Phase-insensitive all-optical transistors based on second-order nonlinearities

We demonstrate and compare two phase-insensitive all-optical transistors (AOT's) based on frequency-degenerate quadratic three-wave interactions. In particular, we demonstrate gain using KTP in a type II geometry. Both AOT's exploit the phase insensitivity inherent to three-wave parametric processes when only two fields are input, providing amplification of a small signal at the operating frequency via the interaction with a second-harmonic wave. The first scheme is based on successive up- and down-conversion (i.e., cascading) while the second relies on parametric down-conversion. We obtain gains of 5 and 160 in the two configurations, respectively, with a significant background and output coherent to the pump in the first case, no background and coherence between output and signal in the second.