1.3-in. active matrix liquid crystal spatial light modulator with 508-dpi resolution

Optical signal processing systems frequently use Fourier transform techniques for correlation, filtering, etc. In practical applications the complex-valued filters placed in the frequency domain plane are often realized by a liquid crystal spatial light modulator (LCSLM). The requirements for such LCSLMs are very demanding, as they should offer a high resolution, high uniformity, low fluctuation in time and an increased number of grayscales or phase steps. We designed and manufactured an active matrix LCSLM (AMLCSLM) and the appropriate driving system meeting these requirements. The AMLCSLM consists of 480 by 480 pixels, each containing an amorphous silicon thin film transistor (aSi-TFT). Although the pixel size is only 50 by 50 micrometer squared, corresponding to a resolution of 508 dpi, an optical aperture of more than 40 percent is achieved. By the use of a transparent storage capacitor the pixel capacitance was greatly increased, resulting in a reduction of the fluctuation in time. The driving circuitry is capable of driving up to 256 grayscales at a frame rate of 100 Hz in dual scan driving mode. For the row scanning the commonly used rectangular impulse shape was substituted by a shortened trapezoidal impulse. As a result a very good uniformity over the area with an overall phase change error of less than 6 percent is obtained.