U.S. patent number RE35,215 [Application Number 08/307,174] was granted by the patent office on 1996-04-23 for frequency converted laser diode and lens system therefor.
This patent grant is currently assigned to SDL, Inc.. Invention is credited to Robert J. Lang, Derek W. Nam, Donald R. Scifres, Robert G. Waarts, David F. Welch.
United States Patent |
RE35,215 |
Waarts , et al. |
April 23, 1996 |
**Please see images for:
( Certificate of Correction ) ** |
Frequency converted laser diode and lens system therefor
Abstract
A compact semiconductor laser light source providing short
wavelength (ultraviolet, blue or green) coherent light by means of
frequency doubling of red or infrared light from a high power diode
heterostructure. The high power diode heterostructure is a MOPA
device having a single mode laser oscillator followed by a
multimode, preferably flared, optical power amplifier. A tunable
configuration having an external rear reflector grating could also
be used. A lens could be integrated with the MOPA to laterally
collimate the light before it is emitted. Straight or curved,
surface emitting gratings could also be incorporated. An
astigmatism-correcting lens system having at least one cylindrical
lens surface is disposed in the path of the output from the MOPA to
provide a beam with substantially equal lateral and transverse beam
width dimensions and beam divergence angles. A nonlinear optical
crystal or waveguide is placed in the path of the astigmatism-free
symmetrized beam to double the frequency of the light. Single pass
or multipass configurations with reflectors could be used, as well
as external resonator and segmented, periodically poled waveguide
configurations.
Inventors: |
Waarts; Robert G. (Palo Alto,
CA), Welch; David F. (Menlo Park, CA), Scifres; Donald
R. (San Jose, CA), Lang; Robert J. (Pleasanton, CA),
Nam; Derek W. (Sunnyvale, CA) |
Assignee: |
SDL, Inc. (San Jose,
CA)
|
Family
ID: |
21745004 |
Appl.
No.: |
08/307,174 |
Filed: |
September 16, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
010279 |
Jan 28, 1993 |
05321718 |
Jun 14, 1994 |
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Current U.S.
Class: |
372/108; 359/326;
359/328; 359/333; 359/344; 359/346; 372/101; 372/21; 372/22;
372/23; 372/50.23; 372/99 |
Current CPC
Class: |
G02F
1/37 (20130101); H01S 5/0265 (20130101); H01S
5/50 (20130101); H01S 5/005 (20130101); H01S
5/0092 (20130101); H01S 5/026 (20130101); H01S
5/06256 (20130101); H01S 5/1064 (20130101); H01S
5/1215 (20130101); H01S 5/141 (20130101); H01S
5/187 (20130101); H01S 5/4025 (20130101); H01S
5/4031 (20130101); H01S 5/4068 (20130101); H01S
5/4087 (20130101) |
Current International
Class: |
G02F
1/35 (20060101); G02F 1/37 (20060101); H01S
5/50 (20060101); H01S 5/00 (20060101); H01S
5/40 (20060101); H01S 3/00 (20060101); H01S
5/026 (20060101); H01S 5/10 (20060101); H01S
5/187 (20060101); H01S 5/0625 (20060101); H01S
5/14 (20060101); H01S 5/12 (20060101); H01S
003/08 (); H01S 003/10 () |
Field of
Search: |
;372/21,22,23,43,49,50,92,96,97,98,99,101,108
;359/333,342,343,344,346,326,328,330,332 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Kozlovsky, W. J. et al., "Generation of 41mW of blue radiation by
frequency doubling of a GaAlAs diode laser", Appl. Phys. Lett. 56
(23), 4 Jun. 1990, pp. 2291-2292. .
Lin, J. T., "Doubled Jeopardy: The Blue-Green Race's New Players",
Lasers & Optronics, Dec. 1990, pp. 34-40. .
Van der Poel, C. J. et al., "Efficient type 1 blue second-harmonic
generation in periodically segmented KTiOPO.sub.4 waveguides",
Appl. Phys. Lett. 57 (20), 12 Nov. 1990, pp. 2074-2076. .
Harada et al., "Generation of blue coherent light from a
continuous-wave semiconductor laser using an organic crystal-cored
fiber", Appl. Phys. Lett. 59 (13), 23 Sep. 1991, pp. 1535-1537.
.
Rikken, G. L. J. A. et al., "Poled polymers for frequency doubling
of diode lasers", Appl. Phys. Lett. 58 (5), 4 Feb. 1991, pp.
435-437. .
Goldberg, L. et al., "Blue Light generation by nonlinear mixing of
Nd:YAG and GaAlAs laser emission in a KNbO.sub.3 resonant cavity",
Appl. Phys. Lett. 56 (21), 21 May 1990, pp. 2071-2073. .
Risk, W. P., "Fabrication and characterization of planar
ion-exchanged KTiOPO.sub.4 waveguides for frequency doubling",
Appl. Phys. Lett. 58 (1), 7 Jan. 1991, pp. 19-21. .
Carts, Y. A., Assoc. Ed., "Nonlinear conversion refines blue-light
devices", Laser Focus World, May 1992, pp. 107-110. .
Bosenberg, W. et al., "Parametric optical generation: from research
to reality", Laser Focus World, May 1992, pp. 165-170..
|
Primary Examiner: Healy; Brian
Attorney, Agent or Firm: Schneck & McHugh
Claims
We claim:
1. A coherent light source comprising
at least one semiconductor optical source generating and emitting
an optical output with a single transverse and lateral mode, said
optical output being emitted from an optical aperture of said at
least one semiconductor optical source with a lateral-to-transverse
emission ratio of at least 10 to 1,
means for exciting said at least one semiconductor optical source
to generate said optical output,
a nonlinear optical material, and
optical coupling means for coupling said optical output from said
at least one semiconductor optical source into said nonlinear
optical material.
2. The coherent light source of claim 1 wherein at least one of
said semiconductor optical source contains a flared optical power
amplifier.
3. The coherent light source of claim 1 wherein at least one said
semiconductor optical source is an at most marginally stable
optical resonator with a spatial mode filter.
4. The coherent light source of claim 1 wherein at least one of
said semiconductor optical sources is wavelength tunable.
5. The coherent light source of claim 1 wherein said optical
coupling means also corrects for astigmatism of said optical output
from said at least one semiconductor optical source between
orthogonal transverse and lateral directions defined by said at
least one semiconductor optical source.
6. The coherent light source of claim 5 wherein said optical
coupling means also focuses said optical output to transverse and
lateral beam waists located within said nonlinear optical
material.
7. The coherent light source of claim 1 wherein said excitation
means includes means for modulating the optical output from at
least one said semiconductor optical source.
8. The coherent light source of claim 7 wherein said excitation
means further includes means for electrically exciting at least one
said semiconductor optical source with at least one different
current density along a length of said semiconductor optical
source.
9. The coherent light source of claim 1 wherein said nonlinear
optical material is located within a resonant optical cavity.
10. The coherent light source of claim 9 wherein said resonant
optical cavity is configured as a ring resonant cavity.
11. The coherent light source of claim 9 wherein said resonant
optical cavity is configured as a linear resonant cavity.
12. The coherent light source of claim 1 wherein said nonlinear
optical material includes a nonlinear material optical waveguide
with polarization domain reversals.
13. The coherent light source of claim 12 wherein said polarization
domain reversals are substantially periodic.
14. The coherent light source of claim 12 wherein said polarization
domain reversals occur with a variable periodicity, whereby
frequency conversion is allowed for a broad band of input
wavelengths.
15. A semiconductor laser comprising
a master oscillator power amplifier (MOPA) device capable of
generating a high power asymmetric coherent light beam of a first
wavelength, and
an optical frequency converter positioned to receive said high
power coherent light beam from said MOPA device and capable of
generating a second light beam from a portion of the power of said
high power light beam, said second light beam having a second
wavelength different from said first wavelength.
16. The laser of claim 15 wherein said MOPA device includes a
single mode DBR laser oscillator.
17. The laser of claim 15 wherein said MOPA device includes a
multimode power amplifier region.
18. The laser of claim 15 wherein said multimode amplifier region
is a flared amplifier region.
19. The laser of claim 15 wherein said frequency converter is a
second harmonic generator.
20. The laser of claim 19 wherein said second harmonic generator
comprises a bulk crystal of optically nonlinear material.
21. The laser of claim 20 wherein said bulk crystal is located
within a resonant optical cavity.
22. The laser of claim 19 wherein said second harmonic generator
includes a nonlinear material optical waveguide.
23. The laser of claim 22 wherein said waveguide has periodic
ferroelectric polarization domain reversals.
24. The laser of claim 22 wherein said waveguide supports only a
single spatial mode of light propagation.
25. The laser of claim 22 wherein said waveguide is a broad area
multimode waveguide.
26. The laser of claim 15 wherein said high power coherent light
beam of said first wavelength has a light path that passes through
said frequency converter only once.
27. The laser of claim 15 wherein said high power beam has a light
path that passes through said frequency converter at least twice,
reflection means being positioned in said light path of said high
power beam for reflecting light of at least said first wavelength
back into said frequency converter.
28. The laser of claim 27 wherein said reflection means is highly
reflective of light of both said first and second wavelengths.
29. The laser of claim 28 wherein said reflection means comprises a
planar mirror, a beamsplitter transmissive of said first wavelength
and reflective of said second wavelength being positioned in a
return light path of said high power light beam and of said second
light beam to couple said second light beam out of said laser as an
output beam.
30. The laser of claim 27 wherein said reflection means comprises
at least one right-angle retroreflector positioned in said light
path to return said high power beam through said frequency
converter along a parallel light path.
31. The laser of claim 30 further including means for focussing
said light beam in said frequency converter and collimating said
light beam for incidence upon said at least one retroreflector.
32. The leer of claim 31 wherein said focussing and collimating
means comprises a pair of lens arrays situated in said parallel
light paths on opposite sides of said frequency converter.
33. A semiconductor laser comprising
a master oscillator power amplifier (MOPA) device capable of
generating a high power coherent light beam of a first
wavelength,
an optical frequency converter positioned to receive said high
power coherent light beam from said MOPA device and capable of
generating a second light beam from a portion of the power of said
high power light beam, said second light beam having a second
wavelength different from said first wavelength, and
an astigmatism-correcting lens system positioned in said light path
between said MOPA device and said frequency converter, said lens
system providing a modified laser beam with substantially equal
lateral and transverse beam width dimensions and substantially
equal lateral and transverse beam divergence angles, where
`lateral` and `transverse` refer to directions respectively
parallel and perpendicular to a plane of an active gain region of
said MOPA device.
34. The laser of claim 33 wherein said lens system comprises a
first spherical lens positioned with a focal plane thereof at an
output surface of said MOPA device so as to collimate the light
beam in the transverse direction and to focus the light beam in the
lateral direction, and a positive cylinder lens positioned beyond
the lateral focus of the light beam where the lateral width
dimension of the light beam has re-expanded to be identical to the
transverse width dimension of the light beam, said cylinder lens
having a positive lateral focal length equal to an effective
optical distance from the lateral focus to said cylinder lens so as
to collimate the light beam in the lateral direction.
35. The laser of claim 34 wherein said first spherical lens is a
compound lens which is diffraction limited for both finite and
infinite conjugate distances of said light beam received from said
MOPA device.
36. The laser of claim 34 wherein said first spherical lens is a
single asphere.
37. The laser of claim 34 wherein said lens system further includes
a negative cylinder lens positioned between said lateral focus of
said light beam and said positive cylinder lens, said negative
cylinder lens having a negative lateral focal length.
38. The laser of claim 33 wherein said lens system comprises a
microlens with two crossed positive cylinder lens surfaces formed
on opposite sides of a transparent substrate, a first of said
cylinder lens surfaces being positioned at a distance from an
output surface of said MOPA device at which the transverse width
dimension of said light beam has expanded to substantially equal
the lateral width dimension of said light beam at a second of said
cylinder lens surfaces, said first cylinder lens surface having a
transverse focal length substantially equal to the distance from
said output surface of said MOPA device to said first cylinder lens
surface so as to collimate the light beam in the transverse
direction, said second cylinder lens surface having a lateral focal
length substantially equal to the effective optical distance of the
light path from an input end of a power amplifier within said MOPA
device through said output surface to said second cylinder lens
surface so as to collimate the light beam in the lateral
direction.
39. The laser of claim 33 wherein said lens system comprises a
microlens with a positive toric lens surface and a planar surface
on opposite sides of a transparent substrate, said toric lens
surface being positioned at a first effective optical distance of
the light path from an output surface of said MOPA device thereto
where the transverse width dimension of said light beam has
expanded to substantially equal the lateral width dimension, said
toric lens surface having a transverse focal length substantially
equal to said first effective optical distance so as to collimate
the light beam in the transverse direction, said toric lens surface
also having a lateral focal length substantially equal to a second
effective optical distance from an input end of a power amplifier
within said MOPA device to said toric lens surface so as to also
collimate the light beam in the lateral direction.
40. The laser of claim 33 wherein said MOPA device includes a lens
integrated therein at an output end of a power amplifier of said
MOPA device, said lens having a lateral focal length in said device
substantially equal to an effective optical distance from an input
end of said power amplifier to said lens so as to collimate the
light in the lateral direction prior to being emitted from an
output surface of said MOPA device.
41. The laser of claim 40 wherein said lens system comprises a
cylinder microlens positioned in the light path at a distance where
the transverse width dimension of the light beam has expanded to
substantially equal the lateral width dimensions and having a
transverse focal length substantially equal to said distance so as
to collimate said light beam in the transverse direction.
42. The laser of claim 40 wherein said lens system comprises a
first spherical lens having a positive focal length equal to a
distance from said output surface of said MOPA device to said first
spherical lens so as to collimate the light beam in the transverse
direction and to focus the light beam in the lateral direction, and
a positive cylinder lens positioned beyond the lateral focus of the
light beam where the lateral width dimension of the light beam has
reexpanded to be substantially identical to the transverse width
dimension of the light beam, said cylinder lens having a positive
lateral focal length substantially equal to an effective optical
distance from the lateral focus to said cylinder lens so as to
collimate the light beam in the lateral direction.
43. The laser of claim 40 wherein said MOPA device further includes
a surface emitting detuned grating output coupler positioned to
receive said laterally collimated light from said integral lens and
to direct the light vertically out of a top or bottom surface of
said MOPA device.
44. The laser of claim 43 wherein said grating output coupler has a
grating pitch and overall length selected so that the vertically
directed output light beam from said MOPA device has substantially
symmetric width dimensions.
45. The laser of claim 15 wherein said MOPA device includes a
surface emitting detuned grating output coupler positioned at an
output end of a power amplifier of said MOPA device to receive
light therefrom and direct the light vertically out of a top or
bottom surface of said MOPA device.
46. The laser of claim 45 wherein said grating output coupler has
plural, periodically spaced grating teeth, each grating tooth being
curved to coincide with a lateral phase front of the light so as to
collimate the vertically directed light beam in the lateral
direction.
47. The laser of claim 45 wherein said grating output coupler has
plural, periodically spaced, parallel, straight grating teeth, said
laser further comprising a cylinder lens in the path of the
vertically directed output light beam with a lateral focal length
substantially equal to an effective optical distance along the
light path to an input end of said power amplifier of said MOPA
device so as to collimate the output light beam in the lateral
direction.
48. A semiconductor laser comprising
a master oscillator power amplifier (MOPA) device having a single
mode laser oscillator and a multimode optical power amplifier
region coupled to said laser oscillator capable of generating a
high power coherent light beam, said light beam characterized by
differing lateral and transverse beam width dimensions and
differing lateral and transverse beam divergence angles, with a
lateral beam waist located at an input end of said power amplifier
region proximate to said laser oscillator and with a transverse
beam waist located at an output surface of said MOPA device, where
`lateral` and `transverse` refer to directions respectively
parallel and perpendicular to a plane of an active gain region of
said MOPA device, and
an astigmatism-correcting lens system positioned in the path of
said light beam output from said MOPA device, said lens system
adapted to provide a modified laser beam with substantially equal
lateral and transverse beam width dimensions and substantially
equal lateral and transverse beam divergence angles.
49. The laser of claim 48 wherein said MOPA device includes a
single mode DBR laser oscillator.
50. The laser of claim 48 wherein said MOPA device includes a
multimode power amplifier region.
51. The laser of claim 50 wherein said multimode amplifier region
is a flared amplifier region.
52. The laser of claim 48 wherein said lens system comprises a
first spherical lens positioned with a focal plane thereof at an
output surface of said MOPA device so as to collimate the light
beam in the transverse direction and to focus the light beam in the
lateral direction, and a positive cylinder lens positioned beyond
the lateral focus of the light beam where the lateral width
dimension of the light beam has re-expanded to be identical to the
transverse width dimension of the light beam, said cylinder lens
having a positive lateral focal length equal to an effective
optical distance from the lateral focus to said cylinder lens so as
to collimate the light beam in the lateral direction.
53. The laser of claim 52 wherein said first spherical lens is a
compound lens which is diffraction limited for both finite and
infinite conjugate distances of said light beam received from said
MOPA device.
54. The laser of claim 52 wherein said first spherical lens is a
single asphere.
55. The laser of claim 52 wherein said lens system further includes
a negative cylinder lens positioned between said lateral focus of
said light beam and said positive cylinder lens, said negative
cylinder lens having a negative lateral focal length.
56. The laser of claim 48 wherein said lens system comprises a
microlens with two crossed positive cylinder lens surfaces formed
on opposite sides of a transparent substrate, a first of said
cylinder lens surfaces being positioned at a distance from an
output surface of said MOPA device at which the transverse width
dimension of said light beam has expanded to substantially equal
the lateral width dimension of said light beam at a second of said
cylinder lens surfaces, said first cylinder lens surface having a
transverse focal length substantially equal to the distance from
said output surface of said MOPA device to said first cylinder lens
surface so as to collimate the light beam in the transverse
direction, said second cylinder lens surface having a lateral focal
length substantially equal to the effective optical distance of the
light path from an input end of a power amplifier within said MOPA
device through said output surface to said second cylinder lens
surface so as to collimate the light beam in the lateral
direction.
57. The laser of claim 48 wherein said lens system comprises a
microlens with a positive toric lens surface and a planar surface
on opposite sides of a transparent substrate, said toric lens
surface being positioned at a first effective optical distance of
the light path from an output surface of said MOPA device thereto
where the transverse width dimension of said light beam has
expanded to substantially equal the lateral width dimension, said
toric lens surface having a transverse focal length substantially
equal to said first effective optical distance so as to collimate
the light beam in the transverse direction, said toric lens surface
also having a lateral focal length substantially equal to a second
effective optical distance from an input end of a power amplifier
within said MOPA device to said toric lens surface so as to also
collimate the light beam in the lateral direction.
58. The laser of claim 48 wherein said MOPA device includes a lens
integrated therein at an output end of a power amplifier of said
MOPA device, said lens having a lateral focal length in said device
substantially equal to an effective optical distance from an input
end of said power amplifier to said lens so as to collimate the
light in the lateral direction prior to being emitted from an
output surface of said MOPA device.
59. The laser of claim 58 wherein said lens system comprises a
cylinder microlens positioned in the light path at a distance where
the transverse width dimension of the light beam has expanded to
substantially equal the lateral width dimensions and having a
transverse focal length substantially equal to said distance so as
to collimate said light beam in the transverse direction.
60. The laser of claim 58 wherein said lens system comprises a
first spherical lens having a positive focal length equal to a
distance from said output surface of said MOPA device to said first
spherical lens so as to collimate the light beam in the transverse
direction and to focus the light beam in the lateral direction, and
a positive cylinder lens positioned beyond the lateral focus of the
light beam where the lateral width dimension of the light beam has
reexpanded to be substantially identical to the transverse width
dimension of the light beam, said cylinder lens having a positive
lateral focal length substantially equal to an effective optical
distance from the lateral focus to said cylinder lens so as to
collimate the light beam in the lateral direction.
61. The laser of claim 58 wherein said MOPA device further includes
a surface emitting detuned grating output coupler positioned to
receive said laterally collimated light from said integral lens and
to direct the light vertically out of a top or bottom surface of
said MOPA device.
62. The laser of claim 61 wherein said grating output coupler has a
grating pitch and overall length selected so that the vertically
directed output light beam from said MOPA device has substantially
symmetric width dimensions.
63. The laser of claim 48 wherein said MOPA device includes a
surface emitting detuned grating output coupler positioned at an
output end of a power amplifier of said MOPA device to receive
light therefrom and direct the light vertically out of a top or
bottom surface of said MOPA device.
64. The laser of claim 63 wherein said grating output coupler has
plural, periodically spaced grating teeth, each grating tooth being
curved to coincide with a lateral phase front of the light so as to
collimate the vertically directed light beam in the lateral
direction.
65. The laser of claim 63 wherein said grating output coupler has
plural, periodically spaced, parallel, straight grating teeth, said
laser further comprising a cylinder lens in the path of the
vertically directed output light beam with a lateral focal length
substantially equal to an effective optical distance along the
light path to an input end of said power amplifier of said MOPA
device so as to collimate the output light beam in the lateral
direction.
66. A laser of claim 48 wherein an optical frequency converter is
positioned to receive said modified laser beam from said
astigmatism-correcting lens system and capable of generating a
second light beam from a portion of the power of said modified
laser beam, said high power coherent light beam from said MOPA
device and said modified laser beam from said lens system being of
a first wavelength, said second laser beam generated by said
optical frequency converter being of a second wavelength different
from said first wavelength.
67. The laser of claim 66 wherein said frequency converter is a
second harmonic generator.
68. The laser of claim 67 wherein said second harmonic generator
comprises a bulk crystal of optically nonlinear material.
69. The laser of claim 68 wherein said bulk crystal is located
within a resonant optical cavity.
70. The laser of claim 67 wherein said second harmonic generator
includes a nonlinear material optical waveguide.
71. The laser of claim 70 wherein said waveguide has periodic
ferroelectric polarization domain reversals.
72. The laser of claim 70 wherein said waveguide supports only a
single spatial mode of light propagation.
73. The laser of claim 70 wherein said waveguide is a broad area
multimode waveguide.
74. The laser of claim 66 wherein said high power coherent light
beam of said first wavelength has a light path that passes through
said frequency converter only once.
75. The laser of claim 66 wherein said high power beam has a light
path that passes through said frequency converter at least twice,
reflection means being positioned in said light path of said high
power beam for reflecting light of at least said first wavelength
back into said frequency converter.
76. The laser of claim 75 wherein said reflection means is highly
reflective of light of both said first and second wavelengths.
77. The laser of claim 76 wherein said reflection means comprises a
planar mirror, a beamsplitter transmissive of said first wavelength
and reflective of said second wavelength being positioned in a
return light path of said high power light beam and of said second
light beam to couple said second light beam out of said laser as an
output beam.
78. The laser of claim 75 wherein said reflection means comprises
at least one right-angle retroreflector positioned in said light
path to return said high power beam through said frequency
converter along a parallel light path.
79. The laser of claim 78 further including means for focussing
said light beam in said frequency converter and collimating said
light beam for incidence upon said at least one retroreflector.
80. The laser of claim 79 wherein said focussing and collimating
means comprises a pair of lens arrays situated in said parallel
light paths on opposite sides of said frequency converter.
81. A semiconductor laser comprising
at least two semiconductor laser oscillators capable of oscillating
at and emitting light beams of different wavelengths,
a semiconductor optical power amplifier optically coupled to said
laser oscillators,
excitation means for pumping said laser oscillators and said
optical power amplifier,
nonlinear optical material optically coupled to said optical power
amplifier to receive high power coherent light therefrom so as to
convert at least one wavelength of said light into another
wavelength by a nonlinear optical process.
82. The laser of claim 81 wherein at least one of said laser
oscillators has means for tuning its oscillation and emission
wavelength.
83. The laser of claim 81 wherein said excitation means for said at
least two laser oscillators are injection currents that are
independent of one another.
84. The laser of claim 81 wherein said amplifier has a lateral
aperture width that permits lightwaves received from said laser
oscillators to expand laterally by diffraction.
85. The laser of claim 84 wherein said amplifier has a wider
lateral aperture at an output end thereof than the lateral aperture
width at an input end thereof adjacent to said laser
oscillators.
86. The laser of claim 81 wherein said excitation means for said
amplifier is at least one injection current applied thereto which
is independent from injection currents applied to said laser
oscillators.
87. The laser of claim 81 wherein said amplifier is differentially
pumped along its length.
88. The laser of claim 81 wherein astigmatism correcting, optical
means are disposed between said amplifier and said nonlinear
optical material for providing an anastigmatic, symmetric light
beam to said nonlinear optical material.
89. The laser of claim 81 wherein said nonlinear optical process is
sum frequency mixing.
90. The laser of claim 81 wherein said nonlinear optical process is
difference frequency mixing.
91. The laser of claim 81 wherein said excitation means drives only
a selected one of said laser oscillators to generate light of a
single selected wavelength, said nonlinear optical process being
frequency doubling.
92. The laser of claim 81 wherein said at least two semiconductor
laser oscillators are closely spaced apart from one another and
each laser oscillator couples directly to said amplifier.
93. The laser of claim 81 wherein means for combining said light
beams of different wavelengths is positioned between said laser
oscillators and said amplifier.
94. A coherent light source comprising
at least two laser sources of different wavelengths, at least one
of said laser sources being a semiconductor master oscillator power
amplifier (MOPA) device,
excitation means for pumping said laser sources to generate high
power coherent light,
a nonlinear optical material, and
means for combining said light from said laser sources in said
nonlinear optical material.
95. The coherent light source of claim 94 wherein an amplifier
portion of said at least one MOPA device has a lateral aperture
width that allows light in said MOPA device to expand laterally at
least along a portion of the length of said amplifier portion.
96. The coherent light source of claim 94 wherein at least one of
said laser sources is wavelength tunable.
97. The coherent light source of claim 94 wherein said excitation
means are capable of providing different excitation levels to
different portions of said laser sources.
98. The coherent light source of claim 94 wherein said nonlinear
optical material is contained within a resonant cavity.
99. A coherent light source comprising
an array of semiconductor master oscillator power amplifiers
(MOPAs), each of said MOPAs in said array emitting light from an
output surface of said MOPA which is asymmetric in its beam width
between orthogonal transverse and lateral directions defined by
said MOPA, with a lateral-to-transverse beam width ratio for said
light output of at least 10 to 1 at said output surface,
a nonlinear optical material capable of generating at least one
light wavelength that is different from a wavelength of light
provided by at least one of said MOPAs, and
optical coupling means for bringing light generated by each of said
MOPAs into said nonlinear optical material with a high power
density.
100. The coherent light source of claim 99 wherein at least two of
said MOPAs emit different wavelengths of light.
101. The coherent light source of claim 99 wherein at least one of
said MOPAs is wavelength tunable.
102. The coherent light source of claim 99 wherein at least one of
said MOPAs is modulatable.
103. The coherent light source of claim 99 wherein said optical
coupling means comprises at least one lens array disposed between
said array of MOPAs and said nonlinear optical material.
104. The coherent light source of claim 103 wherein said optical
coupling means comprises a cylinder lens for focusing light in a
transverse direction perpendicular to said array of MOPAs and a
cylinder lens array for focusing said light in a lateral direction
parallel to said array of MOPAs, said cylinder lens and said
cylinder lens array positioned to provide transverse and lateral
beam waists within said nonlinear optical material.
105. The coherent light source of claim 103 wherein said optical
coupling means comprises a cylinder lens for collimating light in a
transverse direction perpendicular to said array of MOPAs and a
spherical lens array positioned with respect to said cylinder lens
so as to bring light beams from said array of MOPAs to a focus
inside said nonlinear optical material.
106. The coherent light source of claim 99 wherein a single
diffraction lens array brings at least one beam from said array of
MOPAs to a focus in said nonlinear optical material.
107. A coherent light source comprising
at least one monolithic semiconductor master oscillator power
amplifier (MOPA) device,
means for exciting said at least one MOPA device to generate and
emit an optical output, said optical output from said at least one
MOPA device being asymmetric in its beam width between orthogonal
transverse and lateral directions defined by said MOPA device, with
a lateral-to-transverse beam width ratio for said optical output of
at least 10 to 1 at an optical output aperture of said at least one
MOPA device,
a nonlinear optical material contained within a resonant optical
cavity, and
optical coupling means for matching the optical output from said at
least one MOPA device to said resonant optical cavity containing
said nonlinear optical material.
108. The coherent light source of claim 107 wherein at least one
said MOPA device includes a multimode optical power amplifier.
109. The coherent light source of claim 107 wherein at least one
said MOPA device includes a flared optical power amplifier.
110. The coherent light source of claim 107 wherein at least one
said MOPA device is wavelength tunable and said excitation means
includes means for tuning the wavelength of said optical output
from said at least one wavelength tunable MOPA device to match a
resonant condition of said resonant optical cavity containing said
nonlinear optical material.
111. The coherent light source of claim 107 said optical coupling
means focuses said optical output to transverse and lateral beam
waists located within said resonant optical cavity containing said
nonlinear optical material, said transverse and lateral beam waists
of said focused optical output matching beam waists established by
said resonant optical cavity.
112. The coherent light source of claim 111 wherein said optical
coupling means also corrects for astigmatism of said optical output
from said at least one MOPA device between orthogonal transverse
and lateral directions of said optical output defined by said MOPA
device.
113. The coherent light source of claim 112 wherein said optical
coupling means includes an adjustable lens system for providing
said astigmatism correction.
114. The coherent light source of claim 107 wherein said optical
coupling means includes at least one optical element monolithically
integrated with at least one said MOPA device.
115. The coherent light source of claim 114 wherein said
monolithically integrated optical element has means for
electronically controlling an optical parameter of said optical
element so as to be able to optimize coupling of said optical
output to said resonant optical cavity containing said nonlinear
optical material.
116. The coherent light source of claim 107 wherein said resonant
optical cavity is a ring resonant cavity.
117. The coherent light source of claim 107 wherein said resonant
optical cavity is a linear resonant cavity.
118. A coherent light source comprising
at least one monolithic semiconductor master oscillator power
amplifier (MOPA) device,
means for exciting said at least one MOPA device to generate and
emit an optical output,
a nonlinear optical material contained within a resonant optical
cavity, said resonant optical cavity having at least one reflector
with greater than 50% reflectivity to light of the wavelength of
the optical output from said at least one MOPA device,
optical coupling means for matching the optical output from said at
least one MOPA device to said resonant optical cavity containing
said nonlinear optical material.
119. The coherent light source of claim 107 wherein said excitation
means includes means for separately modulating at least one region
of an optical power amplifier of said MOPA device. .Iadd.
120. A coherent light source comprising
a semiconductor optical source generating and emitting a high power
coherent light beam, said light beam characterized by being
astigmatic with lateral and transverse beam waists spaced apart in
different locations, where `lateral` and `transverse` refer to
directions respectively parallel and perpendicular to a plane of an
active gain region of said semiconductor optical source, said light
beam also characterized by being highly asymmetric, with a lateral
beam width dimension that varies along a length of a light path
within said semiconductor optical source, and with different
lateral and transverse beam width dimensions at an output surface
of said semiconductor optical source, wherein a
lateral-to-transverse dimension ratio for said light beam is at
least 10 to 1 at said output surface, and
an astigmatism-correcting lens system positioned in the path of
said light beam emitted from said semiconductor optical source,
said lens system adapted to provide a modified astigmatism-free
light beam. .Iaddend. .Iadd.121. The coherent light source of claim
120 wherein said lens system is further adapted to provide said
modified astigmatism-free light beam with substantially equal
lateral and transverse beam width dimensions and substantially
equal lateral and transverse divergence angles. .Iaddend.
.Iadd.122. The coherent light source of claim 120 wherein said
semiconductor optical source comprises a laser diode with a spatial
mode filter and a gain region that allows lightwaves propagating
therein to
expand laterally toward an output surface thereof. .Iaddend.
.Iadd.123. The coherent light source of claim 122 wherein said
laser diode is a flared-resonator, type Laser diode, in which said
gain region is a flared region with a lateral aperture width that
is wider at an end adjacent to said output surface than at an
opposite end adjacent to said spatial mode filter. .Iaddend.
.Iadd.124. The coherent light source of claim 123 wherein said
spatial mode filter is a single-spatial-mode waveguide optically
coupled to said flared gain region. .Iaddend. .Iadd.125. The
coherent light source of claim 122 wherein said laser diode is a
master oscillator power amplifier (MOPA) device which includes a
single-mode DBR laser oscillator forming said spatial mode filter
and an optical power amplifier region forming said gain region with
a lateral aperture width of said amplifier region that allows
lightwaves received from said laser oscillator to expand laterally
toward an output end of said amplifier region. .Iaddend. .Iadd.126.
The coherent light source of claim 125 wherein said amplifier
region of said MOPA device is a flared region with a lateral
aDerture width that is wider at said output end thereof than at
an input end adjacent to said laser oscillator. .Iaddend.
.Iadd.127. The coherent light source of claim 126 wherein said MOPA
device further includes a surface-emitting detune-dgrating output
coupler Dositioned at said output end of said flared amplifier
region of said MOPA device to receive light therefrom and direct
the light vertically out of a top or bottom surface of said MOPA
device. .Iaddend. .Iadd.128. The coherent light source of claim 120
wherein said semiconductor optical source has means for tuning its
light emission wavelength. .Iaddend. .Iadd.129. The coherent light
source of claim 120 wherein said semiconductor optical source has
means for modulating the output power of said light beam emitted
therefrom. .Iaddend. .Iadd.130. The coherent light source of claim
120 wherein said lens system comprises a positive cylinder lens
positioned in the path of the emitted light beam at a location
where the transverse width dimension of the light beam has expanded
to substantially equal the lateral width dimension and having a
transverse focal length selected so as to partially cotlimate the
light beam in the transverse direction such that the light beam has
substantially equal lateral and transverse diverqence angles.
.Iaddend. .Iadd.131. The coherent light source of claim 130 wherein
a spherically symmetric lens is positioned in the path of the
partially collimated light beam. .Iaddend. .Iadd.132. The coherent
light source of claim 120 wherein said lens system comprises a
spherical lens positioned with a focal plane thereof at an output
surface of said semiconductor optical source so as to collimate the
light beam in the transverse direction ad to focus the light beam
in the lateral direction, and a positive cylinder lens positioned
beyond the lateral focus of the light beam where the lateral width
dimension of the light beam has re-expanded to be substantially
equal to the transverse width dimension of the light beam, said
positive cylinder lens having a positive lateral focal length equal
to an effective optical distance from the lateral focus to said
positive cylinder lens so as to collimate the light beam in the
lateral direction. .Iaddend. .Iadd.133. The coherent light source
of claim 132 wherein said lens system further includes a negative
cylinder lens positioned between said lateral focus of said light
beam and said positive cylinder lens, said negative cylinder lens
having a negative lateral focal length. .Iaddend. .Iadd.134. The
coherent light source of claim 120 wherein said lens system
comprises a microlens with two crossed positive cylinder lens
surfaces formed on opposite sides of a transparent substrate, a
first of said cylinder lens surfaces being positioned at a distance
from an output surface of said semiconductor optical source at
which the transverse width dimension of said light beam has
expanded to substantially equal the lateral width dimension of said
light beam at a second of said cylinder surfaces, said first
cylinder lens surface havinq a transverse focal length equal to the
distance from said output surface of said semiconductor optical
source to said first cylinder lens surface so as to collimate the
light beam in the transverse direction, said second cylinder lens
surface having a lateral focal length substantially equal to the
effective optical distance of the light path from the location of
the lateral beam waist of said light beam within said semiconductor
optical source to said second cylinder lens surface so as to
collimate the light beam in the lateral direction. .Iaddend.
.Iadd.135. The coherent light source of claim 120 wherein said lens
system comprises a microlens with a positive toric lens surface and
a planar surface on opposite sides of a transparent substrate, said
toric lens surface being positioned at a first effective optical
distance of the light path from an output surface of said
semiconductor optical source thereto at which the transverse width
dimension of said light beam has expanded to substantially equal
the lateral width dimension of the light beam, said toric lens
surface having a transverse focal length substantially equal to
said first effective optical distance so as to collimate the light
beam in the transverse direction, said toric lens surface also
having a lateral focal length substantially equal to a second
effective optical distance of the light path from the location of
the lateral beam waist of said light beam within said semiconductor
optical source to said toric lens surface so as to also
collimate the light beam in the lateral direction. .Iaddend.
.Iadd.136. The coherent light source of claim 120 wherein a lens is
integrated into said semiconductor optical source near an output
surface thereof said lens having a lateral focal length within said
source which is substantially equal to an effective optical
distance from said lateral beam waist of the light within said
semiconductor optical source to said lens so as to collimate the
light in the lateral direction prior to being emitted from said
output surface of said semiconductor optical source. .Iaddend.
.Iadd.137. The coherent light source of claim 136 wherein said lens
system comprises a cylinder microlens positioned in the light path
at a distance where the transverse width dimension of the light
beam has expanded to substantially equal the lateral width
dimension of said light beam, said cylinder microlens having a
transverse focal length substantially equal to said distance so as
to collimate said light beam in the transverse direction. .Iaddend.
.Iadd.138. The coherent light source of claim 136 wherein said lens
system comprises a Spherical lens having a positive focal length
equal to a distance from said output surface of said semiconductor
optical source to said spherical lens so as to collimate the light
beam in the transverse direction and to focus the light beam in the
lateral direction, and a positive cylinder lens positioned beyond
the lateral focus of the light beam where the lateral width
dimension of the light beam lens re-expanded to be substantially
equal to the transverse width dimension of the light beam, said
positive cylinder lens having a positive lateral focal length equal
to an effective optical distance from the lateral focus to said
positive cylinder lens so as to collimate the
light beam in the lateral direction. .Iaddend. .Iadd.139. The
coherent light source of claim 136 further comprising a
surface-emitting detuned-grating output coupler integrated into
said semiconductor optical source and positioned to receive said
laterally collimated light from said integral lens to direct the
light vertically out of a top or bottom surface of said
semiconductor optical source. .Iaddend. .Iadd.140. The coherent
light source of claim 120 wherein said semiconductor optical source
includes a surface-emitting detuned-grating output coupler
integrated therein with plural, periodically spaced grating teeth
to direct the light vertically out of a top or bottom surface of
said semiconductor optical source, each grating tooth being curved
to coincide with a lateral phase front of the light so as to
collimate the vertically directed light in the lateral direction.
.Iaddend. .Iadd.141. The coherent light source of claim 120 wherein
said semiconductor optical source includes a surface-emitting
detuned-grating output coupler integrated therein with plural,
periodically spaced, straight grating teeth to direct the light
vertically out of a top or bottom surface of said semiconductor
optical source, said lens system including a cylinder lens in the
path of the vertically directed output light beam with a lateral
focal length selected to collimate the output light beam in the
lateral direction. .Iaddend.
Description
TECHNICAL FIELD
The present invention relates to semiconductor diode lasers whose
red or near infrared light emission is converted to ultraviolet,
blue or green laser light by means of a nonlinear optical device,
such as by frequency doubling with a second harmonic generator
(SHG). The invention also relates to high power laser diodes of the
type known as master oscillator power amplifier (MOPA) devices, and
especially to MOPAs with flared amplifiers.
BACKGROUND ART
Short wavelength (ultraviolet, blue, green) coherent light sources
are desirable for a number of applications, including high density
(.about.1 Gbit/cm.sup.2) optical data storage, color image
processing, such as in laser printers, submicron lithography and
other high resolution laser processing steps for fabricating VLSI
devices, satellite-to-submarine and other underwater optical
communications, and spectroscopy and other optical measurements,
such as interferometric gravity-wave detection and the like. In
many of these applications, compact laser systems are desired, and
in some, relatively high output powers (greater than about 100 mW)
are required. Accordingly, considerable effort has been undertaken
in recent years to develop more compact, short-wavelength coherent
light sources to replace the low-power air-cooled gas lasers, such
as argon ion and helium-cadmium lasers, that are presently the only
practical sources which are available.
Also, mid-infrared wavelength (2-10 .mu.m) coherent light sources
are desirable for a number of applications, including eye-safe
laser ranging and communications, laser surgery, spectroscopy,
environment sensing and diagnostics. Considerable effort has been
undertaken to develop sources in the mid-infrared wavelength range.
Because of their compactness, high electrical-to-optical conversion
efficiency, wavelength tunability and rapid modulation
capabilities, semiconductor laser diodes are being actively studied
to discover whether shorter wavelengths can be generated. At
present, AlGaInP lasers have the shortest practical limit of
wavelength around 600 nm (orange). Potential wide bandgap
semiconductor lasers made of AlGaN or other materials are being
studied, but have not yet been successfully oscillated by current
injection. At present, approaches to directly generating
mid-infrared wavelength are focused on rare-earth doped solid state
lasers, such as thullium (Tm) and holmium (Ho) doped lasers.
Nonlinear optical processes, such as frequency doubling (also
called second harmonic generation) and sum frequency mixing, are
capable of converting red and near infrared light into ultraviolet,
blue and green light. Accordingly, much development work has
focused on using nonlinear frequency conversion techniques to
generate ultraviolet, blue and green light directly from red and
near infrared laser diodes. Direct frequency doubling of laser
diode emission makes possible the extension of available diode
laser wavelengths into the ultraviolet, blue and green regions of
the spectrum, and represents at present the most feasible approach
to developing a compact, efficient, high power coherent source in
those spectral regions. However, in order for this approach to be
successful and useful in practical applications, like those
mentioned above, higher optical conversion efficiencies from the
red and near infrared laser diode wavelengths to the desired
ultraviolet, blue and green wavelengths are needed. Likewise, other
nonlinear optical processes, including optical parametric
generation and difference frequency mixing, are capable of
generating mid-infrared radiation from one or two
shorter-wavelength sources. Again, improved conversion efficiencies
are needed to make these techniques practical.
In general, higher optical conversion efficiencies are associated
with a higher power density or intensity of the fundamental pump
wavelength within the nonlinear optical material. Because of the
relatively low powers available from most diode lasers,
configurations using external resonators or channel waveguide
structures have been preferred. For example, efficient frequency
conversion is possible by coupling laser diode radiation into a
passive external resonator of either a standing wave or
unidirectional ring type that contains a bulk crystal of nonlinear
optical material, such as potassium titanyl phosphate
(KTiOPO.sub.4) or potassium niobate (KNbO.sub.3). The high
circulating intensity that builds up in the crystal located within
the resonator results in efficient frequency conversion of the
laser diode radiation.
W. J. Kozlovsky, et al., in Applied Physics Letters 56 (23), pages
2291-2292 (1990), describe frequency doubling of an 856 nm laser
output from a ridge waveguide, single quantum well, graded index
double heterostructure GaAlAs diode laser in a monolithic
KNbO.sub.3 crystal ring resonator in order to generate 428 nm
(blue) radiation. The ring resonator is a 7 mm long KNbO.sub.3
crystal with curved mirror end faces coated for high reflectivity
at the fundamental wavelength and transmissivity of the frequency
doubled blue light and with a flat total internal reflection
surface parallel to the mirror axes. The crystal resonator is
placed on a thermoelectric cooler so that the temperature can be
stabilized at 15.degree. C. for phase-matched second harmonic
generation along the long direction of the ring path. In order to
achieve efficient power buildup in the KNbO.sub.3 cavity and
generation of stable blue output, the laser output frequency is
locked to the cavity resonance using an elaborate electronic servo
technique that superimposes a small rf current on the dc injection
current to produce weak FM sidebands in the laser output and that
uses a double-balanced mixer for phase sensitive detection of the
optical-heterodyne-spectroscopy signal in the light reflected from
the input surface of the resonator. Such a signal is zero when the
carrier frequency coincides with the cavity resonance. The output
signal of the mixer is amplified and coupled back to the laser
injection current, so that the diode laser's output frequency
tracks the resonance frequency of the KNbO.sub.3 cavity. Using such
a servo technique, a 41 mW blue output (39% optical conversion
frequency) was achieved. However, the technique requires a
significant amount of electronics for it to work properly without
amplitude noise. Elaborate temperature and electronic feedback
controls for matching resonance frequencies are typical of external
resonator systems. Besides being expensive and not very compact, in
attempting to maintain stable operation, they usually introduce
some wavelength jitter into the system.
J. T. Lin, in Lasers and Optronics, December 1990, pages 34-40,
describes diode-pumped self-frequency-doubling (SFD) lasers using
Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 (NYAB) crystals for
the frequency doubling, and compares them against prior single-pass
KTP, external resonator KNbO.sub.3 and channel waveguide
LiNbO.sub.3 second harmonic generator configurations for diode
lasers, as well as other frequency doubled laser systems. Up to 80
mW of output power (up to 8.0% efficiency) at 531 nm is achieved
with NYAB compared to 40 mW of output power at 430 nm for
external-resonator-type second harmonic generation of a 860 nm
diode laser beam. Like diode-pumped solid-state lasers, these SFD
laser systems are not particularly compact, so that a tradeoff
between compactness and greater conversion efficiency must be
made.
Another approach for efficient frequency conversion is to use
ion-diffused channel waveguides of nonlinear optical material, such
as lithium niobate (LiNbO.sub.3) or potassium titanyl phosphate
(KTiOPO.sub.4), to double the frequency of the laser diode
emission. Doubling is relatively efficient if the waveguide is
relatively long (greater than about 1 mm), but phase-matching of
long frequency-doubling waveguides is critical, the available
wavelength range is more limited, and fabrication tolerances are
tight. Periodic poling can ease such requirements and increase
efficiency. Another problem that arises when waveguide systems are
used is that it is difficult to collimate and then focus the diode
laser light to a diffraction-limited spot for efficient coupling
into the waveguide, using conventional spherical lens systems.
However, waveguide systems are compact.
C. J. Van der Poel, et al., in Applied Physics Letters 57 (20),
pages 2074-2076 (1990), describe second harmonic generation in
periodically segmented KTiOPO.sub.4 (KTP) waveguide structures. The
waveguide structures are formed in either flux-grown or
hydrothermally grown KTP substrates by ion exchange through a Ti
mask using various Rb/Tl/Ba nitrate molten salt baths. There are
two segments per period, one segment being bulk KTP with a length
l.sub.1 and a propagation constant mismatch .DELTA.k.sub.1, the
other segment being an ion-exchanged KTP waveguide with a length
l.sub.2 and a propagation constant mismatch .DELTA.k.sub.2, in
which the phase-matching condition .DELTA.k.sub.1 l.sub.1
+.DELTA.k.sub.2 l.sub.2 =2.pi.M is met (M being an integer).
Ferroelectric domain reversals in adjacent segments can also be
included for higher conversion efficiencies. Efficient second
harmonic outputs were observed from 0.38 .mu.m (deep purple) to
0.48 .mu.m (blue-green). W. P. Risk, in Applied Physics Letters 58
(1), pages 19-21 (1991), describes fabrication of optical
waveguides in KTP crystals by an ion-exchange process involving a
molten Rb/Ba nitrate bath. Second harmonic generation from
titanium:sapphire laser light in the 900-1000 nm range was
observed.
A. Harada et al., in Applied Physics Letters 59 (13), pages
1535-1537 (1991), describe second harmonic generation of 442 nm
(blue) light from an 884-nm semiconductor laser using an organic
crystal-cored nonlinear optical fiber coupled to the laser. The
single transverse mode fundamental beam of the diode laser is
collimated by a first objective lens and an anamorphic prism pair,
and then focused into the fiber by a second objective lens. The
fiber was formed by filling a hollow glass fiber by capillary
action with the organic material (DMNP) melt, and then
recrystallizing the polycrystals thus obtained by the
Bridgman-Stockberger single crystal formation method in which the
fiber is pulled out of a 105.degree. C. furnace. The fiber core
diameter and length are 1.4 .mu.m and 5-14 mm, respectively. Output
powers of 0.16 mW (about 1.6% conversion efficiency) were achieved.
G. L. J. A. Rikken, et al., in Applied Physics Letters 58 (5),
pages 435-437 (1991), describe nonlinear optical effects in
sidechain copolymers with methylmethacrylate.
Efficient frequency conversion also requires good spatial and
spectral mode properties of the diode lasers. While grater light
intensities in the nonlinear material are desired, too much power
focused in one place can damage or destroy the nonlinear material.
Poor spatial mode characteristics of the diode laser beam can limit
the mount of focusing that can safely be achieved without damage to
the nonlinear material. Higher power lasers, such as multi-emitter
and broad area laser diodes are a particular problem because of
their highly asymmetric beam characteristics.
In U.S. Pat. No. 4,530,574, Scifres et al. describe an optical
system for collimating and focusing the radiation emitted from a
multi-emitter or broad emitter semiconductor laser so that the
laser beam or beams can be imaged into a single diffraction limited
spot. The optical system includes a first lens system to collimate
or focus the radiation in the near field in the vertical direction
perpendicular to the p-n planar junction, and a second lens system
to collimate or focus the radiation in the far field in the lateral
emission direction parallel to the p-n planar junction. Unwanted
low power interference lobes may be blocked by using an aperture in
the optical system.
An object of the invention is to provide a high power (greater than
100 mW), short wavelength (ultraviolet, blue or green), compact
laser source.
Another object of the invention is to provide a laser source
utilizing a lens system that can collimate and focus the highly
asymmetric and astigmatic beams of higher power laser diode systems
into a diffraction limited-beam for coupling into nonlinear,
frequency converting optical material.
DISCLOSURE OF THE INVENTION
The objects have been met with a compact semiconductor laser light
source having at least one master oscillator power amplifier (MOPA)
type or flared resonator type laser diode for generating one or
more high power coherent light beams of at least a first wavelength
and an optical frequency converter positioned to receive the beam
or beams from the high power laser diode or diodes for generating
an output light beam of a different wavelength from a portion of
the optical power of the beam or beams from the laser diode or
diodes. The MOPA type laser diode preferably includes a single
mode, tunable DBR laser oscillator coupled to a multimode,
preferably flared, power amplifier region. By removing the internal
DBR grating from the laser oscillator, the flared amplifier region
itself can be made to oscillate as an at most marginally stable
resonator with spatial mode filter and to produce a high power
diffraction limited output beam. Both the MOPA type laser diode and
the flared resonator type laser diode are capable of generating at
least 1 W cw of red or near-infrared light output for high
efficiency frequency conversion. Because optical conversion
efficiency increases with the square of the incident optical power,
the system can use a single-pass or nonresonant multi-pass
configuration with a bulk crystal of optically nonlinear material,
and still obtain efficient frequency doubling. A passive external
resonator with the nonlinear material located within the resonant
optical cavity, with its associated resonance matching electronics,
becomes optional. Likewise, the use of a nonlinear material optical
waveguide, with or without periodic ferroelectric polarization
domain reversals, becomes unnecessary and is now optional.
Frequency conversion techniques include frequency doubling and
optical parametric generation using one input beam and sum
frequency mixing and difference frequency mixing using two input
beams.
The objects are also met with a compact semiconductor laser light
source that includes an astigmatism-correcting lens system
positioned in the path of the light beam that is output from either
of the MOPA type or flared resonator type laser diode devices. The
multimode, preferably flared, optical power amplifier region, which
is coupled to a single mode laser oscillator in MOPA type devices,
generates an astigmatic beam with a beam waist in the lateral
direction located at the input end of the amplifier region and
another beam waist in the transverse direction which is located at
the output surface of the MOPA device. Here, `lateral` and
`transverse` refer to directions respectively parallel and
perpendicular to a plane of an active gain region of the MOPA or
flared resonator device. The light beam emitted from the MOPA or
flared resonator device is also highly asymmetric with different
lateral and transverse beam width dimensions and different lateral
and transverse beam divergence angles. The lens system has at least
one nonspherical lens element located where the two beam width
dimensions are equal for independently equalizing the lateral and
transverse characteristics of the beam. A lens could also be
incorporated into the MOPA or flared resonator device or a curved
surface emitting output grating could be integrated into the MOPA
or flared resonator device. The laser diode-lens system combination
is particularly valuable for coupling the high power light at high
intensity into the nonlinear optical material used for frequency
conversion. Because the spatial mode quality of the
lens-system-corrected beam is very good, higher intensities can be
produced in the nonlinear material with less danger of damaging the
material. Coupling of the high power laser beam into a nonlinear
optical waveguide can be achieved with less loss when the
astigmatism-correcting lens system is employed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-4, 5A and 6-8 are schematic side views of eight embodiments
of the present invention, differing principally in the manner in
which the light beam from the high power laser diode passes through
the various nonlinear optical converter configurations in these
embodiments.
FIG. 5B is a top plan view of an embodiment of the present
invention corresponding to the side view seen in FIG. 5A.
FIG. 9 is a perspective view of a master oscillator Power amplifier
(MOPA) type laser diode for use in any of the embodiments shown in
FIGS. 1-8.
FIG. 10 is a top plan view of an alternative flared resonator type
laser diode for use in any of the embodiments shown in FIGS.
1-8.
FIGS. 11A-18A and 11B-18B are respective side and top plan views of
four MOPA device/astigmatism-correcting lens system embodiments of
the present invention which are useful in any of the frequency
convening embodiments shown in FIGS. 1-8.
FIGS. 19-21 are perspective views of yet further surface-emitting
MOPA/lens-system combinations in accord with the present invention,
that include integrating lens and grating elements in the MOPA
device.
FIGS. 22-24 are top plan views of further embodiments of the
present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
With reference to FIG. 1, a semiconductor laser of the present
invention includes a meter oscillator power amplifier (MOPA) type
or flared resonator type laser diode which is capable of generating
a high power coherent light beam 13 having a wavelength in the red
or near-infrared region of the spectrum. A lens system 15, here
represented for simplicity as a single lens, but more typically
being a multi-lens system such as those seen in FIGS. 11-18,
collimates the diverging light 13 from the high power laser diode
11 and possibly focuses the light as well. In most embodiments, it
is necessary for the beam not only to be collimated, but made
symmetric as well. The collimated or collimated and focussed light
beam 13 is coupled into a nonlinear optical frequency converter 17,
which is positioned to receive the high power coherent light beam
13 from the laser diode 11. The optical frequency converter 17 is
capable of generating a second light beam 19 having a wavelength in
the ultraviolet, blue or green range of the spectrum from a portion
of the optical power of the first beam 13. The second beam 19 has a
wavelength which is different from that of the injected beam 13. A
dichroic filter 21 may be placed at the output of optical frequency
converter 17 to block the red or near-infrared fundamental light to
pass the converted ultraviolet, blue or green light as the output
of the laser source.
The configuration shown in FIG. 1 is a single pass second harmonic
generation or frequency doubling configuration in which the
nonlinear optical frequency converter is a bulk crystal of
nonlinear optical material. The nonlinear crystalline material may
be potassium titanophosphate (KTiOPO.sub.4), potassium niobate
(KNbO.sub.3), barium metaborate (BaB.sub.2 O.sub.4), lithium
triborate (LiB.sub.3 O.sub.5), neodymium yttrium aluminum borate
(Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4, as well as various
organic nonlinear optical materials. In this single pass
configuration the high power laser diode light is focused in the
nonlinear crystal 17. The spot size of the infrared beam 13 in the
crystal is optimized for maximum single pass conversion efficiency
by focusing the beam to the smallest possible spot size such that
the beam still remains focused over the length of the crystal. In
the case of a potassium niobate bulk crystal with a length of 1 cm
and a 1 Watt infrared input 13 from the laser diode 11, the output
power of the blue frequency doubled beam 19 is about 10-20 mW.
Using longer crystals the second harmonic generated output can be
increased since the conversion efficiency increases linearly with
the crystal length. However, crystal lengths are currently
available only up to about 2 cm.
With reference to FIG. 2, another embodiment of the present
invention employs a double pass configuration. Again, a MOPA type
or flared resonantor type laser diode 11 emits a red or near
infrared beam 13 which is collimated and focused by a lens system
15. The high power laser diode beam 13 is focused into a nonlinear
crystal 23. The nonlinear crystal 23 has a reflective coating 25
integrated on a surface which is opposite from the input side that
receives the beam 13. The coating 25 reflects the infrared light
from the beam 13 as well as the frequency doubled blue light
generated by the nonlinear crystal material 23. A beamsplitter 27
which is transmissive of the infrared light 13 but reflective of
the frequency doubled light 29 is positioned in the return path of
the light beam between the lens system 15 and the nonlinear crystal
23 in order to couple the frequency doubled blue light 29 out of
the laser system. Care should be taken that the phase of the
reflection from the mirror 25 is the same for the infrared and the
second harmonic generated beam to prevent phase mismatch and
reduced output efficiency. In order to optimize the focusing in the
crystal, the reflector 25 is preferably curved. Assuming double
pass transmission through a 2 cm long potassium niobate crystal and
1 Watt of infrared input, about 40-80 mW cw output power can be
obtained.
With reference to FIG. 3, a multiple pass second harmonic generator
configuration is seen. A MOPA type or flared resonator type laser
diode 11 capable of generating a high power coherent red or near
infrared light beam 13 and a lens system 15 are combined to
collimate and possibly focus the red or near infrared beam 13 into
a nonlinear crystal 31. Retroreflectors 33 and 35 are used to
reflect the beam back through the nonlinear crystal 31 in a
parallel direction so as to remain properly angle tuned for phase
match interaction. The advantage of this retroreflected
configuration is that the interaction length in the nonlinear
crystal 31 can be increased without increasing the crystal length.
Assuming 1 Watt of infrared light 13 input into a 1 cm long
nonlinear crystal 31 in a configuration that includes 10 passes of
the light 13 through the crystal the effective interaction length
is 10 cm and 100-200 mW of second harmonic generated blue light 36
can be obtained.
With reference to FIG. 4, a focused multiple pass configuration
could be used. Again, a MOPA type or flared resonator type laser
diode 11 and a lens system 15 produce a high power, collimated and
focused, laser beam 13 of red or infrared wavelength. The focused
light beam 13 is coupled into a nonlinear optical crystal 39.
Retroreflectors 45 and 47 direct the beam 13 back through the
nonlinear crystal 39 along a parallel path so that the beam 13
passes through the crystal 39 a plurality of times. This
arrangement also includes a pair of lens arrays 41a, 41b, 41c, etc.
and 43a, 43b, 43c, etc. for focusing the light on every pass
through the nonlinear crystal 39. Here lens elements 41a, 43a, 41c,
etc. positioned in the beam path after passage through the
nonlinear crystal 39 collimate the light beam for retroreflection
off of right angle mirrors 45, 47, etc., while lens elements 41b,
43b, etc. positioned in the path of the retroreflected beam prior
to passage through the nonlinear crystal refocuses the beam into
the nonlinear crystal. The focused beam waist is preferably located
in the center of the nonlinear crystal 39 equidistant between the
two lens arrays 41 and 43. Using this focused configuration, the
effective interaction length varies at the length of the nonlinear
crystal times the square of the number of passes. Thus, a 1 cm long
nonlinear crystal 39 receiving 1 W of infrared input 13 from a MOPA
device 11 with 3 passes of the beam, as shown, has an effective
interaction length of 9 cm and provides a second harmonic generated
input beam 49 with about, 90-180 mW power.
With reference to FIGS. 5A and 5B, the nonlinear crystal 51 may be
placed in a passive external resonator to further enhance the
conversion efficiency. A MOPA type laser diode 11 (alternatively a
flared resonator type laser diode) provides a high power beam 13
which is collimated then focused by a lens system 15. The lens
system 15 may include fixed position cylinder fiber lens 16a, a
second, variable position cylinder lens 16b, a variable position
aspherical collimating lens 16c and a spherical focusing lens 16d.
Details of astigmatism correcting lens systems 15 for use in the
invention are described further below With reference to FIGS.
11-19. The nonlinear crystal 51 is positioned in the path of the
focused beam 13 for receiving it. Crystal 51 has reflective
coatings 53 and 55 on two opposed input and output surfaces, which
are tilted slightly away from the perpendicular direction in the
manner of a trapezoid and preferably curved to maintain focusing of
the light 59 in the ring. A third surface 57 parallel to the
principal light direction between coated surfaces 53 and 55 forms a
totally internally reflecting surface. The light path 59 in the
nonlinear crystal 51 describes a unidirectional ring. Reflective
coating 53 on the output surface is highly reflective of the red or
infrared fundamental wavelength of light 13 received from laser
diode 11 and transmissive of the frequency doubled ultraviolet,
blue or green light 61 generated by the nonlinear optical crystal
51. Reflective coating 55 is preferably reflective of the frequency
doubled light generated in the crystal 51 and has a reflectivity to
the fundamental wavelength which is optimized to maximize the
optical power gain from light input by transmission of beam 13
through the coating 55 and fundamental wavelength light loss by
transmission of beam 59 through the coating 55. The resonant cavity
provided by the reflective surfaces 53, 55 and 57 increases the
power in the crystal 51 and increases second harmonic output power
by the square of the incident power on the crystal 51. Given a 1 W
input power from beam 13, the Q of the cavity can be relatively low
compared to resonant cavities that are designed for 100 mW type
laser diodes. This low Q will make the operation of the cavity
easier, leading to a stable output 61 without amplitude noise or
frequency jitter. The laser emission 13 can be maintained at a
wavelength .lambda..sub.1 that is tuned to the cavity resonance in
the nonlinear crystal 51 by adjusting a tuning current I.sub.tun,
applied to the laser diode 11 using an electronic servo tuning
system like that described above in the Background An section with
respect to the Kozlovsky article. The applied tuning current
I.sub.tun is based on a feedback loop signal derived from a
detector 60 with an rf response.
With reference to FIG. 6, high power laser diodes of the MOPA or
flared resonator types could also be used in frequency converting
configurations that use sum or difference frequency mixing (SFM or
DIM) to achieve desired wavelengths with useful efficiency. In FIG.
6, two such high power laser diodes 11.sub.1 and 11.sub.2 are tuned
to emit light of different wavelengths .lambda..sub.1 and
.lambda..sub.2. The light is collimated and focused by a set of
lens systems 15.sub.1 and 15.sub.2 for each diode 11.sub.1 and
11.sub.2 then combined using a dichroic beamsplitter 50 into a
single common light path. The combined beam of both wavelengths
.lambda..sub.1 and .lambda..sub.2 is received by a nonlinear
crystal 52, which is here shown as an external ring resonator with
reflective crystal surfaces 54, 56 and 58 but which could also be
any of the other configurations shown in FIGS. 1-5, 7 and 8.
Difference frequency mixing of the two wavelengths .lambda..sub.1
and .lambda..sub.2 will produce a mid-infrared output of wavelength
.lambda..sub.3 =.lambda..sub.1 .lambda..sub.2
/.vertline..lambda..sub.2 -.lambda..sub.1 .vertline., while sum
frequency mixing will produce blue or green light of wavelength
.lambda..sub.3 =.lambda..sub.1 .lambda..sub.2 /(.lambda..sub.1
+.lambda..sub.2). The reflective properties of mirror surfaces 54
and 56 can be selected to enhance one of these two frequency
conversion operations. The nonlinear material can also be
phase-matched at the sum or difference frequency, as in the
waveguide configurations in FIGS. 7 and 8 discussed below.
Further, some nonlinear crystal materials, including lithium
niobate (LiNbO.sub.3), potassium titanal phosphate (KTiOPO.sub.4),
silver gallium sulfide (AgGaS.sub.2) and silver gallium selenide
(AgGaSe.sub.2), are especially suited to efficient generation and
transmission of the mid-infrared wavelengths of the DFM conversion
technique.
With reference to FIG. 7, nonlinear second harmonic generating
waveguides 65 might also be used with the MOPA type or flared
resonator type laser diode 11. The high power red or near-infrared
beam 13 emitted from the laser diode 11 is collimated by an
astigmatism-correcting lens system 15a, like those seen in FIGS.
10-13, then focused by additional lens elements 15b, typically a
single spherical lens, to a small spot on the entrance to the
narrow single mode waveguide 65 in the nonlinear crystal 63.
Because of the narrow waveguide 65, typically about 3 .mu.m wide,
the light remains focused with a small width over the full
interaction length of the crystal 63. The nonlinear single mode
waveguide 65 preferably is a regularly segmented waveguide formed
by ion exchange through a mask, using a molten salt bath or proton
implantation to introduce the ions to be substituted in the crystal
lattice. The nonlinear crystal material may be lithium niobate
(LiNbO.sub.3), potassium niobate (KNbO.sub.3), potassium titanyl
phosphase (KTiOPO.sub.4) or some other non-linear optical material.
Periodic ferroelectric polarization domain reversals are preferred
for optimum conversion efficiency. Assuming a typical optical
conversion efficiency of 100%-W/cm.sup.2 for second harmonic
generation and neglecting pump depletion effects, a phase-matched
or quasi-phase-matched crystal length of 1 cm and a 1 W input power
for the beam 13 from a MOPA device 11 will provide nearly complete
conversion of the fundamental input wavelength to the frequency
doubled wavelength. The output 67 from the waveguide 65 is
collected and collimated by lens elements 15c, typically a single
spherical lens. A dichroic filter 69 may be placed in the path of
the output beam 67 from the waveguide 65 to block any red or
near-infrared light from input beam 13 that still remains
unconvened and to pass only the frequency doubled ultraviolet, blue
or green light 71 as the output.
With reference to FIG. 8, the nonlinear waveguide 75 may also be a
multimode waveguide which is tens of micrometers wide as compared
to the typical 3 .mu.m width for the single mode waveguide 65 in
FIG. 6. Because of the high input power of the red or near-infrared
beam 13 received from the MOPA type or flared resonator type laser
diode 11, the conversion efficiency of the broader waveguide 75 can
still be very high, even though the optical pump power is less
confined than the single mode waveguide case. The beam 13 emitted
from the laser diode 11 is collimated by the astigmatism-correcting
lens system 15a then focused in only one of two orthogonal
directions by an additional cylinder lens 15d onto the entrance of
the multimode nonlinear waveguide 75. An advantage of the
configuration is that the optical power density within the
waveguide 75 is lower than in the single mode waveguide 65 of FIG.
7, and therefore there is less chance of photorefractive damage.
Further, because of the width of the beam 13, the beam will remain
collimated in wide direction such that very little waveguiding
effect by the waveguide 75 will be necessary in that direction.
Again, the waveguide 75 ran be a regularly segmented, periodically
poled waveguide for efficient second harmonic generation. The
output 77 from the waveguide 75 is allowed to reexpand in the
narrow direction then collimated by another cylinder lens element
15e. A dichroic filter 79 can be used to block any remaining
unconverted red or near-infrared light and to pass only the
frequency doubled ultraviolet, blue or green light beam 81
generated in the nonlinear waveguide 75.
With reference to FIG. 9, a MOPA type laser diode 11 providing a
high power red or near-infrared coherent beam 13 is used in each of
the frequency converting configurations shown in FIGS. 1-8.
Typically, the MOPA device 11 emits 1 W cw of single frequency
optical power. Alternatively, the MOPA device 11 would be operated
in a pulsed mode to provide about 3 W peak output power for even
higher conversion efficiencies than the continuous wave (cw) mode
of operation. A preferred MOPA device capable of generating such
power levels comprises a low power, single-mode DBR laser
oscillator 83 which is monolithically integrated with a multimode,
preferably flared, power amplifier region 115. A typical power
output from the laser oscillator section 83 is about 50 mW. The
laser oscillator has a single mode waveguide 87 driven by an
electrical pump current I.sub.g and bounded by a pair of grating
reflectors 89 and 91 defined in a layer interface proximate to the
waveguide layers 87 so as to interact with lightwaves propagating
therein and form an optical resonator. The highly reflective rear
grating reflector 89 may be wavelength tunable by means of an
injection current I.sub.tun or electrical bias applied across the
grating region. The front grating reflector 91 is partially
transmissive to the propagating light in the waveguide 87 to couple
the light into the amplifier region 85. The amplifier region has
waveguiding layers 93 that are driven by at least an amplifier
current I.sub.amp applied through the conductive surface contact
95. Light coupled from the laser oscillator 83 into the waveguiding
layers 93 of the amplifier region 85 are allowed to freely diffract
in the lateral direction parallel to the plane of the active region
97 as it propagates toward the AR-coated output facet 99.
In FIG. 9, the amplifier region 85 has two split contacts 94 and 95
for optimization of the current density as a function of the length
of the flare. The wider front half of the amplifier region 85 could
thus be pumped with a greater amplifier current I.sub.amp and
current density than the narrower back half of the amplifier region
85 near the laser oscillator 83. Such differential pumping reduces
amplitude noise in the output signal 13, and also improves the
spatial mode quality of the output since optical filamentation is
minimized. Another advantage of the split contacts 94 and 95 is
that the MOPA device 11 can be modulated by means of a modulation
current I.sub.mod applied to the first part of the flared amplifier
85 through contact 94. Output power can then be modulated without
inducing wavelength chirp in the laser oscillator output, which
would adversely affect phase matching in a nonlinear crystal.
Further, modulation can then be accomplished at gigahertz
frequencies with only modest input current I.sub.mod, since the
optical power on the narrow end of the flared amplifier region 85
is still only a few hundred milliwatts.
With reference to FIG. 10, an alternative high power laser source
that could be used for frequency doubling is a flared resonator
type laser diode that includes a light amplifying diode
heterostructure 101 located within an at most marginally stable
resonant optical cavity defined by a tunable grating reflector 103
and a low reflection front end facet 105 of the heterostructure
101. The rear end facet 107 of the heterostructure 101 is
antireflection (AR) coated to prevent self-oscillation of the
heterostructure 107. A lens 109 between the grating reflector 103
and AR coated rear facet 107 of the heterostructure 101 receives
light emitted from a narrow single-mode aperture 111 defined by a
waveguide 113 in the heterostructure 101 and collimates it,
directing the light toward the grating reflector 103 at an
incidence angle .THETA.. The single mode aperture 111 forms a
spatial mode filter for the light oscillating in the cavity. The
lens 109 also receives the reflected light back from the grating
reflector 103 and focuses it upon the rear facet 107, coupling
light of a particular wavelength .lambda., corresponding to the
incidence angle .THETA. of light on the grating 103 through the
narrow aperture 111 into the single mode waveguide 113. The
heterostructure 101 has a flared gain region 115 which is
electrically pumped to amplify the light received from the
waveguide 113 as it freely diffracts within the flared gain region
115. A coherent red or near infrared light beam 117 of wavelength
.lambda. is emitted from the front facet 105 of the heterostructure
101.
Each of the high power coherent sources in FIGS. 9 and 10 are
useful for providing red or near infrared light for efficient
frequency doubling. Diode laser sources of GaAs/InGaAs composition
are capable of providing light ranging from 780 nm to 1030 nm in
wavelength. Using periodically poled nonlinear waveguides like
those shown in FIGS. 7 and 8, the complete range of wavelengths
provided by these sources can be doubled to provide ultraviolet,
blue and green light in the range from 390 nm to 515 nm. Using bulk
crystals, like those shown in FIGS. 1-6, KNbO.sub.3 can be
temperature tuned to frequency double input wavelengths of 840-950
nm and 960 nm-1060 nm, while KTiOPO.sub.4 can be used for
wavelengths longer than about 1000 nm. By frequency tuning the
diode laser sources, while at the same time temperature tuning the
nonlinear crystal material, we can frequency tune the frequency
doubled beam over a wavelength range of about 20-50 nm. A smaller
range of wavelength tuning is also available for segmented and
periodically poled nonlinear optical waveguides. The output from a
detector, such as detector 60 in FIG. 5A, detecting the frequency
convened light can be used to control the temperature of the
nonlinear optical material or the orientation of an external
grating, such as grating reflector 103 in FIG. 10, by means of a
feedback loop to optimize and stabilize the converted output. For
efficient frequency conversion with nonlinear crystals and
particularly for effective light 5 coupling into nonlinear optical
waveguides, good spatial modal quality, especially a symmetric beam
without astigmatism is required. Such high quality beams are also
desired in most other laser applications. Unfortunately, the high
power coherent light sources shown in FIGS. 9 and 10 produce a
highly asymmetric and astigmatic output beam. The light begins to
diverge in the lateral direction parallel to the plane of the
active gain region 97 of these devices in the flared amplifier
regions 85 and 115, and so have a lateral beam waist positioned at
the narrow entrance to the flared regions 85 and 115. However, the
light continues to be guided transversely by the thin waveguide
layers 93 of the flared regions. The light begins to diverge in the
transverse direction perpendicular to the plane of the active
region 97 after it exits from the wide end of the flared amplifier
regions 85 and 118 and from the output surface 99 and 105 of the
source. The transverse beam waist is thus located at the output
surface 99 and 105 of the devices, widely spaced away from the
lateral beam waist. The output beam is considerably wider in the
lateral direction (about 100 .mu.m) than it is in the transverse
direction (about 1 .mu.m) at the output surface 99 and 105, and the
lateral divergence angle (10.degree.-20.degree. FWHM) is smaller
than the transverse divergence angle (30.degree.-40.degree. FWHM)
of the beam. The astigmatism-correcting optical system, shown in
FIGS. 11-19, are presented as a means for correcting the beam
characteristics of the high power sources to produce a symmetric,
astigmatism-free beam for efficient use of the frequency doubling
configurations shown in FIGS. 1-8 and for other laser applications
requiring a good quality high power light beam. While the
discussion is directed to MOPA type laser diodes like that shown in
FIG. 9, the optical systems discussed are also applicable to flared
resonator type laser diodes like that shown in FIG. 10.
With reference to FIGS. 11A and 11B, a preferred
astigmatism-correcting, collimating, lens system for use with the
above-described MOPA type and flared resonator type laser diodes 11
comprises an aspherical cylinder lens 118 together with an
aspherical collimation lens 122. The light 123 from the laser diode
11 is partially collimated by the cylinder lens 118 in the
transverse direction perpendicular to the plane of the active
region 97 of the laser diode 11 so that it has substantially the
same beam divergence in the transverse direction as it has in the
lateral direction parallel to the plane of the active region 97. By
placing the cylinder lens 118 at the position where the transverse
size dimension of the light beam 123 has expanded to equal the
lateral size dimension of the beam 123 and by picking the focal
length of the cylinder lens 118 to achieve the aforementioned
partial collimation, the output beam 126 from the lens 118 is made
symmetric and free of astigmatism. Since the light 123 from the
laser diode 11 has a relatively large transverse divergence, the
cylinder lens 118 will need to have an aspheric surface 120 to
minimize the effect of spherical aberration that is associated with
spherical surfaces. Such aspherical cylinder lens can be made, for
example, by a process of diamond tuning of a large cylinder lens to
create a preform, followed by fiber drawing of this preform, as
taught by James Snyder and Patrick Reichert in SPIE Proceeding
1991, Symposium on Optical Applied Science and Engineering.
After the light 123 from the MOPA type or flared resonator type
laser diode 11 has been made symmetric and anastigmatic with the
single cylinder lens 118, the light 126 can then be collimated by
any diffraction-limited spherically symmetric lens 122 with
sufficient numerical aperture. Since the light 126 provided by the
laser diode - cylinder lens combination typically has a numerical
aperture of about 0.25, the light beam 126 cannot be collimated by
a single spherical lens without introducing spherical aberrations.
Accordingly, the light beam 126 must be collimated by a single lens
122 with an aspheric surface, as shown, or alternatively by a
compound lens element. One advantage of this optical collimation
system is that the focal length of the aspherical collimation lens
122 (or the effective focal length of a compound lens element) is a
free parameter, so that by changing the focal length of the lens,
we can change the spot size of output beam 128. The collimated
light 128 can be focused on a nonlinear crystal by a spherical
focusing lens or lens system, as shown in FIGS. 1-8.
The two lens system in FIGS. 11A and 11B is very compact, since the
individual lenses 118 and 122 are very small, typically less than a
few millimeters in size. Although the alignment of the first
cylinder lens 118 is critical, the lens 118 is small enough to be
easily attached directly to the laser diode 11, thereby providing
the necessary long term stability. In the case of direct
attachment, the thickness of the lens 118 provides the necessary
distance to the aspherical cylinder surface 120 to achieve beam
symmetrization and astigmatism-correction. Because of the small
size of the cylinder lens 118, the "bowtie" effect, which tends to
distort the beam at the comer of the lens, is minimized.
With reference to FIGS. 12A and 12B, after the light 123 from the
laser diode 11 has been made symmetric and anastigmatic by the
aspheric cylinder lens 118, the nearly collimated light 126 may be
focused by a spherically symmetric lens 124 to a small spot for
spatially faltering the light 120. Since the light 126 received
from the first cylinder lens 118 has a numerical aperture of about
0.25, the spherically symmetric lens 124 should have an aspheric
surface to avoid introducing spherical aberration. Alternatively, a
second spherical lens could be placed after the aspheric
collimating lens 122 in FIGS. 11A and 11B to focus the light. A
small pinhole aperture 132 is placed at the focal plane of lens 124
to filter out all light that is not in the main lobe. The light
that is not in the main lobe is due to two effects: (1) amplitude
and phase distortions introduced as the light is amplified in the
flared gain region 95 of the MOPA type or flared resonator type
laser diode 11 and (2) distortions introduced by the cylinder lens
118 due to skew rays at the corners of the cylinder lens 118. This
latter effect is usually referred to as the "bowtie effect." The
pinhole aperture 132 can be a hole in a simple aperture stop, or
alternatively as shown, it can be the entrance surface of an
optical fiber of the type commonly used in fiberoptic transmission.
Light in the main lobe is accepted and transmitted through the core
of the optical fiber, while light outside of the main lobe is not
incident upon the entrance to the fiber core and so is not
transmitted.
Although the amount of light which falls outside of the main lobe
is expected to be relatively small, its effect on beam quality may
be severe if no spatial filter is used. If no spatial filter 134 is
used, the light 130 can still be focused to a small spot, but
outside the focal plane the light in side lobes will interfere with
light from the main lobe and, because of coherent interference
effects, the beam may be severely distorted. This effect has been
verified by measuring brightness of the beam, both with and without
spatial filtering. Brightness is inversely proportional to the size
of the focused spot. It has been found that, without the pinhole
aperture 132, the brightness of the beam is reduced.
The effect of the spatial filter 134 is particularly important for
the application of frequency doubling and other nonlinear frequency
conversion techniques, where the frequency converted beam is
generated over a certain interaction length in the nonlinear
crystal. Any distortion in the input beam will be further amplified
along the length of the crystal, since the non-linear interaction
is proportional to the square of the local input intensity. The
effect of the pinhole spatial filter 34 may be less important in
applications of frequency conversion where the nonlinear crystal is
placed in a resonant cavity, as in FIGS. 5A, 5B and 6. This is
because the cavity will itself act as a spatial filter for modes
which are not matched to the cavity. Light which is not matched to
the cavity will not be resonant and thus will not distort the
frequency converted output.
With reference to FIGS. 13A and 13B, a beam from the MOPA device 11
is corrected by a spherical lens 119 followed by a cylinder lens
121. The light 123 from the MOPA device 11 is collected by a
spherical lens 119 which is positioned with its focal plane at the
output surface 99 of the MOPA device 11. The light in the
transverse direction perpendicular to the plane of the active
region 97 is collimated and the light in the lateral direction
parallel to the plane of the active region 97 is focused to a spot
125 some distance behind the lens. By selecting a cylinder lens 121
with the proper lateral focal length and positioning the cylinder
lens 121 a focal distance from the focal point 125 of the light
123, the output beam can be both circularized as well as corrected
for astigmatism. In particular, the cylinder lens 121 is positioned
beyond the focal point 125 where the lateral beam width has
reexpanded to substantially equal the transverse beam width. The
focal length is then the distance from the focal point 125 to this
position. The first spherical lens 119 can be either an asphere or
a compound lens which is diffraction limited for NA=0.6. Since the
light incident on the lens 119 has large astigmatism, the lens 119
has to be corrected to be diffraction limited for both infinite and
finite conjugate distance.
The optical system seen in FIGS. 13A and 13B may be very long
since, the beam has to diverge in the lateral direction until it
has the same lateral and transverse beam width. The system length
can be made shorter using a short focal length spherical lens 119.
Another method to decrease the length of the optical system is by
including a negative cylinder lens 127 in the system. A schematic
diagram of this configuration is shown in FIGS. 14A and 14B. By
including the negative cylinder lens 127 before a second positive
cylinder lens 129 and with appropriate choice of the relative focal
lengths and the distances between the lenses, we can reduce the
length of the system to 1 or 2 cm. The positive cylinder lens 129
is again positioned where the light beam 123 has been reexpanded by
the negative cylinder lens 127 in the lateral direction to have
substantially identical lateral and transverse beam width
dimensions. The positive cylinder lens has a lateral focal length
which is equal to the effective optical distance from the lateral
focus reimaged by the negative cylinder lens 127 to the position of
the positive cylinder lens 129. The two cylinder lenses 127 and 129
with opposing sign focal lengths are the equivalent to a single
long focal length cylinder lens 121, however the physical distances
are greatly reduced. This is similar to the technique used in
photographic tele-lenses to make the lenses shorter, with
distinction that here we use a cylinder system 127 and 129 and we
use it to correct for astigmatism and beam symmetry. As in the
first configuration shown in FIGS. 13A and 13B, the first lens 119
in FIGS. 14A and 14B may be a compound lens optimized for finite
and infinite conjugate distance, or the lens 119 may be a single
asphere. After the focal length of the lenses and their position
has been determined using paraxial optics, the shape of the lenses
may be optimized such as to minimize the spherical aberrations of
the complete lens system.
With reference to FIGS. 15A and 15B the optical system can be made
even smaller with the use of micro lenses. The schematic diagram in
FIGS. 15A and 15B shows the MOPA device 11 followed by a micro-lens
131 with two crossed cylinder lens surfaces 133 and 135 on a single
substrate. The first cylinder lens surface 133 collimates the light
in the transverse direction, while the second crossed cylinder lens
surface 135 collimates the light in the lateral direction. By
choosing the appropriate focal lengths and the thickness of the
substrate, the output beam from the micro-lens 131 can be made both
symmetric as well as corrected for astigmatism. In particular, the
first cylinder lens surface 133 is positioned at a distance from
the output surface of the MOPA device 11 at which the transverse
width dimension of the light beam 123 has expanded to substantially
equal the lateral width dimension at the second surface 135 (taking
into account the thickness of the microlens substrate). Cylinder
lens surface 133 has a transverse focal length substantially equal
to the distance from the output surface of the MOPA device to the
surface 133. The second cylinder lens surface 135 has a lateral
focal length substantially equal to the effective optical distance
of the light path from the narrow input end of the flared amplifier
region 85 at the output of oscillator 83 within the MOPA device 11
through the output surface of the MOPA device to the second
cylinder lens surface 135.
FIGS. 16A and 16B show a second preferred configuration comprising
a microlens 137 with a single anamorphic lens surface 139 and a
planar surface 141. Since the light 123 emitted from the MOPA
device 11 has different lateral and transverse divergence angles,
the beam 123 will become dimensionally symmetric after a certain
distance. By placing a single anamorphic lens surface 139 at this
distance the beam 123 will be symmetric without astigmatism. This
lens could be made, for example, using binary optics. Binary optics
can also compensate for spherical distortions or bowtie effects
that might otherwise occur with conventional cylinder lenses or
anamorphic lenses.
Another method of optimizing the astigmatic beam is by
incorporating part of the optics within the flared amplifier 85. In
FIGS. 17A, 17B, 18A and 18B, the light in the lateral direction
parallel to the plane of the active region 97 of the MOPA device 11
is collimated by an integrated cylinder lens 143 at the end of the
flared gain region 85. FIGS. 17A and 17B show a configuration where
a bulk spherical lens 145 is used to collimate the light 123 from
the MOPA device 11 in the transverse direction. A configuration
similar to that shown in FIGS. 13A and 13B or FIGS. 14A and 14B can
also be used to make the beam symmetric. The advantage of the
integrated lens 143 is that the light is laterally collimated
before it passes the GaAs-air interface 99 and before it reaches
the spherical lens 145. The low divergence will minimize the
spherical aberrations. Using an integrated lens 143, the light 123
from the MOPA device 11 can also be circularized and corrected for
astigmatism with a single micro cylinder lens 149 as shown
schematically in FIGS. 18A and 18B.
With reference to FIGS. 19-21, the beam 155 can also be coupled out
from the top or bottom surface 151 of a MOPA device 150 via a
detuned diffraction grating 153 or 159.
FIG. 19 shows a way of coupling a collimated beam 155 vertically
out from the surface 151 of MOPA device 150. An integrated lens 157
is fabricated within the MOPA device 150 to laterally collimate the
light on-chip. The resulting beam impinges upon a straight periodic
grating 153 that diffracts a two-dimensionally collimated beam 155
upward.
FIG. 20 shows another way of coupling a collimated beam 155 out of
the surface 181 by using a curved diffraction grating 159. The
curvature and pitch of the grating 159 are chosen so that the
diffracted output beam 155 is perfectly collimated. In particular
the grating teeth curve to match phase fronts of the laterally
diverging light.
FIG. 21 illustrates another method. The expanding beam leaving the
flared amplifier region impinges upon a straight grating 153 and is
diffracted upward. The resulting output beam 155 is subsequently
collimated by an external cylindrical or conical lens 161.
With reference to FIG. 22, a MOPA device 163 has at least two
semiconductor laser oscillators 171 and 172 optically coupled to a
semiconductor optical power amplifier 173. The laser oscillators
171 and 172 have DBR grating reflectors with different grating
pitches .LAMBDA..sub.1 and .LAMBDA..sub.2 that are selected to
cause the laser oscillators 171 and 172 to oscillate at and emit
different wavelengths .lambda..sub.1 and .lambda..sub.2 of light.
One or more of the laser oscillators 171 and 172 could be
wavelength tunable by applying a tuning current or bias voltage to
one or more of the DBR gratings so as to alter their effective
pitch and wavelength reflection response. The laser oscillators 171
and 172 are excited by injection of currents I.sub.1 and I.sub.2
into a gain region of the laser oscillators, which currents I.sub.1
and I.sub.2 could be independent of one another. In this way, one
or both of the laser oscillators 171 and 172 can be selected to
emit light so as to provide one or both wavelengths .lambda..sub.1
and .lambda..sub.2 to the optical power amplifier 173. The
amplifier 173 has an active width or lateral aperture that allows
the lightwaves received from one or both of the oscillators 171 and
172 to expand laterally by diffraction, at least along a portion of
the amplifier region 173. Preferably, the amplifier region 173 is
flared so as to have a wider lateral aperture at its output end
than the aperture width at its input end adjacent to the laser
oscillators 171 and 172. The amplifier 173 is excited to provide
gain to the lightwaves by an injection current I.sub.3 applied to
at least a portion of the amplifier region 173, which preferably is
independent of the currents I.sub.1 and I.sub.2 that are applied to
the laser oscillators 171 and 172. Two or more different currents
could also be applied to different portions of the amplifier region
173. The MOPA device 163 thus provides a high power coherent light
output of one or more selected wavelengths .lambda..sub.1 and
.lambda..sub.2. A lens system 165, like any of the lens systems
shown in FIGS. 11-21, provides a collimated or focused,
anastigmatic, symmetric light beam to a nonlinear crystal 167.
The nonlinear crystal material 167 may be arranged in any of the
configurations shown in FIGS. 1-8 and converts the frequencies or
wavelengths of the light it receives to different frequencies and
wavelengths. In particular, if the MOPA device 163 is selectively
excited to produce high power coherent light at only one of the two
or more wavelengths .lambda..sub.1 and .lambda..sub.2 that it can
generate, then the nonlinear crystal 167 performs frequency
doubling of the input light and efficiently generates an output
beam of wavelength .lambda..sub.1 /2 or .lambda..sub.2 /2. In
another instance, if the MOPA device 163 is selectively excited to
generate both wavelengths .lambda..sub.1 and .lambda..sub.2, then
the nonlinear crystal 167 performs sum or difference frequency
mixing of the two wavelengths .lambda..sub.1 and .lambda..sub.2 and
efficiently generates light at one of two new wavelengths
.lambda..sub.+ or .lambda..sub.-1 as in the configuration in FIG. 6
described above. These new wavelengths are given by ##EQU1## for
sum frequency mixing, and ##EQU2## for difference frequency mixing.
Which of the two wavelengths is generated efficiently depends on
which wavelength .lambda..sub.+ or .lambda..sub.- is phase matched
or quasi-phase matched to the input light. Wavelength tuning of one
or both of the input wavelengths .lambda..sub.1 and .lambda..sub.2
can be used to tune the selected frequency mixed output wavelength
.lambda..sub.+ or .lambda..sub.- provided phase matching or
quasi-phase matching is substantially maintained. While the
nonlinear crystal 167 is shown in a single pass configuration in
FIG. 22, other configurations like the external resonant cavity
configuration of FIG. 6 or waveguide configuration could be
used.
In FIG. 22, the laser oscillators 171 and 172 are spaced apart from
one another by a small center-to-center distance of preferably 10
.mu.m or less. Alternatively, as shown in FIG. 23, two laser
oscillators 175 and 176 having DBR grating reflectors of different
grating pitches .LAMBDA..sub.1 and .LAMBDA..sub.2 and emitting
different wavelengths .lambda..sub.1 and .lambda..sub.2 of light
could be combined in a Y-junction combiner 177 into a single
waveguide 178. The light of both wavelengths .lambda..sub.1 and
.lambda..sub.2 is then optically coupled from the waveguide 178
into a semiconductor optical power amplifier 179. As in FIG. 22,
the power amplifier 179 is preferably flared so as to have a wider
lateral aperture at its output end than the aperture width at its
input end adjacent to the waveguide 178. Different sections of the
MOPA device 164 can be excited by independent currents I.sub.1,
I.sub.2, I.sub.3, I.sub.4 and I.sub.5 to provide independent
driving of the laser oscillators 175 and 176, preamplification by
the waveguide 178 and differential pumping of the amplifier 179.
The high power output of wavelengths .lambda..sub.1 and
.lambda..sub.2 can be collimated and focused by an optical system
like that shown in FIGS. 11-21 into nonlinear optical material and
then converted to light of another wavelength by frequency doubling
or mixing processes in the same manner as described for the system
in FIG. 22.
The semiconductor material in the MOPA devices 163 and 164,
typically GaAs or InGaAs, is itself a nonlinear optical material
which causes a certain degree of frequency doubling and sum or
difference frequency mixing of the light of wavelengths
.lambda..sub.1 and .lambda..sub.2 generated by the laser
oscillators 171, 172, 175 and 176. However, light with higher
energy and shorter wavelengths than the bandbap (typically, about
1.4 eV or 860 nm) are alternated, and the semiconductor material is
transparent only to the wavelengths .lambda..sub.1 and
.lambda..sub.2, when pumped above a transparency threshold, and to
the longer wavelength (mid-IR) difference frequency mixed light
.lambda..sub.-. The difference frequency mixing that does occur is
relatively inefficient, is normally not phase-matched, and the
converted light at wavelength .lambda..sub.- receives no gain
directly from the amplifier region but only from the conversion
itself. Accordingly, this mixing can generally be ignored, the new
wavelengths being provided primarily by the external non-linear
optical material 167.
With reference to FIG. 24, a DBR laser array 181 of the MOPA type
generates a plurality of high power laser beams of either the same
or different wavelengths .lambda..sub.1, .lambda..sub.2,
.lambda..sub.3, etc., the pitches .LAMBDA..sub.1, .LAMBDA..sub.2,
.LAMBDA..sub.3, etc. of the DBR grating reflectors in the array 181
determining the output wavelength or wavelengths .lambda..sub.1,
.lambda..sub.2, .lambda..sub.3, etc. A cylinder lens 183 positioned
approximately one focal length from the output facet 182 of the
array 181 collimates the highly divergent beams in the transverse
direction. The first cylinder lens 183 is followed by a spherical
lens array 185 to bring the beams to a focus inside a nonlinear
optical material frequency doubler 187. The output from the doubler
187 is an array of ultraviolet, blue or green light spots of the
same or different wavelengths .lambda..sub.1 /2, .lambda..sub.2 /2,
.lambda..sub.3 /2, etc. In an alternative configuration, a cylinder
lens array could be used in place of the spherical lens array 185
to provide lateral focusing.
* * * * *