Side-pumped solid-state disk laser for high-average power

A solid state laser module for amplification of laser radiation. The module includes a laser gain medium having a pair of generally parallel surfaces that form a disc-like shape, that receive and transmit laser radiation. At least one perimetral optical medium is disposed adjacent a peripheral edge of the laser gain medium and in optical communication therewith. A source of optical pump radiation directs optical pump radiation into the perimetral optical medium generally normal to the parallel surfaces and the perimetral optical medium transports the optical pump radiation into the laser gain medium to pump the laser gain medium to a laser transition level. Alternative embodiments include arrangements for directing cooling fluids between adjacently disposed laser gain media.

FIELD OF THE INVENTION

This invention relates to solid-state lasers, and more particularly to solid-state disk laser having a side-pumped gain medium cooled on at least one of its facial surfaces.

BACKGROUND OF THE INVENTION

In solid-state lasers (SSL), optical pumping generates a large amount of heat within a laser medium and increases its temperature. Continuous operation of the laser, therefore, requires removal of the waste heat by cooling selected surfaces of the laser medium. Because SSL media typically have a low thermal conductivity, a significant thermal gradient is created between the hot interior and the cooled outer surfaces. This causes a gradient in the index of refraction, mechanical stresses, depolarization, detuning, and other effects, with likely consequences of degraded beam quality, reduced laser power, and possibly a fracture of the SSL medium. Such effects present a major challenge to scaling of SSLs to high-average power (HAP). Pumping by semiconductor laser diodes, which was introduced in the 1990's, greatly reduces the amount of waste heat and paves the way for development of a high-average power (HAP) SSL with good beam quality (BQ). Such lasers are expected to make practical new industrial processes such as precision laser machining with applications ranging from deep penetration welding to processing of aerospace materials.

It has been long recognized that optical distortions caused by transverse temperature gradients (i.e., perpendicular to laser beam axis) degrade beam quality. A class of SSL known as “disk lasers” avoids transverse temperature gradients because waste heat is extracted from the disk in the direction parallel to the laser beam axis. Because of this one-dimensional heat flow, solid-state disk lasers (SSDL) enjoy inherently low susceptibility to thermal lensing and stress birefringence. In addition, their large, round aperture reduces diffraction and beam clipping losses experienced by other SSL configurations. SSDL may use “transmissive” disks having antireflective coatings on each face or “reflective” disks having an antireflective coating on one face and highly reflective coating on the other. In a transmissive disk, waste heat is removed by a coolant (usually gas) flowing through the laser beam path. In a reflective disk, also known as an “active mirror amplifier” (AMA), the back surface of the disk is available for liquid cooling, which can be applied more uniformly, and can easily handle heat fluxes. Both disk laser types (transmissive and reflective) have been in development since the late 1960s, especially as amplifiers for giant pulse lasers for inertial confinement fusion.

In the classical AMA concept, a large aspect ratio, edge-suspended, Nd-Glass disk (or slab) several centimeters thick is pumped by flashlamps and liquid-cooled on the back face. However, this device is not suitable for operation at high-average power because of poor heat removal and resulting thermo-mechanical distortion of the edge-suspended disk. Previous attempts to mitigate these problems and increase the average power output of an SSDL were met with encouraging but limited results. In recent years, the SSDL concept has been revived in the form of a “thin disk laser” AMA introduced by Brauch et al. in U.S. Pat. No. 5,553,088. The thin disk laser uses a gain medium disk which is several millimeters in diameter and 200–400 micrometers in thickness soldered to a heat sink. See, for example, A. Giesen et al., “Scalable concept for diode-pumped high-power lasers,” Appl. Phys. B vol. 58, 365–372 (1994). The diode-pumped Yb:YAG thin disk laser has demonstrated laser outputs approaching 1 kW average power and with beam quality around 12 times the diffraction limit. See, for example, C. Stewen et al., “1-kW CW Thin Disk Laser,” IEEE J. of Selected Topics in Quant. Electr., vol. 6, no. 4, 650–657 (July/August 2000). Another variant of the thin disk laser can be found in L. Zapata et al., “Composite Thin-Disk Laser Scalable To 100 kW Average Power Output and Beyond,” in Technical Digest from the Solid-State and Diode Laser Technology Review held in Albuquerque, N. Mex., Jun. 5–8, 2000.

The applicant's U.S. Pat. No. 6,339,605 titled “Active Mirror Amplifier System and Method for a High-Average Power Laser System”, hereby incorporated by reference, discloses a new SSDL concept, which is suitable for operation at high-average power. The invention uses a large aperture laser gain medium disk about 2.5 mm in thickness and with a diameter typically between 5 cm and 15 cm mounted on a rigid, cooled substrate. Note that the disk thickness in this SSDL concept is about 10 times less than in the classical SSDL and about 10 times more than in the thin disk laser. The substrate contains a heat exchanger and microchannels on the surface facing the laser medium disk. The disk is attached to the substrate by a hydrostatic pressure differential between the surrounding atmosphere and the gas or liquid medium in the microchannels. This novel approach permits thermal expansion of the laser medium disk in the transverse direction while maintaining a thermally loaded disk in a flat condition. The teachings of this patent provide numerous advantages over prior art SSL devices and allow generation of near diffraction limited laser output at very high-average power from a relatively compact device.

The above-mentioned U.S. Pat. No. 6,339,605 also teaches two principal methods for providing pump radiation into the SSDL disk, namely 1) through the large(front or back) face of the disk, or 2) through the sides (edges) of the disk. Side-pumping takes advantage of the long absorption path (approximately same dimension as the diameter of the gain medium disk), which permits doping the disk with a reduced concentration of lasant ions. When quasi-3 level lasing ions are used, reduced doping associated with side-pumped SSDL conveniently reduces pump radiation intensity required to induce laser medium transparency at laser wavelength. In addition, long absorption in a side-pumped SSDL permits the use of laser ions with small pump cross-sections that may be otherwise impractical to use for face-pumping.

While side-pumping is a suitable method for delivering pump radiation, several associated technical challenges still need to be overcome, such as: 1) delivering and concentrating pump radiation into the relatively small area around the disk perimeter; 2) preventing overheating of the disk in the areas where the pump radiation is injected; 3) generating uniform laser gain over the SSDL aperture; and 4) avoiding laser gain depletion by amplified spontaneous emission (ASE) and parasitic oscillations. The significance of these challenges and related solutions disclosed in the prior art are discussed below.

Concentration of Pump Radiation

Modern SSL are optically pumped by semiconductor lasers commonly known as laser diodes. Because each laser diode produces a relatively small optical output (up to a few watts), pumping of SSL for HAP requires the combined output of a great many laser diodes (typically in quantities ranging from hundreds to hundreds of thousands). For this purpose the diodes are arranged in one-dimensional arrays often called “bars” containing about 10 to 100 diodes and two-dimensional arrays often called “stacks” containing several hundred to several thousand diodes. Bars are frequently mounted on water-cooled heat exchangers. Stacks are typically produced by stacking up to about 100 bars and mounting them onto a heat exchanger or by stacking up to about 20 bars already mounted on their individual heat exchangers. A good example of commercially available stacks is the Model SDL-3233-MD available from SDL, Inc., of San Jose, Calif., which can produce 200 microsecond-long optical pulses with a total output of 960 watts at a maximum 20% duty factor. SSL for HAP may require a combined power of multiple diode bars to produce desired pumping effect in the laser gain medium. Regardless of the grouping configuration, individual laser diodes emit optical radiation from a surface, which is several micrometers high and on the order of 100 micrometers wide. As a result, the beamlet of radiation emitted from this surface is highly asymmetric: highly divergent in a direction of the 1 .mu.m dimension (so called “fast axis”) and moderately divergent in the transverse dimension (so-called “slow axis”). This situation is illustrated inFIG. 2. Typical fast axis divergence angles (full-width at half-maximum intensity) range from 30 to 60 degrees, while slow axis divergence angles typically range from 8 to 12 degrees. Optical radiation from an array of diodes has similar properties. High divergence in the fast axis makes it more challenging to harness the emitted power of diode arrays for use in many applications of practical interest. Some manufacturers incorporate microlenses in their laser diode arrays to reduce fast axis divergence to as little as a few degrees. An example of such a product is the lensed diode array Model LAR23P500 available from Industrial Microphotonics Company in St. Charles, Mo., which includes microlenses which reduce fast axis divergence to less than three degrees.

The intensity of the optical output of diode arrays (lensed or unlensed) is sometimes insufficient to pump a SSL gain medium to inversion, and the radiation must therefore be further concentrated. In previously developed systems, optical trains with multiple reflecting and/or refracting elements have been used. See, for example, F. Daiminger et al., “High-power Laser Diodes, Laser Diode Modules And Their Applications,” SPIE volume 3682, pages 13–23, 1998. Another approach disclosed by Beach et al., in U.S. Pat. No. 5,307,430 uses a lensing duct generally configured as a tapered rod of rectangular cross-section made of a material optically transparent at laser pump wavelength. Operation of this device relies on the combined effect of lensing at the curved input surface and channeling by total internal reflection. Light is concentrated as it travels from the larger area input end of the duct to the smaller area exit end. Yet another approach for concentrating pump radiation disclosed by Beach et al. in U.S. Pat. No. 6,160,939 uses a combination of a lens and a hollow tapered duct with highly reflective internal surfaces.

Thermal Control of Disk Perimeter

The surfaces of the laser gain medium that receive pump radiation are susceptible to overheating and, as a result, to excessive thermal stresses. Experience with end-pumped rod lasers shows that a composite rod having a section of doped and undoped laser material provides improved thermal control and concomitant reduction in thermal stresses. See, for example, R. J. Beach et al., “High-Average Power Diode-pumped Yb:YAG Lasers,” UCRL-JC-133848 available from the Technical Information Department of the Lawrence Livermore National Laboratory, U.S. Department of Energy. A suitable method for constructing composite optical materials of many different configurations is disclosed by Meissner in U.S. Pat. No. 5,846,634. Another method suitable for forming glass optics involves casting and fusion bonding doped, undoped, or differently doped sections. Such a method is available from Kigre Inc., Hilton Island, S.C. Yet another method suitable for construction of composite polycrystalline materials is available from Baikowski International, Corp. of Charlotte, N.C.

Uniform Laser Gain Across the Aperture

Due to the exponential absorption of pump radiation, portions of the laser gain medium that are closer to the pump source are susceptible to being pumped more intensely than portions that are further away. Non-uniform deposition of pump energy can result in non-uniform gain. Gain non-uniformities across the laser beam aperture (normal to the laser beam axis) are highly undesirable as they lead to degradation of beam quality. In prior art devices, non-uniform pump absorption has been compensated for in a side-pumped rod laser by the gain medium being fabricated with a radially varying level of doping. Gain uniformity in a side-pumped SSDL can be also improved by appropriately arranging pump diodes around the perimeter of the laser disk. Suitable techniques for this purpose are disclosed by the Applicant in a co-pending U.S. patent application Ser. No. 10/441,373 filed on May 19, 2003. An alternate approach to achieving uniform gain is known as “bleach-wave pumping” which has been proposed by W. Krupke in “Ground-state Depleted Solid-state Lasers: Principles, Characteristics and Scaling,” Opt. and Quant. Electronics, vol. 22, S1–S22 (1990). Bleach wave pumping largely depletes the atoms in the ground energy state and pumps them into higher energy states. Achieving high uniformity of gain becomes even more challenging as the incident laser beam causes saturation-induced change in the spatial distribution of gain. Thus, the weaker portions of the signal are amplified relatively more than the stronger portions because they saturate the medium to a lesser degree.

Amplified Spontaneous Emission (ASE) is a phenomenon wherein spontaneously emitted photons traverse the laser gain medium and are amplified before they exit the gain medium. The favorable condition for ASE is a combination of high gain and a long path for the spontaneously emitted photons. ASE depopulates the upper energy level in an excited laser gain medium and robs the laser of its power. Furthermore, reflection of ASE photons at gain medium boundaries may provide feedback for parasitic oscillations that aggravate the loss of laser power. If unchecked, ASE may become large enough to deplete the upper level inversion in high-gain laser amplifiers. Experimental data suggests that in q-switched rod amplifiers ASE loss becomes significant when the product of gain and length becomes larger than 2.25, and parasitic oscillation loss becomes significant when the product is larger than 3.69. See, for example, N. P. Barnes et al., “Amplified Spontaneous Emission—Application to Nd:YAG Lasers,” IEEE J. of Quant. Electr., vol. 35, no. 1 (January 2000). Continuous wave (CW) or quasi-CW lasers are less susceptible to ASE losses because their upper level population (and hence their gain) is clamped.

A traditional method for controlling ASE losses to an acceptable level is disclosed, for example, by Powell et al. in U.S. Pat. No. 4,849,036. This method involves cladding selected surfaces of the laser gain medium with a material that can efficiently absorb ASE radiation. In particular, it is well known in the art that each of divalent cobalt and divalent samarium ions can absorb ASE in Nd laser operating around 1.06 micrometer wavelength. In addition, divalent cobalt ions can absorb ASE radiation in an Er laser operating at a 1.54 micrometer wavelength. To reduce the reflection of ASE rays at the cladding junction, the cladding material must have an index of refraction at the laser wavelength that is closely matched to that of the laser gain medium. Recently, another method for ASE loss control was introduced. In this method, ASE rays are channeled out of selected laser gain medium surfaces into a trap from which they are prevented from returning. See, for example, R. J. Beach et al., “High-average Power Diode-pumped Yb:YAG Lasers,” supra.

Materials and Methods for Low Waste Heat

To operate a SSL at HAP, it is critical to reduce as much as possible the Stokes shift (difference between the lasing wavelength and the pump wavelength), which is the leading energy loss mechanism contributing to production of waste heat. Waste heat is deposited into the gain medium where it is responsible for thermal lensing, mechanical stresses, depolarization, degradation of beam quality (BQ), loss of laser power, and (in extreme cases) thermal fracture. Consequently, when pumping a HAP SSL, it is highly desirable to use pump absorption features in proximity to the laser emission line.

The most important lasant ions for a HAP SSL operating near 1-micrometer wavelength are trivalent neodymium (Nd3+) and trivalent ytterbium (Yb3+). In addition, trivalent erbium (Er3+), lasing at a 1.54 micrometer wavelength, is important for applications requiring increased eye damage threshold (also known as “eye-safe”). Each Nd, Yb, and Er can be doped into a variety of crystalline, polycrystalline and amorphous host materials.

A side-pumped SSDL disclosed herein makes it possible to reduce the Stokes shift in many important materials and allow Nd and Yb lasers to operate more efficiently. In particular, neodymium ion Nd3+is traditionally pumped by diodes on the 808-nm absorption line that has a large cross-section. In contrast, pumping Nd on a weaker absorption feature around 885 nm deposits energy directly into the upper lasing level. Direct pumping improves Stokes efficiency by nearly 10% and entirely avoids the quantum efficiency loss (estimated at 5% in Nd) associated with energy transfer from the pump band to the upper lasing level. A side-pumped disk is amenable to direct pumping of Nd despite having a low absorption cross-section and narrow width of absorption feature at a wavelength of around 885 nm.

Ytterbium is characterized by a Stokes shift several times smaller than for Neodymium. Yb:YAG and Yb:GGG are traditionally pumped at the broad absorption feature around 941 nm. A more efficient approach is to pump Yb at the zero-phonon line around 970 nm, which offers a smaller Stokes shift and deposits energy directly into the upper laser level. A side-pumped disk laser is amenable to pumping ytterbium despite its rather low absorption cross-sections in many host materials of practical interest, namely YAG, GGG, and glass. Low absorption cross-section makes it more problematic to absorb pump energy in a short distance, as may be desirable for face-pumping of disk and slab lasers or side-pumping rod lasers. A short absorption path in combination with small absorption cross-section necessitates high doping which, in turn, requires very high pump intensities to overcome re-absorption of laser radiation by the ground energy state. This problem is resolved with the side-pumped disk of subject invention, which offers a long absorption path.

Polycrystalline Host Materials: Host materials such as YAG and GGG for use in SSL have been traditionally produced in a single-crystal form typically using the Czochraski method. However, production of large crystals required for HAP SSL is a slow and expensive process. Furthermore, such crystals are very limited in size, contain undesired inclusions, frequently can be produced with uniform doping, and are polluted by trace elements from the melt container. Recently, polycrystalline YAG and other polycrystalline garnets have emerged as viable replacements for single-crystal materials, offering large size products at reduced cost, improved quality, and improved fracture resistance. Such materials have been developed in Japan by Konoshima Chemical Company and are marketed in the US by Baikowski International Corp. of Charlotte, N.C.

Waste heat deposited into a SSL gain medium causes temperature changes which result in thermo-optic distortions that may affect the optical phase-front of the amplified laser beam and degrade its beam quality. Such distortions include thermal expansion, change to the index of refraction (n), and thermal stress-induced birefringence. Materials have been developed that reduce some of these effects. In particular, a glass composition known as athermal glass compensates for the positive coefficient of thermal expansion by a negative coefficient of change to the refractive index (dn/dt) to produce a very low thermal coefficient of optical path. Glass with athermal properties is sold by Kigre Inc. of Hilton Head Island, S.C. under designations Q-98 and Q-100; and by Schott Glass Technologies, Inc., in Duryea, Pa. under designation LG-760.

SUMMARY OF THE INVENTION

In view of the foregoing limitations with previously developed SSDLs, it is an object of the present invention to provide a solid-state disk laser (SSDL) capable of operating at high-average power and with good beam quality (BQ). In particular, the SSDL of the present invention meets a number of significant needs:

a side-pumped SSDL for a HAP;

means for trapping and absorbing ASE rays, thereby significantly reducing feedback to parasitic oscillations;

means for concentrating pump radiation for injection into the AMA disk side;

means for concentrating pump radiation by arranging pump sources around the SSDL perimeter;

means for alleviating thermal stresses and reducing the temperature near surfaces of the laser gain medium where pump radiation is injected;

means for controlling the gain profile across the SSDL aperture;

means for efficient operation of an SSDL-HAP with quasi-3 level laser media such as Yb.sup.3+;

means for efficient operation of an SSDL-HAP with laser media exhibiting high pump saturation intensities;

an SSDL with laser diode pump means that reduces the waste heat load to the solid-state laser medium;

a relatively thin solid-state medium to allow efficient conduction of waste heat;

microchannel cooling of a support substrate for efficient removal of waste heat from the laser gain medium;

a substrate which provides rigid mechanical support for the solid-state laser medium;

the use of concentrator ducts for the delivery of pump power to the sides (edges) of the SSDL laser gain medium;

a composite gain medium assembly for the delivery of pump radiation, reduced thermal distortions and reduced ASE/parasitic losses;

hydrostatic pressure means to maintain the solid-state gain medium in an optically flat condition on said substrate;

attachment means that reduce thermally-induced distortions in the solid-state gain medium;

a longer absorption path for pump radiation, which allows reducing the laser gain medium doping requirements and concomitant reabsorption losses for the gain media of 3-level lasers (e.g., Yb:YAG); and

cooling an SSDL gain medium by flowing gas over the disk face;

operation of an SSDL in a heat capacity mode;

de-correlation of optical path errors in multiple SSDL modules;

alternately operating selected SSDL modules in a bank of multiple SSDL modules to improve heat extraction to gaseous coolant; and

injecting pump light through ASE absorbing material.

The SSDL of the present invention can be used as a building block for construction of laser oscillators as well as laser amplifiers. In one preferred embodiment the invention comprises a laser gain medium having a front surface, a rear surface and a peripheral edge. The rear surface is attached to a cooled support substrate. One or more sources of optical pump radiation are disposed so as to inject optical pump radiation into one or more sections of the peripheral edge of the gain medium. Optionally, a perimetral optical medium may be attached to the peripheral edge of the laser gain medium inbetween the peripheral edge and one of the sources of optical pump radiation. Alternatively, the perimetral optical medium may cover the entire peripheral edge of the laser gain medium.

In one preferred embodiment a plurality of hollow, tapered ducts are arranged inbetween the peripheral edge of the laser gain medium and the sources of optical pump radiation. The hollow, tapered ducts help to direct or “channel” optical pump radiation in the peripheral edge of the laser gain medium. The sources of optical pump radiation are comprised of pluralities of laser diode arrays arranged to direct optical pump radiation through the hollow, tapered ducts.

The precise shape of the laser gain medium may vary considerably, with circular, elliptical, rectangular, hexagonal, octagonal, pentagonal, heptagonal and other polygonal shapes all being possible. The perimetral optical medium may form one or a plurality of sections circumscribing the peripheral edge of the laser gain medium. The section(s) may be secured to the peripheral edge via an optically transparent bond. Various preferred embodiments of the arrangement of the optical pump sources and the laser gain medium are disclosed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

As used herein, “Laser gain medium” refers to an optical material having a host lattice doped with suitable ions, which in the present invention are pumped to a laser transition. Although the present invention is not limited to a specific lasing material or to a specific optical pump source, the preferred host lattice materials are yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), gadolinium scandium gallium garnet (GSGG), lithium yttrium fluoride (YLF), yttrium vanadate, phosphate laser glass, silicate laser glass, athermal glass, sapphire, and transparent polycrystalline ceramic materials. Example of a suitable ceramic host material is polycrystalline YAG available from Baikowski International Corporation. Suitable dopants for this lasing medium include Ti, Cu, Co, Ni, Cr, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. The optical pump source is selected based on the absorption characteristics of the selected laser gain medium. Preferably, semiconductor diode lasers are used for the optical pump source. InGaAs diodes are preferred for pumping of Yb.sup.3+ ions. “Perimetral optical medium” refers to an optical material which is preferably substantially free of substances that can significantly absorb optical pump radiation. Preferably, the perimetral medium is of the same host material as the laser gain medium but it is not doped., In some variants of the invention, perimetral optical medium may be doped with ions which absorb optical radiation at one or more wavelengths of the laser gain transition, but are not pumped to a population inversion. Such ions are intended for absorbing ASE photons and reducing feedback to parasitic oscillations in the laser gain medium. For a Nd laser operating near 1.06 micrometer wavelength the perimetral optical medium can be doped with either divalent samarium or divalent cobalt ions, each of which exhibits strong absorption around 1.06 micrometer wavelength and relatively low absorption at around 808 nanometer and 885 nanometer wavelengths of optical pump sources. The perimetral optical medium may be bonded to selected surfaces of the laser gain medium by a fusion bond, or diffusion bond, or other suitable means. Such bond should be mechanically strong, thermally conductive, and highly transparent at the laser wavelength as well as at pump wavelengths. The refractive index of the updoped optical medium and the bond are preferably closely matched to that of the laser gain medium. A suitable bond can be produced by fusion bonding, diffusion bonding, or optical contacting followed by heat treatment. A fusion or diffusion bond may be produced, for example, by growing a perimetral crystal onto a doped crystal boule or, in the case of glass, by casting perimetral material around the perimeter of a doped core. A method for optical contacting followed by heat treatment is, for example, disclosed by Meissner in U.S. Pat. Nos. 5,441,803, 5,563,899 and 5,846,638 and is commercially available from Onyx Optics in Dublin, Calif. Another method suitable for glass involves casting and fusion bonding doped, undoped, or differently doped sections is available from Kigre Inc., Hilton Island, S.C. Yet another method suitable for construction of composite polycrystalline materials is available from Baikowski International Corp. of Charlotte, N.C.

Referring toFIGS. 3 and 4, there is shown a preferred embodiment of a SSDL module10in accordance with a first preferred embodiment of the present invention. The module10generally comprises a composite gain medium assembly12, a substrate46, optical pump sources68, lenses18, and tapered hollow ducts20. The composite gain medium assembly12and the substrate46form an active mirror assembly21.

Referring now toFIG. 5a,the composite gain medium assembly12has two planar, mutually parallel surfaces, a front surface22and a back surface24, both being ground flat and polished to optical quality. The shape of the composite gain medium assembly12may vary widely but in one preferred form comprises an octagonal disk with a transverse dimension “D” several times greater than its thickness “T”, as indicated inFIG. 5a.Typically, composite gain medium assembly12may have a thickness ranging approximately from 1 millimeter to 10 millimeters and transverse dimensions ranging from about 10 millimeters to 300 millimeters. The composite gain medium assembly12could just as readily be formed in other various shapes such as (but not limited to) polygonal, circular or elliptical shapes if desired. Furthermore, while the use of the term “disk” is used herein to reference this component, it will be appreciated that the composite gain medium assembly12may take other forms which might not be viewed, strictly speaking, as a “disk”.

Referring further toFIGS. 5aand5b,the composite gain medium assembly12comprises a laser gain medium26and eight segments28aof perimetral optical material28. The material of the laser gain medium disk26comprises a suitable solid-state laser gain medium such as, but not limited to, neodymium-doped yttrium-aluminum garnet (Nd:YAG), erbium-doped yttrium aluminum garnet (Er:YAG), ytterbium doped-yttrium aluminum garnet (Yb:YAG), neodymium gadolinium gallium garnet (Nd:GGG), ytterbium-doped gadolinium gallium garnet (Yb:GGG), neodymium-doped glass (Nd:Glass), erbium-doped glass (Er:Glass), ytterbium-doped glass (Yb:Glass), or other suitable laser SSL material. Garnets can be provided in a single crystal or polycrystalline form. The perimetral optical medium28is attached around the perimeter of the laser gain medium26via an optical bond30. The perimetral optical medium28has preferably the same host medium used in the laser gain medium26but does not contain a significant quantity of the dopant laser ions present in the laser gain medium26. The optical bond30must be highly transparent to the optical pump radiation and laser radiation and have good thermal conductivity. Preferred methods for constructing the bond30include the already mentioned fusion bond, diffusion bond, and the method of optical contacting followed by heat treatment. Adjacent segments of perimetral optical medium28do not necessarily need to be joined together. For example,FIG. 5bshows a gap32existing between adjacent perimetral optical media28a.Alternatively, some or all of the segments of the perimetral medium can be jointed to produce an optically and mechanically monolithic unit. Surfaces34, which receive optical pump radiation36(FIG. 3), have a dielectric coating38that is antireflective at optical pump radiation wavelengths. Furthermore, the optical bond30plane and (FIG. 5a) can be machined at a slight angle of 1–5 degrees off normal from surface22to reduce the possibility of direct ASE feedback to parasitic oscillations. Alternate forms of the shape of the perimetral material28are shown inFIGS. 7a–7f.

Referring now toFIG. 4, the back planar surface24has an optical coating40with high reflectivity at a laser wavelength. Such a coating can be dielectric or metallic, or a combination of several layers of metallic and dielectric coatings. The front surface22has an optical coating42that is antireflective at the laser wavelength. Optionally, the coatings40and42can also be individually made reflective at the optical pump wavelength in addition to their already mentioned properties with respect to the laser wavelength. The back surface24is in contact with a surface44of a cooled, rigid substrate46. The surface44contains an array of interconnected microchannels48extending generally over, but not beyond, the contact area between the flat portion of composite gain medium12and the substrate46.

Referring further toFIGS. 3 and 4, the substrate46contains a heat exchanger50that is located below the surface44and not connected to the microchannels48. Coolant52is provided to the heat exchanger50by an inlet header54and drained therefrom by the outlet header56. Internal distribution of the coolant52inside the heat exchanger50is such so as to provide a uniform cooling effect over a large part of the back surface24of the composite gain medium assembly12. Suitable coolants may include liquids such as water, alcohol, members from the Freon.RTM. family, and liquid nitrogen. Coolant fluid connections to the inlet header54and the outlet header56can be provided by pressure-balanced, axially-movable fluid transfer tubes such as disclosed by Eitel in U.S. Pat. No. 4,029,400, the disclosure of which is incorporated by reference herein. Such fluid transfer tubes isolate hydraulic pressure loads from the substrate46and coolant supply so that alignment of substrate46will not be affected. In addition, the fluid transfer tubes balance the hydraulic forces caused by the coolant pressure so that the substrate will not have any significant load placed upon it to interfere with its operation. Furthermore, such fluid transfer tubes permit small axial and lateral adjustments of substrate46such as may be required to optically align the laser gain medium12without affecting the operation of the fluid transfer tubes or placing forces on the substrate from the tubes.

The cooled substrate46is made of a material with good thermal conductivity, preferably copper, tungsten, molybdenum, sapphire, silicon carbide, silicon, but other materials with good thermal conductivity and suitable for microchannel and heat exchanger fabrication can be used. The material of the substrate46can also be chosen to have a coefficient of thermal expansion close to that of the laser gain medium26. Surface44of substrate46is machined to optical flatness except for penetrations created by the microchannels48. Typical dimensions for the microchannels48include a width of about 0.005 inch to 0.040 inch (0.13 mm–1 mm) and a cross sectional area of about 0.000025 inch.sup.2–0.0016 inch.sup.2 (0.00016125 cm.sup.2–0.01032 cm.sup.2). Microchannels48preferably occupy about 50% of the contact area between surface44of substrate46and back surface24of composite gain medium12. The microchannels48may also be formed in a variety of cross-sectional shapes, but preferably have a generally square cross-sectional shape. The thickness of the substrate46is chosen to provide mechanical rigidity necessary to ensure that the surface44remains optically flat under operational conditions.

When optically flat surfaces are brought into contact, they may become bonded even without bonding agents. Such bonds can be attributed to Van der Vaals forces of attraction acting at opposing contact points and surfaces. Such bonding remains stable as long as the components of the composite are not subjected to temperature gradients that cause non-uniform thermal expansion, and resultant stress to overcome this bond strength. However, the bond may also be broken by inserting a strong thin object, for example a razor blade, between the optically contacted surfaces. De-bonding also results when liquids diffuse into the interface from the edge which constitutes the bond line.

In the present invention a positive contact between the back surface24of composite gain medium assembly12and surface44of the substrate46is maintained by a pressure differential between the higher pressure of the atmosphere58surrounding the amplifier module10and the lower pressure inside the microchannels48. The microchannels48can be filled with either liquid (including liquid metals) or gas and are maintained at a pressure substantially lower than that of the atmosphere58. One benefit of using liquid to fill the microchannels48is enhanced heat transfer due to increased thermal conductivity. The required pressure differential to maintain the surfaces24and44in contact over large portions of their areas is typically several tens of PSI. Such a continuous contact ensures that the back surface24will remain optically flat even when composite gain medium assembly12experiences significant thermal load. The continuous contact between surface24and surface44further facilitates the conductive transfer of heat from the gain medium assembly12to substrate46. The substrate46may be further installed in an optical mount60to facilitate easy positioning and alignment.

Apart from the contact between the optically flat surfaces24and44, which in itself provides a good seal, the atmosphere58can be further sealed from the microchannels48by an elastomeric seal62between the perimeter of the contact surface of composite gain medium assembly12and the surface44. Seal62may also hold the composite gain medium assembly12to the substrate46in the absence of a pressure differential, such as during non-lasing conditions. Using a compliant seal in this area also avoids restraining of the composite gain medium assembly12from thermal expansion during lasing and reduces thermal stresses therein. Suitable materials for the elastomeric seal62include RTV.RTM. silicon rubber. Other forms of compliant seals such as an O-ring may also be used. Thermal dSSDLge to the seal62potentially caused by a misalignment of incident laser beam64(FIG. 3) is prevented by a collimator66, which preferably absorbs laser radiation incident on the edge of the laser gain medium26. The collimator66may incorporate suitable cooling means to dissipate absorbed heat. Seal62is preferably protected from optical pump radiation36by extending the high-reflectivity optical coating40to cover at least that portion of the perimetral optical medium28which is in contact with the seal62.

Referring further toFIGS. 3 and 4, during lasing, optical pump source68, which preferably comprises an array of laser diodes, produces and directs optical pump radiation36into cylindrical lenses18. The cylindrical lenses18focus the radiation into the converging hollow ducts20. Internal surfaces72of the ducts20are made highly reflective to the optical pump radiation. Aided by reflections from surfaces72, the optical pump radiation36gradually increases in intensity as it progresses towards the tapered end of the duct20. Optical pump radiation36exiting the tapered end of the duct20enters the perimetral optical medium28and it is transmitted therethrough into the laser gain medium26. Tapered portion28b(FIG. 5a) of the perimetral optical medium28acts as a continuation of the duct20and further concentrates and channels pump radiation into the laser gain medium26. Upon entering laser gain medium26, pump radiation is channeled in a direction generally parallel to the surfaces22and24by multiple internal reflections therefrom. During passage through the laser gain medium26, the optical pump radiation36is gradually absorbed. This absorption process follows Beer's law: I(×)=I.sub.0 exp(−ax), where “×” is the distance into absorbing medium, “a” is the absorption coefficient, “I.sub.0” is the initial intensity of pump radiation, and “I(×)” is pump radiation intensity after traveling distance “×” in the absorbing medium. Preferably, the material of laser gain medium26is doped with absorbing species so that 90% or more of incident pump radiation36is absorbed in the laser gain medium26.

Optical radiation36absorbed by dopant species in laser gain medium26pumps the dopant species to a laser transition. This allows the laser gain medium26to serve as an amplifier of coherent optical radiation. The incident laser beam64, having approximately the same footprint as the aperture in the collimator66, is directed into the laser gain medium26at a generally normal incidence through front surface22and is amplified until it reaches the reflective coating40. On reflection from coating40, the laser beam passes through the laser gain medium26again but in a generally reverse direction. The amplified laser beam64′ exits the laser gain medium26in a direction generally normal to the front surface22. Waste heat dissipated in the laser gain medium26is conducted to the back surface24, through the optical coating40, and transferred to surface44of the substrate46from which it is conducted to the heat exchanger50. Since the front surface22of the laser gain medium26is operating at above ambient temperature, it heats the surrounding atmosphere58. This can lead to formation of pockets of warm atmosphere and/or eddy currents in the atmosphere58. As a result, the laser beam64can experience a variation of its optical path and its optical phase fronts could be perturbed. This situation can be prevented by flowing a stream of gas from atmosphere58over the front surface22, thereby sweeping the pockets of warm atmosphere from the front surface22and preventing the pockets from accumulating and/or forming eddy currents. A suitable stream of gas72can be generated, for example, by a gas jet74shown inFIG. 3, or by other suitable means.

1) Transport of pump radiation: Perimetral optical medium28receives concentrated optical radiation36from the tapered end of duct20and channels it into the laser gain medium26. In this respect, the perimetral optical material28serves as a continuation of the duct20. Surface34of material28can be curved to provide an additional lensing effect.

2) Thermal management of perimeter of the laser gain medium26: The perimetral optical material28is in good thermal contact with the laser gain medium26and provides a heat conduction path to substrate46. This allows it to draw heat away from the perimeter of the laser gain medium26which reduces thermal stresses and distortions therein.

3) Suppression of parasitic oscillations: The perimetral optical medium28is preferably chosen to have an index of refraction closely matching that of laser gain medium26. This allows ASE rays to cross the boundary between the two materials without significant refection. The shape of the perimetral optical medium28can be chosen so as to trap such ASE rays and/or channel them outside the composite gain medium assembly12. By so reducing the feedback of ASE rays from the boundary of composite gain medium assembly12, the feedback mechanism for parasitic oscillations is largely eliminated and oscillations can be suppressed. As already noted above, in some forms of the invention, the perimetral optical medium28can be doped with ions that are absorbing at a laser gain wavelength but not significantly absorbing at optical pump wavelengths. Absorption of ASE rays in the perimetral optical medium28accelerates their decay.

FIGS. 6a–6dshow examples of alternate constructions of the composite gain medium12.FIGS. 6cand6duse two layers of perimetral optical media28and28′. This allows each of the perimetral optical media28and28′ to be either undoped or doped differently with ASE absorbing ions.

FIG. 8is a front view of the amplifier module10showing optical pump source68, lenses18, and tapered hollow ducts20providing optical pump radiation36into the composite gain medium assembly12with octagonal doped laser gain medium26. The circular arrangement of pump source68is produced by placing laser diode arrays68aso as to generally point toward the center of laser gain medium26. This pump source arrangement makes it possible to achieve laser gain that is uniform across large portions of the laser gain medium26. Beamlets produced by individual laser diode elements (as for example shown inFIG. 2) in optical pump source68overlap inside the laser gain medium26and their intensities are summed. The resulting intensity of overlapped beamlets depends on the power output and beamlet divergencies of individual diode elements of each diode array68a,the distance of the diode elements from the center of the laser gain medium26, parameters of the lens18and the duct20, the doping density of laser gain medium26and the radial position (with respect to the center of laser gain medium26) of the location where the intensity is measured.

It is a principal advantage of the present invention that a uniform gain profile is produced in the laser gain medium26across the aperture defined by the collimator66. This is accomplished by choosing an appropriate combination of beamlet divergencies of individual diode elements in optical source68, by the distance of the diode elements of each diode array68afrom the center of the laser gain medium26, by the parameters of the lens18and the duct20, and by the doping density of laser gain medium26. Suitable methods for producing a tailored gain profile in a side-pumped SSDL are disclosed in Applicant's U.S. patent application Ser. No. 10/441,373 filed on May 19, 2003, which is hereby incorporated herein by reference.

FIGS. 9aand9bshow examples of how the radial variation of small-signal gain is affected by choices of divergence of optical pump sources68(in the plane parallel to surface22) and the distance of optical pump sources68from the center of laser gain medium26. Besides producing a uniform small-signal gain across the aperture, the present invention can also be used to provide nearly uniform gain when the medium is saturated. For example, when the invention is used to amplify laser beams with higher intensity in the central portion of the beam, the gain saturation effects near the beam center can be countered by appropriately increasing the pumping intensity (and hence small-signal gain) near the center of laser gain medium26. Specific examples of arrangements of optical pump sources and associated choices of divergence are presented in John Vetrovec, “Compact Active Mirror Laser (CAMIL),” SPIE volume 4630, 2002, and in John Vetrovec et al., “Development of Solid-State Disk Laser for High-Average Power,” in SPIE vol. 4968, 2003, both of which are hereby incorporated by reference.

An alternate version of the first preferred embodiment of the SSDL of the present invention is suitable for operation at increased optical power density. Referring again toFIGS. 3 and 4, in the alternative embodiment, the internal heat exchanger50inside substrate46can be omitted and the coolant52is provided to microchannels48and allowed to directly wet large portions of the back surface24of the composite gain medium assembly12. In this fashion, heat generated in the laser gain medium26is conducted through the surface24and the optical coating40directly into the coolant52. Coolant52is introduced into the microchannels48to provide a uniform cooling effect over a large portion of the back surface24of the gain medium assembly12. The pressure of coolant52is maintained lower than the pressure of atmosphere58to assure attachment of composite gain medium assembly12to substrate46as already explained above.

Yet another alternative to the first preferred embodiment of the SSDL of the present invention that is suitable for tight packaging is shown inFIG. 10.FIG. 10shows an active mirror amplifier module10′ wherein the optical pump sources68and the tapered hollow ducts20are mounted in closer proximity to the substrate46and optical mount60. The composite gain medium assembly12′ incorporates perimetral optical medium28′ having a surface70′ at approximately a 45 degree angle with respect to the surface24(FIG. 5a) of the gain medium assembly12′. The surface70′ has a coating74which is highly reflective at the optical pump radiation wavelengths. Surface34′ of the perimetral optical medium has a coating76which is antireflective at the optical pump radiation wavelengths. Optical pump radiation36is injected into the surface34′ and reflected from coating74into the laser gain medium26.

The efficiency of concentrating pump radiation in duct20can be further improved by stacking laser diode arrays68ain the plane of pump source68to directly point toward the surface34of the composite laser gain medium assembly12as shown inFIG. 11. This configuration of the pump source68also reduces the need for the lens72so that the lens can be omitted from the system. Furthermore, the invention can also be practiced with a solid material lensed duct such as disclosed by Beach et al. in U.S. Pat. No. 5,307,430 in lieu of the hollow duct20. Experience shows that owing to the higher index of refraction, a solid material lensed duct can be more efficient for concentration of pump radiation. However, one drawback of the solid lensed duct in high-average power applications is that low thermal conductivity of the solid duct material (typically optical glass) makes it difficult to remove heat generated therein by pump radiation. If optical pump sources68of sufficient intensity are used, then the need for the concentrator duct20is eliminated.

The invention can also be practiced with a transmissive laser gain medium disk (rather than reflective disk in AMA configuration) and without the substrate46. Referring now toFIG. 12, there is shown a SSDL amplifier module11in accordance with a second preferred embodiment of the present invention. The module11generally comprises a composite gain medium assembly12″, optical pump sources68, lenses18, and tapered hollow ducts20. The composite gain medium assembly12″ is essentially the same as the composite gain medium assembly12, except that both surfaces22and24now have the optical coating42which is antireflective at a laser gain wavelength. The composite gain medium assembly12″ is preferably suspended by the perimetral optical medium28, which is supported by optical mount60′.

The laser gain medium26is pumped to a laser transition by the optical pump sources68in the same manner as in the SSDL amplifier module10. This allows the laser gain medium26to serve as an amplifier of coherent optical radiation. The incident laser beam64, having approximately the same footprint as the aperture in the collimator66, is directed into the laser gain medium26at a generally normal incidence through surface24and is amplified. The amplified laser beam64′ exits the laser gain medium26in a direction generally normal to the surface22. It should be appreciated that the amplifier module11can be also practiced (i.e., positioned) at a Brewster angle with respect to the incident laser beam64. In this case, the antireflection coating42on surfaces22and24can be omitted.

Waste heat dissipated in the laser gain medium26is conducted to both surfaces22and24, through the optical coating42, and transferred to coolant52flowing generally parallel to the surfaces22and24. The coolant52and its flow conditions should be chosen so as to permit a high heat transfer rate while avoiding perturbation to the optical phase front and scattering losses of the laser beams64and64′. A preferred coolant is gaseous helium, although any other suitable coolant could be employed. Experiments with nitrogen and helium flows at subsonic velocities have shown that heat transfer rates on the order of several watts/cm2can be achieved at subsonic flow conditions around Mach 0.2 (see for example S. B. Sutton et al., “Thermal Management in Gas Cooled Solid-State Disk Amplifiers,” UCRL-JC-109280, which can be obtained from the technical library of the Lawrence Livermore National Laboratory.

FIG. 13is a front view of the amplifier module11showing optical pump sources68, lenses18, and tapered hollow ducts20providing optical pump radiation36into the composite laser gain medium assembly12″. The laser gain medium assembly12″ has circular doped laser gain medium26and octagonal perimetral edge28. The octagonal arrangement of pump sources68is produced by placing one or more laser diode arrays68aat each edge of the octagon so as to generally point toward the center of laser gain medium26. This pump source arrangement makes it possible to achieve laser gain that is uniform across large portions of the laser gain medium26(see, for example, J. Vetrovec, “Progress in the Development of Solid-State Disk Laser,” in proc of the 16thAnnual Solid-State and Diode Laser Technology Review, May 20–22, 2003, Albuquerque, N. Mex., paper no. HPAPP-6). It should be appreciated that the arrangement of diode sources68and the perimeter of perimetral medium28could have other polygonal shapes, or be generally circular as shown for example inFIG. 7, and as noted above. Furthermore, a laser gain medium26with a polygonal perimeter can be practiced with a perimetral medium of a different shape (namely circular or elliptical) and conversely, or they can both have the same general shape. An alternate configuration of the SSDL amplifier11that uses higher intensity optical pump sources68can be practiced without the lens18and hollow duct20.

The SSDL amplifiers10,10′ and11of the subject invention can be operated in a thermally continuous mode where the heat deposited into the laser gain medium26is removed in real time by coolant52, or in a semi-continuous heat capacity mode where the laser gain medium26is allowed to gradually warm up to a predetermined limiting temperature. A solid-state laser operating in the “heat capacity laser”, has been disclosed by Albrecht et al. in U.S. Pat. No. 5,526,372. In the heat capacity mode, prior to laser operation, the laser gain medium26is cooled to an initial operating temperature. During laser operation, the laser gain medium26gradually warms up until it reaches its final operating temperature. At that point the laser operation is suspended and the laser gain medium26is allowed to cool again to its initial operating temperature by transferring its stored heat to the coolant52. After reaching this temperature, the process can be repeated. In this fashion, the laser can be operated in a semi-continuous fashion. The length of the laser cycle depends on the rate at which waste heat is deposited into the laser gain medium, the weight and specific heat (cp) of the laser gain medium and the allowable temperature rise. The length of the cooling cycle depends on the effectiveness of the cooling applied to the laser gain medium. When the SSDL amplifier module10,10′ or11operates in the heat capacity mode, the heat can be extracted from the laser gain medium26during laser in operation at a much slower rate than the deposition rate during laser operation.

Any one of the SSDL modules10,10′, and11or variants thereof can be used to construct laser amplifiers as well as laser oscillators. In either case, multiple SSDL modules are required to obtain required laser gain and laser power. Boeing owned U.S. Pat. No. 6,603,793 discloses how SSDL modules in a AMA configuration (e.g.,10and10′) can be arranged in a laser device.FIG. 14shows how SSDL modules11using transmissive composite gain medium assemblies12″ can be used to construct an amplifier bank100. Several composite gain media12″ and associated optical pump sources68are placed adjacent to each other thereby forming between them passages (flow channels)90for flowing a cooling medium52. The spacing between adjacent composite gain medium assemblies12″ and the width of flow channels90are approximately of the same magnitude as the thickness of each gain medium assembly12″. The size of such spacing depends on the design of the perimetral optical medium28and the optical pump source68. In practice, such spacing preferably ranges from about 1 to 10 millimeters. The cooling medium in adjacent channels90can be flowed in the same or in opposite directions. Although the latter approach somewhat complicates the coolant medium delivery and recovery routing, it greatly reduces the transverse component in temperature gradient inside the gain medium26and makes it much easier to maintain alignment and beam quality of the laser beam64.

FIG. 15shows an SSDL amplifier bank100′ that reduces flow requirements for coolant medium52. Here the spacing of adjacent composite gain medium assemblies12″ is approximately the same as in the configuration shownFIG. 14. However, spacers92, mounted on holders80, are installed in the gap between adjacent composite gain medium assemblies12″, thereby reducing the width of the flow channel for coolant medium52. The preferred width of the coolant channels90′ is approximately in the range from 0.5 to 2.0 millimeters. The resulting reduction in coolant medium flow rates allows using a smaller coolant flow system including smaller sizes of the pumps, storage tanks, and piping. Spacers92are each preferably of the same thickness and made of the same material as the host material for the gain medium26and do not contain ions for lasing at the laser wavelength of the gain medium26. Alternatively, the spacers92can be made of a thermal optical material that offers low thermo-optic distortion. Furthermore, spacers92each preferably have an anti-reflective coating on both large faces to avoid excessive loss of power in laser beam64by reflection.

When the amplifier module100′ is operated in a heat capacity mode, spacers90provide additional reservoirs for heat storage. Efficient transfer of heat from each composite laser gain medium assembly12″ to its associated spacer90is made possible by flowing the coolant52through a narrow passage90. Heat storage capacity of spacers90can be increased by appropriately increasing their thickness.

Coolant52flowing in passage90removes heat from the composite laser gain medium assembly12″ and gradually warms up. As a result, down stream portion124of the composite laser gain medium assembly12″ is considerably higher in temperature than the upstream portion122. Temperature affects both the index of refraction and thermal expansion of the laser gain medium26, each in turn affecting the length of optical path through the medium. In the amplifier module100′, coolant52in adjacent gaps90flows in the same direction. As a result, the optical path of the rays near the edge164aof laser beam64is significantly different than the optical path of the rays near the edge164b.This causes an optical path difference (OPD) and perturbation to the optical phase fronts of laser beam64.

FIG. 16shows an arrangement of two amplifier modules100′aand100′bhaving their respective coolants52and52′ showing in opposite directions. This arrangement significantly reduces thermal distortion caused by coolant temperature rise. Another approach for mitigation of the OPD resulting from coolant temperature rise is to counter-flow the coolant.FIG. 17shows an amplifier module100″ where the coolant in adjacent passages90is flowed in opposite directions.

Another type of perturbation to the temperature profile in the laser gain medium26is caused by non-uniform deposition of optical pump radiation36. Computer simulations and laboratory testing performed by the applicant indicates that such perturbations acquire spatial distribution patterns that are related to the arrangement of optical pump sources68around the perimeter of composite gain medium. For example, if optical pump sources68are arranged in octagonal configuration such as shown inFIG. 14, the pump density distribution in the laser gain medium26can be expected to have at least slight spatial non-uniformities that form a generally octagonal pattern. Although the OPD caused by this effect in a single composite gain medium assembly12″ is typically a small fraction of laser wavelength, contributions from multiple disks can sum up to a very significant distortion of optical phasefront. It is well known that random but correlated error contributions add up arithmetically. In particular, if ε is a random statistical error experienced by laser beam64in each laser gain medium26, passage of the beam through N of such laser gain media having mutually correlated errors will result in a total (cumulative) phase front error εN=Nε.

Cumulative phase error can be greatly reduced by reducing the correlation of individual error contributions. In the amplifier module100,100′, and100″, errors in individual composite gain medium assemblies12″ operated with optical radiation sources68arranged in polygonal arrays (such as shown inFIG. 13) can be at least partially de-correlated by slightly rotating (clocking) about the laser beam64each polygonal array with respect to its neighbor. For example, if three octagonal arrays of optical radiation sources68(see e.g.,FIG. 13) in amplifier module100′, each array should be clocked by preferably about 15 degrees with respect to each other. In particular, the first array is clocked by 0 degrees, second array by 15 degrees, and the third array by 30 degrees. If laser beam64encounters random errors ε in each laser gain medium26, passage of the beam through N of such laser gain media having completely de-correlated errors will result in a total (cumulative) phase front error εN=N1/2ε.

The correction of a distorted phase front in laser beam64′ can also be achieved by passing the distorted laser beam through a phase corrector174. The phase corrector174is an optical element with one or more surfaces machined to reverse the effects of OPD in amplifier module100.

Coolant52, for operation of the amplifier modules11,100,100′ and100″, can be provided by a closed or open coolant supply loop. If the coolant flow passage90is made sufficiently narrow (preferably 0.5 to 1.0 millimeters) the coolant flow rate required to operate the amplifier module is rather modest. For example, a flow of helium at Mach 0.2 used for cooling composite gain medium12′ with a 10-cm diameter laser gain medium26and a 0.5-millimeter wide flow passage can extract heat from the laser gain medium at a rate of around 5 Watts per centimeter square. This translates to a flow of 14 standard liters per second per flow passage.

FIG. 18shows a laser device200comprising amplifier modules100′aand100′b,end mirror142and outcoupling mirror144forming an unstable resonator, static phase corrector148, and coolant loop146. Laser beam64″ oscillates between the mirrors142and144, and portion64′″ is outcoupled. The coolant flow loop146further comprises a compressor152, heat exchanger154and interconnecting lines. Warm coolant52exhausted from the amplifier modules100′aand100′bis collected into return line156which feeds the compressor152. The compressor152receives the coolant52, compresses it and discharges compressed coolant into heat exchanger154. The heat exchanger154thermally conditions the coolant to the appropriate temperature and conveys it into feed line158that delivers it to amplifier modules100aand100b.

FIG. 19shows a laser device200′ which is similar to laser device200except that the coolant loop162is open. The coolant loop162comprises a gas storage tank164, control valve166, and interconnecting lines. As demanded for operation of laser device200′, coolant52is released from the storage tank164by control valve166and flowed into the feed line168that delivers it to amplifier modules100′aand100′b.Coolant52exiting from the amplifier modules100′aand100′bforms an exhaust flow168which is vented to atmosphere.

FIG. 20shows an SSDL amplifier bank100′″ that reduces the time-averaged heat load to laser gain medium assemblies12″. The laser gain medium assemblies12″athrough12″ are respectively pumped by optical radiation sources68athrough68hsequentially rather than all at the same time. For example, first the optical pump radiation source68ais energized to pump laser gain medium assembly12′a.Subsequently the optical pump radiation source68ais turned off and, possibly after a pause, optical pump radiation source68bis energized to pump laser gain medium assembly12″b,and so on, and the process is repeated. Alternately, laser gain medium assemblies12″ can be pumped in groups. For example, laser gain medium assemblies12″a,12″c,12″e,and12″gare pumped as one group first and then turned off followed by pumping laser gain medium assemblies12″b,12″d,12″f,and12″h.Since coolant52is flowed preferably in continuous fashion, surfaces of all of the laser gain medium assemblies12″athrough12″ are utilized for heat removal.

FIGS. 21aand21bshow an SSDL amplifier bank100ivthat reduces the time-averaged heat load to laser gain medium assemblies12″. Similar to SSDL amplifier bank100′″ the optical pump in SSDL amplifier bank100ivis also provided to selected laser gain medium assemblies on alternating basis. However, in SSDL amplifier bank100ivthe selection is accomplished by a combination of mechanical and electrical means rather than solely by electrical excitation means. For example, inFIG. 21aoptical pump radiation sources68a,68c,68e,and68gare positioned and energized to pump laser gain medium assembly12″a,12″c,12″e,and12″g.After predetermined pump period (typically 1 to 10 seconds) the optical pump radiation sources68a,68c,68e,and68gare re-positioned and energized to pump laser gain medium assembly12″b,12″d,12″f,and12″h.After predetermined pump period in this position the cycle may be repeated. Alternate means for switching optical pump radiation to selected laser gain medium assemblies is by optical switching that can used reflecting and/or refracting components. Such components can be activated electrically and/or mechanically.