# Toward a Broadband Astro-comb: Effects of Nonlinear Spectral Broadening in Optical Fibers

 Title: Toward a Broadband Astro-comb: Effects of Nonlinear Spectral Broadening in Optical Fibers Author: Chang, Guoqing; Li, Chih-Hao; Phillips, David M.; Walsworth, Ronald L.; Kärtner, Franz X. Note: Order does not necessarily reflect citation order of authors. Citation: Chang, Guoqing, Chih-Hao Li, David F. Phillips, Ronald L. Walsworth, and Franz X. Kärtner. 2010. Toward a Broadband Astro-comb: Effects of Nonlinear Spectral Broadening in Optical Fibers. Optics Express 18(12): 12736-12747. Full Text & Related Files: Chang_TowardBroadband.pdf (196.1Kb; PDF) Abstract: We propose and analyze a new approach to generate a broadband astrocomb 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 $$\approx 100,000$$; 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 1GHz 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 astrocomb is predicted to provide $$1 cm/s (\sim 10 kHz)$$ 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. Published Version: doi:10.1364/OE.18.012736 Other Sources: http://walsworth.physics.harvard.edu/publications/2010_Chang_OptExp.pdf http://arxiv.org/abs/1003.4680 Terms of Use: This article is made available under the terms and conditions applicable to Other Posted Material, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#LAA Citable link to this page: http://nrs.harvard.edu/urn-3:HUL.InstRepos:9367002 Downloads of this work: