Page 377 - Tunable Lasers Handbook

7 Optical Parametric OsciIIators 337

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FIGURE 1 8 Phase-matching cune for AgGaS, for a 2.10-pn pump

lasers. Both increased beam quality and decreased linewidth can lead to an
increased performance for the optical parametric oscillator.
Several different concepts are involved in the assessment of the performance
of an optical parametric oscillator including threshold, slope efficiency, total effi-
ciency. photon efficiency, and pump depletion. Optical parametric oscillators can
be operated either in a cw or a pulsed mode. Of the two modes of operation. the
pulsed mode is much more common since the operation of an optical parametric
oscillator is enhanced by a high power density. The threshold in the cwr mode is
straightforward to define as the amount of pump power required to achieve opti-
cal parametric oscillation. In the pulsed mode. the observable threshold, rather
than the instantaneous threshold. is usually quoted; however. this is not alw ays
made clear. While slope efficiency is sometimes quoted, it could represent either
the ratio of the increase in power at the output wavelength to the increase in
power at the pump wavelength or the increase in power of both the signal and
idler wavelengths to the increase in power at the pump wavelength. In the pulsed
mode. it could be quoted at the instant of peak power or it could be quoted for the
total output energy. Although laser theory usually predicts a nearly linear increase
in the output with increases in the input. optical parametric oscillator theory does
not necessarily predict the same approximation. However, in practice. a linear