Alain Trouillet

Surface plasmon resonance hydrogen sensor using an optical fibre

An optical fibre surface plasmon resonance (SPR) sensor has been developed for the detection of hydrogen leakages. A thin palladium layer deposited on the bare core of a multimode fibre was used as the transducer. In this device, modification of the SPR is due to variation in the complex permittivity of palladium in contact with gaseous hydrogen. This effect is enhanced by using selective injection of high-order modes in the fibre via a collimated beam with non-normal incidence on the input end of the fibre. Measurements of concentrations as low as 0.8% of hydrogen in pure nitrogen have been found to be possible. The response time varies between 3 s for pure hydrogen and 300 s for the lowest concentrations. Such a large range can be explained by the two different crystallographic phases of the palladium-hydrogen system. Moreover, the response of the sensor is dependent on the length of the sensing area. In preliminary experiments, it has been possible to split the sensing area in order to achieve a two-point detection device.

Fibre gratings for hydrogen sensing

Liquid hydrogen has been intensively used in aerospace applications for the past 40 years and is of great interest for future automotive applications. Following major explosive risks due to the use of hydrogen in air, several studies were carried out in order to develop optical fibre sensors for the detection of hydrogen leakage. This paper aims at the presentation of new hydrogen sensors based on the use of fibre Bragg gratings (FBG) and long period gratings (LPG) coated by palladium nanolayers. The sensing principle based on the palladium–hydrogen interaction is presented, as well as experimental results. It is shown that both techniques could be used for hydrogen sensing but with a sensitivity enhanced by a factor up to 500 when using a LPG sensor. FBG sensors appear to be pure strain sensors and LPG sensors are mainly based on the coupling between the cladding modes and evanescent or surface plasmon waves. Preliminary results obtained with an in-fibre Mach–Zehnder interferometer configuration with in-series LPG sensors are also presented. They show potential interest to compensate for the thermal sensitivity of the fibre gratings.

Optical detection of toluene in water by using IGI fibers

The use of a sensor employing a short, straight segment of a silica fiber with an inverted parabolic refractive index profile in the core for the detection of aqueous solutions of toluene was investigated. The sensing layer of the fiber was prepared by depositing a thin polydimethylsiloxane layer on the bare fiber core by the dip-coating technique. The performance of the fiber was examined in relatively long static and dynamic exposures of the fiber to toluene solutions, during which saturated values of the output sensor signal were reached. Detection sensitivities of several milligrams per liter were achieved for both types of exposures.

Detection of refractive-index changes by using a sensing fiber with an inverted parabolic index profile

The application of a novel sensing fiber with an inverted parabolic-index profile to the detection of refractive-index changes of a medium surrounding the fiber core is investigated. The detection experiments were made with a set- up consisting of a fiber connector-based coupler allowing the excitation of the sensing fiber by a light source of a relatively small size placed on the fiber axis in a preset distance from the fiber end face, and an optical cell containing a declad segment of the sensing fiber in an immersion whose refractive index could be varied in a known way by heating. Various light sources, such as outputs of multimode and single-mode fibers were tested as more or less accurate approximations to the ideal point light source. For overcoming problems with the profile imperfections in the central core region appearing in some of the produced fibers and originating from the fiber manufacturing technology, the excitation of the fiber by an inclined beam from a bevelled single-mode fiber was studied as well. For comparison, PCS fibers were used in the experiments, too. The results have shown that in an index range slightly below the index of silica, higher sensitivity can be achieved with the novel fiber than with PCS fibers. The possibility of shifting the measuring range down to a more practical lower index values by using inclined-beam fiber excitation and confinement of the index profile to the outer region at the core will be shown.

Integration of fibre Bragg grating strain sensors into composite electrical insulators

Fiber Bragg Gratings (FBG) sensors embedded in composite insulators used for electrical high voltage applications have been studied. The study included the design of a new method for the manufacturing of composite insulators with optical fibers. The instrumented prototype insulators are intended to keep track of the mechanical and thermal stresses endured by the insulators in actual operating conditions. Optical characterization of the FBG sensors have been made using an erbium laser source and a spectrum analyzer. A 5kN strain resolution in a temperature range between -40°C and +60°C has been achieved using standard SMF-28 optical fibers.

Gaseous hydrogen leakage optical fibre detection system

Liquid hydrogen has been intensively used in aerospace applications during the past forty years and is of great interest for fuel cells technologies and future automotive applications. Following upon major explosive risks due to the use of hydrogen in air, previous studies were carried out in our laboratory in order to develop optical fiber sensors for the detection of hydrogen leakage. This communication is aimed towards a prototype optical fiber system designed for the detection of gaseous hydrogen leakage near the conecting flanges of the liquid hydrogen pipes on the test bench of the engine Vulcain of the rocket ARIANE V. Depending on the configuration, the prototype sensor provides a two-level alarm signal and the detection of gaseous hydrogen leakage is possible for concentrations lower than the lower explosive limit in air (between 0.1 and 4%) with alarm response times lower than 10 seconds in a wide range of temperatures between -35°C and 300°C. The sensing principle based on palladium-hydrogen interaction is presented as well as the detection system composed of an optical fiber probe and an optoelectronic device.

Modified graded-index fibers for chemical sensing

Multimode optical fibers with inverted parabolic refractive- index profiles (IGI fibers) have been shown promising for the fabrication of sensing modules for evanescent-wave chemical detection. Besides IGI fibers with parabolic index profiles, this paper deals with theoretical and experimental investigation of some other types of IGI fibers, particularly with IGI fibers with linear and modified-linear profiles. Refractive-index profiles of the fabricated IGI preforms doped in the core with boron oxide are shown in the paper. The sensitivity of IGI fibers drawn from these preforms to cladding refractive-index changes has been studied by means of the technique of selective excitation of optical models in the fiber immersed in various mixtures of n-butanol and benzyl alcohol. The obtained results are compared with those for reference step-index fibers with uniform pure-silica core. A theoretical model describing the detection process during these measurements has been derived using geometrical optics. On this basis it can be shown that the fiber sensitivity increases in the order of the reference fiber, IGI fiber with linear and modified-linear profiles and IGI fibers with parabolic profiles.