Optical phase conjugation with very weak continuous-wave signals

Summary form only given. We use a high-reflectivity phase-conjugate mirror (PCM) to demonstrate that optical phase conjugation can be performed on continuous-wave signal beams having power spectral densities as low as 5 x 10/sup -18/ W/Hz (i.e., 19 photons/sec.Hz). Our high-reflectivity (>600%), wide-bandwidth (90 MHz) PCM is formed by performing four-wave mixing in a potassium vapor cell using a frequency-stabilized Ti:sapphire laser that is tuned near the D/sub 2/ line. We have measured the noise spectra of the conjugate beam as functions of pump detuning from resonance, probe-pump detuning, pump-probe angular separation, and the number density of potassium atoms.