Thermal buckling of dual-coated fiber

An analysis of buckling is presented for dual-coated fibers within their coating at low temperatures. Buckling causes microbending of the fiber axis, the prime source of added optical loss in the fiber. Buckling is caused by compressive stress exerted on the fiber by the coating, which arises because the thermal expansion coefficient of the coating is substantially larger than that of the fiber. Calculations show that buckling is more likely when the inner primary coating layer is soft and thick. Previous experimental results on fibers in cables indicate that softer, thicker primaries lead to more added loss at low temperatures, contrary to the simple model where lateral pressure imprints irregularities onto the path of the fiber. Such is the evidence that buckling occurs. The theory is applied to various coating designs. The calculated results rank the low-temperature performance reliably, but they also indicate that thermal strain falls short of the buckling strain. Other sources of strain are suggested, including a mechanism whereby irregularities induced by lateral pressure on the outside produce bending moments on the fiber.