Lasers brush up astrophysics spectroscopy

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Menlo Systems and two international teams of scientists have developed a technique using laser frequency combs to improve the precision and stability of wavelength calibration of astrophysical spectrographs.

Teams at the Max Planck Institute of Quantum Optics (MPQ) together with the European Southern Observatory (ESO) and one at the Harvard-Smithsonian Center for Astrophysics together with the Massachusetts Institute of Technology (MIT) helped develop the technique. 

Astronomers use spectrographs to spread the light from celestial objects into its component colours in the same way as water droplets create a rainbow from sunlight. By analysing the resulting spectrum they can measure the velocities of stars, galaxies and quasars, search for planets around other stars, or study the expansion of the universe.  

A spectrograph must be accurately calibrated so that the colours of light can be correctly measured, just like a precise ruler is needed to measure length accurately. In this case the laser frequency comb provides a ruler with an extremely accurate and fine grid, for measuring colours – or more accurately frequencies. 

Such a ruler, a so called 'astro-comb', uses a mode-locked femtosecond laser and a Fabry-Pérot filtering cavity. When linked to an atomic clock it provides a precise and stable wavelength standard against which light from a star can be measured.  

The first successful laboratory demonstration was conducted by the Havard group, by mode filtering using a Menlo Systems Octavius laser. The MPQ/ESO team has tested a prototype device at the VTT (Vacuum Tower Telescope) solar telescope in Tenerife, on 8 March 2008.  

Using a frequency comb generated by a FC1500 fibre-based optical frequency synthesiser from Menlo Systems, they measured the spectrum of the sun in infrared light. These results of the first light for frequency combs on an astronomical telescope are published in science. The technique promises to achieve an unprecedented accuracy and to make many observations that have previously been considered technically unachievable possible.