Agnese Coscetta

Modal analysis of a cantilever beam by use of Brillouin based distributed dynamic strain measurements

In this work we report an experimental modal analysis of a cantilever beam, carried out by use of a Brillouin optical time-domain analysis (BOTDA) setup operated at a fixed pump–probe frequency shift. The employed technique permitted us to carry out distributed strain measurements along the vibrating beam at a maximum acquisition rate of 108 Hz. The mode shapes of the first three bending modes (1.7, 10.8, 21.6 Hz) were measured for the structure under test. The good agreement between the experimental and numerical results based on a finite-element method (FEM) analysis demonstrates that Brillouin based distributed sensors are well suited to perform the modal analysis of a vibrating structure. This type of analysis may be useful for applications in structural health monitoring where changes in mode shapes are used as indicators of the damage to the structure.

Heterodyne slope-assisted Brillouin optical time-domain analysis for dynamic strain measurements

We propose a slope-assisted Brillouin sensing scheme based on the interaction between a phase modulated probe and a pulsed pump. The modulation frequency of the probe is chosen so that both first-order modulation sidebands lie within the Brillouin gain spectrum generated by the pump field. Owing to stimulated Brillouin scattering (SBS), a phase modulation to intensity modulation conversion occurs in the probe field upon interaction with the pump, producing a beat note whose amplitude grows with detuning from the SBS resonance. We show that the ratio between the amplitude of the heterodyne signal and the direct component of the probe gain provides a means for performing pump-power independent dynamic strain measurements, therefore improving the conventional slope-assisted brillouin optical time-domain analysis.

Structural Damage Identification in an Aluminum Composite Plate by Brillouin Sensing

We report the results of an experimental modal analysis aimed to estimate the location of mechanical changes in an aluminum composite panel. A distributed Brillouin optical fiber sensor was used to retrieve the mode shapes of the first two bending modes of the plate. We show that the position of the defect is retrieved from the strain mode shapes, with a resolution determined by the disposition of the fiber across the structure, as well as by the spatial resolution of the interrogation unit.

Experimental modal analysis of an aluminum rectangular plate by use of the slope-assisted BOTDA method

We report an experimental modal analysis of an aluminum rectangular plate (50 cm 30 cm 0:3 cm), carried out by use of a Brillouin optical time-domain analysis (BOTDA) sensor operating in the slope-assisted configuration, i.e. at a fixed pump‐probe frequency shift. Strain measurements were acquired along an optical fiber attached to the structure, at a maximum acquisition rate of 250 Hz, a spatial resolution of 30 cm and a sampling distance of 5 cm in both x- and y-directions. A sequence of dynamic tests, aimed to evaluate the resonant frequencies and strain modal shapes of the structure, were performed on the plate for various boundary conditions (plate clamped with four, three or two bolts). Comparison with finite element method (FEM) analysis and dynamic strain measurements with strain gauges shows that Brillouin based distributed sensors can be usefully employed to perform the modal analysis of a vibrating structure, even if the spatial resolution is comparable with the plate dimensions. (Some figures may appear in colour only in the online journal)

High-Spatial Resolution DPP-BOTDA by Real-Time Balanced Detection

A novel scheme for distributed Brillouin sensing based on differential pump-width pair Brillouin optical time-domain analysis (DPP-BOTDA) with balanced detection of Brillouin gain is proposed and demonstrated. Compared with standard DPP-BOTDA, the proposed method allows us to reduce the measurement time and simultaneously improve the accuracy in Brillouin frequency shift estimation. Experimental results, carried out at 10-cm spatial resolution, demonstrate the effectiveness of the proposed method.

High-Pass Filtering for Accurate Reconstruction of the Brillouin Frequency Shift Profile From Brillouin Optical Frequency Domain Analysis Data

In this paper, we propose and demonstrate a new method to reconstruct the Brillouin frequency shift profile in high spatial resolution Brillouin optical frequency domain analysis (BOFDA) sensors. The method aims to compensate the distorting terms affecting BOFDA measurements, which originate from the modulation impressed on the acoustic wave involved in the scattering phenomenon. We show that these terms can be easily removed by applying a high-pass filter to the data acquired in the frequency-domain, at the cost of a degradation of the signal-to-noise ratio. A numerical analysis, as well as experimental tests carried out at 8-mm spatial resolution, validate the proposed technique.

Simultaneous Strain and Temperature Measurements by Dual Wavelength Brillouin Sensors

The sensitivity of Brillouin frequency shift to strain and temperature has been studied in a low-bend loss silica fiber, at two different wavelengths (850 and 1550 nm). The results show that the ratio between strain sensitivity at the two wavelengths is different from the ratio between temperature sensitivity, which can be conveniently used for simultaneous and distributed measurement of these parameters using a single sensing element. Experimental results, carried out at 1-m spatial resolution, demonstrate the feasibility of the proposed method.

Brillouin Optical Time Domain Analysis in Silica Fibers at 850-nm Wavelength

Brillouin scattering properties of silica fibers at 850-nm wavelength were experimentally investigated using a Brillouin optical time-domain analysis scheme. In the single-mode 780-HP fiber, the Brillouin frequency shift (BFS) and full width at-half maximum of the Brillouin gain spectrum were found to be ~19795 and 90 MHz, respectively. The estimated peak Brillouin gain is 0.32 (mW)-1. The temperature and strain dependences of BFS were determined as well. Distributed sensing capability was demonstrated at a minimum spatial resolution of 1.5 m and a fiber length of 100 m. As a proof-of-principle, the temperature and strain dependences of the BFS in SFM-28 fiber have been determined at 850- and 1550-nm wavelengths, suggesting its possible use for temperature/strain discrimination.

Fiber optic based inclinometer for remote monitoring of landslides: On site comparison with traditional inclinometers

Distributed optical fiber sensors, and in particular those based on stimulated Brillouin scattering, have in recent years been the object of a growing attention for structural and environmental monitoring of large areas because they allow to measure strain and temperature profiles up to tens of kilometers with a strain accuracy of ±10ue and a temperature accuracy of ±1°C. In this paper, we present the application of the above sensing principle to the realization and field testing of a novel inclinometer for the measurement of 3D deformation of soil.

Wind Turbine Blade Monitoring with Brillouin-Based Fiber-Optic Sensors

Wind turbine (WT) blade is one of the most important components in WTs, as it is the key component for receiving wind energy and has direct influence on WT operation stability. As the size of modern turbine blade increases, condition monitoring and maintenance of blades become more important. Strain detection is one of the most effective methods to monitor blade conditions. In this paper, a distributed fiber-optic strain sensor is used for blade monitoring. Preliminary experimental tests have been carried out over a 14 m long WT composite blade, demonstrating the possibility of performing distributed strain and vibration measurements.