由于马赫-曾德尔(Mach-Zehnder)光纤干涉传感系统的定位精度在实际应用中受到初始相位偏移的影响, 本文针对系统的光路结构, 提出了一种基于3×3耦合器的被动数字化解调方法。利用3×3耦合器的2路输出信号构成椭圆曲线, 在约束条件下对信号系数进行最小二乘拟合, 然后采用微分交叉相乘法解调出相位信号。与传统的解调方法相比, 提出的方法降低了对耦合器高对称性的要求, 不需限制其它参数; 约束条件下椭圆拟合的鲁棒性好, 尤其是对于椭圆度较差的数据点具有很强的适应能力, 适用于微弱振动信号的解调。仿真结果和现场实验数据证明该方法切实有效, 运算量小, 利用现场数据解调出的相位信号相关性达到0.992 0; 互相关计算显示其在总长43 km的管道上的定位误差为81.2 m, 有效提高了系统性能。

As the locating accuracy of a Mach-Zehnder fiber sensing system is effected by the initial phase shift in practical use, a passive digitalized demodulation method based on 3×3 fiber couplers was proposed according to the optical path structure. First, two outputs of the 3×3 couplers were used to form an ellipse curve, and then the signal coefficients were estimated by Least Square Method under constraints. Finally, the required phase signal was demodulated by differential and cross-multiplication. Compared with traditional demodulation methods, it lowers the requirements for the symmetry of couplers, and imposes no extra limits on other parameters. The ellipse fitting is robust under constraints, especially adaptable to those data less ideal in ellipse shapes. Simulation and field experiment prove that the method with a small computation is feasible and practical. The demodulated phase signal correlation is 0.992 0 and the position error is 81.2 m over a pipeline with a length of 43 km. The digitalized demodulation method virtually enhances the system function, and shows considerable application values.