Enhanced coherent OTDR for long span optical transmission lines containing optical fiber amplifiers

We have newly constructed an enhanced coherent optical time domain reflectometer (C-OTDR) for use in testing optical cable spans in transmission lines containing erbium-doped fiber amplifiers (EDFA´s), which is based on heterodyne detection using acousto-optic (AO) switches. In order to avoid any optical surges in the EDFA´s in the transmission lines, optical dummy pulses were added between the signal pulses by an AO switch to keep the probe power from the C-OTDR as uniform as possible. We achieved a large single-way dynamic range of 42 dB with 5 dBm less probe power. The measurable portion of the fiber spans was more than 80 km in optical transmission lines containing EDFA´s. This is twice the previously reported value.<>