A phenomenon of phase conjugation (PC) in nonlinear optical processes was discovered in the USSR in 1971 at about the same time in two laboratories—the Institute of Physics of the Belorussian Academy of Science and the Physical Institute of the USSR Academy of Science—for two different conditions, e.g., four-wave interaction and stimulated scattering of light.
These two phenomena differ radically in their physical nature. In the first case, a nonlinear medium is permeated with two counterpropagating pump waves (reference waves). The so-called signal wave subjected to phase conjugation is also directed at this medium. To induce wave interaction in media with cubic nonlinearity, the frequencies of the reference and signal waves are chosen to be near, which gives rise to a fourth wave that propagates in the direction opposite that of the signal wave propagation. This wave, called a phase-conjugated wave, builds on the energies of the reference waves. Reflectivity equal to the power ratio of the phase-conjugated and signal waves can reach values largely exceeding unity. This is a threshold-free method of phase conjugation, as reflectivity has a weak dependence on the signal wave amplitude.