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如图 1所示,光传感器件通过光纤接收从光源输入的光。传输过程当中,由于外界压力的变化,使光纤产生形变,改变了PCF的折射率,光的传播状态也发生变化。光调制器相对应于输入光状态的改变进行光电转换,即重新解调成相位、偏振、幅度等信息。再经过信号处理器,实现压力感测。
在PCF中,根据光纤模式双折射程度的不同分成:相双折射和群双折射。压力作用前,光纤的二重旋转对称性质,使其在受到压力后产生的形变会破坏PCF中的模式简并,致使偏振模存在差异,在x,y方向上的模式偏振态混合。相双折射定义为基模两个偏振态指数的差[13], 即
$ B = {n_{e{\rm{ff}},x}} - {n_{{\rm{eff}},y}} $
(1) 两种正交的x,y偏振态下的有效折射率分别表示为neff, x,neff, y,定义为两种偏振态在PCF内部传输时功率的周期性交换,模式双折射波动量定义为:
$ \Delta B\left( \lambda \right) = \frac{\lambda }{{2\pi }}\left( {{\beta _x} - {\beta _y}} \right) $
(2) 式中,βx, βy表示两个偏振模的模式常数, λ为波长。群双折射波动量定义为:
$ \Delta G\left( \lambda \right) = \Delta B\left( \lambda \right) - \lambda \frac{{{\rm{d}}\Delta B\left( \lambda \right)}}{{{\rm{d}}\lambda }} $
(3) PCF的结构分布,由于压力的作用产生改变,致使PCF中原双折射产生了变化。PCF中传播光脉冲两个正交模的相位差:未受压力时, ΔΦ=Φx-Φy; 受压后,PCF结构发生微小形变,致使其折射率分布发生改变,其相位差ΔΦ(p)=Φx(p)-Φy(p)。其中, Φ表示相位,p表示压力。两个偏振传输态的相位漂移值可以表示为:
$ \Delta \phi = \frac{{2{\rm{ \mathit{ π} }}L}}{\lambda }\left( {{n_{{\rm{eff}},x}} - {n_{e{\rm{ff}},y}}} \right) $
(4) 式中, L表示长度。相位随压力变化的灵敏度可以表示为:
$ K = \frac{{{\rm{d}}\phi }}{{{\rm{d}}p}} = \frac{{2{\rm{ \mathit{ π} }}L}}{\lambda } \times \frac{{\Delta B}}{{\Delta p}} $
(5) 根据(5)式,实现压力增敏的方式之一,即增大光子晶体光纤的双折射随压力的改变度。
当压力作用于PCF上时,内部将会产生应力。在工程应用中,因为PCF的纵向长度远大于截面尺寸。进行应力分析时,通常忽略纵向方向所产生的应力,只考虑横向截面。
填充型增敏式光子晶体光纤压力传感器结构
Structure of sensitized photonic crystal fiber pressure sensor filled with liquid
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摘要: 为了实现光纤传感器压力增敏的效果,设计了一种具有高双折射的光子晶体光纤结构。采用有限元法计算光子晶体光纤在不同应力作用下的有效折射率, 基于光子晶体光纤的压敏特性,分析光子晶体光纤的模式, 并选择该结构里的一个空气孔,填充具有特定折射率的液体材料,构成新型压力传感器。该结构的压力灵敏度由COMSOL软件的仿真得出。结果表明, 经填充液体后,偏振相位灵敏度从72rad/(MPa·m)提升至128rad/(MPa·m),显著提高了77.7%。该研究对增强传感器的力学性能有帮助。Abstract: In order to achieve the effect of pressure sensitization of optical fiber sensor, a photonic crystal fiber structure with high birefringence was designed. The effective refractive index of photonic crystal fiber was calculated by the finite element method when it was applied different stress. Based on the pressure-sensitive characteristics of photonic crystal fiber, the mode of photonic crystal fiber was analyzed. An air hole in the structure was selected and filled with the liquid of specific refractive index. A new pressure sensor was formed by above structures. The pressure sensitivity of the structure was simulated by COMSOL. The simulation results show that the polarization phase sensitivity of photonic crystal fiber sensor increases from 72rad/(MPa·m) to 128rad/(MPa·m). The sensitivity is improved by 77.7% significantly, which contributes to enhance the properties of the sensor.
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