Mirrorless oscillation in photorefractive crystals

In some special wave mixing configurations using photorefractive crystals and forward- and backward-propagating pump waves, one may observe a certain type of self-oscillation without any external feedback, the socalled mirrorless oscillation. Here forward-propagating waves passing through the crystal are enhanced via a parametric amplification process, which in turn causes an enhancement of the backward-propagating waves meeting the same phase-matching conditions. As a result coherent output beams self-develop without any additional input beams apart from the pump beams. Ultimate amplification is only limited by the intensity of the pump beams. In this contribution we describe four configurations which yield mirrorless oscillation for photorefractive BaTiO 3 crystals.