Doppler-Free Spectroscopy of CH4 Using a CW Optical Parametric Oscillator

We present the latest results on using continuous-wave optical parametric oscillators (cw-OPOs) for high-resolution Doppler-free saturation spectroscopy of methane and point out the resulting potential for future optical frequency standards. We also propose a new scheme for combining cw-OPOs with the recently developed femtosecond frequency combs to provide a versatile bridge between optical frequency standards in the visible, near-infrared and mid-infrared spectral ranges.