Frequency shifts in injection-seeded optical parametric oscillators with phase mismatch.

We have observed a frequency shift in the output signal pulses relative to the seed frequency in an injection-seeded, singly resonant, critically phase-matched, pulsed optical parametric oscillator in which phase mismatchwas intentionally introduced. The observed shifts can be large compared with the linewidth of the signal pulse, are approximately linear in phase mismatch, and increase with increasing pump fluence. We observe frequencyshifts of as much as +/-400 MHz for our 532-nm-pumped, potassium titanyl phosphate ring optical parametric oscillator. For zero phase mismatch, we observe nearly transform-limited linewidths of less than 130 MHz. Wecompare the experimental data to a simple analytic model that overestimates the shifts because it ignores pumpdepletion. We also compare our measurements with a numerical model that calculates the two-dimensional, transient electric fields and the resultant spectral distributions while explicitly including walk-off, diffraction, and pump depletion. We find good agreement between the experimental data and the results of this model.