基于双弹光级联差频调制的波片参数测量研究

Waveplate Parameter Measurement Based on Dual Photoelastic Modulators Cascade Differential Frequency Modulation

为了实现对波片快轴方位角和延迟量参数快速、高精度测试,提出了一种基于双弹光级联差频调制的波片参数测量方案。选用两个工作频率不相同的弹光调制器级联,构成偏振分析测量装置。波片的两个参数被加载到偏振分析装置的调制信号中,采用数字锁相技术同时提取调制信号的基频项和差频项,然后完成波片全部参数求解。按照原理分析,搭建了实验系统,并完成了系统初始偏移值定标,完成了632.8 nm的1/4波片,532 nm的1/4波片和1/2波片实验测量。实验结果表明,本文方案的快轴方位角测量最大偏差为0.2°,角度测量标准偏差为0.02°;波片的相位延迟量标准偏差优于5.64×10^(-4)rad,单点数据测量时间仅为200 ms。考虑到波片材料的双折射色散,根据检测激光波长下测量的相位延迟量,进一步计算出应用波长的波片延迟量。测量值与理论值最大偏差不超过1.17 nm,延迟精度优于λ/300。本文方案实现了高速、高精度和高灵敏的波片参数测量,可为波片加工测试和实验定标提供有效手段。

A waveplate is a basic optical component manufactured based on birefringence.It is typically made of quartz crystals,MgF2 crystals,or polymers with tens of micrometers thickness.A waveplate is used to change the polarization state.It is mainly employed in polarization generating devices and polarization analysis devices.It plays an important role in the fields of communication,sensing,and optical storage.The fast-axis azimuth and retardance are the key parameters of the waveplate.Accurate measurement and calibration of the two parameters are key steps in the manufacturing process of the wave plate.These steps directly determine the performance of the polarization optical system using the waveplate.Now,methods such as light intensity,polarization compensation,interferometry,laser feedback,and polarization modulation techniques,are applied to research the measurement of parameters of waveplate.Nevertheless,measurement speed and accuracy still need to be further improved.For the needs of rapid and high-precision parameter measurement of waveplate,a measurement scheme based on dual photoelastic modulators cascade difference frequency modulation is proposed in this paper.Considering the application advantages of photoelastic modulation,such as high modulation frequency,large optical aperture,high modulation purity and stable operation et al.,a novel measurement method using photoelastic modulation is proposed.A simple polarimetry is constructed based on two photoelastic modulators with differential modulation frequencies.The phase retardation and fast axis azimuth angle are loaded into the differential frequency photoelastic modulation signals,and the digital phase-locked technology is used to extract the differential frequency harmonic terms and fundamental frequency harmonic terms of photoelastic modulation at the same time,so as to further solve the phase retardation and fast axis azimuth angle.The principle of the new scheme is analyzed,and an experimental system is built.The initial offset value of the system is calibrated experimentally without any sample.After that,the measurement accuracy and repeatability are measured by using a quarter wave plate for 632.8 nm,a quarter wave plate for 53 nm,and a half wave plate for 532 nm.After the system is built and the initial calibration is completed.A quarter wave plate for 632.8 nm is the first to be determined at different azimuth angles.The fast axis azimuth of the wave plate is adjusted from 0°to 180°at intervals of 10°,the signal amplitudes are obtained by digital phase-locked technique,and the phase retardation and fast axis azimuth angle of the waveplate is further solved.The fast-axis azimuth angle measurements and actual values are in good agreement,as shown in Fig.4,and there is a maximum deviation of 0.11°between the measured value and the actual value,and the standard deviation of angle is 0.02°;throughout the entire process,the measurements of the phase retardation of the waveplate under all fast axis azimuths fluctuated less,the mean value is 1.5727 rad and a standard deviation is 5.57×10^(-4)rad,which indicates that the measurement system in this paper has good stability and repeatability,and the two parameters of fast axis azimuth and phase retardation can be simultaneously determined.A quarter wave plate for 532 nm and a half wave plate for 532 nm are also measured.The time interval of data measurement in the above experiments is only 200 ms.This method is suitable for fast measurement.Moreover,this method adopts a helium neon laser as the detection light source,with a wavelength of 632.8 nm and a spectral bandwidth of less than 2×10-3 nm,which can achieve accurate phase retardation measurements of any wave plates for any wavelength.Taking into account the birefringence dispersion of the waveplate material,the phase retardation at the detection laser wavelength is further calculated to the retardance at the application wavelength.The experimental results show that the maximum deviation between the experimental test value and the theoretical value does not exceed 1.17 nm,and the retardance accuracy is better thanλ/300.In present study,a novel waveplate parameters measurement method based on dual photoelastic modulators cascade difference frequency modulation is demonstrated.The principle of the new scheme is analyzed,and an experimental system is built.The initial offset value of the system is calibrated experimentally.And the measurements of a quarter wave plate for 632.8 nm,a quarter wave plate for 53 nm,and a half wave plate for 532 nm specimens are carried out.The experimental results show that the accuracy of fast axis azimuth measurement is less than 0.2°,the repeatability of fast axis azimuth is 0.02°;the repeatability of phase retardation measurement is 5.64×10^(-4)rad,and the accuracy of retardance is less than 1.17 nm.In addition,the measurement time of single data does not exceed 200 ms.Our study realizes simultaneous measurement of retardation and fast axis azimuth angle for waveplate.This method will be an effective means of processing testing or experimental calibration for waveplate.