Phase Demodulation ?-OTDR Dual-channel Detection Method And Mechanism Research

With the rapid development of Industrial Internet of Things,a new type of distributed fiber optic acoustic sensor based on phase-sensitive optical time-domain reflectometer has been widely studied.Fiber-optic distributed acoustic sensors not only offer the advantages of ordinary optical fiber sensors,but also provide the advantages of being lightweight with a wide monitoring range,excellent concealment,good flexibility,high sensitivity,and accurate positioning.Given these unique characteristics,optical fiber distributed sensors have been widely applied in many fields,including perimeter security,temperature detection,seismic applications,intrusion detection,railway safety monitoring,and other important areas.Compared to point sensors,DAS play an irreplaceable role.To date,different demodulation methods of?-OTDR-based systems such as direct detection demodulation,digital coherent demodulation,and interferometer demodulation technology have been proposed to reconstruct the dynamic-disturbance information.Interferometer demodulation structures are a common demodulation method because of their high resolution and flat frequency response.In the interferometer-based demodulation method,a 3×3 coupler and an unbalanced Michelson interferometer are introduced into the?-OTDR phase demodulation structure.This structure eliminates the need to introduce an external carrier for modulation compared to the phase-generating carrier demodulation method,thus simplifying the demodulation complexity of the sensing system and at the same time reducing the sampling rate requirement as well as increasing the dynamic measurement range of the sensing system.The main purpose of this paper is to provide an in-depth study of the mechanism of a two-way phase demodulation method based on a 3×3 coupler combined with an unbalanced Michelson interferometer structure.Unlike previous?-OTDR demodulation systems based on 3×3 coupler,the?-OTDR demodulation system proposed in this paper only requires two photo-detectors and a dual-channel acquisition device for photoelectric conversion and data acquisition,reducing system hardware costs and facilitating signal processing.In the signal-processing section,we use the improved two-path differential cross-multiplication algorithm to minimize impact of incomplete symmetry of the 3×3 coupler.Theoretical calculations,simulation and experimental results show that the proposed algorithm improves the stability and anti-noise performance of the system signal demodulation result,when the 3×3 coupler is not ideally symmetrical.Based on the designed phase demodulation system,the experimental equipment was then built on an optical platform.Multiple vibration events were simulated by placing multiple piezoelectric ceramics on 2 km sensing fiber,the experimental results show that our proposed system is capable of detecting and recovering actual waveforms of multiple vibration events occurring simultaneously at different locations.The background noise level of our proposed?-OTDR system is about-30.95dB(8.98×10^-4rad/?Hz),and the signal-to-noise ratio(SNR)is about 36.24 dB at 1kHz.And the proposed system is able to reconstructing the disturbance with a linear amplitude response of R²=0.9983.And our system achieves the correct demodulation of disturbance signals in the frequency range from 50 Hz to 10 kHz.This paper proposes a two-way detection system for phase demodulation?-OTDR based on a 3×3 coupler combined with an unbalanced Michelson interferometer and the corresponding two-way DCM demodulation algorithm to improve the stability of the demodulation results and reduce the influence of the incomplete symmetry of the 3×3 coupler on the demodulation results,while the structure is simple and low cost.This solution provides a new option for dynamic acoustic measurements and guarantees the stability and reliability of the phase-demodulated?-OTDR system in practical applications.