Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
2000-02-01
2001-12-11
Davie, James W. (Department: 2881)
Coherent light generators
Particular beam control device
Nonlinear device
C372S036000
Reexamination Certificate
active
06330256
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to cooling techniques for multiple (at least two) nonlinear optical elements employed in laser systems, and, more particularly, to conductive face-cooled nonlinear optical elements for use in laser systems.
BACKGROUND ART
The process of frequency conversion in a nonlinear material generates heat within the nonlinear material medium due to absorption. This heat must be removed if the frequency converter is to operate efficiently at a significant power.
One method of heat removal in solid state crystalline materials employed in laser systems is to remove the heat from the sides of the materials, in a direction transverse to the direction of laser energy propagation. The removal of heat in a transverse direction causes thermal gradients in this direction. This creates two problems. The first problem is that thermal-optical stress and index variations cause thermal aberrations that distort the laser beam. The second problem is that, in most frequency conversion materials, for example, the temperature variation in a direction transverse to the direction of propagation of the laser beam must be maintained to within a very small tolerance range. The presence of a thermal gradient in this direction severely limits the aperture size and the power loading allowed in a laser system design.
U.S. Pat. No. 5,363,391, entitled “Conductive Face-Cooled Laser Crystal”, and issued to Steven C. Matthews et al on Nov. 8, 1994, discloses and claims techniques for passively removing heat from an optical element in a laser system through its optically transmissive faces. Heat is removed by way of optically transmissive heat conducting media disposed adjacent the optically transmissive surfaces of the optical element. Heat is transferred out of the optical element in a direction parallel to the direction of propagation of optical radiation, thus minimizing problems associated with thermal gradients. Devices employing optical elements such as nonlinear frequency conversion crystals and laser crystals may utilize the heat management approach to achieve better performance. Heat is transferred to the heat conducting media by direct contact or through narrow gas-filled gaps disposed between the optical element and the heat conducting media.
While that patent is well-suited for its intended use, improvements are sought to overcome certain remaining problems. Specifically, that patent teaches the use of a traditional dispersive material as a face-cooling medium. However, when two or more crystals are used for efficient second harmonic generation (SHG), for example, the dispersive medium causes the fundamental and the second harmonic beams to be dephased (out of phase with respect to each other) at the output of the face-cooling medium. If a second crystal is placed next to the face-cooling medium, the random phase could cause conversion from the second harmonic back into the fundamental, decreasing the effectiveness of the SHG process. This problem was overcome on earlier multi-crystal testbeds by using the dispersive nature of air to rephase the fundamental and second harmonic. This approach, however, requires separating the face-cooled crystal modules by an air path that is different for each individual product, requiring space (many centimeters of additional beam path) and adding to the manufacturing complexity (active adjustment of crystal spacing).
Thus, there is a need to provide a face-cooling method such as taught in U.S. Pat. No. 5,363,391, but for use with multiple nonlinear crystal formats used primarily for second harmonic generation without the need for air-path rephasing between the crystals.
DISCLOSURE OF INVENTION
In accordance with the present invention, the face-cooling method taught in U.S. Pat. No. 5,363,391 is used with multiple nonlinear crystal formats used primarily for second harmonic generation without the need for air-path rephasing between the crystals. One or more birefringent crystals are cut and oriented such that there is no dispersion between the fundamental and second harmonic wavelengths within each crystal. The birefringent crystals are then disposed in a heat-conducting housing, sandwiched between two or more nonlinear crystals and used as the face-cooling medium. The multiple crystal assembly may be further sandwiched between optically transmissive windows which need not be birefringent or non-dispersive, these windows being used to protect the outermost nonlinear crystals and/or provide additional face cooling. This causes the heat generated in the nonlinear crystals by absorption at the fundamental and second harmonic wavelengths to flow longitudinally (direction of beam propagation) into the face-cooling medium, thereby minimizing any transverse thermal gradient in the nonlinear crystals and the attendant dephasing loss. The crystals can be dry stacked with a very small gas-filled gap as taught in U.S. Pat. No. 5,363,391, immersed in a liquid or gel of suitable refractive index, bonded with suitable optical cement, optically contacted, or diffusion-bonded together to form a composite crystal. For example, MgF
2
is used as a specific case, but the invention is not limited to one particular medium. Other suitable candidates are listed, although it is not an exhaustive list by any means.
Specifically, an optical device for use in a laser system is provided, the optical device comprising:
(a) a heat-conducting housing;
(b) at least two nonlinear optically transmissive optical elements having first and second surfaces disposed in the heat-conducting housing for propagating laser energy in a direction substantially transverse to the plane of the first and second surfaces;
(c) at least one birefringent optically transmissive window cut and oriented so that there is no dispersion between the optical fields involved in the nonlinear interaction occurring within said nonlinear optical elements and that comprises an optically transmissive heat sink, each window disposed adjacent each nonlinear optical element such that each birefringent window is sandwiched by two nonlinear optical elements; and (optionally);
(d) one or two optically transmissive windows that may comprise optically transmissive heat sinks, each window disposed adjacent the exposed faces of the outermost nonlinear optical elements (the outer windows need not be birefringent or oriented for non-dispersion).
Heat generated in the optical elements is passively conducted to the optically transmissive windows in a direction essentially parallel to the direction of laser energy propagation and then to the housing through the optically transmissive windows.
REFERENCES:
patent: 5363391 (1994-11-01), Matthews et al.
patent: 6101201 (2000-08-01), Hargis et al.
patent: 6134258 (2000-10-01), Tulloch et al.
David Eimerl, “High Average Power Harmonic Generation”,IEEE Journal of Quantum Electronics, vol. QE-23, No. 5, May 1987, pp. 575-592.
V.D. Volosov et al, “Suppression of degenerate parametic processes limiting frequency-doubling efficiency of crystals”,Sov. J. Quantum Electron, vol. 6, No. 10, Oct. 1976, pp. 1163-1167.
Digest of Technical Papers, Conference on Lasers and Electro-Optics, Apr. 26-May 1, 1987, OSA/IEEE, Baltimore, Maryland, pp. 258-259.
Mary A. Norton et al, DK*P Frequency Doubler for High Average Power Applications,SPIE, vol. 1223, pp. 75-83, (1990) (No Month).
Marvin J. Weber, Ph.D., “CRC Handbook of Laser Science and Technology, Supplement 2: Optical Materials”, p. 603 (No date).
P.A. Studenikin, “GdVO4as a new medium for solid-state laser: some optical and thermal properties of crystals doped with Cd3+, Tm3+, and Er3+ions”,Quantum Electronics, vol. 25, No. 12, pp. 1162-1165 (1995) (No Month).
Marvin J. Bwver, Ph.D., “CRC Handbook of Laser Science and Technology, vol. V, Part 3: Applications, Coatings, and Fabrication”, pp. 304-316 1987, (No Month).
Byren Robert W.
Sumida David S.
Alkov Leonard A.
Davie James W.
Lenzen, Jr. Glenn H.
Raufer Colin M.
Raytheon Company
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