title: Toward a broadband astro-comb: effects of nonlinear spectral broadening in optical fibers.
abstract: We propose and analyze a new approach to generate a broadband astro-comb by spectral broadening of a narrowband astro-comb inside a highly nonlinear optical fiber. Numerical modeling shows that cascaded four-wave-mixing dramatically degrades the input comb's side-mode suppression and causes side-mode amplitude asymmetry. These two detrimental effects can systematically shift the center-of-gravity of astro-comb spectral lines as measured by an astrophysical spectrograph with resolution approximately 100,000; and thus lead to wavelength calibration inaccuracy and instability. Our simulations indicate that this performance penalty, as a result of nonlinear spectral broadening, can be compensated by using a filtering cavity configured for double-pass. As an explicit example, we present a design based on an Yb-fiber source comb (with 1 GHz repetition rate) that is filtered by double-passing through a low finesse cavity (finesse = 208), and subsequent spectrally broadened in a 2-cm, SF6-glass photonic crystal fiber. Spanning more than 300 nm with 16 GHz line spacing, the resulting astro-comb is predicted to provide 1 cm/s (approximately 10 kHz) radial velocity calibration accuracy for an astrophysical spectrograph. Such extreme performance will be necessary for the search for and characterization of Earth-like extra-solar planets, and in direct measurements of the change of the rate of cosmological expansion.
authors.affiliation: Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Mass Ave Cambridge MA 02139 USA. guoqing@mit.edu
authors.forename: Guoqing
authors.initials: G
authors.lastname: Chang
authors.forename: Chih-Hao
authors.initials: CH
authors.lastname: Li
authors.forename: David F
authors.initials: DF
authors.lastname: Phillips
authors.forename: Ronald L
authors.initials: RL
authors.lastname: Walsworth
authors.forename: Franz X
authors.initials: FX
authors.lastname: Kärtner