Research On The Phase Noise And Noise Suppression Technology In Dual-pulse φ-OTDR System Based On High-frequency Heterodyne Detection

The phase-sensitive optical time domain reflectometry(φ-OTDR)technology based on the coherent Rayleigh scattering is widely used in various fields including perimeter security and resource exploration.With the development of the phase-detection based φ-OTDR technique,φ-OTDR is developing towards the direction of remote sensing with high sensitivity.The phase noise is a bottleneck factor that restricts the system performance.This paper studies the phase noise in the φ-OTDR system,which aims at realizing the remote φ-OTDR system with low noise and high sensitivity.This paper presents and implements a dual-pulse φ-OTDR technique based on highfrequency heterodyne detection.The paper studies the characteristics of the phase noise and signal detection performance of the technology.The phase noise of the system in the400 m sensing fiber is about-71.04 d B.The study finds out the pulse repetition rate has an important impact on the phase noise of the system,which can be attributed to the noise aliasing effect.The results holds a limitation on the phase noise of the remote φ-OTDR system.Based on the full study of the phase noise of the system,the paper testifies the distributed sensing performance of the proposed technology.On the basis of the detailed study of the dual-pulse short-range φ-OTDR based on high-frequency heterodyne detection,the paper studies in detail on the modulation instability(MI)and its suppression technology of the system.Firstly,the paper studies the influence of MI on the output spectrum and Rayleigh scattering signal;Then,the effect of MI on the phase noise of the system is studied,and the results show that the phase noise of the front end of the sensing fiber is not significantly affected by the occurrence of MI,but MI can cause a sharp increase in the noise of the back end of the fiber.This paper proposes a method to suppress MI and MI-induced phase noise in the dual-pulse remote φ-OTDR system.The experimental results verify the effectiveness of the method.Furthermore,the paper establishes a dual-pulse remote φ-OTDR system based on high-frequency heterodyne detection with Raman amplification.The paper studies the effects of forward and backward Raman amplification respectively;The integrated use of Raman amplification and coherent seed injection enhances the signal-to-noise ratio and suppresses the nonlinear effects of the system successfully.The results show that the dualpulse remote φ-OTDR system based on high-frequency heterodyne detection with Raman amplification has excellent distributed sensing performance.The main innovations of the paper are as follows:1.The paper proposes a new φ-OTDR technology based on high-frequency heterodyne detection.Compared to the traditional low-frequency detection technology,the proposed technology has lower phase noise and wider dynamic range.2.The paper proposes a method based on coherent seeds injection to suppress spontaneous MI and MI-induced noise in the dual-pulse remote φ-OTDR system successfully.