Passively and actively mode-locked fibre laser devices
This thesis examines a number of passively and actively mode-locked fibre laser devices. The passive systems that are described incorporate the fast, saturable absorber-like action of a Kerr-type nonlinear optical switch to provide the passive mode-locking mechanism. Two such all-fibre nonlinear switches are described and analysed, namely the nonlinear optical loop mirror and the nonlinear optical gate. The mode-locked fibre laser systems based on the performance of these devices, informally known as the "Figure-of-eight" laser and the "nonlinear polarisation evolution" laser are described and characterised with respect to their modes of operation, self-starting thresholds and resonator parameters. These lasers provide a unique and convenient method for generating fundamental solitons 0.1-5 ps long. A theoretical analysis shows that the resonant loss mechanism, which is responsible for the formation of sidebands in the optical spectrum of the solitons produced by these systems, imposes a limit on the soliton pulsewidth that can be derived from a given cavity. This limit has a square-root dependence on the product of the intracavity dispersion and resonator length. The actively mode-locked fibre laser configurations that are described are 100-1000m long and form part of an investigation into their use for distributed temperature sensing. The sensing method is based on the production of spontaneous Raman backscattered radiation from the mode-locked pulse circulating within the cavity. An extended version of the Kuizenga-Siegman (K-S) analysis of active mode-locking has shown that in systems with large dispersion-length-(bandwidth)2 products there is a significant increase in the pulse durations that can be achieved. However, the accuracy of the K-S theory is also shown to be questionable when used for cavity lengths >50m. Optical time-domain reflectometry measurements on a 4km mode-locked ring laser provided strong supporting evidence for the successful use of long fibre lasers for distributed temperature sensing.