Digital correlation hologram implemented on optical correlator David Abookasis and Joseph Rosen Ben-Gurion University of the Negev Department of Electrical and Computer Engineering P. O. Box 653, Beer-Sheva 84105, Israel ABSTRACT A new kind of computer-generated hologram termed digital correlation hologram (DCH) has been developed and demonstrated with promising results. This hologram is composed of two separate sub-holograms. The reconstructed image is obtained as a result of a spatial correlation between the hologram's two parts. The DCH codes two complex functions which are generated by an iterative optimization procedure. The correlation between the two sub-holograms is implemented by means of a joint transform correlator (JTC). When the double-elements hologram is displayed on the JTC input plane, and illuminated by a plane wave, a desired image is constructed on part of the correlator output plane. The DCH can be used for security and encryption systems, as the desired image will be received in the output plane only when the two specific sub-holograms are placed at the input plane of the JTC. Simulation and experimental results demonstrate the suggested technique. Keywords: Computer Holography, Optical Correlator, Optical Data Processing, Iterative Procedure, Security System 1. INTRODUCTION Generally an image is reconstructed as a result of wave propagation from a hologram through some medium (or optical system) characterized by a determined and parameterized kernel function. For instance, the kernel in the case of the Fresnel hologram 1 is a quadratic phase function which represents a free space medium between the hologram and the image, where the distance between them should satisfy the Fresnel approximation. 2 On the other hand, in the case of the Fourier hologram 3 the light propagates through a linear space-variant system characterized by the Fourier transform phase function. Almost every type of a hologram can be classified according to the kernel associated with the medium (or the system) located between the hologram and the reconstructed image, or between the object and the recorded hologram. Here we propose a significant generalization of this traditional scheme described above. The proposed hologram is divided into two different sub-holograms. One sub-hologram functions as usual as an input of some linear space invariant optical system. The other sub-hologram is used as the kernel function of the same system. The constructed image is obtained on part of the output plane of the system, as a cross-correlation between the two sub-holograms. In other words, between one sub-hologram and the constructed image there is a space-invariant system with an arbitrary kernel function coded inside the other sub-hologram. Both sub-holograms are synthesized by a digital computer and the output image is a result of the correlation process between them. Therefore we call this new hologram a digital correlation hologram (DCH). There are at least two main methods for performing spatial correlation between any two arbitrary masks. One is known as the VanderLugt correlator (VLC) 3 and the other is the joint transform correlator (JTC). 4 In principle, both configurations can be used as a platform to carry out the DCH. However, because of the less restrictive alignment Proceedings of SPIE Vol. 5202 Optical Information Systems, edited by Bahram Javidi, Demetri Psaltis (SPIE, Bellingham, WA, 2003) · 0277-786X/03/$15.00 114 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/12/2013 Terms of Use: http://spiedl.org/terms