Prototype mode scrambler with a four-bar linkage crank-rocker design, built using LEGO Technic elements. Compare with Figure 11. The test fiber is the black cable, and is tied down to bar BQ. Note that here, the motor shaft is not directly attached to point M, but instead a gear system is used with a 40:24 gear ratio. — astro-ph.IM
When coherent light propagates through a multimode optical fiber, the modes interfere at the fiber exit boundary, producing a high-contrast speckle interference pattern called modal noise.
This non-uniform interference pattern introduces systematic errors in fiber-fed precision radial velocity (RV) spectrographs which are detrimental to exoplanet mass measurement. Modal noise can be mitigated by a device called a fiber mode scrambler or fiber agitator, which dynamically perturbs the fiber to change the interference pattern over time, smoothing it over long exposures.
In this paper, we present a prototype optical fiber mode scrambler based on a four-bar linkage crank-rocker mechanism, developed for the GMT-Consortium Large Earth Finder (G-CLEF). G-CLEF is a fiber-fed, high-resolution, precision RV spectrograph for the Magellan Clay Telescope and Giant Magellan Telescope (GMT).
To support this effort, we developed a fiber testing setup capable of imaging the near-field and far-field output of fibers and measuring focal ratio degradation. We designed, built, and tested the mode scrambler, using our setup, on step-index multimode optical fibers with various shapes, including octagonal, square, and rectangular core cross-sections.
We developed custom software utilizing alpha shapes to identify the boundary of an arbitrarily shaped fiber and to compute a signal-to-noise ratio metric for quantifying modal noise.
We investigated the effects of different mode scrambler parameters, such as agitation frequency, on mitigating modal noise. Our results offer valuable insights into optimizing fiber mode scrambling for precision RV spectrographs.
Matthew C. H. Leung, Colby Jurgenson, Andrew Szentgyorgyi, William Podgorski, Mark Mueller, Yahel Sofer Rimalt, Joseph Zajac, Cem Onyuksel, Daniel Durusky, Peter Doherty
Comments: 25 pages, 30 figures, SPIE Optics + Photonics 2025
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2509.18306 [astro-ph.IM] (or arXiv:2509.18306v1 [astro-ph.IM] for this version)
https://doi.org/10.48550/arXiv.2509.18306
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Journal reference: Proc. SPIE 13627, Techniques and Instrumentation for Detection of Exoplanets XII, 136271J (18 September 2025)
Related DOI:
https://doi.org/10.1117/12.3063478
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Submission history
From: Matthew Leung
[v1] Mon, 22 Sep 2025 18:31:56 UTC (6,838 KB)
https://arxiv.org/abs/2509.18306
Astrobiology,