Demonstration of a phase conjugate resonator using degenerate four-wave mixing via coherent population trapping in rubidium

We demonstrate a ring cavity laser using the gain mechanism of a four-wave mixing process, mediated by two-photon Zeeman coherence resulting from coherent population trapping in a rubidium vapor cell. The cell acts as an amplifying phase conjugate mirror at one corner of the ring cavity. Even though the fundamental process requires non-degeneracy between the probe and the conjugate in the atom's frame, they are degenerate in the laboratory frame, via selection of a moving band of atoms for the gratings. As such, no frequency shifting is necessary in the cavity. The polarization orthogonality of the probe and the conjugate is compensated by an intra-cavity quarter wave plate.