In-situ gamma radiation induced attenuation in silica optical fibers heated up to 600 °C

Abstract

In this work, the performance of silica fibers was experimentally studied under continuous gamma irradiation and periodic heating in steps to temperatures up to 600°C. The broadband optical attenuation in 10-meter lengths of three different types (one high-OH, two low-OH types) of multimode silica fibers was continuously monitored in-situ throughout the experiment. All three fiber types survived the experiment both optically and mechanically. Results showed that for the gamma dose rate tested in this experiment (476Gy per hour), gamma radiation primarily affects wavelengths below ~1000nm. For the low-OH silica optical fibers, gamma radiation has little effect on transmission in the infrared, particularly at 1300nm and 1550nm. These two wavelengths are commonly used in existing commercial optical sensing equipment. Thermal effects primarily contributed to the increase in absorption in the infrared region for low-OH silica optical fibers. The large OH concentration in the high-OH fiber results in large intrinsic absorption in the near-infrared, making the high-OH fiber unsuitable for transmission at 1300nm and 1550nm. However, the high-OH fiber had lower radiation-induced attenuation in the UV and visible ranges including 850nm, which is another wavelength that is commonly used for sensing applications. The steady state value for the gamma radiation-induced attenuation in the high-OH fiber at 600nm (the peak wavelength for the absorption due to non-bridging oxygen hole centers) and the time constant for reaching this steady state value both decreased as temperature was increased in the range of 100–400°C, with almost full annihilation of the radiation-induced defects achieved at 400°C. Time constants were on the order of minutes to hours for the gamma dose rate in this experiment and the high-OH fiber that was tested. In general, radiation-induced attenuation was observed to decrease significantly with increasing temperature, particularly at lower wavelengths where most of the defect centers are located. The conditions under which the measurements were made (in-situ as the temperature was varied) also gives insight into the dynamics of the optical attenuation, which may help provide a better understanding of the fundamental physics of radiation-induced optical attenuation. All of the results generated in this work were processed into movies that are available for download.