Distance scale calibration of optical timedomain reflectometers with the recirculating delay line method

An optical pulse typically 10–10 000 ns long is launched into a test fiber via a directional coupler. The backscatter radiation from the fiber is detected and processed and the backscatter signature displayed. Since the backscatter power is directly related to the fiber loss and its arrival time back at the OTDR to the distance traveled, it is for these two parameters that the OTDR needs primarily to be calibrated. Several methods for carrying this out are described in the IEC 61 746 document. In this paper, we present measurements made with the recirculating delay line, which is a passive method among the distance scale calibrating ones.