Modal Noise Mitigation through Fiber Agitation for Fiber-fed Radial Velocity Spectrographs

TL;DR

This paper investigates fiber agitation techniques to reduce modal noise in high-precision spectrographs, finding that quasi-chaotic motion of high-amplitude agitators effectively suppresses noise and improves radial velocity measurements.

Contribution

It explores and optimizes fiber agitation parameters, introducing a new agitation method that significantly reduces modal noise and RV errors in spectrographs.

Findings

Quasi-chaotic agitation best suppresses modal noise.

New agitator design reduces RV error to below 3.2 cm/s.

Optimized agitation improves high-precision spectroscopic measurements.

Abstract

Optical fiber modal noise is a limiting factor for high precision spectroscopy signal-to-noise in the near-infrared and visible. Unabated, especially when using highly coherent light sources for wavelength calibration, modal noise can induce radial velocity (RV) errors that hinder the discovery of low-mass (and potentially Earth-like) planets. Previous research in this field has found sufficient modal noise mitigation through the use of an integrating sphere, but this requires extremely bright light sources, a luxury not necessarily afforded by the next generation of high-resolution optical spectrographs. Otherwise, mechanical agitation, which "mixes" the fiber's modal patterns and allows the noise to be averaged over minutes-long exposures, provides some noise reduction but the exact mechanism behind improvement in signal-to-noise and RV drift has not been fully explored or optimized by the community. Therefore, we have filled out the parameter space of modal noise agitation techniques in order to better understand agitation's contribution to mitigating modal noise and to discover a better method for agitating fibers. We find that modal noise is best suppressed by the quasi-chaotic motion of two high-amplitude agitators oscillating with varying phase for fibers with large core diameters and low azimuthal symmetry. This work has subsequently influenced the design of a fiber agitator, to be installed with the EXtreme PREcision Spectrograph, that we estimate will reduce modal-noise-induced RV error to less than 3.2 cm/s.