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Telescopes and space missions

Optical ‘astrocomb’ could boost searches for Earth-like planets

16 Jan 2018
An example of the spectrum from two laser frequency combs
Fine-toothed: the optical spectrum of two laser frequency combs

A new type of laser frequency comb (LFC) has been developed by scientists in Europe. The prototype device could lead to improvements in how scientists search for Earth-like exoplanets, measure the expansion of the Universe and test the fundamental constants of nature.

LFCs produce spectral lines of light with evenly spaced frequencies and have a wide range of applications in metrology and spectroscopy. The new LFC was developed by Tobias Herr of the Swiss Centre for Electronics and MicrotechnologyFrancesco Pepeof the Geneva Observatory and colleagues. It uses a laser-driven microresonator on a silicon-nitride chip that produces 24 GHz pulses for use in calibrating near-infrared spectrometers. This gives it an advantage over traditional LFCs, which operate at frequencies below 10 GHz and create a line spacing that is too small for astronomical spectroscopy.

The pulses are produced by way of a phenomenon known as temporal dissipative Kerr-cavity solitons (DKSs), which involves trapping ultra-short pulses of light in a circular, micron-sized microresonator. Each time the DKS pulse passes the microresonator’s input-output coupler, some of the pulse is siphoned away and directed towards the spectrometer, producing a series of spectral lines that, in the prototype, are each precisely 24 GHz apart. These lines form a spectral comb and act as a precise calibration tool for the spectrometer.

One popular method of detecting exoplanets is the radial velocity technique. This involves measuring a star’s subtle motion that is caused by the gravitational tug of an orbiting planet. These motions are often no faster than walking pace and require highly accurate spectroscopic measurements of the Doppler shift in the star’s light as it moves. The size of the Doppler shift and the period at which it occurs can tell astronomers both the mass and the distance from the star of the planet. The greater the mass of the star, or the less massive or more distant the planet, the smaller the Doppler shift.

Increasing accuracy

Currently, astronomical spectrometers use hollow-cathode lamps that produce a limited number of noisy calibration lines, or standard, low frequency LFCs that pass through Fabry–Pérot etalons to increase their spectral range at the expense of accuracy. For example, the HARPS instrument on the 3.6 m telescope at the European Southern Observatory in Chile can measure a Doppler shift caused by velocities as low as 30 cm/s. Meanwhile, the ESPRESSO, which saw first light on ESO’s Very Large Telescope in December 2017 can achieve an overall spectral resolution of 10 cm/s. Both use thorium-argon lamps as well as LFCs passed through Fabry–Pérot etalons.

Although the success of the DKS frequency comb also depends on the stability of the spectrometer, it has the potential to measure Doppler shifts of just a few centimetres per second. This means that it could, in principle, be used to discover potentially habitable worlds orbiting Sun-like stars.

“In the future we hope to increase the number of LFC lines to reach the 1 cm/s level,” says Herr. “However, before the technology can be used routinely there are number of technological challenges to overcome,” Herr adds. Among these obstacles is the need to increase the span of the LFC to cover the entire near-infrared and optical bands, and also the need to make the technology less complex and more user-friendly for routine use.

Astronomical applications

However, Pepe says, “instruments such as the upcoming NIRPSspectrograph, which will join HARPs on ESO’s 3.6 metre telescope in 2019, will possibly be equipped with LFCs based on this new technology”. Ultimately, the compact nature of the technology could even see it employed in spectrometers on space-based missions.

Beyond planet-finding, the new technology could improve measurements of the redshifts of galaxies, thereby assisting in providing a more accurate measure of dark energy or the Hubble constant. Meanwhile, by observing galaxies at varying distances and measuring shifts in specific emission lines that are directly dependent upon the properties of the fundamental constants of nature, cosmologists are seeking to discover whether these constants are really variables. Because the shifts related to varying constants, such as the fine structure constant, are so tiny, a DKS laser frequency comb is likely to be the best way of making these measurements.

The new laser frequency comb is described in a preprint on arXiv.

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