Novel optical fibre fabrication techniques for Yb-doped high-power fibre lasers and sensing applications

Abstract

The work presented in this thesis reports on four novel techniques for fabricating speciality silica preforms and optical fibres. The project aims were to conceive new fabrication methods by adapting conventional Modified Chemical Vapour Deposition (MCVD) and optical fibre drawing equipment, and to demonstrate fibre devices for ytterbium (Yb)-doped high-power fibre lasers (HPFL) and optical sensing applications.

Firstly, a new in-situ solution doping technique is presented for fabricating actively-doped fibre-preforms of complex design. The fabrication and characterisation of several multilayered rare-earth (RE)-doped fibres suitable for HPFL applications are reported, including an Yb-doped (>18,000ppm, by weight) fibre with a low effective-NA, which incorporates a pedestal refractive index profile and a unique aluminosilcate (Al:Si) inner-cladding. The vapour-phase deposition of RE ions in fibre-preforms has also been demonstrated using a novel chemical-in-crucible process that is intended for use with precursors of low volatility. Modifications to the standard MCVD setup were made which allows the dopant source to be placed within the substrate glassware and in close proximity to the reaction zone. Preforms with dopant concentrations of up to 25,000 ppm (by weight) of Yb³⁺ ions have been attained using an organometallic precursor, whilst passive Al:Si preforms containing >16mol% of Al have been achieved using gaseous aluminium chloride.

A straightforward fabrication technique for producing silica suspended-core holey fibre (SC-HF) is also presented. The drawn fibre exhibits a relatively low optical loss (of 0.3 dB.m^-1 at λ = 1550nm), and the high air-filling fraction, which was predicted as ~30% (for a core size of 0.8µm), is believed to the highest reported value at the time the work was undertaken. The sensing capability of SC-HF has been demonstrated by constructing an all-fibre acetylene-filled gas cell. The final experimental chapter describes the first example of a novel flat fibre concept. Extended lengths of low-loss planar glass substrates were produced using MCVD and conventional fibre drawing equipment. In combination with direct UV-writing, multifunctional planar waveguiding devices can be fabricated that are mechanically flexible. The potential of the flat fibre platform for sensing applications is discussed.

The reported fabrication techniques have been implemented through the successful demonstration of several fibre devices suitable for Yb-doped HPFLs and optical sensing applications. The developed techniques have future potential in industry and manufacturing, and it is anticipated that the work presented will enable fibres with novel properties and glass compositions to be researched.

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Identifiers

ORCID for J.K. Sahu: orcid.org/0000-0003-3560-6152

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