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Phase coding for holographic memory using a deformable membrane mirror

Phase multiplexing for recording holograms is usually made with the help of digital Hadamard matrices, which have the particularity to be mutually orthogonal. We propose a phase coding for multiplexed holographic memories where the reference uses orthogonal Zernike polynomials generated with the help of a deformable membrane mirror. This membrane mirror is made of silicon nitride and coated with aluminum. It is fifteen millimeters large, less than a micrometer thick and electrostatically activated by thirty-seven electrodes disposed hexagonally. The first fifteen Zernike modes can be efficiently generated by this configuration of electrodes, which allows writing holograms for each of these modes without cross talk. The Zernike modes are measured with the help of a Shack-Hartmann sensor. The pages of data are composed by an LCD spatial light modulator (LCTV with 640x480 pixels) and undergo an optical Fourier transform before being holographically recorded in a photorefractive crystal (Iron doped lithium-niobate) with the phase coded reference beam. The deformable membrane mirror has a frequency response of up to 1 kHz, and therefore the pages of data can be retrieved up to this speed, if the receiving CCD camera is fast enough. Theoretical considerations and experimental results will be presented.