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This explanation has been confirmed by the following experiment. We performed thermal tests (heating from room temperature to 100°C) on fiber samples similar to those discussed in this study but having never been irradiated. These samples were wound on aluminium spools of 54 mm diameter. We measured significant temperature-induced transmission losses in the samples having a polymide or aluminium coating and a thin cladding (1. 1. or 1. 2 cladding to core diameter ratio respectively). We observed negligible temperatureinduced losses in fiber samples having a thick cladding (1. 4 cladding to core diameter ratio). The losses increased with increasing wavelength and were very high in the infrared. The amount of losses depended also on the coating material. We attribute this phenomenon (also observed in the present study) to the dependence of microbending losses on temperature. The small diameter of the fiber coils greatly enhances this phenomenon.
