Abstract

Optimum two-beam coupling gain in photorefractive semiconductors is achieved mainly by the DC field and moving grating technique. Due to the crystals' finite time response, the out of phase component of the refractive-index grating is enhanced. In this work we show that absorption, which is not negligible in these crystals (α ~ cm⁻¹), prematurely saturates and degrades the small - signal gain along the crystal's interaction length. This is due to its' influence on the crystals' response time τ, which is approximately inversely proportional to the light intensity in the crystal, so that τ varies as τ(z) ≈ τ(o)eα^z along the interaction coordinate z. A distributed time constant causes a deviation from t he optimum phase condition along the interaction length when using the moving grating technique. Our model predicts a significantly suppressed small-signal gain, which peaks at an optimum interaction length 1_opτ ≈ 1/α and then degrades for larger 1. This effect can account for discrepancies between measured gain in semiconductors and previous theories. It also presents a fundamental limitation of the moving grating technique in photorefractive waveguides, which would not benefit from long interaction lengths. In this case, the AC field method would be more effective.

© 1990 Optical Society of America