Comprehensive approach to determination of space proton-induced displacement defects in silica optical fiber

Abstract

Silica-based optical fibers presents a variety of applications used in radiation environments such as space, fusion facilities, accelerators and nuclear power plants. The radiation-induced displacement damage in optical fibers resulting in point defects may lead to attenuation signals that is a major concern for these applications. The present study proposes a computational approach to the calculation of the proton-induced displacement damage in vitreous silica. Therefore, Geant4 as a Monte Carlo particle transport code has been used to obtain the knock-on atom distributions caused by the interaction of space trapped proton with vitreous silica during an ISS mission. Moreover, molecular dynamics simulations using ReaxFF potential have been performed to produce the initial vitreous silica structure to evaluate the displacement damage cascades by LAMMPS package. The results show that ReaxFF has an appropriate potential to produce and evaluate the vitreous silica structure that provides better agreement with experimental data at both short-range and medium-range order. Furthermore, ODC(Si3) and NBOHC(O1) are dominant defect species created in the vitreous silica after trapped proton irradiation, where the total number of defects have increased on average by 94 for each keV increasing in PKA energy approximately.

Introduction

Optical fibers provide much superiority over their metallic counterpart, copper cable. The reasons for the superiority lies in the fact that optical fibers are much lighter, immune to electromagnetic interference, higher data bandwidth capacity, noise immunity, low cost, and can transfer large amounts of information [1]. Hence, optical fibers are widely used in innovative technological domains such as telecommunications, sensors for structural health monitoring, diagnostics, and even as radiation dosimeter. Also, they are heavily used in onboard photonic devices and systems in radiation environments as those in space, high energy physics facilities, nuclear power plants [2], [3]. However, the space application of optical fibers is a major concern. Space radiations include trapped particles by the earth’s magnetic field (proton and electron), solar particles, and galactic cosmic rays [2].

One of the most important effects of space radiation on the optical fibers is displacement damage. By radiation of proton, neutron and ion, the interaction of coming particles with lattice atoms transfers energy higher or equal to threshold displacement energy leading to production of high energy recoils of particles known as primary knock-on atoms or PKAs [4], [5]. The collision between PKAs and other atoms causes additional displacements of other atoms which creating non-equilibrium point defects. The presence of defects in a crystalline or amorphous matrix may drastically modify the electrical and optical properties of the host material. In fact, these defects exist in different localized electronic states that can cause optical activities as absorption and luminescence with lower energies than the fundamental absorption edge of the material [6]. Indeed, the displacement damage in the optical fibers creates point defects acting as a color centers at the microscopic scale. Three main basic mechanisms of modifying the macroscopic properties of optical fibers by these point defects are: radiation-induced attenuation, radiation-induced emission, and the radiation-induced refractive index change [7].

Despite the wide experimental studies on radiation-induced defects in the microscopic structure of optical fibers, there is a persuasive need to an accurate modelling method for describing the structural changes, production and evolution of defects during irradiation [8].

Molecular dynamics (MD) simulation is a computer simulation method for analyzing the atomic movements on the microscopic scale used for describing solid networks and defects produced by irradiation in metals and semiconductors [9], [10]. As is well known, the fundamental element in MD simulation is based the definition of a potential energy function or a description of the terms by which the particles in the simulation will interact for the accuracy of the simulated properties. Several potential functions have been developed for different silica polymorphs and vitreous silica and summarized by Schaible [11] and Erikson and Hostetler [12]. The radiation effects on structural properties of vitreous silica by the Feuston and Garofalini (FG) potential have been widely investigated [13], [14]. The FG potential combines a weak three-body interaction with a modified Born-Mayer-Huggins (BMH) ionic two-body interaction. In this potential, a cut-off radius of 5.5 Ã… is applied, which makes it proper to simulating short-range and medium-range scales of vitreous silica features but not necessarily for long-range crystalline ones [7]. Munetoh proposed a new parameter set of Tersoff potential by neglecting the Coulomb interaction terms to the large-scale simulation of Si-O systems [15]. Furthermore, Chowdhury compared the results of MD simulation of the mechanical properties of vitreous silica with experimental values [16]. He observed that different potentials have limitations for predicting the mechanical properties of silica which involves bond/angle deformation/ breakage.

The Reactive force field potential (ReaxFF) based on a bond order will simulate chemical reaction and overcome the limitation of conventional traditional force fields [17]. To bridge the gap between quantum mechanics (QM) and classical methods, the parameters of this potential have been obtained/optimized based on accurate quantum mechanics calculations and experimental data [18]. ReaxFF has been employed widely to study crystalline solids, hydration of systems [19], mechanical and structural properties of various materials [20], glassy silica–water interface [21] and irradiation-induced topological transition in quartz [22]. However, ReaxFF applicability to vitreous materials and radiation damage on them is not yet widely understood.

The present study introduces a comprehensive approach to the quantification of primary microscopic damage on a silica optical fiber by proton irradiation during a 127-day mission at International Space Station (ISS) orbit. For this purpose, we have used ESA’s Space Environment Information System (SPENVIS) and Geant4 to calculate the proton energy spectra and the PKA distribution. Furthermore, a MD simulation by ReaxFF potential has been used to examine the initial silica vitreous structure and to study the PKA induced displacement damage cascades using the LAMMPS package.