Brillouin scattering for refractive index sensing in non-adiabatic tapers

We demonstrate the use of non-adiabatic tapers for refractive index sensing in optical fibers based on Brillouin scattering. By exciting higher order optical modes along the taper, the Brillouin gain spectrum becomes multipeaked, where each peak exhibits a different sensitivity to the refractive index of the surrounding medium. By this method, we demonstrate a sensitivity enhancement of the Brillouin frequency shift to refractive index changes by a factor of ≈ 4, compared to an adiabatic taper with the same waist diameter. Furthermore, the use of the spectral difference between two Brillouin gain peaks provides a temperature-independent measurement of the external refractive index

Quasi-distributed refractive index sensing by stimulated Brillouin scattering in tapered optical fibers

In this paper, we demonstrate that multiple tapers in optical fibers allow for quasi-distributed refractive index sensing via a high spatial resolution Brillouin Optical Frequency-Domain Analysis (BOFDA) configuration. We first characterize, theoretically and experimentally, the variation of the Brillouin frequency shift (BFS) with the diameter of the tapered fiber. Then, we characterize the dependence of the BFS from the outer refractive index in an optical fiber taper with a waist diameter of 10 micron. Finally, we show that more tapers can be realized along the same fiber, in order to provide multi-point refractive index sensing

Distributed measurement of modal birefringence in a few-mode fiber based on stimulated Brillouin scattering

This paper presents a novel technique for the distributed measurement of the modal birefringence in a few-mode fiber (FMF). The method exploits two different phenomena observed in distributed Brillouin measurements: the dependence of the Brillouin frequency shift (BFS) on the effective refractive index (ERI) of the interacting optical beams, and the spatial oscillations of the Brillouin gain deriving from multimodal interference. Using both phenomena, a wide range of ERI separations can be measured, from ≈ 10-7 to 10-2 or more. The measurements have been carried out over a two-mode graded-index FMF, using two photonic lanterns to selectively excite the desired spatial modes. We use the BFS measurements to derive the ERI difference between the LP01 and LP11 mode groups, while the spatial oscillations of the Brillouin gain reveal the birefringence between the vector components (TE01, TM01 and HE21) of the LP11 mode group. The experimental measurements are partly supported by full-vector finite-element-method (FEM) simulations. The reported method may also find application in the field of distributed sensing, by taking advantage of the dependence of modal birefringence from physical parameters such as strain and temperature

Damage detection in an aluminum plate through a phi-OTDR sensor and support vector machines

In this paper, we make use of a phase-sensitive time domain reflectometry (phi-OTDR) sensor with 60-cm spatial resolution to detect the Lamb waves generated by a piezo-ceramic actuator in an aluminum plate. Furthermore, a machine learning algorithm based on Support Vector Machine (SVM) classifiers was employed for damage localization. We show that SVMs are able to identify the characteristics in Lamb wave signals that may be linked to damage location. This study makes full use of the rich information provided by the phi-OTDR sensor, extracting damaged data from diverse damage spots. The results indicate that the proposed technique has the potential to identify and locate damages in thin-plate structures

Dynamic and High-Resolution Strain Measurements Using the Brillouin Optical Frequency-Domain Analysis

In this work, we demonstrate a novel mechanism to localize the stimulated Brillouin interaction over a specific portion of an optical fiber, using a Brillouin Optical Frequency-Domain Analysis (BOFDA) interrogation system and an array of tapers. We show, both numerically and experimentally, that the frequency-domain fiber response, acquired over a narrow range by a vector network analyzer, can be uniquely associated to the multi-taper array whose period matches the swept spectral range. This opens the way to high-resolution (cm-scale) dynamic strain measurements, in addition to fully distributed static strain (or temperature) measurements over the same optical fiber and using the same apparatus

A network analysis optical frequency-domain reflectometer for distributed vibration sensing

We demonstrate the use of a network analysis optical frequency domain reflectometer for dynamic and distributed vibration sensing. Two configurations are assessed and compared, based on either double sideband or single sideband modulation

Health and load monitoring in an aluminium plate through guided waves

Guided wave (GW) based Structural Health Monitoring (SHM) systems are widely promoted in literature for their capability in damage and load monitoring, providing benefits in terms of inspection and maintenance/repair operations. The high sensitivity of GWs to both loads and damages, however, alters in a similar way the measurements gathered by piezoelectric (PZT) sensors network, leading to possible false positives. This paper proposes an experimental tests campaign carried out on a piezo-monitored aluminium plate, 360 mm x 500 mm x 2 mm sized, subjected to a tensile load. The plate was loaded under different load levels, each individually, by using an electromechanical test machine, the Zwick/Roell 250, equipped with a 250 kN load cell, and the diagnostic signals dataset was acquired in a specific frequency range. Load effects were thus analysed along different measurement paths in terms of amplitude and velocity variation, also in presence of a simulated damage

On the Damage Sensitivity of Guided Wave SHM System Under Different Loading Conditions

Guided wave based Structural Health Monitoring (SHM) systems are a reliable and non-invasive approach to monitor structures in several fields. They provide indication about the occurrence of damage, but their large-scale industrial application is still challenging due to some aspects such as the operating loads affecting the structures. Actually, equipping damage tolerant structures with SHM systems can allow the continuous monitoring providing several benefits in terms of conditioned maintenance and repairing operations. This paper presents a numerical modelling technique, based on the Finite Element (FE) method, for the simulation of guided waves in a composite panel affected by loads. Guided wave propagation mechanisms have been analyzed also on a loaded damaged configuration of the panel, in order to investigate the damage sensitivity of the proposed SHM system. The combination of both damage and load allowed considering a scenario closer to the operating conditions of the structure, providing a contribution towards the implementation of SHM in industrial applications

An integrated structural health monitoring system based on Lamb waves

Structural Health Monitoring (SHM) has the potential of providing significant economic benefits through predictive maintenance and identification of damages in structures at an early stage. In particular, active SHM systems based on Lamb wave propagation can be applied to structures of different materials, including composites. In this paper, a portable and totally autonomous SHM system which excites and detects Lamb waves is proposed. Detected signals are processed in-situ and in real time in order to estimate the damage location. The result is then communicated wirelessly to the user, in the form of a spatial map of the damage probability distribution