Development of a Piezoelectric Adaptive Mirror for Laser Beam Control

Abstract

An intra-cavity adaptive mirror is developed for compensation of time-dependent phase aberrations to a laser beam, such as those caused by thermal lensing. The unimorph-type device consists of a metallic disc, with a mirror finish, bonded to a piezoelectric disc, providing a small, low-cost deformable mirror for this application. The mirror is required to be able to deform in the shape of each of the lower order Zernike polynomials, which describe aberrations in optical systems. Numerical modelling is employed to predict the deformation shapes that can be achieved by a unimorph mirror with a particular electrode pattern. The results from a Rayleigh-Ritz model and a finite element model, employing elements including rotational degrees of freedom, are compared to results from a conventional finite element model. The finite element model is applied to model a prototype deformable mirror and produced good agreement with experimental results. Finally, a mathematical optimization routine is proposed to predict the optimal electrode configuration on the free electrode.