Measurement of Thermo-Elastic Deformation of an Optic using a Polarization Based Shearing Interferometer

TL;DR

This paper introduces a polarization-based shearing interferometer that measures thermo-elastic deformation of optics with high sensitivity, using a birefringent crystal for shear and an algorithm for wavefront reconstruction.

Contribution

It presents a novel polarization control method for shearing interferometry that enables full wavefront gradient measurement without precise alignment, and demonstrates high sensitivity in thermal deformation detection.

Findings

Achieved sensitivity better than λ/160 in measuring thermal deformation.

Demonstrated scale invariance of the interferometer's sensitivity.

Developed a method to correct for non-uniform spatial frequency response.

Abstract

A shearing interferometer is presented which uses polarization control to shear the wavefront and to modulate the interference pattern. The shear is generated by spatial walk-off in a birefringent crystal. By adjusting the orientation of the birefringent crystal, the components of the wavefront gradient can be independently measured to allow determination of the full wavefront vector gradient as well as reconstruction of the wavefront. Further, the monolithic nature of the crystal used for shearing allows the interferometer to be setup without need for precise alignment of any components. An algorithm incorporating homodyne detection is presented which analyzes the modulated interferograms to determine the components of the wavefront gradient, from which the wavefront is reconstructed. The thermal deformation of a mirror subject to heating from absorption of a Gaussian pump beam was accurately observed with a sensitivity better than \lambda/160. We show that this sensitivity is scale invariant, and present a method to account for the non-uniform spatial frequency response of the interferometer.