Abstract

At the 1979 IEEE Workshop on Fiber Optic Measurements, we expressed doubt that OTDR or ”backscatter" would ever have sufficient range to penetrate a full repeater section because of launch power limitations and the necessary dynamic range of the apparatus. Analogue signal recovery techniques appeared then to limit the range to about 25 dB of one-way fibre loss. A law of rapidly diminishing returns applies to any straightforward method of increasing the sensitivity, e.g., averaging a larger number of samples. However, we can now report a substantial improvement in the range of OTDR achieved by using a photon counting technique. We have succeeded in locating non-reflecting fibre breaks through one-way attenuations exceeding 40 dB and we believe there is scope for further progress. When there is a reflection associated with the break, the range already exceeds 50 dB which is sufficient for many fibre systems.