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

TL;DR

This paper proposes a method to generate a broadband astro-comb using spectral broadening in nonlinear optical fibers, addressing challenges like side-mode suppression and asymmetry, and demonstrates a design achieving high calibration accuracy for astrophysical applications.

Contribution

It introduces a filtering cavity approach to mitigate nonlinear broadening effects in astro-comb generation, enabling high-precision wavelength calibration.

Findings

Cascaded four-wave-mixing degrades comb quality.

Filtering cavity compensates performance penalties.

Design achieves over 300 nm bandwidth with 16 GHz line spacing.

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 \approx100,000; 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 (~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.