Optical Signal Processing

6

Spatial Filtering Systems

6.1.   INTRODUCTION

In this chapter we discuss some practical aspects of optical spatial filtering systems. We begin with an overview of a general optical processing system and then discuss the basic modules of the optical processor in more detail. We treat the important problem of setting a threshold at the output of the optical processor to detect the signal. We discuss how to handle the usual case of nonuniform noise spectral densities and show how to process signals adaptively. Finally, we illustrate these techniques with various applications and discuss the effects induced by errors in the position of the matched filters in the Fourier plane.

6.2.   OPTICAL SIGNAL PROCESSOR AND FILTER GENERATOR

Consider the basic elements needed in a combined optical signal processor and filter generator, as sketched in Figure 6.1. Light from the laser passes through a shutter and is divided into a reference branch and a processing branch by a beamsplitter. The reference and processing branches are derived from the same laser to ensure that a high degree of spatial and temporal coherence is produced at the filter plane. The transmitted light passes through a second shutter, reflects from a mirror, and expands to fill the Fourier-transform lens, which converges light toward the spatial filtering plane. A spatial light modulator, placed in the convergent beam, accepts data from an electronic source or from an incoherent optical source.

The Fourier transform of the ...