Radiation-resistant characteristics of optical fiber irradiated with gamma rays at low dose rates

Production of GI type fibers, which are now widely used, becomes easy if P is added to them. It is known, however, that P-added fibers suffer a loss in proportion to time when irradiated with gamma rays. This study is aimed at investigating the increase in loss in fibers which is expected to take place under the actual service conditions, where they tend to be exposed to gamma rays at a low dose rate. The sample used is SiO2-GeO2-P2O5 fiber of GI type. Effects of dose rate and irradiation time on induced loss are observed and results obtained are discussed. It is shown that induced loss does not depend on dose rate and that it increases with increasing irradiation time. Accordingly, the maximum permissible total dose would be about 40R and 20R for light of 1.3 μm and 0.85 μm wavelength, respectively, irrespective of the dose rate, if a margine of 0.1 dB/km is taken in the design for the system. It is also observed that the increase in loss is depressed when light of 1.3 μm is used, though the depressed loss is regained when the fiber is left to stand after the irradiation. A model is proposed to explain this phenomenon. (Nogami, K.)