High-power fibre lasers
University of Southampton, Department of Electronics and Computer Science,
This thesis reports on the experimental study of high-power, high-energy, cladding-pumped, rare-earth (Yb3+, Er3+/Yb3+)-doped fibre lasers. Some of the main capabilities of fibre lasers such as: High brightness and thermal properties were exploited for the development of a variety of continuous wave (CW) and Q-switched devices, whose characteristics also includes compactness. Our devices could already be considered an option for several applications.
The 25-year long scientific and commercial evolution that fibre lasers have experienced is discussed in the first two chapters. The invention of Erbium-doped fibre amplifiers (EDFA's) and Internet were two major breakthroughs, which launched the need of WDM systems and laser sources. Fibre lasers, are now considered a flexible and powerful device whose technology has finally reached its maturity.
Cladding pumping is the technique employed in these experiments in order to pump double clad fibre lasers using high power, broad stripes and bars. In this work, several inner cladding shapes have been used to overcome the normally high mismatch between diode laser beams and inner cladding areas of fibre lasers.
Chapter Three consists of a review of cladding-pumped fibre lasers. It describes how inner cladding geometry and pump absorption limits the output power scalability of these devices. Nonlinear effects and amplified spontaneous emission are also studied due to their implication they have over fibre lasers performance.
Results on conventional, continuous wave (CW) fibre lasers including fibre characterization and employed launching techniques are described in Chapter Four. A new method to obtain high intensity laser beam output from an Yb3+-doped, cladding-pumped, highly multimode fibre laser has been proposed. In this experiment, we propose the use of fibre tapers to increase intensity and improve beam quality. In CW regime, our results show an intensity increase of ~3.5 times with a low power penalty of ~1 dB. Also, without tapering, a maximum output power of 21-W was reached with a slope efficiency of >80%.
Using a simple set of optic elements such as a l/2 waveplate, a polarizing beam-splitter and a bulk grating, we investigated the polarization characteristics of an Yb3+ fibre laser, from which we obtained 6.5 W of single polarization tunable output in the range of 1070 to 1106 nm. As a free running laser, the system produced 18 W at 1090 nm and showed a threshold of 1.8 Watts. The experiment is our first approach for developing a reliable high-power Yb3+-doped fibre source, that could be used in conjunction with optical parametric oscillators (OPO) and amplifiers (OPA) to frequency convert to a broad band of wavelengths.
Using a new design of ytterbium-doped fibre made in-house with the conventional modified chemical vapor deposition (MCVD) process, we explored the possibilities of energy storage with such a large mode area (LMA) fibre. The fibre system was capable of delivering energetic pulses of >2 mJ, which could suggest the feasibility of a pulsed fibre laser in the region of tens of milli-Joules. The experiment is described in Chapter Six, on which the experiment that uses the tapered fibre laser in Q-Switched regime is also described and compared to LMA fibre laser. Gaussian-type pulses were obtained which reached pulse energies of 0.6 mJ at 4 kHz using a tapered fibre laser and 1.3 mJ at 500 Hz using conventional laser, corresponding to average powers of 2.1 Watts for the tapered laser and 0.8 watts for the conventional laser.
Er3+/Yb3+-doped fibre lasers were part of our experimental work. This co-doping technique allows pumping of Yb3+ ions using broad-stripe high-power pump sources to reach much higher output power levels. Efficient energy transfer from excited Ytterbium ions into Erbium is achieved. From a preliminary study, the fibre laser showed a threshold of 160 mW and a slope efficiency of 49% with respect to absorbed pump power. The maximum output power was 6.2 watts at 1535 nm and a linewidth of 1 nm. One of our co-doped fibre devices produced 16.8 W of continuous wave, multimode laser power at the interesting wavelength of operation of 1550 nm.
Finally, conclusions and future work are included in Chapter Eight.
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