基于改进型弹光调制结构的VOC气体浓度检测系统

Detection System for VOCs Concentration Based on Improved Photo-Elastic Modulator

由于传统弹光调制系统受调制晶体尺寸的限制,光谱分辨率较低,致使其气体浓度探测精度不高。为了提高系统光谱分辨率,设计了一种改进型弹光调制系统,通过使入射光在晶体内多次反射增大光程差,从而获取更高的光谱分辨率。整个系统由激光器、弹光调制器、起偏器、检偏器及光电探测器构成。弹光晶体上加工一个楔角θ,并在两个面上镀反射膜从实现增大有效光程的效果。计算了楔角对光程、调制相位以及能量的函数关系。最终确定了合适的楔角值,并给出了相应的光程函数表达式。实验分别对三种常见的VOC气体进行浓度分析,样气浓度采用PTM400-VOC型气体分析仪定标,并与传统弹光调制系统进行对比。结果显示,改进型弹光调制系统气体浓度检出限可达0.010mg·m-3,相比传统弹光调制系统高一个数量级以上,气体浓度检测误差为3.4%,也优于传统型。综上所述,改进型弹光调制系统不但具备静态结构高稳定性的优点,同时还大幅提高了系统的光谱分辨能力及浓度检测精度。

Since the size of modulated crystal is limited with size in the conventional photo-elastic modulation system,it’s a low spectrum resolution,so detection accuracy of gas concentration is not high.In order to improve the spectrum resolution of the system,an improved photo-elastic modulation system was designed.This method was carried out multiple reflection of incident light in the crystal.The optical path difference was increased by this method,so that a higher spectrum resolution was obtained.The entire system was constructed with the laser,photo-elastic modulator,polarizer,analyzer and photo-detector.Among them,a wedge angle（θ）was processed on modulation crystal.And a reflection film was plated on both sides;effective optical path length increase was implemented.Through the analysis the wedge angle（θ）was selected,and function on the optical path,the phase of modulation and the energy was calculated for the wedge angle.Ultimately,the appropriate wedge angle values were determined,and the corresponding optical path function expressions were given.In the experiments,the concentration of three common VOC gas was analyzed respectively.The concentration of sample gas was calibration by PTM400-VOC gas analyzer,and the results were compared with conventional photo-elastic modulation system.Experimental results show that gas concentration detection limit is reached to 0.010mg·m-3 for improved photo-elastic modulation system,compared with conventional photo-elastic modulation system it’s an order of magnitude or more.The gas concentration detection error was 3.4%,and it’s also better than the traditional type.In summary,improved photo-elastic modulation system not only has the advantage of high static structural stability,but also a substantial increase in the spectrum resolution and detection accuracy of concentration.