Measurement of long term, radiation induced losses in fibre optics using optical time domain reflectometry

Fiber optics exposed to ionizing radiation exhibit enhanced transmission loss. Recovery processes can take place over a wide time scale and, for extended exposures, as in space, will limit the observed loss. Prediction of the survivability of a fiber optic link in such an environment requires knowledge of the form of the recovery over a time comparable with the mission duration. The use of an optical time domain reflectometer (OTDR) to make the appropriate measurements with the necessary accuracy and sensitivity is discussed, and results are presented for recovery curves, extending to time >1 year, for a number of fiber types. The OTDR technique has been shown to be a useful means of determining radiation-induced losses at times greater than 100 s after irradiation or during extended irradiations of greater duration. Results, compared to those obtained by continuous measurement or by cut back, are precise enough and may show important features not apparent using the other methods. Measurements have shown recovery in transmission to be still occurring in some fibers at 10⁸ s after irradiation, possibly by mechanisms not commencing until 10⁶ s after. Such observations could dramatically change estimations of the survivability of a fiber