Cladding-pumped fibre Raman lasers and amplifiers

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We propose a three year long research project to investigate cladding-pumped fibre Raman lasers and amplifiers for the generation and amplification of light at high powers. We would study the fundamentals and lay a basis for further studies of this new class of devices as well as demonstrate new lasers and amplifiers with greatly improved properties. We would investigate and demonstrate the generation of high-energy pulses, unidirectional pulse amplification, and pulse amplification with very high gain. Furthermore, in the continuous-wave (non-pulsed) regime, we would demonstrate a greatly extended wavelength coverage as well as investigate the potential of power scaling of continuous-wave lasers to very high powers (> 10 kW). These lasers and amplifiers would all be diffraction-limited, which means that they can be focussed to as small a spot as theoretically possible. Key to success would be to develop and fabricate optical fibres that meet new and challenging requirements. High-power fibre lasers (including amplifiers) have already seen a remarkably rapid progress in recent years. They represent a manufacturable technology for real-world applications, and are becoming the laser of choice in several application areas of great scientific and economic importance. To date, these lasers are based on cladding-pumping of rare-earth doped fibres. Doping with rare-earth elements is used to create optical gain in the fibres. Cladding-pumping requires the use of special fibre designs (so-called double-clad fibres). These enable the use of multimode diodes to energise ( pump ) the fibre lasers. Multimode diodes are available at high powers, so that also the fibre lasers pumped by them can reach high powers. Important advantages of high-power fibre lasers include high efficiency and excellent control of temporal and spatial beam parameters, so that well-controlled diffraction-limited beams can be realised. These factors significantly reduce the energy consumption and extend the reach of important materials processing applications. Still, despite the impressive progress, the current approach to high-power fibre lasers and amplifiers, based on doping with a rare earth, has limitations that hampers current and future progress.Cladding-pumped fibre Raman lasers and amplifiers constitute a new class of devices with unique and very interesting properties. They promise to overcome important limitations of current high-power fibre laser technology and greatly expand their reach. Instead of rare-earth doping, stimulated Raman scattering is used to create optical gain. This is a nonlinear scattering process that occurs in all optical fibres. One important advantage of fibre Raman lasers is that whereas high-power rare-earth doped fibre lasers only work in certain wavelength bands, fibre Raman lasers can operate at any wavelength, determined by the pump wavelength and the Raman spectrum of the fibre and limited only by its transparency range. Another advantage is a low thermal load and potentially longer interaction length. These factors both reduce the impact of thermal effects, which is very important at high powers.Since this would be the first significant study of cladding-pumped fibre Raman lasers, the planned investigations are fundamental and exploratory rather than exhaustive. Model development and improved understanding are important objectives. Nevertheless, we see a large number of opportunities with cladding-pumped fibre Raman lasers. We fully expect them to be the subject of further and deeper studies over many years to come, and that the expansion of fibre lasers into new real-world applications will continue.