U.S. patent application number 09/938226 was filed with the patent office on 2002-02-28 for high-power external-cavity optically-pumped semiconductor lasers.
Invention is credited to Caprara, Andrea, Chilla, Juan L., Spinelli, Luis A..
Application Number | 20020024987 09/938226 |
Document ID | / |
Family ID | 23001326 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020024987 |
Kind Code |
A1 |
Caprara, Andrea ; et
al. |
February 28, 2002 |
High-power external-cavity optically-pumped semiconductor
lasers
Abstract
External-cavity optically-pumped semiconductor lasers
(OPS-lasers) including an OPS-structure having a mirror-structure
surmounted by a surface-emitting, semiconductor multilayer
(periodic) gain-structure are disclosed. The gain-structure is
pumped by light from diode-lasers. The OPS-lasers can provide
fundamental laser output-power of about two Watts (2.0 W) or
greater. Intracavity frequency-converted arrangements of the
OPS-lasers can provide harmonic laser output-power of about
one-hundred milliwatts (100 mW) or greater, even at wavelengths in
the ultraviolet region of the electromagnetic spectrum. These high
output powers can be provided even in single axial-mode operation.
Particular features of the OPS-lasers include a heat sink-assembly
for cooling the OPS-structure, a folded resonator concept for
providing optimum beam size at optically-nonlinear crystals used
for frequency conversion, preferred selection of
optically-nonlinear materials for frequency-conversion, and
compound resonator designs for amplifying second harmonic-radiation
for subsequent conversion to third or fourth harmonic
radiation.
Inventors: |
Caprara, Andrea; (Menlo
Park, CA) ; Chilla, Juan L.; (Sunnyvale, CA) ;
Spinelli, Luis A.; (Sunnyvale, CA) |
Correspondence
Address: |
STALLMAN & POLLOCK LLP
Suite 290
121 Spear Street
San Francisco
CA
94105
US
|
Family ID: |
23001326 |
Appl. No.: |
09/938226 |
Filed: |
August 23, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09938226 |
Aug 23, 2001 |
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09263325 |
Mar 5, 1999 |
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6285702 |
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Current U.S.
Class: |
372/92 |
Current CPC
Class: |
H01S 3/1095 20130101;
H01S 3/139 20130101; H01S 3/105 20130101; H01S 5/041 20130101; H01S
5/18383 20130101; H01S 3/082 20130101; H01S 5/0237 20210101; H01S
3/09408 20130101; H01S 5/14 20130101; H01S 3/094053 20130101; H01S
5/02423 20130101; H01S 3/109 20130101; H01S 5/024 20130101; H01S
5/02484 20130101; H01S 5/141 20130101; H01S 3/1083 20130101; H01S
3/09415 20130101; H01S 3/07 20130101; H01S 3/0815 20130101 |
Class at
Publication: |
372/92 |
International
Class: |
H01S 003/08 |
Claims
What is claimed is:
1. A laser, comprising: an OPS-structure having a gain-structure
surmounting a mirror-structure, said gain-structure including a
plurality of active layers having pump-light-absorbing layers
therebetween, said active layers having a composition selected to
provide emission of electromagnetic radiation at a predetermined
fundamental-wavelength between about 425 nanometers and 1800
nanometers when optical-pump light is incident on said
gain-structure, and said mirror-structure including a plurality of
layers of alternating high and low refractive index and having an
optical thickness of about one-quarter wavelength of said
predetermined wavelength; a laser-resonator formed between said
mirror-structure of said OPS-structure and a reflector spaced apart
therefrom, said laser resonator having a longitudinal axis; an
optical arrangement for delivering said pump-light to said
gain-structure, thereby causing fundamental laser-radiation having
said fundamental-wavelength to oscillate in said laser-resonator; a
heat-sink arrangement for cooling said OPS-structure; an
optically-nonlinear crystal located in said laser-resonator and
arranged for frequency-doubling said fundamental laser-radiation
thereby providing frequency-doubled radiation having a wavelength
half of said fundamental-wavelength; and said laser-resonator, said
optically nonlinear-crystal, said OPS-structure, said heat-sink
arrangement and said optical pump-light-delivering arrangement
selected and arranged such that said resonator delivers said
frequency-doubled radiation as output-radiation having a wavelength
between about 212 nanometers and 900 nanometers at an output-power
greater than about 100 milliwatts.
2. The laser of claim 1, wherein said optically nonlinear-crystal,
said OPS-structure and said optical pump-light-delivering
arrangement selected and arranged such that said resonator delivers
said frequency-doubled radiation as output-radiation having an
output-power greater than about 1 Watt.
3. The laser of claim 1 wherein said laser-resonator has an optical
length of at least about 5 cm.
4. The laser of claim 1, wherein said frequency-doubled radiation
is delivered in a single axial-mode.
5. The laser of claim 1 wherein, said active layers of said gain
structure are selected from the group of semiconductor compounds
consisting of In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y,
Al.sub.xGa.sub.1-xAs.sub.yP.sub.1-y, and In.sub.xGa.sub.1-xN where
0.0.ltoreq.x.ltoreq.1.0 and 0.ltoreq.y.ltoreq.1.
6. The laser of claim 5 wherein, said active layers of said gain
structure have a composition of In.sub.xGa.sub.1-xAs where
0.0<x<1.0 and x is selected such that said fundamental
wavelength is between about 900 and 1050 nanometers, and said gain
structure includes separator layers between said active layers,
said separator layers having a composition GaAs.sub.yP.sub.1-y.
7. The laser of claim 6 wherein, said high refractive index layers
of said mirror-structure have a composition GaAs and said low
refractive index layers of said mirror-structure have a composition
AlAs.sub.yP.sub.1-y where 0.0<y<1.0.
8. The laser of claim 5 wherein, said active layers of said gain
structure have a composition of In.sub.xGa.sub.1-xP where
0.0<x<1.0 and x is selected such that said fundamental
wavelength is between about 700 and 900 nanometers, and said gain
structure includes separator layers between said active layers said
separator layers having a composition
In.sub.yGa.sub.1-yAs.sub.zP.sub.1-z where 0.0<y<1.0 and
0.0<z<1.0.
9. The laser of claim 8 wherein, said high refractive index layers
of said mirror-structure have a composition In.sub.pAl.sub.1-pP,
0.0<p<1.0, and said low refractive index layers of said
mirror-structure have a composition Al.sub.qGa.sub.1-qAs, where
0.0<q<1.0.
10. The laser of claim 5 wherein, said active layers of said gain
structure have a composition of In.sub.xAs.sub.1-xP where
0.0<x<1.0 and x is selected such that said fundamental
wavelength is between about 700 and 900 nanometers, and said gain
structure includes separator layers between said active layers said
separator layers having a composition Al.sub.yGa.sub.1-yAs where
0.0<y<1.0.
11. The laser of claim 10, wherein said high refractive index
layers and said low refractive index layers of said
mirror-structure are layers of respectively high and low refractive
dielectric materials transparent to said fundamental
wavelength.
12. The laser of claim 11, wherein said high refractive index
material is zinc selenide and said low refractive index material is
aluminum oxide.
13. The a laser of claim 1, wherein said active layers of said
gain-structure have a gain-bandwidth, said optically nonlinear
crystal has a spectral acceptance-range for frequency-doubling,
said spectral-acceptance range being less than said gain bandwidth,
and said laser resonator further includes a wavelength-selective
element configured and arranged to prevent fundamental
laser-radiation having a wavelength outside of said
spectral-acceptance range from oscillating in said laser
resonator.
14. The laser of claim 13, wherein said wavelength-selective
element is a birefringent filter.
15. The laser of claim 13, wherein said wavelength-selective
element is an etalon.
16. The laser of claim 1, wherein said heat-sink arrangement
includes an actively-cooled member and said actively-cooled member
has a diamond layer in thermal contact therewith, said
mirror-structure of said OPS-structure OPS structure being in
thermal contact with said diamond layer.
17. The laser of claim 16, wherein said actively-cooled member is a
microchannel-cooler.
18. The laser of claim 1 wherein said optical arrangement for
delivering said pump-light to said gain-structure includes at least
one optical-lightguide for transporting pump-light from a source
thereof to an optical-focusing arrangement for focusing said
pump-light, said optical-focusing including at least one lens.
19. The laser of claim 18 wherein said at least one lens is a
radial-gradient-index lens.
20. The laser of claim 19 wherein said optical-focusing arrangement
includes two radial-gradient-index lenses.
21. The laser of claim 1 wherein said optically-nonlinear-crystal
is one of an LBO crystal and a CLBO crystal.
22. The laser of claim 1 wherein said optically-nonlinear crystal
is a crystal of a material selected from the group of
optically-nonlinear materials consisting of LBO, CLBO, BBO, SBBO,
SBO, and BZBO.
23. The laser of claim 1 wherein said longitudinal-axis of said
laser-resonator is folded at an angle by a fold-mirror located
between said reflector and said OPS-structure, and said
optically-nonlinear crystal is located on said longitudinal-axis of
said laser-resonator between said reflector and said
fold-mirror.
24. The laser of claim 23 wherein said fold-mirror is highly
reflective for said fundamental-radiation and highly transmissive
for said frequency-doubled radiation, said fold-mirror thereby
serving as an output-coupling mirror for delivering said
frequency-doubled output-radiation from said laser-resonator and
essentially preventing frequency-doubled radiation from reaching
said OPS-structure.
25. The laser of claim 24 wherein said fold mirror is convex for
focusing the light into the optically-nonlinear crystal.
26. The laser of claim 1 wherein said OPS-structure is configured
to minimize parasitic lateral oscillation of fundamental radiation
therein.
27. The laser of claim 26, wherein said OPS-structure is in the
form of a rectangular chip having an emitting-face, said pump-light
being delivered onto said emitting-face in a predetermined area
thereon, said chip having two pairs of parallel end-faces at right
angles to said emitting-face, said parallel end faces being spaced
apart by a distance at least three times the longest dimension of
said predetermined pump-light-delivery area to minimize parasitic
lateral oscillation of fundamental radiation therein.
28. The laser of claim 26, wherein said OPS-structure is in the
form of a rectangular chip having an emitting-face and having two
pairs of parallel end-faces at right angles to said emitting-face,
said parallel end-faces being roughened to prevent spectral
reflection of fundamental radiation therefrom, thereby minimizing
said parasitic lateral oscillation.
29. The laser of claim 1, wherein, in said mirror-structure, the
refractive index, thermal conductivity, and number of said layers
thereof are selected to provide maximum thermal conductivity of
heat-generated in said gain-structure to said heat-sink
arrangement, while providing sufficient reflectivity to cause
build-up of fundamental radiation in said laser resonator.
30. The laser of claim 1, wherein said mirror-structure further
includes a layer of a highly-reflective metal, said metal-layer
being closest to said heat-sink arrangement.
31. The laser of claim 1, wherein said pump-light is delivered on
said gain-structure of said OPS-structure in a predetermined pump
spot-size and said resonator is configured such that the spot-size
at said gain structure of said oscillating fundamental
laser-radiation is about equal to said pump spot-size.
32. The laser of claim 31, said frequency-doubled output radiation
is delivered in a single axial-mode.
33. A laser, comprising: an OPS-structure having a gain-structure
surmounting a mirror-structure, said gain-structure including a
plurality of active layers having pump-light-absorbing layers
therebetween, said active layers having a composition selected to
provide emission of electromagnetic radiation at a predetermined
fundamental-wavelength between about 425 nanometers and 1800
nanometers when optical-pump light is incident on said
gain-structure, and said mirror-structure including a plurality of
layers of alternating high and low refractive index and having an
optical thickness of about one-quarter wavelength of said
predetermined wavelength; a laser-resonator formed between said
mirror-structure of said OPS-structure and a reflector spaced apart
therefrom, said laser resonator having a longitudinal axis; an
optical arrangement for delivering said pump-light to said
gain-structure, thereby causing fundamental laser-radiation having
said fundamental-wavelength to oscillate in said laser-resonator; a
heat-sink arrangement for cooling said first OPS-structure; a first
optically-nonlinear crystal located in said laser-resonator and
arranged for frequency-doubling said fundamental laser-radiation,
thereby providing frequency-doubled radiation having a wavelength
half of said fundamental wavelength; a second optically-nonlinear
crystal located in said laser-resonator an arranged for mixing said
frequency-doubled radiation and said fundamental laser-radiation
thereby providing frequency-tripled radiation having a wavelength
one-third of said fundamental-wavelength; and said laser-resonator,
said optically nonlinear-crystal, said OPS-structure, said
heat-sink arrangement and said optical pump-light-delivering
arrangement selected and arranged such that said resonator delivers
said frequency-tripled radiation as output-radiation having a
wavelength between about 142 nanometers and 600 nanometers at an
output-power greater than about 100 milliwatts.
34. The laser of claim 33 wherein said longitudinal-axis of said
laser-resonator if folded at an angle by a fold-mirror located
between said reflector and said OPS-structure, and said first and
second optically-nonlinear crystals are located on said
longitudinal-axis of said laser-resonator between said reflector
and said fold-mirror.
35. The laser of claim 32 wherein said fold-mirror is highly
reflective for said fundamental-radiation and highly transmissive
for said frequency-doubled radiation and said frequency-tripled
radiation, said fold mirror thereby serving as an output-coupling
mirror for delivering said frequency-tripled output-radiation from
said laser-resonator and essentially preventing frequency-doubled
and frequency tripled radiation from reaching said
OPS-structure.
36. The laser of claim 33, wherein said first optically-nonlinear
crystal is a crystal of a material selected from the group of
optically-nonlinear materials consisting of LBO, and CLBO.
37. The laser of claim 33 wherein said second optically-nonlinear
crystal is a crystal of a material selected from the group of
optically-nonlinear materials consisting of LBO, CLBO, BBO, SBBO,
SBO, and BZBO.
38. A laser, comprising: a laser-resonator having a resonator axis
and being terminated by first and second mirrors; an OPS-structure
having a surface-emitting gain-structure, said gain-structure
including a plurality of active layers having separator layers
therebetween said active layers having a composition selected to
provide emission of electromagnetic radiation at a predetermined
fundamental-wavelength when optical-pump light is incident on said
gain-structure, and said laser-resonator being configured to
include said gain-structure of said OPS-structure; an optical
arrangement for delivering said pump-light to said gain-structure,
thereby causing fundamental laser-radiation having said
fundamental-wavelength to oscillate in said laser-resonator; a
heat-sink arrangement for cooling said OPS-structure; an
optically-nonlinear crystal located in said laser-resonator and
arranged for frequency-doubling said fundamental laser-radiation
thereby providing frequency-doubled radiation having a wavelength
half of said fundamental-wavelength; and said laser-resonator, said
optically nonlinear crystal, said heat-sink arrangement and said
optical pump-light-delivering arrangement selected and arranged
such that said resonator delivers said frequency-doubled radiation
as output-radiation at an output-power greater than about 100
milliwatts.
39. A laser as recited in claim 38 wherein said OPS structure
surmounts a mirror structure, and wherein said mirror structure
functions as a fold mirror in the laser resonator.
40. A laser, comprising: an OPS-structure having a gain-structure
surmounting a mirror-structure, said gain-structure including a
plurality of active layers having pump-light-absorbing layers
therebetween, said active layers having a composition selected to
provide emission of electromagnetic radiation at a predetermined
fundamental-wavelength when optical-pump light is incident on said
gain-structure, and said mirror-structure including a plurality of
layers of alternating high and low refractive index and having an
optical thickness of about one-quarter wavelength of said
predetermined wavelength; a laser-resonator formed between said
mirror-structure of said OPS-structure and a reflector spaced apart
therefrom, said laser resonator having a longitudinal axis, said
longitudinal axis being folded by a fold-mirror located thereon
between said OPS-structure and said reflector; an optical
arrangement for delivering said pump-light to said gain-structure,
thereby causing fundamental laser-radiation having said
fundamental-wavelength to oscillate in said laser-resonator; and a
first optically-nonlinear crystal located in said laser-resonator
between said fold-mirror and said reflector said fold-mirror being
configured to focus said oscillating fundamental radiation into
said optically-nonlinear crystal, said optically-nonlinear crystal
being arranged for frequency-doubling said fundamental
laser-radiation thereby generating frequency-doubled radiation
having a wavelength half of said fundamental-wavelength.
41. The laser of claim 40, further including a heat-sink
arrangement for cooling said OPS-structure, and wherein said
laser-resonator, said optically nonlinear-crystal, said
OPS-structure, said heat-sink arrangement and said optical
pump-light-delivering arrangement selected and arranged such that
said resonator delivers said frequency-doubled radiation as
output-radiation at an output-power greater than about 100
milliwatts.
42. The laser of claim 40, wherein said fold-mirror is highly
reflective for said fundamental laser-radiation and highly
transmissive for said frequency-doubled radiation, whereby said
frequency-doubled radiation exits said resonator as output
radiation.
43. The laser of claim 40, wherein said composition of said active
layers of said gain-structure is selected such that said
fundamental-wavelength is between about 425 nm and 1800 nm, said
frequency-doubled radiation correspondingly having a wavelength
between about 212 and 900 nm.
44. The laser of claim 40, further including a second
optically-nonlinear crystal located in said laser-resonator an
arranged for mixing said frequency-doubled radiation and said
fundamental laser-radiation thereby providing frequency-tripled
radiation having a wavelength one-third of said
fundamental-wavelength.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates in general to external-cavity
optically-pumped semiconductor lasers (hereinafter, OPS-lasers)
including a surface-emitting, semiconductor multilayer (periodic)
gain-structure. The invention relates in particular to arrangements
of such lasers which can provide fundamental laser output-power of
about two Watts (2.0 W) or greater, and intracavity
frequency-converted arrangements of such lasers which can provide
harmonic laser output-power of about one-hundred milliwatts (100
mW) or greater.
DISCUSSION OF BACKGROUND ART
[0002] The term OPS-lasers, as used herein, refers to a class of
vertical-cavity surface-emitting semiconductor lasers wherein
optical gain is provided by recombination of electrical carriers in
very thin layers, for example, about 150 Angstrom units (.ANG.) or
less, of a semiconductor material. These layers are generally
termed quantum-well (QW) layers or active layers.
[0003] In an OPS-laser, several QW layers, for example, about
fifteen, are spaced apart by separator layers also of a
semiconductor material, but having a higher conduction band energy
that the QW layers. This combination of active layers and separator
layers may be defined as the gain-structure of the OPS-laser. The
layers of the gain-structure are epitaxially grown. On the
gain-structure is an epitaxially-grown multilayer mirror-structure,
often referred to as a Bragg mirror. The combination of
mirror-structure and gain-structure is referred to hereinafter as
an OPS-structure.
[0004] In an (external cavity) OPS-laser, another (conventional)
mirror, serving as an output-coupling mirror is spaced-apart from
the OPS-structure, thereby forming a resonant cavity with the
mirror-structure of the OPS-structure. The resonant cavity,
accordingly, includes the gain-structure of the OPS-structure. The
mirror-structure and gain-structure are arranged such that QW
layers of the gain-structure are spaced apart by one
half-wavelength of the fundamental laser wavelength, and correspond
in position with antinodes of a standing-wave of the fundamental
laser-radiation in the resonator. The fundamental-wavelength is
characteristic of the composition of the QW layers.
[0005] Optical pump-radiation (pump-light) is directed into the
gain-structure of the OPS-structure and is absorbed by the
separator layers of the gain-structure, thereby generating
electrical-carriers. The electrical-carriers are trapped in the QW
layers of the gain-structure and recombine. Recombination of the
electrical-carriers in the QW layers yields electromagnetic
radiation of the fundamental-wavelength. This radiation circulates
in the resonator and is amplified by the gain-structure thereby
generating laser-radiation.
[0006] OPS-lasers have often been used in the prior art as a means
of conveniently testing QW structures for later use in
electrically-pumped semiconductor lasers. More recently, OPS-lasers
have been investigated as laser-radiation sources in their own
right. The emphasis of such investigation, however, appears to be
on providing a compact, even monolithic, device in keeping with the
generally compact nature of semiconductor lasers and packaged
arrays thereof.
[0007] The gain-structure of OPS-structures may be formed from the
same wide range of semiconductor-materials/substrate combinations
contemplated for diode-lasers. These include, but are not limited
to, InGaAsP/InP InGaAs/GaAs, AlGaAs/GaAs, InGaAsP/GaAs and
InGaN/Al.sub.2O.sub.3, which provide relatively-broad spectra of
fundamental-wavelengths in ranges, respectively, of about 960 to
1800 nanometers (nm); 850 to 1100 nm; 700 to 850 nm; 620 to 700 nm;
and 425 to 550 nm. There is, of course, some overlap in the ranges.
Frequency-multiplication of these fundamental-wavelengths, to the
extent that it is practical, could thus provide relatively-broad
spectra of radiation ranging from the yellow-green portion of the
electromagnetic spectrum well into the ultraviolet portion.
[0008] In conventional solid-state lasers, fundamental-wavelengths,
and, accordingly, harmonics thereof (produced by frequency-doubling
or frequency-mixing) are limited to certain fixed wavelengths
characteristic of a particular dopant in a particular crystalline
or glassy host, for example, the well-known 1064 nm wavelength of
neodymium-doped yttrium aluminum garnet (Nd:YAG). While one of
these characteristic wavelengths may be adequate for a particular
application, it may not be the optimum wavelength for that
application.
[0009] OPS-lasers provide a means of generating wavelengths, in a
true CW mode of operation, which can closely match the optimum
wavelength for many laser applications, in fields such as medicine,
optical metrology, optical lithography, and precision laser
machining. Prior-art OPS-lasers, however, fall far short of
providing adequate power for such applications. It is believed that
the highest fundamental output-power that has been reported, to
date, for an OPS-laser is 700 mW at a wavelength of about 1000 nm
(Kuznetsov, et al., IEEE Photonics Tech. Lett 9, 1063 (1997)). For
an intracavity frequency-doubled OPS-laser, it is believed that
highest output-power that has been reported is 6 mW at a wavelength
of about 488 nm (Alford et al. Technical Digest of the IEEE/OSA
Conference on Advanced Solid State Lasers, Boston Mass., Feb. 1-3
1999, pp 182-184). It believed that there has been no report to
date of generation of continuous wave (CW) ultraviolet (UV)
radiation in an OPS-laser, either directly or by
frequency-multiplication.
[0010] However flexible an OPS-laser may be in potentially offering
a wide selection of wavelengths, in order to be competitive in
applications in which solid-state and other lasers are currently
used, at least an order-of-magnitude, and preferably two
orders-of-magnitude increase in power over that offered by
prior-art OPS-lasers is required. This power increase must also be
achieved without sacrifice of output-power stability and
beam-quality. Further, in order to be applicable in the broadest
range of applications the range of OPS-laser wavelengths available
at high-power and with high beam-quality must be extended into the
UV region of the electromagnetic spectrum, preferably to
wavelengths less than 300 nm.
SUMMARY OF THE INVENTION
[0011] The present invention is directed to providing high-power
OPS-lasers, including high-power OPS-lasers providing ultraviolet
radiation, i.e., at wavelengths less than about 425 nm. In one
particular aspect, an OPS-laser in accordance with the present
invention comprises an OPS-structure having a gain-structure
surmounting a mirror-structure. The gain-structure includes a
plurality of active layers having pump-light-absorbing layers
therebetween. The active layers have a composition selected to
provide emission of electromagnetic radiation at a predetermined
fundamental-wavelength between about 425 nanometers and 1800
nanometers when optical-pump light is incident on the
gain-structure. The mirror-structure includes a plurality of layers
of alternating high and low refractive index and having an optical
thickness of about one-quarter wavelength of the predetermined
wavelength.
[0012] A laser-resonator is formed between the mirror-structure of
the OPS-structure and a reflector spaced apart therefrom. An
optical arrangement is provided for delivering the pump-light to
the gain-structure, thereby causing fundamental laser-radiation
having the fundamental wavelength to oscillate in the
laser-resonator. A heat-sink arrangement is provided for cooling
the OPS-structure. An optically-nonlinear crystal is located in the
laser-resonator and arranged for frequency-doubling the fundamental
laser-radiation, thereby providing frequency-doubled PATENT
radiation having a wavelength half of the fundamental
wavelength.
[0013] The laser-resonator, the optically nonlinear-crystal, the
OPS-structure, the heat-sink arrangement and the optical
pump-light-delivering arrangement are selected and arranged such
that the resonator delivers the frequency-doubled radiation as
output-radiation having a wavelength between about 212 nanometers
and 900 nanometers at an output-power greater than about 100
milliwatts. The laser preferably has a resonator length greater
than about 5.0 cm
[0014] In one embodiment of a high-power OPS-laser in accordance
with the present invention, stable, single axial-mode, CW laser
output-power of about 4.0 W at 488 nm wavelength is achieved by
intracavity frequency-doubling 976 nm radiation from a single
OPS-structure using an optically-nonlinear crystal of lithium
triborate (LBO) in a resonator having a length of about twenty-five
centimeters (cm). The OPS-structure has active layers of an
In.sub.0.18Ga.sub.0.82As composition, and pump-light-absorbing
(separator) layers of a GaAs.sub.0.978P.sub.0.022 composition. The
laser is pumped by about 34.0 W of 808 nm radiation from two
diode-laser arrays. Numerical models indicate that the same
resonator may be modified by including an optically-nonlinear
crystal for mixing the fundamental and frequency-doubled radiation
to produce about 120.0 mW of (frequency tripled or third-harmonic)
325 nm UV radiation. A simplified configuration of the resonator of
this example was used without an optically nonlinear crystal to
deliver fundamental output-power of about 10 W.
[0015] The 4.0 W of frequency-doubled output-power represents over
two orders-of-magnitude increase over what is believed to be the
highest reported frequency-doubled output-power of any prior-art
OPS-laser. It is believed that frequency-tripled output of any
power has not been achieved in a prior-art OPS-laser.
[0016] Numerical models indicate that in a resonator similar to the
resonator of the example above, an OPS-structure having a
gain-structure In.sub.xGa.sub.1-xP quantum wells with
In.sub.yGa.sub.1-yAs.sub.zP.sub.1-- z separator layers
therebetween, wherein x is selected to provide fundamental
radiation at 750 nm may be pumped by 12.0 Watts of pump-light at
670 nm, using a 5 mm-long LBO crystal for frequency doubling to
provide output-power in excess of 1 Watt at the frequency-doubled
wavelength of 375 nm.
[0017] This remarkable increase in OPS-laser output-power and the
ability to generate high, CW, UV output-power, either by
frequency-doubling or frequency-tripling, is achieved without
sacrifice of beam-quality. Single mode operation provides that
OPS-lasers in accordance with the present invention can have a beam
quality less than 2.0 times, and as low as 1.2 times the
diffraction limit. This high-beam quality makes the inventive
OPS-lasers ideal for applications in which the output radiation
must be focused to a very small spot for making precise incisions
in inorganic or organic material, or must be efficiently coupled
into an optical fiber for transport to a location where it is to be
used.
[0018] In another aspect of an OPS-laser in accordance with the
present invention, the laser includes first and second resonators
arranged such that a portion of the resonator axes of each are on a
coaxial path. The first resonator includes an OPS-structure
arranged outside the coaxial path to provide a selected
fundamental-wavelength of laser radiation. Located on the coaxial
path of the first and second resonators is an optically-nonlinear
crystal arranged for frequency-doubling the fundamental radiation.
The first and second resonators are interferometrically matched to
maintain optimum phase-matching between fundamental and
frequency-doubled radiation in the optically-nonlinear crystal.
Fundamental-wavelength radiation and frequency-doubled radiation
circulate together only along the coaxial path. An
optically-nonlinear crystal is located in the second resonator
outside the coaxial path for doubling the frequency of the
frequency-doubled radiation thereby providing frequency-quadrupled
radiation. Numerical models indicate that by using the 976 nm
OPS-structure and pumping arrangement of the above-described first
embodiment, this second embodiment is capable of providing about
2.0 W of frequency-quadrupled (244 nm) radiation.
[0019] In yet another aspect of an OPS-laser in accordance with the
present invention, the laser includes first and second
interferometrically-matched resonators arranged such that a portion
of the resonator axes are on a coaxial path. The coaxial path
includes a first optically-nonlinear crystal arranged for
frequency-doubling as discussed above with respect to the second
embodiment. Fundamental-wavelength radiation and frequency-doubled
radiation circulate together only along the coaxial path. A second
optically-nonlinear crystal is located in the coaxial path of the
first and second resonators for mixing the fundamental radiation
with frequency-doubled radiation thereby providing
frequency-tripled radiation. Numerical models indicate that by
using the 976 nm OPS-structure and pumping arrangement of the
above-described first embodiment, this third embodiment is capable
of providing about 4.0 W of frequency-tripled (325 nm)
radiation.
[0020] Other, more general, aspects of OPS-lasers in accordance
with the present invention include but are not limited to: design
of heat-sink configurations and bonding methods for cooling an
OPS-structure which allow the above exemplified high pump-powers to
be directed on the OPS-structure while maintaining a safe operating
temperature therefor; design of optically-long resonators for
providing a relatively large fundamental mode-size at OPS-structure
to take advantage of a larger pumped-area, thereby increasing laser
output-power; design of specific, folded-resonator configurations
for optimizing output of frequency-converted radiation and
preventing reflection of the frequency-converted radiation back
into the OPS-structure where it would be lost through absorption;
selection of a specific ratio of pumped area to mode-size at the
OPS to optimize use of gain, and to prevent generation of
transverse modes of oscillation; use of an intracavity
wavelength-selective element for preventing oscillation of
fundamental radiation at wavelengths outside the spectral range of
acceptance of optically nonlinear crystals; selection of optically
nonlinear materials for maximum spectral acceptance to allow the
use of efficient and tolerant wavelength-selective devices for the
former; configuration of OPS-structures to eliminate parasitic
lateral oscillation which would otherwise reduce output power;
design of OPS-structures for minimum net stress and reliability
under high power operation; use of radial-index gradient lens to
optimize multiple optical-fiber delivery of pump-light; and design
of mirror-structures for the inventive OPS-structure for maximum
thermal-conductivity thereby facilitating cooling of the
OPS-structures.
[0021] It will be particularly evident from the detailed
description of the present invention presented below that for
achieving the high powers discussed above, OPS-laser resonators in
accordance with the present invention depart radically from the
"compactness" philosophy of prior-art OPS-lasers and are
inventively configured for intracavity frequency multiplication. It
will also be evident that significant attention is directed to
thermal management of OPS-structures, to the design of
OPS-structures themselves, and to selection of frequency
multiplication materials, in order to achieve the remarkable
output-power levels, and stability of the inventive OPS-lasers. It
will further be evident that certain inventive aspects of the
invention are applicable to both high and low-power OPS-lasers, or
even to laser types other than OPS-lasers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The accompanying drawings, which are incorporated in and
constitute a part of the specification, schematically illustrate a
preferred embodiment of the present invention, and together with
the general description given above and the detailed description of
the preferred embodiment given below, serve to explain the
principles of the invention.
[0023] FIG. 1 schematically illustrates a preferred embodiment of
an OPS-laser in accordance with the present invention having a
resonator including an OPS-structure and an optically-nonlinear
crystal arranged for intracavity frequency-doubling the
fundamental-wavelength of the OPS-structure.
[0024] FIG. 2 is a graphical representation of the mode-size of
fundamental radiation as function of axial position in the
resonator of FIG. 1
[0025] FIG. 3 schematically illustrates details of one preferred
arrangement and composition of semiconductor layers in the
OPS-structure of FIG. 1.
[0026] FIG. 4. schematically illustrates another preferred
embodiment of an OPS-laser in accordance with the present invention
having a resonator including an OPS-structure and two
optically-nonlinear crystals arranged for intracavity
frequency-tripling the fundamental-wavelength of the
OPS-structure.
[0027] FIG. 5. schematically illustrates yet another preferred
embodiment of an OPS-laser in accordance with the present invention
having a first resonator including an OPS-structure and a second
resonator having a common optical path with the first resonator,
the common optical path of the resonators including two
optically-nonlinear crystals arranged PATENT for intracavity
frequency-tripling the fundamental-wavelength of the
OPS-structure.
[0028] FIG. 6 schematically illustrates still another preferred
embodiment of an OPS-laser in accordance with the present invention
having a first resonator including an OPS-structure and a second
resonator having a common optical path with the first resonator,
the resonators being arranged for intracavity frequency-quadrupling
the fundamental-wavelength of the OPS-structure.
[0029] FIG. 7 schematically illustrates a further preferred
embodiment of an OPS-laser in accordance with the present invention
having a straight resonator including an OPS-structure and a
birefringent filter.
[0030] FIG. 8 schematically illustrates the OPS-structure of any of
FIGS. 1 and 4-7 bonded to a heat-sink assembly in accordance with
the present invention.
[0031] FIG. 9 is a graph depicting isothermal contours produced by
uniform heat-flow into a selected area of a surface of a
relatively-massive copper heat-sink.
[0032] FIG. 10 is a graph depicting isothermal contours produced by
uniform heat-flow into the selected area of FIG. 7 on a surface of
a CVD-diamond layer bonded to a relatively-massive copper
heat-sink.
[0033] FIG. 11 is a perspective view schematically illustrating
parasitic lateral oscillation in an OPS-structure in accordance
with the present invention.
[0034] FIG. 12 is a graph schematically illustrating computed
conversion efficiency as a function of wavelength for LBO and
potassium niobate (KNbO.sub.3) crystals of equal-length.
[0035] FIG. 13 schematically illustrates a further preferred
embodiment of an OPS-laser in accordance with the present invention
having a resonator including two OPS-structures and an
optically-nonlinear crystal arranged for intracavity
frequency-doubling the fundamental-wavelength of the
OPS-structure.
[0036] FIG. 14 schematically illustrates a further preferred
embodiment of an OPS-laser in accordance with the present invention
having a folded resonator terminated by first and second external
mirrors and being folded by a mirror-structure of an
OPS-structure.
DETAILED DESCRIPTION OF THE INVENTION
[0037] Turning now to the drawings, wherein like components are
designated by like reference numerals. FIG. 1 schematically
illustrates an OPS-laser 20 in accordance with the present
invention. Laser 20 includes a resonator 22 having a longitudinal
axis 24 thereof folded by a fold-mirror 26. Resonator 22 is
terminated at one end thereof by a flat mirror or reflector 28, and
at the other end thereof by a mirror portion (mirror-structure) 30
of an OPS-structure 32. A gain portion (gain-structure) 34 of
OPS-structure 32 is thus located in the resonator in contact with a
resonator mirror, i.e., mirror-structure 30.
[0038] Gain-structure 34 of OPS-structure 32 is an
epitaxially-grown monolithic semiconductor (surface-emitting)
multilayer structure including a plurality of active layers (not
shown in FIG. 1) spaced apart by pump-light-absorbing
separator-layers (also not shown in FIG. 1). It should be noted
here that the terminology "spaced apart by pump-light-absorbing
separator layers" in the context of this description and the
appended claims does not preclude there being other layers between
the QW layers. Depending on the composition of the QW layers, one
or more other layers may be included for strain-management,
carrier-confinement and the like. Any such arrangement is
applicable in the context of the present invention.
[0039] In prior-art OPS-lasers, the mirror-structure of the
OPS-structure is typically also an epitaxially-grown multilayer
structure grown from different compositions of material having a
general composition Al.sub.xGa.sub.(1-x)As (abbreviated AlGaAs),
wherein increasing x increases the bandgap of the material, and
lowers the refractive index. While this is one form of
mirror-structure which could be used for mirror-structure 30, it is
neither the only form contemplated, nor the preferred form
contemplated. Mirror-structure 30 in an OPS-structure 32 in
accordance with the present invention, need not be epitaxially
grown and may include dielectric or metal layers. If epitaxially
grown, it need not be formed entirely from materials in the
(ternary) AlGaAs system. Preferred mirror-structures are discussed
in detail further hereinbelow.
[0040] Continuing now with reference to FIG. 1, OPS-structure 32 is
bonded in thermal contact with a heat-sink 36. Heat-sink 36 is
preferably an actively-cooled heat-sink such as a
microchannel-cooler. A particularly preferred arrangement of heat
sink 36 is discussed in detail further hereinbelow.
[0041] OPS-structure 32 is optically pumped, preferably, by
pump-light delivered from one or more diode-laser arrays (not
shown). In FIG. 1, pump-light is delivered from two diode-laser
arrays via two optical fibers (or fiber bundles) 40. Pump-light 42
diverges as it exits a fiber 40. In each case, the diverging
pump-light is directed by a mirror 44, through focusing lenses 46
and 48, to be focused (only an axial ray shown) on gain-structure
34 of OPS-structure 32. It should be noted, that while two
pump-light-delivery fibers 40 and associated focussing optics are
illustrated in FIG. 1, this should not be considered as limiting
the present invention. Only one, or more than two
pump-light-delivery fibers and associated focusing-optics may be
used, and even different pump-light sources with or without fiber
delivery may be used, without departing from the spirit and scope
of the present invention. Further it should be noted that optical
fibers and fiber bundles are just one preferred means of
transporting pump-light from a source thereof. Other forms of what
may be generally termed "a lightguide", for example, solid or
hollow light-waveguides maybe used without departing from the
spirit and scope of the present invention.
[0042] Mirrors 26 and 28, and mirror-structure 30 of OPS-structure
32, each have maximum reflectivity at a fundamental (emission)
wavelength characteristic of the composition of (active layers of)
gain-structure 34 of OPS-structure 32. Energizing gain-structure 34
of OPS-structure 32 causes laser radiation having the fundamental
-wavelength (fundamental-radiation) to circulate in resonator 22.
This fundamental-radiation is indicated in FIG. 1 by single arrows
F.
[0043] Included in resonator 22, in folded portion 22A thereof,
proximate, but spaced apart from mirror 28, is an
optically-nonlinear crystal 50 arranged for frequency-doubling
(halving the wavelength of) the fundamental radiation. This
generates frequency-doubled or second-harmonic (2 H) radiation
indicated in FIG. 1 by double arrows 2 H. 2 H-radiation is
generated both on a first pass of fundamental laser radiation
therethrough and on a return pass of the fundamental-radiation
after it is reflected from mirror 28. Fold-mirror 26 is transparent
to the 2 H-radiation and, accordingly, serves to couple the 2
H-radiation out of resonator 22.
[0044] It should be noted here, that this folded-resonator
arrangement of an OPS-laser in accordance with the present
invention is particularly important, considering the high-power
operation of the device. The folded-resonator arrangement allows
formation a resonating beam in resonator 22 having optimum
characteristics at OPS-structure 32 and at the optically-nonlinear
crystal 50. In one arrangement the pump-light spot-size at
OPS-structure 32 preferably has a gaussian shape, preferably with a
1/e.sup.2 radius of about 230 micrometers (.mu.m). In order to
maximize overlap and obtain optimum power-extraction extraction in
fundamental transverse mode, the resonating fundamental-radiation
at OPS-structure 32 preferably has a similar size of 230 .mu.m
(1/e.sup.2 radius). Extensive numerical simulations, corroborated
by experimental results, indicate that the spot-size of
fundamental-radiation in optically-nonlinear crystal 50 is
preferably of the order of 50 .mu.m (1/e.sup.2 radius) for optimum
second harmonic generation. FIG. 2 depicts mode-size (radius) as a
function of axial position within resonator 22 of laser 20. of FIG.
1. It can be seen that this preferred resonator arrangement of
plane end mirrors (mirror-structure 30 and external mirror 28)
folded by a concave fold-mirror 26 (here, having a radius of 100
mm), provides above-discussed preferred spot or mode-sizes at
OPS-structure 32 and in optically-nonlinear crystal 50. The
folded-resonator arrangement, as noted above, also allows two
passes of fundamental radiation through optically-nonlinear crystal
50, thereby increasing the amount of 2 H-radiation generated.
Extraction of the 2 H-radiation via fold-mirror 36 prevents 2
H-radiation from being lost by absorption in OPS-structure 32.
[0045] In an intracavity frequency-doubled laser in accordance with
the present invention, it is preferable to include a
wavelength-selective element, such as a birefringent filter or an
etalon in the resonator for forcing the resonator to oscillate
precisely at the wavelength that the optically-nonlinear crystal is
arranged to frequency-double. This is discussed in detail further
hereinbelow with respect to selection of a preferred material for
optically-nonlinear crystal 50. In laser 20, such a
wavelength-selective element is depicted in the form of a
birefringent filter 52 arranged at Brewster's angle (here
57.1.degree.) to axis 24 of resonator 20. It is emphasized here
that the purpose of this wavelength-selective element is not
axial-mode selection, as this is accomplished by a combination of
the unique properties of OPS-structure 32 combined with its
location in resonator 22. Rather, birefringent filter 52 is used to
effectively spectrally narrow the gain-bandwidth of gain-structure
34 of OPS-structure 32 to a bandwidth narrower than a spectral
acceptance region over which the optically-nonlinear crystal 50 is
effective. This prevents laser 20 from oscillating at wavelengths
where the optically-nonlinear crystal is ineffective. This aspect
of OPS-lasers in accordance with the present invention is also
discussed in detail further hereinbelow.
[0046] In one example of a laser 20 in accordance with the
arrangement of FIG. 1, OPS-structure 32 (see FIG. 3) has a
gain-structure 34 comprising fifteen QW or active-layers of an
In.sub.0.18Ga.sub.0.82As composition, having a thickness of about
75.0 Angstrom Units (.ANG.) providing a nominal fundamental
(emission) wavelength of 976 nm. Between the QW layers are
pump-light-absorbing (separator) layers of a
GaAs.sub.0.978P.sub.0.022 composition having a thickness of 1217
.ANG.. Between the QW layers and the separator layers is a
strain-relieving layer of GaAs having a thickness of about 50
.ANG.. Mirror-structure 30 comprises 27 pairs or periods of
alternating layers of GaAs having a refractive index of about 3.51
and AlAs.sub.0.96P.sub.0.04 having a refractive index of about 2.94
and an optical thickness of one-quarter wavelength at the
fundamental-wavelength. Gain-structure 34 also includes a
carrier-confinement layer of Ga.sub.0.51In.sub.0.49P, having a
thickness of 1588.ANG., PATENT between the last separator layer and
mirror-structure 30. At an opposite extremity of gain-structure 34
there is also a carrier-confinement layer of
Ga.sub.0.51In.sub.0.49P having a thickness of 1588 .ANG..
[0047] OPS-structure 32 is epitaxially grown on an n-type GaAs
wafer (substrate), gain-structure 34 being grown first, beginning
with the carrier confinement layer. Mirror-structure 30 is
epitaxially-grown on the gain-structure. After the OPS-structure is
grown, the wafer is etched away. The first-grown confinement layer
serves as an etch-stop layer when the substrate is removed by
etching. The wafer, and structures grown thereon, is diced into
several OPS-structures 32 in the form of square "chips" about 2.0
mm by 2.0 mm.
[0048] An OPS-structure (chip) is first bonded to a
microchannel-cooler (cooler 36). One preferred microchannel-cooler
is a Model SA-2, available from Saddleback Aerospace Corporation of
Los Alamitos, Calif. Before bonding the OPS-structure to the
microchannel-cooler, a relatively thin (about 0.3 mm thick)
synthetic diamond (CVD-diamond) layer, is bonded to the
microchannel-cooler. The synthetic diamond layer has a particular
thermal-management function which is discussed in further detail
hereinbelow.
[0049] In prior-art methods for bonding to diamond a gold
overcoated platinum metalization is applied to the diamond before
bonding. Indium is solder used for bonding. Under the operating
conditions contemplated for a bonded OPS-structure in accordance
with the present invention, the gold overcoating can dissolve in
the indium bonding material, eventually causing long-term
instability of the bonded joint, with an associated deformation of
the OPS structure. One preferred bonding method in accordance with
the present invention which is believed may provide superior
long-term stability and avoid potential deformation is to provide
metalization on the diamond comprising a titanium layer overcoated
by a platinum layer only without a gold overcoating. Bonding is
then performed using indium solder. It is believed that such a
bonding method would be superior to prior-art chip-bonding methods,
not only under the high-power operating conditions contemplated for
OPS-lasers in accordance with the present invention but for any
OPS-laser.
[0050] Another possible bonding method would be to use a more
stable, gold-tin eutectic solder rather than indium solder. It is
possible, here, however, that a coefficient of thermal expansion
(CTE) mismatch with diamond may limit its use with diamond
heat-sinks. The CTE mismatch problem may be avoided by use of
alternate heat-sink materials having a closer CTE match to the
gold-tin eutectic, such as copper-tungsten (Cu-W), cubic boron
nitride, silicon-diamond composite, and the like. Some compromise
in output-power performance may be expected here, however, due to
the lesser heat conduction efficiency of these materials compared
to diamond. The CTE mismatch problem becomes greater the larger the
chip.
[0051] After the OPS-structure is bonded to the
diamond-layer/microchannel- -cooler, the GaAs substrate is removed
by etching. Preferably, an antireflection coating is deposited on
thus-exposed gain-structure 34 to improve entry of pump-light into
the gain-structure.
[0052] Regarding optical pumping of OPS-structure 32, each fiber 40
delivers about 17.0 W of 795 nm radiation from a FAP-30C-800-B
diode-laser-array package available from Coherent Semiconductor
Group of Santa Clara, Calif. Mirrors 44 are dielectric-coated
mirrors having greater than 99.9%. reflectivity at 795 nm and
28.degree. angle of incidence. Lenses 46 are cemented doublets
having a focal length of 40.0 mm and a diameter of 18.0 mm. Lenses
48 are cemented doublets having a focal length of 21.0 mm and a
diameter of 14 mm . These lenses are available from Melles Griot of
Irvine, Calif. The pump-light is focused by the mirrors and lenses
into an area of OPS-structure. A total of 34 Watts of pump-light in
the pumped area has a substantially Gaussian intensity profile with
a radius of about 260 .mu.m at the 1/e.sup.2 points. The
exemplified diode-laser-array packages providing the pump-light
require about 50.0 Watts each of so-called "wallplug"
electrical-power input generate 20.0 Watts of pump radiation
coupled into transport fibers 40.
[0053] Birefringent filter 52 is quartz plate having a thickness of
3.08 mm and oriented as depicted in FIG. 1 at 57.1 degrees to axis
24, with the quartz optical axis in the plane of the plate. Such a
filter is available as Part No. BF254-6T from VLOC Company of Port
Richey, Fla. This orientation of birefringent filter 52 provides
that fundamental radiation F is polarized perpendicular to the
plane of FIG. 1 as illustrated by arrow P1.
[0054] Birefringent filter 52 has narrow transmission-peaks
separated by about 35 nm, each with a full width at half maximum
transmission (FWHM) of about 3 nm. Maximum selectivity is achieved
by keeping the quartz optic-axis at an angle of about 45 degrees
from an axis defined by the intersection of the vertical plane with
the plane of the plate. The wavelength of the transmission peaks
can be shifted by rotating the plate slightly around an axis normal
to its faces, thus achieving tuning of the filter. The computed
tuning rate, confirmed experimentally, is about 5.6 nm per degree
of rotation
[0055] Mirror 28 is a plane mirror and fold-mirror 26 is a concave
mirror having a radius of curvature of 100 mm. Mirrors 26 and 30
are axially separated by a distance of 202 mm. Mirrors 26 and 28
are axially separated by a distance of 56 mm. Accordingly,
resonator 22 has an axial length of 258 mm (25.8 cm).
[0056] Optically nonlinear crystal 50 is a 5 mm-long crystal of
lithium triborate (LBO) having a cross-section of 3 mm.times.3 mm.
The crystal is cut for type-1 phase matching for 976 nm radiation.
Propagation of the fundamental beam is in the crystallographic X-Y
plane. The propagation direction is at an angle of 17.1 degrees
from the X-axis. The fundamental-radiation is polarized
perpendicular to the X-Y plane (parallel to the Z-axis).
Second-harmonic radiation is polarized in the X-Y plane, as
illustrated by arrow P2.
[0057] The above-specified exemplary OPS-laser generated an
intracavity power of fundamental (976 nm) radiation of about 300
watts yielding frequency-doubled (488 nm) radiation at an output
power of 5 Watts in single longitudinal (axial) mode and single
transverse mode. The beam divergence was measured at 1.2 times the
diffraction limit (M.sup.2=1.2). The quantity M.sup.2 is a
numerical measure which represents a ratio of the size of the beam
to a diffraction-limited limited size. A high quality beam may be
regarded as a beam having an M.sup.2 of about 2.0 or less. The
high-beam quality available with the inventive OPS-lasers make them
useful for applications in which the laser output-radiation must be
focused to a very small spot for making precise incisions in
inorganic or organic material, or must be efficiently coupled into
an optical-fiber or guided by an articulated arm for transport to a
location where it is to be used.
[0058] From the power point-of-view alone, this represents about a
one-thousand-fold increase over the above-discussed 6.0 mW output
of a prior-art IC, frequency-doubled OPS-laser. Surprisingly, the
fundamental radiation, and, accordingly, the frequency-doubled
radiation was in a single axial (longitudinal) mode. Temporal
stability of the output-radiation is estimated in terms of
output-fluctuation or noise of less than about 0.05% RMS over a
band DC to 10 MHz. It is believed that in other embodiments
OPS-lasers in accordance with the present invention described
hereinbelow a noise of less than about 0.1% can be obtained.
Pump-light to second-harmonic efficiency is about ten percent.
Electrical to second-harmonic efficiency is about four percent.
Generally, a pump-light to second-harmonic efficiency of 3.0
percent or greater may be achieved in appropriate embodiments of
inventive OPS-lasers discussed herein.
[0059] Detuning of the cavity (moving mirror 28 axially away from
fold-mirror 36) to reduce the beam spot-size at OPS-structure 32,
forced operation in multiple transverse mode, allowed obtaining up
to 7.5 Watts of output-power at 488 nm, thereby obtaining even
higher power and efficiency at the expense of a reduced
beam-quality due to the multiple mode operation. The relationship
of mode-size to pumped-area (pump-spot size) in connection with
ensuring single axial-mode operation is discussed in detail further
hereinbelow.
[0060] The high output-power at 488 nm of the inventive IC double
OPS-laser is obtained with a total input electrical-power to the
pump-light-providing diodes of about 100 Watts. In prior-art
lasers, multi-Watt, high-beam-quality radiation at 488 nm was only
obtainable as the output of argon ion (gas) lasers. These lasers
typically require several thousands of Watts (up to 30 Kilowatts)
of electrical-power input. Accordingly, the above-exemplified laser
in accordance with the present invention provides an improvement in
the "electrical-to-optical" efficiency of two orders of magnitude
for 488 nm lasers.
[0061] Continuing now with the description of preferred embodiments
of OPS-lasers in accordance with the present invention, an
important aspect of the above-discussed example of an OPS-laser is
the discovery that such a high-fundamental power can be generated
from an OPS-structure, by paying attention, inter alia, to thermal
management of the intensely-pumped OPS-structure in combination
with increasing the dimensions of the resonator well beyond those
of prior-art OPS-lasers.
[0062] Having established experimentally that such a high
fundamental and 2 H-power can be provided, it is possible to
numerically-model other inventive OPS-lasers based on the same or
different OPS-structures 32 and "long resonator" arrangements,
similar to that of laser 20, which generate other fundamental and 2
H wavelengths at similar output power.
[0063] In particular, high-power CW radiation in the ultraviolet
spectral region, for example, at about 375 nm, can be generated by
using an OPS-structure 32 having a gain-structure 34 having
In.sub.xGa.sub.1-xP quantum wells with
In.sub.yGa.sub.1-yAs.sub.zP.sub.1-z separator layers therebetween.
Such an OPS-structure, pumped with 670 nm radiation from
fiber-coupled diode-lasers (for example, SDL part number 7470 P-5
from SDL,Inc., of San Jose, Calif.) provides gain at 750 nm. A
resonator similar to resonator 22 of FIG. 1 may be used, with a
similar birefringent filter 52, suitably angle-tuned, and with an
optically-nonlinear crystal 50 of LBO arranged for frequency
doubling the 750 nm radiation. In this regard, the LBO crystal is
preferably cut for propagation in the X-Y crystallographic plane,
at an angle of 37.5 degrees from the X-axis. The polarizations of
the fundamental and 2 H-radiation are the same as for the (976 nm
fundamental-wavelength) laser 20 exemplified above.
[0064] The efficiency of conversion for 750 nm radiation for the
LBO crystal is computed to be slightly higher (17% higher) than the
efficiency of conversion for 976 nm radiation for equal lengths of
the optically-nonlinear crystal. This results from a balancing of
two opposing factors. The effective nonlinearity
(nonlinear-coefficient) of LBO at 750 nm is lower, at about 1.0
picometer per volt (pm/V) compared with a value of 1.2 pm/V at 975
nm, due to the different angle of propagation. The overall
conversion-efficiency, however, is proportional to the square of
the nonlinear-coefficient, but is also proportional to the inverse
of the square of the wavelength. Accordingly, the shorter
wavelength (750 nm), is instrumental in providing the
above-discussed improvement of the overall
conversion-efficiency.
[0065] In one numerically-simulated example of a frequency-doubled,
750 nm fundamental-wavelength laser 20, 12.0 Watts of pump-light at
670 nm, obtained from 4 fiber-coupled diode-laser arrays or
modules, using a 5 mm-long LBO crystal for optically-nonlinear
crystal 20, resulted in computed output-power in excess of 1 Watt
at the 2 H-wavelength of 375 nm. Using higher pump-power from more
the same number of more-powerful diode-laser arrays, or from more
arrays of the same power, can provide 375 nm output-power
comparable to the 488 nm output power exemplified above for the
same resonator. Use of more pump-light arrays is possible using an
alternative pump-light delivery arrangement discussed in detail
further hereinbelow.
[0066] Having established experimentally, and numerically
corroborated, that such a high fundamental power can be generated,
it is possible to configure other "long-resonator" type, inventive
OPS-lasers based on the same OPS-structures, for generating third
harmonic (3 H) and fourth harmonic (4 H) radiation, and to compute,
with confidence, what output-power can be generated at such third
and fourth harmonic-wavelengths in the UV region of the
electromagnetic spectrum. Descriptions of such resonator
configurations are set forth below, exemplifying generation of the
third harmonic of 750 nm radiation, and the fourth harmonic of 976
nm radiation.
[0067] Referring now to FIG. 4, a laser 60 including a resonator 23
is schematically illustrated. Laser 60 is identical in most regards
to laser 20 of FIG. 1, with the following exceptions. Mirror 28 is
coated for high reflectivity at the fundamental and second (2 H)
harmonic wavelengths. Mirror 26 is maximally reflective at the
fundamental-wavelength and highly transmissive at the 2 H and 3
H-wavelengths. An optically-nonlinear crystal 62, is located
between optically-nonlinear crystal 50 and mirror 26 in folded arm
23A of resonator 23. Optically-nonlinear crystal 62 is arranged to
mix the fundamental and 2 H wavelengths (frequencies), thereby
generating frequency-tripled radiation, indicated in FIG. 4 by
triple arrows 3 H. Both 2 H and 3 H-radiation leave resonator 23
via fold-mirror 26. The 2 H and 3 H-radiations are then separated
by a dichroic beamsplitter 64.
[0068] Now, third-harmonic generation (mixing of fundamental and 2
H frequencies) is known to be proportional (dependent on
optically-nonlinear crystal characteristics) to the product of the
fundamental and second-harmonic power. So, based on experimentally
established values for these powers in this resonator, and on
documented values for 3 H-generation efficiency of various
optically-nonlinear materials, it can be predicted with a high
degree of confidence that the above-described example of a laser 60
using .beta.-barium borate (BBO) for optically-nonlinear crystal 62
can generate about 150 mW of true-CW, single-mode, laser
output-power at a wavelength of 250 nm, assuming an intracavity
fundamental power of about 300 W at 750 nm generating about 5.0 W
of 2 H-radiation (375 nm).
[0069] A BBO crystal (crystal 62) suitable for this process is cut
for propagation at an angle of 48 degrees from the optic axis, in a
plane containing the optic (Z) axis and making an angle of 30
degrees with the X-Z crystallographic plane. Fundamental-radiation
and 2 H-radiation propagate with polarization perpendicular to the
optic axis, (ordinary polarization). The 3 H-radiation is generated
with extraordinary polarization (normal to the fundamental
polarization).
[0070] Computations based on published data from crystal
manufacturers (CASIX) indicate that by focussing 300 watts of 750
nm radiation down to a radius of 60 .mu.m in optically-nonlinear
crystal 62 (in arm 20A of resonator 20), together with 5 Watts of 2
H-radiation (375 nm) focused coaxially in the same crystal to a
radius of 50 nm, about 150 mW of 250 nm (3 H) radiation is
generated. The above-described BBO crystal employs type-I phase
matching. This means that the fundamental and the 2 H radiation
must have the same polarization within the crystal.
[0071] The second-harmonic-generating LBO crystal
(optically-nonlinear crystal 50), however, generates 2 H-radiation
with polarization orthogonal to the polarization of the
fundamental-radiation. The polarization of the 2 H-radiation,
accordingly, must be rotated by 90 degrees before it enters
optically-nonlinear crystal 62. This may be achieved by a
birefringent quartz plate (polarization rotator) of such design
that it has retardation of an even integer multiple of .pi. for the
fundamental radiation, and retardation of an odd integer multiple
of .pi. for the 2 H radiation.
[0072] Such a polarization-rotator (depicted in FIG. 4 in phantom
as polarization-rotator 65) is inserted in the common path (arm
20A) of the fundamental and 2 H-radiation between optically
nonlinear crystals 50 and 62. Polarization-rotator 65, leaves the
polarization of the fundamental-radiation unaltered, and rotates
the linear polarization of the 2 H-radiation by an amount equal to
twice the angle between the initial polarization and the direction
of the optic-axis of the retardation plate.
[0073] As an alternative, a BBO crystal may be cut for type-II
mixing, so that polarization-rotator 65 is not required to modify
the polarization of the 2 H-radiation. Conversion in this case,
however, can be expected to be less efficient.
[0074] Providing a resonator mirror or reflector on any optically
nonlinear crystal in a OPS-laser resonator in accordance with the
present invention is not precluded. However, additional measures
such as temperature control of the crystal may be required to
resolve the conflicting alignment requirements for the resonator
and the cyrstal.
[0075] In resonator 23 of FIG. 4, third-harmonic generation from
970 nm fundamental-radiation is also possible using the
alternatives discussed above for 750 nm fundamental radiation.
Numerical simulations indicate that such third-harmonic generation
could generate about 150 mW of 325 nm radiation. Those skilled in
the art, from the above discussed example can determine appropriate
nonlinear crystal parameters without further detailed description.
Accordingly, no such detailed description is presented herein.
[0076] The 150 mW of third-harmonic radiation exemplified above
represents more than a twenty-times increase in power on
third-harmonic generation over the maximum believed to have been
reported in the prior art for only second-harmonic generation. The
arrangement for generating this third harmonic radiation
represented by laser 60, however, is arguably a somewhat
inefficient approach to IC generation of third harmonic radiation.
Significant improvements in harmonic output-power are possible in
another family of OPS-laser resonators in accordance for the
present invention discussed below, beginning with reference to FIG.
5
[0077] In FIG. 5, yet another embodiment 70 of an OPS-laser in
accordance with the present invention is schematically depicted.
Laser 70 includes two resonators 25 and 27 having a common, axial
path 35 (indicated in bold line in FIG. 5) over a portion of their
total length. Resonator 25 may be defined as a "fundamental
resonator" and is terminated at one end thereof by mirror 28 and at
the other end thereof by mirror-structure 30 of OPS-structure 32.
Resonator 25 is folded by a fold-mirror 26. OPS-structure 32 is
mounted and pumped as described above for lasers 20, and 60. Only
one pump-light-delivery fiber and associated focussing optics are
depicted in FIG. 5, for clarity.
[0078] Resonator 27 may be defined as a "second-harmonic
resonator". It is used to build up 2 H-radiation to levels much
higher than are achieved in the simple double-pass arrangements of
above-described resonators 22 and 23. Resonator 27 is terminated at
one end thereof by mirror 28 and at the other end thereof by a
mirror 74. Resonator 27 is also folded by a fold-mirror 26.
Fold-mirror 26 in this embodiment is coated for maximum reflection
at the fundamental and 2 H-wavelengths and maximum transmission at
the 3 H-wavelength. Mirror 28 in this embodiment is coated for
maximum reflection at the fundamental, 2 H and 3 H-wavelengths. The
axes of the resonators are folded together by a dichroic
beamsplitter 72 coated for maximum reflection at the 2 H-wavelength
and maximum transmission at the fundamental-wavelength.
Accordingly, the common (here, folded) axial path of resonators 25
and 27 extends between beamsplitter 72 and mirror 28. It is pointed
out here, that all coatings mentioned above are believed to be
within the capabilities of commercial suppliers of optical coating
services. One such supplier is Coherent Auburn Group, of Auburn,
Calif.
[0079] Located in the common axial-path of resonators 27 and 27,
proximate beamsplitter 72, is an optically-nonlinear crystal 50
arranged for doubling the fundamental-wavelength. The 2 H-radiation
generated by the frequency-doubling circulates and builds up in
resonator 27, thereby greatly increasing the amount of intracavity
2 H-radiation compared with the simple "double-pass" arrangement of
laser 20 of FIG. 1. Located in common, folded portion 27A of
resonators 25 and 27, (wherein, of course, both fundamental and 2
H-radiation are circulating) is an optically-nonlinear crystal 62
arranged for mixing the fundamental and 2 H-radiation to generate 3
H-radiation. 3 H-radiation so generated escapes the resonators via
fold-mirror 26. Regarding the resonators in general, the following
should be noted.
[0080] It is preferable that resonators 25 and 27 have about the
same length and are similarly optically configured. This provides
that the mode-size for fundamental and 2 H-radiation is about the
same, thereby maximizing mixing efficiency. It is also preferable
that the resonators are interferometrically-matched to maintain an
optimum phase-relationship between the circulating fundamental and
2 H-radiation at the location of optically-nonlinear crystal 50.
This is required to ensure that 2 H-radiation generated in
optically-nonlinear crystal 50 is added, in phase, to the 2
H-radiation circulating in the 2 H-resonator. This ensures that
circulating 2 H-radiation can grow to high levels, thereby
providing a high conversion-efficiency in the BBO mixing-crystal
(optically nonlinear crystal 62). In this regard, it is also
preferable that fundamental resonator 25 operate in only a single
axial-mode to avoid loss of power from mode-competition. Conditions
for providing single axial-mode operation are discussed in detail
further hereinbelow.
[0081] One arrangement for providing interferometric-matching of
the resonators is to provide an axial driver 76, such as a
piezo-electric driver, for mirror 74 of resonator 27. As mirror 74
is axially-driven, circulating 2 H-radiation rises to a maximum
level and falls to a minimum level (which may be zero or near-zero)
corresponding respectively to a maximally-in-phase condition and a
maximally-out-of-phase condition at optically-nonlinear crystal 50
for the fundamental and 2 H-radiation (standing waves).
Correspondingly, 3 H-output-power would rise and fall. A
beamsplitter 78 directs a small sample-fraction 3 HS of the 3
H-output to a detector 80. Detector 80 is cooperative with a
controller 82 which adjusts driver 76 to maintain peak
output-power.
[0082] As noted above, third-harmonic generation is proportional to
the product of the fundamental and second-harmonic power. By
incorporating a second resonator for 2 H-power, laser 70 provides
that a much greater 2 H-power is available for mixing than is the
case in laser 60 of FIG. 4, and, further, without any increase
being required in the fundamental-power. So, based on
above-discussed values for available fundamental-powers in this
resonator and documented values for 2 H-generation efficiency of
LBO it can be numerically determined that 300 W of circulating 750
nm radiation would generate about 200 W of circulating 375 nm
radiation in resonator 27. The double-pass combination of these in
an optically non-linear crystal 62 of .beta.-barium BBO can
generate about 4 W of true-CW, single-mode, laser output-power at a
wavelength of 250 nm. Similar output-power levels for 325 nm
radiation can be determined for 3 H-conversion of 976 nm radiation.
This is a more than a six-hundred-times increase in power on
third-harmonic generation over the maximum believed to have been
reported for only second harmonic generation in prior-art
OPS-lasers.
[0083] An optically-nonlinear (mixing) crystal preferable for laser
70 is the same as that discussed above with reference to laser 60.
Care should be exercised in the design of resonator 25 of laser 70,
however, in particular concerning the sizes of the beams within the
optically-nonlinear crystals 50 and 62.
[0084] The fundamental and 2 H-beams are preferably focused tightly
in optically nonlinear crystal 62 to maximize conversion (mixing)
efficiency. An optimum range of larger beam sizes, however, is
preferred for fundamental-radiation in optically-nonlinear
(doubling) crystal 50. Extensive numerical modeling of coupled
resonators 25 and 27 indicates that for an LBO crystal having a
length of 15 mm, the spot-sizes of the fundamental beam at the
OPS-structure and in the doubler crystal are preferably about the
same. This result simplifies the design of the resonator, since it
makes it possible to place the doubling crystal in the proximity of
the OPS-structure (as illustrated in FIG. 5), without having to
create, in the common optical path of resonators 25 and 27, a
tightly-focused waist in which to place optically-nonlinear
(doubling) crystal 50. In one preferred resonator configuration,
the fundamental beam has a spot-size of about 200 .mu.m at
OPS-structure 32, a spot-size of about 200 .mu.m in
optically-nonlinear crystal 50, and a spot-size of about 50 .mu.m
in optically-nonlinear crystal 62. The 2 H-resonator 27 is designed
so that the spot-size of the 2 H-beam in optically-nonlinear
crystal 50 is about 150 .mu.m, and in optically-nonlinear crystal
62 is about 40 .mu.m.
[0085] Referring now to FIG. 6, still another embodiment 90 of an
OPS-laser in accordance with the present invention is depicted.
Laser 90 includes two resonators 29 and 31 having a common axial
path 37 (indicated in bold line in FIG. 4) over a portion of their
total length. Resonator 29 may be defined as a "fundamental
resonator" and is terminated at one end thereof by mirror 26 and at
the other end thereof by mirror-structure 30 of OPS-structure 32.
OPS-structure 32 is mounted and pumped as described above for
lasers 20, and 60. Only one pump-light-delivery fiber and
associated focussing optics are depicted in FIG. 6, for
clarity.
[0086] Resonator 31 may be defined as a "second harmonic
resonator". Resonator 31 is terminated at one end thereof by mirror
74, and at the other end thereof by a mirror 92. Resonator 31 is
folded by a fold-mirror 94. Mirror 28 in this embodiment is coated
for maximum reflection at the fundamental-wavelength. Mirror 74 is
coated for maximum reflection at the 2 H-wavelength. Mirror 92 is
coated for maximum reflection at the fundamental, 2 H, and, fourth
harmonic (4 H) wavelengths. Mirror 94 is coated for maximum
reflection at the 2 H-wavelength and maximum transmission at the 4
H-wavelength
[0087] The axes of the resonators are folded together by dichroic
beamsplitters 72 and 73, each thereof coated for maximum reflection
at the 2 H-wavelength and maximum transmission at the
fundamental-wavelength. Accordingly, the common axial-path of the
resonators 29 and 31 extends only between beamsplitters 72 and 73.
For reasons discussed above, mirror 74 is mounted on a driver 76
controlled by a controller 82 in accordance with power detected by
detector 80 to interferometrically-match resonators 29 and 31.
[0088] Optically-nonlinear crystal 50 is arranged for
frequency-doubling fundamental-radiation circulating along common
path 37 of resonators 29 and 31. Frequency-doubled radiation so
generated circulates and builds up in resonator 31. Another
optically-nonlinear crystal 67 is located in folded portion 31A of
resonator 31 between mirrors 92 and 94 thereof. Optically-nonlinear
crystal 67 is arranged for frequency-doubling circulating
frequency-doubled radiation 2 H thereby generating
frequency-quadrupled (4 H) radiation. The 4 H-radiation exits
resonator 31 via fold-mirror 94.
[0089] Such a resonator configuration is advantageous in the
generation of 244 nm radiation through fourth-harmonic generation
of 976 nm. A preferred optically-nonlinear (doubling) crystal 50
for this application is an LBO crystal having a length of about
15.0 mm, with orientation of the crystal-axes identical to those
described above with reference to laser 20 of FIG. 1.
Optically-nonlinear (quadrupling) crystal 67 is preferably a BBO
crystal having a length of about 8.0 mm. This crystal is preferably
cut for propagation at an angle of 54 degrees from the optic-axis,
in a plane containing the optic (Z) axis and making an angle of 30
degrees with the X-Z crystallographic plane of the
optically-nonlinear crystal.
[0090] The 2 H-radiation propagates with polarization perpendicular
to the optic axis, (ordinary polarization) the 4 H radiation is
generated with extraordinary polarization (normal to the 2
H-polarization). Preferably mirror 26 has a radius of curvature of
30 cm., mirror 94 has a radius of curvature of 10 cm, and mirrors
74 and 92 are flat mirrors.
[0091] Similar considerations apply for the beam spot-size in
optically-nonlinear crystals 50 and 67 as apply for crystals 50 and
63 of FIG. 5. Numerical simulations indicate that an optimum,
relatively large, beam-size exists for the beams inside the
optically-nonlinear (doubler) crystal 50. The fundamental-radiation
spot-size in optically-nonlinear (doubler) crystal 50 is
preferentially the same as the spot-size at OPS-structure 32, for a
crystal length of about 15 mm. The "second harmonic resonator" 31
is dimensioned so as to generate a 2 H spot-size in
optically-nonlinear crystal 67 equal to the fundamental beam
spot-size divided by the square root of two (1.414), so that a beam
sustained by the resonator 31 has the same transverse size as beam
generated by optically-nonlinear crystal 50. The 2 H-radiation is
focused by concave fold-mirror 94 into the optically-nonlinear
crystal 67, to a spot-size of about 50 microns (1/e.sup.2
radius).
[0092] Another preferred material for optically-nonlinear crystal
67 is cesium lithium borate (CLBO). This material can
advantageously substitute BBO for frequency-doubling 488 nm
radiation in this and other applications. A CLBO crystal for
frequency-doubling 488 nm radiation is preferably cut for
propagation of the beam at an angle of 75.7 degrees from the
Z-axis, in a plane containing the Z axis and at an angle of 45
degrees from the X-Z plane. The 244 nm radiation so generated is
polarized in the plane defined by the direction of propagation and
the Z-axis (extraordinary polarization), the 488 nm radiation is
polarized perpendicular to the 244 nm radiation. The
nonlinear-efficiency of CLBO for this application is the same as
that of BBO. CLBO, however, has a significant advantage of a
five-times-greater angular acceptance than BBO, and a four-times
smaller walk-off angle than BBO. A greater acceptance angle and
smaller walk-off angle both contribute to increasing the net
conversion-efficiency of an optically nonlinear crystal. First
order considerations of acceptance angle and walk-off angle
indicate that the conversion-efficiency of CLBO may potentially be
several times higher than the efficiency of BBO. These first-order
considerations are applicable in general to frequency-doubling
radiation having a wavelength between about 425 nm and 525 nm in
any resonator. The radiation in that wavelength range may be the
fundamental-radiation or harmonic radiation of any gain-medium.
[0093] While embodiments of high-power OPS-lasers in accordance
with the present invention are described above with respect to
intracavity frequency-multiplication arrangements, it will be
evident to one skilled in the art that high-power OPS-laser in
accordance with the presentation may be operated, without
intracavity optically-nonlinear crystals, simply as an efficient
source of high-power single-mode fundamental radiation. A
fundamental output power of at least 2 W and even greater than 5 W
may be achieved in fundamental resonator embodiments of the an
OPS-laser in accordance with the present invention.
[0094] By way of example, 10 W of single axial-mode fundamental
radiation at 976 nm was generated in an OPS-laser 20F (see FIG. 7)
including a simple linear (not folded) resonator 22F formed by a
concave output-coupling mirror 28F and mirror structure 30 of
OPS-structure 32. Mirror 28F has a radius of curvature of 30 cm and
is spaced apart from OPS-structure 32 by a distance of about 15.0
cm. A birefringent filter 52 is preferably included in resonator
22F. This allows for output-wavelength adjustment which may be
required if manufacturing tolerances of OPS-structure 32 provide
peak gain at some wavelength slightly displaced from a desired
output-wavelength.
[0095] Such an inventive OPS-laser could be used as a
radiation-source for extracavity frequency-multiplication using
either a single-pass arrangement of optically-nonlinear crystals or
by directing the fundamental-radiation into a prior-art extracavity
PATENT ring-resonator, including, or formed from, an
optically-nonlinear material. A high-power OPS-laser in accordance
with the present invention may also provide a source of fundamental
(pump) radiation for an optical parametric oscillator (OPO),
co-linearly or non-colinearly pumped. Such arrangements are
disclosed in co-pending U.S. patent application No. 09/179,022 by
the same inventors, the complete disclosure of which is hereby
incorporated by reference.
[0096] Proceeding now with a description of mode-control and other
scaling aspects of the present invention, a surprising discovery
was that single axial-mode operation of the inventive lasers was
simply achieved even at the relatively long resonator-lengths
required for the exemplified high-power operation. The long
resonator length is required for reasons, inter alia, as
follows.
[0097] The gain-structure of an OPS has some inherent limitations
due to the need to strongly absorb pump-light in layers separating
the QW layers. These QW layers are preferably optically-spaced
apart by a half-wavelength of the fundamental radiation. Prior-art
investigations of such structures seem to have resulted in a
general belief among practitioners of the relevant art that a
number of about fifteen such QW layers is about optimum. This
results at least from a consideration that the pump-light, through
absorption in the structure, may not penetrate effectively more
than fifteen half-wavelengths deep. It should be noted here that
use of gain-structures with 15 QW layers in examples of OPS-lasers
in accordance with the present invention should not be construed as
limiting the inventive OPS-lasers or indicating that optimization
of the depth and QW content of such gain-structures has reached a
limit. In developing the inventive OPS-lasers, emphasis has simply
been placed, with evident success, at addressing other scaling
issues and other structural issues of OPS-structures. A discussion
of such other issues is set forth below.
[0098] Considering, for the purposes of this description that some
limit, whatever it is, to the depth or QW layer content of OPS
gain-structures exists, then another approach to increasing
available gain (accordingly power) in the gain-structure is
necessary. One such approach to increasing output-power in
accordance with the present invention is to increase the
pumped-area on the OPS-structure.
[0099] In order to take advantage of a larger pumped-area, however,
a resonator must be increased in length such that the resonator
mode-size is sufficient to extract all of the gain available in the
pumped-area. Resonator length is increased in proportion to the
square of the increase in mode size. Accordingly, with increasing
power being derived, inter alia, from increasing mode size, the
dimensions of the inventive OPS-resonators radically depart from
the prior-art compactness which appears from prior-art teachings to
be an attractive feature of OPS-lasers. Indeed, dimensions of the
inventive OPS-lasers become comparable with the dimensions of
prior-art diode-pumped solid-state (DPSS) laser-resonators of
similar output-power and beam-quality. As will be evident from this
description, however, these inventive resonators, particularly
those arranged for IC frequency-conversion, have certain
characteristic design features particularly related to the use of
an OPS-structure as a gain-medium.
[0100] One originally anticipated problem in drastically increasing
the length of an external OPS-laser resonator is the problem of
mode-control, specifically, operating in a single axial-mode.
Operation in a single axial-mode is preferred, inter alia, for
maximizing and decreasing temporal fluctuation of output-power. As
noted above operating a "fundamental resonator" in a single
axial-mode is particularly preferred in multiple-resonator IC
frequency-multiplication embodiments of OPS-lasers in accordance
with the present invention.
[0101] In above-mentioned prior-art DPSS-lasers, as the length of a
resonator is increased, the number of possible axial (longitudinal)
modes of oscillation is also increased, absent any measures for
preventing this. It would also be anticipated, on prima facie
consideration, that this problem would exacerbated in an OPS-laser
because of the greater gain-bandwidth of a semiconductor
gain-medium compared with a solid-state (dopant:host) gain-medium
of a DPSS-laser.
[0102] Typically, a large number of axial-modes leads to a
fluctuating (noisy) power output as the modes compete chaotically
for gain in the gain-medium. Prior-art measures which have been
taken in DPSS-lasers to limit the number of oscillating modes and
reduce output-noise include selective placement of the gain-medium
in the resonator in positions where phase-relationships between
adjacent modes limit this mode-competition. This approach has met
with only limited success in prior-art DPSS-lasers. It is believed
that, practically, true single mode operation has only been
possible in prior-art lasers by mechanically stabilizing the
resonator and including a highly wavelength-selective element in
the resonator, with its attendant problems of resonator loss,
maintenance of tuning, and the like.
[0103] Surprisingly it has been found that single axial-mode
operation is possible in a high power OPS-laser in accordance with
the present invention apparently independent of the physical length
of the resonator, at least up the 25 cm length of the above
exemplified inventive laser. It is believed that this length could
be significantly further extended, for example, up to 100 cm or
greater, while still maintaining single-axial-mode operation
without any special provision for-providing such operation.
[0104] It is believed, without being limited to a particular
theory, that this surprising discovery may be explained as follows.
In all above-described embodiments of the present invention,
gain-structure 34 of OPS-structure 32 is located precisely at the
end of the "fundamental-resonator", a position in which all
possible axial-modes of operation (standing waves) have a node,
i.e., are exactly in phase.
[0105] An OPS gain-structure of the present invention, is only
about seven wavelengths "long". The resonator itself may be tens of
thousands or even hundreds of thousands of wavelengths long.
Accordingly, even widely numerically separated modes (i.e., N and
N+m where m is an integer) do not become appreciably out of phase
within the gain-medium.
[0106] Now, the OPS gain-structure, with half-wavelength-spaced QW
layers only provides gain in narrow region one-half-wavelength
apart Accordingly, as the actual wavelengths of adjacent possible
modes differ fractionally, at most, by only a few millionths of a
wavelength, or a few hundredths of the width of the QW layers, all
possible modes have about an equal chance to extract the available
gain. That mode which begins to oscillate first wins the
"mode-competition" and deprives the others of the gain needed to
oscillate, thereby forcing single axial-mode operation.
[0107] To ensure true single-mode (single-frequency or TEMOO)
operation in an-OPS-laser in accordance with the present invention,
it is preferable to design resonators, wherein, as exemplified
above, the pumped-area (pump-spot) and fundamental spot-size at
OPS-structure 32 are about the same, i.e., the ratio of pump
spot-size to mode-size is about unity (1.0) or less. This provides
that there is insufficient gain available outside the TEM.sub.00
spot-size to support other transverse-modes of oscillation of
different frequencies while still deriving maximum gain from the
pumped area.
[0108] Above-described OPS-lasers in accordance with the present
invention have been described in terms of having physically long
resonators, it being understood here, that what is referred to as
resonator-length is the axial-length or distance between resonator
end-mirrors. One skilled in the art, however, will recognize that
increasing the physical resonator length is not the only way to
provide for a large mode-size in the gain-structure 34 of
OPS-structure 32 for increasing power. Incorporating a suitable
optical system within a resonator can produce a large "equivalent
length" for the resonator, even when the actual (physical) length
of the resonator is relatively short. This may be used to provide a
large spot-size at an OPS-structure in accordance with the present
invention with a resonator having a relatively short physical
length. An example of such a resonator arrangement used in a
prior-art, flashlamp-pumped Nd:YAG laser, employing a telescopic
arrangement of lenses in the resonator, is described in a paper by
Hanna et al., Opt. Quantum Electron. 13, 493 (1981).
[0109] Ray optics considerations indicate, however, that in any
such complex resonator, wherein a mirror-structure 30 of an
OPS-structure 32 forms at one extremity of the resonator, the
complex resonator is equivalent (as seen from mirror-structure 30)
to a simple resonator consisting of a mirror (generally curved)
placed at a specific distance from mirror-structure 30 of the
OPS-structure 32. In other words, any arbitrarily complex resonator
can always be reduced, for the purpose of determining mode-size at
gain-structure 34 of OPS-structure 32, to an equivalent simple
resonator having mirror-structure 30 of OPS-structure 32 at one end
thereof, and a curved mirror at the other end thereof. Accordingly,
in long-resonator-type OPS-lasers in accordance with the present
invention there can be defined an equivalent or optical
resonator-length necessary to provide a desired mode-size at
OPS-structure 32. This optical length definition encompasses the
relatively simple resonator structures described above, wherein the
optical length is close to the physical length as well- as any more
complex arrangements for shortening physical length while
maintaining a sufficiently large optical length. A preferred
optical length for the resonator of an OPS-laser in accordance with
the present invention is at least about 5 cm and preferably greater
than about 10 cm.
[0110] The most straightforward way to increase the mode or
spot-size at an OPS-structure 32 in an OPS-laser in accordance with
the present invention is to increase the optical length of the
resonator by simply increasing the physical length of the
resonator, with an appropriate choice of the radius of curvature of
resonator mirrors. The beam spot-size required for the generation
of multi-watt radiation in the above-exemplified laser 20 of FIG. 1
was obtained by employing a resonator length of 25 cm, which, even
though much larger than resonator lengths of prior art OPS-lasers,
is still small enough to allow for convenient packaging of the
laser while still allowing operation in a single axial-mode.
[0111] Turning next to thermal management issues, as discussed
briefly above, thermal-management of OPS-structure 32 in an
OPS-laser in accordance with the present invention is particularly
important. Accordingly, a discussion of some important aspects of
the thermal management is set forth below with reference to FIG. 8.
FIG. 8 depicts, in detail, OPS-structure 32 and heat-sink 36 on
which it is bonded.
[0112] The thermal-management problem may be summarily defined as
follows. Pump-light 42 is absorbed in gain-structure of
OPS-structure 32. That portion of the absorbed pump-light which
does not generate electrical-carriers, i.e., does not provide
laser-radiation, generates heat in gain-structure 34. This heat
must be conducted away by heat-sink 36 at a rate sufficient to keep
the temperature of gain-structure preferably below a temperature of
about 90.degree. C. It should be noted here that this temperature
is merely provided for guidance and should not be considered
critical or limiting.
[0113] Mirror-structure 30 impedes the conduction of heat from
gain-structure 32 to heat-sink 36. As discussed above, in a
preferred, inventive heat-sink arrangement, OPS-structure 32 is
bonded, by a solder layer 110, to a layer 112 of synthetic diamond,
preferably having a thickness of about 0.3 mm. Diamond-layer 112,
in turn is bonded, by another solder layer 114, to a copper-bodied
microchannel-cooler 116.
[0114] In developing a heat-sink configuration for OPS-structure in
accordance with the present invention, consideration was first
given to the heat-sink properties of massive substrates of copper
and diamond. From calculations considering the pump-power density
provided by the above exemplified 34 W of pump-power delivered
directly on the surface of a massive (essentially infinite in
extent) copper heat-sink it was estimated that surface temperature
on such a heat-sink would be 110.degree. C. In practice, the 34 W
of pump power would not be delivered directly to the surface but
would be impeded by the materials of mirror-structure 30. These
materials have a relatively low thermal-conductivity. Accordingly,
the temperature of gain-structure 34 could be expected to be about
15.degree. C. higher than the highest temperature at the surface of
the heat-sink.
[0115] From calculations considering the pump-power density
provided by the above exemplified 34 W of pumping delivered
directly on the surface of a massive CVD-diamond (having a
thermal-conductivity 2.5 times higher than copper) heat-sink, it
was estimated that the surface temperature on such a heat-sink
would be 45.degree. C. This would be a tolerable temperature for
gain-structure 34 of OPS-structure 32. However, a diamond (even
synthetic) sufficiently large to be considered of infinite
extent(would be prohibitively expensive, if at all obtainable.
[0116] Giving consideration to a microchannel-cooler, it was
determined that the surface temperature on such a cooler provided
by the above exemplified 34 W of pumping delivered directly thereto
would be about 110.degree. C. It was then determined that if a
relatively thin layer of synthetic diamond were bonded to the same
microchannel-cooler, the surface temperature (on the diamond layer)
would be only 55.degree. C. degrees, i.e., comparable-with the
calculated temperature for the above-discussed hypothetical massive
diamond heat-sink. This determination was significant in enabling
the pump-power densities required for OPS-lasers in accordance with
the present invention to be safely delivered to OPS-structure
32.
[0117] On a prima facie consideration, it may appear that at a
constant pump-power density, the above-exemplified temperatures in
OPS-structure 32 may be expected to,be remain the same as the
pumped-area thereon is increased, making power-scaling beyond the
above identified levels merely a matter of increasing resonator
length. On closer investigation, however, this proves not to be the
case.
[0118] By way of illustration of the pumped-area scaling problem,
FIGS. 9 and 10 depict computed isothermal contours (assuming radial
symmetry) in respectively a relatively-massive (1.0 mm-thick)
copper heat-sink and in a 0.3 mm-thick CVD-diamond layer bonded on
a 0.7 mm-thick copper-heat-sink. The isothermal contours represent
response to a uniform surface heating at a rate Q of 180 Watts per
square mm (W/mm.sup.2) within a circle H of 0.25 mm radius.
(corresponding to the above-exemplified pump-power density on
OPS-structure 32). Isothermal contours are at 5.degree. C.
intervals. The terminology "relatively massive", as used above,
means in relationship to the dimensions of OPS-structure 32.
[0119] It can be seen from both FIGS. 9 and 10 that the temperature
rises from edge to center of the heated (pumped) area. Increasing
the pumped-area at the same pump-power density, accordingly, would
increase the peak temperature of the OPS-structure. Fortunately,
however, this increase is proportional to the square-root of the
increase in area. This allows some trade-off between pump-power
density and increasing pumped-area.
[0120] The effectiveness of the CVD-diamond layer, of course is
also evidenced by the lower peak temperature of between 45.degree.
C. and 50.degree. C. achieved with the diamond layer (see FIG. 10)
compared with a peak temperature of about 100.degree. C. without
the CVD-diamond layer (see FIG. 9).
[0121] Synthetic diamond in single crystal form has twice the
thermal-conductivity of the CVD form. Accordingly, by substituting
a single crystal diamond layer for the CVD-diamond layer a further
surface temperature reduction of about 20.degree. C. could be
expected. Synthetic diamond in CVD and single crystal form is
available from Sumitomo Electric Industries (USA) Ltd, of New York,
N.Y.
[0122] The high thermal-conductivity of the above discussed
inventive arrangement of heat-sink 36 ensures not only low
temperature in gain-structure 34 of OPS structure 32, but also low
transverse temperature gradients in the plane of the OPS-structure.
Transverse temperature gradients cause, through temperature
dependence of the index of refraction of the OPS material,
transverse variation of the optical path seen by the
fundamental-radiation on propagation in the OPS-structure. Such
gradients manifest themselves in a solid-state gain medium as
"thermal-lensing" which means that the gain-medium behaves, in a
first order approximation, as a positive spherical lens. This
phenomenon is well known in prior-art DPSS lasers, and complicates
the design of resonators for such lasers, as these resonator
designs must take into account both the thermal-lensing and its
variation as the pump-power level is changed. By way of example, a
typical neodymium-doped yttrium vanadate (Nd:YVO.sub.4) gain-medium
crystal can exhibit thermal-lensing as high as seven diopters under
typical pump conditions.
[0123] In considering the design of heat-sink 36 it was recognized
that careful design could lead to exceptionally low values for the
thermal-lensing in an OPS-laser in accordance with the present
invention compared with values encountered in prior-art DPSS
lasers. By way of example, in the above-described example of laser
20 (FIG. 1), employing a CVD diamond layer on a microchannel cooler
as a heatsink, a thermal lensing of significantly less than one
diopter was measured. The use of synthetic, single-crystal diamond
instead of CVD diamond would reduce this already small thermal
lensing by a factor of two. Such low thermal-lensing simplifies
considerably the design of the laser resonator. Generally, in
designing a resonator for an OPS-laser in accordance with the
present invention, it may be assumed that there is will be no
thermal lensing.
[0124] Turning now to the heat-transfer-impeding effect of
mirror-structure 30 of OPS-structure 32, in the example of the
inventive OPS-laser discussed above, the total thickness of
mirror-structure 30 of OPS-structure 32 is about 4.1 .mu.m.
Computations, based on this total thickness; bulk
thermal-conductivity values of GaAs and AlAsP; and the structure of
cooled-substrate 36 as depicted in FIG. 6, indicated that because
of the heat-transfer-impeding effect of mirror-structure 30, the
temperature of gain-structure 34 would be between about 18.degree.
C. and 20.degree. C. higher than would be the case, were the
gain-structure directly-bonded to diamond layer 112 of
cooled-substrate 36. Accordingly, a substantial portion of the
potential reduction in surface temperature offered by the bonded
diamond layer is sacrificed to the mirror-structure. From
experimental observations of the shift of the fluorescence spectrum
of gain-structure 34 under the operating conditions of the above
described practical example of inventive OPS-laser, it has been
determined that gain-structure 34 is at a temperature of about
90.degree. C.
[0125] It is clear, that an improvement in the thermal-conductivity
of mirror-structure 30 of OPS-structure 32 could allow a higher
pump-power to be delivered to gain-structure 32. This would allow
further increases in output-power. Some approaches to improving
this thermal-conductivity are set forth below.
[0126] An advantage of growing OPS-structure 32 in the manner
described above is that it is neither necessary that
mirror-structure 32 be epitaxially-grown, nor necessary that it be
formed entirely, if at all, from semiconductor materials. Given
this flexibility, it is possible to design mirror-structures 30 of
OPS-structures 32 in accordance with the present invention using a
wide selection of materials for forming high and low refractive
index layers. Accordingly, values of absolute refractive index of
the materials, and thermal-conductivity of the materials, can be
selected such that such that the thermal-conductivity in the axial
direction of the mirror-structure (as a whole) is maximized.
Clearly, of course, the materials selected must have minimal
absorption (be highly transparent) for the fundamental-wavelength
such that mirror-structure is capable of providing maximum
reflectivity for the fundamental-wavelength. Maximum reflectivity
in this regard may be defined as a value preferably about 99.9
percent or greater.
[0127] One option for increasing thermal-conductivity of
mirror-structure 30 is to substitute a layer of a highly reflective
metal such as aluminum, silver, gold, copper or magnesium, for
several high and low refractive index layer-pairs of transparent
materials, and to allow an absorption in the metal layer of about
0.1 percent. Dielectric multilayer overcoated aluminum mirrors of
similar structure, for providing inexpensive high-efficiency
reflectors for optical scanners, copiers and the like, are often
referred to in the optical-coating art as "enhanced
metal-mirrors".
[0128] A metal layer in a mirror-structure 30 in accordance with
the present invention would be the last-deposited (mirror) layer
and would be furthest from the gain-structure. As this metal layer,
in a completed OPS-structure 32, would be in direct contact with
the heat-sink, it is believed that at least 0.1 percent absorption
would be tolerable.
[0129] By way of example, using the GaAs and AlAsP materials
exemplified above, with a 1000 .ANG. layer of gold as the final
layer, 99.9% reflectivity could be provided by a structure having
only nine pairs of GaAs and AlAsP layers . Such a structure would
have a total thickness of only about 1.47 .mu.m, i.e., about
one-third the thickness of the mirror-structure of the
above-described example. Accordingly up to a three-fold increase in
thermal-conductivity of the mirror-structure as a whole can be
anticipated.
[0130] It is believed that further reductions in thickness could be
possibly be obtained by using a material having a much lower
refractive index than AlAsP as a low refractive index material, for
example, barium fluoride (BaF.sub.2) having a refractive index of
about 1.48. Using a 1000 .ANG.-thick layer of gold as a final
layer, 99.9% reflectivity could be provided by a mirror-structure
30 having as few as two pairs of GaAs and BaF.sub.2 layers. Such a
mirror-structure would have a total thickness of only about 0.54
.mu.m, i.e., about one-eighth the thickness of the OPS-structure of
the above described example. What is gained in thermal-conductivity
by the total thickness reduction, however, will be at least
partially offset by the lower conductivity of BaF.sub.2 compared
with AlAsP and the greater physical thickness of BaF.sub.2 required
to provide one-quarter wavelength optical thickness at the
fundamental-wavelength.
[0131] Such "enhanced metal-mirrors" may be particularly useful in
OPS-structures which have fundamental emission at wavelengths
shorter than about 0.7 Am where semiconductor materials have too
great an absorption to be used effectively in multilayer
reflectors. For these wavelengths, use must be made of dielectric
materials such as titanium oxide (TiO.sub.2) and niobium, oxide
(Nb.sub.2O.sub.5) to provide high index layers, in combination with
materials such as silicon dioxide (SiO.sub.2) as a low index layer.
It is useful to note that a mirror using only a combination of
TiO.sub.2 and SiO.sub.2 layers, having the same reflectivity as a
mirror made from only GaAs and AlAsP layers, would have a total
thickness about fifty percent greater.
[0132] It should be noted that, depending on the selection of a
highly reflecting metal for an "enhanced-metal" type
mirror-structure, and the type of bonding material selected for
bonding the OPS-structure to heat-sink 36, it may be necessary to
add a layer, preferably of a suitable metal, on the reflective
metal layer to prevent that layer from reacting, alloying or
dissolving in the bonding material. As this additional layer, would
be (as far as incident laser radiation is concerned) "behind" the
reflective metal layer it is not important that it be highly
reflecting.
[0133] As discussed above, OPS-structures such as OPS-structure 32
are grown over essentially the entire area of a semiconductor
wafer. After the OPS-structure is grown, the wafer is diced into
chips of OPS-structure. A prior-art-preferred dicing method
involves scribing and breaking the wafer to form the chips.
Breakage takes place, cleanly and smoothly, along a crystalline
plane of the wafer and the epitaxially grown OPS-structure thereon.
Referring now to FIG. 11, an OPS-structure chip (designated here,
for consistency by reference numeral 32) formed as described above,
has what may be termed as an emission face 119 on gain-structure 34
and has smooth end-faces 120 in resulting from the scribe-and-break
dicing method. End faces 120 are at right angle to emission-face
119 and have a relatively-high Fresnel reflectivity (about 30%) due
to the relatively-high refractive index of semiconductor materials
of the gain-structure. At operating powers contemplated for
OPS-lasers in accordance with the present invention, absent any
preventive arrangements, laser action can take place laterally
(indicated in FIG. 11 by arrows B) in resonators formed by parallel
pairs of end-faces 120.
[0134] During operation, such parasitic lateral oscillation would
rob gain-structure 34 of OPS-structure 32 of gain which would
otherwise be available for providing laser-action in the intended,
surface-emitting direction (indicated in FIG. 11 by arrows C).
Certain measures may be taken to suppress such parasitic lateral
oscillation. One such measure comprises employing a dicing method
wherein the wafer is cut or scribed completely from one side to the
other with a diamond-scribe, diamond-saw, or like abrasive-cutting
device. Following such scribing, end-faces 120 will be sufficiently
rough or irregular that they are ineffective as mirrors.
Accordingly, parasitic lateral oscillation will effectively
suppressed. Other methods of roughening the end faces are not
precluded.
[0135] If end-faces 120 are left smooth as a result of
scribe-and-break dicing, coating the end-faces with a simple
antireflection coating of material having a lower refractive index
than the end-faces, such as aluminum oxide (Al.sub.2O.sub.3) or
yttrium oxide (Y.sub.2O.sub.3), can reduce the reflectivity of
end-faces 120 to a level where they no longer are effective as
mirrors, thereby effectively suppressing parasitic lateral
oscillation.
[0136] Yet another means of suppressing parasitic lateral
oscillation is to ensure that the longest linear dimension
(end-face length) of chip 32 (OPS-structure 32) is significantly
larger than the pumped-area 122. At power levels contemplated for
OPS-lasers in accordance with the present invention, the length of
an end-face 120 is preferably greater than about three-times the
diameter of the pumped area. Intrinsic absorption of the unpumped
OPS-structure materials and the increased distance required for
radiation to travel between end-faces 120 can provide sufficient
loss to effectively suppress parasitic lateral oscillation even if
end-face 120 are smooth.
[0137] Deliberately increasing absorption of the materials of
gain-structure 34 outside the pumped area for suppressing parasitic
lateral oscillation is not without the bounds of possibility. This
may be accomplished, for example, after growth of OPS-structure 32,
by selective doping or ion implantation. Clearly, any of the
above-described measures for reducing parasitic lateral oscillation
may be use alone or, where appropriate, in combination.
[0138] Turning now to selection of frequency-conversion materials
for OPS-lasers in accordance with the present invention, as noted
above, the range of wavelengths available from external cavity
OPS-lasers may be extended by frequency multiplication, doubling or
mixing. This is preferably accomplished by intracavity (IC)
frequency-conversion (harmonic generation), in which an
optically-nonlinear crystal is included in a resonator formed
between the mirror-structure of an OPS-structure and a (external to
the structure) conventional mirror. A particularly preferred
optically-nonlinear crystal material for IC frequency converted
OPS-lasers in accordance with the present invention is LBO.
[0139] Because of the limited power available in prior-art
OPS-lasers, it has been the practice in such lasers to use an
optically-nonlinear material with as high a frequency
conversion-efficiency as possible, typically, KNbO.sub.3.
KNbO.sub.3, however, has certain disadvantageous properties that
make it a less-than-ideal choice for use in a high-power
frequency-doubled laser in accordance with the present invention.
These disadvantageous features are a certain fragility, and a
limited spectral-acceptance range. The spectral-acceptance range of
an optically-nonlinear material is that range of wavelengths which
would be frequency-converted for a crystal of the material.
[0140] FIG. 12 depicts the computed, normalized spectral-acceptance
of KNbO.sub.3, and of the significantly-more-durable, but less
efficient, LBO. Each is assumed to be in the form of a 5 mm-long
crystal. Also depicted is the gain-bandwidth of active layers of
InGaAs having a composition selected to provide laser action around
976 nm. It can be seen that the spectral-acceptance of the
KNbO.sub.3 is about one-tenth that of the LBO, and is very narrow
by comparison with the gain-bandwidth of the active layers.
[0141] Now, as the frequency-doubling mechanism of an intracavity
optically-nonlinear crystal represents a loss for the
fundamental-wavelength, absent any other constraint, the resonator
will tend to shift (hop) its oscillation wavelength to a wavelength
where there is no loss. When this happens, frequency-doubled output
will drop catastrophically, if not cease altogether, and
fundamental power will surge correspondingly.
[0142] Wavelength-hopping will result in a very noisy
frequency-converted output of lower average power than would be the
case in the absence of wavelength-hopping. This wavelength-hopping
can be limited by including in the resonator a highly
wavelength-selective element such as a birefringent filter, an
etalon, or the like. Any such device having the necessary
selectivity, however, would introduce significant resonator losses,
thereby limiting output-power. Either way, this would mean that the
nominally high conversion-efficiency of the KNbO.sub.3 may not be
realized, in practice, in its full measure. Further, the
wavelength-selective device would probably require an active
measure such as temperature control to satisfactorily stabilize its
peak-transmission wavelength.
[0143] The LBO material, having the much wider acceptance would
still not be completely immune from wavelength-hopping, without the
wavelength-restraining benefit of an intracavity
wavelength-selective device. Such a wavelength-selective device in
the case of LBO, however, can be ten-times less selective than
would be required for the KNbO.sub.3; would not require active
stabilization; and would not introduce significant cavity
losses.
[0144] For completeness, it is pointed out here that the
spectral-acceptance of an optically-nonlinear crystal is inversely
proportional to the length of the crystal. The
second-harmonic-generating (SHG) efficiency of such a crystal is
roughly proportional to the square of a material constant
d.sub.eff, multiplied by the square of the length of the crystal.
As the value of d.sub.eff for KNbO.sub.3 is about ten-times greater
than the value of d.sub.eff for LBO, then, in theory at least, a
0.5 mm-long KNbO.sub.3 crystal would have the same peak
SHG-efficiency and spectral-acceptance as a 5.0 mm-long LBO
crystal. It is believed, however, that because of the fragility of
the KNbO3, a 0.5 mm long KNbO3 crystal may be somewhat impractical
to fabricate and handle. The use of KNbO3 crystals in resonators in
OPS-laser resonators in accordance with the present invention,
however, is not precluded.
[0145] Generally, for reasons discussed, above LBO and chromium
lithium triborate (CLBO) are preferred for frequency-doubling and,
alternatively, for frequency tripling. BBO is preferred for
frequency quadrupling and, alternatively, for frequency tripling.
For doubling or tripling wavelengths shorter than about 670 nm,
optically nonlinear materials including strontium barium borate
(SBBO), strontium borate (SBO), and barium zinc borate (BZBO) are
believed to be effective. These optically-nonlinear material
preferences should not, however, be considered as limiting IC
frequency-converted lasers in accordance with the present
invention.
1TABLE 1 Fundamental Bragg Wavelength Mirror QW/Separator Substrate
(nm) 1 In.sub.xGa.sub.1-xAs/GaAs.sub.yP.sub.1-y GaAs 900-1050 2
In.sub.xGa.sub.1-xP/In.sub.yGa.sub.1-yAs.sub.zP.sub.1-z GaAs
700-900 3 InAs.sub.xP.sub.1-x/InGa.sub.yP.sub.1-y InP 930-1800
[0146]
2 TABLE 2 Bragg Mirror Materials Structure High Refractive-Index
Low Refractive-Index Number Layers Layers 1 GaAS
AlAs.sub.xP.sub.1-x 2 In.sub.xAl.sub.1-xP Al.sub.yGa.sub.1-yAs 3
Dielectric Dielectric
[0147] Continuing now with a description of preferred materials for
high-power OPS-structures in accordance with the present invention,
TABLE 1 lists preferred QW layer and separator-layer materials for
gain-structure 30. Each set of materials is associated with a
particular mirror-structure 30 (Bragg mirror) number, substrate and
fundamental-wavelength range. Details of Bragg mirror, composition
are given in TABLE 2.
[0148] The structures represented in TABLES 1 and 2 represent a
departure from prior-art-preferred structures for the same
wavelength ranges, in regard to both mirror-structures and
gain-structures. In TABLES 1 and 2 compound composition proportions
(subscripts) x, y, and z all have a value between 0.0 and 1.0. In
each case, materials (compounds) are selected such that the stress
of the OPS-structure as a whole is as low as practically possible,
by ensuring that the stress-thickness product of all tensile
strained layers and all compressive strained layers is about of
equal magnitude. This is of particular importance in OPS-structures
in accordance with the present invention which are subject to the
stress of high operating temperatures, and correspondingly large
temperature-cycles between periods of operation and inactivity.
[0149] The OPS-structure (see FIG. 2) of the above-exemplified IC
frequency-doubled OPS-laser in accordance with the present
invention is an example of the first-listed family of structures
wherein GaAs "transition" layers between the QW and separator
layers, alleviate the stress-difference which would otherwise exist
between these layers. The gain-structure of the OPS-structure of
FIG. 2 can be generally categorized as being formed from components
of the III-V quaternary system InGaAsP wherein the substrate
material is a binary III-V compound of the components (here GaAs),
and wherein the QW layers are formed from a one possible ternary
III-V compound including both components of the substrate material,
and the separator layers are formed from the other possible ternary
III-V compound including both components of the substrate material.
Type 3 structures of TABLE 1 (on InP substrate material) also fall
into this category. This is believed to provide the optimum stress
compensation for OPS structures formed from this quaternary system.
For type 3 structures of Table 1 a mirror-structure including
dielectric materials is preferable. One preferred structure
includes high refractive index layers of zinc selenide (ZnSe) and
low refractive index layers of aluminum oxide. This combination
provides about the same reflectivity per number of layer-pairs as
the combination of TiO.sub.2 and SiO.sub.2 used in general coating
applications in the prior-art, but with a lower total physical
thickness and higher thermal conductivity. It is emphasized here
that the mirror structures of TABLE 2 are preferred structures for
use with QW/separator structures of TABLE 1, the use of other
mirror structures with the QW/separator structures of TABLE 1 in
OPS-lasers in accordance with the present invention is not
precluded.
[0150] Further, from the description of inventive OPS-lasers
provided above, those skilled in the art may select other
OPS-structures for providing fundamental-radiation at wavelengths
between about 425 nm and 1800 nm without departing from the spirit
and scope of the present invention. Such structures may include, by
way of example, QW or active layers selected from a group of
semiconductor compounds consisting of,
In.sub.xGa.sub.1-xAs.sub.yP.sub.1-y,
Al.sub.xGa.sub.1-xAs.sub.yP.sub.1-y, and In.sub.xGa.sub.1-xN where
0.0.ltoreq.x.ltoreq.1.0 and 0.ltoreq.y.ltoreq.1. This list,
however, should not be considered as limiting the present
invention. Second, third and fourth harmonic wavelengths, of
course, would be between about 212 and 900 nm; 142 and 600 nm; and
106 and 225 nm. Performance in the shorter wavelength half of the
latter range may be somewhat limited by the availability of
suitable optical materials.
[0151] Turning next to a discussion of further aspects of
increasing output-power in OPS-lasers in the present invention, in
embodiments of OPS-lasers described above, only one OPS-structure
32 is included. Several approaches to increasing output-power to
beyond the levels achieved or confidently predicted have been
discussed without regard to this. It will be evident to one skilled
in the art from these discussions, however, that exploiting the
above-discussed approaches alone may lead to a point of diminishing
returns. Should this prove to be the case, or simply as an
alternative, output-power (fundamental or harmonic) may be
increased by incorporating at least a second OPS-structure in a
suitably-configured resonator. A description of one such resonator
is set forth below.
[0152] Referring to FIG. 13, a further embodiment 128 of an IC
frequency-doubled OPS-laser in accordance with the present
invention is depicted. Laser 128 comprises a tightly-folded
resonator 130 including two OPS-structures, 32 and 32X. Resonator
130 is terminated at one end thereof by mirror-structure 30 of
OPS-structure 32 and at the other end thereof by a mirror 28.
Resonator 130 includes a birefringent filter 52 for reasons
discussed above. The OPS-structures are mounted on above-described
heat-sink assemblies 36. Resonator axis 132 is folded once by
mirror-structure 30X of OPS-structure 32X, and again by a
fold-mirror 26. An optically-nonlinear crystal 50, arranged for
frequency-doubling, is located in arm 130A of resonator 130, i.e.,
between fold-mirror 26 and end mirror 28. Mirror 28 is coated for
maximum reflectivity at the fundamental-wavelength. Mirror 26 is
coated for maximum reflectivity at the PATENT
fundamental-wavelength and maximum transmission at the 2
H-wavelength.
[0153] It should be noted here that the tight folding of the
resonator is selected to maintain as close to normal incidence as
possible on OPS-structure 32X. This is to minimize interference in
gain structure 34X between counterpropagating fundamental beams.
Such interference can lead to the formation of interference-fringes
(a sort of "lateral spatial hole-burning") in the plane of the QW
layers, thereby limiting gain extraction from these layers. In this
regard, it is also preferable to adjust spacing of the QW layers in
gain-structure 34X and the thickness of layers of mirror-structure
30X to compensate for the effective refractive index change due to
non-normal incidence, thereby maintaining effective half-wavelength
spacing of the QW layers.
[0154] Continuing with reference to FIG. 13, a further difference
between laser 128 and other above described OPS-lasers in
accordance with the present invention is the manner in which
pump-light is delivered to the OPS-structures. In laser 128,
pump-light is transported along fibers 40 as in other
above-described embodiments. Fold-mirror 44 and focusing-lenses 46
and 48 of these other embodiments, however, are replaced with an
assembly of two radial-gradient-index lenses 146 and 148. Such
lenses are often referred to generically as "selfoc". It has been
found such lenses can provide the same optical performance as
lenses 46 and 48 in the above-described example of laser 20. In one
example lens 146 is a Part No. 06LGS216 available from Melles Griot
Inc of Irvine Calif. Lens 148 is a Part No. SLW180-020-083-A2. This
selfoc lens arrangement provides convenient access for multi-fiber
(fiber-bundle) pumping in the case of tightly-folded resonators
such as resonator 28. The arrangement may be used in any other
above-described embodiment simply to reduce the volume of
pump-light delivery optics or to permit power to be delivered by
more fibers than would be possible because of the bulk of the
conventional focusing-optics.
[0155] From the foregoing description, those skilled in the art to
which the present invention pertains may devise OPS-lasers
including more than two OPS-structures without departing from the
spirit and scope of the present invention. Such OPS-lasers may be
configured for delivering output-radiation at the fundamental
wavelength of the OPS-structures or at harmonic wavelengths
thereof.
[0156] From the description of laser 128 it will also be evident to
those skilled in the art that a folded laser-resonator may be
formed between two external mirrors, and include one or more
OPS-structures 32 only as fold mirrors. Such a folded
laser-resonator is depicted in FIG. 14. Here, an OPS-laser 140 in
accordance with the present invention includes a laser-resonator
142 terminated by mirrors 28F and 144 and tightly-folded (for
reasons discussed above) by mirror structure 30 of OPS-structure
32. Heat-sink and optical-pumping arrangements are the same as
described above for laser 128. Laser resonator optionally includes
a birefringent filter 52.
[0157] The laser-resonator arrangement of laser 140 may be useful
if it desired to configure a laser-resonator which is terminated by
two non-flat mirrors. It will be evident from the arrangement of
laser 140 that similar arrangements wherein a resonator is not
terminated at one end thereof by a mirror-structure of an
OPS-structure may be devised for any other above described
embodiments of OPS-lasers in accordance with the present
invention.
[0158] In summary, several embodiments of high-power, IC
frequency-converted, OPS-lasers in the accordance with the present
invention have been described above. Generally, embodiments of the
inventive laser can generate second, third or fourth harmonic
output-power from fundamental radiation at a power of 100 mW or
greater, and even at 1 W or greater. These high-harmonic output
powers can be achieved in single axial mode or TEM.sub.00 operation
with a beam-quality of less than about twice the diffraction-limit.
Particularly notable is the ability of the inventive lasers to
generate UV output-radiation at these high output-powers, more
particularly, at better than one percent pump-to-harmonic
efficiency by second-harmonic generation from fundamental radiation
in a 700 nm to 800 nm wavelength ranges.
[0159] The inventive OPS-lasers, can provide a means of generating
wavelengths, in a true CW mode of operation, which can closely
match the optimum wavelength for many laser applications, in fields
such as medicine, optical metrology, spectroscopy, optical
lithography, and precision laser machining. By way of example, an
inventive OPS laser can efficiently produce CW output radiation in
the 590 to 600 nm wavelength range by second-harmonic generation
from an OPS-structure emitting in the 1180 to 1200 nm range. 590 to
600 nm is advantageous for ophthalmic surgical application. This
wavelength range can not be efficiently generated by prior-art
lasers. The ability to generate high-power UV radiation is
applicable in particular in areas such as direct writing or
patterning of printed circuit boards or conventional UV plates. In
such applications the inventive OPS-lasers offer not only power,
efficiency and beam quality, but also the ability for many
applications to "tailor" an output wavelength to an absorption-peak
of material to be exposed, cut, ablated, heated, photochemically
altered or otherwise treated, thereby reducing the absolute power
required for the applications. From the above presented description
of the OPS lasers those skilled in the appropriate arts may
conceive many other uses for the inventive lasers without departing
from the spirit and scope of the present invention.
[0160] The present invention is described and depicted herein with
reference to a preferred and other embodiments. The present
invention is not restricted, however, to those embodiments
described and depicted. Rather, the invention is limited only by
the claims appended hereto.
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