Compensation of inherent wavefront distortion in zero-twist LCoS spatial light modulators.

A good and complete calibration is the most important step in the electrically addressed spatial light modulator (SLM) initialization. It is clear that the quality and effectiveness of the optical component addressed to the SLM strongly depends on the knowledge of the device response. In fact, the signal must be modified before addressing it to the SLM to compensate for the distortions internally introduced by the device and hence, to eventually reproduce the optical component with the desired performance. In this work we deal with the spatial calibration of reflective zero-twist liquid-crystal on silicon (LCoS) SLM by testing the aperture of the device to characterize the inherent phase distortion. The inherent distortion of the reflective LCoS SLM may need not only the compensation for the backplane curvature but also for other possible non-uniformities caused by thickness variations of the liquid crystal layer across the aperture. Firstly, we build a global Look-Up Table (LUT) of phase modulation versus the addressed grey level for the whole device aperture. Secondly, when a lack of spatial uniformity is observed, we define a grid of cells onto the SLM aperture and develop a multipoint calibration. The relative phase variations between neighbour cells for a uniform grey level lead us to build a multi-LUT for an improved compensation. Multipoint calibration can be done using either phase-shift interferometry or Fourier diffraction pattern analysis of binary phase gratings. Experimental results that give evidence of the compensation progress are provided. We discuss the results obtained when the SLM is used to display diffractive optical elements, with and without compensation of inherent wavefront distortion, and either with global LUT or with multi-LUT. [ABSTRACT FROM AUTHOR]