High-Temperature Fibre Sensor Potential and Low Noise Fibre Laser

Abstract

This thesis presents a comprehensive investigation into the physical phenomena that define the operational limits of two distinct classes of fibre optic systems: distributed sensors and random lasers. The research addresses fundamental challenges in both domains, focusing first on the characterization of material stability in sensors deployed in extreme thermal environments, and second on the development of all-optical, passive methodologies for controlling the intrinsic dynamic instabilities of disordered laser systems. The unifying theme is the progression from identifying performance-limiting mechanisms to harnessing a quantitative understanding of these mechanisms for enhanced characterization and control.

The first part of the thesis investigates the performance of silica optical fibre as a distributed sensor using Optical Frequency Domain Reflectometry (OFDR) under intense thermal loads where material degradation is the primary constraint. Measurements in an electrical arc plasma, combined with non-linear calibration, enabled spatially resolved temperature sensing up to approximately 1300 ℃. This established OFDR as a multi-parameter diagnostic tool, yielding simultaneous measurements of temperature, thermally-induced residual stress, and dynamic optical path length changes. The study further demonstrates that the fundamental limit of cross-correlation sensing is governed by the fibre's fictive and glass transition temperatures, beyond which structural relaxation induces waveguide breakdown and signal decorrelation. To enable continuous operation, a study in a hydrogen flame was conducted to formalize the mechanism of sensor failure. The irreversible, thermally-driven rearrangement of the fibre's glass network was quantified, leading to the establishment of decorrelation time as a new, physically-grounded metric for sensor durability. Measurements indicating a 50% loss of signal correlation in approximately 20 seconds at 1000 ℃ provide a quantitative framework for assessing the measurement validity window of fibre sensors at their material limits.

The second part of the thesis addresses the control of chaotic modal behaviour in Erbium-doped Random Fibre Lasers (EDRFLs). Two distinct, passive, all-optical strategies are developed and demonstrated. The first is a feedback-based approach, where Self-Injection Locking (SIL) to a novel Random-Fibre-Grating Ring (RFGR) resonator is used to discipline the laser's dynamics by slaving its frequency to the stable resonances of the external passive system. This method was shown to be effective, suppressing mode competition to achieve stable, single-frequency operation with a 1.23 kHz linewidth and a 70 dB side-mode suppression ratio. The second is a nonlinearity-based approach that harnesses the laser's internal dynamics. By engineering a two-section cavity, a robust Optical Bistability (OB) was established and exploited as a mechanism for passive, self-stabilization, resulting in a measured order-of-magnitude reduction in intensity noise.

This thesis establishes a framework for sensor durability assessment, demonstrates two paradigms for the all-optical control of disordered lasers, and introduces a methodology for the internal characterization of active photonic devices. A central element of this work is the application of OFDR, which served both as the sensing technology in the first part of the research and as the in-situ diagnostic tool in the second. This diagnostic application provided the first spatially resolved map of internal refractive index dynamics within an active Random Fibre Laser (RFL), which identified the origin of the observed bistability as a hybrid mechanism of population-driven (Kramers-Kronig) and localized, intensity-driven (Kerr) nonlinearities.