  We present an experimental technique allowing to stabilize the frequency of
the non resonant wave in a singly resonant optical parametric oscillator (SRO)
down to the kHz level, much below the pump frequency noise level. By comparing
the frequency of the non resonant wave with a reference cavity, the pump
frequency noise is imposed to the frequency of the resonant wave, and is thus
subtracted from the frequency of the non resonant wave. This permits the non
resonant wave obtained from such a SRO to be simultaneously powerful and
frequency stable, which is usually impossible to obtain when the resonant wave
frequency is stabilized.
