THERMALLY INDUCED OPTICAL DISTORTION IN HIGH-ENERGY LASER SYSTEMS

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 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.