Abstract

We report on the use of second-order Raman amplification to assist a phase-sensitive optical time domain reflectometer (phi OTDR) used for vibration measurements over very long distances. The sensor was able to measure vibrations of up to 380 Hz (limit set by the time of flight of light pulses) in a distance of 125 km with a resolution of 10 m and no post-processing. Balanced detection is used to reduce the relative intensity noise. A comparison with a sensor using first-order Raman amplification under similar conditions is presented and a clear improvement of performance is demonstrated.