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.
A fiber Bragg grating has been inscribed in a 100-µm diameter sapphire optical fiber with the phase mask technique and a fs-laser emitting at 800 nm. The grating was placed inside a sealed alumina capillary to protect the fiber from the environment. Then the fiber was set inside an oven and cycled up 7 times to a maximum temperature of 1500°C during 2 h. We observed that after two cycles, the grating is stabilized and no more hysteresis on the Bragg wavelength is observed. However, the temperature uncertainty is as high as 15°C and is principally due to modal interference. Then the grating is submitted to a 3-day annealing and two annealing successive 4-day annealing – for a total of eleven days – at a temperature of 1500°C. During this treatment, the grating amplitude remained constant and the Bragg wavelength showed no significant drift. As a conclusion, the packaged grating did not exhibit any erasure during these annealing experiments and perform reliable temperature measurement up to 1500°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.
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.
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.
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