Abstract

In modern signal processing environments the ability to excise certain frequencies from a given rf signal, known as notch filtering, is a useful function. Programmability of the notch frequencies is a crucial capability in many applications, which at the same time is difficult using conventional electronic methods. Hybrid electrooptical (EO) methods have been proposed and demonstrated where spatial light modulators are used in conjunction with an acoustooptic (AO) device to perform the desired function via Fourier optics. In this paper, we describe a new approach using photorefractive two-beam coupling with EO modulators. Two-beam coupling is a non-linear optical phenomenon where two coherent beams of light can exchange energy in a photorefractive medium.¹ Our system uses a generalization of this effect for temporally modulated beams which has been theoretically characterized elsewhere.¹ Our system uses a two-beam coupling apparatus in which one beam (the pump) is modulated with the received signal to be filtered and the other (the probe) is modulated with synthesized sinusoids at the notch frequencies. The modulation is performed with either AO or EO devices. The two-beam interaction causes those portions in the pump beam with the same frequency content as those present in the probe beam to become almost completely diverted into the probe beam direction so that the pump beam that emerges from the photorefractive crystal is nearly devoid of those temporal frequencies. This beam is then directed into a heterodyne detector apparatus to recover the notch filtered signal.

© 1991 Optical Society of America