Ewan Allan | Searcharxiv

arXiv2026

Demonstration of a Single-Laser-Diode-Pumped Ti:Sapphire Astrocomb on the Southern African Large Telescope

Astrocombs -- broadband lasers comprising thousands of narrow, uniformly spaced and atomically-referenced spectral lines -- offer gold-standard wavelength calibration for ground-based optical telescopes, with the potential for the cm/s precision needed for radial-velocity follow-ups of exoplanet candidates from photometric space missions like Kepler, TESS and PLATO. Current astrocombs, particularly those operating in the visible spectrum, are complex and expensive, putting them beyond the reach of many observatories. Here, we present the first on-instrument demonstration of a new and simple astrocomb concept, providing calibration light across nearly the entire 550 nm to 890 nm red channel of the Southern African Large Telescope's High Resolution Spectrograph (SALT-HRS). An octave-spanning supercontinuum generated in a silicon nitride waveguide by a GPS-referenced diode-pumped Ti:sapphire laser is filtered to 21 GHz before fibre delivery to the spectrograph. Using the astrocomb, we obtain the wavelength solution for one order of SALT-HRS and the wavelength-dependent line-spread function of the instrument. With potential for further extension into the blue spectral region, this uniquely simple architecture brings visible-to-infrared astrocomb technology within the reach of a wider range of astronomical observatories.

astro-ph.IM↗

arXiv2026

Visible octave frequency combs in silicon nitride nanophotonic waveguides driven by Ti:sapphire lasers

Nonlinear nanophotonic waveguides have opened a route to compact frequency combs for precision metrology, spectroscopy and astronomy, yet broadband comb access to the visible remains challenging on CMOS-compatible platforms. Silicon nitride is widely accessible and low loss into the visible, but most demonstrations rely on telecom pumping and thick stress-managed films, where the large spectral gap to the visible dispersive wave raises the soliton order and power required for efficient conversion. Here we show that pumping closer to the visible provides a complementary route. Starting from crack-free 400 nm SiN films, we implement dispersion-engineering with air-clad nanophotonic waveguides whose enhanced geometric dispersion opens an anomalous-dispersion window across the Ti-sapphire tuning range. Femtosecond Ti-sapphire pulses then drive octave-spanning combs from the visible to the near-infrared, with the visible edge and overall bandwidth lithographically tuned by the waveguide width at pulse energies of only tens of picojoules. The air-clad geometry also produces strong polarization-dependent dispersion, enabling switching between all-normal and soliton-dominated broadening in the same device, and support octave-spanning combs at 1 GHz repetition rates directly driven by a compact diode-pumped Ti-sapphire oscillator. These results position air-clad SiN nanophotonic waveguides as an efficient interface between emerging short-wavelength integrated gain platforms and fully integrated visible frequency-comb engines.

physics.optics↗