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Xiao, Boyu (2025) Mid-infrared chalcogenide glass fibre lasers for operation beyond 4 μm wavelength. PhD thesis, University of Nottingham.

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Abstract

The 3–50 μm Mid-infrared (MIR) wavelengths have vast potential across many applications, including healthcare, environmental monitoring, industrial process control and security systems, due to their overlap with the “fingerprint region” of molecular absorption. However, fibre-based light sources in the MIR range are currently limited, particularly beyond 4 μm. Chalcogenide glass optical fibres are promising candidates for enabling lasing in the beyond 4 μm, thanks to their broad MIR transmission window and capacity for rare earth dopant solubility. This project focuses on the development of fibre lasers beyond 4 μm, using Ce3+ doped step-index chalcogenide glass fibre. The MIR Photonics Group at the University of Nottingham reported the first fibre laser operating beyond 4 μm in 2021 but the power output was not measured and also there were difficulties in repeating the result with many fibre samples indicating an issue with fibre geometry or quality.

An initial focus of this thesis was on identifying four key types of defects observed in chalcogenide glass fibres, produced both prior to and during this study. These defects – cladding defects, hole defects at the core/cladding interface, scale-like defects on the fibre outer surface, and particle formations on cleaved chalcogenide fibre end-faces – were believed to have negative impacts on the key properties of chalcogenide fibres i.e. optical loss, mechanical strength, laser damage threshold. Among these defects, the hole defects at the core/cladding interface were considered the most concerning due to their proximity to the fibre’s core which is the most critical portion of the fibre.

To produce hole-defect-free Ce3+ doped step-index chalcogenide glass fibres, a new fibre fabrication process, termed the co-extruded-rod-in-tube method by the author in this project was developed. This technique introduced an additional inner-cladding layer in the traditional core-cladding structure; this inner-cladding layer was co-extruded with the core material to form a first core-cladding structure which was then inserted in the outer-cladding tube to be drawn to fibre. Scanning electron microscopy analysis of the cleaved fibre samples made using the co-extruded-rod-in-tube confirmed the successful elimination of hole defects at the core/inner-cladding interface.

Lasing was achieved at ∼4.65 μm, pumped by a 4.15 μm quantum cascade laser. The newly fabricated Ce3+ doped step-index chalcogenide fibre produced a maximum output power of ∼7.2 ± 0.1 mW, with a slope efficiency of 2.7 ± 0.1% and a threshold of ∼49.8 ± 0.5 mW. These results represent a significant step towards the realisation of efficient fibre-based MIR laser sources beyond the 4 μm wavelength.

This project has made substantial progress in the understanding and management of defects in chalcogenide fibres, bringing the goal of developing highly efficient MIR fibre lasers closer to fruition.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Seddon, Angela B. Furniss, David Phang, Sendy
Keywords:	Chalcogenide glass optical fibres; Fibre lasers; Mid-infrared transmission; Fibre defects
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) > TA1501 Applied optics. Phonics
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Electrical and Electronic Engineering
Item ID:	80321
Depositing User:	Xiao, Boyu
Date Deposited:	31 Jul 2025 04:40
Last Modified:	31 Jul 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/80321

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