Research On The Polarization-relevance Noise And Multipoint High Frequency Detection Of Φ-OTDR

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) utilizes the Rayleigh backscattering in the fiber to sense the disturbance along the sensing fiber. Compare to Brillouin backscattering and Raman backscattering, Rayleigh backscattering has more higher power, so Φ-OTDR just needs few decades average. In essential, Φ-OTDR realize the fully distributed sensing by the Rayleigh coherent fading. In the fiber, the phase of probe is key influence factor to Rayleigh coherent fading. The disturbance can be detected by the Φ-OTDR as long as the fiber length is changed in the level of wavelength, which indicate the high sensitivity of Φ-OTDR. Nowadays, Φ-OTDR mainly measures the dynamic disturbance, obtaining the location and frequency of the dynamic disturbance. But, following the in-depth application, two disadvantages of Φ-OTDR have come up.In general,Φ-OTDR is considered only sensitive to the phase change. In application, Φ-OTDR is also sensitive to the polarization. In the classic model of 1-D pulse response, the polarization in the probe pulse is considered to be consistent. Because the birefringence exist in the fiber, the assumption in the classic model is incorrect. While external disturbance transforms the local birefringence characteristic of fiber by squeezing, twisting or bending, the polarization sate of probe pulse will be modulated during going through this disturbance section. And the following Rayleigh backscattering will be superposed by the polarization-relevance noise, which can deteriorate the ability of the fully distributed sensing of Φ-OTDR. The final result of Rayleigh power detection will reflect this effect of PRN. This paper improves the classic Φ-OTDR model by introducing the polarization-relevance item, reveals the generation mechanism of the polarization-dependence of Φ-OTDR. To suppress the polarization-dependence, we put forward a detection method based on orthogonal polarization sate of pulse pair (OSPP). We set up the whole theory model of OSPP, and the validity also is demonstrated in the theory analysis. In this paper, the system of Φ-OTDR based on OSPP is designed. The theory analysis and the result of experiment both desmontrate the validity of the OSPP scheme. In the experiment, the fiber length is 1.5 km, the 3Hz bend disturbance is arranged at 360m, and the 5Hz stretch disturbance is arranged at 1380m. We find that the PRN was superposed in the following Rayleigh signal of the bend disturbance, and the fiber stretch cannot be distinguished. The experiment result have shown the suppression ratio 9.2 dB can be obtained and distinguished the fiber stretch disturbance.The length of sensing fiber limits the detectable maximum frequency of Φ-OTDR. To overcome this limit while realizing multipoint high frequency disturbance detection, this paper also proposes a method based on long-short time division multiplexing probe pulses. To demonstrate this method, we set up the Φ-OTDR model of long pulse, which includes the influence factors of polarization, loss during probe pulse and the frequency drift. This model reveal the relation bwteen the scattering point number participated in the Rayleigh superposition and the probe propagation time in the fiber. This model also improves the classic Φ-OTDR theory. In the meantime, we also analyze the process and result of the phase of Rayleigh backscattering being modulated by multipoint disturbance, which may be meaningful to the Φ-OTDR research in the future. To demonstrate the validity of this method, we design experiments demonstration. The fiber length is 7.5km, and two disturbance with the frequency 100kHz and 65kHz is arranged at the location of 500m and 5.5km respectively. The Experiment results have shown this method succeed in demodulating the high frequency of two disturbance and their corresponding location.This dissertation research the PRN and multipoint high frequency disturbance detection to overcome the two disadvantages in Φ-OTDR. The OSPP scheme and long-short time division multiplexing probe pulses have been proposed. The theory analysis and experiment result both demonstrate the validity of them. The two schemes not only expand the application of Φ-OTDR but also improve Φ-OTDR theory model.