Aldo Minardo

Dynamic strain measurement in optical fibers by stimulated Brillouin scattering

We present a technique for dynamic strain measurements in optical fibers based on the stimulated Brillouin scattering interaction between two counterpropagating optical pulses. The technique allows for a high sampling rate and permits to addressing dynamically and randomly the position at which vibration is measured. Preliminary experimental results carried out with a perturbation frequency up to 98 Hz demonstrate the validity of the proposed technique.

Response of fiber Bragg gratings to longitudinal ultrasonic waves

In the last years, fiber optic sensors have been widely exploited for several sensing applications, including static and dynamic strain measurements up to acoustic detection. Among these, fiber Bragg grating sensors have been indicated as the ideal candidate for practical structural health monitoring in the light of their unique advantages over conventional sensing devices. Although this class of sensors has been successfully tested for static and low-frequency measurements, the identification of sensor performances for high-frequency detection, including acoustic emission and ultrasonic investigations, is required. To this aim, the analysis of feasibility on the use of fiber Bragg grating sensors as ultrasonic detectors has been carried out. In particular, the response of fiber Bragg gratings subjected to the longitudinal ultrasonic (US) field has been theoretically and numerically investigated. Ultrasonic field interaction has been modeled, taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to the elasto-optic effect. Numerical results, obtained for both uniform and Gaussian-apodized fiber Bragg gratings, show that the grating spectrum is strongly influenced by the US field in terms of shape and central wavelength. In particular, a key parameter affecting the grating response is the ratio between the US wavelength and the grating length. Normal operation characterized by changes in the wavelength of undistorted Bragg peak is possible only for US wavelengths longer than the grating length. For US wavelengths approaching the grating length, the wavelength change is accompanied by subpeaks formation and main peak amplitude modulation. This effect can be attributed to the nonuniformity of the US perturbation along the grating length. At very high US frequencies, the grating is not sensitive any longer. The results of this analysis provide useful tools for the design of grating-based ultrasound sensors for meeting specific requirements in terms of field intensity and frequencies.

A Simple Technique for Reducing Pump Depletion in Long-Range Distributed Brillouin Fiber Sensors

We present a technical solution, capable of alleviating the problem of pump depletion in a long-range Brillouin distributed fiber sensor. This solution takes advantage of the presence of two sidebands in the probe wave to generate a dual gain-loss Brillouin interaction, giving rise to reduced pump depletion. Experimental results, carried out by using a Brillouin optical frequency-domain analysis configuration, demonstrated the improvement offered by the gain-loss technique.

A reconstruction technique for long-range stimulated Brillouin scattering distributed fibre-optic sensors: experimental results

The experimental validation of a numerical technique for temperature/strain profile reconstruction based on Brillouin optical-fibre time-domain analysis (BOTDA) sensors is presented. In this approach, we search directly for the Brillouin frequency shift profile along the fibre that matches the measured data. The algorithm is based on a harmonic expansion of the unknown profile, whose coefficients are determined by means of a multidimensional minimization. Experimental measurements have been carried out in order to reveal the influence of nonlocalities in Brillouin measurements, and to prove the capability of the proposed algorithm to compensate for these effects.

Distributed Sensing at Centimeter-Scale Spatial Resolution by BOFDA: Measurements and Signal Processing

In this paper, we demonstrate high spatial (≈3 cm) and spectral (≈30 MHz) resolution Brillouin sensing by use of Brillouin optical frequency-domain analysis (BOFDA) and signal processing. An iterative method is employed to correct the acquired data from spurious effects associated to acoustic wave modulation and relevant in the high spatial resolution regime. Experimental tests demonstrate that the proposed algorithm allows to reconstruct the Brillouin shift profile with the full spatial resolution allowed by the system bandwidth.

Low distortion Brillouin slow light in optical fibers using AM modulation

Stimulated Brillouin scattering (SBS) has been recently shown to offer a mechanism for generating tunable all-optical delays in room-temperature single-mode optical fibers at telecommunication wavelengths. This technique makes use of the rapid variation of the refractive index that occurs in the vicinity of the Brillouin gain resonance. When the slow light pulse delay is subject to a constraint on the allowable pulse distortion, it has been shown that the use of a pair of closely-spaced Brillouin gain lines can increase the distortion-constrained delay, with respect to the single-line configuration. In this paper, we numerically and experimentally demonstrate that the same experimental apparatus usually employed for generating a Brillouin gain doublet, can also be used for achieving three equally-spaced Brillouin gain resonances, further increasing the distortion-constrained pulse delay.

Distributed Temperature Sensing in Polymer Optical Fiber by BOFDA

Distributed temperature measurements in a perfluorinated graded-index polymer optical fiber (POF) with 50- μm core diameter are reported for the first time, to the best of our knowledge. Brillouin optical frequency-domain analysis is shown to be able to resolve spatially the temperature-dependent Brillouin frequency shift profile along a 20-m POF fiber sample, at a nominal spatial resolution of 4 m. The results indicate that POFs are potentially useful for distributed temperature measurements based on stimulated Brillouin scattering.

An Instrumented Flume to Investigate the Mechanics of Rainfall-Induced Landslides in Unsaturated Granular Soils

The mechanics of rainfall-induced flowslides in pyroclastic soils have yet to be completely clarified. The complexity of phenomena (rainfall-induced failure in initially unsaturated granular deposits, post-failure transition to flow-like landslide) requires the use of a well-equipped small-scale flume. To this aim, flume experiments at the Second University of Naples were performed to analyze the fundamental aspects of such phenomena. A new experimental program is now being carried out to assess the performance of a time domain reflectometry device and optical fibers as indicators of impending failure. The paper describes the instrumented flume and the procedures adopted for monitoring the major aspects of slope behavior. Our first experimental results are very promising in this respect.

Planar Waveguides for Fluorescence-Based Biosensing: Optimization and Analysis

Optimization of planar waveguides for fluorescence biosensing is presented in this paper. In particular, the authors show that optical (refractive index) and geometrical parameters have a strong influence on the efficiency of excitation and collection of fluorescent signals. Numerical analyses show that a single-mode slab waveguide, operating at its fundamental transverse magnetic mode and near its cutoff point, results in an efficient fluorescence excitation when employed as evanescent wave biosensor. A high-refractive index contrast is demonstrated to be the key parameter for an efficient fluorescence collection. Other geometries that are an alternative to the classical slab waveguide may result in an improvement of the fluorescence excitation and collection efficiencies

Stimulated Brillouin scattering frequency-domain analysis in a single-mode optical fiber for distributed sensing.

A numerical and experimental analysis of the stimulated Brillouin scattering in a single-mode optical fiber for distributed sensing applications is carried out in the frequency domain. The theoretical model describing the Brillouin interaction is solved by taking into account the temporal dynamics of the acoustic wave that is involved. The simulations and the experimental results reveal the role played by the ac component of the acoustic wave, which is responsible for significant changes of the small-signal stimulated Brillouin scattering transfer function that occur when the modulation frequency rises above the natural Brillouin gain spectrum linewidth. One should take these effects into account to perform accurate signal processing of frequency-domain signals in high-resolution distributed sensing applications.