Wave front generation using a phase-only modulating liquid-crystal- based micro-display with HDTV resolution

摘要

Liquid-crystal (LC) based micro-displays can be used to modulate incoming light waves with respect to amplitude, phase and polarization. Twisted-nematic LC displays produce a combined phase-polarization modulation so that it is difficult to achieve pure phase modulation without amplitude modulation. We present a new phase-only modulating LCOS (Liquid Crystal On Silicon) spatial light modulator (SLM) based on an electrically controlled birefringence (ECB) liquid crystal mode. The device has a HDTV (1920x1080) resolution and a small pixel pitch of only 8μm (87% fill factor) on a digital silicon back plane. The LC molecules are aligned parallel to the electrodes and an applied electric field forces them to tilt towards the direction of the field. This leads to a pure phase modulation with a phase retardation of 2π for wavelengths between 420 and 1064nm, with negligible polarization change (＜ 1%) if the light is linearly polarized parallel to the director axis of the LC molecules. The shape of the back-plane of the LCOS micro-display was investigated using a Twyman-Green interferometer and the observed deviation from a plane surface was compensated by addressing the inverse spatially resolved phase retardation function. The interferometer was then used to measure wave fronts that were generated with the micro-display, representing optical elements like e.g. single lenses, lens arrays and tilted mirrors.