Abstract
Multiple internal reflection effects on the optical modulation of a commercial reflective parallel-aligned liquid-crystal on silicon (PAL-LCoS) spatial light modulator (SLM) are analyzed. The display is illuminated with different wavelengths and different angles of incidence. Non-negligible Fabry-Perot (FP) effect is observed due to the sandwiched LC layer structure. A simplified physical model that quantitatively accounts for the observed phenomena is proposed. It is shown how the expected pure phase modulation response is substantially modified in the following aspects: 1) a coupled amplitude modulation, 2) a non-linear behavior of the phase modulation, 3) some amount of unmodulated light, and 4) a reduction of the effective phase modulation as the angle of incidence increases. Finally, it is shown that multiple reflections can be useful since the effect of a displayed diffraction grating is doubled on a beam that is reflected twice through the LC layer, thus rendering gratings with doubled phase modulation depth.
Highlights
Parallel-aligned liquid-crystal spatial light modulators (PAL-LC-SLM) may be ideally regarded as pixelated linear retarders with tunable retardance, useful for applications where pure phase modulation is a key feature [1]
PAL-SLMs ideally offer pure phase modulation when illuminated with light that is linearly polarized parallel to the LC director
They are usually designed to produce at least 2π phase modulation at the operating wavelength. Their phase response versus addressed gray level is sometimes linearized at the electronic driver level
Summary
Parallel-aligned liquid-crystal spatial light modulators (PAL-LC-SLM) may be ideally regarded as pixelated linear retarders with tunable retardance, useful for applications where pure phase modulation is a key feature [1]. PAL-SLMs ideally offer pure phase modulation when illuminated with light that is linearly polarized parallel to the LC director They are usually designed to produce at least 2π phase modulation at the operating wavelength. For small diameter beams illuminating the device under a large-enough angle of incidence, multiple reflected beams can be spatially separated In this situation, it will be shown how the effect of a displayed diffraction grating is doubled on a beam that has been reflected twice through the LC layer.
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