Incoherent Optical Frequency Domain Reflectometry for Distributed Thermal Sensing

This thesis reports the main results from an investigation of a fibre-optic distributed temperature sensor based on spontaneous Raman scattering. The technique used for spatial resolving is the incoherent optical frequency domain reflectometry, where a pump laser is sine modulated with a stepwise increasing frequency, after which the inverse Fourier transform is applied to the signal from the backscattered light.

This technique is compared with the more conventional optical time domain reflectometry, where a short pulse is sent through the fibre, and the location of the scattering section is determined by the time difference from the emission to the detection of light. A temperature sensor with a range of 2-4km comprising a step-index multi-mode fibre and a high-power 980nm pump laser existed prior to the start of the PhD study.

In this study, a sensor range of approximately 10km, and a spatial resolution of order 1m is strived to be achieved. These demands are attempted to be reached by employing a low-loss telecom-grade transmission single-mode fibre or an alternative fibre as the sensing fibre, and a pump laser operating in the low loss region of silica.

An analysis of the optical module comprising a pump laser, optical filters, optical fibre and photo-detectors are presented. Limitations, trade-offs and optimisation processes are described for setups having different specifications with respect to range, resolution and accuracy. The analysis is conducted using computer simulation programs developed and implemented in Matlab.

The computer model is calibrated and tested, and describes the entire system with high precision. Noise analysis and digital processing of the detected signal are discussed as well. An equation describing the standard deviation of the measured temperature is derived for the device, and shows the dependency of the accuracy on measurement time, resolution, range, attenuation and detector parameters.

Temperature measurements on 7.8km low-loss graded-index multimode fibre and 14km standard single-mode fibre with a spatial resolution varying from 0.5m to 6m are given as representative examples of the achieved accuracy (0.5oC to 5oC) and range, with measurement time of a few minutes. Measurements on 25km were conducted as well to demonstrate this possibility; with a short measurement time it was not possible to obtain a reasonable accuracy though.

In addition to the Raman based temperature sensor, a quasi distributed fibre-optic temperature and strain sensor based on an array of fibre Bragg gratings interrogated by a tuneable laser is also developed and analysed. This type of fibre-sensor is particularly suitable for strain monitoring on large concrete constructions such as bridges.

The sensor is built and tested in a field trial on a bridge, and the estimated accuracy of around 2με is achieved.