High optical gain using counterpropagating beams in iron and terbium-dopedphotorefractive lithium niobate

Exceptionally high values of photorefractive pain coefficient of up to 100 cm(-1) have been observed in Fe-doped and Fe/Tb-doped crystals of photorefr active lithium niobate. It is believed that these are the highest observed coefficients of any crystalline photorefractive medium. Accurate measuremen ts of gain coefficient have been possible for the first time by using a spe cially cut triangular crystal which allows the use of short interaction len gths and the complete elimination of surface reflections by having the beam s incident at Brewster's angle. Experimental results are consistent with a simple model of photorefractive beam coupling up until the onset of noise w hich is observed to deplete the pump for interaction lengths longer than 1 mm.