基于双声光调制器Φ-OTDR系统的抑制频漂方法

Method of suppressing frequency drift in Φ-OTDR system based on dual acousto-optic modulators

摘要:

相位敏感光时域反射系统(Φ-OTDR)中激光器和声光调制器都存在频率漂移现象,从而导致解调后的信号出现相位偏移和振幅波动,影响振动信号检测精度。本文提出了基于双声光调制器的Φ-OTDR系统结构,采用外差检测构造了具有相同频漂的本振和传感信号,克服了频漂对传感光纤后向瑞利散射光信号的频漂影响。激光器输出的光信号被分为上下两个支路,分别经两个声光调制器和两个分光器产生四路移频光脉冲。下支路的一路光脉冲进入传感光纤,其后向瑞利散射信号和上支路的一路光脉冲相干后生成的传感拍频信号,另外两路脉冲相干后生成的本振拍频信号,二者具有相同的频漂效应,通过正交相位解调即可解决频漂问题。本文建立了抑制频漂的正交解调模型,理论推导分析了该模型抑制频漂的机理,仿真验证了双声光调制器系统抑制频漂影响的可行性。

Abstract:

The frequency drift phenomenon exists in both the laser and the acousto-optic modulator of the phase sensitive photosensitive time-domain reflection system (Φ-OTDR), which leads to phase shift and amplitude fluctuation in the demodulated signal, affecting the accuracy of vibration signal detection.This paper proposes a Φ-OTDR system structure based on dual acousto-optic modulators, and adopts heterodyne detection to construct the local oscillator and sensing signals with the same frequency drift, thereby overcoming the influence of frequency drift in the backward Rayleigh scattering optical signal of the sensing fiber.The optical signal output by the laser is divided into two branches, the upper and lower branches. The signals in these two branches are then passed through two acousto-optic modulators and two optical splitters respectively, generating four frequency-shifted optical pulses in total.One of the optical pulses in the lower branch enters the sensing fiber, and its backward Rayleigh scattering signal interferes with one of the optical pulses in the upper branch, generating a sensing beat signal.The other two pulses generate the local oscillator beat frequency signal after coherence,both of them have the same frequency drift effect.The frequency drift issue can be resolved through quadrature phase demodulation.This paper establishes an orthogonal demodulation model to suppress frequency drift, theoretically deduces and analyzes the mechanism of this model to suppress frequency drift, and verifies the feasibility of the dual acousto-optic modulator system to suppress the influence of frequency drift through simulation.