A high-power semiconductor diode laser or array of semiconductor diode lasers (14) optically end-pumps a compact, tunable, solid-state laser (28) with a pumping beam (74) well-matched to the absorption bandwidth and mode volume (78) of the solid-state laser (28). Tilted birefringent plates (210) positioned within the solid-state resonator cavity (16) are employed to control the spectral bandwidth and wavelength output of the waveguide pumping beam (204). Infrared output (100) generated by such a solid-state laser (28) is coupled into a nonlinear waveguide (200) and converted to visible output (206) through the process of second-harmonic generation.

TECHNICAL FIELD 
The present invention relates to solid-state lasers optically pumped by 
diode lasers and, in particular, to a method and an apparatus for 
generating laser output within a 720-920 nm range and second harmonic 
generation laser output within a 360-460 nm range for use in medical and 
electronic processing applications. 
BACKGROUND OF THE INVENTION 
A variety of methods have been employed for optically pumping solid-state 
lasers, such as those containing neodymium-doped lithium yttrium fluoride 
(Nd:YLF) or neodymium-doped yttrium aluminum garnet (Nd:YAG) lasants. A 
common method is to use an arc lamp or other similar light source to 
excite a laser rod. The light source and laser rod are positioned within 
and at different foci of a highly reflective housing of elliptical 
cross-section. This method typically requires relatively large diameter 
laser rods to efficiently absorb enough of the pumping light emitted by 
the light source to allow solid-state laser operation. Another limitation 
of this pumping method is the relative inefficiency caused by poor overlap 
of the optical emission spectrum of the pumping light source with the 
absorption bandwidth of the solid-state lasants. For some industrial 
operations such as processing electronic materials, compact diode-pumped 
solid-state lasers offer numerous advantages. For example, large gas 
lasers or arc-pumped Q-switched YAG lasers that typically require water 
cooling systems are largely incompatible 17 with clean room conditions 
often necessary for link processing of dynamic random access memory 
devices. 
There are several different methods for diode-pumping solid-state lasers. 
In U.S. Pat. No. 3,982,201, Rosenkrantz et al. describe a solid-state 
laser that is pumped by single diode lasers or arrays of diode lasers to 
which the solid-state laser rod is end-coupled. Because the output 
wavelength of the diode laser array is a function of its temperature, the 
diode lasers are operated in a pulsed mode at a low duty cycle to maintain 
the array at a sufficiently stable temperature so that its output 
wavelength remains matched to the absorption bandwidth of the solid-state 
laser rod. The output power characteristics of this laser system are 
limited by the relatively inefficient match between the output of the 
diode lasers and the mode volume of the solid-state laser rod. 
In "Efficient LiNdP.sub.4 0.sub.12 Lasers Pumped with a Laser Diode," 
Applied Optics, vol. 18, No. 23 (Dec. 1, 1979), Kubodera and Otsuka 
describe the well-known practice of collecting the output light of a diode 
laser and focusing its expanded output light using conventional lenses, 
such as two microscope condenser lenses. This method is particularly well 
suited for applications where the emitter width and beam divergence of the 
diode laser are small. However, as the emitter dimensions and beam 
divergence increase, it becomes increasingly difficult to efficiently 
collect the output beam with collimating lens or lenses. It also becomes 
more difficult to focus the expanded beam into the solid-state lasant 
crystal with sufficient depth of focus to allow efficient overlap of the 
pump beam throughout the resonator mode volume within the lasant. 
In U.S. Pat. No. 4,710,940, Sipes, Jr. describes a Nd:YAG solid-state laser 
that is end-pumped by a diode laser array or by two diode laser arrays 
that have been combined by use of polarizing beam-splitting cubes. Sipes, 
Jr., cites the analysis of D. G. Hall in "Optimum Mode Size Criteria for 
Low Gain Lasers," Applied Optics, 579-1583, vol. 20, (May 1, 1981), to 
suggest that the "pump profile shape does not matter much as long as all 
the pump light falls within the resonator mode." Sipes, Jr., notes, 
however, that Hall's analysis does not account for the divergence 
properties of Gaussian beams, so Sipes, Jr., suggests that, if required, 
the cross-section of the pump beam could be modified by use of a 
cylindrical lens. 
In U.S. Pat. No. 4,761,786, Baer describes a Q-switched, solid-state laser 
that is end-pumped by a diode laser or diode laser array. The output light 
from the pump source is collected by a collimating lens and directed by a 
focusing lens to end-pump the laser rod. Baer notes that "other lenses to 
correct astigmatism may be placed between the collimating lens and 
focusing lens." Baer also describes an alternate embodiment that employs a 
remotely positioned diode laser pumping source coupled through an optical 
fiber, the output of which is focused by a lens into the laser rod. 
In U.S. Pat. No. 4,763,975, Scifres et al. describe two optical systems 
that produce bright light output for a variety of applications, including 
pumping a solid-state laser such as a Nd:YAG. Scifres et al. describe an 
optical system that employs a plurality of diode lasers, each of which is 
coupled into one of a plurality of fiber-optic waveguides. The waveguides 
are arranged to form a bundle and the light from the diode laser sources 
is emitted at the output end of the bundle. Optics, such as a lens, may be 
used to focus the light into a solid-state laser medium. Alternatively, 
the fiber bundle may be "butt"-coupled to the laser rod. Butt-coupled 
means end-coupled at a position very close to or in contact with the laser 
rod. 
Scifres et al. describe another optical system that employs a diode laser 
bar, broad-area laser, or other elongated source to pump a solid-state 
laser. The diode laser bar light output is coupled into a fiber-optic 
waveguide having an input end that has been squashed to be elongated and 
thereby have core dimensions and lateral and transverse numerical 
apertures that correspond respectively to those of emission dimension and 
lateral and transverse divergence angles of the diode laser bar. The 
output light from the fiber-optic waveguide is either focused by a lens 
into the end of the solid-state laser rod or butt-coupled to the rod. 
Scifres et al. state that either end of the fiber-optic waveguide can be 
curved. Although these methods attempt to match the output light from the 
fiber-optic waveguide to the resonant cavity mode of the solid-state 
laser, they are limited in efficiency by the numerical aperture of the 
sources that can be effectively collected and guided by the fiber-optic 
waveguides. 
Certain methods are known for coupling the output of high-power diode 
lasers into solid-state lasants. High-power diode lasers are necessarily 
broad-area devices or arrays of narrow-width diode lasers because the 
potential for catastrophic optical damage to the mirrors dictates that the 
optical outputs be limited typically to 10 to 20 mW per micron of emission 
stripe width. Typical high-power diode lasers used to pump solid-state 
lasants include aluminum gallium arsenide (AlGaAs) diode lasers. Examples 
of such diode lasers include Model No. SDL-2480-P1 with continuous wave 
(CW) output power of 3.0 watts (W) and an emission width of 500 .mu.m; 
Model No. SDL-2462-P1 with CW output power of 1.0 W and an emission width 
of 200 .mu.m; and Model No. SDL-2432-P1 with CW output power of 0.5 W and 
an emission width of 100 .mu.m, all of which are manufactured by Spectra 
Diode Labs, 80 Rose Orchard Way, San Jose, Calif. Use of AlGaAs 
semiconductor diode lasers to optically pump solid-state lasers has led to 
development of compact, solid-state lasers. 
Broad-area lasers are described by Thompson in "A Theory for Filamentation 
in Semiconductor Lasers", Optoelectronics, 257-310, vol. 4, (1972) and by 
Kirkby, et al. in "Observations of Self-Focusing in Stripe Geometry 
Semiconductor Lasers and Development of a Comprehensive Model of Their 
Operation," IEEE Journal of Quantum Electronics, 705-719, vol. QE-13 
(1977). Such broad-area lasers (emission width of typically greater than 5 
.mu.m) exhibit a filamentary structure in their optical nearfield 
patterns. The filament structures arise from a nonlinear interaction 
between the carriers and the optical field in the active area of the 
laser. The process of stimulated emission effectively reduces the gain 
profile within the active area and results in an increase in the 
refractive index in the portion of the active area contributing most 
strongly to the optical mode. This region of increased refractive index is 
bounded by regions of the active area that do not contribute so strongly 
to the optical mode and are characterized by smaller refractive index 
values. This lateral variation in refractive index in a local region 
within the active area of the diode laser can form a local lateral index 
guide. 
When the active area is broader than about 5-10 .mu.m, as is the case in 
typical high-power laser diodes used for solid-state laser pumping, 
several, or in some cases, many such index-guided regions may form. 
Stimulated emission within each such lateral index-guided region within 
the active area may occur in the form of a filament that is only partly 
spatially coherent or is spatially incoherent with respect to neighboring 
filaments. This filamentation phenomena is, therefore, a fundamental 
source of lateral spatial incoherence in high-power laser diodes and, 
consequently, places limits on the optical brightness obtainable from such 
devices. 
Although these methods have with varying degrees of efficiency been used to 
optically pump solid-state laser mode volumes and been used to produce 
useful solid-state laser output at a variety of emission wavelengths, 
improved methods for coupling the optical output from diode lasers into 
solid-state lasants are highly desirable. 
A method for theoretically obtaining high-power, nearly diffraction-limited 
optical output from a high-power diode laser has recently been described 
by Tilton, . . . DeFreez, et al., in "High Power, Nearly 
Diffraction-Limited Output from a Semiconductor Laser with an Unstable 
Resonator," IEEE Journal of Quantum Electronics, 2098-2108, vol. 27, No. 
9, (September 1991). The high-power AlGaAs diode laser described therein 
demonstrates high power (greater than 1 watt from both facets) and nearly 
diffraction-limited optical output. The reference states that "[f] or many 
semiconductor laser applications such as solid-state laser end pumping. . 
. , single-lobed, diffraction limited beams of hundreds of milliwatts are 
required." Coupling the optical output from such an unstable resonator 
into a solid-state laser has not heretofore been attempted, but is 
described in a concurrently filed U.S. patent application Ser. No. 
07/873,411 of Baird and DeFreez for Method and Apparatus for Efficient 
Operation of a Solid-State Laser Optically Pumped by an Unstable 
Semiconductor Laser, assigned to assignee of the present invention. 
Many important laser applications require laser operation at visible or 
ultraviolet wavelengths. Moreover, a compact source of coherent visible or 
ultraviolet light output suitable for use in hospital operating rooms and 
similar medical environments is also highly desirable for use in a wide 
range of medical treatments, such as photo-activation of therapeutic 
drugs. 
A variety of methods have been described for generating laser output in the 
400 nm to 600 nm wavelength range from solid-state lasers and diode lasers 
by utilizing the nonlinear process of second-harmonic generation (SHG). 
For example, several methods have been described for producing SHG laser 
output in the 520-540 nm wavelength range from diode-pumped, solid-state 
lasers containing a neodymium-doped lasant. Baer, et al. in U.S. Pat. No. 
4,653,056 describe one such method in which an AlGaAs diode laser 
end-pumps a solid-state laser resonator containing a Nd:YAG rod and 
potassium titanium phosphate (KTP) nonlinear crystal to produce SHG laser 
output at 532 nm. As described in "Second Harmonic and Sum-Frequency 
Generation to 4950 and 4589 A.degree. in KTP," IEEE Journal of Quantum 
Electronics, vol. QE-24, No. 1 (January 1988), such bulk KTP crystals are 
phase-matchable for type-2 second-harmonic generation down to 495 nm. For 
SHG wavelengths shorter than 495 nm, other nonlinear materials are 
required. 
Kozlovsky, et al. in "Efficient Second Harmonic Generation of a 
Diode-Laser-Pumped CW Nd:YAG Laser Using Monolithic MgO:LiNbO.sub.3 
External Resonant Cavities," IEEE Journal of Quantum Electronics, vol. 24, 
No. 6 (June 1988), describe producing about 30 mW of SHG output at 532 nm 
by using a diode-pumped Nd:YAG, single-mode ring laser operating at 1064 
nm to pump an external monolithic cavity of nonlinear magnesium 
oxide:lithium niobate (MgO:LiNbO.sub.3). 
Another method of producing SHG laser output at 532 nm is described by 
Schutz, et al. in "Miniature Self-Frequency-Doubling CW Nd:YAB Laser 
Pumped by a Diode-Laser," in Optics Communications, vol. 77, No. 2, 3 (15 
Jun. 1990). Schutz, et al. describe producing a SHG output of about 10 mW 
at 532 nm by end-pumping a laser resonator containing the 
self-frequency-doubling lasant neodymium:yttrium aluminum boron (Nd:YAB) 
with 870 mW emitted by a AlGaAs diode laser array operating at an output 
wavelength of 807 nm. 
Risk and Lenth in "Room-Temperature, Continuous-Wave, 946-nm Nd:YAG Laser 
Pumped by Laser-Diode Arrays and Intracavity Frequency Doubling to 473 
nm," Optics Letters, Vol. 12, No. 12 (December 1987), describe a method to 
pump a 1 mm length rod of Nd:YAG with two 0.25 W diode laser arrays whose 
output are combined using a polarizing beamsplitter cube arrangement. The 
method employs a 5 mm long crystal of lithium iodate (LiIO.sub.3) cut for 
Type I phased-matched frequency doubling of 946 nm output at room 
temperature in the solid-state laser resonator cavity and produces 
approximately 100 .mu.W of SHG blue light at 473 nm. Risk, Pon, and Lenth 
in "Diode Laser Pumped Blue-Light Source at 743 nm Using Intracavity 
Frequency Doubling of a 946 nm Nd:YAG Laser," Applied Physics Letters, 
vol. 54, No. 17 (24 Apr. 1989), describe further work on a similar method 
employing a single 0.5 W laser diode to end-pump a solid-state laser 
resonator containing a 1 mm long Nd:YAG rod and a 3.7 mm long KNbO.sub.3 
nonlinear crystal to produce. 3.1 mW of blue output at 473 nm. 
Methods have also been described in which the laser output from diode 
lasers are directly frequency doubled. Kozlovsky, et al. describes such a 
method in "Generation of 41 mW of Blue Radiation by Frequency Doubling of 
a GaAlAs Diode Laser," Applied Physics Letters, vol. 56, No. 23, (4 Jun. 
1990). They employ a monolithic ring resonator of KNbO.sub.3 to convert 
105 mW of incident diode laser power at 856 nm to 41 mW of blue 428 nm 
output power. This method, however, requires use of a diode laser 
operating in a single longitudinal mode. High-power, gain-guided diode 
lasers typically do not operate in a single-longitudinal mode and single 
spatial mode and therefore are not likely to be useful for this method in 
efforts to achieve higher SHG output powers. In addition, the 
room-temperature wavelength limit for noncritical phase-matching in 
KNbO.sub.3 of about 860 nm is likely to prevent SHG wavelengths 
significantly shorter than the wavelengths they describe from being 
produced using KNbO.sub.3 and similar methods. 
In "Blue Second Harmonic Generation in KTP, LiNbO.sub.3 and LiTaO.sub.3 
Waveguides," Phillips Journal of Research, vol. 46, 231-265 (1992), 
Jongerius, et al. describe conversion of an infrared pump beam into a blue 
beam by SHG through coupling the pump beam into channel waveguides that 
have been diffused into the surface of KTP, LiNbO.sub.3, or lithium 
tantalate (LiTaO.sub.3) substrates. They describe achieving 6 mW of 460 nm 
output power from a periodically segmented domain-inverted KTP waveguide 
by pumping the waveguide with 920 nm output from a Ti:Sapphire solid-state 
laser. 
As those skilled in the art will appreciate, AlGaAs diode lasers typically 
operate within the wavelength range 770 nm to 840 nm. The efficiency of 
high-power AlGaAs lasers is typically poorer at shorter wavelengths in 
comparison to higher wavelengths due to active region heating effects. 
Therefore, conversion of the output power from an AlGaAs diode laser 
utilizing SHG is likely to be limited to wavelengths greater than 385 nm, 
unless methods to produce SHG conversion at wavelengths lower than those 
conventionally demonstrated can be found. 
Accordingly, it would be desirable to find a method to produce a compact, 
diode-pumped solid-state laser which has useful laser output in a more 
extensive range, such as the 720-920 nm, that can be converted using the 
nonlinear process of SHG to visible or ultraviolet laser output in the 
360-460 nm range. 
New chromium-doped, solid-state laser materials such as chromium:lithium 
calcium aluminum fluoride (Cr:LiCAlF) and chromium:lithium strontium 
aluminum fluoride (Cr:LiSAlF) have been shown to provide optical output in 
the 720-920 nm range. These solid-state laser materials are described by 
S. A. Payne, et al., in "LiCaAlF.sub.6 :Cr.sup.3+ : A Promising New 
Solid-State Laser Material," IEEE Journal of Quantum Electronics, 
2243-2252, vol. 24, No. 11, (November 1988); S. A. Payne, et al., in 
"Laser Performance of LiSrAlF.sub.6 :Cr.sup.3+," in Journal of Applied 
Physics, 1051-1055, vol. 66, No. 3; and by S. A. Payne et al. in U.S. Pat. 
No. 4,811,349. 
Such inhomogenously broadened materials have been optically pumped by 
aluminum gallium indium phosphide (AlGaInP) diode lasers as described by 
Scheps, et al., in "Cr:LiCaAlF.sub.6 Laser Pumped by Visible Laser 
Diodes," IEEE Journal of Quantum Electronics, 1968-1970, vol. 27, No. 8 
(August 1991) and by Scheps, et al., in "Diode-Pumped Cr:LiSrAlF.sub.6 
Laser," Optics Letters, 820-822, vol. 16, No. 11 (Jun. 1, 1991). However, 
the relatively low stimulated emission cross-section-fluorescence lifetime 
product of these materials consequently requires relatively large pump 
powers to obtain laser operation threshold by pumping with such a broad 
area, high-power diode laser. This requirement results from the relatively 
large pumping beam radius inherent from the lateral spatial incoherence 
typical of such devices. The optical output of such a broad-area, 
high-power diode laser coupled via conventional methods into such an 
inhomogenously broadened solid-state lasant material is insufficient to 
generate optical output of usable power from such a solid-state lasant. 
Thus, improved methods for coupling the optical output of high-power diode 
lasers, especially those having improved lateral spatial coherence, into 
the mode volumes of a solid-state lasant such as Cr:LiCAlF or Cr:LiSAF are 
highly desirable. 
SUMMARY OF THE INVENTION 
An object of the present invention is, therefore, to provide a compact, 
tunable, solid-state laser that is optically end-pumped by a diode laser 
or by an array of diode lasers. The method of optical pumping is chosen to 
provide a pumping beam with excellent spatial overlap with respect to the 
resonator mode volume. Both the pump beam radius within the lasant and the 
resonator mode radius are tailored to be small to produce TEM.sub.00 mode 
operation at low diode laser output powers. 
In one embodiment, the very bright output from an unstable resonator 
semiconductor diode laser (URSL) is collected and focused using 
conventional lenses into a solid-state laser resonator mode volume. 
Another object of the invention is to provide a method for coupling 
infrared laser output within the 720-920 nm range generated by such a 
diode-pumped, tunable, solid-state laser into a nonlinear waveguide in 
order to convert the infrared output beam to a visible or near ultraviolet 
output beam within the 360-460 nm range by utilizing the nonlinear process 
of second-harmonic generation (SHG). 
A further object of this invention is to produce a longitudinally 
URSL-pumped solid-state laser in which the laser rod is of a Cr:LiCAF or 
Cr:LiSAF type and in which the solid-state laser reaches threshold laser 
operation at low pumping power output. 
In another embodiment, the output of a high-power diode laser or array of 
diode lasers is collected using a nonimaging concentrator fabricated from 
a crystalline, high refractive index material that is close-coupled to the 
mode volume of the solid-state lasant. This method is described in 
concurrently filed U.S. patent application Ser. No. 7/873,449 of Baird, 
DeFreez, and Sun for Method and Apparatus for Generating and Employing a 
High Density of Excited Ions in a Lasant, which is assigned to assignee of 
the present application. The high-power diode laser in the preferred 
embodiment described by Baird et al. is of a type that is typically 
gain-guided in the lateral plane of the device and index-guided in the 
transverse plane. Accordingly, the diode laser is typically spatially 
incoherent in the lateral plane, thus limiting its optical brightness. The 
surface of the lasant that is close-coupled to the output aperture of the 
nonimaging concentrator has a dichroic coating, which is transparent at 
the pumping wavelength and highly reflective at the solid-state laser 
wavelength, and forms one mirror of the solid-state laser resonator 
cavity. This surface may be fabricated with a radius of curvature. A 
second mirror, which is partly reflecting and partly transmitting at the 
laser wavelength, and which may be fabricated with a radius of curvature, 
serves to complete the resonator cavity and to couple laser light out of 
the resonator cavity. 
Yet another object of the invention is to employ the laser output power 
generated by such methods to process electronic materials, such as dynamic 
random access memories (DRAMs), and to photo-activate drugs sensitive at 
these wavelengths for use in therapeutic medical applications. 
In another embodiment, a Q-switch is inserted into the solid-state laser 
resonator cavity to produce a laser output of short (&lt;100 ns), Q-switched 
pulses. 
In another embodiment, the continuous wave (CW) laser output power from the 
diode-pumped tunable solid state laser is coupled using conventional 
lenses into a nonlinear waveguide to convert the infrared laser output 
beam from the solid-state laser into a visible laser output beam by use of 
the nonlinear process of second-harmonic generation. A further embodiment 
couples the Q-switched output from the solid-state laser into the 
nonlinear waveguide to produce a laser output of visible or near 
ultra-violet, short-pulsewidth, Q-switched pulses. 
Additional objects and advantages of the present invention will be apparent 
from the following detailed description of the preferred embodiments 
thereof, which proceeds with reference to the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
With reference to FIGS. 1, 2A, and 2B, laser system 10 preferably includes 
a power supply 12 for supplying electrical current to a high-power, 
unstable resonator semiconductor laser (URSL) 14 with an optical output of 
greater than 250 mW to pump a solid-state laser resonator cavity 16 having 
a cavity length 18 of about 15 mm. High-power URSL 14 forms part of a 
diode laser package 20 that is connected to a heat sink 22. High-power 
URSL 14 is positioned so that its optic axis 24 (FIGS. 2A and 2B)is 
substantially coaxial to an optic axis 26 extending through resonator 
cavity 16 of a solid-state laser 28. A processing unit (PU) 30 determines 
the power level and other signal levels supplied by power supply 12 to 
high-power URSL 14. 
High-power URSL 14 can be fabricated by focused ion beam micromachining, as 
described in Tilton, . . . DeFreez, et al., of one or both mirrors 40 and 
42 and to provide mirror surfaces with respective radii of curvature 44 
and 46 such that the combination of mirror curvatures 44 and 46 imparts a 
greater than unity lateral magnification to an optical field propagating 
within high-power URSL 14. For example, high-power URSL 14 may be a 
broad-area, high-power AlGaInP semiconductor diode laser that typically 
emits at wavelengths in the range 610 nm to 690 nm. Such a high-power URSL 
14 may have a cavity length 50 of 500 .mu.m, an active area width 52 of 
200 .mu.m, an active area thickness 54 of 0.005-2.0 .mu.m, and a mirror 42 
with a spherical radius of curvature 46 of infinity. Mirror 40 of 300 
.mu.m of such a high-power URSL 14 may be micromachined to have a 
spherical radius of curvature 44 of 2200 .mu.m with maximum sag depth 56 
of 5.1 .mu.m along URSL optical axis 24 with respect to the unmachined 
mirror plane 58. Such a high-power URSL 14 may have a resonator 
magnification of 2.5. 
Unlike conventional high-power diode lasers and arrays of diode lasers used 
for pumping solid-state lasants, high-power URSL 14 exhibits lateral 
spatial coherence as well as transverse spatial coherence. This 
improvement in spatial coherence results in high-power URSL 14 generating 
a high-power optical output 60 that can be efficiently collected by a lens 
system that typically includes a collimating lens 68 and a cylindrical 
lens 70. Optical output 60 is subsequently focused by an objective lens 72 
to form an optical pumping beam 74 that has its radius and depth of focus 
selected to be well-matched to the radius and length of a lasant mode 
volume 76. The lasant mode volume 76 constitutes the portion of mode or 
beam volume 78 of resonator cavity 12 that is contained within solid-state 
lasant 80. Skilled persons will appreciate that in FIG. 1, mode or beam 
volume 78 is shown greatly enlarged for ease of visualization and does not 
represent a true path through the other elements in FIG. 1. 
Analyses suggest that adjustments to the radii of curvature 44 and 46 of 
the respective mirrors 40 and 42, in combination with adjustments to the 
cavity length 50, can modify lateral divergence angle 92 originating from 
virtual point source 90 in the lateral plane to make angle 92 
substantially equal to transverse divergence angle 96 originating from 
real point source 94 in the transverse plane of high-power URSL 14. A 
highly advantageous feature of using a high-power URSL 14 is that it 
allows use of the very small cavity mode and pump mode radii (less than 50 
.mu.m) that contributes to the reduction of the threshold pumping power 
required for operation of solid-state laser 28. 
This arrangement effectively reduces the power of optical output 60 
required from high-power URSL 14 to obtain threshold operation of 
solid-state laser 28. Furthermore, optical pumping beam 74 is preferably 
selected with lasant mode volume 76 to produce TEM.sub.00 mode laser 
operation, a useful property which allows optical output beam 100 
(preferably greater than 100 mW) from resonator cavity 16 to be readily 
focused by a lens assembly 102 employing conventional optical methods. 
FIG. 3 is a graphical representation of exemplary analytical data depicting 
output power vs. diode-pumping power applied to a Cr:LiCAF laser. The data 
illustrate the threshold operation of a Cr:LiCAF laser achieved with 
pumping power supplied by a broad-area diode laser (slope A) and by an 
URSL (slope B), each having equal emission widths and cavity lengths. 
Slope A demonstrates that an URSL of the present invention provides not 
only a reduction in threshold, but also provides a better slope 
efficiency. A person skilled in the art will appreciate these differences 
are generally true regardless of changes made to the numerous variables 
used to calculate the data illustrated in FIG. 3. 
Alternatively a high-power, gain-guided AlGaInP diode laser could be 
employed in place of URSL 14. As those skilled in the art will appreciate, 
and as shown in FIG. 3, the reduced lateral spatial coherence of this type 
of device will typically result in a larger threshold pumping power 
required for laser operation of the solid-state laser. 
The optical brightness of a high-power gain-guided diode laser or array of 
diode lasers is typically limited by lateral spatial incoherence of the 
optical output arising from filamentation. Filamentation of the optical 
output arises from the reduction of the gain profile within the active 
area of the diode laser because of stimulated emission of the optical 
mode. The resulting increase in the refractive index in the portion of the 
active area contributing most strongly to the optical mode results in an 
increase in the refractive index bounded by the smaller refractive index 
values in the surrounding portions of the active region that are not 
contributing as strongly to the optical mode. This variation in the index 
profile within the active area can form an index-guided region. When the 
active area is broader than about 5-10 .mu.m, which is typical in the case 
of high-power laser diodes, several, and in some cases many, such 
index-guided regions may form. Stimulated emission within each such 
index-guided region of the active area may be in the form of a filament 
that is spatially incoherent with respect to neighboring filaments. This 
filamentation phenomenon, therefore, is a fundamental source of lateral 
spatial incoherence in high-power diode lasers, and consequently, 
contributes to limiting the optical brightness of such devices. 
With reference to FIG. 1, resonant cavity 16 of solid-state laser 28 
preferably includes a solid-state lasant 80 that comprises a 
chromium-doped crystal, such as Cr:LiSrAlF.sub.6 or Cr:LiCaAlF, positioned 
along optic axis 26. The preferred dopant level for Cr:LiSrAlF.sub.6 or 
Cr:LiCaAlF.sub.6 lasants employed in the present invention is greater than 
1.0% atomic. Skilled persons will appreciate that lasant 80 may be any 
chromium-doped fluoride composition ofCr.sup.3+ :XYZF.sub.6 composition 
wherein X is selected from Li.sup.+, Na.sup.+, K.sup.+, and Rb.sup.+, Y is 
selected from Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Cd.sup.2+, and Mg.sup.2+, 
and Z is selected from Al.sup.3+, Ga.sup.3+, and Sc.sup.3+. 
The length of solid-state lasant 80 is typically selected such that the 
mathematical product of the lasant length and the absorption coefficient 
of the lasant at the preselected diode laser pump wavelength is greater 
than or equal to one. The pumping beam radius within the lasant is 
selected to substantially overlap the resonator cavity mode radius. 
A dichroic coating 104 is applied to a preferably curved surface 106 of a 
rear resonator mirror 108. Dichroic coating 104 is highly transmissive at 
the preselected high-power URSL pump wavelength such as 650 nm and highly 
reflective at a preselected lasant emission wavelength such as 780 nm. 
Lasant surfaces 112 and 114 may be coated for high transmission at the 
lasant emission wavelength, and may have respective wedge angles 116 and 
118 which may be Brewster's angle defined by the lasant emission 
wavelength and polarization. An output coupling mirror 120 that is partly 
transmissive at the lasant emission wavelength and which may have a radius 
of curvature, forms the opposite end of resonator cavity 16. In an 
alternate embodiment, resonator mirror 108 is eliminated and dichroic 
coating 104 is applied to lasant surface 112 so that it forms one 
reflective surface of resonator cavity 16. When used as one of the 
reflective surfaces of resonator cavity 16, lasant surface 112 is 
fabricated with an appropriate radius of curvature. 
The radii of curvature are chosen in conjunction with cavity length 18 and 
the geometry of lasant 80 to provide a resonator mode beam waist or radius 
waist that permits low threshold laser operation. In the preferred 
embodiment, resonator mirror 108 has a radius of curvature of 100 mm, and 
output coupling mirror 120 has a radius of curvature 20 mm. Lasant 80 has 
a length of about 5 mm and has a rectangular cross section of 4 mm.times.5 
mm. A TEM.sub.00 mode radius waist of less than 40 .mu.m is located within 
lasant mode volume 76 near lasant surface 112. Optical pumping beam 74 is 
focused to have a beam radius well-matched to the TEM.sub.00 mode radius 
throughout lasant mode volume 76. 
FIG. 4 is a schematic diagram of an alternate embodiment of laser system 
130 of the present invention. FIG. 5 is an enlarged, elevated, 
fragmentary, cross-sectional view of a portion of the laser system of FIG. 
4 showing a nonimaging concentrator coupled to a lasant. Some elements of 
laser system 130 are labeled with reference numerals that correspond to 
similar elements in laser system 10 of FIG. 1. Although these elements may 
not be identical in dimension or composition, their functional 
descriptions will be omitted in the interest of brevity. 
With reference to FIGS. 4 and 5, laser system 130 preferably includes a 
power supply 12 for supplying electrical current to a high-power, AlGaInP 
diode laser 134 with typical optical output power of greater than 250 mW 
to pump solid-state laser resonator cavity 136. High-power diode laser 134 
is positioned along an optic axis 138 and emits a pumping beam 140 that 
travels substantially collinearly with and proximal to optic axis 138. 
Pumping beam 140 emitted by high-power diode laser 134 is typically 
astigmatic and has a noncircular radiation pattern whenever diode laser 
134 is of the gain-guided type. High-power, gain-guided diode lasers are 
easier to manufacture, more widely available, less expensive, and 
typically available with higher optical output power than high-power, 
index-guided diode lasers. 
The output wavelength of high-power diode laser 134 may be adjusted by 
temperature tuning, i.e. controlling the voltage supplied to the 
thermoelectric cooler package 142 to which high-power diode laser 134 is 
mounted. Temperature tuning is well known in the art and is described in 
"Laser Diode Guide Book," Sony Corporation of America, p. 52. The output 
wavelength of high-power diode laser 134 is typically adjusted to a 
preselected wavelength that is within the absorption bandwidth of 
solid-state lasant 150. 
A dichroic coating 152 is applied to rear surface 154 to form one end of 
resonator cavity 136. Output surface 156 of solid-state lasant 150 may 
have a wedge angle 158, which may be the Brewster's angle defined by the 
emission wavelength and polarization, to minimize retroreflections of the 
pumping beam directed towards high-power diode laser 134. Further, output 
surface 154 may be coated to be highly transmissive at the lasant emission 
wavelength. A concave output coupling mirror 160, partly transmissive at 
the lasant emission wavelength, forms the opposite end of resonator cavity 
136. 
A coupling stage 164 is mounted into resonator housing 166 and is adapted 
to receive a nonimaging concentrator 170 and solid-state lasant 150 to 
facilitate coupling and alignment of nonimaging concentrator 170 to 
high-power diode laser 134 and solid-state lasant 150. Coupling stage 164 
preferably provides for nonimaging concentrator 170 to be closely 
end-coupled to solid-state lasant 150 with only a small air gap of about 
10 .mu.m between them. Coupling stage 164 is machined to allow minor 
orientation adjustments of solid-state lasant 150 so that its .pi. and 
.rho. crystallographic axes are properly aligned with respect to optic 
axis 138. An epoxy or solder is also preferably applied at coupling stage 
points 172 and fill holes 174 to secure nonimaging concentrator 170 and 
lasant 30 to coupling stage 164. 
Nonimaging concentrator 170 preferably is of cylindrical shape with linear 
taper, which is "best fit" to an ideal compound parabolic concentrator, 
and is fabricated from sapphire or other high refractive index, 
crystalline dielectric material. Persons skilled in the art will 
appreciate that such crystalline material, such as sapphire, is unlike 
amorphous materials, such as those used for fiber-optic waveguides, and 
cannot readily be squashed to form elongated shapes such as described by 
Scifres et al. Nonimaging concentrator 170 also preferably includes a 
microcylindrical lens 176 ground onto its input aperture 178 to reduce the 
transverse divergence angle, which is typically larger than the lateral 
divergence angle, of pumping beam 140 to allow efficient coupling of it 
from high-power diode laser 134 into lasant mode volume 180 (the portion 
of the resonator mode volume contained within solid-state lasant 150) in 
resonator cavity 136. 
FIG. 6 is a graph showing the numerical aperture of a nonimaging 
concentrator 170 of refractive index 1.74 as a function of the cladding 
index. FIG. 6 is not corrected for tilt angle of the concentrator taper, 
which typically is very small. FIG. 6 demonstrates that a nonimaging 
concentrator 170 with refractive index 1.74 in air collects all rays that 
are not reflected at the interface. Those skilled in the art will 
recognize that antireflection coatings can be applied to microcylindrical 
lens 176 and the output aperture 182 to greatly reduce reflections at 
these interfaces. This coupling embodiment is a significant improvement 
over use of single-mode or multi-mode fiber-optic waveguides, such as 
described by Scifres et al. in U.S. Pat. No. 4,673,975. Moreover, the 
relatively low refractive index step between the transparent core and 
cladding material characteristic of such fiber-optic waveguides results in 
a significant numerical aperture limit for Scifres et al.'s devices. 
Preferably, nonimaging concentrator 170 has an input aperture 178 that is 
larger than the width of the active area of emission 184 of high-power 
diode laser 134 and of sufficient diameter to intercept essentially all 
radiation emitted from the high-power diode laser 134. In addition, the 
nonimaging concentrator 170 has an output aperture 182 that is less than 
the diameter of lasant mode volume 180. The nonimaging coupling of pumping 
beam 140 from high-power diode laser 134 into lasant mode volume 180 
results in an efficient overlap of pumping beam 140 with the lasant mode 
volume 180, thus minimizing losses which would otherwise occur through 
divergence of the pumping beam outside lasant mode volume 180. 
In an alternate embodiment, microcylindrical lens 176 is an independent 
lens, fabricated from sapphire or similar index material and positioned 
between nonimaging concentrator 170 and diode laser 134. The nonimaging 
coupling of pumping beam 140 from high-power diode laser 134 into lasant 
mode volume 180 results in an effective overlap of pumping beam 140 with 
lasant mode volume 180, thus minimizing losses which would otherwise occur 
through divergence of the pumping beam outside lasant mode volume 180. 
Although the following description proceeds with reference to FIGS. 1 and 
4, elements common to both laser systems 10 and 130, but having different 
reference numerals will be identified by the reference numerals appearing 
in FIG. 1. With reference to FIGS. 1 and 4, a nonlinear waveguide 200 is 
placed external to solid-state laser resonator cavity 16. Nonlinear 
waveguide 200 is selected from a group including periodically segmented 
domain-inverted KTP waveguide, periodically segmented LiNbO.sub.3, and 
segmented LiTaO.sub.3. The solid-state laser optical output 100, which is 
linearly polarized with polarization direction parallel to the 
crystallographic c-axis of lasant 80, is collected and focused using 
conventional lens assembly 102 to form a waveguide pumping beam 204. 
Nonlinear waveguide 200 is mechanically oriented in a mount (not shown) 
such that the waveguide pumping beam 204 polarization direction is in 
correct orientation to be confined within waveguide 200. A periodically 
segmented domain-inverted KTP waveguide 200 is preferred to convert the 
infrared waveguide pumping beam 204 emitting in the wavelength range 720 
to 920 nm to a visible or near ultraviolet output beam 206 operating in 
the wavelength range 360 to 460 nm by utilizing the nonlinear process of 
second-harmonic generation. 
The efficiency with which the optical power of the waveguide pumping beam 
204 is converted into optical power of the visible output beam 206 can be 
optimized by selection of the radius and spectral bandwidth of the 
waveguide pumping beam 204 to best match a selected segmentation period, 
waveguide length, and waveguide width as described by Jongerius, et al. 
For the purpose of controlling the spectral bandwidth and wavelength 
output of the waveguide pumping beam 204, tilted birefringent plates or 
other tuning elements 210, may be introduced into resonator cavity 16, 
according to the analyses well-known to the art of Bloom in "Modes of a 
Laser Resonator Containing Tilted Birefringent Plates" Journal of the 
Optical Society of America, vol. 64, No. 4, 447-452. Plates 210 may be 
fabricated from quartz or similar birefringent materials. The plates 210 
are tilted at Brewster's angle, and each plate 210 has its optic axis 
aligned perpendicular to the plane defined by the p polarization of the 
Brewster surfaces. 
For the purposes of controlling the radius of waveguide pumping beam 204, 
the effective focal length of lens assembly 102 can be selected to provide 
focused spot sizes of less than 10 .mu.m. For example, solid-state laser 
beam 100 may have a TEM.sub.00 radius of 57 .mu.m at the output coupler 
mirror 120 and have a TEM.sub.00 radius of 72 .mu.m at lens assembly 102, 
which may be a single or double convex lens, and which then focuses 
solid-state laser output beam 100 to form waveguide pumping beam 204. 
Waveguide pumping beam 204 is focused to a 5 .mu.m spot size in nonlinear 
waveguide 200. Other methods known to the art may be employed to control 
the spectral bandwidth and waveguide pumping beam radius without departing 
from the scope of the present invention. 
With reference to FIGS. 1, 4, and 5, resonator cavity 16 also preferably 
includes a modulator 220, shown in phantom, interposed along optic axis 26 
between lasant 80 and output coupling mirror 120. Skilled persons will 
appreciate that the solid-state resonator cavity and high-power URSL 
dimensions previously set forth describe a specific continuous wave 
embodiment of laser system 10 not actually shown in FIG. 1. It is noted 
that inclusion of modulator 220 within solid-state resonator cavity 16 
would require adjustment of most of the dimensions to compensate for the 
elongated solid-state resonatory cavity 16 depicted in FIG. 1. 
Modulator 220 is preferably a Q-switch that employs an acousto-optic medium 
222, preferably quartz or SF-57 glass or other medium having an acceptable 
figure of merit such as fused silica or TeO.sub.2, that is bonded to an 
acoustic wave transducer 224. Acoustic wave transducer 224 receives an RF 
signal from an impedance-matched RF power amplifier 226 that is responsive 
to signals from processing unit 30. 
When an RF signal is present, transducer 224 generates an acoustic wave 
that propagates through medium 222 transverse to the optic axis 26 in 
resonator cavity 16. The acoustic wave functions as a diffraction grating 
that substantially increases the optical losses in resonator cavity 16 to 
effectively prevent lasing and consequently allow energy to be stored in 
the lasant 80. Whenever the RF signal to transducer 224 is interrupted, 
the grating is removed and the optical losses in resonator cavity 16 are 
diminished. While employing this method, a laser system of the present 
invention can generate short optical pulses having a duration of typically 
less than 50 ns with high peak power. 
High peak power pulses generated by the method described in the above 
embodiments can be coupled into nonlinear waveguide 200. Using the 
nonlinear process of second harmonic generation, pulsed infrared waveguide 
pumping can be converted to an output beam 206 of visible or 
near-ultraviolet high peak power pulses. 
Persons skilled in the art will appreciate that crystal length and dopant 
level are selected concurrently with the pump beam radius, resonant mode 
radius, and output coupler transmission to generate advantageous values 
for the pumping beam power required at laser threshold. Skilled persons 
will also appreciate that as good quality higher dopant level laser 
crystals become available, the length of the lasant can be reduced in 
order to utilize smaller pump beam and resonant mode radii to even further 
reduce the pumping beam output power required at threshold. 
It will be obvious to those having skill in the art that various changes 
may be made in the details of the above-described embodiments of the 
present invention without departing from the underlying principles 
thereof. For example, high-power URSL 14 can be composed of other 
light-emitting semiconductor material such as InGaAsP or ZnSe. The scope 
of the present invention should be determined, therefore, only by the 
following claims.