A combination of continuous-wave optical parametric oscillator and   femtosecond frequency comb for optical frequency metrology
  We combine a tunable continuous-wave optical parametric oscillator and a femtosecond Ti:Sapphire laser frequency comb to provide a phase-coherent bridge between the visible and mid-infrared spectral ranges. As a first demonstration of this new technique we perform a direct frequency comparison between an iodine stabilized Nd:YAG laser at 1064 nm and an infrared methane optical frequency standard at $3.39 \mu$m. 
Evgeny Kovalchuk, Thilo Schuldt and Achim Peters