30-Hz relative linewidth watt output power 1.65-$\mu$m continuous-wave singly resonant optical parametric oscillator

We built a 1-watt cw singly resonant optical parametric oscillator operating at an idler wavelength of 1.65~$\mu$m for application to quantum interfaces. The non resonant idler is frequency stabilized by side-fringe locking on a relatively high-finesse Fabry-Perot cavity, and the influence of intensity noise is carefully analyzed. A relative linewidth down to the sub-kHz level (about 30 Hz over 2 s) is achieved. A very good long term stability is obtained for both frequency and intensity.