Optical parametric oscillator

An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave (called "pump") with frequency ${\displaystyle \omega _{p}}$ into two output waves of lower frequency (${\displaystyle \omega _{s},\omega _{i}}$) by means of second-order nonlinear optical interaction. The sum of the output waves' frequencies is equal to the input wave frequency: ${\displaystyle \omega _{s}+\omega _{i}=\omega _{p}}$. For historical reasons, the two output waves are called "signal" and "idler", where the output wave with higher frequency is called signal. A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, ${\displaystyle \omega _{s}=\omega _{i}=\omega _{p}/2}$, which can result in half-harmonic generation when signal and idler have the same polarization.

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