Thermally Induced Optical Distortion In High-Energy Laser Systems

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Abstract
High-energy laser (HEL) systems involve an optical train consisting of mirrors and windows, which may compromise the system's operation because of unavoidable irradiance-mapping aberrations resulting from the absorption of some fraction of the incident laser-beam energy. This paper describes an analytical investigation of relevant processes and discusses how laser-driven mirror/window distortions may affect the performance of HEL systems with regard to focal intensities and on-target fluences. An approximate expression for the brightness at the Gaussian focus is derived on the basis of a far-field degradation model and shown to be simple enough to allow HEL system designers to assess the capability of a contemplated optical train and to evaluate its behavior as a function of beam-power level and laser run-time. Two figures of merit are introduced for the purpose of characterizing the response of power-optics mirrors and windows in terms of thermally induced wavefront errors. For cooled mirrors, it is proposed to consider a figure of merit FoMm = K'eff/ (Am), where K'eff relates to the thermal conductivity, the heat-transfer coefficient, and the thickness of the faceplate, whereas AM measures the mirror absorptance, and E is the distortion coefficient. Similarly, the window figure of merit is FoMw = Cp/ (AW'x), where C'P represents the specific heat per unit volume, AW measures the window absorptance, and x is a distortion coefficient as defined in earlier investigations. On using these figures of merit, an important result emerges: if the window material has a positive distortion coefficient, window lensing suppresses the steady-state mirror-induced aberration at time tc = N(D/d)2FoMw/FoMm, where N is the number of relay mirrors and D/d is the telescope magnification. Under normal circumstances, the far-field irradiance exhibits strongly nonlinear features. Refocusing can eliminate the effect of mirror distortion but, ultimately, window distortion dominates and

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