1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,
2. Department of Physics, Harvard University, 17 Oxford Street, Cambridge, Massachusetts 02138, USA
3. Department of Electrical Engineering and Computer Science and Research Laboratory for Electronics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
4. MenloSystems Inc., 69 Stickles Pond Road, Newton, New Jersey 07860, USA

Correspondence to: Ronald L. Walsworth^1,2 Correspondence and requests for materials should be addressed to R.L.W. (Email: rwalsworth@cfa.harvard.edu).

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 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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