Overview of radiation induced point defects in silica-based optical fibers

Silica-based optical fibers, fiber-based devices and optical fiber sensors are today integrated in a variety of harsh environments associated with radiation constraints. Under irradiation, the macroscopic properties of the optical fibers are modified through three main basic mechanisms: the radiation induced attenuation, the radiation induced emission and the radiation induced refractive index change. Depending on the fiber profile of use, these phenomena differently contribute to the degradation of the fiber performances and then have to be either mitigated for radiation tolerant systems or exploited to design radiation detectors and dosimeters. Considering the strong impact of radiation on key applications such as data transfer or sensing in space, fusion and fission-related facilities or high energy physics facilities, since 1970′s numerous experimental and theoretical studies have been conducted to identify the microscopic origins of these changes. The observed degradation can be explained through the generation by ionization or displacement damages of point defects in the differently doped amorphous glass (SiO₂) of the fiber's core and cladding layers. Indeed, the fiber chemical composition (dopants/concentrations) and elaboration processes play an important role. Consequently, identifying the nature, the properties and the generation and bleaching mechanisms of these point defects is mandatory in order to imagine ways to control the fiber radiation behaviors. In this review paper, the responses of the main classes of silica-based optical fibers are presented: radiation tolerant pure-silica core or fluorine doped optical fibers, germanosilicate optical fibers and radiation sensitive phosphosilicate and aluminosilicate optical fibers. Our current knowledge about the nature and optical properties of the point defects related to silica and these main dopants is presented. The efficiency of the known defects to reproduce the transient and steady state radiation induced attenuation between 300 nm and 2 µm wavelength range is discussed. The main parameters, related to the fibers themselves or extrinsic - harsh environments, profile of use - affecting the concentration, growth and decay kinetics of those defects are also reviewed. Finally, the main remaining challenges are discussed, including the increasing needs for accurate and multi-physics modeling tools.