Tiny Doppler shifts in the light from a star can reveal the wobble in its position caused by a transiting planet. But to measure these shifts astronomers need to precisely calibrate their spectrometers, which is not easy. Help comes from photonics: two groups, both writing in Nature Photonics, report on microresonator laser frequency combs that enable spectrometer calibration with a precision high enough to potentially spot Earth-like planets.

Credit: Suh, M.-G. et al. (2019), Springer Nature Limited

Frequency combs are like rulers in frequency space: sources with spectra consisting of discrete, equally spaced frequencies that are known precisely. Ten years ago, laser frequency combs were first used for calibration in astronomy. However, they have not yet reached their full potential because of the complexity of coupling them to astronomical spectrometers. Due to the laser repetition rate, laser frequency combs typically have a comb spacing of a few GHz, too narrow for astronomical spectrometers to resolve. Although it’s possible to work around this using techniques such as spectral filtering, doing so is complicated and introduces unwanted optical effects.

The new ‘astrocombs’, reported by Tobias Herr and co-workers and Kerry Vahala, Charles Beichman and co-workers, produce spectral lines with spacings close to 20 GHz, ideally suited for spectrometers. The combs use microresonators based on dissipative Kerr solitons, ultra-short optical pulses arising from the balance between dissipation and nonlinearity, and between parametric gain and loss. The solitons’ short round-trip time in the laser cavity leads to a high repetition rate.

Although the potential of Kerr soliton microresonators for astronomical spectroscopy has been recognized for some time, their practical implementation has proved challenging. This is because it is not only comb spacing that matters: to be useful for spectrometer calibration, the combs must also be stable and robust. “Until now, operating a microresonator-based comb for precision spectroscopy outside the protected setting of a photonics research laboratory has not been attempted,” says Herr. Their group tested the comb with the GIANO-B high-resolution near-infrared spectrometer at the Telescopio Nazionale Galileo in Spain. “This would not have been possible without the combined expertise of multiple groups in optical frequency metrology, microfabrication and astronomy,” says Herr. At the Keck Observatory in Hawaii, Suh and co-workers used their microresonator astrocomb to calibrate the NIRSPEC near-infrared spectrometer.

Next-generation spectrometers will make it possible to take full advantage of the precision afforded by microresonator astrocombs. In addition, the astrocombs are power-efficient and can be miniaturized to a few cm3 — small enough to be used in space-based telescopes.