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光纤被覆层的重要作用是避免施加于光纤的侧向力所引起的微弯损耗。高模量被覆则能减弱光纤对微弯的敏感性。为了在宽温度范围内保护光纤,一次被覆的模量应该低;二次被覆的模量应该高,而且均要与温度无关。但即使选用最合适的有机被覆料,也是引起光损耗的一个新的根源。由于二氧化硅和有机被覆料的线性热膨胀系数约相差两个数量级,因此温度的变化会引起轴向应力。所以冷却会使柔软的一次被覆光纤出现弯曲,进而增加传输损耗。选用玻璃化转变温度>80℃的高模量二次被覆料更会出现这种结果。对于这类被覆料,一次被覆和二次被覆从固化温度冷却期间的径向收缩差颇为重要。本文还讨论了一次被覆厚度对光纤的影响。
An important role of the optical fiber coating is to prevent microbending losses caused by lateral forces applied to the optical fiber. High modulus cladding reduces the susceptibility of the fiber to bends. To protect the fiber over a wide temperature range, the modulus of primary coating should be low; the modulus of secondary coating should be high and temperature independent. But even choosing the most suitable organic coating material is a new source of light loss. Since the coefficients of linear thermal expansion of silica and the organic coating are about two orders of magnitude different, temperature changes cause axial stresses. So the cooling will make a soft coated fiber bending, thereby increasing the transmission loss. Use of glass transition temperature> 80 ℃ high modulus secondary coating will appear even more this result. For this type of coating material, the difference in radial shrinkage between the primary coating and the secondary coating from the curing temperature during cooling is of importance. This article also discusses the impact of a coating thickness on the optical fiber.