Fiber Bragg gratings are widely used for optical sensing applications, including their deployment in harsh environments. The use of Type III femtosecond gratings shows a prominent interest due to their ability to withstand very high temperatures (over 1000°C for 100’s of hours). These Type III fiber Bragg gratings correspond to a periodic structure of micro-voids generated by femtosecond-laser in silica-based optical fibers, fabricated by the point-by-point technique. The physicochemical characteristics of the micro-voids were investigated by quantitative phase microscopy technique and their thermal stability monitored through isochronal annealing experiments up to 1250°C.
We investigate the behavior and stability of fiber Bragg gratings written by femtosecond laser pulses in Ge-doped fused silica optical fibers, using both the phase mask and point-by-point techniques, during their annealing at 1200°C for 30 min and subsequent aging at 1000°C during 43 hours. Bragg wavelength drifts and reflected peak amplitude variations were shown to drastically differ depending on the writing scheme and thermal history. Particularly, we show that amplitude decay of point-by-point gratings at 1200°C may be easily mitigated by tuning the writing pulse energy. Future work may be pursued in order to finely unravel the high temperature mechanisms regarding the stability of fs-written fiber Bragg gratings used as temperature sensors in order to improve measurement stability and accuracy.
Femtosecond laser point-by-point writing is a commonly used method for fabrication of Fiber Bragg Grating sensors dedicated to harsh environments, such as high temperature or irradiation. In addition, a femtosecond laser platform allows for inscription of compact fiber optic diffraction gratings that consist of micro-voids or filaments formed into the fiber core and cladding by focusing laser pulses using microscope objectives. Light propagating in the fiber is coupled to radiation modes due to Mie scattering, thus providing wavelength dispersion in free space. Chirping the grating period further allows focusing of the outcoupled light in a given plane. Such an all-fiber focusing grating forms a compact photonic device permitting its use for FBG sensor interrogation. In this paper, fabrication of such spectrometers operating at 850 and 1550 nanometers is described. A characterization setup allowing measurements of spectra of FBGs at those wavelength bands is presented, and results corresponding to various focusing distances, grating lengths and chosen microscope objectives are exposed. Azimuthal distribution of scattered light is discussed, as well as focusing distance versus grating period chirp and spectrometer resolution versus grating length. Finally, spectra reflected by pointby- point FBG sensors are presented, thus demonstrating the great potential interest of such gratings for FBG interrogation.
Fiber Bragg gratings can be used to monitor temperature or strain in harsh environments. We investigate the effect of Xrays on type III gratings – also called void gratings –which are known for their capacity to withstand high temperatures. The tested gratings are inscribed in a SMF28 germanosilicate optical fiber using the point-by-point method and a frequency-doubled Yb femtosecond laser emitting at 515 nm. The tested FBGs are separated in two groups depending on their reflectivity levels (Low/High). Half of each group is pre-annealed at a temperature of 750°C during 30 min. We have irradiated all the gratings up to 100 kGy(SiO2) at a dose-rate of 10 Gy/s at two different irradiation temperatures: 25°C and 150°C. For all the irradiations, the grating radiation response is identical independently of the chosen writing and preannealing conditions. When the irradiation is performed at 25°C, a Bragg wavelength shift of 10 pm is observed for all the gratings, which represents an error of less than 1°C at the total dose of 100 kGy while at 150°C the Bragg peak shift only of less than 4 pm corresponding to an error of 0.3°C.
This paper presents the development of a CO2 laser-based heating system to simulate fast thermal phenomena on high temperature resistant FBGs dedicated to high temperature profiles monitoring within components exposed to hot plasma inside Tokamak.. We have demonstrated the reliability of two optical fiber based sensing techniques to control spatial and temporal annealing measurement during high and fast thermal process.
We investigated the Bragg Wavelength Shift (BWS) induced by X-rays in a large set of conventional FBGs up to 100kGy dose. Obtained results give some insights on the influence of irradiation parameters such as dose, dose rate as well as the impact of some writing process parameters such as thermal treatment or acrylate recoating on the FBG radiation tolerance.
For the first time, a gold coated single mode optical fiber has been used to detect a liquid sodium leakage on a pipe of secondary circuit pipe mock-up of nuclear fast reactor (Gen IV) by means of Optical Frequency Domain Reflectometry-based on Rayleigh backscattering. During 150 min of the experiment we were able to detect and monitor the evolution of a liquid sodium leakage on the surface of the pipe.
The regeneration of Tilted Fiber Bragg Gratings photowritten in a singlemode SMF-28e fiber is investigated. Three tilted FBGs, with respectively 0°, 4° and 8° external tilt angles are annealed according to a conventional high temperature regeneration protocol. We monitor the evolution of distinct spectral features during the thermal annealing process. We also propose a method in order to follow the regeneration process of tilted FBG, without focusing on individual spectral resonances. Regeneration of tilted FBGs presents numerous interests for the realization of high temperature sensors and devices, but also to improve their resistance to preliminary manufacturing processes requiring high temperature steps.
Regenerated Fiber Bragg Gratings have been annealed at high temperatures up to 890°C during 9000 hours in continuous. This test has been conducted in order to experimentally assess the long-term stability of regenerated FBGs dedicated to measurements in high temperature environments. The evolution of both reflectivities and Bragg wavelengths of four wavelength-multiplexed regenerated FBGs simultaneously annealed at respectively 760°C, 810°C, 850°C and 890°C is reported.
High temperature gradient of a conventional tubular furnace is characterized using a single fiber sensing line with
wavelength-multiplexed short-length regenerated Fiber Bragg Gratings. The multiplexed gratings are simultaneously
regenerated using a high temperature annealing process. Temperature calibration from ambient temperature up to 850°C
is conducted leading to a standard deviation of 0.33°C after polynomial fitting of the wavelength shift with temperature.
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