Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
2001-01-22
2003-09-23
Paladini, Albert W. (Department: 2125)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S035000, C372S072000, C372S075000
Reexamination Certificate
active
06625193
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to solid-state lasers, and more particularly to an active mirror amplifier laser having a side-pumped gain medium disposed in contact with an actively cooled substrate.
2. 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 last decade, greatly reduces the amount of waste heat and paves the way for development of a HAP-SSL with good beam quality. 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 “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 the generation of a 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 an AMA were 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. The thin disk laser uses a gain medium disk which is several millimeters in diameter and 200-400 &mgr;m 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 patent application Ser. No. 99/505,399 titled Active Mirror Amplifier System and Method for a High-Average Power Laser System, hereby incorporated by reference, discloses a new AMA 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 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 teachings of this patent application provide numerous advantages over prior art SSL and allow generation of near diffraction limited laser output at very high-average power from a relatively compact device.
The above-mentioned patent application Ser. No. 99/505,399 also 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. The former method is often referred to as “face pumping” and is further elaborated on in J. Vetrovec, “Diode-pumped Active Mirror Amplifier For High-Average Power,” in proc. from Lasers 2000 Conference held in Albuquerque, N.Mex., Dec. 4-8, 2000. This publication describes a face-pumped AMA with pump radiation from a diode array injected into the laser gain medium through an optically transparent substrate.
To make face pumping efficient, the AMA disk must absorb a large fraction of the pump radiation injected. This condition can be met by a certain combination of disk thickness and doping density of lasant ions. However, in many cases of interest it is impractical (or undesirable) to make the necessary increase in disk thickness or lasant doping level. For example, doping a yttrium-aluminum garnet (YAG) crystal with neodymium (Nd
3+
) ions beyond about 1.5% of atomic concentration is known to reduce the fluorescence time, broaden the line-width, and excessively stress the crystal due to a mismatch in size between the Nd atoms and yttrium atoms (the latter being replaced in crystal lattice). Increasing the disk thickness is often undesirable as it also increases thermal impedance and leads to higher thermal stresses. These considerations limit design parameters of face-pumped AMA to a relatively narrow regime. Face pumping is also impractical in conjunction with ytterbium (Yb
3+
) lasant ions, which require very high pump intensities to overcome re-absorption of laser radiation by the ground energy state. For example, a 2.5 mm-thick AMA disk made of YAG crystal would require about 10% atomic doping concentration of Yb
3+
ions to absorb 90% of face-injected pump radiation in two passes. Such a high Yb concentration would require an unreasonably high pump intensity of about 34 kW/cm
2
to induce medium transparency at 1.03 &mgr;m wavelength, and several times this level to efficiently operate the laser. In this situation, injecting the pump radiation into the disk side (i.e., edge or perimeter) becomes an attractive alternative. 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. This in turn reduces requirements for pump radiation intensity.
While side-pumping may be 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 AMA 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.
1. Concentration of Pump Radiation
Modern SSL are optically pumped by semiconductor laser
Harness & Dickey & Pierce P.L.C.
Paladini Albert W.
The Boeing Company
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