保偏光纤应力传感特性的理论分析

Theoretical Analysis of Stress Sensing Based on Polarization Maintaining Fiber

对保偏光纤应力传感器中应力方向对于拍长、双折射主轴、串音以及偏振态演化等特性进行了理论分析。从光弹效应和弹性力学理论出发,推导出同时存在几何双折射、内应力以及外应力情况下的折射率分布,并采用等效阶跃光纤近似法,理论分析和数值仿真了横向应力的方向对于保偏光纤拍长、偏振主轴方向、偏振耦合强度(串音)以及输出偏振态演化的影响。仿真结果表明:1外应力的大小和方向对保偏光纤的拍长都将产生影响,在靠近慢轴上施加压力会减少双折射,增大拍长;靠近快轴上施加压力会增大双折射,减小拍长。2外应力方向与快慢轴不重合时,随着应力的逐步增大,保偏光纤新的双折射轴逐步向外应力方向靠近。3输出偏振态在邦加球上的演化轨迹类似于长短轴逐步增大的椭圆,其周期与应力方向直接相关。利用这一现象,可以通过测量输出偏振态的演化轨迹同时测量出外应力的大小和方向。

The relationship between polarization maintaining fiber （PMF） characteristics, such as beat length, principle axes of birefringence, polarization crosstalk and polarization state evolution, with stress direction is studied theoretically. Based on principle of photo elastic and elasticity, we have deduced refractive-index distribution which is joint result of geometrical birefringence, inner stress and external stress, moreover, we theoretically analyzed and simulated the effect of the transversal stress direction on beat length, principal axes of hirefringence, coupling intensity （ polarization crosstalk） and polarization state evolution. The numerical simulation results demonstrated ： ① The external stress values and direction are effect on PMF＇ s beat length. It will reduce birefringence and increase beat length when a stress along with the slow axis is exerted, otherwise, it will increase birefringence and reduce beat length when the stress along with the fast axis. ② With increasing external stress, the new principal axes of birefringence will grad- ually close to the direction of external stress. ③ The evolutionary track of the output polarization on the Poincare Sphere is similar with ellipse which the major and minor axis of ellipse are gradually increase, and the periodicity is di- rectly related to the direction of stress. According to the principle, we can measure the stress values and direction sim- ultaneously by means of measuring the evolutionary track of the output polarization.