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Optical cryptography of gray-level image information using QPSK modulation and digital holography

We propose a novel optical cryptography of gray-level information image using QPSK digital modulation method and digital holographic technique. A gray-level information image is digitized into 8-bits binary information data by ASCII encoding method and these binary information data are expressed by four pair of quadrature phase values in a block having 2×2 pixels by QPSK digital modulation. After encoding and modulation, the size of data to be encrypted expands two times more than the original size of gray-level image. The modified information with corresponded phase values is displayed on a phase-type spatial light modulator and is encrypted with a security key by using optical digital holography. The security key is expressed with random binary phase. Digital hologram in this method is Fourier transform hologram and is recorded on CCD camera with 256 gray-level quantized intensities. These encrypted digital holograms are able to be stored by computer and be transmitted over a communication network. With this encrypted digital hologram, the phase values are reconstructed with the same security key by holographic technique and are decrypted into the original gray-level information image by decoding. Simulation results show that the proposed method can be used for a cipher and security system.