Measurement of Laser-Induced Thermo-Elastic Deformation In an Optic Using Polarization-Based Lateral Shearing Interferometry

Abstract

A polarization-based shearing interferometer is presented that is capable of measuring the thermal deformation of a mirror subject to heating from absorption of a Gaussian laser beam. The shear is generated by spatial walk-o↵ in a birefringent crystal. By adjusting the orientation of the crystal, the components of the wavefront gradient can be independently measured to allow determination of the full wavefront vector gradate as well as reconstruction of the wavefront. Furthermore, the monolithic nature of the birefringent crystal allows non-critical alignment of the interferometer's components. The interferogram is modulated, and a homodyne detection algorithm analyzes the modulated interferograms to extract the components of the wavefront gradient from which the wavefront is reconstructed. The thermal deformation of the laser-heated mirror was accurately observed with a sensitivity better than /160. The sensitivity of this interferometer is scale invariant, and we present a method to account for the non-uniform spatial frequency response of the interferometer.