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


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 calibration2,3,4, with recent encouraging results5. 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.

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Figure 1: Block diagram of the astro-comb.
Figure 2: Example astro-comb output spectrum with 37-GHz line spacing.
Figure 3: Suppression of extraneous source-comb lines for the astro-comb.

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This project is supported by the Harvard University Origins of Life Initiative, the Smithsonian Institution, and DARPA.

Author contributions D.S., A.S. and R.L.W. initiated the project. C.-H.L., D.F.P. and R.L.W. designed the comb filtering strategy. C.-H.L. and D.F.P. designed the Fabry–Pérot stabilization strategy. R.L.W. supervised the project. C.-H.L. constructed the comb filtering system and acquired the filtered-comb data. A.J.B., P.F. and F.X.K. provided the 1-GHz laser comb and assisted C.-H.L. in its integration with the comb filtering system. C.-H.L. performed the initial data analysis. C.-H.L., A.G.G., F.X.K., D.F.P., D.S., A.S. and R.L.W. interpreted the data and assessed systematic effects. D.S. and A.S. advised on the astrophysical relevance of the data.

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Correspondence to Ronald L. Walsworth.

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Li, CH., Benedick, A., Fendel, P. et al. A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1. Nature 452, 610–612 (2008).

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