We report on an experimental investigation and numerical analysis of the spectral properties of pulsed nanosecond optical parametric oscillation. The optical parametric oscillator (OPO) used in the experimental investigations consisted of a 2.5-mm-long crystal of beta-barium borate (pumped by the third harmonic of a Q-switched Nd:YAG laser) and a 3.5-mm-long optical cavity of two plane mirrors. Despite the short crystal, this OPO provided an efficiency of 26% at pump-pulse intensities of 30 mJ. Owing to the short cavity, the longitudinal mode spacing was ∼1 cm-1. The complete mode spectrum of individual OPO pulses could thus be recorded with a 1-m grating spectrometer having a spectral resolution of 0.25 cm-1. If the OPO was unseeded, the energy distribution in the mode spectrum varied considerably from pulse to pulse. Near threshold, the fluctuations of the energy of individual modes are close to 100%. The origin of these fluctuations are the statistical fluctuations of the vacuum field that initiate the OPO oscillation. The measured mode spectra were numerically simulated by solving the coupled amplitude equations for the signal and the idler modes and the pump field. The numerical results are in good agreement with the measurements. In further investigations, the OPO was seeded with either pulsed or cw laser radiation. The experimental results demonstrate that seed pulse energies of a few nanojoules or cw seed powers of a few milliwatts were sufficient for reliable single-longitudinal-mode operation. The mode spectra of the seeded OPO as well as the spectra recorded for seed powers in the regime between seeded and unseeded OPO operation are in good agreement with the spectra predicted by numerical analysis.
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