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.<
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