A nematic liquid crystal phase and amplitude spatial light modulator for optical signal processing applications
Spatial light modulators (SLMs) are two-dimensional programmable transparencies for wavefront control; however, a significant disadvantage of SLM technology has been the lack of an SLM that independently and simultaneously modulates a wavefront's amplitude and phase. To circumvent this limitation, indirect encoding techniques are used to provide amplitude and phase control; however, indirect encoding techniques increase the computational requirements and reduce the SLM's useable space-bandwidth-product. In this dissertation we use liquid crystal display fabrication technology to produce an SLM with independent control of both the amplitude and phase. This new SLM provides independent phase and amplitude modulation of a 100 x 100 array of pixels with greater than 16 phase modulation levels and a contrast ratio greater than 1200:1. Scaling to larger array sizes is feasible.^ This research combines liquid crystal SLM technology, liquid crystal deformation theory, Fresnel image theory, and Fourier optics theory to solve interesting technical problems uncovered during this research. For example, the phase-only SLM was corrupted by liquid crystal inversion walls that formed near the pixel edges. These wavefront fidelity corrupting walls are experimentally characterized and then modeled. To avoid illumination of the walls, a Fresnel image array illuminator is used. Related Fresnel image investigations yielded new discoveries including a diagnostic technique for measuring the phase versus voltage SLM response, and a closed form relationship to describe Fresnel diffraction from spatially quantized tilt, spherical phase, and periodic grating transparency patterns. The diagnostic technique and the spherical phase modulation are experimentally demonstrated.^ Application related demonstrations of the SLM include two-dimensional optical phased array beam deflection, two-dimensional programmable lensing using a single phase-only SLM, and high fidelity wavefront reconstruction using the combined amplitude and phase SLM. In these demonstrations, the reduced active area of the pixelized SLMs produces significant higher order diffraction lobes. A simple aperture filling phase plate, fabricated using binary optic techniques, is shown to minimize the higher order diffraction lobes. The wavefront reconstruction fidelity remains high with the phase plate. ^
Engineering, Electronics and Electrical|Physics, Optics
William James Cassarly,
"A nematic liquid crystal phase and amplitude spatial light modulator for optical signal processing applications"
(January 1, 1990).
Dissertations available from ProQuest.