<title>Enhancing the response time for photorefractive beam fanning</title>

Abstract

Two types of optical limiting devices which have been demonstrated using photorefractive crystals are the 'beam fanning' limiter and the 'two-beam coupling' limiter. Experimental demonstrations of these two devices have revealed an interesting difference in behavior between those two limiter types. The two-beam coupling limiter is consistently faster than the beam fanning limiter for the same incident intensity. This is somewhat surprising since the beam fanning limiter is based on two-beam coupling phenomena. Our recent experiments show, however, that the relative speed of the two devices can be made more alike using a phase grating at the entrance face of the beam fanning limiter. While the phase grating scatters only about 1% to 5% of the incident light is apparently provides sufficient seeding to significantly enhance the beam fanning response time. Since the diffracted light is only about 1% the distortion to vision through the limiter due to dispersion is minimal. In the presentation we will present results from an experimental investigation of the effect of seeding on the 'beam fanning' limiter response time. In particular, we have examined the response time dependence on the intensity and direction of the seed beam. Comparison with theory will also be presented.