Near-IR frequency comb to characterize acetylene-filled fiber-based frequency references*

Optical frequency combs have revolutionized the field of optical frequency metrology. Typically Ti:sapphire lasers form the basis of these combs, but Cr:forsterite offers an interesting alternative in the near-IR, lasing at $\sim $1250 nm, and broadened in a highly nonlinear fiber (HNLF) to span an octave between 1020 and 2040 nm. We have demonstrated the first self-referenced Cr:forsterite laser that uses prisms for dispersion compensation, and observe narrower carrier-envelope offset beatnotes than in the case of the first Cr:forsterite laser to be self-referenced [1]. We will use this comb to characterize optical frequency references based on acetylene-filled hollow photonic bandgap optical fibers. Furthermore, we have developed a simplified technique for observing sub-Doppler features in these fibers, called a ``reflected pump'' technique, and compare it to more conventional methods. [1] K. Kim \textit{et al}., ``Stabilized frequency comb with a self-referenced femtosecond Cr:forsterite laser,'' Opt. Lett., \textbf{30}, 932 (2005).