Optical Parametric Oscillators Information
Optical parametric oscillators (OPO) use parametric amplification in nonlinear crystals to produce optical gain. The output of an OPO can be varied over wide spectral ranges. The device consists of an optical resonator and a nonlinear optical crystal. OPOs use a nonlinear response of a medium to a driving field to convert photons of one wavelength to photons of longer wavelengths.
The bandwidth is determined by the bandwidth acceptance of the nonlinear crystal. Basic designs usually exhibit relatively broad bandwidths of about 15 cm-1. Introducing frequency selective elements into the cavity helps reduce bandwidth.
Optical parametric oscillators can be used in several applications including spectroscopy to cover very wide spectral regions, military applications including the generation of broadband high-power light for blinding heat-seeking missiles, and in digital projection displays.
An optical parametric oscillator is a light source similar to a laser in that it also uses a resonator, but based upon optical gain from parametric amplification in a nonlinear crystal rather than from stimulated emission. It converts an input laser wave (a "pump") into two output waves of lower frequency (idler and signal) by means of second-order nonlinear optical interaction. In the nonlinear optical crystal, 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 sum of the output waves' frequencies is equal to the pump wave frequency.
A main attraction of OPOs is that the signal and idler wavelengths, which are determined by a phase-matching condition, can be varied over wide ranges. Thus it is possible to access wavelengths (e.g. in the mid-infrared, far-infrared, terahertz) 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.
OPOs can be classified as continuous-wave or pulsed. The latter are easier to build, since the high intensity lasts only for a tiny fraction of a second, which damages the nonlinear optical material and the mirrors less than a continuous high-intensity device. An important feature of the OPO is the coherence and the spectral width of the generated radiation. When the pump power is significantly above threshold, the two output waves are, to a very good approximation, coherent states (laser-like waves). The linewidth of the resonated wave is very narrow (as low as several kHz). The non-resonated generated wave also exhibits narrow linewidth if a pump wave of narrow linewidth is employed. Narrow-linewidth OPOs are widely used in spectroscopy.