Development of a high spatial resolution temperature compensated distributed strain sensor

Abstract

Optical fibre sensors have offered such unrivalled distributed sensing features that they continue to be successfully exploited in various industries for performing continuous measurements of the physical parameters, such as, temperature and strain. However, there arise certain conditions in engineering or materials manufacturing and characterisation environments, when the user seeks to extract the knowledge about a single physical parameter only, say strain, whilst the environment is also subject to temperature. This thesis explores techniques to offer solutions under such conditions by developing a high spatial resolution temperature compensated distributed strain sensor.

The preliminary exploration involved exploiting the high spatial resolution Brillouin frequency based Brillouin optical correlation domain analysis technique in combination with the anti Stokes Raman intensity based optical time domain reflectometry technique. This work resulted in achieving a temperature compensated strain resolution of 46 µε with a spatial resolution of 24cms over a sensing length of 135m. Tackling the impact of modest pump depletion effects on the Raman backscattered signal, a beneficial normalisation protocol was identified. During discussions the possible limitation imposed by the weaker backscattered Raman signal towards achieving any better results on the strain and spatial resolution values, seemed to emerge. In order to enhance the performance of the sensor, anti Stokes Brillouin, which is a stronger back scattered signal, was exploited for the time domain reflectometry measurements. This resulted in 22µε of temperature compensated strain resolution with a spatial resolution capability of 10cms.

Although the combined performance of the Brillouin frequency in combination with Brillouin intensity proved better than the performance of the Brillouin frequency in combination with Raman intensity, but since the electronic detection system was changed together with pulse widths between the two techniques that it was unclear as to which combinatory technique was best suited for designing the sensor. It is with this view that a theoretical analysis on the performance of the R-OTDR and B-OTDR was carried under similar sensor parameters. B-OTDR is identified as a better technique compared to the R-OTDR towards providing high spatial resolution temperature compensation feature to the Brillouin frequency based distributed strain measurements.

The exploration also gave an opportunity to experimentally study for the first time the impact of simultaneously varying temperature and strain on the four Brillouin coefficients. This study proved useful in identifying the corrections to the Brillouin coefficients in order to estimate the true value of strain and temperature in a temperature controlled variable strain environment. The thesis culminates with a summary of work, discussing the thresholds of various non linear effects together with means to improve the performance of the sensor. With the view of enhancing the sensor applicability, schemes of loss compensation are also discussed. In conclusion work for the future is highlighted.

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Identifiers

ORCID for Mohammad Belal: orcid.org/0000-0001-5175-3158

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