Blue laser action in Tm-doped fluoride fibre

Abstract

This thesis describes the development of a fibre laser capable of converting infra-red light into coherent blue light in a simple and efficient way. This was achieved in a fluoride fibre, ZBLAN, doped with the rare-earth ion Tm³⁺ which has broad emission around 480nm when pumped by infra-red light with a wavelength near 1140nm. Multiple absorption of pump photons has been proposed as the upconversion process.

The spectroscopy of Tm³⁺-doped ZBLAN glass has been investigated. Specific attention has been paid to the absorption strength near 1140nm from both the ground state and the excited states, ³H₄, and ³F₄, of the Tm³⁺ ion. This spectroscopy has allowed a rate equation model of the process to be produced which aided the selection of the optimum pump wavelength and enabled the small-signal gain and laser threshold to be calculated.

The most efficient blue laser fabricated had a slope efficiency of 31% and a laser threshold of 90mW. both with respect to 1140nm pump power launched into the fibre core. By variation of the laser cavity parameters the laser threshold could be lowered at the expense of the slope efficiency; the lowest threshold achieved was 11 mW, which demonstrates the suitability of this laser for diode pumping. These results were all obtained with fibre doped to 1000 ppm by weight with Tm³⁺ ions. Fibres of a higher doping level displayed a photodarkening effect when pumped with infra-red light; this has also been investigated.

A laser source with a broad gain bandwidth, 1100 to 1150 nm, has been produced to pump the upconversion fibre laser. This was based on Yb³⁺-doped germanosilicate fibre and utilised fibre grating technology to select the laser wavelength within the gain bandwidth. The knowledge of Yb³⁺-doped germanosilicate glass obtained was further used in a short investigation into fabricating a single frequency laser, operating at 1083nm, based on a short fibre laser cavity formed with fibre grating reflectors. An output power of approximately 5mW was achieved in a single frequency.

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