Laser frequency combs aid the search for exoplanets

Garching, Germany--A team of scientists headed by Theodor Hänsch from the Laser Spectroscopy Division at the Max Planck Institute of Quantum Optics has collaborated with researchers from the European Southern Observatory (ESO), the Instituto de Astrofísica de Canarias, and Menlo Systems GmbH (Martinsried) in modifying the optical frequency-comb technique in a way that it can be applied for the calibration of astronomical spectrographs.¹ Hänsch is one of the inventors of the optical frequency comb.

The new instrument has been tested successfully with the High Accuracy Radial velocity Planet Searcher (HARPS), a spectrograph at the 3.6 m telescope at the La Silla Observatory in Chile. A tenfold improvement of precision was obtained as compared with traditional spectral lamp calibrators. This should greatly enhance the chances to find earthlike planets outside our solar system.

These planets cannot be imaged directly, even with the largest telescopes. One of the most successful detection methods is the measurement of the small Doppler shifts in the spectrum of the parent star due to the recoiling motion caused by the planet. The star's light contains numerous spectral lines characteristic of the different chemical elements in the star’s gas atmosphere. When the star is moving towards or away from the observer, these lines are shifted to slightly higher or lower frequencies; this shift is what is measured to determine the presence of an exoplanet.

Better calibration with frequency combs

Measuring this parameter is equivalent to comparing it with a calibrated standard. Previously, the precision in frequency measurements was limited by slow drifts such as from aging of the calibration source (for example, a thorium spectral lamp). The laser frequency combs greatly improve the precision of frequency measurements; in 2005 MPQ and ESO decided to cooperate on the development of frequency combs for the calibration of astronomical spectrographs. As the first test measurements carried out at the VTT in Tenerife in 2008 turned out to be very promising, the scientists began to work on a frequency comb for the HARPS-spectrograph at the La Silla Observatory.

The adoption of the laser-frequency comb for astronomical spectroscopy posed a few major technical challenges. Even precision spectrographs like HARPS provide a limited frequency resolution, typically of 105. Hence the lines of the frequency comb to be developed have to be spaced at intervals of more than 10 GHz, otherwise the spectrograph cannot resolve them. Furthermore, astronomical spectrographs operate in the visible spectral region.

In order to ensure robust and stable operation fiber-laser systems were chosen as the basis of the frequency comb. Fiber-laser systems, however, emit light in the IR region, with spectral distances of a few 100 MHz. The scientists were able to change these properties by implementing a cascade of several spectral filters and using advanced fibers (developed by Philip Russell from the Max Planck Institute for the Science of Light, Erlangen). This resulted in a frequency comb with the desired mode spacing and a broad spectrum in the visible region. The calibration of the HARPS spectrograph with this frequency comb resulted in a sensitivity for velocity changes as small as 2.5 cm/s. This was demonstrated in a series of measurements in November 2010 and January 2011. By observing a star with a well known planet for a couple of nights, the team could prove the high stability of the system over time.

For the near future the scientists pursue a task that is even more demanding than looking for planets. Astronomical observations have clearly shown that the universe is not static but instead expanding continuously. New results on the microwave background radiation and the observation of supernovae suggest that this expansion is accelerating over time. However, the change of the velocity is expected to be very small -- on the order of 1 cm/s per year. Such precision is to be delivered by the next ESO project, the European Extremely Large Telescope (E-ELT), which is planned to be constructed in Chile in the next decade. High-precision frequency combs will be at the heart of its CODEX spectrograph, providing a calibration precision of one part per 300 billion.

REFERENCE:

1. Tobias Wilken et al., Nature, 31 May 2012; DOI:10.1038/nature11092.