Transmission variations in liquid crystal spatial light modulators caused by interference and diffraction effects

We report on the characteristics of a newly developed high-resolution (640x 480 pixels) parallel-aligned liquid crystal spatial light modulator (LCSLM) as a function of wavelength. Phase-only operation over a range of 2 pi rad is easily achieved by operating at shorter wavelengths. We also measure an unexpected effect - the transmitted light intensity changes with applied v oltage. Our experiments show that thin-film interference and pixel diffract ion effects are responsible for this observed behavior. The diffraction eff ect is caused by a nonuniform electric field across each pixel. This nonuni form electric field introduces a blazing effect that changes the intensity distribution in the various diffracted orders as a function of applied volt age. These same kinds of effects have been observed with several other twis ted-nematic LCSLMs. Because of the complicated polarization effects caused by these twisted-nematic liquid crystal devices, however, the diffraction a nd interference effects are more easily studied using the parallel-aligned LCSLM. (C) 1999 Society of Photo-Optical Instrumentation Engineers. [S0091- 3286(99)01006-5].