A research team from the Laser Spectroscopy Division of Professor Theodor W. Hänsch at the Garching-based Max Planck Institute of Quantum Optics, in partnership with the Instituto de Astrofísica de Canarias, and the Martinsried-based Menlo Systems, the European Southern Observatory (ESO), has upgraded the frequency comb technique.
This technique can now be used for calibration of astronomical spectrographs. The new instrument has been successfully validated with the High Accuracy Radial velocity Planet Searcher (HARPS), at Chile’s La Silla Observatory. Up to 10 times more accuracy was achieved with this technique than with traditional spectral lamp calibrators. It will lead to investigating and identifying earth-like planets beyond the solar system.
However, these planets cannot be directly imaged and therefore detection methods such as measuring the small Doppler shifts in the spectrum of the parent star can be carried out. Distant stars generate light including numerous spectral lines that feature a variety of chemical elements in the star’s gas atmosphere. These lines are shifted to higher or lower frequencies based on the movement of the star with respect to the observer. By measuring the Doppler shifts, the star’s movement can be determined.
The invention of the laser-frequency combs during the late 90's led to increased precision of frequency measurements. In 2005, MPQ and ESO jointly developed the frequency combs, making it effective for calibration of astronomical spectrographs. Laser-based light with a spectrum containing numerous lines at constant intervals generates a frequency comb. The electronic feedback circuits enable each line to be defined with respect to an atomic clock. Slight variations of the star light can be observed by comparing the spectral lines of a star with the lines of the laser ruler.
The lines of the potential frequency comb need to be spaced at intervals over 10 GHz, enabling measurement by spectrograph as astronomical spectrographs function in the visible spectral region.
Frequency comb uses fibre-laser systems as the basis to assure robust operation. By using an array of several spectral filters and advanced fibres, the scientists altered these properties to create a frequency comb with the preferred mode spacing and a wide spectrum in the visible region. Based on this frequency comb, calibration of HARPS spectrograph was carried out, providing sensitivity for velocity changes up to 2.5 cm/s. The research team was able to determine the high stability of the system by studying a star with a familiar planet for two nights.
Source: http://www.mpq.mpg.de/