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 nm to 2400 nm, and its HIRES spectrograph 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 down-conversion of the comb to 1600 nm using an optical parametric oscillator (OPO), followed by broadband super continuum generation in highly nonlinear fiber. Here, we present an example of this approach in the form of a 10-GHz astrocomb comprising a Fabry-Pérot-filtered super-continuum, derived from a degenerate OPO and spanning 1.15–1.80-ߤm. We characterize the astrocomb using Fourier-transform spectroscopy, enabling the mode orders within the filtered comb to be identified.