Double-clad rare-earth doped fibre devices
This thesis reports on the exploitation of the cladding pumping technique to scale the output power of rare-earth (Er3+, Er3+/Yb3+, Yb3+, and Nd3+) doped fibre amplifiers and lasers, and on the study of alternative pumping schemes and applications. In the first three chapters, the introduction of the cladding pumping scheme, its basic fibre geometries and fundamental analyses of double-clad fibres and their devices are presented. Chapter 4 describes an investigation of various Er3+-doped and Yb3+ co-doped fibre devices. An Er3+/Yb3+-doped SM fibre amplifier pumped in the 820 nm band can not only avoid the excited state absorption (ESA) peaks of Er3+ at 800 and 840 nm, but also have a higher ratio of ground state absorption to ESA than that of Er3+ fibres. This enables a high gain amplifier with a much shorter fibre length. An experimental study of cladding pumped Er3+/Yb3+ fibres in amplifier and laser forms shows the simplicity of power scaling and brightness conversion from high-power large-area multimode laser diodes into a single diffraction-limited fibre mode, and thus scales the output powers. Efficient operation of a three-level Er3+-doped double-clad fibre in both laser and amplifier forms is described. The fibre design of a low area ratio of the inner cladding to the core reduces the threshold to an acceptable level. The study of bending effects in large area ratio double-clad fibres indicates that a fibre having a concentric geometry can have nearly as high an efficiency as an eccentric core geometry if periodic bending is employed to promote mode scrambling. In contrast, double-clad fibres with eccentric cores and rectangular inner claddings were found to be relatively insensitive to bending. In order to obtain high-peak-power pulsed sources, various kinds of novel pulsed fibre lasers have been investigated. These include a cladding pumped Nd3+ Q-switched fibre laser, an enhanced Q-switched double-clad fibre laser, a Q-switched Er3+ fibre laser double-clad fibre with a large mode-area core, and a picosecond mode-locked Yb3+fibre laser. Cladding pumped amplification of short pulses can extract high energy stored in doped fibres, thus obtain pulses with high peak power and energy.