Ultra Long Span φOTDR Sensing System based on ROPA Technology

【Objective】For the problem of limited sensing distance of distributed optical fiber sensing system, a Phase-Sensitive Optical Time Domain Reflectometry (φOTDR)distributed optical fiber sensing system based on Remote Optically Pumped Amplifier (ROPA) technology was proposed.【Methods】Adopting polarization diversity reception technology to obtain orthogonal polarization signals to solve the problem of polarization mismatch, Frequency division band processing suppresses coherent fading and polarization fading. Applying high-order Raman pumping technology and cascaded ROPA technology, three remote gain units are selected with appropriate gain medium lengths. Introduced at the appropriate positions in the transmission link, the pump light and signal light are coupled and sent by the remote pumping unit to excite erbium ionst for unrepeatered amplification, realizing ultra long single span vibration signal sensing and solving the problem of insufficient single-span sensing distance of existing φOTDR.【Results】The experimental results show that In-phase and Quadrate (IQ) demodulation algorithm can be used to achieve linear recovery of Piezoelectric Transducer (PZT) disturbed 100 Hz signal.【Conclusion】The results show that this system can achieve vibration signal sensing of 176.6 km ultra long single span, breaking the existing φOTDR single span sensing distance recording. The vibration curve of the first 176 km is stable and the amplitude is below 0.4 rad. The undisturbed position in the three-Dimensional (3D) map is relatively stable, and the disturbance signal added by PZT at the end of the link can be accurately perceived. After Fast Fourier Transform (FFT) transformation processing, the signal-to-noise ratio of the 100 Hz disturbance signal is 8.9 dB. The experimental results are crucial for building ultra long single-span φOTDR sensing system and provides a certain reference value for the development of unrepeatered ultra long-distance sensing in OTDR systems.