Abstract: A second harmonic generation apparatus provides a polarization element transmitting only light polarized in an extra-ordinary axis direction on the proceeding path of a second harmonic emitted from a resonator. The light polarized in the extra-ordinary axis direction is transmitted to a beam splitter, and a part of the polarized light is fed back to a feedback circuit, so that the temperature of a non-linear birefringent crystalline element inside resonator can be stably controlled and the output second harmonic can be stably produced.

Abstract: A second harmonic generation method and apparatus is capable of stabilizing an output. The apparatus provides a first beam splitter on the proceeding path of a second harmonic output, and other beam splitters on the proceeding path of the reflecting beam of the second harmonic and that of the transmitted beam. A second and third beam splitters are arranged on the proceeding paths of the second harmonic output and the beam separated from the second harmonic output to satisfy ##EQU1## where K is a constant, R.sub..parallel. is reflectivity with respect to p-polarization parallel to the incident surface of the first and second beam splitter, and R.sub..perp. is reflectivity with respect to s-polarization perpendicular to the incident surface of the first and second beam splitters.