Optical-phase conjugation in difference-frequency generation

This study presents experiments performed in β-BaB₂O₄ (BBO) at frequency degeneracy (ω_1,2 = ω) in which the intense plane wave to be used as the pump field (ω₃ = 2ω) is provided by the frequency-doubled output of a high-coherence pulsed Nd:YLF laser (mod. Green Star, Neva Technologies Ltd., St. Petersburg, Russia). The residual fundamental pulse provides the seed field. The phase-modulation imprinted onto E₁ (0,t) is either that of a biconvex lens or that of a diffusing glass plate. The work shows that in the former case the DFG field E₂ (r,t) exiting the non-linear crystal reconstructs the focal spot of the seed field; in the latter one E₂ (r,t) upon reflection back through the diffusing glass plate transforms into a field with spatial spectrum virtually identical to that of the seed field before diffusion.