect and couple a portion of said light wave in said one
region into other regions of said active .[.layer.]. .Iadd.medium
.Iaddend.spatially displaced from said one region and wherein said
first mentioned means comprises a plurality of spaced mesa
structures including .[.all.]. .Iadd.portions .Iaddend.of said
layers .Iadd.and said active medium .Iaddend., said mesa structures
separated from each other by a medium of lower refractive index
than said active .[.layer.]. .Iadd.medium .Iaddend., and said
second mentioned means comprises a plurality of interconnecting
mesa structures .Iadd.intermediate of the ends of said mesa
structures .Iaddend., at least one such interconnecting mesa
structure being disposed between adjacent mesa structures. 47. In a
monolithic laser device wherein one or more layers of semiconductor
materials are fabricated on a substrate, one of said layers forming
.[.an.]. active .[.layer.]. .Iadd.medium .Iaddend.for light wave
generation and propagation, means for forward biasing said active
.[.layer.]. .Iadd.medium .Iaddend.to produce a light wave in at
least one region of said active .[.layer.]. .Iadd.medium
.Iaddend.and means to deflect and couple a portion of said light
wave in said one region into other regions of said active
.[.layer.]. .Iadd.medium .Iaddend.spatially displaced from said one
region and wherein said first mentioned means comprises a plurality
of spaced channels in the surface of said substrate, said second
mentioned means comprises a plurality of interconnecting channels
in said substrate .Iadd.intermediate of the ends of the ends of
said spaced channels .Iaddend., at least one such interconnecting
channel
being disposed between adjacent channels. 48. In a monolithic laser
device wherein one or more layers of semiconductor materials are
fabricated on a substrate, one of said layers forming .[.an.].
active .[.layer.]. .Iadd.means .Iaddend.for light wave generation
and propagation, means for forward biasing said active .[.layer.].
.Iadd.means .Iaddend.to produce a light wave in at least one region
of said active .[.layer.]. .Iadd.means .Iaddend.and means to
deflect and couple a portion of said light wave in said one region
into other regions of said active .[.layer.]. .Iadd.means
.Iaddend.spatially displaced from said one region and wherein said
.[.second.]. .Iadd.third .Iaddend.mentioned means is a periodic
grating disposed in said layers such that said light wave interacts
with said grating being at an angle relative to the direction of
light wave
propagation. 49. In a monolithic laser device wherein one or more
layers of semiconductor materials are fabricated on a substrate,
one of said layers forming .[.an.]. active .[.layer.]. .Iadd.medium
.Iaddend.for light wave generation and propagation, means for
forward biasing said active .[.layer.]. .Iadd.medium .Iaddend.to
produce a light wave in at least one region of said active
.[.layer.]. .Iadd.medium .Iaddend.and means to deflect and couple a
portion of said light wave in said one region into other regions of
said active .[.layer.]. .Iadd.medium .Iaddend.spatially displaced
from said one region and wherein said first mentioned means
comprises a plurality of current confining channels, said second
mentioned means characterizes said current confining channels as
each being disposed
at an angle relative to the longitudinal axis of said device. 50.
The device of claim 49 wherein said current confining channels are
parallel.
. The device of claim 49 wherein said current confining channels
are angularly disposed relative to each other and coupled to an
adjacent
current confining channel by interconnecting channel means. 52. The
device of claims 49, 50 or 51 wherein said current confining
channels are contact
stripes. 53. In a monolithic laser device wherein one or more
layers of semiconductor materials are fabricated on a substrate,
one of said layers forming .[.an.]. active .[.layer.]. .Iadd.means
.Iaddend.for light wave generation and propagation, means for
forward biasing said active .[.layer.]. .Iadd.means .Iaddend.to
produce a light wave in at least one region of said active
.[.layer.]. .Iadd.means .Iaddend.and means to deflect and couple a
portion of said light wave in said one region into other regions of
said active .[.layer.]. .Iadd.means .Iaddend.spatially displaced
from said one region and wherein said regions are impurity in said
device, said impurity profile defining a plurality of .Iadd.linear
.Iaddend.optical cavities, said .Iadd.linear optical
.Iaddend.cavities being coupled to one or more adjacent cavities by
an interconnecting cavity formed by said profile .Iadd.and
intermediate of the ends of said cavities .Iaddend.. .Iadd.54. The
device of claim 31 wherein said linear cavities are coupled by said
light coupling regions intermediate of the ends of said linear
cavities. .Iaddend. .Iadd.55. The device of claim 31 wherein said
linear cavities are angularly disposed relative to adjacent, linear
cavities and are coupled to said adjacent, linear cavities by said
light coupling regions at the ends of said cavities. .Iaddend.
.Iadd.56. The device of claim 43 wherein said linear emitting
cavities are coupled by said deflection means intermediate of the
ends of said linear emitting cavities. .Iaddend. .Iadd.57. The
device of claim 43 wherein said linear emitting cavities are
angularly disposed relative to adjacent, linear cavities and are
coupled to said adjacent, linear cavities by said deflection means
at the ends of said cavities. .Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates generally in injection lasers and, more
particularly, to heterostructure injection lasers having
multi-emission capability.
Higher power outputs are being sought in semiconductor junction
lasers to meet requirements necessary for optical fiber
transmission, optical disk writing and integrated optical
components and circuits. To achieve higher output powers from
injection lasers, a wide contact stripe region has been proposed
wherein stripe widths in excess of, for example, 20 .mu.m, were
employed in conventionally known double heterojunction and single
heterojunction injection lasers. The width of the stripe was
increased to spread the current density over a larger region of the
light guiding layer of the device thereby spreading out the
developed power by virtue of the larger emitting area. This also
reduced the potential of structural damage and degradation of the
laser device due to higher current and power densities established
where narrower stripe geometries are employed.
Injection lasers have been known to have a stripe width of
approximately 75 .mu.m, achieving pulsed output powers of
approximately 650 mW.
A disadvantage of these wide stripe lasers has been that transverse
mode operation along the p-n junction plane is not stable. On one
hand these broad stripe lasers operate in one or more higher order
transverse modes exhibiting a broad divergence in the far field
radiation pattern, which pattern may fluctuate with time or with
driving current. On the other hand, multiple filaments may be
simultaneously established in the pumped regions of the light
guiding active layer resulting in uncontrolled optical interference
fringes in the laser beam.
Greater power outputs have been realized where more than one
contact stripe may be employed on the same laser device and if
their stripe separation is small enough, optical coupling can be
achieved due to transverse optical wave overlapping. This is
disclosed in U.S. Pat. No. 3,701,044 and in Applied Physics
Letters, Volume 17, Number 9, pages 371-373. With such overlapping,
the two established lasers, upon pumping, operate in a phase-locked
state. However, as indicated in these disclosures, several
transverse modes were present so that stable beam output was not
achieved.
It has already been known that with very narrow stripe geometry,
such as 2 .mu.m wide, lowest order or fundamental transverse mode
can be achieved at least at current pumping levels near threshold.
See Japanese Journal of Applied Physics, Volume 16, Number 4,
April, 1977, pages 601-607. While such narrower stripe geometry may
be used in a multistripe configuration, higher order transverse
modes may appear at higher current levels and a variable range of
beam divergence occurs in the far field pattern over a wide range
of pumping currents.
OBJECT AND SUMMARY OF THE INVENTION
It is the general object of this invention to improve the power
output level of semiconductor junction laser devices.
It is a further object of this invention to provide a monolithic
laser device that has a plurality of emitting regions which provide
a narrow beam divergence over a wide range of pumping currents.
In accordance with the present invention, means are provided to
deflect and couple a portion of optical waves established in the
active layer in a monolithic laser device into other spatially
displaced regions of the active layer. Such means provides for a
portion of the optical waves established in any one emitting region
or cavity of the active layer to be deflected and coupled into one
or more adjacent or spatially established emitting cavities of the
active layer. This strong direct deflective coupling of light into
other regions or into established emitting cavities in the active
layer of the laser device provides for (1) better coherence
resulting in lower beam divergence in the far field optical
interference fringe pattern over a wide range of pumping currents
and (2) more uniform simultaneous attainance of filament
establishment among the several emitters for given current
threshold.
By direct light deflective coupling, it is meant that some portion
of the light wave is directly split off, stripped, redirected,
steered or deflected from one emitting region of the active layer
to an adjacent emitting region.
In all embodiments to be hereinafter described, the deflection
means employed provides a refractive index change in the laser
device, the effect of which is to provide direct light deflective
coupling as just defined.
Direct light deflective coupling of the emitting cavities can be
accomplished by interconnecting current confining geometry, or by
interconnecting impurity profile, or by multichannel geometry in
the substrate of the device, or by a deflection grating or by
current confining geometry positioned at an angle relative the
cleaved ends of the device.
Other objects and attainments together with a fuller understanding
of the invention will become apparent and appreciated by referring
to the following description and claims taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a perspective schematic diagram of a monolithic injection
laser having multi-emitter capability with current confining means
to provide direct light deflection according to the invention.
FIG. 1a is a perspective schematic diagram of a monolithic
injection laser having multi-emitter capability with an impurity
profile to provide direct light deflection according to the
invention.
FIG. 2 is a perspective schematic diagram of a multi-channeled
substrate injection laser having multi-emitter capability and light
deflection means according to the invention.
FIG. .[.2.]. .Iadd.3 .Iaddend.is a partial perspective view of the
detail of the multi-channeled substrate of the laser shown in FIG.
2.
FIG. 4 is a perspective schematic diagram of a mesa injection laser
having multi-emitter capability and light deflection means
according to the invention.
FIGS. 5a to 5h are schematic illustrations of different contact
stripe geometries that may be employed as means to provide
deflective optical coupling among multi-emitting cavities of a
monolithic injection laser device. FIGS. 5e and 5f show, in
addition, a deflection grating to accomplish such coupling.
FIG. 6 illustrates the pulsed output power in milliwatts per facet
for the laser shown in FIG. 1.
FIG. 7 illustrates the far field radiation pattern along the p-n
junction plane for the laser shown in FIG. 1.
FIG. 8 illustrates the half power beam divergence angle along the
p-n junction plane for several types of multi-emitter injection
lasers, including the injection laser shown in FIG. 1, as a
function of the ratio of driving current over threshold
current.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The laser devices to be now described are of the double
heterostructure type. However, the means of deflective light
coupling disclosed may be included in other laser devices, such as,
the distributed feedback type, the buried heterostructure type, the
single heterostructure type, the homojunction type, the large
optical cavity type, the twin guide type, the transverse junction
stripe type or others well known in the art.
Referring to FIG. 1, there is schematically shown in monolithic
laser device 10 in accordance with one illustrative embodiment of
this invention. The fabrication of this device 10, as well as other
laser structures hereinafter described, may be fabricated by liquid
phase epitaxy, molecular beam epitaxy or metalorganic processes,
which techniques are known in the art. Deposited on substrate 12
are layers 14, 16, 18 and 20 may comprise, respectively, n-GaAs;
n-Ga.sub.1-z Al.sub.z As; p-Ga.sub.1-y Al.sub.y As, p-Ga.sub.1-x
Al.sub.x As and n-GaAs, where x and z are greater than y and x and
z may be equal. For example, layers 14 and 18 may be, respectively,
n-Ga.sub.0.65 Al.sub.0.35 As and p-Ga.sub.0.65 Al.sub.0.35 As and
layer 16 may be p-Ga.sub.0.9 Al.sub.0.1 As, making this the active
layer with the highest index of refraction and the lowest bandgap
to provide a waveguide for light wave propagation under lasing
conditions along the plane of the p-n heterojunction 22. Layers 14,
18, 20 may be approximately 2 .mu.m thick while active layer 16 may
be 0.1 .mu.m thick.
As well recognized in the art, the conductivity type of these
layers may be reversed, which is also true for later described
embodiments.
Fabrication of the device 10 is completed by depositing a silicon
nitride layer 24 on layer 20. The desired contact stripe geometry
is provided first by forming the geometry through a
photolithographic mask by means of plasma etching. This is followed
by zinc diffusion through n-type layer 24 into layer 18, as
illustrated at 28 in FIG. 1. This diffusion helps confine the
current during pumping.
Conductive layer 26 is then deposited on the selectively etched
layer 24 to provide a metallization for electrode connection and
current pumping. Layer 26 may be gold and chromium. Also the bottom
surface of substrate 12 metallized to provide a contact for the
other electrode connection. This deposited conductive layer 30 may
be a gold-tin alloy.
After contact deposition, the ends of device 10 are cleaved to a
desired length, such as 375 .mu.m.
The current confining channel geometry shown in FIG. 1 comprises
ten parallel contact stripes 32. The number of stripes 32 is
significant from the point of desired power output. Increase in the
number of stripes 32 will proportionally increase the optical power
output. Also the higher the number of emitting cavities 33, the
higher the obtainable peak power output and the lower the
divergence angle of the resultant beam in the far field.
Upon current pumping of device 10, emitting cavities 33 are
produced in active layer 16 below each contact stripe 32.
Stripe separation 36 from stripe center to center may typically be
from 2 .mu.m to 25 .mu.m and stripe width 34 may be from 1 .mu.m to
6 .mu.m. Fabrication of the device of FIG. 1 has been done with
stripe widths 34 of 3 .mu.m and stripe separations 36 of 10
.mu.m.
If the stripes 32 are closely spaced, that is approximately 8 .mu.m
or less, optical coupling will occur because the lateral extent of
each fundamental transverse mode in the emitting cavities 33 under
each contact stripe 32 will overlap to couple a portion of the
light wave generated under one stripe 32 into an adjacent contact
stripe 32. As a result, phase locking of the operating modes of all
generated light waves in the active layer may occur.
However, coherence can be enhanced, reducing beam divergence and
provide a higher power peak output beam by employing with the
monolithic laser device 10, means to directly deflect and couple a
portion of the optical wave developed in each of the lasing
cavities produced in layer 16 into one or more adjacent
cavities.
Such deflection means may take several forms, one of which is shown
in FIG. 1. Each of the contact stripes 32 are directly coupled by
interconnecting contact stripes 38. Interconnecting stripes 38 are
provided in layer 24 during photolithographic and etching processes
carried out for contact stripes 32.
The interconnecting stripes 38 may be curved as shown in FIG. 1 or
may be lateral and straight. However, there are many other
geometrical configurations, as will be explained later in
connection with FIGS. 5a through 5d, that would provide suitable
direct light deflective coupling.
The curved interconnecting contact stripes 38 may have, for
example, a radius of curvature of 1 millimeter from one parallel
contact stripe to an adjacent parallel contact stripe.
The curved interconnecting stripes 38 between adjacent stripes 32
provide very strong optical coupling between the emitting cavities
33 formed in active layer 16. The coupling is strong because
portions of the optical wave in one particular cavity will split
off and be deflected into different spatially displaced emitter
cavities. Such wave portions may be deflected to an adjacent
parallel cavity 33 or deflected into one or more additional
spatially displaced cavities 33. This manner of deflection of
optical wave portions of established optical waves in cavities 33
is also true for all other embodiments to be described
hereinafter.
It is noted that contact stripes 32 need not necessarily be
positioned close together to obtain fundamental mode overlapping to
provide the benefits obtained by interconnecting contact stripes
38. The interconnecting stripes 38 provide deflective coupling of
light to provide the improved structure without the need of close
stripe geometry, such as, 8 .mu.m or less. It is, however,
beneficial to have narrower stripe widths 34 between 2 .mu.m to 4
.mu.m to stabilize fundamental mode operation.
Typical pumping operation of the laser device 10 may be at
300.degree. K. with current pulses having a pulse width at 800
nanoseconds and a frequency at 10 KHz. Current thresholds,
I.sub.th, for the cavities 33 may range between 350 and 450
milliamps with threshold attained for all cavities 33 with 5% of
initial threshold current for the first lasing emitting cavity.
Alternatively, rather current pumping, the laser device may operate
with a greater duty cycle or continuous wave while employing a heat
sink or thermo-electric cooler.
In summary, the multiple stripe layer device 10 provides current
confinement defined by the contact stripes 32 and 38 for producing
a plurality of light beams 40 in the near field and producing far
field optical interference fringe pattern having improved high
power and low beam divergence. With 10 cavities, it has been found
that the power output is also approximately 10 times the power
output of a single emitter cavity. The output power of
approximately 1 watt at 65% differential quantum efficiency with
far field divergence being within a few degrees over a wide range
of pumping currents.
In FIG. 6, the power per facet versus pumping current
characteristics are shown for the laser device 10 of FIG. 1. As
shown, threshold current, I.sub.th, is approximately 400 milliamps.
The curve 42 is linear, exhibiting no current kinks, that is,
abrupt changes in power output unpon uniform increase of pumping
current.
In FIG. 7, the angular far field, optical interference fringe
pattern 44 is shown for the output of the laser device 10 of FIG.
1. This pattern is generated by rotating an apertured light pipe in
an arc parallel to and centered upon the p-n heterojunction 22. The
operating current employed during the generation of pattern 44 was
420 milliamps, about 5% above threshold current. Of interest, is
the fact that there are a plurality of peaks 46. The number of
peaks 46 equals the number of emitting cavities 33 which indicates
the phased locked operation of all of cavities 33.
Also, there are two major diffraction intensity lobes 47 and 48
separated by an angle of approximately 5.6.degree.. These lobes are
at approximately -2.degree. and +4.degree. and are different
diffracted orders arising due to the periodic nature of the
established emitter cavities 33. A minimum intensity point is
observed at 0.degree.. The emission of the largest lobe 48 is
believed due to an inherent phase delay between adjacent cavities
33.
In FIG. 8 shows a comparison of the full half-power width,
divergence angle for three different types of multi-emission laser
devices as a function of pumping current. Curve 50 represents laser
device 10 shown in FIG. 1 having interconnecting contact stripes
38. Curve 52 represents a laser structure similar to that shown in
FIG. 1 except that there are no interconnecting contact stripes 38.
The laser device for curve 52 has the same parallel stripe geometry
as the device 10 of FIG. 1 where stripe width 34 is 3 .mu.m and
stripe separation 36 is 10 .mu.m. Thus, any wave coupling and
resulting phase locked operation depends solely on transverse mode
wave overlapping and not on actual wave deflection and coupling
accomplished in laser device 10.
Curve 54 also represents a laser structure similar to that shown in
FIG. 1 but, like the laser device represented by curve 52, there
are no interconnecting contact stripe geometry. The stripe geometry
comprises a plurality of parallel contact stripes similar to
contact stripes 32 but with a stripe width 34 of 3.5 .mu.m and a
stripe separation 36 of 8 .mu.m. Here, overlapping of the
transverse mode wave is much more pronounced because of smaller
stripe separation as compared to the laser of curve 52.
In FIG. 8, the relative pumping current is represented by the ratio
of actual pumping current, I, over threshold current, I.sub.th.
The half power width, divergence angle is measured in the far field
and is the angle in degrees measured from the cleaved facet of the
laser device that subtends the half width output of the far field
pattern of the type shown in FIG. 7. Of interest is the lower beam
divergence of approximately 1.9.degree. for laser device 10 at
threshold current and maintaining a relatively low divergence angle
(approximately 2.degree. to less than 5.degree.) over a wide range
of pumping currents, i.e., up to about 4.5 times threshold current,
I.sub.th. On the other hand, the range of divergence angle for the
laser devices represented by curves 52 and 54 were generally higher
(except for curve 54 near threshold) and vary much more
significantly over a wide range of pumping currents.
Also significant from FIG. 8 is the fact that, the smaller the
optical coupling between emitting cavities, the less coherence and,
therefore, the wider the beam divergence in the far field.
The establishment of emitting regions or cavities in the active
layer of the laser device may be formed using other fabrication
techniques. They can be formed by diffusion, ion implantation,
chemical etching, preferential crystal growth, sputtering and ion
beam milling.
In FIG. 1a, the laser device 51 has the same substrate and
fabricated layers as laser device 10 of FIG. 1, except that there
is no silicon nitride layer 24 Contact layer 26 uniformly covers
n-type layer 20. In order to provide waveguiding and create optical
cavities 33 in the active layer 16, an impurity profile 53, such as
a diffusion (zinc, for example) or ion implantation, may be
extended through layer 18 to active layer 16. Profile 53 creates a
refractive index change in the plane of layers 16 and 18. Profile
53 as illustrated at the facet 55 of laser device 51 extends
throughout the device. The geometry of the profile is illustrated
by dotted lines 57 on the surface of layer 26. Geometry 57 is the
same as the geometry for the contact stripes 32, 38 shown in FIG.
1. The interconnection of geometry 57 at 59 illustrates the
interconnecting impurity profile within laser device 51 to provide
for direct light deflective coupling among the established emitting
cavities 33.
The impurity profile 53 need only extend downwardly into laser
device 51 to sufficiently permit the light wave established in
emitting cavities 33 to overlap into and interact with the profile.
The profile provides a change in refractive index (both real and
imaginery) which stimulates and guides the light wave according to
geometry 57. Interconnecting profiles at 59 provide a steering
mechanism for portions of the propagating light wave to be
deflected and coupled into one or more adjacent emitting cavities
33.
Referring to FIG. 2, laser device 60 comprises substrate 62 and
sequentially deposited layers 64, 66, 68, 70 and 72. These layers
may, respectively, comprise n-GaAs; n-Ga.sub.0.65 Al.sub.0.35 As;
p-Ga.sub.0.95 Al.sub.0.05 As; p-Ga.sub.0.65 Al.sub.0.35 As; p-GaAs
and layer of metalization of a gold-chromium alloy. A bottom
metallic contact 74 may comprise a gold-tin alloy.
A plurality of parallel channels 76 are ion milled or etched into
substrate 62 prior to growth or deposition of layers 62 through 72.
Channel 76 provide mesas 78 therebetween. As best shown in FIG. 3,
at a point along the length of the channels 76, there is provided a
series of interconnecting channels 80 between adjacent channels 76.
These interconnecting channels may be curved, as in the case of
interconnecting stripes 38 of FIG. 1, or may be transversely
disposed relative to channels 76 as illustrated in FIG. 3. What is
important is that the channels 76 be interconnected in a manner to
split off, deflect and guide a portion of developed optical wave
from one emitter cavity 82 to one or more other adjacent cavities
82.
In laser device 60, T, the thickness of the active layer 66, may
typically be 200A to 0.4 .mu.m or greater; S, the thickness of
layer 64 above mesas 78, may be 0.2 .mu.m; H, the depth of channels
76, may be 1 .mu.m; D, the periodic width of a channel-mesa
combination, may be approximately 10 .mu.m and W, the width of the
channel basin, should be slightly less than D, such as, 8 .mu.m or
less.
Upon current pumping, the portions of the active layer 66 above the
channels 76 will provide a waveguide for light wave propagation
under lasing conditions and, thus, the establishment of emitter
cavities 82. The channels 76 and mesas 78 provide in effect a
transverse refractive index profile along the plane of the p-n
junction 22. The regions above mesas 78 provide areas in layer 64
of less thickness compared to areas in layer 64 above channels 76.
Light waves propagating in emitter cavities 82 are stabilized in
the fundamental transverse mode in the areas above mesas 78 at the
adjacent sides of the established cavities 82. Higher order modes
do not oscillate because they are absorbed into these adjacent
areas and the propagating wave is induced to stay within the
confines of the cavity 82.
The same induced losses and light guiding is also obtained through
interconnecting channels 80 so that portions of optical wave
developed in any particular cavity 82 is split off and deflected
into one or more adjacent cavities 82 bringing about phased locked
operation and stronger coherence.
The laser device 90, shown in FIG. 4, is similar in semiconductor
material and current channeling as shown in FIG. 1 except there are
only four emitting cavities 91 rather than ten and the device is
fabricated to provide spacing 102 between adjacent cavities. The
fabrication of laser device 90 is as follows. Layers 92, 96, 98 and
100 are sequentially deposited using conventional techniques as
previously indicated. No isolating or contact layers need be formed
on layer 100. Using conventional photolithographic techniques, a
mask is prepared on layer 100, exposed and thereafter controlled
etching provides spacings 102. The depth 104 of spacings 102 is
controlled to be established within layer 94 adjacent to active
layer 96. The resulting structure appears as a plurality of
parallel mesa structures 101 coupled by interconnecting mesa
structures 103.
Appropriate contacts can be provided on layer 100 and on the bottom
of substrate 92. The resulting structure, upon current pumping,
will operate in the same manner as laser device 10. Spacings 102
provide better current confinement properties than possibly
obtainable in connection with laser device 10 of FIG. 1.
The spacing 102 between provides for a medium, air, which has a low
index of refraction than active layer 96. This spacing may also be
filled with a semiconductor material deposited during a second
sequence of growth. The material chosen should have a lower index
of refraction than active layer 96.
It is of interest to mention at this point that the laser devices
10 and 90 provide current confining channels to establish the
emitting cavities while laser device 51 provides a impurity profile
to establish emitting cavities and laser device 60 provides a
material thickness change to establish the emitting cavities. In
any case, what is effectively being accomplished is a change in the
refractive index (both the real and imaginary parts) at regions
where the emitting cavities are created. The stripe geometry of
laser device 10 can be combined with the identical channel geometry
of laser device 60. Also, other types of current confining
channels, such as, buried heterostructure geometry, buried stripe
geometry, substrate stripe and implanted stripe, may be employed in
the substrate of these laser devices to provide direct deflective
coupling. Such buried stripe fabrication is disclosed in U.S. Pat.
No. 4,099,999 issued July 11, 1978 and assigned to the assignee
herein.
There are many ways of employing interconnecting current confining
channel geometry as a means to deflect and couple portions of split
off optical waves among the various emitting cavities. In FIGS. 5a
through 5d, various examples of interconnecting geometry is
illustrated. IN FIG. 5a, the parallel contact stripes 106 are
interconnected by means of criss-cross interconnecting stripes 108.
In this embodiment, deflection may occur in either direction of
light wave propagation. In FIG. 5b, parallel contact stripes 110
are provided with curved sections 112 ending in a single stripe
section 114. These sections 112 and 114 form a y-shaped
interconnecting means. Curved sections 112 aid in establishing
fundamental mode operation and provide means to split off and
deflect portions of the developed optical wave into one or more
adjacent cavities.
In FIG. 5c, the parallel contact stripes 116 are interconnected in
an offset manner along the length of the device by transversely
disposed interconnecting stripes 118. Each interconnecting stripe
118 is displaced relative to an adjacent interconnecting stripe 118
along the length of the laser device.
In FIG. 5d, the parallel contact stripes 120 are interconnected by
a single transverse contact stripe 122. Stripe 122 has a larger
width than individual stripes 120 and may be disposed laterally at
an angle across stripes 120 as well as perpendicular thereto as
shown in the Figure.
The laser device 130 shown in FIGS. 5e and 5f illustrates the use
of a grating as a means of splitting off and deflecting a portion
of the established light wave in the active layer into other
regions of the active layer. Rather than establishing emitting
cavities in the active layer, the entire active layer is pumped.
However, parallel contact stripes could be provided on the surface
of the device 130 to establish separate emitting cavities and the
grating used to deflect and couple light waves from one or more
cavities into one or more other adjacent cavities.
Structurally, laser device 130 may be similar to laser device 10 in
FIG. 1, i.e., layers 132, 134, 136, 138 140, and 142 of device 130
are comparable to layers 12, 14, 16, 18, 20 and 26 of device 10.
Contact layer 142 covers the entire surface of device 130. A
grating pattern 146 is provided, for example in layer 134, adjacent
to active layer 136, to deflect and couple portions of an
established optical light wave in one region of the active layer
into other regions of the active layer 136 in a manner depicted by
arrow 147.
The grating pattern 146 may be disposed at 45.degree. relative to
the cleaved ends 148 of the device 130. However, other angles may
be used as well. The grating period .LAMBDA. is given by
.LAMBDA.=p.lambda.o sin.theta./n.sub.eff where .lambda..sub.o is
the free space laser wavelength, .theta. is the angle of the
grating relative to the lasing beam path, n.sub.eff is the
effective refractive index seen by the laser light and p is an
integer. If the grating is at 45.degree. to the beam direction and
if the laser operates at 8000A with n.sub.eff =3.6, a grating
period of integer multiples of 1571A may be used.
In FIG. 5g, the plurality contact stripes 150 are positioned at an
offset angle .phi. relative to the longitudinal length of the laser
device. The angularity of the stripes 150 is exaggerated for
purposes of explanation. In practice, this angle may be 1.degree.
with the contact stripes having a 2 .mu.m width and an 8 .mu.m
separation from stripe center to center. The length of the device
may be approximately 250 .mu.m.
A portion of the optical wave propagating along an emitting cavity
of the active layer will be deflected, upon light beam reflection
at either cleaved facet 152 of the device, into an adjacent
emitting cavity. This is because the angle of incidence of the
optical wave is not normal to the cleaved facets so that a portion
of the optical wave will be reflected from the mirror surfaces of
facets 152 into an adjacent cavity as depicted by arrow 154.
In FIG. 5h, the contact stripes 156 are not parallel but positioned
angularly relative to each other in a manner that a portion of the
light wave incident to the cleaved facet 158 is reflected and
coupled into an adjoining emitting cavity. This deflective coupling
is accomplished by interconnected stripe geometry 160 at the ends
of stripes 156. Arrow 162 represents portions of light deflected
from one established emitting cavity to an adjoining cavity.
In this embodiment as well as other embodiments of FIGS. 5, rather
than employing current confining channels, such as, contact stripes
158, a diffused or implanted material composition change providing
a refractive index profile corresponding to such channel geometry
sufficiently diffused and implanted into the device to effectively
interact with the light wave will also produce the direct light
deflective coupling provided by these channel geometries.
Although in all the foregoing illustrations, the means to provide
deflective coupling have been illustrated in an equally spaced
manner, they may be positioned to have variable or unequal spacing
to provide beam focusing or to provide side lobe suppression
thereby enhancing the fundamental lobe (such as, lobe 48 at
+4.degree. in FIG. 7) in the far field.
Also, the current confining channels need not be defined by oxide
or contact stripes as shown in the Figures. Any other well known
current confining technique such as ion implantation, diffusion,
substrate stripes, planar stripes, mesa stripe, internal strip
transverse junction stripe etc. may be used.
Also, as shown in the Figures the deflective means for coupling
light from one region of the active layer to another has been shown
to be present near or within the active layer. However, the
deflective means could be removed from close proximity to the
active layer by coupling light from the active layer into a
transparent waveguide layer. A number of methods for coupling light
into transparent waveguide layers such as twin guide lasers, taper
coupled lasers and others are well known in the art. Once in the
transparent waveguide, the light deflective means such as is
provided by a refractive index change could be used to deflect the
light back into other spatially displaced regions of the active
layer thus allowing for strong optical coupling of spatially
separated regions of the active layer.
Although all the foregoing embodiments have been described in
connection with semiconductor materials of GaAs and GaAlAs, other
light emitting materials may be employed, such as InGa, AsP, GaAlP,
GaAlSb, and PbSnTe.
While the invention has been described in conjunction with specific
embodiments, it is evident that many alternatives, modifications
and variations will be apparent to those skilled in the art in
light of the foregoing description. Accordingly, it is intended to
embrace all such alternatives, modifications, and variations as
fall within the spirit and scope of the appended claims.
* * * * *