Emmanuel Marin

Strain and Temperature Sensing Characteristics of Single-Mode–Multimode–Single-Mode Structures

We present a comprehensive study of the strain and temperature-sensing characteristics of single-mode-multimode-single-mode (SMS) structures based on the modal interference of guided modes of graded index multimode fiber (MMF) section spliced in between two single-mode fibers. A detailed theoretical study of the structures in terms of the refractive index distribution, effect of dopant and their concentrations, and the variation of core diameter has been carried out. Our study shows that for the SMS structure with a GeO2-doped MMF there exists a critical wavelength on either side of which the spectrum shows opposite spectral shift with a change in temperature/strain, whereas for structures with a P2O5-doped MMF it shows monotonic red shift with increasing temperature/strain. It has been found that the critical wavelength shifts toward higher wavelengths with decreasing ldquoqrdquo value/doping concentration. Using different MMFs, both the red and blue spectral shifts have been observed experimentally. It has also been found that the SMS structure has higher sensitivity toward this critical wavelength. The study should find application in designing strain-insensitive high-sensitive temperature sensors or vice versa.

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.

Side-Polished Optical Fiber Grating-Based Refractive Index Sensors Utilizing the Pure Surface Plasmon Polariton

In this paper, we present the ambient refractive index (ARI) sensing characteristics of metal-coated side-polished optical fiber gratings based on the excitation of the pure surface plasmon polariton. The resonance wavelength shift as a function of the ARI and the grating lengths required for a fixed minimum transmittivity (30%) for the TM-like mode are obtained for different metal and residual cladding thicknesses. It is found that a long-period grating (LPG)-based sensor is about 5-20 times more sensitive to the change in the ARI and requires much shorter grating lengths for a given sensitivity than the one based on fiber Bragg grating (FBG). Further, unlike a FBG-based device, an LPG-based sensor is found to have maximum sensitivity at an optimum value of metal thickness, the reason for which is also explained. This paper should find application in the design of sensitive fiber-optic bio/chemical sensors.

Discriminated measures of strain and temperature in metallic specimen with embedded superimposed long and short fibre Bragg gratings

We propose a superimposed fibre Bragg gratings device to measure, localize and discriminate strain and temperature effects simultaneously for structural health monitoring. Long period grating (LPG) and fibre Bragg grating (FBG) exhibit different responses to an applied perturbation; thus, strain and temperature influences can be determined separately by measuring the corresponding wavelength shifts. In this paper, we present a configuration based on the use of these two grating types: a LPG and a FBG written in the same fibre section which allows us to discriminate the contributions of these two main perturbations. The sensor is calibrated in a temperature range from 22 °C to 120 °C and in a strain range from 0 to 1400 μe. The reported errors are estimated to be within ± 0.4 °C and ±5 μe respectively. Our sensor is compared to those suggested in the literature for the discrimination between strain and temperature with Bragg gratings. We propose a parameter E which allows us to compare the relative uncoupling efficiency of those techniques. These sensors were embedded and tested in metallic material plates for the purpose of validating structural health monitoring.

Radiation tolerant fiber Bragg gratings for high temperature monitoring at MGy dose levels

We report a method for fabricating fiber Bragg gratings (FBG) resistant to very severe environments mixing high radiation doses (up to 3 MGy) and high temperatures (up to 230°C). Such FBGs have been written in two types of radiation resistant optical fibers (pure-silica and fluorine-doped cores) by exposures to a 800 nm femtosecond IR laser at power exceeding 500 mW and then subjected to a thermal annealing treatment of 15 min at 750°C. Under radiation, our study reveals that the radiation induced Bragg wavelength shift (BWS) at a 3 MGy dose is strongly reduced compared to responses of FBGs written with nonoptimized conditions. The BWS remains lower than 10 pm for temperatures of irradiation ranging from 25°C to 230°C without noticeable decrease of the FBG peak amplitude. For an applicative point of view, this radiation induced BWS corresponds to an additional error on the temperature measurements lower than 1.5°C, opening the way to the development of radiation-tolerant multi-point temperature sensors for nuclear industry.

Highly Sensitive Miniaturized Refractive Index Sensor Based on Au-Ag Surface Gratings on a Planar Optical Waveguide

We present the theoretical study of a novel highly sensitive, miniaturized, integrated optic refractive index sensor based on a Au-Ag surface grating. The grating is considered to be made of alternate layers of equi-thick Au and Ag regions along the direction of propagation, on the surface of the waveguide. Due to the same thickness of both the metals, the surface plasmon polaritons (SPP) for both metals have their field maxima at the same transverse distance, leading to an increased modal overlap in the grating region and hence a reduced grating length. An exact coupled-mode-theory based on the local mode matching has been used to analyze the mode coupling between the guided mode and the SPP. It has been shown that the proposed design requires nearly one fourth of the grating length as compared to the corrugated metal grating for the same metal thickness. Further, for co-propagating mode coupling (LPG based sensor) the structure is found to be maximum sensitive at an optimum metal thickness, however, such an optimum metal thickness does not exist for counter-propagating coupling (FBG based sensor).

Critical Wavelength in the Transmission Spectrum of SMS Fiber Structure Employing GeO

We demonstrate and explain the existence of a critical wavelength in the transmission spectrum of single-multi-singlemode fiber structures, employing GeO2-doped multimode fibers (MMFs), such that the spectrum shows opposite spectral shift on either side of this wavelength with increasing temperature. Further, the shift is maximum for peaks/dips nearest to this wavelength. The variation of critical wavelength with temperature is also measured. A theoretical account of the observed behavior has been presented and the results are found to be in excellent agreement with the experiments. It is shown theoretically that for a given MMF there should be two critical wavelengths of the opposite nature separated by a large wavelength difference.

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Vulnerability of Optical Frequency Domain Reflectometry (OFDR) based sensors to high γ-ray doses (up to 10 MGy) is evaluated with a specific issue of a radiation-hardened temperature and strain monitoring system for nuclear industry. For this, we characterize the main radiation effects that are expected to degrade the sensor performances in such applicative domain: the radiation-induced attenuation (RIA), the possible evolution with the dose of the Rayleigh scattering phenomenon as well as its dependence on temperature and strain. This preliminary investigation is done after the irradiation and for five different optical fiber types covering the range from radiation-hardened fibers to highly radiation sensitive ones. Our results show that at these high dose levels the scattering mechanism at the basis of the used technique for the monitoring is unaffected (changes below 5%), authorizing acceptable precision on the temperature or strain measurements. RIA has to be considered as it limits the sensing range. From our vulnerability study, the OFDR sensors appear as promising candidates for nuclear industry even at doses as high as 10 MGy.

-Doped Multimode Fiber

The modal characteristics of silica-based photonic crystal fibres (PCF) are examined through a scalar modal analysis combined with an effective V-parameter model using a finite-difference method. Cut off frequencies of the lowest-order modes and the normalised dispersion curves are given and are found to be quasi-independent of the hole-size-to-pitch ratio. A simple and accurate empirical relation for the cutoff wavelength is given for the first higher-order mode for fibres with hole-size-to-pitch ratio below 0.7. These results should be very useful in designing PCFs for various applications.

Vulnerability of OFDR-based distributed sensors to high γ-ray doses

We report on the space-selective precipitation of silver nanoparticles in silica-based and silver-exchanged soda-lime glasses by simultaneous continuous wave ultraviolet exposure and heat treatment. In silica-based glasses, we explain that simultaneous treatments lead to much higher silver nanoparticles concentration than similar treatments performed into two separated steps by minimizing the detrimental influence of oxidation on the nanoparticles’ growth. In the case of silver-exchanged soda-lime glasses, nanoparticles are observed with both small and larger diameters of about 1 and 7 nm, whose concentrations depend both on the laser power density and on the heating temperature.