An Asynchronous Image Subtracting Optically Addressed Spatial Light Modulator
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AN ASYNCHRONOUS IMAGE SUBTRACTING OPTICALLY ADDRESSED SPATIAL LIGHT MODULATOR PIERRE R. BARBIER AND GARRET MODDEL
Department of Electrical and Computer Engineering and Optoelectronic Computing Systems Center, University of Colorado, Boulder, CO 80309-0425, USA. ABSTRACT
The operation of image-subtraction is useful in optical processing. We report the first asynchronous image-subtracting optically addressed spatial light modulator incorporating a ferroelectric liquid crystal and a color-sensitive a-Si:H p-i-n-i-p photodetector. We present experimental results characterizing the sensitivity of the photodetector current-voltage characteristics to wavelength, and demonstrate image subtraction. Finally, we propose a model that explains the operation of the image-subtracting OASLM. BACKGROUND
An optically addressed spatial light modulator (OASLM) alters a reflected or transmitted output light beam in response to an input image. OASLMs are key elements in optical computing and image processing. The combination of a-Si:H and ferroelectric liquid crystal (FLC) has proved to be highly successful in forming high performance general purpose OASLMs. 1 An image-subtracting optically addressed spatial light modulator (OASLM) may be used to subtract one image from another, a desirable function in several optical processing applications. An FLC is a birefringent material whose optical axes can be rotated under the influence of an electric field. This optical-axes rotation can be converted into an intensity modulation by placing the FLC between crossed-polarizers. An FLC exhibits bistability: it switches to one of two bistable states when an electric field is applied and remains in this state after the electric field is removed. An FLC OASLM is fabricated by sandwiching a -1 IRm thick FLC layer and a -2.5 gtm thick a Si:H p-i-n photodiode between two optical flats which have been coated with transparent-conducting-oxide (TCO) electrodes. 1 An FLC OASLM may be driven by applying a 1 kHz, -10 V to +10 V voltage
square wave between the TCO electrodes. When the a-Si:H photodiode is forward biased, the positive applied voltage drops entirely across the FLC, causing the molecules to rotate to what we may define as the off state. When the photodiode is reverse biased in the dark, the FLC remains under positive voltage, and stays in its previous off state. When the photodetector is illuminated with a write light, it produces a photocurrent which induces a negative voltage across the FLC This write-light-induced negative voltage tends to rotate the molecules into their on state forming a molecular-orientation replica of the write image. This replica is transformed into an amplitude-modulated image by placing the OASLM between crossed-polarizers. Although FLC is intrinsically bistable, continuous gray levels may be achieved by controlling its switching time with the write-light intensity. 2 In this -case, the OASLM optical response may be integrated over several frame periods with a slow photodetector such as a CCD camera. Local and contr
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