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Chapter 6:
Spatial Light Modulators as Optical Elements
The importance of liquid crystal materials in display technologies cannot be overstated, and in the closely related form of a SLM, we are just beginning to appreciate the many applications that are possible. The SLM is an electrooptical modulation device that can be used to provide laser beam shaping, aberration control, optical computing, and much more. When operated in the electronically controlled birefringence mode, a SLM will only modulate the phase of the light, without affecting the amplitude; however, the electronics used to control the individual pixels can reduce the usable size of a pixel to just 50% of its actual size. As a result, strong diffractive effects can occur in optical systems using SLMs. Many of the optical devices that could be emulated using a SLM are able to affect the optical path difference by many wavelengths of light, while the SLM is usually limited to about 1 wave. As such, the required phase modulation is generated using a modulo 2p scheme, often referred to a phase wrapping. In this way, the SLM acts as a diffractive optical element. The behavior of a SLM as a diffractive optical element can be exploited to create many electrically controlled devices that have the performance of many practical optical elements. Electronically controlled lenses, gratings, apertures, masks, and shutters are all possible. While these devices can be used with active control, they can also be used with preprogrammed configurations, allowing rapid switching between configurations.
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