TESLA-Report No. 2000-25 August 2000

INTRODUCTION Optical fibers are used in a wide field of applications, for instance in communication networks. Their degradation by ionizing radiation is often an unwanted effect. The main radiation effect is a wavelength-dependent increase of attenuation by absorption and scattering processes. Despite the fact that the different types of modern telecommunication fibers show strongly increased radiation hardness (1), compared with the early ones, there still exist some single-mode (SM) and multi-mode graded index (MM GI) fibers with high and only slowly annealing loss increase. Such fibers can be used for in situ local radiation dosimetry also at particle accelerators, to observe the emission of radiation along the beam line where radiation-sensitive equipment might have to be installed (2, 3). The attenuation of the optical fiber can be measured with an Optical Time Domain Reflectometer OTDR (figure1). A fraction of the incoming laser pulse will be reflected along the fiber. The scat