Development of components and fibres for the power scaling of pulsed holmium-doped fibre sources

Abstract

In this thesis the optimisation and peak power scaling of pulsed holmium-doped fibre lasers were investigated with the aim of demonstrating a fibre gain medium that is able to address the requirements of applications that currently rely on bulk crystalline Ho:YAG or Ho:YLF solutions.

Conventional fibre processing techniques such as cleaving, end-capping and component fabrication were improved upon using CO2 laser processing. The resulting components and processes are also characterised under high power operating conditions and have enabled subsequent experimentation and demonstrations.

Holmium-doped silica fibres were fabricated and characterised with the aim of reducing impurity contaminations, improving composition and achieving efficient operation at 2.1 μm. These fibres were characterised passively using transmission spectroscopy and actively in a laser configuration. The most efficient of these compositions operated with a 77% slope efficiency in a core-pumped laser up to average powers of 5 W and was then processed into a double-clad geometry. The cladding-pumped fibre was operated at 70 W output power with a slope efficiency of 67% and represents one of the highest power and most efficient cladding-pumped holmium-doped fibres demonstrated to-date.

Small-signal amplifiers utilising both thulium-doped and holmium-doped silica fibres were demonstrated. These amplifiers offered a broad wavelength coverage spanning 490 nm at 15 dB gain from 1660 nm – 2150 nm. This remarkably broad wavelength coverage is attractive for a large number of disciplines looking to exploit this previously difficult-to-reach wavelength range.

In addition to these devices, the average power and peak power scaling of 2 μm fibre sources was investigated. A thulium-doped fibre laser operating at 1950 nm with >170 W of output power, a tuneable holmium-doped fibre laser producing >15 W over the wavelength span from 2040 nm – 2171 nm and a pulsed holmium-doped fibre amplifier with >100 kW peak power at 2090 nm are reported.

Finally we review the requirements for efficient scaling of mid-infrared optical parametric oscillators and analyse the non-linear effects that arise when attempting to scale the peak power in silica fibres in the 2 μm spectral region. We implement a range of strategies to reduce the onset of nonlinear effects and demonstrate a holmium-doped fibre amplifier with peak power levels exceeding 36 kW in a 5 ns pulse with a spectral width of <1 nm. This represents the highest spectral density achieved for nano-second pulse duration from pulsed holmium-doped fibre sources. This preliminary result provides an excellent platform for further peak power scaling and also in replacing conventional Q-switched Ho:YAG lasers.

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