Broadband Astrocomb with Feed-Forward Integration of a cw Laser Frequency Marker

Calibrating high-resolution spectrographs traditionally relies on absorption cells or hollow cathode lamps, but their precision is constrained by their irregular spacing and line intensities [1]. Laser frequency combs (LFCs) offer a transformative alternative with atomically referenced, drift-free, and equally spaced lines [2], [3]. While LFCs with comb-mode spacings <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(f_{\text {rep }}\right)$</tex> between 250–1000 MHz can deliver thousands of low-noise, high-contrast calibration lines, the multi-GHz resolution of astronomical spectrographs requires Fabry-Pérot (FP) pre-filtering to extract a sub-set of this primary spacing to a multiple, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$m f_{\text {rep }}$</tex> (typically > 10 GHz), that allows individual comb modes to be resolved. A several-nm length change is sufficient to adjust the FP transmission condition between adjacent comb-mode sub-sets, but there is no a priori way to determine which of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$m$</tex> sub-sets has been filtered to the spectrograph. In this work, we present a Ti:sapphire-based astrocomb in RIZ band (650–1050 nm) integrated with a single-frequency laser using a feed-forward technique. We show how this approach precisely integrates the cw laser mode into the LFC, providing a fiducial marker and enabling Pound-Drever-Hall (PDH) locking of the FP cavity by the cw laser to automatically ensure broadband LFC filtering when the FP length is chosen appropriately.