An optical parametric oscillator comprising: an optical cavity; a semiconductor gain- medium (14) located within the optical cavity, such that together they form a semiconductor laser, and a nonlinear material (24) located within the cavity such that the nonlinear material continuously generates down- converted idler- and signal-waves in response to a pump-wave continuously generated by the semiconductor gain-medium, wherein the pump wave is resonant within the optical cavity and one or other but not both of the down-converted waves is resonant within the pump wave cavity or a further optical cavity. Brewster plates (40) ensure singly resonant OPO and a birefringent filter (30) is used for frequency setting. Coupled cavities alllow for setting the photon lifetime in the cavity that relaxation oscillations are prevented.