Measurement facility for the evaluation of the backscattering in fiber: Realization of an OTDR operating at single photon level

This paper presents the INRIM e®ort related to the implementation of an optical time domain re°ectometer (OTDR) that exploits backscattered light to test an optical ¯ber operating at single photon level, using two di®erent avalanche photodetectors: a free-running Id220 and a gated Id201. Our tests show that the use of the Id220-based OTDR could be very helpful for QKD application, since it allows testing much longer portions of an optical link. This represents a great advantage for QKD systems, where single components and connections are often embedded in a non-separable optical system.