Abstract

We report measurements of the second harmonic dependence on the input fundamental light power in optical fibers possessing a χ⁽²⁾ induced by seeding.¹ Our results are explained by including self-phase and cross-phase modulation (SPM and CPM) in the equations governing second harmonic generation. The index changes due to SPM and CPM do not cancel out and result in an intensity-dependent phase mismatch, whose strength also depends on the relative polarizations of the fundamental and second harmonic lights. During the seeding stage, this effect detunes the χ⁽²⁾ grating toward negative phase mismatch. When generating the second harmonic through such a detuned χ⁽²⁾ grating, the intensity-dependent term first brings the interaction closer to exact phase matching, resulting in a more than quadratic dependence of second harmonic power on the input power. At very high intensity, the induced phase mismatch becomes positive at the peak of the pulses and frequency conversion saturates. The polarization dependence of the saturation is also discussed.

© 1989 Optical Society of America