Journal Article

A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1

Li, Chih-Hao, Sasselov, Dimitar, Benedick, Andrew J., Kärtner, Franz X., Glenday, Alexander G., Szentgyorgyi, Andrew, Fendel, Peter, Phillips, David F., Walsworth, Ronald L.

2008

Overview

Combing the sky for 'earths' The current count of extrasolar planets (on planetquest.jpl.nasa.gov) stands at 277, none of them Earth-like. Most were detected as a Doppler shift in stellar spectral lines, a method that 'sees' planets down to about five times the mass of the Earth. If Earth-sized planets are to be revealed by this observational approach, better Doppler shift resolution via improved spectrograph wavelength calibration will be required. A newly developed instrument, the 'astro-comb', achieves just that by adapting the laser frequency comb, a device that has revolutionized laboratory spectroscopy, to the needs of astrophysics. This involves reducing the density of comb lines, without compromising spectral resolution. A performance test of the astro-comb is reported in this issue, and in May 2008, the new device joins the search for 'exoearths' in earnest. Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s −1 , sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. The fabrication of an 'astro-comb' that should allow a precision as high as 1 cm s −1 in astronomical radial velocity measurements is reported Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s -1 (ref. 1 ), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earth-like orbit, a precision of ∼5 cm s -1 is necessary. The combination of a laser frequency comb with a Fabry–Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration 2 , 3 , 4 , with recent encouraging results 5 . Here we report the fabrication of such a filtered laser comb with up to 40-GHz (∼1-Å) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or ‘astro-comb’, is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s -1 in astronomical radial velocity measurements.

• Publisher: Nature Publishing Group UK,Nature Publishing,Nature Publishing Group
• Subject: Astronomy / Astrophysics / Biological and medical applications / Calibration / Earth / Earth, ocean, space / Exact sciences and technology / Extrasolar planetary systems / Extrasolar planets / Fabrication / Frequencies / Fundamental areas of phenomenology (including applications) / Humanities and Social Sciences / Instruments, apparatus, components and techniques common to several branches of physics and astronomy / Lasers / letter / Mercury / Metrological applications / multidisciplinary / Optics / Physics / Planets / Science / Science (multidisciplinary) / Spaceborne and space research instruments, apparatus and components (satellites, space vehicles, etc.) / Stars