U.S. patent application number 10/689544 was filed with the patent office on 2004-07-15 for injection laser.
Invention is credited to Shveykin, Vasily I..
Application Number | 20040136424 10/689544 |
Document ID | / |
Family ID | 20209064 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040136424 |
Kind Code |
A1 |
Shveykin, Vasily I. |
July 15, 2004 |
Injection laser
Abstract
An injection laser comprises a laser heterostructure, at least
one radiation inflow region adjoining the laser heterostructure,
and reflectors that together form an optical resonator. The laser
heterostructure comprises an active layer, cladding layers, and
ohmic contacts. The radiation inflow region adjoining the laser
heterostructure is transparent to the laser radiation and is
located on at least one side of the active layer. The radiation
inflow region additionally includes at least one optical facet, an
outer surface, and an inner surface. Radiation generated in the
active layer flows into the radiation inflow region and exits the
laser.
Inventors: |
Shveykin, Vasily I.;
(Moscow, RU) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
20209064 |
Appl. No.: |
10/689544 |
Filed: |
October 20, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10689544 |
Oct 20, 2003 |
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09781017 |
Feb 9, 2001 |
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09781017 |
Feb 9, 2001 |
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PCT/RU99/00275 |
Aug 5, 1999 |
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Current U.S.
Class: |
372/44.01 |
Current CPC
Class: |
H01S 5/10 20130101; H01S
5/18 20130101; H01S 5/142 20130101; H01S 5/1085 20130101; H01S
5/4025 20130101; H01S 5/14 20130101; H01S 5/2027 20130101; H01S
5/141 20130101 |
Class at
Publication: |
372/044 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 10, 1998 |
RU |
98114581 |
Claims
What is claimed is:
1. An injection laser comprising at least one gain region having a
longitudinal gain axis and outputting laser radiation at an outflow
angle .phi., said injection laser comprising: a laser
heterostructure comprising: an active layer forming said at least
one gain region; cladding layers comprising at least one layer
having a refractive index, and ohmic contacts; and at least one
radiation inflow region adjoining said laser heterostructure that
is transparent to said laser radiation, has a refractive index
n.sub.RIR, and is located on at least one side of said active
layer, said radiation inflow region including at least one optical
facet, an outer surface, and an inner surface, said optical facet
being oriented at an angle of inclination .psi. with respect to a
plane perpendicular to said longitudinal gain axis; and reflectors
that together form an optical resonator at least part of which
coincides with at least part of said radiation inflow region and at
least part of said gain region, wherein said laser heterostructure
and said adjoining radiation inflow region together have an
effective refractive index n.sub.eff such that n.sub.RIR exceeds
n.sub.eff; arccos(n.sub.eff/n.sub.R-
IR).ltoreq.arccos(n.sub.eff-min/n.sub.RIR); and n.sub.eff-min is
greater than n.sub.min, where n.sub.eff-min is the minimum value of
n.sub.eff for laser heterostructures with radiation inflow regions
that produce outflow of radiation from the active layer into the
radiation inflow region, and n.sub.min is the smallest of the
refractive indices in the cladding layers of the
heterostructure.
2. The injection laser of claim 1, wherein said active layer
comprises at least one sublayer.
3. The injection laser of claim 2, wherein said cladding layers are
respectively disposed on a first surface and on an opposite second
surface of said active layer and comprise cladding sublayers
I.sub.i and II.sub.j, respectively, with refractive indices
n.sub.Ii and n.sub.IIi and bandgaps E.sub.Ii and E.sub.IIi,
respectively, where i=1, 2, . . . , k and j=1, 2, . . . , m are
defined as integers that designate the sequential numbers of the
cladding sublayers counted from the active layer with at least one
cladding sublayer within each cladding layer.
4. The injection laser of claim 3, wherein at least one of said
cladding sublayers disposed between said active layer and said
radiation inflow region is electrically conductive and has an ohmic
contact formed therewith.
5. The injection laser of claim 4, wherein at least two of said
cladding sublayers disposed between said active layer and said
radiation inflow region comprise electrically conducting
semiconductors having bandgaps of varying sizes, said ohmic contact
being formed with said sublayer having the smallest bandgap.
6. The injection laser of claim 3, wherein that at least one of
said cladding sublayers has a refractive index at least as large as
the refractive index n.sub.RIR of said radiation inflow region.
7. The injection laser of claim 1, wherein said at least one
optical facet is oriented so as to subtend an acute angle with the
inner surface.
8. The injection laser of claim 1, wherein said laser
heterostructure includes barrier regions.
9. The injection laser of claim 1, wherein said gain region
comprises a strip-type gain region.
10. The injection laser of claim 1, wherein: said gain region has a
length L.sub.GR and a width W.sub.GR; and said inner surface of the
radiation inflow region has a length L.sub.RIR and a width
W.sub.RIR respectively at least as large as the length L.sub.GR and
width W.sub.GR of said gain region.
11. The injection laser of claim 10, wherein said radiation inflow
region includes a first portion having a thickness no greater than
W.sub.GR that borders said laser heterostructure and is
electrically conductive, said radiation inflow region having a
second portion that comprises material having an optical loss
factor .alpha..sub.RIR of no more than about 0.1 cm.sup.-1.
12. The injection laser of claim 11, wherein an ohmic contact is
formed with said electrically conductive portion of said radiation
inflow region.
13. The injection laser of claim 1, wherein said radiation inflow
region has a thickness ranging between about 2 to about 50,000
micrometers (.mu.m).
14. The injection laser of claim 1, wherein said radiation inflow
region comprises an optically homogeneous material.
15. The injection laser of claim 14, wherein said radiation inflow
region comprises a semiconductor having a bandgap E.sub.RIR (eV)
and said active layer comprises a semiconductor having bandgap
E.sub.a (eV), said bandgap E.sub.RIR (eV) exceeding said bandgap
E.sub.a (eV) of the active layer by more than about 0.09 eV.
16. The injection laser of claim 1, wherein said radiation inflow
region comprises a substrate.
17. The injection laser of claim 1, wherein said radiation inflow
region is electrically conductive.
18. The injection laser of claim 17, wherein an ohmic contact is
formed with said radiation inflow region.
19. The injection laser of claim 1, wherein a portion of said
radiation inflow region comprises a plurality of layers oriented
parallel to said inner surface, said plurality of layers comprising
materials having different refractive indices.
20. The injection laser of claim 1, wherein said radiation inflow
region has an optical loss factor .alpha..sub.RIR of no more than
about 0.1 cm.sup.-1.
21. The injection laser of claim 1, wherein: said gain region is
bounded at two opposite ends by two side surfaces; at least one of
said side surfaces is inclined at an angle with respect to said
inner surface, said angle being identical to an angle formed
between one of said adjacent optical facets with said inner
surface; and said side surface has a reflection coefficient equal
to that of said adjacent optical facet.
22. The injection laser of claim 1, wherein said at least one
optical facet comprises one of said reflectors of said optical
resonator and is formed such that said angle of inclination .psi.
is equal to the outflow angle .phi., which is equal to
arccos(n.sub.eff/n.sub.RIR).
23. The injection laser of claim 1, wherein: said at least one
optical facet is oriented such that said angle of inclination .psi.
is equal to (.pi./4)-(.phi./2) where
.phi.=arccos(n.sub.eff/n.sub.RIR); and at least part of said outer
surface of the radiation inflow region that coincides with a
projection of said optical facet formed thereon forms one of said
reflectors of the optical resonator.
24. The injection laser of claim 1, wherein: said at least one
optical facet is oriented such that said angle of inclination .psi.
is equal to (.pi./4)+(.phi./2), where
.phi.=arccos(n.sub.eff/n.sub.RIR); and at least part of one surface
of said injection laser opposite to said inflow region that
coincides with a projection of said optical facet formed thereon,
forms one of said reflectors of said optical resonator.
25. The injection laser of claim 1, wherein one of said optical
facets is oriented such that said angle of inclination .psi. is
equal to zero.
26. The injection laser of claim 25, wherein said optical facet
comprises a reflective coating formed thereon.
27. The injection laser of claim 1, wherein at least one of said
reflectors of said optical resonator comprises an external
reflector.
28. The injection laser of claim 27, wherein said radiation inflow
region comprises two optical facets each optical facet oriented
such that said angle of inclination .psi. with respect to said
plane perpendicular to said longitudinal gain axis is equal to
zero.
29. The injection laser of claim 28, wherein at least one of said
reflectors of said optical resonator comprises a cylindrical
mirror.
30. The injection laser of claim 27, wherein at least one of said
reflectors of said optical resonator comprises a plane mirror.
31. The injection laser of claim 27, wherein at least one of said
reflectors of said optical resonator comprises a diffraction
grating.
32. The injection laser of claims 1, wherein at least one of said
reflectors of said optical resonator comprises a spherical
mirror.
33. The injection laser of claim 1, comprising at least two gain
regions each having a longitudinal gain axis and being disposed
adjacent an inner surface of at least one radiation inflow
region.
34. The injection laser of claim 33, wherein an independent ohmic
contact is formed with each of said gain regions, each ohmic
contact positioned opposite said radiation inflow region.
35. The injection laser of claim 33, comprising at least two
sequences of gain regions and a common radiation inflow region for
each sequence of gain regions, each sequence of gain regions
comprising at least two gain regions, said longitudinal gain axis
of each gain region in each sequence being parallel to each other
and being disposed at right angles to a line of intersection
between active layers and an extension of a plane of an optical
facet of said common radiation inflow region.
36. The injection laser of claim 35, further comprising ohmic
contacts adjoining strips of metallization for respective sequences
of gain regions, said strips of metallization formed on at least
part of the outer surface of at least one of said common radiation
inflow regions.
37. The injection laser of claim 35, further comprising independent
ohmic contacts formed with strips of metallization that are
positioned parallel to said gain axes of said gain regions and that
are insulated from each other, said strips of metallization being
located opposite said radiation inflow region.
38. The injection laser of claim 35, wherein said gain regions are
formed along at least one line parallel to said longitudinal gain
axes of the gain regions.
39. The injection laser of claim 38, wherein: said radiation inflow
region has an outer surface and a thickness d.sub.RIR, said gain
regions having leading edges closest to one side of said
heterostructure, said gain regions being separated by a distance
equal to 2d.sub.RIR/tan .phi. when measured from respective leading
edges of said gain regions; and at least part of said outer surface
coinciding with a projection of said gain region onto it is
optically reflective.
40. The injection laser of claim 35, wherein: at least two adjacent
gain regions are electrically isolated by a nonconductive part of
said radiation inflow region, and ohmic contacts associated with
each of said gain regions are electrically coupled by a
metallization layer.
41. The injection laser of claim 1, further comprising at least two
gain regions each having longitudinal gain axis and adapted to
output light at identical outflow angles .phi., said gain region
formed on opposite surfaces of said radiation inflow region along
two lines that are parallel to each other and to said longitudinal
gain axes, said gain regions having leading edges nearest to a
common side of said heterostructure, said gain regions on opposite
sides of the inflow region being separated by a distance of
d.sub.RIR/sin .phi. when measured from said leading edges, where
d.sub.min is the thickness of said radiation inflow.
42. The injection laser of claim 1, further comprising a
semiconductor layer formed on said radiation inflow region, said
semiconductor layer having a refractive index less than said
effective refractive index n.sub.eff.
43. The injection laser of claim 42, wherein said semiconductor
layer has a refractive index approximately equal to n.sub.min.
44. The injection laser of claim 1, wherein said radiation inflow
region has a thickness d.sub.RIR in a range between about 2 and 100
micrometers.
45. The injection laser of claim 1, wherein said at least one said
gain region flares outward from a portion having a first width to a
portion having a second width larger than said first width.
46. The injection laser of claim 1, wherein said at least one gain
region comprises at least three gain regions, said at least three
gain regions including a middle gain region surrounded by first and
second end gain regions.
47. The injection laser of claim 46, wherein one of said cladding
layers is thicker along a portion adjacent said middle gain region
than along portions adjacent said first and second end gain
regions.
48. The injection laser of claim 46, wherein both of said cladding
layers has a substantially identical thickness along said middle
gain region and said first and second end gain regions.
49. The injection laser of claim 46, wherein said ohmic contacts
comprise at at least a first ohmic contact associated with said
middle gain region, a second ohmic contact associated with said
first end gain region, and a third ohmic contact associated with
said second end gain region.
50. The injection laser of claim 46, wherein said ohmic contacts
include one continuous ohmic contact associated with the middle
gain regain and the first and second end gain regions.
51. The injection laser of claim 46, wherein said radiation inflow
region comprises a rectangular parallelepiped.
52. The injection laser of claim 46, further comprising a
semiconductor layer having a refractive index less than said
effective refractive index n.sub.eff formed on said radiation
inflow region.
53. The injection laser of claim 46, wherein said at least one
radiation inflow region comprises at least one radiation inflow
region on each of opposite sides of said active layer.
54. The injection laser of claim 53, further comprising
semiconductor layers having refractive indices less than said
effective refractive index n.sub.eff formed adjacent each of said
radiation inflow regions.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/718,017, filed on Feb. 9, 2001, which is a
continuation-in-part of International Application No.
PCT/RU99/00275 filed at the Russian Receiving Office on Aug. 5,
1999, published in a language other than English under PCT Article
21(2) as WO 00/10235 on Feb. 24, 2000. Accordingly, priority under
.sctn.365(c) is claimed to International Application No.
PCT/RU99/00275 filed Aug. 5, 1999. Additionally, priority under
.sctn.119 is claimed to Russian Patent Application No. 98114581
(now U.S. Pat. No. 2,142,665) filed Aug. 10, 1998.
FIELD OF THE INVENTION
[0002] The invention relates to quantum electronic technology, and
more specifically to efficient semiconductor sources of radiation
with a narrow radiation patterns.
BACKGROUND OF THE INVENTION
[0003] The injection laser (hereinafter referred to as "the laser")
is a device that converts electrical energy into the light
possessing a narrow spectral composition and high directivity.
[0004] Different types of lasers are known: lasers with a
strip-type active lasing region and with radiation output through
the mirror of an optical resonator (S. S. Ou et. al., Electronics
Letters (1992), Vol. 28, No. 25, pp. 2345-2346),
distributed-feedback lasers (Handbook of Semiconductor Lasers and
Photonic Integrated Circuits, edited by Y. Suematsu and A. R.
Adams, Chapman-Hill, London, 1994, pp. 44-45 and 393-417), laser
amplifiers, including a master oscillator power amplifier (MOPA)
(IEEE J. of Quantum Electronics (1993), Vol. 29, No. 6, pp.
2052-2057), and lasers with curved resonators and radiation output
through a surface (Electronics Letters (1992), Vol. 28, No. 21, pp.
3011-3012). Further expansion of the applications of such lasers is
impeded by insufficiently high output power, efficiency, operating
life, and reliability, including situations with monomode
lasing.
[0005] The laser described in U.S. Pat. No. 4,063,189, issued to D.
R. Scifres et al. in 1977 includes a laser heterostructure
(hereinafter referred to as a "heterostructure") that contains an
active layer of GaAs positioned between two optically homogeneous
cladding layers. The gain region of the operating laser, of length
L.sub.GR, in practice coincides with the thick active layer into
which non-equilibrium carriers are injected by means of ohmic
contacts. As the term is conventionally employed, a gain region is
that part of the heterostructure which includes the active layer
and from which radiation is spreading in a activated laser. The
gain region, hereinafter referred to as "the GR", is the medium of
the optical resonator. The length of the GR along the longitudinal
gain axis is bounded by flat end surfaces that act as reflectors.
The length L.sub.OR of the optical resonator (Fabry-Perot)
coincides with the length L.sub.GR, so that the ratio
.mu.=L.sub.OR/L.sub.GR (1)
[0006] is equal to one. Reflective coatings with a coefficient of
reflection close to one (hereinafter referred to as "reflective
coating") are applied to the reflectors of the optical resonator.
The radiation inflow region (hereinafter referred to as "RIR"), as
which a substrate of GaAs is used, borders on a surface of one of
the cladding layers that is distant from the active layer. The
inner surface of the RIR, whose area is equal to the area of the
GR, is located on the cladding layer adjacent to the RIR. The flat
optical facets of the RIR are a continuation of the planes of the
reflectors of the optical resonator and are perpendicular to the
longitudinal gain axis of the GR. A coating with a reflection
coefficient close to zero (hereinafter referred to as
"antireflective coating") is applied to one of the optical facets
(hereinafter referred to as "the facet"), while a reflective
coating is applied to the other facet. The facet with the applied
anti-reflective coating is the output surface. The RIR is made
electrically conductive, and an ohmic contact is made with its
outer surface, which is opposite the inner surface. Another ohmic
contact is made from the direction of the heterostructure.
[0007] When direct current is supplied to the laser, nonequilibrium
carriers are injected into the active layer, and induce the
generation of radiation of a specified wavelength .lambda. and mode
composition in the medium of the optical resonator. Part of the
laser radiation from the GR exits the laser through the RIR. The
angle of outflow of radiation is define by the following
equation:
.phi.=arccos(n.sub.eff/n.sub.RIR) (2)
[0008] (see J. K. Buttler et al., IEEE Journ. of Quant. Electron.
(1975), Vol. QE-11, p. 402). Note that the use of identical
compositions for the active layer and RIR (both made of GaAs)
restricted the range of ratios n.sub.eff/n.sub.RIR from more than
0.9986 to 1, and the outflow angle .phi. to the range from
3.degree. to 0.degree., respectively.
[0009] The following basic parameters were obtained for the
fabricated laser (see D. R. Scifres et al., U.S. Pat. No.
4,063,189, 1977, as well as D. R. Scifres et al., Applied Physics
Letters (1976), Vol. 29, No. 1, pp. 23-25): a threshold current
density j.sub.thr of 7.7 kA/cm.sup.2, a threshold current J.sub.thr
of 7.0 A for a length L.sub.OR of 400 .mu.m, a short-pulse output
power of 3 W, a differential efficiency on the order of 35-40%, and
an angle of divergence .THETA..sub.1 of 2.degree. in the vertical
plane for laser radiation output through the face. The vertical
plane referred to is the plane that passes through the longitudinal
gain axis and that is perpendicular to the active layer. The
corresponding horizontal plane is perpendicular to the vertical
plane.
SUMMARY OF THE INVENTION
[0010] An injection laser comprising at least one gain region
having a longitudinal gain axis and outputting laser radiation at
an outflow angle .phi. comprises a laser heterostructure, at least
one radiation inflow region adjoining the laser heterostructure,
and reflectors that together form an optical resonator. The laser
heterostructure comprises an active layer, which forms at least one
of the gain regions, cladding layers comprising at least one layer
having a refractive index, and ohmic contacts. The radiation inflow
region adjoining the laser heterostructure is transparent to the
laser radiation, has a refractive index n.sub.RIR, and is located
on at least one side of the active layer. The radiation inflow
region additionally includes at least one optical facet, an outer
surface, and an inner surface, the optical facet being oriented at
an angle of inclination .psi. with respect to a plane perpendicular
to the longitudinal gain axis. At least part of the optical
resonator coincides with at least part of the radiation inflow
region and at least part of the gain region. The laser
heterostructure and the adjoining radiation inflow region together
have an effective refractive index n.sub.eff such that
[0011] n.sub.RIR exceeds n.sub.eff,
[0012]
arccos(n.sub.eff/n.sub.RIR).ltoreq.arccos(n.sub.eff-min/n.sub.RIR),
and
[0013] n.sub.eff-min is greater than n.sub.min,
[0014] where n.sub.eff-min is the minimum value of n.sub.eff for
laser heterostructures with radiation inflow regions that produce
outflow of radiation from the active layer into the radiation
inflow region, and n.sub.min is the smallest of the refractive
indices in the cladding layers of the heterostructure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIGS. 1-7 schematically depict longitudinal sections (along
the optical gain axis of the GR) of different designs of lasers
with GR end surfaces made in the form of planes that extend the
planes of the corresponding facets of the RIR, and with one-way
radiation output.
[0016] Specifically, in FIGS. 1-3, the facets of the RIR are
implemented as reflectors of the optical resonator;
[0017] in FIG. 1, the laser has two inclined facets all shown with
a positive angle of inclination .psi. equal to .phi.;
[0018] in FIG. 2, the laser includes an external reflector in the
form of a plane mirror to one of the inclined facets; and
[0019] in FIG. 3, the laser has one facet with an angle of
inclination .psi. equal to zero;
[0020] FIGS. 4-7 depict lasers wherein parts of the outer surface
of the RIR that are implemented as reflectors of the resonator;
[0021] in FIG. 4, the laser has two inclined facets of the RIR with
negative angles of inclination .psi. equal to (.pi./4-.phi./2),
[0022] in FIG. 5, the laser has an external reflector in the form
of a diffraction grating to one of the inclined facets;
[0023] in FIG. 6, the laser has a facet with an angle of
inclination .psi. equal to zero; and
[0024] in FIG. 7, the laser has one inclined facet of the RIR with
a positive angle of inclination and another inclined facet with a
negative angle of inclination.
[0025] FIGS. 8-9 schematically depict longitudinal sections of
designs with parts of the outer surface of the heterostructure that
are implemented as reflectors of the optical resonator;
[0026] in FIG. 8, the laser has two inclined facets with angles of
inclination .psi. equal to (.pi./4+.phi./2); and
[0027] in FIG. 9, the laser has one inclined facet with an angle of
inclination .psi. equal to (.pi./4+.phi./2), and the other with an
angle of inclination .psi. equal to zero.
[0028] FIG. 10 schematically depicts a longitudinal section of the
design of a laser with two facets with angles of inclination .psi.
equal to zero, and with outer inclined reflectors in the form of
plane mirrors.
[0029] FIGS. 11-13 schematically depict cross-sections of lasers
with ohmic contacts positioned differently with respect to the
RIR;
[0030] in FIG. 11, the ohmic contact is on the outer surface of the
RIR;
[0031] in FIG. 12, the ohmic contact is located on the electrically
conductive sublayer of the cladding layer adjacent to the RIR;
and
[0032] in FIG. 13--the ohmic contact is on the electrically
conductive part (layer) of the RIR adjacent to the
heterostructure.
[0033] FIGS. 14-15 schematically depict a longitudinal section
(FIG. 14) and perpendicular cross-section (FIG. 15) of the lasers
with six GRs that are galvanically series-parallel connected and
have a single common RIR.
[0034] FIGS. 16-18 schematically depict longitudinal sections, and
FIG. 19 depicts a perpendicular cross-section of the laser with a
multiplicity of GRs independently galvanically controllable, with
separate RIRs for three different sequences of GR;
[0035] in FIG. 16, each laser has two inclined facets on the RIR
with positive angles of inclination (see FIG. 9) and the output of
laser radiation is in the direction perpendicular to the plane of
the active layer;
[0036] in FIG. 17, each laser has one inclined facet forming part
of the optical resonator with a positive angle of inclination and a
second inclined facet with a negative angle of inclination (see
FIG. 8) and the output of laser radiation is in the direction
perpendicular to the plane of the active layer; and
[0037] in FIG. 18--each laser has two inclined facets on the RIR
with positive angles of inclination (see FIG. 1) and the output of
laser radiation is at an angle .phi. with respect to the plane of
the active layer.
[0038] FIGS. 20-21 schematically depict longitudinal sections of
the laser, with GRs that are galvanically series connected and that
are disposed along their own gain axes and to a separate RIR;
[0039] in FIG. 20, the three GRs is on one surface of the RIR;
and
[0040] in FIG. 21, the placement of four GRs is on two opposite
surfaces of the RIR.
[0041] FIG. 22 schematically depicts the longitudinal section of
the laser with two inclined facets and three GRs with a different
thickness of the cladding layer for the middle GR and the end
GRs.
[0042] FIG. 23 schematically depicts the longitudinal section of
the laser with two inclined facets and three GRs with an identical
thickness of the cladding layer for the middle GR and the end
GRs.
[0043] FIG. 24 schematically depicts the longitudinal section of
the laser in the shape of a rectangular parallelepiped with one
thin-layer RIR and three GRs and with a different thickness of the
cladding layer for the middle GR and the end GRs.
[0044] FIG. 25 schematically depicts the longitudinal section of
the laser in the shape of a rectangular parallelepiped with two
thin-layers RIR on both sides of the active layer and with an
identical thickness of the cladding layer for the middle GR and the
end GRs.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0045] Embodiments of the invention will now be described with
reference to the accompanying Figures, wherein like numerals refer
to like elements throughout. The terminology used in the
description presented herein is not intended to be interpreted in
any limited or restrictive manner, simply because it is being
utilized in conjunction with a detailed description of certain
specific embodiments of the invention. Furthermore, embodiments of
the invention may include several novel features, no single one of
which is solely responsible for its desirable attributes or which
is essential to practicing the inventions herein described.
[0046] One preferred embodiment of the invention comprises a laser
with a lower current-density threshold, increased differential
efficiency, reduced astigmatism and angles of divergence of the
output radiation in the vertical and horizontal planes. This laser
possesses enhanced spectral characteristics of the laser radiation,
an expanded range of direction of laser-radiation output, and
increased effective length of the optical resonator. In the
aggregate, the result is increased power, efficiency, operating
life, and reliability. This laser may comprise a multibeam laser.
Note also that the technology for fabricating this laser is
simplified.
[0047] The laser comprises an injection laser which includes a
laser heterostructure that contains an active layer and cladding
layers, in which it is possible to form a gain region. The laser
also includes reflectors, which form an optical resonator, ohmic
contacts, and on at least one side of the active layer a radiation
inflow region (RIR) whose inner surface borders on the
corresponding cladding layer. The condition for radiation outflow
from the active layer into the radiation inflow region is fulfilled
if the refractive index n.sub.RIR of the radiation inflow region
exceeds the effective refractive index n.sub.eff for the aggregate
consisting of the laser heterostructure and the adjoining radiation
inflow region. This condition is fulfilled when
arccos(n.sub.eff/n.sub.RIR) is greater than zero. Accordingly, the
composition and thicknesses of the active layer, cladding layers,
and RIR are selected so that the condition for outflow of radiation
from the active layer into the RIR is fulfilled, i.e., that the
refractive index n.sub.RIR of the RIR exceeds the effective
refractive index n.sub.eff of the heterostructure and of RIR 7 or
.phi.=arccos(n.sub.eff/n.sub.RIR)>- 0. At least part of the
medium of the optical resonator is made of at least part of the
inflow region and of at least part of the gain region. At least one
of the reflectors of the optical cavity is made with a reflection
coefficient selected from the range greater than zero and less than
one. Lasing occurs during operation of this injection laser which
includes at least one inflow region transparent to laser radiation
produced by the laser. In addition, from at least one part, the at
least one gain region formed is characterized by a gain
G.sub.outflow (cm.sup.-1) of outflowing radiation from the active
layer into the inflow region, the latter being characterized by an
introduced threshold loss factor .alpha..sub.RIR-thr (cm.sup.-1).
The value of the gain G.sub.outflow (cm.sup.-1) is selected to be
greater than the factor .alpha..sub.RIR-thr (cm.sup.-1). The
condition for radiation outflow from the active layer into the
radiation inflow region is defined by the relations:
arccos(n.sub.eff/n.sub.RIR).ltoreq.arccos(n.sub.eff-min/n.sub.RIR),
[0048] where n.sub.eff-min is greater than n.sub.min, and where
n.sub.eff-min is the minimum value of n.sub.eff out of all possible
n.sub.eff for the multiplicity of laser heterostructures with
radiation inflow regions that are of practical value. The index
n.sub.min is the smallest of the refractive indices in the
heterostructure cladding layers.
[0049] The laser is unique in its choice of the medium for the
optical resonator, which includes at least part of the RIR and at
least part of the GR. Negative optical feedback is assured by the
fact that at least one of the reflectors in the optical resonator,
which is disposed, for example, on one of the facets of the RIR, is
made with a reflection coefficient selected from the range greater
than zero and less than one. The choice of the compositions,
thicknesses, and number of layers and sublayers of the
heterostructure and RIR in the working laser assures outflow of
directional spontaneous radiation or emission from the active layer
(or GR) into the RIR with an intensity sufficient to fulfill the
condition for lasing in the optical resonator. For any laser as
well as this particular embodiment, a condition for lasing is that
the radiation gain exceed the losses. This condition for operating
the injection laser will be realized, in terms introduced above,
when the value of the gain G.sub.outflow (cm.sup.-1) of the
outflowing radiation from the active layer into the RIR exceeds the
value of the threshold loss factor .alpha..sub.RIR-thr (cm.sup.-1);
this threshold loss factor .alpha..sub.RIR-thr characterizes the
radiation inflow region and moreover all radiation losses in the
optical resonator at the lasing threshold. The lasing-threshold
condition obtained may be written as:
G.sub.outflow-thr=.alpha..sub.RIR-thr=(.mu..multidot..alpha..sub.RIR)+.alp-
ha..sub.out+.alpha..sub.diffr, (3)
[0050] where G.sub.outflow-thr (cm.sup.-1) is the value of the gain
G.sub.outflow at the lasing threshold, .alpha..sub.RIR is the
optical loss factor of the laser radiation in the RIR (which
includes, e.g., absorption, scattering), .alpha..sub.out is the net
loss factor, which is related to the laser radiation output from
the optical resonator and which is equal to:
.alpha..sub.out=(2L.sub.GR).sup.-1.multidot.1n(R.sub.1.multidot.R.sub.2).s-
up.-1, (4)
[0051] .alpha..sub.diffr is the coefficient of diffraction losses
of laser radiation as it exits from the optical resonator. More
specifically:
.alpha..sub.diffr=(L.sub.OR).sup.-1.multidot.1n{1-(.lambda..multidot.L.sub-
.OR/n.sub.RIR.multidot.S.sub.refl)}.sup.-1, (5)
[0052] S.sub.refl is the area of the reflectors of the optical
cavity, L.sub.OR is the length of the optical resonator, which is
equal to (.mu..multidot.L.sub.GR) (see equation (1)), and R.sub.1
and R.sub.2 are the coefficients of reflection for the reflectors
of the optical resonator.
[0053] Depending on the laser, the ratio .mu., equation (1), may
range from approximately 0.8 to 3.0. The current density through
the laser at which equation (3) is fulfilled is the threshold
current density j.sub.thr.
[0054] Note that the lasing preferably does not occur in the GR,
which, in both the laser disclosed in U.S. Pat. No. 4,063,189,
issued to D. F. Scifres et al. in 1977, as well as in other known
lasers, corresponds to the medium of the optical resonator. This
objective is easily accomplished by selecting the mode of intensive
outflow, in which the value of G.sub.outflow (cm.sup.-1) is
selected to be close to the value of the total gain G.sub.GR
(cm.sup.-1) of the radiation propagating in the GR. Or more
accurately, the following condition is fulfilled: the difference
between the gains (G.sub.GR-G.sub.outflow) is smaller than the
total loss factor .alpha..sub.GR-thr (cm.sup.-1) of the radiation
propagating in the GR. Moreover, the intensive outflow mode is
preferable for the lasers, since it results in increased
efficiency. In addition to the mode of intense outflow of radiation
from the GR, the laser is also characterized as producing an
outflowing radiation mode having a wide range of outflow angles
.phi. (see equation (2)), and accordingly of the ratios
(n.sub.eff/n.sub.RIR). Preferably, the upper bound of the outflow
angles .phi..sub.max is defined by the relations:
arccos(n.sub.eff/n.sub.RIR).ltoreq.arccos(n.sub.eff-min/n.sub.RIR)=.phi..s-
ub.max, (6)
for n.sub.eff-min>n.sub.min, (7)
[0055] where n.sub.eff-min is the minimum value of n.sub.eff out of
all possible n.sub.eff for the multiplicity of heterostructures
with inflow regions that are of practical value, and n.sub.min is
the smallest of the refractive indices in the heterostructure
cladding layers. Using numerical calculations for a heterostructure
based on InGaAs/GaAs/AlGaAs compounds with a RIR of GaAs, which
emits at wavelengths of between about 0.92-1.16 .mu.m, the maximum
outflow angle .phi..sub.max was found to be approximately
30.degree..
[0056] The laser with the aforementioned features is unique in that
it uses spontaneous radiation directed at an angle to the
longitudinal gain axis in the GR, followed by amplification thereof
and lasing in the optical resonator. The path of the laser rays in
such a resonator experiences a "bending" ("refraction"), with
simultaneous amplification of the radiation. The angle .phi. of
radiation outflow from the gain region in the RIR is made equal to
the radiation inflow angle .xi. for laser (and spontaneous) rays
that are reflected from the reflectors of the optical resonator and
that are directed from the RIR back into the GR. The aforementioned
feature of optical-ray propagation is preferably used for all
designs of the laser, based on the known optical principle of the
reversibility of the ray path in optical systems, which is applied
to the multilayer laser heterostructure in aggregate with a
RIR.
[0057] The aggregate of the characteristic features determines the
uniqueness of the operation and the advantages of the lasers.
Lasing and the process of generation of the corresponding modes in
their optical resonator occur mostly as the laser rays propagate in
the homogeneous, weakly absorbing volume of the RIR. Only after the
laser rays reflected from the facets of the RIR are incident on the
heterostructure (which generally is asymmetrical) does local
amplification and total internal reflection occur. In conventional
lasers the corresponding modes of laser radiation are formed in a
fundamentally different way, namely lasing occurs entirely in the
thin active dielectric waveguide of a GR bounded by the end
reflectors of the optical resonator, while amplification occurs
along the entire propagation path in the GR. In view of the
foregoing, the laser may be called an "injection laser (or diode
laser) with a resonant cavity."
[0058] The aforementioned distinctions determine the basic
advantages of the lasers. In comparison with conventional lasers,
for example, the threshold current density j.sub.thr can be
reduced. This feature is due to the fact that if there are equal
net losses to radiation output, defined by the factor
.alpha..sub.out (see equation (4)), the optical losses in the
volume of a homogeneous RIR which are characterized by the
coefficient .mu..multidot..alpha..sub.RIR (cm.sup.-1), can be made
smaller than the internal optical losses in a multilayer gain
region (the factor .alpha..sub.GR). This is possible, at least in
part, because the composition of the RIR may be selected to differ
from the composition of the semiconductor layers of the
heterostructure. In addition, by selecting the Fresnel number N
(which is equal to .lambda..multidot.L.sub-
.OR/n.sub.RIR.multidot.S.sub.refl) to be much larger than one, the
known diffraction losses can be made negligibly small (see equation
(5)). Also see A. Meitland and M. Dann, Introduction to Laser
Physics [in Russian], Nauka Publishers, 1978, pp.102-118.
[0059] There are additional possibilities for reducing the
threshold current density in the lasers. This threshold current
density is reduced not only in comparison with the threshold
currents of a laser with a small outflow angle .phi. (see U.S. Pat.
No. 4,063,189, issued to D. R. Scifres et al. in 1977) but also in
comparison with the threshold currents of modern lasers with
quantum-dimensional active layers, as for example discussed in S.
S. Ou et al. (Electronics Letters (1992), Vol. 28, No. 25, pp.
2345-2346). One possibility is related to the selection of larger
outflow angles .phi. (from the range 0<.phi.<.phi..sub.ma-
x), which is determined by relations (2), (6), and (7) and results
in an increase in the optical radiation localization coefficient
.GAMMA., which in turn inevitably leads to an additional decrease
in the threshold current densities (see, e.g., T. M. CocKerill et
al., Appl. Phys. Lett. (1991), Vol. 59, pp. 2694-2696).
[0060] The differential efficiency also can be increased with these
lasers. The following estimating formula has been obtained to
characterized this differential efficiency:
.eta..sub.d=.eta..sub.1.multidot..eta..sub.2. (8)
[0061] Here, the coefficient .eta..sub.1 corresponds to the
efficiency of output of the outgoing radiation from the GR into the
RIR, and is equal to
.eta..sub.1=G.sub.outflow/(G.sub.outflow+.alpha..sub.GR). (9)
[0062] In contrast, .eta..sub.2 corresponds to the efficiency of
output of the laser radiation from the RIR, and is equal to
.eta..sub.2=.alpha..sub.out/(.alpha..sub.out+(.mu..multidot..alpha..sub.RI-
R)+.alpha..sub.diffr). (10)
[0063] The factor .alpha..sub.GR includes of the optical radiation
losses in the GR (the factor .alpha..sub.GRF) and the losses to
radiation output at the start and end of the GR (the factor
.alpha..sub.GRO). Specifically, .alpha..sub.GR is defined by the
equation:
.alpha..sub.GR=.alpha..sub.GRO+.alpha..sub.GRF. (11)
[0064] By selecting a gain G.sub.outflow significantly greater than
.alpha..sub.GR, we will obtain an efficiency .eta..sub.1 (see
equation (9)) close to one. Note that in U.S. Pat. No. 4,063,189,
issued to D. R. Scifres et al. in 1977, the increase in the gain
G.sub.outflow is constrained by the increase in the lasing
threshold current. For the lasers designed as described above,
however, this constraint does not exist. If .alpha..sub.out is
selected to be much larger from the sum
(.mu..multidot..alpha..sub.RIR)+.alpha..sub.diffr, the efficiency
.eta..sub.2 can be made close to one (see equation (10)). The
efficiency .eta. is defined, herein, as the total efficiency of the
laser without regard for losses due to electrical resistance, for
which the following equation obtains:
.eta.=.eta..sub.d.multidot..eta..sub.thr, (12)
where
.eta..sub.thr=(1-j.sub.thr/j.sub.oper), (13)
[0065] and j.sub.oper is the operating current density through the
laser. It follows from relations (12) and (13) that simultaneously
increasing .eta..sub.1 and .eta..sub.2 (equation (8)) and
decreasing j.sub.thr will lead to an increase in the efficiency
.eta..
[0066] The spatial and spectral characteristics of the laser
radiation can also be improved in these laser devices. In the
original design of the optical resonator, the mode of the laser
radiation is basically generated in the homogeneous volume of the
RIR (or of homogeneous parts thereof) in the absence of
nonequilibrium majority carriers. In ordinary lasers, the
concentration of injected carriers and the optical gain affect the
value of the refractive index of the directional waveguide modes of
the radiation. This attribute significantly determines the
stability or lack thereof of the spatial and spectral
characteristics of injection lasers (M. Osinski et al., IEEE Journ.
of Quant. Electronics, Vol. 23, 1987, pp. 9-29). In the lasers
described above, however, the aforementioned gain and injection
processes are distributed, and they occur only on a very small part
of the total optical path length. Therefore, the lasing mode of one
spatial mode with respect to the transverse index in the direction
parallel to the layers of the laser heterostructure (with a
corresponding decrease in the angle of divergence .THETA..sub.2 in
the horizontal plane) can be preserved for significantly larger
dimensions of the strip than in ordinary lasers. Consequently, the
divergence of the output radiation will be significantly reduced in
not only the vertical but also the horizontal plane. As defined
herein, the horizontal plane corresponds to the plane that is
perpendicular to the vertical plane and that is located on the
output surfaces. In the general case, the output surfaces are
defined as the laser surface for radiation output.
[0067] Furthermore, stable generation of single-frequency laser
radiation can be achieved in these lasers over a wider range of
currents. Moreover, the so-called "chirp effect," the frequency
shift of the laser radiation resulting from changes in the
pumping-current amplitude (see, e.g., T. L. Koch and J. E. Bowers,
Electronics Letters, Vol. 20, 1984, pp. 1038-1039), which is
undesirable for a number of applications, can also be reduced
significantly.
[0068] Note also that the lasers described above have a feature
that significantly simplifies the technology for making them in
comparison with conventional laser designs. For example, to
eliminate undesirable losses in the lasers disclosed in U.S. Pat.
No. 4,063,189 issued to D. R. Scifres et al. in 1977, one reflector
of the optical resonator must be made with a reflective coating,
and the optical facet of the RIR that is an extension thereof must
be made, by contrast, with an antireflective coating. This design
is difficult to fabricate in view of the micron dimensions of the
reflector. Since the intensive outflow mode
(G.sub.outflow.congruent.G.sub.GR) is implemented for the lasers
and since laser radiation is not generated in the GR, it is
possible to simplify the technology for fabricating the lasers
without significantly effecting the output characteristics. This
goal can be achieved by making the angles of inclination .psi. and
the reflection coefficients of the faces of the RIR and adjacent
end surfaces the same.
[0069] Also in the lasers described above, the active layer is
preferably formed of at least one sublayer, and the active layer
can be implemented as one or several active sublayers, including
sublayers having quantum-dimensional thicknesses separated from
each other by barrier sublayers. The cladding layers, which are
respectively positioned on the first surface and opposite second
surface of the active layer, are respectively formed from cladding
sublayers I.sub.i and II.sub.j, where i=1, 2, . . . , k and j=1, 2,
. . . , m are defined as integers that designate the sequential
number of the cladding sublayers, counted from the active layer,
with refractive indices n.sub.Ii and n.sub.IIj, respectively, and
with bandgaps E.sub.Ii and E.sub.IIj. At least one cladding
sublayer is made in each cladding layer. If the active layer
comprises sublayers, the cladding layers generally are made of two
or more sublayers on each side of the active layer. For the case
where the active layer comprises one sublayer with a thickness of
approximately 50 nm or more, each cladding layer comprise one
sublayer. We consider the gradient layers used (see, e.g., S. S. Ou
et al., Electronics Letters (1992), Vol. 2F, No. 25, pp. 2345-2346)
to be the final number of sublayers of a cladding layer with
corresponding n.sub.Ii and n.sub.IIj obtained by layering each
sublayer in the gradient layer. Here, in general the refractive
indices of the cladding layers usually are smaller than the
refractive indices of the active sublayers. By selecting the
aforementioned versions of the active and cladding layers (for the
specified heterostructure) it also is possible to effect
improvement of the efficiency and threshold current density, and to
ensure the necessary value of n.sub.eff used in equations (2), (6),
and (7).
[0070] In preferred embodiments, at least one of the angles of
inclination .psi. of the facets of the RIR is greater than zero in
absolute value as defined with respect to the plane perpendicular
to the longitudinal gain axis called the normal plane. The angle of
inclination .psi. is arbitrarily designated positive if the facet
subtends an acute angle with the inner surface (or with the active
layer, since the output surface and the active layer are parallel
to each other), and to be negative if the facet subtends an obtuse
angle with the inner surface. The inclined facets introduced into
the design and which have specified angles of inclination .psi.
make it possible, by using easy to implement technological
solutions, to obtain an efficient design of an optical resonator
with negative feedback, that provides for different directions of
laser radiation output, as well. For some laser designs, barrier
regions are introduced into the heterostructure. The introduction
of barrier regions makes it possible to create lasers with a strip
gain region (i.e., in the form of a strip of width W.sub.GR) that
incur small losses of injection current to spreading. Barrier
regions also enable multibeam lasers to be formed.
[0071] To eliminate current losses, the length L.sub.RIR of the
inner surface of the RIR, which is determined along the
longitudinal gain axis of the GR, and the width W.sub.RIR of the
RIR, are made to be at least of length L.sub.GR and at least of
width W.sub.GR of the GR, respectively. The thickness d.sub.RIR of
the RIR depends on the outflow angle .phi., the length L.sub.GR,
and the angles of inclination of the optical facets. This thickness
may be varied over a wide range from about 2 .mu.m to about 50,000
.mu.m or more.
[0072] Since the RIR actually is the passive volume of the optical
resonator, transparency of the RIR is necessary for the functioning
of the laser. To obtain higher efficiency of the laser, the RIR (or
a part thereof) must be made of an optically homogeneous material,
and the optical radiation losses in it (to absorption and
scattering) must be much smaller than the net losses to laser
radiation output from the optical resonator. Specifically, the
condition .alpha..sub.RIR<<.al- pha..sub.out must be
fulfilled, or
.alpha..sub.RIR<<(2.mu..multidot.L.sub.GR).sup.-1.multidot.1n(R.sub.-
1.multidot.R.sub.2).sup.-1. (14)
[0073] If the RIR is made of a semiconductor material, in addition
to the requirements of homogeneity, to fulfill equation (14) the
RIR must have a bandgap E.sub.RIR larger than the bandgap E.sub.a
for the active layer, which determines the wavelength .lambda. of
the laser radiation. Losses to absorption are known to decrease
approximately exponentially, depending on the difference between
E.sub.RIR and E.sub.a (see, e.g., H. C. Huang et al., Journ. Appl.
Phys. (1990), Vol. 67, No. 3, pp. 1497-1503). To reduce the optical
loss factor .alpha..sub.RIR (cm.sup.-1) and consequently to attain,
in addition to high differential efficiency .eta..sub.d and low
j.sub.thr, an increase in output radiation power (as a result of an
increase in the effective length of the optical resonator), it is
desirable for E.sub.RIR to exceed E.sub.a by at least about 0.09
eV. In this case the optical loss factor for absorption may reach
values on the order of about 0.1 cm.sup.-1 or less. In the general
case, the RIR may be made not only of semiconductor materials. It
is preferred that its characteristics, particularly the refractive
index n.sub.RIR and optical loss factor .alpha..sub.RIR for
absorption and scattering, satisfy the relations in equations (2),
(6), (7), and (14).
[0074] To simplify the technology for making the laser, the RIR may
comprise a substrate on which a heterostructure is grown.
Furthermore, the RIR may be made electrically conductive, and in
this case, an ohmic contact is formed with the surface of the
RIR.
[0075] In cases where the RIR is nonconductive, in order to obtain
low values of .alpha..sub.RIR (cm.sup.-1) and increase the
effective length L.sub.OR of the optical resonator and the output
radiation power P (W), part of the volume of the RIR that borders
on the heterostructure is preferably made electrically conductive,
and the remaining volume preferably comprises a material with an
optical loss factor .alpha..sub.RIR of no more than about 0.1
cm.sup.-1. This part of the RIR that borders the heterostructure
preferably has of a thickness no greater than W.sub.GR (.mu.m).
[0076] Hereinafter, this part of the volume of the RIR that borders
on the heterostructure and that has the specified thickness is
referred to as the first layer of the RIR. The other parts of the
RIR may be the second layer of the RIR, which is adjacent to the
first, third, and subsequent layers. Preferably, the parts of the
RIR that are layers parallel to its inner surface comprise
materials with different refractive indices. In such cases, when
the difference in the properties of the aforementioned layers of
the RIR includes not only electrical conductivity but also
different refractive indices, the outflow angles .phi. (2) in the
layers of the RIR can be controlled. Hence the thickness of the RIR
and its layers also can be controlled. The outflow angle
.phi..sub.i in the i-th layer (where i=2, 3, . . . , s are
integers) with a refractive index n.sub.RIR is equal to
arccos(n.sub.eff/n.sub.RIR) (see equation (2)). Therefore, if for
example the refractive index n.sub.RIR1 of the first layer is
smaller than n.sub.RIR2 for the second layer, the thickness of the
second layer may be made smaller than for the first layer, and vice
versa. Decreasing the thickness d.sub.RIR may lead to
simplification of the technology and to a lowering of the costs of
making the RIR. In both cases, an ohmic contact is made with the
electrically conductive part of the RIR, and the thickness of this
part of the RIR is preferably no greater than the width W.sub.GR of
the GR.
[0077] In lasers whose RIR has an optical loss factor
.alpha..sub.RIR of no more than about 0.1 cm.sup.-1, it is possible
to achieve a large volume of the active GR (by increasing L.sub.GR
to approximately 1 cm) with small laser-radiation losses in the
RIR, and to obtain large values of the output radiation power. In
lasers having a width W.sub.GR of micron dimensions, to simplify
the fabrication, an ohmic contact from the direction of the RIR is
made with one of the electrically conductive cladding sublayers
that is located between the active layer and the RIR, preferably
with the electrically conductive sublayer that has the smallest
bandgap.
[0078] For those laser designs in which, for large values of
L.sub.GR, it is undesirable to have a large thickness d.sub.RIR
(.mu.m), preferably at least one of the cladding sublayers has a
refractive index no smaller than n.sub.RIR. This leads to an
increase in the value of n.sub.eff and consequently to a decrease
in the outflow angle .phi. (see equation (2)) and in d.sub.RIR.
Small thicknesses d.sub.RIR lead to savings of material in the
RIR.
[0079] To simplify the technology for fabricating the lasers,
preferably the end surface of the GR, on at least one side has the
same angle of inclination .psi. and the same reflection coefficient
as the adjacent optical facet of the RIR. As noted previously, in
practice, this feature will not degrade the performance of the
laser.
[0080] Different embodiments of the injection laser are described
below.
[0081] In one laser, for example, preferably at least one optical
facet of the RIR comprises a reflector of the optical resonator and
has a positive angle of inclination .psi. equal to the outflow
angle .phi., which is equal to arccos(n.sub.eff/n.sub.RIR). This
configuration provides the ability to make the lasers with an
outflow angle .phi. throughout the entire range of its values, as
defined in equations (2), (6), and (7), up to .phi..sub.max. Within
this range is included values of the angles .phi. larger than the
angle of total internal reflection .sigma.. Hence, one can reduce
the threshold current density, increase the efficiency and power,
decrease the angle of divergence in the vertical plane, and effect
output of laser radiation through the optical facet of the RIR when
the radiation is normally incident on the optical facet.
[0082] For the same purposes, but with implementation of laser
radiation output perpendicular to the plane of the active layer, at
least one optical facet is formed with a negative angle of
inclination .psi. equal to (.pi./4)-(.phi./2), and at least part of
the outer surface of the RIR comprises a reflector of the optical
resonator. This part of the outer surface of the RIR preferably
corresponds at least to the portion where the projection of the
optical facet is formed on the RIR. Alternatively, at least one
optical facet of the RIR is formed with a positive angle of
inclination .psi. equal to (.pi./4)+(.phi./2), and at least part of
the laser surface opposite the RIR comprises a reflector of the
optical resonator. This part of the laser surface opposite to the
RIR preferably corresponds at least to the portion where the
projection of the optical facet is formed thereon. In the first
case, radiation output will occur through the outer surface of the
RIR, and in the second case it will occur in the diametrically
opposite direction.
[0083] In a number of other cases, in the aforementioned laser
designs the other optical facet of the RIR is formed with an angle
of inclination .psi. of about zero. A reflective coating is formed
on the other optical facet of the RIR (when the angle .phi. is less
than the angle of total internal reflection .sigma. on the output
surface). This leads to one-way output of laser radiation, and also
to a decrease in the length L.sub.GR and in the angle of divergence
.THETA..sub.1 of the output laser radiation in the vertical
plane.
[0084] To enhance the spatial and spectral characteristics of the
laser, preferably, at least one of the reflectors of the optical
resonator comprises an external reflector. If the angle .phi. is
smaller than the angle .sigma., the use of an external reflector
(outside the RIR) makes it possible to create a laser in which both
facets of the RIR subtend an angle of inclination .psi. equal to
about zero. This simplifies the technology for fabricating the
laser, since there is no need to make the facets for the RIR
sloped. In this case, negative feedback is generated by using an
external reflector (or reflectors) made with the corresponding
angle of inclination.
[0085] It is also possible that one of the reflectors of the
optical cavity, which is formed either as an external reflector, or
on the output surfaces of the RIR or heterostructure, comprises a
plane mirror, a cylindrical mirror, a spherical mirror, or a
diffraction grating.
[0086] Note that it is accepted practice, see, e.g., I. I.
Bronshtein and K. A. Semendyayev, Handbook of Mathematics (in
Russian), p. 170, 1953 to measure the angles of inclination .psi.
formed between two half-planes (a facet and the inner surface) as
the angle between two perpendiculars drawn in both half-planes from
one point on the line of their intersection. The accuracy with
which the angle of inclination .psi. is preferably made is
determined by the dispersion angle of divergence .DELTA..phi.. The
diffraction angle of divergence may be ignored because of its
smallness in comparison with the angle .DELTA..phi.. The dispersion
angle is determined instead by the spread of the outflow angle
.phi. as a function of the wavelength .lambda., which varies over
the range of the spectral band .DELTA..lambda. for spontaneous
radiation. The angle .DELTA..phi. has been determined by a
numerical calculation using equation (2) for the known dependences
of n.sub.eff and n.sub.RIR on .lambda. in the range
.DELTA..lambda.. The calculations show that for the most frequently
used heterostructures with .DELTA..lambda. of about 20-50 nm, the
angle .DELTA..phi. lies in the range from about 0.5.degree. to
about 1.5.degree..
[0087] Preferred embodiments of the lasers include designs with two
or more, i.e., plurality of gain regions.
[0088] A unique feature of one multibeam laser is that at least two
gain regions are formed on the inner surface of at least one RIR
preferably so as to produce identical outflow angles .phi.. The
gain regions, which may (but not necessarily) have a rectangular
shape, are positioned with specified periods, including those in a
mutually perpendicular direction. In a number of cases, an
independent ohmic contact is made with each GR from the direction
opposite the location of the RIR. Such a laser using an output
reflector of the RIR will have a multiplicity of laser beams,
including beams that are spatially separated from each other and
that can be turned on independently by the operating current.
[0089] In another multibeam laser, a multiplicity of laser beams
form a two-dimensional matrix in which each beam is independently
controllable by the operating current. The gain regions are formed
from at least two sequences of GRs. In each at least two GRs are
disposed so that the gain axes of each GR in each sequence are
parallel to each other. These gain axes are positioned at a right
angle to the line of intersection of the active layer with the
extension of the plane of the facet of the common RIR for each
sequence of GRs. Furthermore, from the direction of the RIR, on at
least part of the outer surfaces of the common RIRs, ohmic contacts
and metallization layers electrically connected to them are formed.
At least the metallization layers are formed in strips, one for
each sequence of GRs. From the direction opposite the location of
the RIR, the metallization layers connected to the independent
ohmic contacts are formed in strips that are insulated from each
other and are positioned parallel to the gain axes of the GRs.
[0090] In another laser, several GRs are successively connected to
a unified optical resonator. In this case, the GRs are formed along
at least one line parallel to the longitudinal gain axes of the
GRs. The spacing between the starts of the GRs is 2d.sub.RIR/tan
.phi., and the outer surface is optically reflective, at least at
points on the projection of the gain regions onto the outer surface
at the outflow angle .phi.. It also is possible to form at least
one GR having identical outflow angles .phi., on opposite surfaces,
of RIRs along two lines that are parallel to each other and to the
longitudinal gain axes of the GRs; here, the shortest distance
between the starts of the GR on opposite sides of the RIR is
selected to be d.sub.RIR/sin .phi.. These modifications make it
possible to increase the radiation output power while at the same
time reducing the thickness of the RIR and improving the conditions
of heat removal.
[0091] In lasers with either one or a multiplicity of gain regions,
the gain region or at least two adjacent GRs be galvanically
isolated all the way to the nonconductive part of the volume of the
RIR, and that the ohmic contacts of the gain regions be
galvanically coupled by a metallization layer. This arrangement
makes it possible to increase the value of the supply voltage and
to perform effective matching of the lasers to power sources.
[0092] The laser described above includes an original design of an
optical resonator the volume (bulk) of whose medium comprises not
only an active layer but also the volume of the gain region with
intense outgoing radiation, as well as the passive volume of the
radiation inflow region, which are formed with appropriately made
compositions and thicknesses. These features along with the number
of layers of the laser heterostructure, the configuration of the
radiation inflow region and its optical facets, the ohmic contacts,
and the metallization layers, make possible practical delimitation
of the region of generation of laser-radiation modes and the region
of injection and stimulated recombination of nonequilibrium
carriers.
[0093] This unique laser design offers many advantages. These
include a decrease in the threshold current density, an increase in
efficiency (including differential efficiency), and an improvement
in the level of astigmatism. This laser produces small angles of
divergence close to the diffraction angles for two mutually
perpendicular directions of output radiation, and provides an
increase in the stability of monomode lasing, a significant
decrease in the dependence of the wavelength of the generated laser
radiation on the pumping-current amplitude, and the ability to
increase the effective length of the optical resonator and
radiation output power. Many of these advantages accrue from the
unique design of the laser and, in particular, by rejecting the
traditional dielectric waveguide design with an active layer
inside, as a lasing medium for generating directional waveguide
modes. Additional advantages of the laser include the ability to
obtain different directions of laser radiation, including those
perpendicular to the plane of the active layer, as well as
increased service life or lifetime and operational reliability.
These lasers are also highly manufacturable. An additional
advantage of the laser with a multiplicity of gain regions is that
the integrated technology can be employed for fabricating these
devices.
[0094] The technical implementation of the invention is based on
known basic production processes, which by now are well developed
and are used extensively in laser manufacture. The range of
radiation wavelengths of the lasers that have been put to use to
date extends from the infrared to the ultraviolet. Depending on the
wavelength, appropriate heterostructures are used for different
sections of the wavelength range. For example, heterostructures
based on semiconductor compounds in the AlGaN/GaN/GaInN system, and
also in the ZnCdSSe/GaAs system, are most effective for
ultraviolet, blue, and green radiation (0.36
.mu.m<.lambda.<0.58 .mu.m); compounds in the AlGaInP/GaAs
system are most effective for red and yellow (0.58
.mu.m<.lambda.<0.69 .mu.m); compounds in the AlGaAs/GaAs
system and in the InGaAs/GaAs/AlGaAs system are most effective for
infrared (0.77 .mu.m<.lambda.<1.2 .mu.m); compounds in the
GaInAsP/InP system are most effective for infrared (1.2
.mu.m<.lambda.<2.0 .mu.m); and compounds in the
AlGaInSbAs/GaAs system are most effective for infrared (2.0
.mu.m<.lambda.<4.0 .mu.m). In each of the aforementioned
ranges, appropriate materials for the RIR that satisfy conditions
(2), (6), (7), and (14) must be selected, depending on the
wavelength .lambda. used and the heterostructure chosen. Among
semiconductor materials for RIRs, preferred material include: GaN
for the AlGaN/GaN/GaInN system; ZnSe for the ZnCdSSe/GaAs system;
GaP for the AlGaInP/GaAs system; GaP for the AlGaAs/GaAs system;
GaAs and GaP for the InGaAs/GaAs/AlGaAs system; Si and GaAs for the
GaInAsP/InP system; and Si and GaAs for the AlGaInSbAs/GaAs system.
These laser designs can be successfully implemented by using the
recently developed "wafer bonding" technology; see, e.g., H. Wada
et al., IEEE Photon. Technol. Lett., Vol. 8, p. 173 (1996). The
designs for efficient injection lasers are applicable for at least
all the foregoing ranges of laser-radiation wavelengths and
heterostructure systems.
[0095] With reference now to FIGS. 1 and 11, the preferred laser 1
includes a heterostructure 2, which comprises an active layer 3
positioned between two cladding layers 4 and 5, respectively, with
sublayers I.sub.i and II.sub.j (not shown). Active layer 3 consists
of two active sublayers and a barrier sublayer that separates them
(not shown in FIG. 1). The length of the L.sub.GR is about 4000
.mu.m. The length of the optical resonator L.sub.OR is about 3760
[.mu.m (see equation (1)), and the ratio .mu. (equation (1)), is
equal to about 0.9397. The width W.sub.GR in the form of a strip 30
(or mesa strip) bounded on the sides by barrier regions 6 is about
400 .mu.m. The total width of the laser crystal is about 1000
.mu.m. A semiconductor RIR 7, bounded on the end faces by facets 8
and 9, which are implemented as reflectors of the optical
resonator, is located on the surface, distant from active layer 3,
of sublayer II.sub.m of cladding layer 5. RIR 7 is implemented as a
substrate 10 to which the required form is imparted. Both facets of
RIR 7, the first facet 8 and second facet 9, are inclined with a
positive angle of inclination .psi. equal to the outflow angle
.phi., which is about 200. This angle .psi. is calculated from the
normal plane, which is perpendicular to the longitudinal gain axis
in the GR. If the angle of inclination .psi. is positive, facets 8
and 9 subtend an acute angle with active layer 3. A reflective
coating 11 with a reflection coefficient R.sub.1 of about 0.999 is
formed on the first facet 8, and a partially reflective coating 12
with a reflection coefficient R.sub.2 of about 0.01 is formed on
the second facet. The end surfaces 13, which determine the length
of the gain region L.sub.GR, are an extension of the corresponding
planes of the inclined facets 8 and 9 and have the same
inclinations and the same reflection coefficients as facets 8 and
9. This simplifies the fabrication technology. This simplification
does not have any practical effect on the parameters of laser 1,
since strong outflow is realized in the preferred laser 1, and
there is practically no laser radiation through end surfaces 13.
The accuracy with which the facets 8 and 9 are inclined may range
from about 19.6.degree. to 20.4.degree.. The thickness d.sub.RIR of
RIR 7, for which the relation
d.sub.RIR.gtoreq.(L.sub.RIR.multidot.tan .phi./(1+tan.sup.2.phi.)),
(15)
[0096] is fulfilled, is about 1,286 .mu.m. Contact layer 14 is
positioned on the surface of sublayer I.sub.k of cladding layer 4,
and an ohmic contact 15 is formed on it. An ohmic contact 16 is
made on the opposite side of RIR 7, on its outer surface 17 of RIR
7 (in this case, on the surface of substrate 10). Inner surface 18
borders on heterostructure 2, and is parallel to the plane of
active layer 3.
[0097] Heterostructure 2, which comprises a number of semiconductor
layers and sublayers 19-27, together with contact layer 14, may be
grown by the conventionally known method of Metal Organic Chemical
Vapor Deposition (MOCVD) on substrate 10 from electrically
conductive gallium arsenide. The composition, thicknesses,
refractive indices, type, doping concentrations, and absorption
coefficients of layers 19-27 of heterostructure 2, contact layer
14, and RIR 7 are presented in the Table. This heterostructure 2
also is used in the following embodiments (with the changes
specified for the separate embodiments). The operating wavelength
for this heterostructure 2 is about 980 nm. In FIG. 1 and in
subsequent FIGS. 2-10, 14, 16-18, and 20-25 conventional arrows
show the directions of propagation of laser radiation in RIR 7 and
outside it. The laser 1 is mounted on a thermally conductive slab
(not shown in FIGS. 1-25) against the side of ohmic contact 15. The
power is supplied to ohmic contacts 15 and 16.
[0098] The basic parameters for both the laser 1, and modifications
of it, were obtained by numerical modeling performed according to a
program employing a matrix method of solving Maxwell's equations
with the corresponding boundary conditions for the multilayer laser
heterostructures (see, e.g., J. Chilwall and I. Hodkinson, Journ.
Opt. Soc. Amer., A (1984), Vol. 1, No. 7, pp. 742-753). The
calculations employed the following initial parameters: the
material gain in active layer 3 for achieving inversion was about
200 cm.sup.-1, the proportionality factor between the gain and
injected-electron concentration in active layer 3 was about
5.times.10.sup.-16 cm.sup.2, and the lifetime of the nonequilibrium
electrons in active layer 3 was 1 ns.
[0099] The calculations also assumed: an optical loss factor
.alpha..sub.GRO in the GR of about 3 cm.sup.-1 (see, e.g., D. Z.
Garbuzov et al., IEEE Journ. of Quant. Electr. (1997), Vol. 33, No.
12, pp. 2266-2276) and in accordance, for example, with H. C. Huang
et al. (Journ. Appl. Phys. (1990), Vol. 67, No. 3, pp. 1497-1503),
and with a value of the optical loss factor .alpha..sub.RIR of the
laser radiation in RIR 7 of about 0.1 cm.sup.-1. The losses to
output of spontaneous radiation through end surfaces 13 of the gain
region were not taken into account in the calculations because of
their smallness, so that .alpha..sub.GR (equation (11)), is about 3
cm.sup.-1. The values adopted for the parameters are typical of the
laser heterostructure 2 in question, which is based on
InGaAs/GaAs/AlGaAs. For a heterostructure 2 using other compounds,
such as GaInPAs/InP, these parameters may change somewhat.
[0100] The following results were obtained by numerical
calculation:
[0101] the threshold current density j.sub.thr was determined to be
about 89.3 A/cm.sup.2 (equation (3) is fulfilled for this current
density);
[0102] the outflow angle .phi. is about 20.degree.;
[0103] the dispersion angle of divergence .DELTA..phi. for the
outgoing spontaneous radiation (for the .DELTA..lambda. of 30 nm
used in the calculation) was found to be about 0.8.degree.;
[0104] the effective refractive index n.sub.eff is about
3.3124;
[0105] the total gain G.sub.GR and gain G.sub.outflow for the
outgoing radiation (reached for a current density j.sub.oper of
about 2,500 A/cm.sup.2 through laser 1) are about 320.80 cm.sup.-1
and 320.03 cm.sup.-1, respectively; here the difference
(G.sub.GR-G.sub.outflow), which is about 0.73 cm.sup.-1, is smaller
than .alpha..sub.GR (equation (11)), which is about 3 cm.sup.-1,
which means that the mode of intensive outflow has been selected,
and that lasing did not occur in the GR;
[0106] the net loss factor .alpha..sub.out (equation (4)), for the
output laser radiation from RIR 7 is about 11.515 cm.sup.-1;
[0107] the optical loss factor, defined as
.mu..multidot..alpha..sub.RIR, is about 0.09397 cm.sup.-1;
[0108] the threshold loss factor .mu..multidot..alpha..sub.RIR-thr
(equation (3)), in RIR 7, defined as the sum
(.alpha..sub.out+(.mu..multi- dot..alpha..sub.RIR)+(60 .sub.diffr),
is equal to about 11.609, and the .alpha..sub.diffr calculated from
equation (5) is negligibly small;
[0109] the area of the gain region S.sub.GR is about
1.6.times.10.sup.-2 cm.sup.2;
[0110] the threshold current J.sub.thr, defined as
(j.sub.thr.multidot.S.s- ub.GR), is about 0.8 A;
[0111] the differential efficiency .eta..sub.d is about 0.9827 (see
equation (8)), and its components .eta..sub.1 (equation (9)) and
.eta..sub.2 (equation (10)) are about 0.9907 and 0.9919,
respectively; and
[0112] the threshold loss factor .eta..sub.thr (equation (13)) for
a j.sub.oper of about 2,500 A/cm.sup.2 is 0.9643, and the
efficiency .eta. (equation (12)) of laser 1 is 0.9476.
[0113] The output power P (W) of laser radiation is determined
as:
P=.eta..multidot.J.multidot.(hv), (16)
[0114] where J is the operating current through the device, equal
to about 40 A, and (hv) is the photon energy in volts, equal to
about 1.265 V. The value obtained for P is about 47.95 W. The
radiation near-field area (output aperture) S.sub.ap, defined as
(d.sub.ap.multidot.W.sub.GR), is equal to about
0.51.times.10.sup.-2 cm.sup.2, where d.sub.ap is equal to
L.sub.GR.multidot.sin .phi., specifically 1,368 .mu.m, and W.sub.GR
is approximately 400 .mu.m. The angles of divergence .THETA..sub.1
and .THETA..sub.2 of the output radiation were respectively
estimated as the wavelength .lambda., divided by d.sub.ap, and the
wavelength .lambda., divided by W.sub.GR. In the vertical plane,
the angle .THETA..sub.1 was found to be about 0.72 mrad. In the
horizontal plane the angle .THETA..sub.2 is about 2.45 mrad for the
range of currents for which monomode lasing can be maintained. The
output of laser radiation on the output facet 9, defined as
P/S.sub.ap, is about 9,402 W/cm.sup.2.
[0115] Another embodiment of laser 1 (see FIG. 2) differs from the
one described above with regard to FIG. 1 in that an antireflective
coating 28 was applied to facet 8, and outer reflector 29 was
implemented as a plane mirror measuring approximately
2,500.times.2,500 .mu.m.sup.2, with a reflection coefficient of
about 0.999, which was parallel to facet 8 and approximately 10,000
.mu.m from it. The width W.sub.GR was 1,368 .mu.m, and the total
width of the laser crystal was approximately 3,000 .mu.m. The basic
parameters of laser 1 in this embodiment that differ from the first
embodiment are: S.sub.GR, J.sub.thr, J.sub.oper, P (equation (16)),
and S.sub.ap are respectively equal to about 5.47.times.10.sup.-2
cm.sup.2, 1.43 A, 136.75 A, 163.9 W, and 1.87.times.10.sup.-2
cm.sup.2, and .THETA..sub.1 and .THETA..sub.2 are identical and are
equal to approximately 0.72 mrad.
[0116] The difference between the laser depicted in FIGS. 10 and 11
and the one shown in FIG. 3 is that facets 8 and 9 of RIR 7 were
made with an angle of inclination .psi. of zero. As a result of the
increase in the thicknesses of layers 21 and 25 of heterostructure
2 (see Table), the outflow angle .phi. was reduced to approximately
12.degree., which is smaller than the angle .sigma.. Here, the
plane of external reflectors 29 for providing negative feedback was
adjusted with an angle of inclination relative to the normal plane
of about 42.3.degree., and the angle .DELTA..phi. was about
1.4.degree.. Laser 1 had unidirectional radiation output, since one
reflector had a reflective coating 11 with R.sub.1 of about 0.999,
and the other had a coating 12 with R.sub.2 of about 0.05. In
addition to the known advantages inherent in lasers with external
resonators (such as, the enhancement of spatial and spectral
characteristics), another advantage is simplified fabrication.
[0117] The laser 1 of FIG. 4 and FIG. 11 differed from the last
shown in FIG. 1 in that both facets 8 and 9 of RIR 7 have a
negative angle of inclination .psi. equal to (.pi./4-.phi./2),
specifically 35.degree.. The length L.sub.RIR is approximately
1,000 .mu.m and the width W.sub.GR is approximately 340 .mu.m. The
thickness d.sub.RIR of RIR 7 is about 500 .mu.m, and its length
L.sub.ORIR is about 1,700 .mu.m. A reflective coating 11 with a
reflection coefficient R.sub.1 of about 0.999 is formed on the
outer surface 17 at the location of the projection of facet 8 onto
it, and a partially reflective coating 12 with a reflection
coefficient R.sub.2 of approximately 0.02 is formed at the point of
the projection of the other facet 9. The length L.sub.OR of the
optical resonator, defined as .mu..multidot.L.sub.GR (equation
(1)), is about 1,940 .mu.m, and the ratio .mu. equals 1.94. The
following parameters were obtained for this laser 1:
[0118] the threshold current density j.sub.thr is about 42
A/cm.sup.2;
[0119] the loss factors .alpha..sub.out and
.mu..multidot..alpha..sub.RIR are about 3.913 cm.sup.-1 and 0.194
cm.sup.-1, respectively;
[0120] the .alpha..sub.diffr calculated from equation (5) is
negligibly small, and consequently .alpha..sub.RIR-thr (equation
(3)) is about 4.107 cm.sup.-1;
[0121] the efficiencies .eta..sub.1, .eta..sub.2, .eta..sub.d,
.eta..sub.thr, and .eta. are respectively 0.9907, 0.958, 0.9439,
0.9832, and 0.9280;
[0122] the threshold current J.sub.thr is about 0.143 A for an
S.sub.GR of about 0.34.times.10.sup.-2 cm.sup.2; and
[0123] the operating current J.sub.oper is approximately 8.5 A (for
a j.sub.oper of 2500 A/cm.sup.2), and the output power P (equation
(16)) of laser 1 under conditions of a single spatial mode is about
9.98 W, while the angles of divergence .THETA..sub.1 and
.THETA..sub.2 are identical and are equal to about 2.8 mrad.
[0124] The laser 1 depicted in FIGS. 5 and 11 differs from the one
in FIG. 4 in that one of the reflectors of the optical resonator is
external and is implemented as a reflective diffraction grating 29.
This grating enables single-frequency lasing.
[0125] In laser 1 shown in FIGS. 8 and 11 both facets 8 and 9 are
inclined at a positive angle .psi. equal to (.pi./4+.phi./2),
specifically about 55.degree., and the output radiation is directed
at a right angle to the plane of active layer 3 in the direction
toward the heterostructure 2. The GR, of width about 15.3 .mu.m and
length 45 .mu.m, is located in the middle of inner surface 18,
whose width W.sub.IRIR is about 25 .mu.m and whose length
L.sub.IRIR is about 135 .mu.m. Coatings 11 and 12, whose reflection
coefficients are about 0.999 and 0.90, respectively, are applied to
the inner surface 18 at the points of projection of facets 8 and 9.
A barrier region 6 is formed on the remaining area of inner surface
18, which is free of heterostructure 2 and coatings 11 and 12. The
thickness of RIR 7 is 23 .mu.m, the length of the outer surface 17
is about 73.8 .mu.m, and the length L.sub.OR of the optical
resonator was about 129 .mu.m. The aforementioned design changes
determined the following parameters: j.sub.thr is about 125
A/cm.sup.2, and .alpha..sub.out, .alpha..sub.diffr, .mu., and
(.mu..multidot..alpha..sub.- RIR) are respectively about 11.7
cm.sup.-1, 4.22 cm.sup.-1, 2.86, and 0.286 cm.sup.-1, and
consequently .alpha..sub.RIR-thr is about 16.2 cm.sup.-1. The
threshold current J is about 0.861 mA, the operating current
J.sub.oper is about 13.8 mA, the output power P (equation (16)) of
the laser radiation is about 11.66 mW, and .eta..sub.d (equation
(8)) and .eta. (equation (12)) are about 0.7139 and 0.6693,
respectively. This laser 1 generates in a single longitudinal mode,
its wavelength is practically independent of the pumping current
over a wide range thereof, and the angles of divergence
.THETA..sub.1 and .THETA..sub.2 are identical and are equal to 6.4
mrad (0.37.degree.). This laser, in addition to the others (see
FIGS. 4 and 6-9) is competitive with conventional lasers having
vertical resonator (see, e.g., B. Weigl et al., Electronics
Letters, Vol. 32, No. 19, pp. 1784-1786, 1996).
[0126] The laser 1 depicted in FIGS. 7 and 11 differs from those
shown in FIGS. 10, 4, and 8 in that the facets 8 and 9 are made
different in sign but identical in absolute value of the angle of
inclination .psi., which is approximately 30.degree.. This design
makes it possible to obtain output radiation directed at a right
angle to the plane of active layer 3, but for smaller values of the
ratio .mu., see, e.g., equation (1), than for some of the lasers
described above.
[0127] What is common to the lasers 1 in FIGS. 3, 6, and 9 is that
facet 8 of RIR 7 is made with an angle of inclination .psi. of
about zero. For the lasers 1 made in accordance with the design in
FIGS. 1, 4, and 8, this configuration leads to a doubling of the
linear size of the radiation output aperture, with a corresponding
decrease in the angle of divergence .THETA..sub.1 in the vertical
plane, provided that the lengths L.sub.GR are kept the same.
[0128] In the lasers shown in FIGS. 1-10, mesa strip 30, which
determines the dimensions of the GR, may be made of a specified
width by using barrier regions 6 (see FIGS. 11-13). For a laser 1
with a mesa strip 30 with a width of micron size, ohmic contact 16
can be made, for example, with sublayer 25 of cladding layer 5, as
shown in FIG. 12. In this case, RIR 7 may be an insulating region,
which facilitates the selection of RIR 7 with a small coefficient
.alpha..sub.RIR. If a two-layer RIR 7 comprising a first
electrically conductive layer 31 and a second insulating layer 32
is present, ohmic contact 16 is made with the first layer 31 (FIG.
13). In this case, the second layer 32 of RIR 7 may be not only an
insulating layer, but also a layer that differs in composition from
first layer 31, which also facilitates the selection of both small
values of .alpha..sub.RIR2 and corresponding thicknesses d.sub.RIR2
of layer 32, provided that the refractive index of the layers of
RIR 7 are selected appropriately.
[0129] The lasers 1 in FIGS. 14-21 differ from the ones described
above in that their designs include two or more (i.e., a
multiplicity of) gain regions. A characteristic feature of the
multibeam laser 1 shown in FIGS. 14 and 15 is that 32.times.30 GRs
are disposed on the inner surface 18 of RIR 7 and are series and
parallel electrically coupled to together for the passage of the
operating current therethrough. The length and width of each GR are
approximately 290 and 85 .mu.m, respectively. They are arranged in
the form of a rectangular grating whose spacings along (x) and
across (y) the length L.sub.IRIR of the RIR are approximately 300
and 100 .mu.m, respectively. The length L.sub.IRIR and width
W.sub.IRIR of the inner surface 18 of RIR 7 are approximately 9,600
and 3,000 .mu.m, respectively, the thickness of RIR 7 is 3,214
.mu.m, the length L.sub.OR of the optical resonator is
approximately 10,216 .mu.m, and the reflection coefficients of
coatings 11 and 12 on facets 8 and 9 are about 0.999 and 0.32,
respectively. The basic parameters for the laser beams from each
GR, j.sub.thr, .alpha..sub.out, .mu., and
(.mu..multidot..alpha..sub.RIR), have values of about 182
A/cm.sup.2, 19.95 cm.sup.-1, 35.2, and 3.52 cm.sup.-1,
respectively, and consequently .alpha..sub.RIR-thr is about 23.47
cm.sup.-1; the threshold current J.sub.thr is about 43.7 mA, the
operating current J.sub.oper is selected to be approximately 600
mA, and the output power of laser monomode radiation P (equation
(16)) is about 592.6 mW. Here, .eta..sub.d (equation (8)) is
0.8421, and .eta. (equation (12)) is about 0.7808, and the angles
of divergence .THETA..sub.1 and .THETA..sub.2 are about 9.0 mrad
and 8.28 mrad, respectively.
[0130] Thirty GRs, which are in each of 32 rows of the grating that
are positioned across the length of the RIR, are electrically
interconnected in series, and the rows themselves are connected in
parallel. The galvanic series coupling of the aforementioned GRs is
implemented (see FIG. 15 and FIG. 14) by introducing an
electrically conductive first layer 31 (with a carrier
concentration of approximately 10.sup.18 cm.sup.-3) of RIR 7, an
insulating region 35 of width 15 .mu.m, metallization layers 36 to
ohmic contacts 15, connecting ohmic contacts 15 of two such
adjacent GRs, between which is an insulating region 35 that borders
(in contrast to barrier region 6) on the insulating region with
layer 32 of RIR 7 (see FIG. 15). The aforementioned metallization
layers 36, which also provide a parallel current connection of the
aforementioned 32 rows of GRs, are implemented as 16 lines
approximately 180 .mu.m wide and 9,600 .mu.m long. In the working
device, to each GR there will correspond a radiation near field and
an output laser beam on the reflector with a partially reflective
coating 12. If the dimensions, the length of the optical resonator
and the length of the GR itself are chosen properly, they will not
overlap (see FIG. 14). The total operating current through laser 1
is about 3.2 A for an operating voltage of 48 V (1.6 V for each
GR), and the total output power of all laser beams is about 568.9
W.
[0131] A characteristic feature of the lasers 1 depicted in FIGS.
16-19 is that a common RIR 7 with common facets 8and 9 is formed
for a linear sequence (or linear array) of GRs. The line of
intersection of the plane of the active layer with the extension of
the planes of facets 8 and 9 subtends a right angle with the gain
axes of the gain regions in the aforementioned linear array. The
device may contain a fairly large number of such linear arrays,
which are monolithically joined by heterostructure 2 with barrier
regions 6, which is common to all the linear arrays. A separate
laser beam will correspond to each GR in each such line of GRs. The
direction of radiation output from such multibeam lasers 1, with a
multiplicity of separate GRs, may be either perpendicular to the
plane of active layer 3 (see FIGS. 16 and 17) or at an angle .phi.
(see FIG. 18). In contrast to lasers 1 in FIGS. 14 and 15, the
thickness of RIR 7 in lasers 1 FIGS. 16-19 may be reduced
significantly, and the density of the laser beams per square
centimeter may be significantly increased.
[0132] In the laser 1 depicted in FIGS. 16 and 19, 32 linear
sequences or array of GRs, each of which contains 30 GRs that have
the same dimensions and parameters as the laser 1 in FIGS. 8 and
11, are made on the inner surfaces 18 of the 32 RIRs 7. With the
exception of its width of approximately 750 .mu.m, RIR 7 has the
same dimensions and characteristics as the laser 1 in FIGS. 8 and
11. On each of four sides, the GRs are current separated by barrier
regions 6, and an independent ohmic contact 15 with contact layer
14 is made by known methods. The spacing of the GRs in the line is
about 25 .mu.m, and the spacing between lines is about 135 .mu.m.
For independent supply of operating current to each GR, 30
longitudinal strips of metallization layers 36 are made to ohmic
contacts 15, and 32 transverse strips of metallization layers 37,
which are directed transverse to the optical gain axes of the GRs,
are formed to the ohmic contacts 16 of each of the 32 RIRs. When an
electrical signal is supplied to an arbitrary combination of two
mutually perpendicular metallization strips 36 and 37, the laser
beam is generated with the involvement of that GR which is disposed
between intersecting strips 36 and 37 of the selected metallization
strips. Each laser beam (of which there are 960 in all) has the
same parameters as in the laser 1 in FIGS. 9 and 11.
[0133] The laser 1 shown in FIGS. 20 and 21 include several GRs
that are connected in series, one after another, into a common
optical resonator. This is accomplished by virtue of reflections
from the outer optical surface 17 of RIR 7 (see FIG. 20) and by
virtue of reflections, with corresponding gain in laser radiation
from gain regions disposed on both surfaces of RIR 7 (see FIG. 21).
These embodiments provide an increased effective length of the
optical resonator for smaller thicknesses of RIR 7, including
situations with large outflow angles .phi.. Furthermore, in them
conditions more conducive to heat removal are provided as a result
of the distributed nature of the heat sources in the gain
regions.
[0134] Note also that for the laser 1 described with reference to
FIGS. 14-21, certain electrical connections of the longitudinal and
transverse strips of metallization layers 36 and 37 can be used to
obtain series, parallel, or series-parallel galvanic coupling of
the gain regions. This design makes it possible to perform more
effective matching of high-power multibeam lasers to power
sources.
[0135] The following laser 1, unlike the ones described above in
connection with FIG. 20, has one strip-type GR with length L.sub.GR
and width W.sub.GR. The RIR 7 is in the form of a rectangular
parallelepiped and includes one layer with a thickness d.sub.RIR
that is located above the lower contact layer 14 (which may be
necessary, depending on the materials employed in the laser 1). The
other ohmic contact 16 (with the corresponding metallization
layers) is located on the outer optic surface 17, except on
portions of the outer surface of the RIR where the strip-type GR is
projected onto this outer surface. The reflectors of the optical
resonator are disposed on optical facets of the RIR. The size of
the optical facets 8, 9 is at least equal to the product of the
thickness d.sub.RIR and the width W.sub.GR.
[0136] In another embodiment of the laser 1, the entire outer
surface 17 of the RIR 7 is covered with a semiconductive layer with
a refractive index smaller than n.sub.eff. The outer surface 17 of
the RIR 7 may, for example, be covered with a material having a
refractive index equal to n.sub.min, i.e., the lowest refractive
index of the semiconductive materials of the cladding layers 4 and
5 of heterostructure 2. Preferably, the contact semiconductive
layer 14 (if necessary) has a band gap smaller than that of the
semiconductor layer on the outer surface 17 of the RIR 7. The ohmic
contact 16 is located above this semiconductor layer on the outer
surface having index n.sub.min.
[0137] In yet another embodiment of the laser 1, the region of
injection or region of gain comprises a portion that starts out as
a rectangular strip about 2 to 5 .mu.m in width and from about 0.5
to several millimeters in length but which widens to form a
funnel-shaped injection region. The funnel shaped region is flared
at an ranging from about 5.degree. to 15.degree. over a length
which may range from about 0.5 to 2 mm or more. Such a
configuration enables the reduction of the threshold currents of
laser 1, leading to the further increase in its efficiency. In
addition, this configuration allows for the significant reduction
of the angle of divergence .theta..sub.2 of laser radiation in the
horizontal plane at the exit of the funnel.
[0138] A laser is also possible having an RIR 7 on both sides of
the active layer 3 of the heterostructure 2.
[0139] In these four embodiments described above, as well as in the
laser 1 depicted in FIG. 20, wherein a semiconductor is disposed on
the outer surface 17 of the RIR 7, the radiation flows into the RIR
7 approximately in the form of two flat waves directed at outflow
angles .phi. in two opposite directions, see, e.g., equation (2).
In addition, if the thickness d.sub.RIR is smaller than L.sub.IRIR
multiplied by tan.phi., a portion of the outflowing radiation
would, after falling on the semiconductor layer of the outer
surface 17 of the RIR with an index n.sub.min be completely
reflected from it. Subsequently, the reflected radiation will
return to the GR of the heterostructure 2 where it is re-radiated
back into the RIR 7. Once in the RIR 7, the light will be totally
internally reflected from the semiconductor layer with an index
n.sub.min once again. This process will continue to repeat itself.
The number of such radiation re-reflections is determined mainly by
the value of the angle .phi., determined by equation (2), the
thickness d.sub.RIR of RIR 7, and the length, L.sub.GR, of the
injection region.
[0140] The resulting radiation into the RIR 7 will be directed
approximately along the respective perpendiculars to the reflectors
of the optical resonator of the laser 1. These reflectors are
located on the optical facets of the RIR 7 through which laser
radiation will primarily be emitted.
[0141] The shape and size of the near-field radiation pattern of
the laser 1 at on the optical facets of the RIR 7 will be
restricted by the size of the rectangular facets, one of the sides
of which is slightly greater than the thickness d.sub.RIR, the
other side being slightly greater than the width W of the strip of
the injection region. Accordingly, the angle of divergence
.THETA..sub.1 in the vertical plane may be estimated as
approximately equal to the wavelength .lambda. of the radiation
divided by the thickness d.sub.RIR of the RIR 7. By selecting the
thickness d.sub.RIR, it is possible to reduce the angle of
divergence .THETA..sub.1 as well as the density of radiation, which
is important for obtaining high output power and a high reliability
for the Laser 1. This thickness d.sub.RIR preferably ranges from
about 2 to 100 .mu.m and in certain cases may be smaller than about
2 .mu.m.
[0142] It is also important to note that, with respect to the
embodiments of Laser 1 described above, as a result of the
reduction of the thickness d.sub.RIR of RIR 7, it is not necessary
to rely on wafer bonding technology. In contrast, the
heterostructure 2, along with the RIR 7, the semiconductor layer
with index n.sub.min formed thereon, as well as the contact layer
14 may be deposited in the single process of epitaxial growth. This
approach simplifies the manufacturing process and enables less
expensive manufacturing equipment to be employed to fabricate a
variety of laser which operate at different wavelengths
.lambda..
[0143] The laser depicted in FIGS. 22 and 11, in contrast to the
ones described above with regard to FIGS. 1 and 11 has three
strip-type gain regions (GR), a middle GR surrounded by two end GRs
with contacts 14 at the end of the cladding layer 4. The thickness
d.sub.RIR of RIR 7 has been selected to be at least as large as the
value of thickness d.sub.RIR by formula (15) where L.sub.IRIR is
replaced by the length of the end GR designated hereinafter as
L.sub.GRE. The thickness of the cladding layer 5 of the middle GR
is greater than the thickness of the end GRs, for example, the
thickness of the layer 5 in the middle GR may be equal to the
thickness of layer 4 as shown in FIG. 22. In an operating laser 1,
at the current densities discussed above, the intensity of outgoing
radiation in the end GRs is significantly greater than the
intensity of outgoing radiation in the middle GR, where it is
practically nil. As a result of this phenomenon, stimulated
(super-luminescent) radiation of a sufficient power is introduced
from the middle GR to the end GRs. This stimulated
(super-luminescent) radiation enables electrical power introduced
into the end GRs to be converted into a directed radiation in a
highly efficient process which generates laser radiation. This
generation occurs in the optical resonator which includes the
middle and end GRs, as well as a portion of the RIR 7. The
reflectors of the optical resonator comprise one optical facet 8
having a reflective coating 11 thereon and the other optical facet
9 with a partially reflective coating 12 adhere thereto. This
design advantageously has a low threshold current density
j.sub.thr, a high conversion efficiency .eta., and a thickness
d.sub.RIR of the RIR 7 that is small as compared to the thickness
of the RIR in the laser 1 discussed above with regard to FIGS. 1
and 11. Having a small thickness d.sub.RIR allows for the growth of
the RIR layer 7 by the methods of epitaxial growth, thereby
simplifying the process of manufacture.
[0144] The laser 1 shown in FIGS. 23 and 11 differs from that of
FIG. 22 in that an identical single-layer RIR 7 and an identical
cladding layer 5 are associated with all three GRs, the middle and
two ends. Nevertheless, the current density in the middle GR of the
operating laser 1 has a value that allows for super-luminescent
radiation flowing from the middle GR into the end GRs to be much
higher than the radiation outgoing from the middle GR. The
selection of heterostructures achieving these requirements was made
by employing numerical methods.
[0145] Another embodiment of the laser 1 (not shown) is similar to
the laser shown in FIG. 22 except that the contact layer 14 and the
ohmic 15 are continuous. This design produces equal current
densities through the middle and end GRs and advantageously offers
simplicity of operation.
[0146] Note that these design features extend to the laser 1
depicted in FIGS. 2-10 and 14-19, and 20-21 as well as to the laser
of FIG. 1.
[0147] The laser 1 depicted in FIGS. 24 and 11, unlike the laser 1
in FIG. 22, has an RIR 7 shaped as a rectangular parallelepiped
with a small thickness d.sub.RIR, which preferably ranges from
about 2 to about 10 .mu.m. This thickness can be much smaller than
the length of the end GR, L.sub.GRE, multiplied by tan.phi.. A
semiconductive layer 38 with a refraction factor n.sub.min is
located on the outer surface 17 of the RIR 7 similar to the one
described above. The construction of the RIR 7 and the functioning
of the semiconductor layer formed on the outer surface 17 of the
RIR are similar to those described above in connection for example
with the laser 1 of FIG. 20. The design of FIG. 24 advantageously
reduces the generation threshold and increases efficiency.
[0148] The laser 1 depicted in FIG. 25 differs because it contains
two outflow regions 7 located on both sides of the active layer 3
which like the RIR in the laser of FIG. 24 comprises relatively
thin layers. The two outflow regions 7 also have associated
therewith adjacent semiconductive layers 38. In this embodiment,
the layers of RIR 7 and cladding layers 5 are identical for all
three GRs. Functionally, this embodiment is similar to the laser 1
depicted in FIG. 23; in the operating laser 1, the current density
in the middle GR is greater than in the end GRs.
[0149] In addition to the embodiments described above, lasers 1
with two GRs could be also be used, the first of these GRs being
similar in construction to the middle GR, while the second one
being similar to the end GR. While the first GR preferably
comprises the strip-type, the second one can preferably is
funnel-shaped, widening from the strip of the first GR.
[0150] Thus, in the lasers 1 described herein, the output power of
the laser radiation is increased manifold, the threshold current
densities are reduced significantly and the angles of divergence in
two mutually perpendicular directions are reduced, the effective
lengths of optical resonators are increased, near-maximum values of
the efficiencies (including the differential efficiency) are
attained, and the dependence of the generated wavelength of the
laser radiation on the pumping-current amplitude is reduced
significantly. High-efficiency lasers with output of laser beams,
including output in the direction perpendicular to the plane of the
active layer, and multibeam lasers, including those turned on
independently, also are obtained.
[0151] In general injection lasers are used in fiber-optic
communications and data-transmission systems, in ultra-high-speed
optical computing and switching systems, in the design of medical
equipment, laser process equipment, and frequency-doubled lasers,
and for the pumping of solid-state and fiber lasers. Accordingly,
the injection lasers described above can be advantageously employed
in these various application as well as numerous others.
[0152] Moreover, the present invention may be embodied in other
specific forms without departing from the essential characteristics
as described herein. The embodiments described above are to be
considered in all respects as illustrative only and not restrictive
in any manner. The scope of any invention is, therefore, indicated
by the following claims rather than the foregoing description. Any
and all changes which come within the meaning and range of
equivalency of the claims are to be considered in their scope.
1TABLE Names & Nos. of layers, Thickness of Carrier Absorption
sublayers, Composition layer Refractive Type of concentration
coefficient & regions of layer d (.mu.m) index n conduction
N.sub.e (cm.sup.-3) .alpha. (cm.sup.-1) 1 2 3 4 5 6 7 Contact layer
14 GaAs 0.3 3.525 P 2 .times. 10.sup.19 140 Cladding layer, 4 19
Al.sub.0.6Ga.sub.0.4As 0.9 3.20 P 1 .times. 10.sup.18 7.0 sublayers
20 Al.sub.0.6Ga.sub.0.4As 0.6 3.20 P 2 .times. 10.sup.17 3.50 21
GaAs 0.06 3.525 -- -- 3.0 Active layer, 3 22 In.sub.0.2Ga.sub.0.8As
0.008 3.63 -- -- 3.0 sublayers 23 GaAs 0.012 3.525 -- -- 3.0 24
In.sub.0.2Ga.sub.0.8As 0.008 3.63 -- -- 3.0 Cladding layer, 5 25
GaAs 0.06 3.525 -- -- 3.0 sublayers 26 Al.sub.0.6Ga.sub.0.4As 0.1
3.20 N 5 .times. 10.sup.17 2.0 27 Al.sub.0.6Ga.sub.0.4As 0.25 3.20
N 1 .times. 10.sup.18 3.0 Radiation inflow 7 GaAs 1286.0 3.525 N 1
.times. 10.sup.18 0.1 region
* * * * *