Mitigating Modal Noise in Multimode Circular Fibres by Optical Agitation using a Galvanometer

• Main Authors: Ghosh, Supriyo, Boonsri, Chantira, Martin, William, Jones, Hugh R. A, Choochalerm, Piyamas, Usher, Sarah, Yerolatsitis, Stephanos, Wocial, Thomas, Wright, Thomas
• Format: Journal Article
• Language: English
• Published: 28-12-2023
• Subjects: Physics - Earth and Planetary Astrophysics; Physics - Instrumentation and Methods for Astrophysics; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2312.16996

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 reimage 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 micron step-index, 50 micron graded-index, and a
combination of 50 micron 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% mitigation of modal noise at a cost of about 5%
output light-loss. Our solution is easy to use and can be implemented in
fibre-fed spectrographs.