Wavelength selection and transverse mode control in high power fibre lasers

Abstract

In this thesis we explore the wide parameter space of thulium doped silica fibre lasers, looking at various techniques for the control of operating wavelength and bandwidth as well as transverse beam profile. We demonstrate the extremely broad tunability of thulium fibre sources. Through various device architectures we show wavelength coverage over a 450nm wavelength range from 1660nm to 2115nm. With a simple external cavity containing an electronically controllable acousto-optic tunable filter we construct a wavelength agile tunable fibre source. This source demonstrates rapid wavelength selection and coverage within the thulium gain band including multiple simultaneous wavelength selection. Under fixed wavelength operation we show the generation of high power and efficiency 1726nm light in a monolithic thulium fibre source with output powers of up to 12.6W and internal laser slope efficiencies of 69%.

Exploiting the fast cavity dynamics of pulsed fibre sources we demonstrate the generation of a bandwidth and wavelength flexible ASE source. Under pulsed operation we generate peak powers of greater than 2kW with tunable bandwidth from 24nm to 0.28nm corresponding to a change in coherence length from ~5mm down to ~60µm. Applying this source to nonlinear frequency conversion, we show the ASE seeded supercontinuum generation at output powers of up to 1.5W, covering a wavelength range of more than one octave from 1000nm to greater than 2400nm. Looking further ahead with this ASE source, we investigate the effects of propagation and amplification within a multimode waveguide, highlighting the potentially detrimental effects of modal interference and through spectral bandwidth tailoring, successfully supresses these effects. Showing stable high peak power amplification within a multimode thulium fibre amplifier.

Finally, we introduce a novel mode selection technique utilising multimode fibre Bragg gratings. Allowing the selection of individual modes within a multimode fibre oscillator. We experimentally demonstrate this technique within a multimode thulium fibre source showing the electronically controllable selection and arbitrary switching between the fundamental and next higher order mode at switching speeds of up to 20kHz and output powers of greater than 5W. Extending this technique, we show the generation of near arbitrary superpositions of the first two guided modes with electronically tunable beam ‘flatness’ and other parameters. The prospects for further core area scaling are also discussed.

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