Distributed feedback fibre laser strain and temperature sensors

Abstract

This thesis presents the development of two new types of polarimetric distributed feedback (DFB) fibre laser sensors for simultaneous strain and temperature measurements. These fibre Bragg grating (FBG) based sensors offer strain and temperature measurement accuracies of ±0.3 - ±15 µ.epsilon and ±0.04 - ±0.2°C which are suitable for many applications. The main advantage of these DFB fibre laser sensors over other FBG based sensors is the simplicity of their interrogation system. The first type of sensor operates stably in a single longitudinal mode which splits into two orthogonally polarised modes. This sensor utilises the wavelength of one polarisation mode and the RF beat frequency between the two polarisation modes. The system complexity is reduced to a minimum in the dual longitudinal mode polarimetric DFB fibre laser sensor which utilises the RF beat frequencies between two longitudinal modes and their associated orthogonal polarisations, therefore requiring only a simple and cost effective frequency counter.
The ability of measuring strain and temperature simultaneously is demonstrated with prototype sensors embedded in a concrete test specimen. Miniature sensor packages are used to protect the sensors from their harsh environment and to enable handling by personnel who is inexperienced with fibre optic sensors.
The reduction of measurement resolution caused by external feedback into DFB fibre laser sensors, which is likely to occur in serially multiplexed sensor networks and remote sensors, is investigated.
Furthermore, operation of DFB fibre lasers up to 400 °C is demonstrated. This yields important information about grating decay in sensor and telecommunication applications, the required annealing temperatures to prevent this decay and the initial grating strength to obtain the maximum output power of DFB fibre lasers.
This experimental work is supported by extensive theoretical modelling. For the first time an Er³⁺:Yb³⁺ DFB fibre laser model is presented which takes homogeneous upconversion of paired Er³⁺-ions and pump excited state absorption into account. An extended version of this model incorporates, for the first time, self-heating in DFB fibre lasers which is caused by non-radiative decays. The performance of DFB fibre lasers employed in telecommunication applications is likely to benefit from these modelled results, which are also verified by experimental data.

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ORCID for Michael Zervas: orcid.org/0000-0002-0651-4059

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