Tunable 30 GHz laser frequency comb for astronomical spectrograph characterization and calibration

TL;DR

This paper presents a tunable 30 GHz laser frequency comb capable of broadband coverage from 700 to 1300 nm, designed for precise calibration of astronomical spectrographs and correction of detector artifacts.

Contribution

It introduces a method for full deterministic tunability of a 30 GHz electro-optic comb with supercontinuum generation, applicable to high-precision spectrograph calibration.

Findings

Achieved full tunability of the frequency comb across 700-1300 nm.

Demonstrated control of laser frequency and mode spacing.

Applicable to calibration of astronomical spectrographs.

Abstract

The search for earth-like exoplanets with the Doppler radial velocity technique is an extremely challenging and multifaceted precision spectroscopy problem. Currently, one of the limiting instrumental factors in reaching the required long-term $10^{-10}$ level of radial velocity precision is the defect-driven sub-pixel quantum efficiency variations in the large-format detector arrays used by precision echelle spectrographs. Tunable frequency comb calibration sources that can fully map the point spread function across a spectrograph's entire bandwidth are necessary for quantifying and correcting these detector artifacts. In this work, we demonstrate a combination of laser frequency and mode spacing control that allows full and deterministic tunability of a 30 GHz electro-optic comb together with its filter cavity. After supercontinuum generation, this gives access to any optical frequency across 700 - 1300 nm. Our specific implementation is intended for the comb deployed at the Habitable Zone Planet Finder spectrograph and its near-infrared Hawaii-2RG array, but the techniques apply to all laser frequency combs used for precision astronomical spectrograph calibration and other applications that require broadband tuning.