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optical parametric oscillators, OPO
Acronym: OPO
Definition: coherent light sources based on parametric amplification within an optical resonator
An optical parametric oscillator (OPO) [1,?2] is a light source similar to a laser, also using a kind of laser resonator, but based on optical gain from parametric amplification in a nonlinear crystal rather than from stimulated emission. Like a laser, such a device exhibits a threshold for the pump power, below which there is negligible output power (only some parametric fluorescence).
Figure 1: Schematic of an optical parametric oscillator.
A main attraction of OPOs is that the signal and idler wavelengths, which are determined by a phase-matching condition, can be varied in wide ranges. Thus it is possible to access wavelengths (e.g. in the mid-infrared, far-infrared or terahertz spectral region) which are difficult or impossible to obtain from any laser, and wide wavelength tunability (often by affecting the phase-matching condition) is also often possible. This makes OPOs very valuable, for example, for laser spectroscopy.
A limitation is that any OPO requires a pump source with high optical intensity and relatively high spatial coherence. Therefore, a laser is essentially always required for pumping an OPO, and as the direct use of a laser diode is in most cases not possible, the system becomes relatively complex, consisting e.g. of laser diodes, a diode-pumped solid-state laser, and the actual OPO.
Figure 2: Setup of a typical optical parametric oscillator with a ring resonator. The pump beam is injected through a dichroic mirror. The signal beam is resonant, whereas the idler is usually ejected by at least of the resonator mirrors.