Fiber modal noise mitigation by a rotating double scrambler

TL;DR

This paper introduces a rotating double scrambler to effectively reduce fiber modal noise in high-precision spectrographs, especially at wavelengths with limited fiber modes, offering an alternative to traditional agitation methods.

Contribution

The paper presents a novel rotating double scrambler design that enhances modal noise mitigation by continuously varying mode excitation, improving performance over traditional fiber agitation.

Findings

Prototype design successfully reduces modal noise in laboratory tests

Rotating scrambler achieves more efficient mode averaging

Potential for improved radial velocity measurements in astronomy

Abstract

Fiber modal noise is a performance limiting factor in high-precision radial velocity measurements with multi-mode fiber-fed high-resolution spectrographs. Traditionally, modal noise is mitigated by agitating the fiber, this way redistributing the light that propagates in the fiber over many different modes. However, in case of fibers with only a limited number of modes, e.g. at near-infrared wavelengths or in adaptive-optics assisted systems, this method becomes very inefficient. The strong agitation that would be needed stresses the fiber and can lead to focal ratio degradation. As an alternative approach, we propose to use a classic optical double scrambler and to rotate the scrambler's first fiber end during each exposure. Because of the rotating illumination pattern of the scrambler's second fiber, the modes that are excited vary continuously. This leads to very efficient averaging of the modal pattern at the fiber exit and to a strong reduction of modal noise. In this contribution, we present a prototype design and first laboratory results of the rotating double scrambler.