A 12.5 GHz-spaced optical frequency comb spanning >400 nm for near-infrared astronomical spectrograph calibration

A 12.5 GHz-spaced optical frequency comb locked to a global positioning system disciplined oscillator for near-infrared (IR) spectrograph calibration is presented. The comb is generated via filtering a 250 MHz-spaced comb. Subsequent nonlinear broadening of the 12.5 GHz comb extends the wavelength range to cover 1380-1820 nm, providing complete coverage over the H-band transmission window of earth's atmosphere. Finite suppression of spurious sidemodes, optical linewidth, and instability of the comb has been examined to estimate potential wavelength biases in spectrograph calibration. Sidemode suppression varies between 20 and 45 dB, and the optical linewidth is similar to 350 kHz at 1550 nm. The comb frequency uncertainty is bounded by +/- 30 kHz (corresponding to a radial velocity of +/- 5 cm/s), limited by the global positioning system disciplined oscillator reference. These results indicate that this comb can readily support radial velocity measurements below 1 m/s in the near IR. (C) 2010 American Institute of Physics. [doi:10.1063/1.3436638]