Coherent light generators – Particular active media – Plural active media or active media having plural dopants
Reexamination Certificate
2001-02-13
2004-10-26
Jackson, Jerome (Department: 2815)
Coherent light generators
Particular active media
Plural active media or active media having plural dopants
C372S066000, C372S040000, C372S035000
Reexamination Certificate
active
06810060
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to laser amplifiers, and more particularly to a solid-state laser incorporating multiple disk-shaped laser gain media (subapertures) placed adjacent to each other to fill an optical aperture of an AMA module.
BACKGROUND OF THE INVENTION
Thermomechanical effects present a major challenge to scaling of a solid-state laser (SSL) to high-average power (HAP). In particular, optical distortions caused by transverse temperature gradients (i.e., perpendicular to laser beam axis) are known to degrade beam quality, which may render the beam useless for many important applications. A class of SSL known as “active mirror amplifier” (AMA) originally disclosed by Almasi et al. in U.S. Pat. No. 3,631,362 (1971) has shown effective reduction of transverse temperature gradients and demonstrated generation of laser output with very good beam quality. See, for example, J. Abate et al., “Active Mirror: A Large-Aperture Medium Repetition Rate Nd:Glass Amplifier,” Appl. Opt. Vol. 20, no. 2, 351-361 (1981) and D. C. Brown et al., “Active-Mirror Amplifier: Progress and Prospects,” IEEE J. of Quant. Electr., vol. 17., no. 9, 1755-1765 (1981). 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 HAP 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 AMA have been met with encouraging but limited results. In recent years, the AMA concept has been a revived in the form of a “thin disk laser” introduced by Brauch et al. in U.S. Pat. No. 5,553,088 (1996). The thin disk laser uses a gain medium disk 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). A 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 first co-pending patent application Ser. No. 99/505,399 entitled “Active Mirror Amplifier System and Method for a High-Average Power Laser System”, which is hereby made a part hereof and incorporated herein by reference, discloses a new AMA concept suitable for operation at high-average power. This invention uses a large aperture laser gain medium disk about 2.5 mm in thickness and with a diameter typically between 5 and 15 cm, mounted on a rigid, cooled substrate. Note that the disk thickness in this AMA concept is about 10 times less than in the classical AMA 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 above-mentioned patent application Ser. No. 99/505,399 teaches two principal methods for providing pump radiation into the AMA disk, namely: 1) through the large (front or back) face of the disk, or 2) through the sides (edges) of the disk. AMA using the former method, which is often referred to as “face pumping,” is further elaborated, for example, in J. Vetrovec, “Active mirror amplifier for high-average power,” to be published in SPIE vol. 4270 (2000).
FIG. 1
shows such a face-pumped AMA where pump radiation from a diode array is injected into the laser gain medium through an optically transparent substrate.
The applicant's second co-pending patent application, entitled “Side-Pumped Active Mirror Solid-State Laser for High-Average Power”, Docket No. 00-173 filed on Jan. 22, 2001, which is hereby made a part hereof and incorporated herein by reference, discloses a composite AMA wherein optical pump radiation is injected into the peripheral edge of a composite gain medium disk. Side-pumping takes advantage of the long absorption path (approximately the same dimension as disk diameter), which permits doping the disk with a reduced concentration of lasant ions and a corresponding reduction in pump radiation intensity. The composite gain medium is formed by bonding an undoped optical medium to the peripheral edges of the laser gain medium disk. This construction facilitates improved coupling between the source of optical pump radiation and the laser gain medium, as well as concentration of optical pump radiation, cooling of the peripheral edge of the laser gain medium disk, and providing a trap for amplified spontaneous emission (ASE). In that invention, sources of optical pump radiation are placed around the perimeter of the composite gain medium. Tapered ducts may be disposed between the sources of optical pump radiation and the composite gain medium for the purpose of concentrating optical pump radiation. With the proper choice of laser gain medium doping, pump source divergence and geometry, a uniform laser gain is achieved across large portions of the gain medium.
The teachings of the two above-mentioned co-pending patent applications of the Applicant provide numerous advantages over prior art SSL and allow generation of near diffraction limited laser output at very high-average power from a relatively small device. However, these co-pending patent applications disclose only AMA modules utilizing single monolithic laser gain medium covering the device optical aperture. The term “aperture” as used herein is the one typically used in optics, namely: “The diameter of the objective of a telescope or other optical instrument”, as defined in McGraw-Hill Dictionary of Scientific and Technical Terms, 4
th
edition, published by McGraw-Hill, Inc.
To obtain higher average laser power from an AMA module, it is beneficial to increase the size of the optical aperture. However, the size of a single monolithic laser gain medium disk required to fill the optical aperture is limited by available fabrication technology. In particular, YAG crystal boules can be reliably grown only to about 5 cm diameter and GGG crystals to about 15 cm diameter. The difficulties, limitations, and cost of growing large crystals pertinent to the subject invention are discussed, for example, in D. Dawnes, “Nd:YAG—The Versatile High-power Solid-state Laser Crystal,” published in Industrial Laser Review in March 1995.
Another consideration associated with using large AMA disks is a tradeoff between transverse dimensions of the disk and the producible laser gain. In particular, a larger size AMA disk can produce higher average laser power but to avoid excessive ASE losses, this must be done with much lower gain than a comparably smaller AMA disk. This can be a significant limitation when optimizing an ultrahigh-average power system, where a very large number of AMA stages with monolithic apertures would be needed to meet a particular output power and gain requirements for efficient operation of a laser resonator.
SUMMARY OF THE INVENTION
The present invention provides an apparatus and method for achieving improved performance in a solid-state laser. The solid-state laser of the present invention uses multiple disk-shaped laser gain media (subapertures) placed adjacent to each other to fill an optical aperture of an AMA module. The perime
Harness & Dickey & Pierce P.L.C.
Jackson Jerome
Landau Matthew C
The Boeing Company
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