Phase Only Liquid Crystal Light Modulator And Its Application In The Fourier Plane Of Optical Correlation Systems

Abstract

ABSTRACT It has been shown theoretically that optical correlation systems which use phase only filters are capable of producing sharp correlation peaks and good discrimination between different input objects, whilst at the same time having high optical efficiency. In addition to this, the resolution required in the filter plane is very much lower than that required for an equivalent conventional Van der Lugt correlator using a matched filter which has been made holographically. Here, we describe a flexible phase only filter implemented using a special-purpose liquid crystal spatial light modulator, and present results which demonstrate how it can be applied in optical correlation systems. 1. INTRODUCTION There has recently been a great deal of interest in phase only filters as applied to optical correlation systems. This type of filter modifies the phase distribution of light passing through it, without altering the amplitude distribution. It has been shown by computer simulation that, when applied in the Fourier Plane of coherent optical,correlation systems, phase only^fliters are capable of giving very sharp correlation peaks and high optical efficiency .The work of Psaltis et. al. 3 and Flannery et. al. 45 ' has demonstrated very effectivelythat useful optical correlators can be constructed using phase only filters implemented with spatial light modulators which have limited pixel counts (typically 128 X 128 pixels).However, these authors used magneto-optic devices , which only allow binary phase modulation of the incoming light - i.e. the only phase modulation values allowed for each pixel are zero or PI. Whilst binary phase only filters can give good correlation performance in most situations, with some types of object (particularly self-symmetric, displaced, reference objects) they can give undesirably^ ghigh noise levels and even false correlation peaks in the output correlation plane ' . Even though the amount of information required to write binary phase only filters is lower, in many situations - particularly where safety ^considerations are involved - the true phase only filter is preferable. In previous work we have shown how it is possible to use a commercially available twisted nematic liquid crystal television panel as a continuous phase only modulator, but construction of a practical correlator using this technique would be very difficult since the phase modulation depth available is very limited, and the aberrations of the panel very high.In this paper, we describe a new special-purpose liquid crystal panel which can be used as a continuous phase only spatial light modulator, and it's application in an optical correlator system. The panel is not twisted nematic, and so phase modulation can be achieved in one pass with essentially very little change in the polarisation state of the light passing through it. The panel we used was designed to have a phase modulation depth in excess of 2PI, and low aberrations, and this enabled us to use it with high efficiency and with no aberration correction.