Research On Key Technologies And System Of High-precision Polarization Lidar

Lidar(light detection and ranging)has become an important tool for atmospheric remote sensing.Due to the ability to obtain depolarization information,which is critical to atmospheric remote sensing,polarization lidars are indispensable to the lidar family.With reference to polarization radar,the first polarization lidar was born in 1971.So far,the basic principle about polarization lidars has been mature.As well as other polarization measurements,however,the detection accuracy of polarization lidars is limited by systematic errors.In the thesis,we focus on the development of high-precision polarization lidars for atmospheric remote sensing.A general theoretical model is established on the basis of the Mueller calculus.Using Monte Carlo(MC)method,all systematic errors of polarization lidars are analyzed.The polarization properties of several critical optical components are studied.A series of key technologies are proposed to help polarization lidars to achieve high-precision measurements.Finally,we build a high-precision polarization lidar and validate it via experimental measurements.The main contents are as follows:A general theoretical model for polarization lidars is established.According to the Stokes-Mueller calculus,polarization lidars are divided into seven components,i.e.,laser,emitting module,the atmosphere,receiving module,calibration module,polarization analysis module and detecting module.The polarization properties of each part mentioned above are characterized by a Stokes vector or a Mueller matrix.In this way,all system parameters are connected with the measurement results of polarization lidars.Due to the proposed theoretical model and MC method,systematic errors of polarization lidars are analyzed quantificationally and the effects of every system parameter on the detection accuracy of polarization lidars are characterized.As a result,seven critical system parameters are picked out,which provide the way to achieve a high-precision polarization lidar.A new method named ?45° method is proposed to calibrate the gain ratio in polarization lidars.Gain ratio bears directly on the measurement results of polarization lidars and is one of the most important system parameters in polarization lidars.The ?45° method is validated via both theoretical calculations and experimental measurements.Careful comparisons between the ?45°method and the existing ±45 method are performed.It is found that the ?45° method possesses obvious advantages over the ±45°method in terms of signal-noise-ratio,the independence of both atmospheric conditions and other systematic errors such as the cross talk of the polarizing beam splitters(PBS).Effects of a nonideal half-wave plate(HWP)on the gain ratio calibration measurements in polarization lidars are performed,which helps to build a better error model for polarization lidars.Quantitative criterion is determined to judge the polarization properties of an HWP,which offers a big help to choose a qualified HWP for polarization lidars whose detection accuracy is specified.Moreover,several external errors,including ambient temperature variations,wavelength differences,and tilt angles of the HWP,are also studied in detail.It is found that true-zero HWPs are the best choices to calibrate the gain ratio in polarization lidars.The polarization properties of several critical optics in the receiving module and polarization analysis module of polarization lidars are studied.The Mueller matrices of receiving telescopes are obtained by ray tracing with space vectors.The relationship between the measurement errors of the atmospheric depolarization parameter and the elements of the Mueller matrices of receiving telescopes is deri-ved.The polarization properties of receiving telescopes in terms of laser wavelength,coating material,orientation,and F number are analyzed,respectively.The angular distribution of backscatter signal of lidars is obtained and the polarization properties of receiving telescope with different field of views are characterized.By comparing two common receiving telescopes in linear and circular polarization lidars,it is found that the measurement errors caused by the Newton telescopes in circular polarization lidars are significantly greater than those in linear polarization lidars,while the performances of the Cassegrain telescopes in the two lidars are almost identical.What is more,the measurement errors caused by the Cassegrain telescopes are much less than the counterparts caused by the Newton telescopes.According to the comparison results,the optimal telescopes are respectively presented for polarization lidars working in different polarization states and laser wavelengths.In addition,the extinction ratio of commercial PBSs,which has significant effects on the polarization lidars,is too low to be competent.On the basis of common PBSs,a high-precision polarizing beam splitting module is proposed,whose extinction ratios in both reflection channels and transmission channels are bigger than 10000.This module is examined via theoretical calculations,simulations and experimental measurements.A polarization lidar is developed in Zhejiang university.It is designed very carefully.All aforementioned key technologies are applied in every module of the polarization lidar.In a snowy night,the polarization lidar successfully obtains stable backscatter signal whose atmospheric depolarization ratio is measured to be 0.0088.Since the bandwidth of the involved filter in the lidar is 3 nm,therefore,the measurement results are very close to theoretical depolarization ratio of the clear air.This fact clearly demonstrates that the polarization lidar enables to achieve high-precision atmospheric depolarization measurements.