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Title: Fibre laser sources with low quantum defect
Author: Yao, Tianfu
ISNI:       0000 0004 5356 5670
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2014
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High power fibre lasers with efficient amplification of optical signals have been widely used for various applications for many years. A great advantage of fibres is their thermal properties, which are so good that fibres are often quoted as being immune to thermal degradation. However, powers keep increasing and the fibre geometry is becoming more similar to their “bulk” (non-waveguiding) counterparts. Thermal problems are severe in bulk lasers, and are, unsurprisingly, becoming more important also for fibres. Heating is inevitable in the laser cycle due to the energy difference(quantum defect) between pump and signal photons. Nevertheless, there is substantial room to reduce the heating by minimising the quantum defect, with pump and signal wavelengths as close as possible. In this thesis, I demonstrate low-quantum-defect fibre amplifiers and lasers based on two different energy conversion processes, i.e., in high-brightness (tandem) pumped ytterbium-doped fibre and in short-wavelength-pumped Raman fibre. The latter approach increases the photon energy relative to the thermal energy and vibrational energy of the host. Firstly, as it comes to tandem-pumping of ytterbium-doped fibre amplifiers, the challenge is to make the pump and signal wavelengths as close as possible, while keeping the signal gain and pump absorption sufficiently high. With the optimum average ytterbium excitation level and high pump brightness, the quantum defect can be as small as 0.6% when pumped at 1030 nm, according to theoretical calculations. Subsequently, an experimental amplifier core-pumped by a single-mode laser source is presented. A 2% quantum defect is reached, with pump and signal wavelengths at 1030 nm and 1050 nm, respectively. The slope efficiency reaches 95% to 96%. Initial investigations show low photodarkening with tandem-pumping, with some dependence on the dependence on ytterbium ions concentration. Secondly, Raman conversion of pulses in a diode-pumped highly nonlinear fibre is studied in a ring-laser cavity configuration. The quantum defect is 3.5% with 806 nm pump and 835 nm Stokes wavelengths. A slope efficiency of 65% is obtained with 600 m long fibre and 100 ns pulse width. Then, I study experimentally and theoretically 975 nm continuous-wave-pumped fibre Raman lasers based on a graded-index and a double-clad fibre. Both lasers emit at 1019 nm (4.3% quantum defect) with improved brightness. A record laser output of 6 W and 19% slope efficiency from the double-clad Raman fibre and 20 W from the graded-index fibre shows further scaling of singlemode power is possible with improved cavity and fibre design.
Supervisor: Nilsson, Lars Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; TK Electrical engineering. Electronics Nuclear engineering