Optical frequency link between an acetylene stabilized laser at 1542 nm and an Rb stabilized laser at 778 nm using a two-color mode-locked fiber laser

We have demonstrated optical frequency link of two frequency-stabilized laser diodes operating at 1542 nm (a rotational-vibrational line of isotope acetylene molecule) and 778 nm (an Rb two photon transition) using a two-color mode-locked fiber laser. The laser produces a frequency comb (300 fs pulse) in the 1560 nm region and, through the second harmonic generation (SHG) process, another frequency comb (100 fs pulse) in the 780 nm region. Beat note signals have been observed between each of the stabilized laser radiation and a mode of the mode-locked fiber laser in the vicinity of the stabilized laser frequency. The S/N ratios were 25 dB and 30 dB with 100 kHz resolution bandwidth for beat notes at λ=1542 nm and λ=778 nm, respectively. Preliminary result has shown the potential of the system for frequency measurements with the accuracy of below 100 kHz. This limitation will be overcome by improving the frequency counting system using a phase-locked tracking oscillator and installing a PZT element in the fiber laser cavity. Supposing the accuracy of the mode spacing is similar to the case of a mode-locked Ti:sapphire laser, the accuracy of kHz level is very presumable.