We theoretically consider photorefractive degenerate four–wave mixing oscillators and show that, for this particular system, the linear stability analysis technique leads to wrong results. The reason for that astonishing failure lies in the unphysical predictions of the undepleted pump approximation, which appears naturally during the linearization process of the full model. As a consequence, photorefractive four–wave mixing does not admit a perturbative treatment that permits the derivation of mean–field models, and one is forced to use the full model in order to make sensible predictions.

We report on experimental demonstration of optical transient detection (OTD) based on photorefractive two-wave mixing of femtosecond pulses. The demonstrated technique also combines nonlinear-crystal-based OTD with up-conversion from infrared into the visible range. The approach enables measurement of phase changes of a dynamic signal in the infrared using GaP- or Si-based detectors while suppressing stationary background. Experimental results reveal existence of the relation between input phases in the infrared and output phases in the visible wavelength range. We further present experimental evidence of additional merits of up-converted transient phase analysis under noisy conditions, such as residual continuous-wave emission affecting the ultrashort pulses from the laser.