An '''optical parametric oscillator''' (OPO) is a [[parametric oscillator]] which oscillates at optical frequencies. It converts an input [[lasersecond order]] wave (called "pump") into two output waves of lower frequency (<math>\omega_s, \omega_i</math>) by means of [[nonlinear optics| nonlinear optical interaction]]. The sum of the output waves frequencies is equal to the input wave frequency: <math>\omega_s + \omega_i=\omega_p</math>. For historic reasons, the two output waves are called "signal" and "idler", where the wave with bigger frequency is called signal. A special case is the degenerate OPO, when the output frequency is one-half the pump frequency, <math>\omega_s=\omega_i=\omega_p/2</math>.
