Continuous Ultraviolet to Blue-Green Astrocomb

TL;DR

This paper presents a novel method for generating a continuous, broadband ultraviolet to blue-green optical frequency comb using nonlinear waveguide techniques, enabling precise spectrograph calibration for astronomical observations.

Contribution

It introduces a new approach combining second-harmonic generation and sum-frequency-mixing in a waveguide to produce gap-free UV-visible light from an infrared laser comb.

Findings

Achieves gap-free 390-520 nm spectrum from a 1 GHz laser.

Demonstrates effective elimination of spectral gaps in the comb.

Requires only ~100 pJ pulse energies for broadband generation.

Abstract

The characterization of Earth-like exoplanets and precision tests of cosmological models using next-generation telescopes such as the ELT will demand precise calibration of astrophysical spectrographs in the visible region, where stellar absorption lines are most abundant. Astrocombs--lasers providing a broadband sequence of ultra-narrow, drift-free, regularly spaced optical frequencies on a multi-GHz grid--promise an atomically-traceable, versatile calibration scale, but their realization is challenging because of the need for ultra-broadband frequency conversion of mode-locked infrared lasers into the blue-green region. Here, we introduce a new concept achieving a broad, continuous spectrum by combining second-harmonic generation and sum-frequency-mixing in an aperiodically-poled MgO:PPLN waveguide to generate gap-free 390-520 nm light from a 1 GHz Ti:sapphire laser frequency comb. We lock a low-dispersion Fabry-Perot etalon to extract a sub-comb of bandwidth from 392-472 nm with a spacing of 30 GHz, visualizing the thousands of resulting comb modes on a high resolution cross-dispersion spectrograph. Complementary experimental data and simulations demonstrate the effectiveness of the approach for eliminating the spectral gaps present in second-harmonic-only conversion, in which weaker fundamental frequencies are suppressed by the quadratic \{chi}^((2)) nonlinearity. Requiring only ~100 pJ pulse energies, our concept establishes a practical new route to broadband UV-visible generation at GHz repetition rates.