Hybrid BOFDA/BOCDA system for distributed static and dynamic strain measurements.

We present a distributed optical fiber sensor based on a hybrid Brillouin optical frequency/correlation-domain analysis (BOFDA/BOCDA) configuration for both static and dynamic strain measurements. Distributed static strain (or temperature) measurements are realized using the conventional BOFDA method, i.e.

Distributed Temperature Sensing through Network Analysis Frequency-Domain Reflectometry.

In this paper, we propose and demonstrate a network analysis optical frequency domain reflectometer (NA-OFDR) for distributed temperature measurements at high spatial (down to ≈3 cm) and temperature resolution. The system makes use of a frequency-stepped, continuous-wave (cw) laser whose output light is modulated using a vector network analyzer. The latter is also used to demodulate the amplitude of the beat signal formed by coherently mixing the Rayleigh backscattered light with a local oscillator.

Tuning of the Brillouin scattering properties in microstructured optical fibers by liquid infiltration.

We demonstrate the possibility to modify the Brillouin scattering properties of a microstructured pure-silica core optical fiber, by infiltrating a liquid inside its holes. In particular, we show that the dependence of the Brillouin frequency shift (BFS) on the temperature can be reduced by infiltration, owing to the large negative thermo-optic coefficient of the liquid. Infiltrating a chloroform-acetonitrile mixture with a refractive index of 1.

Innovative Photonic Sensors for Safety and Security, Part III: Environment, Agriculture and Soil Monitoring.

In order to complete this set of three companion papers, in this last, we focus our attention on environmental monitoring by taking advantage of photonic technologies. After reporting on some configurations useful for high precision agriculture, we explore the problems connected with soil water content measurement and landslide early warning. Then, we concentrate on a new generation of seismic sensors useful in both terrestrial and under water contests.

Innovative Photonic Sensors for Safety and Security, Part I: Fundamentals, Infrastructural and Ground Transportations.

Our group, involving researchers from different universities in Campania, Italy, has been working for the last twenty years in the field of photonic sensors for safety and security in healthcare, industrial and environment applications. This is the first in a series of three companion papers. In this paper, we introduce the main concepts of the technologies employed for the realization of our photonic sensors.

Innovative Photonic Sensors for Safety and Security, Part II: Aerospace and Submarine Applications.

The employability of photonics technology in the modern era's highly demanding and sophisticated domain of aerospace and submarines has been an appealing challenge for the scientific communities. In this paper, we review our main results achieved so far on the use of optical fiber sensors for safety and security in innovative aerospace and submarine applications. In particular, recent results of in-field applications of optical fiber sensors in aircraft monitoring, from a weight and balance analysis to vehicle Structural Health Monitoring (SHM) and Landing Gear (LG) monitoring, are presented and discussed.

Automated Damage Detection Using Lamb Wave-Based Phase-Sensitive OTDR and Support Vector Machines.

In this paper, we propose and demonstrate a damage detection technique based on the automatic classification of the Lamb wave signals acquired on a metallic plate. In the reported experiments, Lamb waves are excited in an aluminum plate through a piezoelectric transducer glued onto the monitored structure. The response of the monitored structure is detected through a high-resolution phase-sensitive optical time-domain reflectometer (ϕ-OTDR).

Lamb Wave Detection for Structural Health Monitoring Using a ϕ-OTDR System.

In this paper, the use of a phase-sensitive optical time-domain reflectometry (ϕ-OTDR) sensor for the detection of the Lamb waves excited by a piezoelectric transducer in an aluminum plate, is investigated. The system is shown to detect and resolve the Lamb wave in distinct regions of the plate, opening the possibility of realizing structural health monitoring (SHM) and damage detection using a single optical fiber attached to the structure. The system also reveals the variations in the Lamb wave resulting from a change in the load conditions of the plate.

Distributed Optical Fiber Sensor Applications in Geotechnical Monitoring.

We report the experimental application of distributed optical fiber sensors, based on stimulated Brillouin scattering (SBS), to the monitoring of a small-scale granular slope reconstituted in an instrumented flume and subjected to artificial rainfall until failure, and to the monitoring of a volcanic rock slope. The experiments demonstrate the sensors' ability to reveal the sudden increase in soil strain that foreruns the failure in a debris flow phenomenon, as well as to monitor the fractures in the tuff rocks. This study offers an important perspective on the use of distributed optical fiber sensors in the setting up of early warning systems for landslides in both rock and unconsolidated materials.

Long-Term Monitoring of a Tunnel in a Landslide Prone Area by Brillouin-Based Distributed Optical Fiber Sensors.

This paper shows the results of the monitoring of the deformations of a tunnel, carried out using a distributed optical fiber strain sensor based on stimulated Brillouin scattering. The artificial tunnel of the national railway crosses the accumulation zone of an active landslide, the Varco d'Izzo earthflow, in the southern Italian Apennines. Severely damaged by the landslide movements, the tunnel was demolished and rebuilt in 1992 as a reinforced concrete box flanked by two deep sheet pile walls.

The Role of Tapered Light-Diffusing Fibers in Plasmonic Sensor Configurations.

In this work, we experimentally analyzed the effect of tapering in light-diffusing optical fibers (LDFs) when employed as surface plasmon resonance (SPR)-based sensors. Although tapering is commonly adopted to enhance the performance of plasmonic optical fiber sensors, we have demonstrated that in the case of plasmonic sensors based on LDFs, the tapering produces a significant worsening of the bulk sensitivity (roughly 60% in the worst case), against a slight decrease in the full width at half maximum (FWHM) of the SPR spectra. Furthermore, we have demonstrated that these aspects become more pronounced when the taper ratio increases.

A Nanoplasmonic-Based Biosensing Approach for Wide-Range and Highly Sensitive Detection of Chemicals.

In a specific biosensing application, a nanoplasmonic sensor chip has been tested by an experimental setup based on an aluminum holder and two plastic optical fibers used to illuminate and collect the transmitted light. The studied plasmonic probe is based on gold nanograting, realized on the top of a Poly(methyl methacrylate) (PMMA) chip. The PMMA substrate could be considered as a transparent substrate and, in such a way, it has been already used in previous work.

Distributed Static and Dynamic Strain Measurements in Polymer Optical Fibers by Rayleigh Scattering.

We demonstrate the use of a graded-index perfluorinated optical fiber (GI-POF) for distributed static and dynamic strain measurements based on Rayleigh scattering. The system is based on an amplitude-based phase-sensitive Optical Time-Domain Reflectometry (ϕ-OTDR) configuration, operated at the unconventional wavelength of 850 nm. Static strain measurements have been carried out at a spatial resolution of 4 m and for a strain up to 3.

Two-wavelength phase-sensitive OTDR sensor using perfect periodic correlation codes for measurement range enhancement, noise reduction and fading compensation.

We demonstrate a two-wavelength differential-phase-measuring OTDR sensor that uses perfect periodic correlation phase codes to enhance the measurement performance. The two-wavelength technique extends the measurement range of OTDR sensors by synthesizing a virtual longer-wavelength measurement from two simultaneous measurements of phase using different lasers. This increases the range free from phase unwrapping errors.

Distributed Dynamic Strain Sensing Based on Brillouin Scattering in Optical Fibers.

Over the past three decades, extensive research activity on Brillouin scattering-based distributed optical fiber sensors has led to the availability of commercial instruments capable of measuring the static temperature/strain distribution over kilometer distances and with high spatial resolution, with applications typically covering structural and environmental monitoring. At the same time, the interest in dynamic measurements has rapidly grown due to the relevant number of applications which could benefit from this technology, including structural analysis for defect identification, vibration detection, railway traffic monitoring, shock events detection, and so on. In this paper, we present an overview of the recent advances in Brillouin-based distributed optical fiber sensors for dynamic sensing.

An Fiber Bragg Grating-Based Monitoring System for Slope Deformation Studies in Geotechnical Centrifuges.

Centrifugal model tests, which can reproduce the deformation process of the slope, play a crucial role in investigating the mechanism of slope failure. The FBG-based sensors, with high precision, electromagnetic resistance, light weight and small size, have been introduced into geotechnical centrifuge monitoring. The slope evolution is a complex multi-parameter dynamic process which involves the interaction of displacement, stress and strain.

Long-Term Monitoring with Fiber Optics Distributed Temperature Sensing at Campi Flegrei: The Campi Flegrei Deep Drilling Project.

Monitoring volcanic phenomena is a key question, for both volcanological research and for civil protection purposes. This is particularly true in densely populated volcanic areas, like the Campi Flegrei caldera, which includes part of the large city of Naples (Italy). Borehole monitoring of volcanoes is the most promising way to improve classical methods of surface monitoring, although not commonly applied yet.

Refractive index sensing by Brillouin scattering in side-polished optical fibers.

In this Letter, we demonstrate the possibility to measure the refractive index of a liquid, using the stimulating Brillouin scattering in a 3-cm-long side-polished optical fiber. In addition, we show that by depositing a high-refractive index layer on the polished surface the sensitivity of the Brillouin frequency shift (BFS) can be increased due to a higher penetration of the evanescent field in the outer medium. Experiments show a maximum BFS change of about 11 MHz when varying the refractive index of the external medium from 1 (air) to 1.

Proposal of Brillouin optical frequency-domain reflectometry (BOFDR).

We demonstrate a Brillouin optical frequency-domain reflectometry (BOFDR) technique, which can measure the strain and/or temperature along an optical fiber with one-end access, by detecting the spontaneous Brillouin scattering from a sinusoidally modulated pump light. Compared to the Brillouin optical frequency-domain analysis (BOFDA), we show that BOFDR measurements are free from the distorting components related to acoustic wave modulation, thus simplifying data processing.

Cost-effective method for fast Brillouin optical time-domain analysis.

A new Brillouin optical time-domain analysis (BOTDA) technique for acquiring the full Brillouin gain spectrum (BGS) at high speed is proposed and demonstrated. The method employs a frequency swept microwave source for the generation of the probe wave, so that the entire BOTDA measurement is taken within the duration of the frequency sweep itself. By properly setting the duration of the sweep, the repetition rate of the pump pulses and the number of averages, truly distributed and dynamic measurements of the BGS are possible using a set-up at a fraction of the cost and complexity of the previously reported fast-BOTDA methods.

Experimental and numerical study on stimulated Brillouin scattering in a graded-index multimode fiber.

Numerical and experimental results on stimulated Brillouin scattering (SBS) in a graded-index multimode silica fiber are reported. The Brillouin Gain Spectrum (BGS) is shown to strongly depend on the pump and probe modal content. By use of a numerical model, the BGS at varying launching conditions of both pump and probe beams is computed.

Real-time monitoring of railway traffic using slope-assisted Brillouin distributed sensors.

The application of a Brillouin distributed sensor for the monitoring of railway traffic is presented in this work. The field test is performed on the Italian regional line San Severo-Peschici, operated by Ferrovie del Gargano. A single-mode optical fiber sensor was attached along a rail length of about 60 m.

Limitations and strategies to improve measurement accuracy in differential pulse-width pair Brillouin optical time-domain analysis sensing.

In this work, we analyze the effects of Brillouin gain and Brillouin frequency drifts on the accuracy of the differential pulse-width pair Brillouin optical time-domain analysis (DPP-BOTDA). In particular, we demonstrate numerically that the differential gain is highly sensitive to variations in the Brillouin gain and/or Brillouin shift occurring during the acquisition process, especially when operating with a small pulse pair duration difference. We also propose and demonstrate experimentally a method to compensate for these drifts and consequently improve measurement accuracy.

Long-range distributed Brillouin fiber sensors by use of an unbalanced double sideband probe.

We propose and demonstrate a long-range Brillouin Optical Time-Domain Analysis (BOTDA) distributed sensing system making use of an unbalanced double sideband probe formed by a Stokes and an anti-Stokes line. In particular, we show that for each measuring condition an optimal Stokes /anti-Stokes input power ratio exists, allowing a larger suppression of nonlocal effects induced by pump depletion. Experiments on a 50 km single-mode sensing fiber with 5 meters spatial resolution are reported.

Numerical analysis of single pulse and differential pulse-width pair BOTDA systems in the high spatial resolution regime.

A numerical analysis of conventional and differential pulse-width pair Brillouin optical time domain analysis systems is reported. The tests are focused on determining the performance of these systems especially in terms of spatial resolution, as a function of the pulse characteristics. A new definition of spatial resolution is given, based on analysis of the shape of the Brillouin gain spectrum.