Interference Signal’s Digital Demodulation And Application For Dual Mach-Zehnder Optic Fiber Interferometer

Long-distance oil and gas pipeline is the most important means of transportationin energy supply. With China’s rapid economic development, oil and gas resourcesare under growing demand, and pipeline’s total mileage is increasing, which makepipeline precaution and leak detection technology become a research hotspot. Thus,timely detection and precise positioning of external invasion and leakage along thepipeline has important social significance and economic value. Our research grouphas developed a distributed optical fiber sensing system based on dual Mach-Zenhderinterferometer structure. The system can alarm, identify and locate third-party damageincidents along the pipeline, and has been applied to pipelines in many places.In field experiments, the correlation between detection signals shows randomvariation, causing errors in positioning, whose performance relies on cross correlationcalculation, and seriously affects system’s operation stability. In this paper, the causesof signal correlation deterioration were studied, and a passive demodulation methodbased on3×3fiber coupler was further proposed. The research contents mainlyinclude the following aspects:(1) The modulation principle of the optical signal in fiber was analyzed. Thenthe reason of polarization-induced phase shift and the polarization-induced signalfading phenomenon were illustrated by the system’s Jones matrix model. Meanwhile,3×3coupler output signal mathematical model was established due to the coupler’sproperty, to explain the working principle of3×3coupler phase signal demodulation,which was aimed at the sinusoidal modulation and frequency multiplication in fiberoptic interferometer.(2) The optical structure was modified by adding two3×3couplers in thesystem, and a passive digital demodulation method based on ellipse fitting wasproposed. The scheme used the two output signals of the3×3coupler to constitute anelliptic curve, and adopt least squares fitting under constraint to figure out the signal’scoefficients. Then a voting mechanism was taken to select the best fitting results, andto obtain the cosine modulated term of the phase signal, followed by digital arctangentmethod and differential cross-phase multiplication method to recover original phase signal. Finally, simulation of single harmonic signal was conducted to verify theellipse fitting scheme.(3) The processing flow of3×3fiber coupler digital demodulation method wasillustrated thoroughly. The demodulation scheme was verified by field experimentaldata, and the demodulated phase signal was applied to locate the vibration. Outcomesshowed that the demodulation method can effectively eliminate the signal distortion’sinterference on system’s positioning. The correlation index of demodulation signalswas close to1, the ellipse fitting was robust, and positioning accuracy was high andsteady, well improving system stability.