Optical correlators may become a less-expensive prospect if a new approach by researchers at Displaytech and Black Forest Engineering (Longmont and Colorado Springs, CO, respectively) makes it into the next generation of devices. The engineers have integrated spatial light modulators (SLMs) and a camera onto a single chip. In addition, the cover glass, which is used to keep the liquid crystal in the SLM in place, doubles as a diffractive lens for each element on the die. The result is a system that requires alignment of only two mirrors.
The integrated design is a classic Van der Lugt correlator.1 An incoming image is displayed on an SLM, which is "read out" via a laser beam. A lens Fourier transforms the image, and the transform meets a second SLM. This modulator contains the Fourier transform of an image that was stored and processed digitally: the target. Where the transforms match, light continues through the system to be detected at the camera plane, creating a correlation peak at the location of the matching object or objects.
Design problems
In the integrated design, the correlator is folded back on itself via a mirror, with diffractive lenses on top of each element performing the necessary Fourier transforms (see figure). Among the first problems that engineers had to overcome was the reflective/diffractive nature of the modulators and associated lenses. Because the SLMs consist of small pixels, the liquid-crystal-on-silicon devices cause diffracted orders to propagate through the system, even when all the pixels are turned off. In the same way, the diffractive lenses inherently create unwanted beams of light. This problem was resolved by making two small mirrors instead of one big one, to tweak the system alignment.
Another problem caused by the diffractive nature of the lenses was the light source, originally intended to be a laser diode. Rather than tackle a temperature-control problem related to diode light sensitivity, the researchers used a continuous-wave, solid-state diode-pumped Nd:YVO4 laser.
The demonstrator consisted of two 212 ¥ 212-pixel light modulators with a pixel pitch of 13 µm and a 256 ¥ 256 camera to detect the correlation peaks. The volume required for the optical path is approximately 3 mm (the height of the camera element) ¥ 14 mm (the width of the three active components and the space between them combined) ¥ 34 mm (the distance from the modulator/camera chip to the mirrors). Researchers calculate that, with 7.6-µm pixels, they could create a 1024 ¥ 1024 image-correlator subsystem operating at up to 2000
