Increase in the brightness of laser radiation due to phase conjugation as a result of frequency conversion

The fraction of the energy propagating within an angle close to the diffraction limit was measured by a double-pass laser system with phase conjugation. The measurements were carried out at the fundamental frequency and at harmonics of this frequency. The brightness of the harmonics was considerably higher than that of the fundamental-frequency radiation. An analysis was made of the factors preventing the attainment of the theoretical brightness maximum. The results obtained were interpreted by assuming a combined influence of wave aberrations and a partial spatial coherence of the original fundamental-frequency radiation.