Enhancement of dynamic strain range based on phase-sensitive OTDR with time-delay double probe pulses

Phase-sensitive optical time domain reflectometry (φ-OTDR) can be used to recover external vibration information through phase demodulation. However, with the restriction of the unwrapping algorithm, traditional φ-OTDR can not demodulate signals correctly when the external dynamic strain is large. To solve this problem, an φ-OTDR system with time-delay double probe pulses scheme is proposed and demonstrated in this paper. In the scheme, time-delay double probe pulses with different frequency are employed, the phase signals with large dynamic strain can be effectively restored by subtracting the two phase signals of Rayleigh backscattered (RBS) signals arise from the time-delay double probe pulses. In our scheme, the digital correction method is also used to eliminate the influence of disturbance frequency on the demodulation results. The simulation and experimental results show that the system can measure vibration amplitude of 15.55 με, the vibration frequency of 800 Hz, the spatial resolution of 5 m. And then the strain range of 7.55 με to 15.55 με disturbance signals are successfully recovered on a 2 km single-mode fiber. The maximum measured dynamic strain is 58 times that of conventional φ-OTDR, and which is limited to the amplitude of the existing piezoelectric transducer (PZT) in our laboratory. Moreover, the sensor can work while vibration frequency ranges from 200 Hz to 1.4 kHz under large stain in experiments and the phase difference is linear with the value of time-delay. The proposed method will extend the scope of application fields of φ-OTDR, in which requires large dynamic strain recognition.