Absolute-frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser.

A frequency comb is generated with a chromium-doped forsterite femtosecond laser, spectrally broadened in a dispersion-shifted highly nonlinear fiber, and stabilized. The resultant evenly spaced comb of frequencies ranges from 1.1 to beyond $1.8 \mathit{\mu }\mathrm{m}$. The frequency comb was referenced simultaneously to the National Institute of Standards and Technology’s optical frequency standard based on neutral calcium and to a hydrogen maser that is calibrated by a cesium atomic fountain clock. With this comb we measured two frequency references in the telecommunications band: one half of the frequency of the $d/f$ crossover transition in ${ }^{87}\mathrm{Rb}$ at 780 nm, and the methane ${\nu }_2+2{\nu }_3$ $R\left(8\right)$ line at 1315 nm.