Luigi Zeni

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.

Silicon electro-optic modulator based on a three terminal device integrated in a low-loss single-mode SOI waveguide

We analyze, from a theoretical point of view, a novel silicon optical amplitude-phase modulator integrated into a SOI (silicon on insulator) optical waveguide and based on a three terminal electronic structure which gives rise to definite advantages in comparison with a classical p-i-n diode based modulator. The proposed device utilizes the free carrier dispersion effect to produce the desired refractive index and absorption coefficient variations. The MEDICI two-dimensional (2-D) semiconductor device simulator has been used to analyze the electrical operation, with reference to the free carrier concentration injected into the optical channel, its uniformity and the required current density and electrical power. The optical investigation was carried out by means of FDM (finite difference method), EIM (effective index method), and BPM (beam propagation method) tools, giving rise to a complete evaluation of the properties of our device. We report the results for both the amplitude and phase modulators, paying attention to the static and the dynamic behavior. In particular, an amplitude modulation of 20%, with an injection power of about 126 mW, and a switching time of 5.6 ns can be achieved theoretically, Furthermore, as a phase modulator, the device exhibits a very high figure of merit, predicting an induced phase shift per volt per millimeter of about 215/spl deg/, for a injection power of about 43 mW, and a switching time shorter than 3.5 ns.

Low Cost Sensors Based on SPR in a Plastic Optical Fiber for Biosensor Implementation

This paper reports the fabrication and testing of two configurations of optical sensor systems based on Surface Plasmon Resonance (SPR) at the interface of a liquid sample and sandwiched structures realized starting from the exposed core of a Plastic Optical Fiber (POF). The proposed geometries have proven to be suitable for measuring the refractive indexes of liquids whose refractive index falls around 1.35. Furthermore, the proposed sensing head, being low cost and relatively easy to realize, may be very attractive for biosensor implementation.

Microfluidic sensor based on integrated optical hollow waveguides

A simple integrated optical refractometric sensor based on hollow-core antiresonant reflecting optical waveguides is proposed. The sensor uses the antiresonant reflecting guidance mechanism and permits one to measure the refractive index of a liquid filling the core by simply monitoring the transmitted spectrum. The device has been made with standard silicon technology, and the experimental results confirm numerical simulations performed in one- and two-dimensional geometry. The sensor exhibits a linear response over a wide measurement range (1.3330-1.4450) and a resolution of 9 x 10(-4) and requires a small analyte volume.

Silicon micromachined hollow optical waveguides for sensing applications

Novel micromachined optical waveguides useful for sensing applications are proposed. The waveguide is designed as hollow-core antiresonant reflecting optical waveguide (ARROW) and can be easily fabricated using standard silicon micromachining techniques. The hollow structure permits to use the core to confine simultaneously the light and the substance to be probed, leading to an increase of the interaction efficiency. Numerical simulations, performed using finite element method technique, show that with a suitable design these waveguides can be used in sensing applications, where the substances under test can be gases or liquids.

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.