Estudio de los métodos de mejora de sensores Brillouin distribuidos en fibra óptica mediante amplificación Raman

Resumo

The great development experienced by the optical fiber sensors over the last years have led them to being very attractive in certain applications of civil engineering, in energetic transport, in fire detection and pollutants in adverse conditions and distantly, etc., being able to compete with the traditional electrical and electronic sensors. The main advantages of this technology take root in the good properties of the optical fiber: low attenuation, immunity to electromagnetic noise and deflagrations, high speed of transmission, small size and weight, flexibility, possibility of multiplexing... Between the optical fiber sensors, the distributed sensors are very adequate for the monitoring of big infrastructures because they allow, with only one cable of optical fiber and a sole interrogator, to have thousands of points of monitoring distributed through the structure. Specifically, the distributed optical fiber sensors based on stimulated Brillouin scattering, known in the bibliography as BOTDA (Brillouin Optical Time Domain Analysis), are adapted for the auscultation of civil structures of great size, and particularly for those of great length as the railway structures. This type of sensor provides us information relative to the temperature and deformation, as well as of those physical magnitudes dependent on the first ones, on every point of the fiber along all its length. One of the limitations for this application that the BOTDA sensors present is its scope. When it tries to support spatial resolutions of the meter order (1-2 meters) it is not possible to reach monitorizable lengths highier than 50 kilometres. In this PhD work we present an ingenious method to increase the mentioned range using Raman distributed amplification. The Raman distributed amplification has been incorporated by us into the BOTDA system. We have used it in first order using a Raman pump with a 1455 nm wavelength, and amplifying at 1550 nm (in three different configurations). In the second order configuration we have used a Raman pump with a 1365 nm and amplifying at 1550 nm, in bidirectional configuration. The results obtained with first order Raman amplification, supposed in the moment of its spreading in magazines and scientific conferences, a record in relation between the monitored distance, the spatial resolution and the uncertainty of measure (75 km, 2 m, ±1.2◦C). By means of the use of Raman amplification in second order on the BOTDA system it is possible to achieve an approximately constant signal/noise relation with the distance, making that the optical fiber behave as a line virtually without losses. With this system we have managed to monitor 100 kilometres with spatial resolution of two meters and to achieve that the signal/noise relation is practically constant with the distance, in comparison with the BOTDA system assisted with first order Raman amplification