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.

We study the localisation and control of high frequency sound in a dual-core square-lattice photonic crystal fibre preform. The coupled states of two neighboring acoustic resonances are probed using an interferometric set up, and experimental evidence is obtained for odd and even symmetry trapped states. Full numerical solutions of the acoustic wave equation show the existence of a two-dimensional sonic band gap, and numerical modelling of the strain field at the defects gives results that agree well with the experimental observations. The results suggest that sonic band gaps can be used to manipulate sound with great precision and enhance its interaction with light.

This review focuses on silica-based optical fibers, guiding light through the total internal reflection mechanism, that are currently used in telecommunications and sensor networks. Section I briefly introduces the fiber and fiber sensor technologies. Section II presents their various applications in radiation environments. In section III, the radiation effects on optical fibers are explained, first at a microscopic scale, with the description of point defects generated by irradiation. Then, the macroscopic consequences of the appearance of these point defects are discussed, in particular the phenomena of radiation-induced attenuation (RIA), radiation-induced emission (RIE) and radiation-induced refractive index change (RIRIC). RIA impacts nearly all applications under irradiation. We then detail the various parameters, intrinsic or extrinsic to the optical fibers that influence their amplitudes and kinetics. This improved knowledge paved the way for the radiation hardening of optical fibers through various approaches, either at material, component or system levels, that are described in section IV. Recent advances are presented for passive and for active rare-earth doped optical fibers and systems. Then, the use of fiber optic sensors is introduced, starting in section V with the latest advances in radiation-hardened point or distributed sensors. Then, the potential of this technology for radiation detection, beam instrumentation and dosimetry applications is assessed in section VI. Finally, the future prospects and the main challenges to be met in order to accelerate the implementation of this technology in the harshest environments are discussed.

In the framework of the development by CEA and SCK·CEN of a Fabry Perot Sensor (FPS) able to measure dimensional changes in Material Testing Reactor (MTR), the SAKE 1 (Smirnof extention-Additional Key-tests on Elongation of glass fibres) campaign was performed. Its first goal was to measure the fiber linear compaction under high fast neutron fluence. This compaction is known to cause a noticeable measurement error of a FPS through a radiation-induced drift and an accurate quantification of this effect is required to optimize the FPS design. To achieve this, an innovative approach was used. Ad hoc samples of various fiber types have been prepared and exposed in the SCK.CEN BR2 reactor (Mol Belgium). After 22 days of irradiation at 291°C a total fast (E > 1MeV) fluence of 3 to 5 × 10 ¹⁹ n _fast /cm ² , depending on the sample location, was accumulated. A precise measurement of each fibre sample length was made before the irradiation and compared to the post irradiation measurement highlighting a decrease of the fibres' length corresponding to about 0.25% of linear compaction. The amplitude of the observed changes is independent of the neutron fluence, which indicates that the compaction effect may saturate below the lowest considered fluence. A second goal was to perform preliminary evaluation of the feasibility to implement distributed temperature measurement in MTR using fibre Bragg gratings (FBG). Several FBG written in radiation hard fibres using a femtosecond laser have been elaborated with a procedure allowing to enhance their radiation tolerance. First promising results on their vulnerability to these extreme conditions are discussed.