  We present a reliable, narrow linewidth (100 kHz) continuous-wave optical
parametric oscillator (OPO) suitable for high-resolution spectroscopy
applications. The OPO is based on a periodically-poled lithium-niobate crystal
and features a specially designed intracavity etalon which permits its
continuous tuning and stable operation at any desired wavelength in a wide
operation range. We demonstrate Doppler-free spectroscopy on a rovibrational
transition of methane at 3.39 um.
