Continuous ultraviolet to blue-green astrocomb

Article https://doi.org/10.1038/s41467-024-45924-6 Continuous ultraviolet to blue-green astrocomb Received: 25 July 2023 Accepted: 7 February 2024 1234567890():,; 1234567890():,; Check for updates Yuk Shan Cheng1, Kamalesh Dadi 1, Toby Mitchell1, Samantha Thompson Nikolai Piskunov3, Lewis D. Wright4, Corin B. E. Gawith 4,5, Richard A. McCracken 1 & Derryck T. Reid 1 2 , 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 infraredlaser-based technology. Here, we introduce a concept achieving a broad, continuous spectrum by combining second-harmonic generation and sumfrequency-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 secondharmonic-only conversion. Requiring only ≈100 pJ pulses, our concept establishes a new route to broadband UV-visible generation at GHz repetition rates. In the field of extreme precision radial velocity (EPRV) measurements, the aim is to push the detection threshold to 10 cm s−1. This 10 cm s−1 goal is set to enable the detection of low-mass, longer-period extrasolar planets (exoplanets), in particular those that are similar to our own Earth. On some of the next-generation instruments, such as ANDES on the ELT1, the aim is to achieve measurements at the 1–2 cm s−1 level, to enable high precision and high accuracy cosmological observations like the Sandage–Loeb Test2. To detect an exoplanet via the radial velocity (RV) method requires the collection of high-resolution spectra of its host star, over a timescale corresponding to at least three orbits of the exoplanet. The shift in wavelength of all the spectral lines is measured (Doppler shift) and recorded over time. For example, for the Earth in orbit around the Sun (viewed edge-on), this radial velocity timeseries would appear as a sinusoid with an amplitude of 9 cm s−1 and a period of 1 year. To put a 10 cm s−1 Doppler shift in perspective, the size of a single pixel on many current high-resolution spectrographs used for RV measurements is of the order of 1 km s−1, illustrating how these measurements require extremely precise wavelength calibration. Astrocombs—broadband laser frequency combs with multi-GHz spacing—represent the ideal wavelength calibrator3 for these tasks. To enable the detection of an Earth-analog exoplanet, the RV measurement must be stable over many years, have a single measurement precision of ≈ 1 m s−1, have spectral features that are regular and sufficiently spaced, unresolved (i.e., narrow intrinsic width), and cover the entire spectral format so that maximum information can be extracted from the stellar spectrum. Laser frequency combs can provide all these features, by virtue of their stability, atomically-referenceable accuracy, sub-100-kHz linewidths, multi-GHz spacing, and potential for broadband spectral coverage4,5. 1 Institute of Photonics and Quantum Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK. 2Astrophysics Group, Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0HE, UK. 3Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden. 4Covesion Ltd, Unit F3, Adanac North, Adanac Drive, Nursling, Southampton SO16 0BT, UK. 5Optoelectronics Research Centre, University of Southampton, Southampton, Hampshire e-mail: SO17 1BJ, UK. Nature Communications | (2024)15:1466 1 Article Radial velocity precision depends on the number of strong spectral features6, and for solar-like stars, the density of such lines increases towards the blue end of the spectrum. This is why the most advanced RV instruments today, e.g., HARPS (380–690 nm7), EXPRES (380–680 nm8), and ESPRESSO (378–789 nm9), reach out to the blue part of the spectrum. Future science cases such as the Sandage–Loeb test use the cosmological redshifted Lyman-alpha forest, with the wavelengths starting at ≈365 nm. Converting this advantage to RV precision requires accurate and reproducible wavelength calibration in the blue and near-ultraviolet. Alternative calibration methods (principally hollow-cathode lamps) do not provide intrinsic accuracy, homogeneous wavelength coverage, or stability of an LFC3. Blue-visible astrocombs have previously been demonstrated using a variety of approaches. The second-harmonic generation of near-infrared mode-locked lasers has produced narrowband astrocombs at 400 nm10,11. Supercontinuum generation in photonic-crystal fiber has enabled blue-green (435–600 nm12), green (530–560 nm13), and green-red (500–620 nm14) astrocombs. Recent improvements in the manufacture of silicon nitride (SiN) and periodically-poled lithium niobate (PPLN) waveguides have led to on-chip approaches which exploit χ ð2Þ and χ ð3Þ processes. Obrzud et al.15 demonstrated 400–600 nm of gap-free coverage at 10 GHz mode spacing through χ ð3Þ triple-sum-frequency-generation of an electro-optic modulator comb in SiN; however the intrinsically wide mode spacing of the pump source required amplification, broadening, and compression stages to achieve sufficient peak power for nonlinear conversion, despite the enhancement offered by waveguide confinement. Nakamura et al.16 reported a 30 GHz astrocomb which extended down to 350 nm via second, third, and fourth harmonic generation of a 1.5 µm source in a chirped PPLN waveguide, however, wide gaps remained in the spectral coverage, and the 250 MHz pump laser required multiple filtering and amplification stages to achieve the desired mode spacing. A similar approach has recently been demonstrated using an EOM pump source17. In this work, we propose and experimentally demonstrate a new approach that achieves a broad, continuous visible spectrum by combining second-harmonic generation (SHG) and sum-frequencymixing (SFM) in an aperiodically-poled MgO:PPLN waveguide. On its own, SHG suppresses weaker spectral features contained in the fundamental light, since it is a quadratic (χ ð2Þ ) nonlinearity, but by using a strong auxiliary pulse, SFM can be used to linearly transfer weak but broad infrared components into the visible. We illustrate this experimentally by using a Ti:sapphire laser frequency comb operating with a repetition frequency of f rep = 1 GHz to generate 390–520 nm light from an infrared supercontinuum, achieving a gap-free frequency bandwidth of >90 THz. The wide mode spacing of this visible comb means that a relatively low finess (...truncated)