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[[Category:Nonlinear optics]]
A '''continuous-wave optical parametric oscillator''' (cw-OPO) converts a [[Continuous_wave | continuous-wave]] input [[Laser | laser]] wave (called "pump") into two [[Continuous_wave | continuous-wave]] output waves of lower frequency (<math>\omega_s, \omega_i</math>) by means of [[nonlinear optics | nonlinear-optical interaction]]. The sum of the output waves frequencies is equal to the input wave frequency: <math>\omega_s + \omega_i=\omega_p</math>. For historic reasons, the two output waves are called "signal" and "idler". A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, <math>\omega_s=\omega_i=\omega_p/2</math>.
The OPO consists essentially of an [[Optical_cavity|optical resonator]] and a [[Nonlinear Optics|nonlinear optical crystal]]. The optical resonator serves to resonate at least one of signal and idler waves. In the nonlinear optical crystal, the pump, signal and idler waves overlap. The interaction between these three waves leads to amplitude gain for signal and idler waves (parametric amplification) and a corresponding deamplification of the pump wave. The gain allows the resonating wave(s) (signal or idler or both) to oscillate in the resonator, compensating the loss that the resonating wave(s) experience(s) at each round-trip. This loss includes the loss due to outcoupling by one of the resonator mirrors, which provides the desired output wave. Since the (relative) loss is independent of the pump power, but the gain is dependent on pump power, at low pump power there is insufficient gain to support oscillation. Only when the pump power reaches a particular threshold level, oscillation occurs. Above threshold, the gain depends also on the amplitude of the resonated wave. Thus, in steady-state operation, the amplitude of the resonated wave is determined by the condition that this gain equals the (constant) loss. The circulating amplitude increases with increasing pump power, and so does the output power.
The photon conversion efficiency, the number of output photons per unit time in the output signal or idler wave relative to number of pump photons incident per unit time into the OPO can be high, in the range of tens of percent.
Typical threshold pump power is between tens of milli-Watt to several Watt, depending on the type of OPO. Output powers of several Watt can be achieved.
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