Polarization lidar gain ratio calibration method: a comparison
doi: 10.37188/CO.2020-0136
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摘要: 增益比定标误差是影响偏振激光雷达退偏比精度的主要因素之一，观测前必须进行准确的增益比定标。本文分析了现存多种增益比定标方法的基本原理，并通过实验对比了+45°法、±45°法、∆45°法、旋转拟合法与退偏器法等增益比定标方法的定标准确性与优缺点。实验结果表明：∆45°法、±45°法与旋转拟合法在对准偏失角较小的情况下定标相对准确，但±45°法与旋转拟合法操作较为繁琐。+45°法在无对准偏失角的情况下定标误差仍较大。退偏器法操作最简便，但会受到非理想退偏器的制约。通过理论分析与实验对比，本文给出了增益比定标方法的最佳选择，即在一般情况下采用∆45°法定标，在有高精度退偏器的情况下采用退偏器法定标。Abstract: Gain ratio calibration error is one of the most significant factors affecting the accuracy of a polarization lidar depolarization ratio. This paper analyzes the basic principles of various existing gain ratio calibration methods and compares the advantages and disadvantages of the +45° method, ±45° method, ∆45° method, rotation fitting method and pseudo-depolarizer method in practice though experiments. Results show that: the ∆45° method, ±45° method and rotation fitting method are relatively accurate when the misalignment angle is small, but the operation of the ±45° method and rotation fitting method are more complicated. The +45° method still has a large calibration error without a misalignment angle. The pseudo-depolarizer method is the easiest to operate, but it is restricted by a non-ideal pseudo-depolarizer. Through comparison of theory and experiment, this paper provides a suggestion for the best choice of gain ratio calibration method. It is recommended that the ±45° method be used for calibration with a half-wave plate, and the pseudo-depolarizer method be used for calibration with a high-precision depolarizer.
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Key words:
- polarization lidar /
- gain ratio /
- calibration /
- depolarization ratio
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图 1 偏振激光雷达系统基本原理与结构图
Figure 1. Basic principle and structure diagram of polarization lidar system
图 2 +45°法原理图。(a)半波片旋转之前；(b)半波片旋转+45°之后
Figure 2. Schematic diagram of +45° method. (a) Before HWP rotation. (b) After HWP rotation by +45°
图 3 ±45°法原理图。(a)半波片旋转之前；(b)半波片旋转+22.5°之后；(c)半波片旋转−22.5°之后
Figure 3. Schematic diagram of ±45° method. (a) Before HWP rotation. (b) After HWP rotation by +22.5°. (c) After HWP rotation by −22.5°
图 4 ∆45°法原理图。(a)半波片旋转之前；(b)半波片旋转45°之后
Figure 4. Schematic diagram of ∆45° method. (a) Before HWP rotation. (b) After HWP rotation by 45°
图 7 5种定标方法相对误差随对准偏失角的变化曲线
Figure 7. Relative errors changing with the misalignment angle for the five calibration methods
图 8 实际测量退偏比随对准偏失角的变化曲线图。其中蓝色圆圈代表在不同
$\theta $ 情况下测量的${\delta ^{\rm{*}}}\left( \theta \right)$ ，红色虚线代表拟合曲线，绿色虚线代表${\theta _{{\rm{init}}}}$ Figure 8. Curve of the actually measured depolarization ratio changing with the misalignment angle, where the blue circle represents the
${\delta ^{\rm{*}}}\left( \theta \right)$ values measured at different$\theta $ angles, and the red and green dotted lines represent the fitting curve and${\theta _{{\rm{init}}}}$ , respectively
图 9 退偏器法增益比定标结果。蓝色圆圈代表半波片在不同角度下测量的增益比，红色虚线代表余弦函数多项式拟合曲线
Figure 9. Gain ratio calibration result of pseudo-depolarizer method. The blue circles represent the gain ratios measured at different misalignment angles, and the red dotted line represents the cosine polynomial fitting curve
图 5 旋转拟合法原理图。(a)半波片旋转之前；(b)半波片旋转
${\theta _{{\rm{h}},j}}$ 角之后Figure 5. Schematic diagram of rotation fitting method. (a) Before HWP rotation. (b) After HWP rotation by
${\theta _{{\rm{h}},j}}$ 表 1 Main parameters for polarization lidar system
Table 1. Main parameters for polarization lidar system
Main parameters Value Laser center wavelength 532 nm Laser energy 5 mJ Repetition frequency 10 Hz Pulse width 8 ns Diameter of telescope primary mirror 210 mm Field of view of telescope 1 mrad Focal length of telescope 2000 mm Filter bandwidth 3 nm 
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表 2 Calibration results of five methods at
${\theta _{\rm{h}}}{\rm{ = }}{0^\circ }$ Table 2. Calibration results of five methods at
${\theta _{\rm{h}}}{\rm{ = }}{0^\circ }$ Calibration method +45° ±45° △45° Rotation fitting Pseudo-depolarizer Calibration result 1.2185±0.1379 1.2679±0.1518 1.2676±0.1524 1.2716±0.0250 1.1977±0.1483
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