Abstract

As a method of optically addressed spatial light modulation (SLM), real-time holography in photorefractive materials offers a highly desirable combination of dynamic range, resolution, and sensitivity. Unfortunately, true holographic recording requires a reference beam that is coherent with the information-bearing beam. This requirement was eliminated with the development of the photorefractive inchoherent-to-coherent converter (PICOC) device,¹ which used an incoherent information-bearing input beam to locally erase a photorefractive grating. In attaining compatibility with incoherent sources, however, this device sacrificed the wide dynamic range of real-time holography. In particular, uniformity of response is obtained only with weak signal beams—a situation inherently resulting in a low-contrast output. To circumvent this difficulty we have devised a new photorefractive SLM technique in which high-contrast and incoherent-source compatibility are simultaneously achieved. Specifically, by superimposing our input image on an external fixed-amplitude grating and incoherently imaging the composite result into a photorefractive crystal, we may write a corresponding index grating into the material. We show that this grating may be read out by means of standard holographic techniques to yield a coherent replica of the image without the contrast loss associated with the PICOC device. In particular, we show that although this new device offers resolution equivalent to that of the PICOC, substantial improvements in both the dynamic range and the available range of response speed are achieved.

© 1990 Optical Society of America