Optical phase-conjugation property of three-photon excited backward stimulated emission

Three-photon pumped cavityless lasing experiments have shown that the forward and backward stimulated emission (cavityless lasing) beams manifest different divergence angles and spatial/temporal fluctuation behaviors. When a dye solution as the gain medium is pumped by a pulsed laser beam with ∼ 1500-nm wavelength and ∼ 150-fs pulse duration, the backward stimulated emission beam exhibits a smaller divergence angle and much better spatial/temporal stability than the forward stimulated emission beam. These apparently unexpected results can be explained reasonably by assuming that the backward stimulated emission beam exhibits an optical phase-conjugation property, so that influences from a random spatial/temporal change of the refractive-index in the lasing medium can be automatically removed for this backward lasing beam. A physical model of wave-front reconstruction via pump-beam induced holographic grating in an aberrated gain medium is proposed, which is supported by a rigorous theoretical analysis.