Fiber-optic sensors based on Mandelstam-Brillouin backscattering: metrological characteristics and development trends

Fiber-optic sensors operating on Mandelstam-Brillouin backscattering represent a promising class of distributed optical sensors for simultaneous temperature and deformation monitoring in extended infrastructure objects. To date the key sensor metrological characteristics have been analyzed without establishing correlations between them. However, a complex analysis of the main metrological characteristics is required when designing the sensors under discussion. A comprehensive analysis makes it possible to more accurately predict the actual metrological characteristics of a sensor under the simultaneous influence of several factors. The relevance is dictated by the need to move from estimated measurements to system design in order to ensure the required accuracy and stability of the sensor in operating conditions. A complex analysis of the sensor key functional characteristics is conducted including spatial resolution, dynamic range, measurement errors, and measurement speed. It is found that the spatial resolution of the investigated fiber-optic sensors depends on the acoustic phonons relaxation time and is limited to the value of approximately 1 m, however, the application of differential pulse methods and synthetic spectrum analysis makes it possible to obtain a resolution of 0.1–0.4 m. The sensitivity of fiber-optic sensors to the temperature changes and mechanical deformation is analyzed. The linear dependences of the Brillouin frequency shift on deformation are determined. However, the mutual sensitivity of the parameters requires the use of hybrid methods for their separation. The methods of optimizing the signal-to-noise ratio by using fiber amplifiers, single photon detectors and algorithmic filters are investigated. It is shown that the use of combined Raman amplification allows extending the measurement range up to 120 km while maintaining a spatial resolution of 5 m and a temperature error within ±(1–2) °C. The compromise relations between the key metrological characteristics of fiber-optic sensors operating on the Mandelstam-Brillouin backscattering are investigated in details. The data of the metrological characteristics analysis are critically important for defining the main trends in the sensor developments. The results obtained are of practical importance for designing the fiber-optic sensors of this type which are applied in structural monitoring the complex engineering infrastructures, including bridges, dams, tunnels. 

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