Distributed optical fiber sensors (DOFS) represent a very effective technology for the continuous monitoring of structures. Conventionally, DOFS make use of some form of light scattering in silica glass optical fibers. Rayleigh scattering is caused by random fluctuations in the refractive index of the fiber, and can be used to perform high-sensitivity, vibration (dynamic) sensing. In such a case, the amplitude and/or the phase of the backscattered light excited by a coherent laser pulse is acquired at one or more lasing wavelengths. Mechanical vibrations or acoustic perturbances are usually identified by comparing consecutive phase sensitive OTDR (?-OTDR) acquired traces, as they induce variations in the backscattered light at the corresponding position.
A different mechanism of light scattering is the Brillouin scattering, which involves the interaction between optical and acoustic fields in the optical fiber: any changes in the tensional state or temperature of the fiber alter its elastic properties, and in particular, the frequency of the acoustic wave generated either thermally (in case of spontaneous scattering) or by electrostriction (in case of stimulated scattering). This frequency coincides with the Doppler shift experienced by the incident light. Thus, the strain or the temperature of the fiber is recovered by measuring the frequency shift between the incident and scattered light, which is known as the Brillouin Frequency shift (BFS). Brillouin methods are capable of high spatial resolution (down to the mm range), and very long sensing lengths (up to one hundred km and more). Furthermore, differently from Rayleigh based sensors, they allow absolute (not reference-based) measurements. However, they cannot capture weak vibrations with nε-level strain, mainly because of the relatively poor sensitivity of the BFS to strain (∼ 50 kHz/µε).
Hybrid sensors, exploiting (at least) two scattering mechanisms for multiparameter sensing, may provide more valuable information and a comprehensive identification of fault events. Furthermore, the measurement schemes can share some parts, such as those required for the probe pulse generation, as well as those aimed at data acquisition. Therefore, in the last years there has been a growing interest in developing hybrid sensor schemes, capable of integrating the two technologies in the same interrogation setup. In this presentation, the recent advances in hybrid Rayleigh/Brillouin based sensors will be discussed. We will show that the integration of the two technologies permits to realize cost-effective solutions for simultaneous, multiparameter sensing.