Multi-Level Lossless Phase Modulation in Liquid Crystals for High-Speed Spatial Light Modulators

Doktorsavhandling, 2006

The spatial light modulator (SLM) is a compact device that enables highly precise control of light propagation with low drive voltages and no moving parts. By imposing a certain phase modulation onto the incident lightwave, functions such as multi-beam steering, beam shaping, and air turbulence corrections can be achieved in real time. Thanks to the rapid development of liquid crystal (LC) displays, the LC-based SLM is the primary technology for phase modulating SLMs. Future important SLM applications may include novel beam steerers, optical correlators, and adaptive optics, but these require faster SLMs than are presently available, preferably with analog phase modulation. Today, commercial SLMs have response times of ~20 ms or more for analog phase modulation and ~1 ms for binary modulation. We first describe our efforts to achieve multi-level, or even analog, phase modulation without loss, using LC cells that already had proven to allow fast amplitude modulation. We found that anti-ferroelectric LC (AFLC) cells can be used to obtain three almost equidistant phase levels, ideally without any induced loss. Further, we used the analog mode of ferroelectric LCs (FLCs), referred to as V-shaped switching (V-FLC), to obtain analog phase modulation up to pi rad and 2pi rad, for a simulated transmissive and reflective device, respectively; these values being 0.8pi rad and 1.6pi rad for slightly non-ideal fabricated cells. Also, fast (~1 ms) four-level phase modulation was realized by combining two binary FLC SLMs. In addition, a practical diffraction-based characterization method for general SLMs is presented, the accuracy of which is comparable to established but laborious interferometer methods. Further, an improved design method for the spatial modulation realized by the SLM is presented, which incorporates the phase and concurrent amplitude modulation as well as pixel gap and pixel proximity effects. Finally, a field-experiment is presented in which an LC-based SLM is used to achieve beam tracking in a retro-communication system.