基于双芯光子晶体光纤的激光多普勒多点速度测量研究

Study on Multi-Point Velocity Measurements of Laser Doppler Velocimetry Based on Dual-Core Photonic Crystal Fiber

为了实现微流体流场中多点速度测量,提出基于双芯光子晶体光纤的激光多普勒速度计研究。理论分析基于双芯光子晶体光纤的相干特性和控制体特征。计算结果表明纤芯间距为7.4μm,在控制体内可产生5条干涉明纹,与实验结果保持一致。但由于噪声的存在,实验所得各级条纹的强度比值比理论计算结果小。增大纤芯间距,可增加控制体内的干涉条纹数量,从而提高速度测量精度。从双芯光子晶体光纤出射的光斑至相干过程的成像结果表明,速度计的工作距离约30μm。分析基于双芯光子晶体光纤的激光多普勒多点速度测量原理,并进行单点、两点和三点速度测试。单点测量时,粒子平均速度为0.980 m/s,相对不确定度为0.5%;两点测量时,粒子平均速度分别为1.761 m/s、1.769 m/s;三点测量时,粒子平均速度分别为2.106、2.084、2.097 m/s。实验结果表明,基于双芯光子晶体光纤的激光多普勒速度计可进行多点速度测量,而且具有探头微型化、系统简单的特点,因此可嵌入微系统进行流场的多点速度测量。

To realize multi-point velocity measurements in micro fluid flow, this paper presents a study on laser Doppler velocimetry based on the dual-core photonic crystal fiber. The coherent characteristics and property of measurement volume based on the dual-core photonic crystal fiber are theoretically analyzed. Calculation results show that there are five bright interference fringes in measurement volume as the spacing between two fiber cores is about 7.4 μm, which is in agreement with the experiments. Due to the noise in experiment, intensity ratio of each fringe in experiment is lower than that based on the calculation. Enlarging the spacing between two fiber cores results in the increase of interference fringes, thus leads to increase the accuracy of measuring velocity. According to the images during the process from two separated faculae to their interference, the working distance for this velocimetry can be identified as about 30 μm. The principle of multi-point measurements of laser Doppler velocimetry based on the dual-core photonic crystal fiber is analyzed. The test based on the velocimetry is then executed to measure the single point velocity, two-point velocities and three-point velocities. For single point measurement, the averaged velocity of the particle is 0.980 m/s with the relative uncertainty of 0.5%. For two-point measurements, the averaged velocities of the particles are 1.761 m/s and 1.769 m/s, respectively. For three-point measurements, the averaged velocities of the particles are 2.106 m/s, 2.084 m/s and 2.097 m/s, respectively. Experimental results demonstrates that the laser Doppler velocimetry based on the dual-core photonic crystal fiber can realize multi-point velocity measurements. The velocimetry has the advantages with micro sensing head and simple systematic configuration over classical one. Therefore, the velocimetry can be embedded in the micro system for multi-point velocity measurements of micro fluid flow.