基于达曼光栅的快照式斯托克斯偏振测量方法

Snapshot Stokes Polarimetry Method Based on Dammann Grating

斯托克斯偏振测量常被用于获取光束的偏振特性。提出一种利用偏振无关的达曼光栅快照式测量偏振光束斯托克斯参量的方法。偏振光束通过达曼光栅后在空间对称的位置上被分成4束,这4束光经波片和线偏振器调制后,最终被CCD采集。将单次快照采集的光强图简单叠加运算就可计算得到偏振光束的斯托克斯参量,并可进一步计算得到偏振光束的偏振分布、矢量质量因子(VQF)和模间相位。所提测量方法对不同椭圆偏振光的测量结果与商用偏振测量仪的测量结果之间的平均相对误差为6.97%。所提方法的测量装置简单,无需转动任何器件,单次快照就可完成测量,具有可靠的测量精度。

Objective Polarization is a key parameter of light,accurate and rapid measurement of which plays a significant role in a variety of areas such as remote sensing technology,Mueller matrix measurement,and biological diagnosis.Stokes parameters directly reflect the light intensity of the polarization component of light,and all parameters can be directly determined by the measurement of light intensity.On this basis,the polarization distribution,vector quality factor,and intra-modal phase can be measured.The measurement methods of Stokes parameters mainly include the divisionoftime and divisionof-amplitude methods.The divisionoftime method refers to the measurement of required intensities one by one,which is only applicable to static polarized light.The divisionof-amplitude method can overcome shortcomings encountered by the divisionoftime method,but it also faces many problems such as the complexity of the device,the uneven distribution of light intensity,and different propagation distances.In this study,a Stokes polarimetry method based on a polarization-insensitive Dammann grating is proposed.The Stokes parameters of the polarized beam can be calculated by the intensity spots in a single snapshot,and the polarization distribution,vector quality factor,and intramodal phase of the polarized beam can be further obtained.This method has simple measuring equipment and does not need any rotating components.The measurement can be completed with a snapshot and has reliable accuracy.Methods The measurement principle of the proposed method is described in Fig.1.Formulas for calculating Stokes parameters,the polarization distribution,the vector quality factor,and the intra-modal phase of the polarized light are derived.The experimental setup is built upon the measurement principle.In the experiment,different vector polarized beams,circularly polarized beams,and elliptically polarized beams are generated to measure the target polarized beam.The polarization-insensitive Dammann grating is used to divide the incident polarized beam into four identical beams in a spatially symmetrical position.After being collimated by a convex lens,four beams are modulated by wave plates and a polarizer and finally captured by a CCD.Via the captured intensity images,Stokes parameters of the measured polarized beam are calculated,and the polarization distribution,vector quality factor,and the intra-modal phase of the polarized light beam are obtained.Finally,according to the measurement principle,we analyze the influence of the phase retardation deviation and fast axis azimuth deviation of wave plates and the transmittance axis deviation of the polarizer on Stokes parameter measurement.Results and Discussions First,the generated radially polarized beam is used for the initial calibration,which is divided into four identical beams,as shown in Fig.2.Four light spots recorded in Fig.2 are used to calibrate the center position of the light spot,which is beneficial to the subsequent measurement.After that,the radially polarized beam is measured.On the basis of the four light spots recorded in a snapshot(Fig.3),Stokes parameters of the radially polarized beam are calculated(Fig.4),and then the polarization distribution is reconstructed(Fig.5).The above experimental measurement results are all compared to the corresponding theoretical simulation results,and they have a good agreement.Then,more polarized beams are measured,and their experimental measurement results of reconstructed polarization distribution conform well to the theoretical simulation results(Fig.6).The measurement results of the generated elliptically polarized beams are compared with those of the commercial polarimeter to verify the feasibility and accuracy of the measurement method.Table 1 shows the relative measurement errors of different elliptically polarized beams between the proposed measurement method and the polarimeter,and the average relative error is 6.97%,which indicates the feasibility and accuracy of the proposed method.Additionally,the vector quality factor and intra-modal phase of different polarized beams are measured.At last,the analysis of the influence of some existing errors on Stokes parameter measurement shows that the phase retardation deviation and fast axis azimuth deviation of wave plates and the transmittance axis deviation of the polarizer could bring about a maximum measurement error of around 9% to Stokes parameter measurement.Conclusions This study proposes a method of measuring Stokes parameters of arbitrary polarized beams based on a polarization-insensitive Dammann grating with a snapshot.The Dammann grating is used to divide the incident polarized beam into four identical beams in a spatially symmetrical position.After being collimated by the lens,the four beams pass through different wave plates and a polarizer and are eventually captured by a CCD.The Stokes parameters of the polarized beam can be calculated by a simple superposition of the intensity spots in a single snapshot,and the polarization distribution,vector quality factor,and intra-modal phase of the polarized beam can be further obtained.The experimental measurement results are in good agreement with the theoretical simulation results.The average relative error of elliptically polarized beams between the proposed measurement method and the commercial polarimeter is 6.97%,which verifies the feasibility and accuracy of this measurement method.The Dammann grating used in this method is designed according to the wavelength of the incident beam and the required separation angle.It can accept the spot diameter of the incident beam in a wide range and has no requirements for the polarization of the incident beam.Hence,the measuring device has certain universality.The proposed Stokes polarimetry method is simple and can obtain reliable results without all-digital devices,which effectively reduces the cost.The subsequent use of wave plates with higher precision of phase retardation and rotation mounts with higher precision of rotation can further improve the measurement accuracy.