基于干涉解调技术的光纤激光器水声传感系统

Fiber Laser Acoustic Pressure Sensing System Based on Interferometric Demodulation Technique

分析了基于Michelson干涉解调技术的光纤激光器水声传感的原理,在一段掺铒光纤中写入具有π相移的光纤光栅构成光纤激光器,水声压力作用在激光器上引起激光工作波长的变化;采用基于3×3耦合器的偏振无关的非平衡光纤Michelson干涉仪将激光波长变化转化为干涉仪的相位变化;干涉仪的输出由光电探测器转换后使用DSP进行信号解调。针对3×3耦合器分光比不对称的问题,本文提出利用实时调整幅度的2路干涉信号进行解调的方案,该方案不需要3×3耦合器有严格的分光比,消除了外界环境对解调输出的非线性影响。水声探测实验表明,光纤激光器水声传感系统的声压灵敏度为-166.5dB(参考值1rad/μPa),解调结果与水声信号具有良好的线性关系。

The theoretical acoustic sensing characteristics of a fiber laser are described based on Michelson interferometric demodulation technique. The fiber laser sensor is made by writing Bragg grating with π phase-shift into a section of Er-doped fiber. Acoustic pressure applied on the fiber laser induces laser wavelength shift. And a polarization-insensitive unbalanced Michelson fiber interferometer based on the 3×3 coupler is employed to transform the wavelength shift into phase shift of the interferometer. The signals are processed by Digital Signal Processing （DSP） to demodulate the phase shift. Based on the asymmetry characteristic of 3×3 coupler output signal, a novel algorithm for demodulating the phase shift is proposed. By employing two output signals whose amplitude can be adjusted in real time, it don＇t need uniform splitting ratio of coupler. The sensitivity experiment shows that the sensitivity is -166.5 dB（re 1 rad/μPa） when a 200 m path imbalanced readout interferometer is used, and the system output is linear to the pressure applied on the fiber laser sensor.