An Investigation of Untapered and Tapered Fibre Transmission Properties

This thesis is about investigation of untapered and tapered fibre transmission properties by experimentation and simulation. Tapered fibre fabricated from commercial products and custom fibre from Bath University was investigated including different fibre type, taper length, and taper ratios. Simulations of untapered and tapered fibre by consideration of geometrical optics using Zemax are presented and compared to electromagnetic waveguide simulations using COMSOL. In addition, the impact of modal noise in tapered fibre is investigated, especially graded-index tapers, by quantifying macro- and micro-bending loss. Moreover, the fabrication of build-inhouse connector tapered fibre is introduced as a robust and cost-saving tool for spectrograph link. Lastly, a tapered fibre is tested with a compact EXOplanet high-resolution SPECtrograph (EXOhSPEC) of the University of Hertfordshire using Tungsten and ThAr lamps. Although the cladding light is detected clearly in custom graded-index taper, the light throughput in EXOhSPEC is improved. Overall these results indicate the potential for reasonable fibre-fed spectrograph performance using a ’single tapered fibre’

Incoherent light in tapered graded-index fibre: a study of transmission and modal noise

© 2022 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license, http://creativecommons.org/licenses/by/4.0/We investigated the impact of taper length on light transmission through tapered graded-index fibres. We tested commercial fibres from Thorlabs and a custom graded-index fibre using both coherent and incoherent light sources. Our experimental results show optimum performance for taper transition lengths of 25 mm, although our simulations suggest further improvement may be possible for even shorter transition lengths. We also measured the modal noise power fluctuations caused by bending the fibre. Here, we observe that the custom fibre tapers have the highest transmission but suffer from the most modal noise. Accordingly, we find that the commercial graded-index fibre tapers promise practical usage as a beam mode-field converter, as they have lower power fluctuations but retain relatively high transmission if compared to commercial small core step-index fibre.Peer reviewe

Transmission properties of tapered optical fibres: Simulations and experimental measurements

© 2021 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license. https://creativecommons.org/licenses/by-nc-nd/4.0/We measured the transmission of tapered and untapered optical fibres as a function of input beam numerical aperture at 635 nm. The tapered fibres were fabricated with an adiabatic tapering process from graded and step-index fibres with 50 m core diameters to form a 100 mm long taper with 5:1 taper ratio. We tested tapered graded-index and step-index fibres fabricated from commercial Thorlabs products and a custom graded-index taper. The 5:1 tapered graded-index fibre can give a transmission greater than 0.4 for Thorlabs and 0.6 for the custom taper. We simulated the transmission of the tapered fibres and found reasonable agreement with the measured graded-index tapered fibre results across the numerical aperture range of interest. Experimentally, step-index tapered fibres performed relative poorly and considerably below modelling expectations. Based on our examinations this arises because the properties of step-index fibre were not robust to the tapering process. Suitably tapered graded-index fibres may offer a new route for efficient focal ratio reduction of fibre optic signals, e.g., in fibre-fed spectrographs, though we stress that our measurements have been limited to monochromatic light in this work.Peer reviewe

A small actively-controlled high-resolution spectrograph based on off-the-shelf components

© 2021. The Astronomical Society of the Pacific. The original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. https://creativecommons.org/licenses/by/3.0/We present the design and testing of a prototype in-plane echelle spectrograph based on an actively controlled fiber-fed double-pass design. This system aims to be small and efficient with the minimum number of optical surfaces—currently a collimator/camera lens, cross-dispersing prism, grating and a reflector to send light to the detector. It is built from catalog optical components and has dimensions of approximately 20 × 30 cm. It works in the optical regime with a resolution of >70,000. The spectrograph is fed by a bifurcated fiber with one fiber to a telescope and the other used to provide simultaneous Thorium Argon light illumination for wavelength calibration. The positions of the arc lines on the detector are processed in real time and commercial auto-guiding software is used to treat the positions of the arc lines as guide stars. The guiding software sends any required adjustments to mechanical piezo-electric actuators which move the mirror sending light to the camera removing any drift in the position of the arc lines. The current configuration using an sCMOS detector provides a precision of 3.5 milli-pixels equivalent to 4 ms −1 in a standard laboratory environment.Peer reviewe

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

© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/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.Peer reviewe