Analysis of uncoated LPGs written in B-Ge doped fiber under proton irradiation for sensing applications at CERN

Abstract

In this contribution, a complete dissertation concerning the behavior of a Long Period Grating (LPG) inscribed in a B-Ge co-doped optical fiber by means of an excimer laser and exposed to proton irradiation during a recent extensive campaign performed at the European Organization for Nuclear Research (CERN) with a fluence of 4.4·10 15 p·cm −2 is provided. The experimental results have been thus combined for the first time to the best of our knowledge with numerical simulations in order to estimate the variations of the major parameters affecting the grating response during the ultra-high dose proton exposure. From the correlation between experimental and numerical analysis, the irradiation exposure was found to induce a maximal variation of the core effective refractive index of ~1.61·10 −4 , responsible of a resonance wavelength red shift of ~44 nm in correspondence of the highest absorbed radiation dose of 1.16 MGy. At the same time, a relevant decrease close to ~0.93·10 −4 in the refractive index modulation pertaining to the grating was estimated, leading to a reduction of the resonant dip visibility of ~12 dB. The effect of the proton beam on the spectral response of the LPG device and on the optical fiber parameters was assessed during the relaxation phases, showing a partial recovery only of the wavelength shift without any relevant change in the dip visibility revealing thus a partial recovery only in the refractive index of the core while the reduction of the refractive index modulation observed during the irradiation remained unchanged.www.nature.com/scientificreports www.nature.com/scientificreports/ 5 %RH, i.e. where both miniaturized capacitive sensors and polyimide coated FBGs substantially lose accuracy. Nevertheless, to assess their possible application in the new generation of detectors foreseen within the HL-LHC project, a clear understanding of the physical and optical mechanisms involved during the LPGs radiation exposure is mandatory, together with a systematic study of their behavior under realistic radiation levels with respect to those expected at HL-HLC.Certainly, massive knowledge concerning the effects of irradiation on optical fibers and optical fiber-based devices has been collected over the years, but still not all the factors involved are completely clear and known. In 2018 Girard et al. published a very interesting review about the recent advancements on the radiation hardened fiber optic-based systems 13 , highlighting the potential and the future challenges of this innovative technology for the application in harsh environments. In ref. 13 the authors also provided an overview of the main radiation-induced effects on several classes of fiber optics and fiber optic-based technology, including both point sensors and distributed sensors. It is well-assessed that radiation alters the fiber properties by creating point defects in silica-based material due to ionization or displacement damage processes leading to structural modifications in the pure or doped amorphous host silica matrix...

Radiation Effects on Long Period Fiber Gratings: A Review

“…Concerning the attenuation band depth, it seems to show trivial changes in most of experiments except for a few cases. In fact, an UV-written LPG showed a significant band decreasing under proton beam [87]. It is reasonable to believe that this effect can be attributed to the grating type rather than radiation type; the radiation induces a uniform refractive index increase that reduces the grating RI modulation.…”

“…The results were strongly dependent on fiber type and were consistent between the two kinds of radiations. Finally, very recently the first report about proton irradiation of LPGs at high doses has been released [87].…”

“…Recently, in [86,87] the group of Cusano reported the first demonstration concerning an LPG investigated under proton irradiation. For the analysis, they selected the photosensitive single mode B/Ge codoped fiber PS1250/1500 manufactured by Fibercore UK.…”

Recent Advances in Optical Fiber Enabled Radiation Sensors

“…In 2018, a temperature-compensated radiation sensor based on two LPG fiber sensors was developed by Stancălie et al [79]. In 2020, Berruti et al achieved an uncoated LPG radiation sensor based on a commercial B-Ge codoped optical fiber for multi-parametric sensing in high radiation environments [80]. The sensitivity of the LPG device to an external factor can be expressed as…”

Numerical Analysis of Radiation Effects on Fiber Optic Sensors

“…It is well known that radiation significantly affects the basic characteristics of FBG sensors such as peak wavelength and spectral width, and the amplitude and magnitude of these changes largely depend on the grating type and fabrication technique [8]. Similar to FBG, radiation also changes the structural parameters of LPG; again, it depends on the inscription technique, grating type, and composition of fibers [13,28,35]. It has been reported that fs-etched compared to conventional UV irradiated gratings can survive for a long time in radiation environment without significant degradation of gratings parameters [8][9][10].…”