Optical EngineeringMaterials for high-power laser optics: figures of merit for thermally induced beam distortions
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High-power, high-energy laser (HEL) systems include an optical train consisting of mirrors and windows, which must be capable of transporting and directing the beam without seriously degrading the nominal performance. Since catastrophic failure modes are not a major threat at beam-power levels of current interest, the system’s performance as measured in terms of achievable target irradiances may degrade as a result of thermal lensing, that is, the wavefront distortion caused by thermally induced phase aberrations. Analytical investigations that address the problem of evaluating the nature of laser-driven mirror/ window distortions are reviewed and updated. In this context, a method is proposed for obtaining simple figures of merit (FoM) for rating the thermal lensing performance of mirror-substrate materials as well as window-material candidates. The performance of cooled HEL mirrors reflects their ability to minimize irradiance-mapping wavefront distortions, which leads to defining a thermal distortion coefficient j that controls the out-of-plane growth of the faceplate. It is then straightforward to derive equations for characterizing the root mean squared surface deformation and to assess the merit of mirror-faceplate materials in a pulsed or a cw environment. Since state-of-the-art heat exchangers exhibit relatively modest Biot numbers, the thermal conduction is not a critical parameter but the modulus of elasticity must be properly factored into the FoM for cw operation. Window-induced wavefront deformations require special attention because they involve not only position-dependent variations of the window thickness but also position- and polarization-dependent variations of the refractive index. This situation leads to introducing symmetric and antisymmetric distortion coefficients, which can be combined into an effective optical distortion coefficient x that specifies the relative weight of birefringence compared to all other sources of distortion and shows that zero distortion can be achieved only with stressbirefringence- free material having a negative dn/dT. As in the case of mirror-faceplate materials, FoMs for the prediffusion and the steadystate regimes emerge in a direct manner and demonstrate that fluorozirco- aluminate glass by far outperforms other window-material candidates in implementing the zero-distortion goal.