Power-scaling of wavelength-flexible two-micron fibre sources

Abstract

In this thesis we explore thulium doped silica fibre based sources, focusing on laser and amplified spontaneous emission behaviours. We analyse new ways of improving fibre cavity performances by first demonstrating a novel way of manufacturing doped fibres showing high pump absorption whilst retaining ease of use for cleaving and splicing. This new process offers a trade-off between circular and non-circular fibre geometries by maintaining advantages of both configurations. An implementation process on fibre drawing towers is detailed for future large scale production of highly circular active fibres with high mode scrambling, resulting in high pump absorption comparable, and potentially higher, to octagonal fibre. We then introduce a new way of improving fibre tip movement insensitivity in free-space feedback arms by utilising corner-cubes as reflective elements, with results showing transverse fibre tip movement of more than a millimetre in specific configurations whilst maintaining high feedback efficiency. Output power variation of less than 35% was demonstrated over a translation window of ±1.2mm in some cases.

Exploiting the movement insensitive properties offered by a corner-cube, a tunable ring laser based on fibre tip movement and a Fabry-Perot etalon is demonstrated. Up to 5nm of quasi-continuous fine tuning is proven with a theoretical accuracy of 8pm/μm fibre tip movement and a linewidth lower than 1.5GHz is demonstrated. Potential for rapid wavelength scanning and for much broader tuning over a window of few tens of nm is proposed with further modifications of the experimental setup to allow wider fibre tip movement without feedback losses appearing.

Focus is then centred on the broad tuning capabilities on thulium doped silica fibres and a CW laser source allowing tuning over more than 130nm in the 2μm band is described. Tuning is achieved by the use of a digital micro-mirror device (DMD) coupled with a diffraction grating, allowing further spectral shaping. Up to 8.5W of output power is displayed, pump power limited, with capabilities for multi-wavelength emission and spectral power density shaping by adjustment of the micro-mirror matrix reflective pattern. Modification of the system was explained in order to achieve different requirements, either by improving tuning range, accuracy or minimal line width.

Utilising the fast dynamics of a doped fibre cavity, a cavity is built around an acousto-optic modulator to frustrate lasing in order to create a feedback tolerant pulsed amplified spontaneous emission (ASE) source. This source was designed to allow generation of a pulsed wavelength-controllable ASE via the use of a DMD coupled with a diffraction grating. A core-pumped setup is demonstrated, reaching tuning from 1860 to 1950nm and a cladding-pumped architecture is built for longer wavelengths generation to improve compatibility with amplifier stages. This source displayed tuning performances from 1940 to 2020nm with peak power of up to 1.5kW and pulses shorter than 100ns. Multi-waveband behaviour is demonstrated and output bandwidth is controlled through the DMD. A cladding-pumped amplification stage is described and amplification of the ASE output by 15dB, reaching up to 72W, pump limited, was demonstrated corresponding to peak powers of more than 5kW. Prospects for pumping of a ZGP OPO cavity with an ASE are discussed, detailing the potential benefits of utilising a bandwidth-adjustable ASE source for mid-infrared generation

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Identifiers

ORCID for Jacob Mackenzie: orcid.org/0000-0002-3355-6051

ORCID for Peter Shardlow: orcid.org/0000-0003-0459-0581

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

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