A Study for Estimating Thermal Strain and Thermal Stress in Optical Fiber Coatings

Optical fibers are drawn with resin coatings that protect the glass. In general UV-curable resins are used for the coatings, and in most single-mode (SM) fibers a dualcoating structure is adopted. Each coating layer has an independent function in the protection of the glass. The coating in direct contact with the glass is known as the primary (P) coating, with the role of acting as a buffer against the external forces that can cause microbending. Accordingly the primary coating is designed to be soft, with a Young’s modulus of 1 MPa or less, and in consideration of the conditions of use, the glass transition temperature Tg is lower than room temperature. The primary coating is then covered by a hard secondary (S) coating for protection against external forces, and generally speaking a material having a Young’s modulus of 500 MPa or more and a Tg of 60°C or above is used. During the drawing process the optical fiber is heated by an exothermic reaction as the UV-curable coating cures and by heat irradiated by the UV lamp, so that the temperature of the coatings rises above the Tg of the secondary coating. After UV lamp irradiation, the fiber is selfcooled to room temperature and then wound on a spool 1)~4). During the cooling process following curing, the secondary coating, since its Tg is higher than room temperature, transforms from the rubbery state to the glassy state. The primary coating, for its part, remains in the rubbery state as it cools to room temperature. The CTE of a polymer changes significantly at its Tg 1). This means that at temperatures below the Tg of the secondary coating. The coefficient of thermal shrinkage of the primary coating is greater than that of the secondary coating. This CTE mismatch gives rise to strain at the primary/secondary interface, resulting in a force acting to delaminate the primary coating from the glass. Figure 1 is a schematic of CTE mismatch between the primary and secondary coatings, where β is cubic CTE, its subscript indicates the type of coating, and ΔT is the difference between the Tg of the secondary coating and room temperature. Knowing the thermal strain of optical fiber coatings would be of great importance in terms of fiber reliability. The purpose of this study is to measure the CTEs of each of the coatings from the coatings of actual optical fibers, and further to determine the actual thermal strain produced in the coatings and further, the residual stress. Accordingly, for the measurement of CTE we used a thermo-mechanical analyzer (TMA) and to estimate residual A Study for Estimating Thermal Strain and Thermal Stress in Optical Fiber Coatings