Coherent light generators – Particular temperature control
Reexamination Certificate
1998-04-30
2001-02-27
Font, Frank G. (Department: 2877)
Coherent light generators
Particular temperature control
C372S034000, C372S089000, C372S075000, C372S035000, C372S072000, C372S038060, C372S051000, C372S055000, C372S059000, C372S036000
Reexamination Certificate
active
06195372
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to solid-state lasers. More particularly, this invention pertains to the thermo-mechanical properties of the optical material YAG, an acronym for yittrium aluminum garnet (Y
3
Al
5
O
12
), the most commonly used solid-state laser material in the world.
Conventional YAG lasers, diode-pumped or flashlamp-pumped, have limits to the amount of average power they can produce. The limits are determined by the amount of heat generated during the optical pumping process, and by the strength of the material being pumped. Heat generation in such lasers is described in T. Y. Fan, “Heat Generation in Nd:YAG and Yb:YAG”, IEEE Journal of Quantum Electronics, Volume 29, pages 1457-1459, 1993 and D. C. Brown, “Heat, Fluorescence, and Stimulated-Emission Power Densities and Fractions in Nd:YAG”, IEEE Journal of Quantum Electronics, Volume 34, pages 560-572, 1998.
During high-average-power operation, a great deal of heat is generated. Heat is normally removed from the barrel of the rod or the surfaces of a slab. The rod or slab is conventionally cooled by flowing temperature controlled water through a surrounding channel or sheath. This causes a temperature gradient between the edge of the rod or slab which is coolest and the center which is hottest. The temperature gradient in turn produces stresses in the rod or slab that can become comparable to the intrinsic strength of the YAG. The strength of YAG is lowest on its barrel and faces that have been optically finished or ground. Rod barrels are typically ground but can also be polished. In either case defects, scratches and voids are found in the barrel that reduce the strength. Because polished surfaces have statistically smaller defects and scratches, they have higher strength than ground or roughened barrels. Reference is made to J. Marion, “Strengthened Solid-State Laser Materials,” Applied Physics Letters, Volume 47, pages 694-696, 1985. Barrel roughening is often performed to eliminate or reduce parasitic oscillations in pulsed high-gain lasers. For CW lasers, all polished surfaces are usually acceptable.
When the thermally-induced stresses on the rod barrel become equal to the strength of the surface, the YAG material shatters and must be replaced. This limits the average power to input pumping below the fracture-limit. The amount can be calculated exactly and varies with the optical material composition, the size of the rod and the way in which the barrel surface has been prepared. The material thermal expansion coefficient, thermal conductivity, Poisson's ratio and Young's modulus figure in the calculations.
SUMMARY OF THE INVENTION
It has been discovered that it is possible to significantly increase the average power capability of a rod of YAG by cooling the material with fluids at low or cryogenic temperatures. Finally, it is also found that if a rod is operated with the same heat power density at 77° K as at 300° K, the thermal distortions due to thermal effects are significantly reduced. Similar characteristics are achieved in slab-type laser systems.
Briefly stated, the invention in a preferred form is a system for increasing the average power capability of a solid-state laser by cooling the laser crystal to a cryogenic temperature and then pumping the laser crystal. Cryogenic fluid is circulated along a path which is disposed in heat exchange relationship with the laser crystal. For embodiments which employ a rod, the fluid path is generally parallel to the longitudinal axis of the rod. For slab-type laser configurations, the fluid path is generally parallel to the plane of the slab and may be disposed on opposing sides of the crystal slab. The laser crystal may be pumped along a radiation path that traverses through the cryogenic fluid as well as some of the structures which define the cryogenic fluid channels.
The invention has applicability in connection with pumping sources which employ a diode, a diode array, a flashlamp, a laser or multiple lasers. The solid-state lasing crystal may comprise YAG with a lasing ion selected from the group consisting of Nd, Yb, Er, Tm, Ho and Cr. The lasing element may be of a rod, slab, active-mirror or disc amplifier form.
A fluid circuit connects a cryogenic reservoir with the laser pump chamber. A closed loop or open loop circulation system may be employed. For some embodiments, a refrigerator is disposed in the circuit connecting the cryogenic reservoir and the laser pump chamber. The pump chamber is internally configured to define the cryogenic fluid flow path in a manner which provides highly efficient pumping and cooling of the lasing element. The structures which define the cryogenic fluid flow path are constructed from materials which are optically transparent at the pump wavelength.
An object of the invention is to provide a new and improved laser system which has increased average power output capability.
Another object of the invention is to provide a new and improved solid-state laser system which operates in an efficient manner to produce a favorable power output by optimizing certain thermo-mechanical properties of the lasing element.
A further object of the invention is to provide a new and improved solid-state laser system which incorporates an efficient and cost effective cooling system for increasing the average power capability of conventional lasing crystals.
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Alix Yale & Ristas, LLP
Flores Ruiz Delma R.
Font Frank G.
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