Continuous ultraviolet to blue-green astrocomb

Yuk Shan Cheng, Kamalesh Dadi, Toby Mitchell, Samantha Thompson, Nikolai Piskunov, Lewis D. Wright, Corin B. E. Gawith,Richard A. McCracken & Derryck T. Reid

Abstract

Cosmological and exoplanetary science using transformative telescopes like the ELT will demand precise calibration of astrophysical spectrographs in the blue-green, where stellar absorption lines are most abundant. Astrocombs— lasers providing a broadband sequence of regularly-spaced optical frequencies on a multi-GHz grid—promise an atomically-traceable calibration scale, but their realization in the blue-green is challenging for current infrared laser-based technology. Here, we introduce a concept achieving a broad, continuous spectrum by combining second-harmonic generation and sum frequency-mixing in an MgO:PPLN waveguide to generate 390–520 nm light from a 1 GHz Ti:sapphire frequency comb. Using a Fabry-Pérot filter, we extract a 30 GHz sub-comb spanning 392–472 nm, visualizing its thousands of modes on a high-resolution spectrograph. Experimental data and simulations demonstrate how the approach can bridge the spectral gap present in second harmonic-only conversion. Requiring only ≈100 pJ pulses, our concept establishes a new route to broadband UV-visible generation at GHz repetition rates.