Gain and phase-conjugation fidelity of a four-wave Brillouin mirror based on methane

We report measurements of the gain and phase-conjugation fidelity of a nondegenerate four-wave Brillouin mirror (FWBM) based on high-pressure methane. In the experiments we used a Nd:YAG laser that produced single-mode pulses of 13-nsec duration. Depending on the energy of the pump we measured signal reflectivities between 10 and 10(3), constant over six orders of magnitude of signal energy. We also determined the conjugation fidelity of the FWBM by measuring the Strehl ratio of the conjugated return as a function of the energy of the input signal. We show that the Strehl ratio reduces to 0.5 for an input energy of 1.5 x 10(-13) J/mode of the FWBM. This noise-equivalent signal energy constitutes the limiting sensitivity of the FWBM. Our experiments show that a gas-filled FWBM could be used as a phase conjugator and high-repetition-rate optical image intensifier.