1. Field of the Invention
The present invention relates to an article for reducing or eliminating parasitic oscillation (PL) and amplified spontaneous emission (ASE) in solid state lasers. More specifically, it relates to a thin film attached to strategic facets of a solid state laser gain material to frustrate the total internal reflection of parasitic oscillation and amplified stimulated emission.
2. Description of Related Art
The architecture of most high average power solid state lasers consists of a laser medium interposed between a highly reflecting mirror and an output coupler. Solid state zigzag slab lasers are typically used when high average power is needed because the optical path utilizes more gain medium and tends to average out index of refraction gradients caused by the thermal loading. Diode lasers or flashlamps are often used to optically excite the gain medium. In slab lasers, the medium is optically excited through the two largest exposed planes which are parallel to the optical axis. The laser medium must be cooled due, in part, to the high optical fluxes coupled into the slab by the pumping mechanism. If not properly designed, the stored energy density in slab lasers decreases. This performance reduction is caused by the complementary effects of amplified spontaneous emission (ASE) and parasitic oscillations that occur in the slab laser. [see for example, "Fluorescence Amplification and Parasitic Oscillation Limitations in Disk Lasers", by J. B. Trenholme, NRL Memorandum Rep.2480, July 1972; J. E. Swain et al., J. Appl. Phys., Vol. 40 p.3973, (1969); and J. M. McMahon et al., IEEE J. Quantum Electron., QE-9, p.992, (1973); and U.S. Pat. No. 4,849,036].
Edge claddings are used on solid state laser material to absorb the amplified spontaneous emission and to suppress the onset of parasitic oscillations that would otherwise reduce the stored energy. In general, these claddings consist of a material that is refractive index matched to the laser glass and which contains a dopant that absorbs at the laser (ASE) frequency. A number of different materials have been used for cladding, ranging from sprayed-on glass frits to liquids to castings of monolithic glass. [See, for example, G. Dube and N. L. Boling, Applied Optics, Vol. 13, p.699, (1974); G. Guch, Jr.,Applied Optics, Vol. 15, p.1453, (1976); and D. Milam et al., "Platinum Particles in the Nd:doped Disks of Phosphate Glass in the Nova Laser", Laser Induced Damage in Optical Materials 1985: Proceedings of the Boulder Damage Symposium, Boulder, Colo., November 1985] In the 100-kJ Nova pulsed laser system at Lawrence Livermore National Laboratory, claddings of monolithic glass doped with ionic copper that absorbs at 1 micrometer have been used. Although the performance of this latter cladding works well in high peak power lasers, the heat transfer properties are not adequate for use in high average power solid state lasers.
Parasitic lasing (PL) severely limits the performance of high gain solid state zigzag slab lasers by depleting energy in the upper laser level. The problem is enhanced because solid state laser materials have a higher index of refraction than the surrounding media. The traditional method for suppressing ASE and PL, as stated supra, has been to bind a relatively thick layer of weakly absorbing material with closely matched index of refraction to the edges. Index matching suppresses the reflection of light rays at the boundary between the two media, enabling the absorption of radiation within the bonding material. These traditional edge absorbers for ASE and PL suppression work adequately for high peak power, low average power lasers, but, due to unacceptable heat transfer properties, are not suitable for high average power lasers.
It is desirable to have a thin film attached onto the otherwise reflective surfaces of a solid state laser medium to bring about suppression of total internal reflections and reduction in the internal reflectivity for wide angular incidence without use of the doped index matching material. The present invention provides such an article.