Passive optical fibers for nuclear instrumentation
RADIATION-HARDENED FIBER AT HIGH IONIZING DOSES
Many high energy physics facilities, the nuclear or dismantling industries require single-mode or multi-mode optical fibers operating in the infrared domain, most often at Telecom wavelengths: 1310 nm and 1550 nm. These optical fibers are used for data transfer or serve as sensitive elements to distributed sensors of temperature, strain, liquid levels... Fig.1 lists the harsh environments of interest for optical fibers and fiber sensors , characterized by very different doses, dose rates and temperatures.
At the macroscopic scale, the irradiation affects the optical fiber properties via three main mechanisms . The first is radiation- induced attenuation (RIA), which results in a large increase in the fiber transmission losses. The second is the radiation-induced light emission (RIE) which corresponds to a parasitic signal superimposed on the one of interest. The third is the change in the refractive index of glass, particularly observed at high neutron fluences.
For high gamma irradiation doses (> 100 kGy(SiO2) and dose rates below 1 kGy/h, RIA remains the limiting factor for fiber-based nuclear instrumentation. The level and kinetics of RIA depend on many parameters relating to the optical fibers, their profile of use and considered harsh environments .
Fig.1. Various radiation environments of interest for optical fibers and optical fiber sensors.
Ad hoc optical fibers have been developed for different actors of the nuclear field
Radiation-hard single-mode optical fibers with acrylate or polyimide coatings, have been developed for ANDRA for use as temperature and strain sensors in the severe environment associated with radioactive waste disposal: doses up to 10 MGy, T <100 °C, presence of hydrogen.
Other radiation hardened optical fibers have been elaborated for AREVA (now ORANO) to be integrated as water level sensors into nuclear pools (doses up to 10 MGy, T <100 °C) or to their functionalization in the form of resistant fiber Bragg grating sensors.
CURRENT PERFORMANCES OF HARDENED FIBERS
Fibers with pure silica and fluorine doped cores exhibit the best performance in terms of radiation resistance. Fig.2 compares the measured RIA levels for different optical fiber compositions at 1550 nm versus the deposited dose at 25 °C . Typical losses for this category of optical fibers are around ~40 dB/km at 1550 nm. New optical fibers, developed via the iXblue SPCVD process, show improved performance with an RIA of less than 10 dB/km between 1310 nm and 1625 nm . At these high doses, the mechanical strength of optical fibers must also be studied, particularly for environments combining radiative and thermal stresses.
LabH6 members studied the degradation of different types of optical fiber coatings (acrylate, carbon, polyimide, aluminum) in an environment combining irradiation and temperature [4,5]. This study has established that some of these coatings allow to withstand doses exceeding MGy (SiO2) up to temperatures of the order of 250 ° C, without detrimental degradation of their post-irradiation mechanical strength/response measured by the bench shown in Fig.3.
 S. Girard et al., J. Optics, 20 093001, 2018
 S. Girard et al., TNS, 60 (3) 2015, 2013
 S. Girard et al., Reviews in Physics (4) 100032, 2019.
 G. Mélin et al., RADECS 2019, submitted
 G. Mélin et al., IEEE TNS, Early Access, 2018
Optical fibers with RIA <10 dB/km between 1310 and 1625 nm after a dose of 1 MGy are available:
Fig.2. RIA at 1550 nm according to the dose for different fiber optic compositions .
Fig.3. Illustration of the test bench for the mechanical strength of optical fibers before/after irradiation.