Phase-sensitive optical time-domain reflectometric system based on a single-source dual heterodyne detection scheme.

A phase-sensitive optical time-domain reflectometric (ϕ-OTDR) system based on a novel single-source dual heterodyne detection scheme is proposed and demonstrated. It uses the optical beat-frequency signals as the local oscillator signal containing the modulated frequency, frequency drift and phase fluctuation, while the signal to be detected contains all the forgoing spectral components, in addition to the vibration signal under measurement. Frequency mixing serves to isolate the pure vibration signal from the omnipresent residual frequency and phase fluctuations caused by a less strictly synchronous clock, inherent characteristics of the laser and the acousto-optical modulator, and environment temperature changes. With a reduced burden on data processing, better real-time performance is achieved as well. Using probe light pulses of 4 kHz repetition rate and 80 ns pulse width, a 9 m spatial resolution has been achieved on a 24.6 km sensing fiber, with a detectable frequency range from 5 Hz to 1.715 kHz, with a signal-to-noise ratio greater than 23.5 dB. All the above parameters are close to the maximum theoretical values. The drastically improved system demodulation characteristics foreshadow better performance and improved reliability in engineering applications.