Optical data encryption in a photorefractive crystal

Abstract

Methods of protecting stored data from unauthorized users have been in use for as long as methods for storing data have existed. Encryption describes the process of converting data into a form, called a ciphertext, which is inaccessible without the proper access key.
Recently, the need for large data capacities has led to the use of photorefractive materials as optical storage media. Numerous techniques have been reported to exploit the theoretical capacity of a single crystal, which is upward of 1013 bits. For these large holographic memories to be practically implemented, data security is an issue that must be addressed. Several optical encryption techniques for holographic media have already been reported. One technique adds a random phase to the image before recording with a plane-wave reference beam in a holographic film. Another technique encrypts an image using a focused reference beam with a random phase, while another relies on phase conjugation to decrypt an image recorded with a random phase.
In this paper, we present a method for encrypting optical data in a photorefractive crystal that precludes the need for the addition of a random phase for encryption and phase conjugation during decryption. Our set-up uses a pattern bearing reference beam to record an image in the crystal and we show that complete reconstruction is impossible without using the proper reference pattern.