The fracture mechanisms of slab lasers are examined and the critical defects, induced during optical fabrication, are identified. A rationale for determining an appropriate operating stress for the slab laser is outlined, based on Weibull statistics, and this method is experimentally assessed in full-sized slab fracture tests. Techniques for achieving strong slabs are then examined. We determine that strengthening by subsurface damage minimization has the highest potential for strengthening, but that slab durability must also be enhanced in order for the slab to remain strong in practice. Good chemical durability is achieved by the use of water-proof overcoats. Good mechanical durability is achieved by the use of compressive surface layers. The compressive surface layers prevent the deterioration in slab strength from physical damage to the slab surface.
Recent advancements in high repetition rate and high average power laser systems have put increasing demands on the development of improved solid state laser materials with high thermal loading capabilities. We have developed a process for strengthening a commercially available Nd doped phosphate glass utilizing an ion-exchange process. Results of thermal loading fracture tests on moderate size (160 x 15 x 8 mm) glass slabs have shown a 6-fold improvement in power loading capabilities for strengthened samples over unstrengthened slabs. Fractographic analysis of post-fracture samples has given insight into the mechanism of fracture in both unstrengthened and strengthened samples. Additional stress analysis calculations have supported these findings. In addition to processing the glass' surface during strengthening in a manner which preserves its post-treatment optical quality, we have developed an in-house optical fabrication technique utilizing acid polishing to minimize subsurface damage in samples prior to exchange treatment. Finally, extension of the strengthening process to alternate geometries of laser glass has produced encouraging results, which may expand the potential for strengthened glass in laser systems, making it an exciting prospect for many applications.
Gadolinium scandium gallium garnet (GSGG) codoped with neodymium (Nd) and chromium (Cr) is a new laser material that can make more efficient lasers. I compare it to Nd-doped yttrium aluminum garnet (YAG) for two types of systems: large zig-zag slab lasers and small rod lasers. Nd:Cr:GSGG has a significant advantage over Nd:YAG in large systems because of its high efficiency and availability in large sizes. In small rod systems, though, the larger thermal lensing and birefringence in Nd:Cr:GSGG make it less desirable than Nd:YAG from an optical standpoint, but its high efficiency means that laser system size and weight can be reduced. I review progress in obtaining large Nd:Cr:GSGG slabs. In this effort, Allied-Signal Corp. has successfully grown 5-inch diameter Nd:Cr:GSGG boules.
Floating zone technique using an arc image furnace was applied for growing Ti3+ :Al203 single crystals with high doping levels. Up to 1 atom % of Ti was incorporated with relatively uniform concentration distribution. The disturbing optical absorption around 800 nm was found to increase parabolically with Ti concentration and its possible origin was suggested in relation to clustering in the crystal. Detection of no appreciable inclusions in the crystal eliminated the possibility of the scattering loss. The concentration quenching of the fluorescence lifetime was found to take place beyond 0.3 atom %.
A moving slab geometry Nd:Glass laser is capable of operating at much beyond the thermal stress fracture limit for a conventional fixed slab laser. Average power output of 43.8 W input power and at 2.06% slope efficiency in the first prototype. The moving slab laser has the potential for scaling to kilowatt average power levels.
The zigzag slab geometry is an established method of reducing thermo-optical effects in solid-state lasers. Achieving predicted improvements requires careful control of the slab temperature distribution. Our conduction cooled zigzag laser design is an attempt to provide a Nd:Glass slab with uniform pumping and cooling within a protective environment. This paper reports the results of initial tests of this design.
Lasers for some space applications have operational requirements for lifetime, efficiency and power output which can only be met by development of all solid-state laser sys-tems. Laser diode pumping of neodymium host materials has demonstrated good efficiency and lifetime at low power levels. This paper addresses the issues involved in scaling to the kilowatt power level. A design configuration for stand-off coupling of rack and stack laser diode arrays to a laser slab is presented which satisfies all of the above criteria. Design examples for near term efficiencies of >5% and long term efficiencies >15% for both cw and pulsed type lasers in the one kilowatt range are used as illustrations of the technique.
It is well-known that under uniform heating, a thin plate develops a quadratic temperature profile with the thermal gradient normal to the plate surface (Figure 1). The central part of the plate is warmer than the surfaces, and also undergoes thermal expansion. The thermal expansion normal to the surface produces no stress, but the thermal expansion parallel to the plate surface puts the surface in tension and the central part in compression. A simple but useful model of the plate treats it as a set of three isothermal plates arranged in the order cold-warm-cold (Figure 2). The warm part is AT warmer than the cold surfaces, and left to itself it would expand by aAT. Being constrained by the surface, it is in compression. If the warm part had infinite modulus, the resulting strain in the surface would be aAT. Because it relaxes, the actual surface strain is reduced by 1/3. By Hooke's law the stress is proportional to the strain, so that the surface stress is simply 2/3.EaAT, where E is Young's modulus.
Techniques and results for high second harmonic conversion efficiencies (>60%) of solid state lasers, using a single KD*P crystal are presented. This 60% approaches the theoretical limit for beams with both a Gaussian temporal and spatial profile. Designs using segmented transverse flow cooled plates, which are capable of handling the high average powers available from slab laser systems, are also discussed.
Slab lasers offer an inherent advantage over conventional devices when operated in the total internal reflection mode. Further improvements can be gained with these devices using SBS Optical Phase Conjugation. This paper describes experimental and theoretical studies of this combined approach for several geometries. Limitations imposed by thermal distortion, passive losses, birefringence, optical damage, parasitics and extraction efficiencies are addressed. Non-linear frequency conversion with emphasis on improved beam quality is also discussed.
Parasitic oscillations and amplified spontaneous emission (ASE) can often strongly influence the operation and efficiency of laser devices, as has been shown previously for disk and active-mirror amplifiers. Here we report the first comprehensive investigation of those phenomena in total internal reflection (TIR) face-pumped lasers. The results to be presented here were made possible by the development of two three-dimensional computer codes. The first (PARA) systematically searches for parasitic oscillations in slab lasers and determines the gain required to reach threshold. Our second code (AMSPE) is a three dimensional raytrace model which includes temporal gain and allows for non-uniform gain profiles. AMSPE calculates the gain depletion as well as changes in spatial gain profile and thus the decrease in amplifier efficiency as a function of a number of critical parameters such as slab aspect ratio, spontaneous emission spectral profile, and slab face angle. In this paper we first review the classes of parasitics in slab lasers and show how symmetry breaking can significantly increase the energy storage capability of such deyices. We then review the construction of the AMSPE code and show how it may be used to identify maximum efficiency slab laser configurations.
Results are presented for direct solar pumping of a Nd:YAG rod laser. Stable CW output of more than 60 watts was obtained with slope efficiencies exceeding 2%. Results are consistent with predictions based on a simple solar laser model we have developed. Using this model, performance projections and design concepts for higher power and higher efficiency solar-pumped solid state lasers are presented. It is shown that existing laser materials with broadband absorption characteristics (e.g., alexandrite and Nd:Cr:GSGG) can have better than 10% overall conversion efficiencies when solar pumped. The utility of solar lasers for various laser applications in space is briefly discussed.
A ground-based DIAL sensor for demonstrating vertical atmospheric water vapor meas-urements, in the 940 nm region, is described. The system is based on a tunable Nd:glass laser and a hydrogen Raman Cell, and has the potential for being suitable for Space applications. Preliminary results in the development of the 940 nm transmitter and atmospheric backscatter measurements are presented.