Stabilization of Femtosecond Lasers for Optical Frequency Metrology and Direct Optical to Radio Frequency Synthesis

We have actively stabilized the comb of frequencies from a mode-locked femtosecond laser using a Fabry-Perot reference cavity. This technique offers the ability to synthesize a comb of highly stable radio frequencies directly from optical transitions. The measured fractional frequency instability of the components of the frequency comb relative to the reference cavity was $<5\mathrm{}\times {10}^{-13\mathrm{}}$ in 0.1 sec. The variation of the optical mode spacing versus frequency of the reference cavity was also directly measured using this technique.