Three-dimensional numerical modeling of residual stresses in metal-coated optical fibers

A three-dimensional numerical analysis was carried out to study the stress-strain state of a single-mode optical fiber with a copper protective coating during cooling from the drawing temperature to room temperature. The problem was solved within a quasi-stationary thermoelastic formulation without axial symmetry, where the initial temperature of each structural layer was defined by its softening or melting point. In the calculations, the softening temperature of each layer was taken according to the impurity content in the quartz glass. The simulations were performed using COMSOL Multiphysics v.6.2. Residual stress and strain fields were obtained, as well as the longitudinal and transverse strain components in the fiber core. Additional simulations for fibers with silica cladding diameters of 125 μm and 200 μm without the metal coating to analyze the cooling process of the silica part of the fiber from its softening temperature to ambient conditions. This approach makes it possible to isolate the contribution of the metallic coating to the overall deformation, since variations in the cladding diameter can affect the stress distribution in the core even before the copper layer is applied. It was found that increasing the cladding diameter from 125 μm to 200 μm, while keeping the copper thickness constant, decreases the transverse strain in the core by about 36 %. The proposed thermoelastic model is relatively simple but physically consistent and requires further experimental verification. With refinement, it may serve as a basis for the design and analysis of metal-coated optical fibers intended for operation in harsh environments.