High-power cladding-pumped Raman and erbium-ytterbium doped fibre sources
Raman fibres and rare-earth doped silica fibres have been investigated for many years as gain media for the amplification of optical signals in telecommunications thanks to their broad gain bandwidth. They are now widely used for that application. It is only recently that power scaling of fibre laser sources has taken place with the development of double-clad fibres and highpower laser diode pump sources. Cladding-pumped fibre lasers are now a rapidly expanding and emerging technology with a wide range of applications, where high-power and high-brightness laser sources are required. Nevertheless, so far, most high-power lasers have been based on highly efficient ytterbium-doped fibre, while progress to power-scale other rare-earth doped fibres and fibre Raman laser has been much more modest. This thesis can be divided into two main themes. The first concerns the power-scaling and study of erbium-ytterbium doped fibre laser sources for optical amplification or as laser sources. The second theme concerns the development and study of high-power Raman fibre lasers and amplifiers based on the novel concept of a cladding-pumped Raman fibre. The themes are jointed in that the cladding-pumped Er:Yb doped lasers, developed in the first theme, are used as pump sources in this second part for the Raman devices. They are also jointed in that they both concern power-scaling of “eye-safe” sources at around 1.6 mm. Firstly, in collaboration with co-workers, high-power, large core, erbium-ytterbium doped fibre laser sources are developed. Output powers in excess of 70 W are obtained. Good beam quality output is achieved thanks to a tapered fibre section. The taper is compatible with standard single-mode fibre which enabled the realization of tuneable fibre lasers free from bulk external grating. The laser’s tuning characteristics are investigated in the C- and L-band range. Subsequently, a master-oscillator power-amplifier (MOPA) based on large core Er:Yb doped fibres is developed for the generation of high-energy pulses. The details of the MOPA are studied and presented. With careful design considerations, pulses free from non-linear effects, with energy up to 1 mJ and peak powers up to 6.6 kW, with narrow spectral linewidth, are obtained at 1535 nm. Secondly, using a double-clad fibre, consisting of raised index, germanium doped, core and inner cladding, with a pure silica outer cladding, a high-power CW single mode Raman fibre laser, pumped by a multi-mode erbium-ytterbium doped fibre laser, is demonstrated for the first time. The laser slope efficiency is 67% and the output power is in excess of 10 W. An experimental and theoretical study of the laser is performed. Then, the pulse amplification in a cladding-pumped Raman fibre is studied in a single pass amplifier configuration. The effects on the laser performance of the pump and Stokes seed powers, fibre length and four-wave mixing are presented. The Stokes’s small-signal gain can be as high as 50 dB. Using this configuration, 700 ns long pulses are amplified up to 10 mJ which shows that, potentially, optical pulses could be amplified to much higher energy. Finally, these results together, let predict that, soon, cladding-pumped Raman fibre could be used as direct brightness converter.