Mitigating modal noise in multimode circular fibres by optical agitation using a galvanometer

Supriyo Ghosh, Chantira Boonsri, William Martin, Hugh R.A. Jones, Piyamas Choochalerm, Sarah Usher, Stephanos Yerolatsitis, Thomas Wocial, Thomas Wright

Research output: Contribution to journalArticlepeer-review


Modal noise appears due to the non-uniform and unstable distribution of light intensity among the finite number of modes in multimode fibres. It is an important limiting factor in measuring radial velocity precisely by fibre-fed high-resolution spectrographs. The problem can become particularly severe as the fibre's core become smaller and the number of modes that can propagate reduces. Thus, mitigating modal noise in relatively small core fibres still remains a challenge. We present here a novel technique to suppress modal noise. Two movable mirrors in the form of a galvanometer re-image the mode-pattern of an input fibre to an output fibre. The mixing of modes coupled to the output fibre can be controlled by the movement of mirrors applying two sinusoidal signals through a voltage generator. We test the technique for four multimode circular fibres: 10 and 50 m step-index, 50 m graded-index, and a combination of 50 m graded-index and 5:1 tapered fibres (GI50t). We present the results of mode suppression both in terms of the direct image of the output fibre and spectrum of white light obtained with the high-resolution spectrograph. We found that the galvanometer mitigated modal noise in all the tested fibres, but was most useful for smaller core fibres. However, there is a trade-off between the modal noise reduction and light-loss. The GI50t provides the best result with about 60 per cent mitigation of modal noise at a cost of about 5 per cent output light-loss. Our solution is easy to use and can be implemented in fibre-fed spectrographs.

Original languageEnglish
Pages (from-to)8-18
Number of pages11
JournalRAS Techniques and Instruments
Issue number1
Early online date1 Jan 2024
Publication statusPublished - 2 Jan 2024

Data Availability Statement

The data underlying this article will be shared on reasonable request to the corresponding author.


  • Galvanometer
  • Instrumentation
  • Modal Noise
  • Optical Fibres
  • Spectrograph

ASJC Scopus subject areas

  • Earth and Planetary Sciences (miscellaneous)
  • Astronomy and Astrophysics
  • Artificial Intelligence
  • Engineering (miscellaneous)

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