Laser-frequency-comb calibration for the Extremely Large Telescope: an OPO-based infrared astrocomb covering the H and J bands

The Extremely Large Telescope (ELT) will address an unprecedented optical wavelength range from 370 to 2400 nm, and its high-resolution spectrograph (HIRES) will require a laser frequency comb calibrator of comparable coverage. An architecture based around a Ti:sapphire master comb in principle enables wavelengths across this range to be obtained by a combination of second-and third-order nonlinear effects. In this scheme, near-infrared wavelength coverage can be addressed by downconversion of the comb to 1600 nm using an optical parametric oscillator (OPO), followed by broadband supercontinuum generation in highly nonlinear fiber. Here we present an example of this approach in the form of a 10 GHz astrocomb comprising a Fabry Perot-filtered supercontinuum derived from a degenerate OPO and spanning 1.15 1.80 m. We characterize the astrocomb using Fourier-Transformspectroscopy, enabling the mode orders within the filtered comb to be identified.