Evaluation of phase-only liquid crystal spatial light modulator for phase modulation performance using a Twyman-Green interferometer

In order to evaluate the phase modulation performance of a 256 x 256 pixel reflecting liquid crystal spatial light modulator purchased from the US Boulder Nonlinear Systems, we identify the linear range of phase shift and evaluate the spatial nonuniformity of the modulator by measuring both phase and intensity with a Twyman - Green interferometer. Experimental results show that the 50 - 210 grey scales linear ranges of the phase shift established by phase and intensity measurements are in good agreement with each other, which proves that more accurate phase modulation can be achieved. The inherent backplane curvature of the modulator is less than lambda/3 and the root- mean- square value of the phase nonuniformity across the modulator aperture is less than lambda/10, so the backplane curvature of the modulator is the main contributor to phase distortion due to the modulator. Analysing the deviation of the root-mean-square value of the phase nonuniformity indicates that the stability of the modulator decreases with increasing grey scales. It is therefore concluded that the modulator calibrated using a single interferometer can be used for beam steering, wave-front correction and transformation.