Massimo Leonardo Filograno

The design, development, and testing of a low-cost phase optical time-domain reflectometry (Phase-OTDR) system, intended for use in structural health monitoring (SHM) applications, are presented. Phase-OTDR is a technology that is growing and evolving at an impressive rate. Systems based on this principle are becoming very sensitive and elaborate and can perform very accurate condition monitoring, but at the same time, they are critically alignment-dependent and prohibitively costly to be considered as viable options in real field applications. Certain Phase-OTDR systems have been applied in real field studies, but these examples are mostly a proof-of-concept. The system presented here is the result of a compromise between performance and cost, using commercial components, specifically combined and tuned for SHM applications. The design and implementation of all the electronic and optoelectronic steps are presented, and the operation of the system is demonstrated, achieving a spatial resolution of ~6 m over 5 km. This work provides useful engineering guidelines for the low-cost implementation of Phase-OTDR systems. It is anticipated that the affordable development of such interrogation systems will promote their use in a wide range of SHM applications with moderate monitoring requirements and will assist the penetration of Phase-OTDR technology in the industry.

We present a fiber optic magnetic field sensor conceived for magnetic resonance imaging (MRI) applications. The sensor is based on the integration of fiber optic strain sensors (fiber Bragg gratings-FBGs) with a sensing material (Terfenol-D). The response of an FBG integrated with a block of Terfenol-D was preliminarily investigated by taking into account the dependence of the Terfenol-D magnetostrictive response on both the longitudinal and transversal magnetic fields, with different preloads. Based on the performed characterizations, a triaxial magnetic field sensor was designed, characterized, and fabricated. An algorithm enabling the demodulation of the magnetic field from the readout of the three FBGs was also implemented, by taking into account the interdependence among the different sensor responses. Experimental results demonstrate the ability of the triaxial sensor to measure the magnetic field. Performance assessment and critical analysis are reported as well, elucidating both the abilities and limitations of the implemented sensing configuration. Finally, as proof of principle, a sensing system constituted of 20 triaxial sensors has been fabricated and used to map the magnetic field strength distribution in an MRI diagnostic centre.

Smart or multi-functional materials are receiving more and more attention in the last decade, due to their strong potentialities to provide new and remarkable functionalities. With the aim to identify the operator by a systematic approach based on first order reversals, the device reported has been developed. The box contains the magnetostrictive material equipped with a strain sensor based on Fiber Bragg Grating. A screw is also employed to apply a specified prestress to the sample. The availability of an electromagnet with air-gap fields up to 2T allows to apply sufficiently large fields able to lead the Terfenol samples close to saturation, for a wide set of applied pre-stresses. The magnetostrictive response of the Terfenol sample for a wide range of pre-stresses is displayed.

We report on the use of second-order Raman amplification to assist a phase-sensitive optical time domain reflectometer\ud ( φOTDR ) used for vibration measurements over very long distances. The sensor was able to measure vibrations of up to\ud 380 Hz (limit set by the time of flight of light pulses) in a distance of 125 km with a resolution of 10 m and no postprocessing.\ud Balanced detection is used to reduce the relative intensity noise. A comparison with a sensor using first-order\ud Raman amplification under similar conditions is presented and a clear improvement of performance is demonstrated

In this study, the authors present an experimental and theoretical description of the use of first order Raman amplification to improve the performance of a Phase-sensitive optical time domain reflectometer (φOTDR) when used for vibration measurements over very long distances. A special emphasis is given to the noise which is carefully characterized and minimized along the setup. A semiconductor optical amplifier and an optical switch are used to greatly decrease the intra-band coherent noise of the setup and balanced detection is used to minimize the effects of RIN transferred from the Raman pumps. The sensor was able to detect vibrations of up to 250 Hz (close to the limits set by the time of flight of light pulses) with a resolution of 10 m in a range of 125 km. To achieve the above performance, no post-processing was required in the φOTDR signal. The evolution of the φOTDR signal along the fiber is also shown to have a good agreement with the theoretical model.

Phase-sensitive optical time domain reflectometry (φOTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. Up to now, φOTDRs have been used mostly for the measurement of sub-kHz vibrations, normally in the context of intrusion sensing. In this paper, the authors present an experimental and theoretical description of a high-visibility φOTDR and its performance when used for ultrasonic vibration measurements. The use of a semiconductor optical amplifier in the setup allows to suppress coherent noise and also to improve the spectral response of the pump pulses. These two advantages greatly decrease the detected intra-band noise thus allowing frequency measurements in the limits set by the time of flight of the light pulses while maintaining the simplicity of the scheme, as no post-processing, extremely high coherence lasers or coherent detection methods are required. The sensor was able to measure vibrations of up to 39.5 kHz with a resolution of 5 m over a range which could go up to 1.25 km. This is the first time to our knowledge that a fully distributed measurement of ultrasonic waves was achieved. The statistical behavior of the system was also described theoretically and characterized experimentally.

Wheel flats are a major source of problems in railway systems since they cause strong wear both in the infrastructure and in the train vehicles. In this paper, we present field tests concerning the application of Fiber Bragg Grating (FBG) sensors for the detection of out-of-roundness in high-speed train wheels. Wheel flats are shown to cause high-energy impacts on the rails that can be monitored using FBGs located in the rails. The results point out a great potential of this technology for the development of reliable wheel flat detection systems.

In structural health monitoring, the propagation of ultrasonic waves along a structure can reveal interesting data relevant to the integrity of the whole structure. The availability of a system for distributed sensing of ultrasonic waves could, in some cases (e.g. pipelines) provide extremely valuable information to civil engineers. Phase-sensitive optical time domain reflectometry (ΦOTDR) is a simple and effective tool allowing the distributed monitoring of vibrations along single-mode fibers. Up to now, ΦOTDRs have been used mostly for the measurement of sub-kHz vibrations. In this work, the authors present an experimental characterization of a high-visibility ΦOTDR and its performance when used for ultrasonic vibration measurements. The sensor was able to measure vibrations of up to 39.5 kHz with a resolution of 5 m over a range which could go up to 1.25 km. This is the first time to our knowledge that a ΦOTDR is demonstrated for distributed measurement of ultrasonic waves.

In this paper we present the field testing results of a low-cost all-fibre polarimetric current sensor for the monitoring of current in the European high-speed railway network. The sensor fulfils the requirements of robustness, sensitivity, accuracy and cost required for the monitoring of catenary current in changeover sections. Changeover sections are nonfed sections of the catenary that are placed between sections fed with different phases and that introduce discontinuities in the current collection done by the train. Since the train passes at high-speed between the two sections, an electric arc can be formed that may lead to significant damage of the infrastructure. To avoid this situation, it is essential to ensure the switch-off of the current collection before arriving and switch-on again when the changeover section is passed. An adequate protection system that monitors the current in the catenary before the changeover section allows to trigger the necessary protection mechanisms in the infrastructure. Efficient, robust and lightweight electrical current sensors are therefore essential for this security system. The sensor proposed here uses the Faraday magneto-optic Effect with a well-known polarimetric interrogation method. The optical configuration is extremely simplified through the use of few cost-effective, all-fibre devices with a simplified alignment. It allows high sensitivity for low current values, demonstrating a resolution below the ampere level with a dynamic range up to 500 A. The setup is electronically self-referenced to reduce the effect of small misalignments in the polarization, power variations in the optical source, temperature changes, birefringence effect and bending-induced attenuation in the lead fibre. A set of temperature tests in a climatic chamber were also performed in a range of temperature between -20 up to 80ºC, to assess the robustness of the device to operating temperature variations. The field tests include tests during commercial operations and special tests simulating fault conditions.