1 LCOS Spatial Light Modulators: Trends and Applications

Spatial light modulator (SLM) is a general term describing devices that are used to modulate amplitude, phase, or polarization of light waves in space and time. Current SLM–based systems use either optical MEMS (microelectromechanical system, [1]) or LCD technology [2]. In this chapter, we review trends and applications of SLMs with focus on liquid crystal on silicon (LCOS) technology. Most developments of liquid crystal (LC) microdisplays are driven by consumer electronics industry for rear–projection TVs, front projectors, and picoprojectors. Also,MEMS technologies such as digitalmicromirror device (DMD, [3]) and grating light valve (GLV, [4]) are driven by these industries, except for membrane mirrors. Some industrial applications have forced MEMS development for scanning, printing technologies, and automotive applications [5]. But the major R&D-related driving force for new SLM technologies is the defense industry. Technological advances in lithography are the basis for MEMS developments. Phase modulators based on 2D pistonlike mirror arrays [6, 7] or ribbonlike 1D gratings [8] show high performance in frame rate. Unfortunately, the availability of these technologies is limited because they are developed either company-internal or within defence projects. The major advantages of MEMS are frame rate, spectral range, and an efficient use of nonpolarized light. Phase modulators and other optical implementations are still niche markets for the MEMS industry. Even now, customized MEMS developments are quite challenging and expensive. LC panels still have an advantage out of their projection applications in terms of resolution and minimal pixel size for 2D displays. Only LC-based technology is able to modulate intensity, phase, and/or polarization because of polarization rotation and/or electrically controlled birefringence (ECB). LCOS technology [9] was developed for frontand rear(RPTV) projection systems competing with AMLCD (active matrix LCD) and DMD. The reflective arrangement due to silicon backplane allows putting a high number of pixels in a small panel, keeping the fill factor ratio high even for micrometer-sized pixels.