Spatial light modulators (SLM's) have many uses including but not limited to projection, holography, optical switching, and optical computing.
SLM's maybe static or temporal. SLM's come in a wide variety of types including film based, liquid crystal device (LCD) based, and micro-electro-mechanical (MEM's) based. A diffraction grating is a simple example of a static SLM; a holographic lens or hologram is a less simple example. FIG. 1 shows a sketch of a plane wave light beam (or parallel light beam) 10 having flat planes of equal phase shown by lines 12 incident on a transmission diffraction grating 14. Light is diffracted from diffraction grating 14, and two orders of diffraction 16 and 18 are shown. The parallel light beam 10 can be considered a beam of light uniform in space and in time having only one of the two parts which make up a more complex beam. By convention, we call the two parts of the light beam the “real” and the “imaginary” part, since in the mathematical representation of the light fields the two parts are described by a real number and an imaginary number, and in graphical representations the light field is represented by a vector in the two dimensional complex plane which has a “real” axis and an “imaginary” axis. The vector has two components, a “real” component and an “imaginary” component. The amplitude of the electric field vector of the light wave is given by the length of the vector, and the phase of the light field by the angle which the vector makes with, by convention, the “real axis”. The phase of the light field is given in degrees (from 0 to 360 degrees) or by radians (from 0 to 2 π radians). By choice, we call the light beam 10 a beam having only a real component. The real component is spatially modulated by letting only certain spatial components through the transmission grating, and the spatial modulation of the real part of the light beam 10 produces diffracted light in various orders, of which only two are shown.
FIG. 2 shows a more complicated thick film hologram 24 where most of the incoming light 10 is thrown in one diffraction order 28. The surfaces of equal phase are shown as more complicated surfaces than planes, and the information contained in the thick film hologram is carried in the outgoing wave. The hologram 24 produces an output wave 28 which has both real and imaginary parts, and both the real and imaginary parts may have spatial modulation. The production of such holograms 24 is well known to one of skill in the holographic art, and the resultant holographic reconstructions of the original complicated optical signal have been on view since the 1960's. The diffraction grating of FIG. 1 can be thought of as a simple hologram, where a light wave having only a real component incident generates a light wave also having only a real component. The reconstructed light wave signals shown in FIGS. 1 and 2, however, are static and do not change in time.
While there are many different types of temporal SLM's, they can loosely be divided into two categories, transmissive and reflective. Transmissive SLM's are exemplified by Liquid Crystal Display (LCD) SLM's such as are found in the screens of most laptop computers. An example of an LCD SLM is made by Boulder Nonlinear Systems. LCD SLM's can be filled with various liquid crystal types and topologies to spatially and temporally modulate amplitude of a light beam. These types of devices carry with them the drawbacks of state of the art LCD's including limited contrast ratios and limited switching speeds.
Reflective SLM's are exemplified by arrays of micromirrors. An example of a micromirror SLM is made by Texas Instruments. The micromirrors can be made to adjust their positions and/or angles to modulate the amplitude of portions of a light beam. LCD's can be operated in a reflective mode where the light passes twice through the liquid crystal material. Such LCD SLM's are exemplified by liquid crystal on silicon (LCOS) devices used in front and back projection systems.
The current state of the art of Micromirrors is in a constant state of improvement and the edge of the curve is difficult to characterize. A micromirror array can be as simple as a prior art array of static mirrors fabricated on an absorbing substrate using standard MEMs techniques. A hologram can be displayed, for example, by an array having pattern of missing mirrors. Such holograms are equivalent to the diffraction grating of FIG. 1 which modulate only one component of the light beam, and which are called “thin film holograms”
It is useful, of course, if the mirrors can be made to turn on and off to modify the light beam temporally. One way to do this is to tilt the mirrors of the array. In the “on” state, mirrors reflect light into the target or a lens to be projected on to a target. In the “off” state, mirrors are tilted to throw away light into a direction other than the target, generally to an absorber to absorb the light and prevent stray light. An example of the technology is the Texas Instruments Digital Light Processor (DLP) micro mirror Array. One of their very successful products employs square mirrors of 16 microns on a side, spaced 17 microns apart. During the last year, the standard mirror size has been reduced to 14 microns, indicating that the industry is continuing to shrink mirror size and continue general developmental progress. The type of contribution made by the light reflected by an individual mirror can depend on the application.
When used in a lensed projection system, for example, each mirror will reflect a small amount of light that corresponds to a single pixel in the spatial domain in the same way that a single light valve corresponds to a single pixel in a LCD display. In the “on” state, a mirror reflects light into the projection screen. In the “off” state, the mirror is tilted to throw away its light into a direction other than the projection screen. This can be considered a form of magnitude-only amplitude modulation in the spatial domain.
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