This paper describes excess loss of single-mode jacketed optical fibers at low temperature. A structural model is proposed for the jacketed fiber in order to investigate the relationship between the structure and excess loss. This model clarifies the excess loss increase mechanism that any initial irregularity existing in the jacketed fiber increases due to shrinkage of the jacket and causes excess loss. These results show that reducing the buffer diameter and controlling the fiber deformation inside the jacket are effective in suppressing excess loss at low temperature.

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Coated single-mode fibers with various coating thicknesses were manufactured, and loss increases due to cooling were measured. It was found that the excess loss increases steeply in the small V-value region at low temperatures. A simple model for fiber bends in the coating was introduced, and the increases in loss were related to the strain resulting from use of the model. It would appear that the axial compressive strain has a serious influence on the loss increases. A suitable silicone buffer layer for preventing loss increases has been determined. The selected silicone layer diameter is 0.45 mm for a 0.9-mm nylon outer layer diam.

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