A simpler approach to distributed sensing with ultra-low reflectivity fibre points

Abstract: Distributed fibre sensors based on the frequency-domain analysis of Rayleigh backscattered light are well established. They exhibit excellent performance in both sensitivity and spatial resolution, but their application can be limited due to their cost and the complexity of the analysis. This work presents a system based on coherent optical frequency-domain reflectometry used in Rayleigh distributed sensors, but with some modifications to the fibre and the implementation of signal-conditioning algorithms that enable the use of more readily available components and simplified analysis. A sensing fibre is prepared by printing uniformly spaced (in this instance), ultra-low reflectivity points. When swept-wavelength light is introduced into the fibre, the reflections from the ultra-low reflectivity points interfere with the reflection from the tip of the fibre. These reflections can be processed with the techniques used in coherent optical frequency-domain reflectometry, providing information about the state of the fibre with regards to a parameter (such as temperature) between the reflective points. The function of the ultra-low reflectivity points is to provide stronger reflections than those produced by Rayleigh backscatter. The ultra-low reflectivity points are fibre Bragg gratings that act as reflectors and not as sensors per se. They are manufactured to reflect the same wavelength and, because of their low reflectivity, they also have a wider reflective spectral range than commonly used high-reflectivity fibre Bragg gratings. This removes the need for specialised detection equipment. Use of a reference interferometer and signal-processing algorithms (linearisation and phase fixing) makes it possible to replace a high-precision linearly tuneable laser with a standard tuneable laser or even a distributed feedback laser diode as the optical source. A novel technique is also presented for identifying the initial phase in the signal, post-acquisition, by means of a fibre Bragg grating in the reference interferometer. In combination with the linearisation and phase fixing algorithms, this locks the phase of the signal prior to analysis, dispensing with the need for precise synchronisation between the optical source and signal acquisition. The best spatial resolution that can be achieved by the system is 0.36 mm, and the best temperature resolution achieved (with a spatial resolution of 60 mm) had a standard deviation of...M.Ing. (Electrical and Electronic Engineering