Advanced trends in nonlinear optics applied to distributed optical-fibre sensors

The distributed optical-fibre sensors based on the properties of Brillouin scattering is the central object of this thesis. In the past decade, optical fibres have gained a large interest as sensors: attractive solutions based on the non-linear stimulated Brillouin scattering have been proposed in the early 90s and the possibility to achieve long-range fully distributed strain measurements has been extensively demonstrated. The Brillouin interaction is responsible for the coupling between two optical waves and an acoustic wave when a resonance condition is fulfilled. Since the resonance condition is strain and temperaturedependent, by determining the resonance frequency we directly get a measure of temperature or strain. Local information about the acousto-optical resonance condition is typically obtained by using pulsed lightwaves and a classical time-of-flight technique (BOTDA technique). The main goal of this work has been the development of an innovative technique for the generation of optical signals, using a set of locked lasers – instead of the traditional techniques using external modulators. The utilisation of the injection-locking of semiconductor lasers is the key of the entire set-up and represents an entirely new and original approach, since it brings significant improvements in terms of SNR and costs. As long as intense pulses propagate along the fibre, the optical signals can be seriously degraded by several nonlinear interactions occurring inside the fibre; we show that the nonlinear effect exhibiting the lowest threshold power is the modulation instability (MI) process. From the study of the dynamic behaviour of MI we could observe the Fermi-Pasta-Ulam (FPU) recurrence over few periods in very comfortable conditions. One original application of Brillouin sensing has been the dosimetric measurement of ionising radiations in a nuclear environment. The measurement campaign has not only shown that distributed sensors based on Brillouin spectral analysis are radiation tolerant up to very high doses, but has also revealed the first observation – to our knowledge – of the negative compaction of silica in fibres. Distributed fibre sensors based on stimulated Brillouin scattering offer a unique capability for the analysis of optical signals and nonlinear phenomena in optical fibres. We present a generalised theoretical approach to the problem of localised sensing and report on the first distributed measurement – to our knowledge – of the parametric gain in a single-pump fibre-optics parametric amplifier (FOPA)

A Novel All-Fiber Configuration for a Flexible Polarimetric Current Sensor

This set-up is particularly simple and makes possible the use of a flexible cable that may be plugged to the instrument using connectors for easy field applications. Measurements of AC and DC currents are demonstrated

A novel all-fibre configuration for a flexible polarimetric current sensor

In this paper a polarimetric fibre-optics current sensor in a new highly versatile all-fibre configuration is described. The optical configuration is particularly simple and is based on a back-and-forth propagation through a twisted sensing fibre. This results in a very flexible sensing cable which can be adapted to a great number of existing power installations, by simply winding the fibre around the uninterrupted electrical conductor. The prototype presented here is designed to measure ac and dc currents with an excellent accuracy over more than three decades. © 2004 IOP Publishing Ltd

Novel configurations based on laser injection locking applied to Brillouin fibre sensing

Injection locking of two DFB semiconductors opens new possibilities to generate effective signals for fibre sensing. This is illustrated through the application to distributed Brillouin sensing that shows significant progress with respect to established techniques

The effects of gamma-radiation on the properties of Brillouin scattering in standard Ge-doped optical fibres

We have experimentally studied the effects of gamma-radiation up to very high total doses on the physical properties of Brillouin scattering in standard commercially available optical fibres. A frequency variation of about 5 MHz for both Brillouin frequency and linewidth has been measured at the total dose of about 10 MGy. The radiation-induced shift has a negligible practical impact and makes Brillouin scattering very immune to radiation, so that distributed sensors based on this interaction exhibit an interesting potential for use in nuclear facilities. © 2006 IOP Publishing Ltd

Complete experimental characterization of stimulated Brillouin scattering in photonic crystal fiber

International audienceWe provide a complete experimental characterization of stimulated Brillouin scattering in a 160 m long solid-core photonic crystal fiber, including threshold and spectrum measurements as well as positionresolved mapping of the Brillouin frequency shift. In particular, a three-fold increase of the Brillouin threshold power is observed, in excellent agree-ment with the spectrally-broadened Brillouin gain spectrum. Distributed measurements additionally reveal that the rise of the Brillouin threshold results from the broadband nature of the gain spectrum all along the fiber and is strongly influenced by strain. Our experiments confirm that these unique fibers can be exploited for the passive control or the suppression of Brillouin scattering

Mesure distribuée du gain d'un amplificateur paramétrique à fibre optique par rétro-diffusion Brillouin

Nous avons réalisé, pour la première fois à notre connaissance, une mesure distribuée du gain paramétrique d’un amplificateur à fibre optique. La technique repose sur une analyse de la rétrodiffusion Brillouin dans le domaine temporel. Elle a permis de mettre en évidence différents régimes de gain paramétriques