Optical Parametric Oscillators (OPOs) are based on multiwave interaction in a nonlinear medium. They have been realized in a variety of configurations, giving rise to an extended range of new dynamical problems. Like lasers, OPOs admit a steady state bifurcation at threshold and, in addition, they may exhibit bistability or Hopf bifurcations. Moreover, thermal effects may be dominant in cw oscillators leading to interesting slow–fast responses where the temperature is a new dynamical variable. Second-harmonic generation (SHG) is in a sense the inverse process of degenerate parametric amplification. Devices based on SHG are described by similar evolution equations but show different phenomena.

Parametric processes

An OPO is a light source similar to a laser, 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. 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. We may thus 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 we may also realize wide wavelength tunability. The downside is that any OPO requires a pump source with high intensity and relatively high spatial coherence. Therefore, we always need a laser as the pump source, generally a diode-pumped solid state laser.