Real-time rail-track monitoring system employing innovative wavelength diversity technique in distributed optical fibre sensors

Lalam, Nageswara (2018) Real-time rail-track monitoring system employing innovative wavelength diversity technique in distributed optical fibre sensors. Doctoral thesis, Northumbria University.

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Abstract

Brillouin based distributed fibre sensors gained a lot of attention in recent years for structural health monitoring applications, due to their higher sensing range over tens of kilometres and distributed measurement capability of simultaneous strain and temperature. In Brillouin optical time domain reflectometry (BOTDR) system, the signal-to-noise ratio (SNR) determines the sensing performance of the system. However, the SNR is limited by the restricted maximum input pump power, which is limited by the non-negligible nonlinear effects, such as stimulated Brillouin scattering.

In this research, a novel wavelength diversity technique is proposed to enhance the SNR, hence improve the strain and temperature measurement accuracies, which is required for accurate rail-track condition monitoring. In addition, this research work presents the following contributions (i) a simple, low-cost passive depolarizer is adopted to reduce the polarization noise; (ii) an inline erbium-doped fibre amplifier (EDFA) is employed at a certain distance to amplify the attenuated pulse in order to improve the sensing range; and (iii) a cost-effective reference Brillouin ring laser is used in BOTDR system to overcome the complexity of the receiver bandwidth reduction.

The proposed wavelength diversity BOTDR system combined with a passive depolarizer and Brillouin ring laser is validated over a 50 km sensing fibre with a 5 m spatial resolution. The SNR is improved by 5.1 dB, which corresponds to 180% improvement compared to a conventional BOTDR system. Whereas, the strain and temperature accuracies at a 50 km fibre distance are ±10 με and ±0.45oC, respectively. Furthermore, for the first time, simultaneous integration of long-term evolution (LTE) radio-over-fibre (RoF) data system and BOTDR sensing system using a single optical fibre is proposed and demonstrated. The error vector magnitude (EVM) performance of LTE-RoF data system is analysed for three modulation formats of QPSK, 16-QAM and 64-QAM in the presence of various BOTDR sensing powers.

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