The parametric oscillator has at least one optical non-linear crystal, e.g. a barium borate crystal, with its input and output faces (E,A) lying in planes at an acute angle to one another. The angle between the input and output face of the non-linear crystal can lie between 1 to 13 degrees and is selected to ensure high resonation of the optical parametric oscillator for the signal wave and low resonation for each other wave.
