Improved stabilization of a 13 µm femtosecond optical frequency comb by use of a spectrally tailored continuum from a nonlinear fiber grating

We report significant enhancement $(+24\mathrm{dB})$ of the optical beat note between a 657 nm cw laser and the second-harmonic generation of the tailored continuum at 1314 nm generated with a femtosecond Cr:forsterite laser and a nonlinear fiber Bragg grating. The same continuum is used to stabilize the carrier-envelope offset frequency of the Cr:forsterite femtosecond laser and permits improved optical stabilization of the frequency comb from 1.0 to $2.2\mu \mathrm{m}$. Using a common optical reference at 657 nm, a relative fractional frequency instability of $2.0\times {10}^{-15}$ is achieved between the repetition rates of Cr:forsterite and Ti:sapphire laser systems in 10 s averaging time. The fractional frequency offset between the optically stabilized frequency combs of the Cr:forsterite and Ti:sapphire lasers is $\pm (0.024\pm 6.1)\times {10}^{-17}$.