Continuous-wave fiber optical parametric oscillator with sub-MHz linewidth

A continuous-wave fiber optical parametric oscillator with a ring cavity containing a highly nonlinear fiber and two cascaded narrowband filters is demonstrated. We report a 3-dB laser linewidth of less than 400 kHz at 1549 nm with single longitudinal mode operation. Wavelength stability of the laser is <±100 MHz, governed mainly by the narrowband filters.