Optical parametric oscillator
A continuous-wave optical parametric oscillator (cw-OPO) converts a continuous-wave input laser wave (called "pump") into two continuous-wave output waves of lower frequency () by means of nonlinear-optical interaction. The sum of the output waves frequencies is equal to the input wave frequency: . For historic reasons, the two output waves are called "signal" and "idler".
The OPO consists essentially of an optical resonator and a 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 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, 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.
In order to change the output wave frequencies, the phasematching properties of the crystal must be changed. This is accomplished by changing its temperature or orientation or quasi-phasematching period (see below). In addition, the resonator may contain elements to suppress mode-hops of the resonating wave and to change the optical path length of the resonator. Mode-hop suppression often requires active control of some element of the OPO system.
As nonlinear optical crystals, crystals using quasi-phasematching (QPM) can be employed. With a suitable range of these, output wavelengths from 700 nm to 5000 nm can be generated. Common pump sources are Neodymium lasers at 1 m or 0.5 m.
An important feature of the OPO is the coherence and the spectral width of the generated radiation. The two output waves are, to a very good approximation, coherent states (laser waves). The linewidth of the resonated wave is very narrow (as low as several kHz). The nonresonated generated wave also exhibits narrow linewidth if a pump wave of narrow linewidth is employed.
The output waves can be tuned in frequency by changing the path length of the optical resonator or by tuning the pump frequency.