Spatial Light Modulators and Applications: Summaries of Papers Presented at the Spatial Light Modulators and Applications Topical Meeting Held on March 15-17, 1993 in Palm Springs, California

Abstract

'a," t .i iiagr~e I hour Per response, including tkte time for review. Design and fabrication aspects of a longitudinal effect PLZT spatial light modulator is described.Experimental data covering the performance in the visible wavelength region is presented. This paper presents the design and fabrication aspects of a longitudinal effect PLZT spatial light modulator designed for operation in the visible and near infrared. Included are experimental data showing the electric field induced phase shift characteristics of a 16 x 16 square array spatial light modulator. Nd:YAG laser damage threshold of bare PLZT is also presented. It is concluded that longitudinal effect PLZT spatial light modulators are rugged devices that can be used in adaptive optics, laser beam quality enhancement and beam steering applications.The following operational parameters were selected as design objectives for the demonstration of the first spatial light modulator:Number Design Aspects--A nonferroelectric PLZT material having a quadratic electro-optic and electrostrictive effects was selected to be the modulator material' 2 . The spatial light modulator configuration is schematically shown in Figure 1. In this configuration, a PLZT wafer is coated with two conductive electrodes; one electrode is optically transpsarent and the other is highly reflective. The application of a voltage in between electrodes results in an index and thickniess change in the PLZT wafer. The differential phase shift (i.e., the phase shift of light upon reflection from the back phase of PLZT when the applied voltage is zero or V) is given by a simple quadratic expression:A--n(n 2 R+2M)V 2 /Ikt where n is the index of refraction of PLZT, X refers to the wavelength of light, and R and M refer to quadratic electrostriction and electro-optic constants. By using reflected light from the second interface a factor of two increase in phase shift is obtained for an applied voltage vs a purely transmissive device. The index of refraction of PLZT (-2.5) enters as a multiplier ;n the phase shift equation. This formula also predicts that the PLZT wafer thickess needs to be minimized, consistent with voltage breakdown, -saturation, and optical-.fabrication concerns, in order to provide high phase shifts with the minimum amount of drive voltage, i.e. the device is electric field driven. AR CoatingSubstrate AR Coating .... Transparent Con-W ductive CoatingGold Pixels Figure 1. Schematic of a longitudinal effect PLZT phase modulator.The spatial light modulator, being a pixel addressable phase shift device, requires application and removal of voltages from individual pixels which can be modeled as an array of capacitors. To bring the dynamic voltage levels into an acceptable value, supported by state of the art commercial drivers, and to linearize the response, the spatial light modulator is operated with a biasing electric field. The phase shift between different pixels is then achieved by the application (removal) of differential voltages from the common bias voltage applied to ...