The performance and operation of high-speed, liquid crystal spatial light modulators are discussed in relation to a variety of system-level aspects. The issues involving the use of these devices in optical processing systems are the primary focus of this paper, but other applications such as ultra-video imaging and beamsteering are also included in the discussion.

The wide range of electro-optical properties such as electrically adjustable optical activity, birefringence and scattering make liquid crystals (LC) suitable for defined alternation of amplitude, phase polarization and spectral composition. Optically and electrically addressable LC-SLM being the data input or light modulating devices result in arrangements which can be used for real-time image processing, holographic applications and optical computing systems. The range of SLM applications is dependent on such parameters as sensitivity, resolution capability, switching time, flatness and thermal-mechanical stability. The parameters of optical addressable a-Si:SLMs, MIS-SLMs, and CdS-SLMs with liquid crystals as light modulating layers and their technological dependence are in discussion.

Recent advances in diffractive liquid crystal (LC) modulation structures for projection display service have generated diffractive spatial light modulator (SLM) devices capable of high performance in transmission and/or reflection. Patterning of the LC domain alignments has resulted in the generation of near perfect phase diffraction grating structures in the lC material, which can be controlled by a single electrode or by an array of segmented electrodes, such as an array of pixels. Consequently, diffractive artifacts from inter-electrode gaps can be eliminated and/or suppressed in a wider spectrum of applications with enhanced performance relative to previous structures. Transmissive diffractive LC modulators suitable for display or switch applications with modulation efficiencies in excess of 90 percent and contrast ratios of greater than 1500:1 in un-polarized light have been demonstrated. Operation of this type of modulator in reflection has also been verified and demonstrated. Comparison of patterned alignment, LC device performance with MEMS and other diffractive structures will be given. Extensions of these techniques to devices which include beam scanners and controllable diffractive optics will be proposed.

Electrically controlled diffractive gratings are developed on the basis of cholesteric liquid crystal confined between two transparent electrodes. The electrodes are coated with unidirectionally treated alignment layers. The initial state is planar, with helix axis oriented normally to the electrodes. The applied field causes reorientation of molecules and creates structures modulated in the plane of the cell. Surface alignment provides unidirectional uniformity of the modulation. The parameters of the modulated structures and light diffraction are controlled by the cholesteric pitch, cell thickness and applied voltages. In the device of the first type, the modulated state produces Raman-Nath (RN) diffraction and allows a the modulated structure depends on the applied field. Diffraction regimes of both RN and Bragg types are demonstrated for this geometry. In the RN regime, the electric field allows one to control continuously the deflection angle. This effect can be used in various beam steering devices. The variation in the diffracted beam direction is more than 15 degrees. Typical working voltages are less than 10V. We present both the experimental results and 3D computer simulations of modulated structures caused by the field.

Light-addressed spatial light modulators with various photosensitive layers have been investigated using both holographic and projection techniques. The ways of reaching the compromise between high resolution and high speed of the device have pointed out.

A new type of electro-optical device, a switchable hologram, has emerged from the combination of two technologies, namely, photopolymer holographic materials and polymer- dispersed liquid crystals (PDLCs). Starting from a simple homogeneous mixture containing a polymerizable monomer, a liquid crystal (LC), and a photoinitiator dye, this system cures under holographic illumination to form well defined channels of PDLCs interspersed between dense, LC-free polymer channels. These periodic PDLC planes produce diffraction of light in the Bragg regime with surprisingly good optical quality. LC droplets within the PDLC channels can be remarkably small, ranging in size from 20 nm to 200 nm. Even within this small domain, the molecules can be reoriented by an applied field, leading to electro-optical modulation of the diffraction efficiency. The polarization properties and sharp threshold switching of the diffracted light may be partially explained by the unique shape and consequent nematic ordering of the LC droplets. Complex holograms can be written in this material, and electro- optical switching at < 5 V/micrometers with response and relaxation times in the 20-40 microsecond(s) regime have been consideration. Spatial light modulators which modulate the intensity can make use of these switchable holograms. Some of the potential applications are in fiber optic switches, programmable optical interconnects, digital zoom lenses, optically assisted true-time-delay phased array radar, flat panel display, and dynamic filters.

THis paper presents optical testing of polysilicon surface micromachined piston micromirror arrays. Similar piston micromirror arrays were fabricated using two different commercially available surface micromachining foundry processes: the DARPA supported multi-user MEMS processes (MUMPs), and Sandia Ultra-planar Multi-level MEMS Technology (SUMMiT). All test arrays employ square reflecting elements in an 8 X 8 element 203 micrometers square grid. Fabrication constraints limit the MUMPs designs to fill-factors of less than 80 percent. The chemical mechanical polishing planarization step integral to the SUMMiT process allows an as-drawn fill-factor of 95 percent to be easily achieved. MUMPs designs employ both the standard gold metallization and maskless sputtered chromium/gold post-process metallization, while post process metallization is the only option for the SUMMiT design. Testing of the micromirror arrays focuses on microscope interferometer characterization of mirror topography, and measurement of the far field diffraction pattern for each. The measured results show that control of the individual micromirror element surface topography is more important for imaging applications than maximizing the as-drawn fill-factor.

This paper describes the design and characterization of several types of micromirror devices to include process capabilities, device modeling, and test data resulting in deflection versus applied potential curves and surface contour measurements. These devices are the first to be fabricated in the state-of-the-art four-level planarized polysilicon process available at Sandia National Laboratories known as the Sandia Ultra-planar Multi-level MEMS Technology. This enabling process permits the development of micromirror devices with near-ideal characteristics which have previously been unrealizable in standard three-layer polysilicon processes. This paper describes such characteristics which have previously been unrealizable in standard three-layer polysilicon processes. This paper describes such characteristics as elevated address electrodes, various address wiring techniques, planarized mirror surfaces suing Chemical Mechanical Polishing, unique post-process metallization, and the best active surface area to date.

The digital micromirror device (DMD) is a micro-optical- electro-mechanical structure consisting of an array of 16 micrometers X 16 micrometers square mirrors positioned on a 17 micrometers pitch. Each individual mirror can be tilted +/- 10 degrees relative to the DMD substrate; the tilt is along the diagonal direction of the micromirror. The device was invented and manufactured by Texas Instruments (TI), Inc. TI packages the DMD as an OEM product for use in projection displays. We are investigating the use of the DMD as a spatial light modulator for precision imaging and spectroscopy applications. This includes optical characterization of the device, as well as systems engineering to operate the device. Some of the performance metrics to be considered are the diffraction efficiency, optical-switching contrast, background scattering properties, mirror crosstalk, and the modulation transfer function.

GaAs asymmetric Fabry-Perot vertical cavity modulators are useful in a wide variety of applications. Vertical cavity devices have employed amplitude or phase modulation for optical switching. Amplitude modulators have been demonstrated in large format arrays. Additional uses for the pixels include directional modulation, detection, and light- emitting capability. When arrays of these deices are integrated with electronic circuits - most significantly silicon CMOS VLSI - at the pixel level, large, complex optical spatial light modulators, detectors, transceivers, computation devices, and emitters can be created for a wide variety of applications. These applications range from target recognition to SAR radar processing, to optical data routing, to optical interconnect systems, to optical memory access.

Surface plasmon tunable filter is a new technology under development at the Jet Propulsion Laboratory. This technology can also be used to build an electronically tunable mirror. When white light is incident on a metal/electro-optic material interface under certain conditions, surface plasmon waves can be excited at the interface. Photons in the wavelength range of the surface plasmon resonance will be converted into the energy of free electrons in the metal. When using nickel or a rhodium/aluminum bilayer as the metal, the bandwidth of the surface plasmon resonance can cover all of the visible spectrum. This surface plasmon resonance depends on the dielectric constants of both the metal and the electro-optic materials Therefore, application of a voltage to the electro optic material to change its dielectric constant can theoretically result in a change in the reflectivity of the interface from less than 0.5 percent to over 80 percent. The experimental results show a contrast ratio of 50:1 and a maximum reflection of 50 percent.

The alignment of ferroelectric liquid crystal (FLC) is heavily influenced by the FLC flow rate during SLM cell filling. This flow rate is affected by a number of factors, one aspect of which is the structure of the silicon backplane. Even when the device has been planarized the structure of the pixelated top layer metal still influences the FLC flow rate and therefore the FLC alignment. We have produced a flat silicon backplane substrate using damascene processing to manufacture the mirror/electrodes. Damascene processing is a metal polishing technique. In this process the oxide layer which has already been polished is etched to create trenches in the desired pattern of the metal layer, a blanket deposition of metal is then performed, which fills the trenches and covers the wafer surface, finally CMP is performed, which removes the excess material on the wafer surface leaving the metal in the trenches and the top surface flat. There are some problems associated with damascene processing which will affect its suitability in the micro-=fabrication of SLM backplanes. The softer metal material is prone to dishing and scratching and the harder oxide material can be eroded. THese effects are dependent on the level of control of the CMP process. A process is being developed, using novel slurry chemistries, to allow the incorporation of this technique into our post-processing procedure. The results of the application of this process to test structures and an analysis of the suitability of this technique in the microfabrication of SLM silicon backplanes will be presented.

An IR reflective optical modulator has been fabricated. This has been achieved by coating a thick film resistor network with a thin film of vanadium dioxide via a reactive sputtering process. Vanadium dioxide undergoes a semiconductor to metal phase transition at approximately 68 degrees C, therefore to switch the reflective optical modulator the thick film resistors in the network are driven electrically. As the resistors heat up to beyond the transition temperature, the vanadium dioxide undergoes its transition from a transparent semiconductor state to a highly reflective metallic state. Provided the thick film heater network, underneath the vanadium dioxide, is non- reflective, then a significant change in reflectivity is observed upon undergoing the transition, and hence the modulation of reflected IR radiation is achieved. The useful waveband of operation of the device encompasses the region 2-25 micrometers , this is primarily limited by the transparency of the semiconductor state of the vanadium dioxide. Correct stoichiometry of the thin film of vanadium dioxide is critical in producing good modulation depth in the reflective mode. Several devices have been fabricated and tested. They show a reflectivity increase of approximately 13:1 upon switching. The devices to date have demonstrated switch on speeds of 0.1 s and switch off speeds of 0.2 s. This has been achieved without any form of substrate temperature control apart from that produced by the electrical drive. Very slight changes in the stoichiometry of the vanadium dioxide thin film can greatly increase the temperature range and hysteresis of the semiconductor to metal phase transition. This has been utilized to allow partial phase transitions to occur, yielding partial increases in reflectivity, and hence the ability to generate grey levels in the reflected IR radiation.

Most spatial light modulators (SLM) are limited in that they cannot produce arbitrary complex modulations. Because phase and amplitude are usually coupled, it is difficult to computer design appropriate modulation patterns fast enough for the real-time applications for which SLMs are suited. Dramatic computational speedups can be achieved by using encoding algorithms that directly translate desired complex values into values that the modulator can produce. For coherently illuminated SLMs in a Fourier transform arrangement pseudorandom encoding can be used. Each SLM pixel is programmed in sequence by selecting a single value of pixel modulation from a random distribution having an average that is identical to the desired fully complex modulation. While the method approximates fairly arbitrary complex modulations, there are always some complex values that are outside the encoding range for each SLM coupling characteristic and for each specific pseudorandom algorithm. Using the binomial distribution leads to methods of evaluating and geometrically interpreting the encoding range. Evaluations are presented of achieving fully complex encoding with SLMs that produce less than 2 (pi) of phase shift, identifying an infinite set of encoding algorithms that encode the same value, identification of the maximum encoding range, and geometric interpretation of encoding errors.

Unlike in the microwave domain, where open loop phase control is adequate, a phased array antenna working at optical frequencies will require precise closed loop control of each element pixel to realize a well defined high brightness far-field antenna pattern. We describe and present experimental data for a design that permits precision, to < (lambda) /100, phase control with a high bandwidth that compensates for temperature, mechanical effects, delay times of each phase shift element, and non- linear response. Experimentally, the output of a phase measurement system is used in an electronic feedback loop to dynamically linearize an inherently non-linear liquid crystal. The experiment consisted of a spatial heterodyne, temporal homodyne, fiber optic Mach-Zehnder interferometer to recover phase of a single nematic liquid crystal element. The resulting phase measurement, represented as an analog voltage, is used in a feedback loop to correct for the non- linear drive voltage-to-phase retardance response of the liquid crystal. A demonstration of this technique using several periodic drive waveforms at frequencies of 10-100 Hz was performed. Data are presented showing a phase retardance resolution of < 1 nm which enabled a significant improvement in the linearity of the liquid crystal response.

The number of applications in product inspection is already large and grows rapidly. Fabrication processes run at high speeds, and demands on accuracy and quality are rising. Today, electronic vision system often do not provide enough processing performance to satisfy real-time requirements of industrial applications. This motivates the development of hybrid vision systems which, by utilization of parallel optical filtering, have enough processing power to become applicable in automated product inspection. We have developed a programmable optical processor which is designed to extract defects on technical surfaces by means of optical image analysis. The processor is capable of analyzing video image sequences in real-time using liquid-crystal spatial light modulator technology. Structural defects are visually enhanced by an adaptive wavelet filtering method. We have implemented a demonstrator device and showed its operation by an example of practical relevance. The experiments confirm that optical image processing is an attractive way to do quality control in real-time.

we demonstrate that with a single manufacturing process and custom FLC materials, individual reflective FLC SLMs can be optimized for a wide range of chosen wavelength regions. One lot of 256 X 256 SLM cells were prepared from a single silicon wafer. These cells were filed wit five different FLC materials having birefringence spanning a range from 0.129 to 0.218. The resulting retardance variation allowed SLM characteristics to be tailored to give optimized performance in any wavelength region from 400nm to 1000nm.

Ways of the improvement in dynamic characteristics of the nematic liquid-crystal cells have been considered under various conditions at the solid-liquid crystal interface.