Residual reflections of the idler wave in nominally singly resonant optical parametric oscillators can lead to fluctuations in the output because the parametric conversion process is sensitive to the phases of the reflected waves. The energy fluctuations in a pulsed optical parametric oscillator are studied experimentally and numerically for single- or multi-longitudinal mode pump beams. We find that the fluctuations are reduced by a multi-mode pump so this may be preferable when unwanted reflections are present. We also observe that the parametric conversion process leads to serious self-focusing of the pump beam, and this limits the maximum safe pump energy.
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