As commonly used spectroscopic elements, the dichroic mirror and the polarization beam splitter have been widely used in polarization lidar in recent years. Due to the non-ideal performance of the two optical components and the polarization error angle during installation, the depolarization ratio of the backscattered light in atmospheric detection will be affected to a certain extent. In the mainstream polarization lidar calibration methods, the influence of the polarization beam splitter on the polarization signal of the system is mainly considered, but in multi-wavelength polarization lidar systems, there will be two optical devices, namely the dichroic mirror and the polarization beam splitter. In addition, few studies discuss the effect of the depolarization ratio of the dichroic mirror on the atmospheric backscatter signal. The article focuses on the problem that only the influence of the polarization beam splitter is usually considered in the calibration of polarization lidar. Through the simulation method, the influence of the dichroic mirror, the polarization beam splitter, and the cascade of the two on the depolarization ratio of aerosols in the atmosphere is analyzed, with the error analysis given. We hope that relevant research in this article can be used to improve the detection accuracy of polarization lidar and the design of calibration methods.

In this article, we use the Stokes-Miller matrix method to analyze the influence of optical devices on the signal depolarization ratio. The simulated light of the backward scattered light of dust and cirrus clouds at the wavelengths of 532 nm and 1064 nm is used as the input, and the data on the depolarization ratio of dust and cirrus clouds are obtained by consulting the literature. We chose the dual-wavelength polarized lidar (532 nm and 1064 nm) receiving system as an example. The model of the selected dichroic mirror is DMPL900 produced by THORLABS, and the parameters of the polarization beam splitter are given by the relevant literature.

The results show that the commonly used long wave-pass dichroic mirror will produce a change of 7.111% in the depolarization ratio under the transmission channel of 1064 nm, and the reflection channel of 532 nm has a change of 3.012% in the depolarization ratio (Table 2). When the position of the main polarization of the signal and the plane of incidence is changed, the depolarization ratio error does not change significantly. As the polarization error angle increases, the depolarization ratio error will be further increased (Fig. 4). When the error angle reaches 10°, the depolarization ratio error will increase by more than 15% on the original basis (Fig. 5). For polarization beam splitter, the depolarization ratio of dust detected at 532 nm and 1064 nm will produce relative error changes of 21.333% and 27.3%, respectively; the depolarization ratio of cirrus clouds detected at 532 nm will produce a relative error change of 14.2% (Table 4). By making the main polarization state of the signal perpendicular to the plane of incidence, the error of the depolarization ratio will be greatly reduced. With the increase in polarization error angle, the change in depolarization ratio is similar to the dichroic mirror (Fig. 9). Under the cascade of the two optical elements, the depolarization ratio of the simulated light also indicates an increase in accumulative errors (Fig.10).

The dichroic mirror and polarization beam splitter are important optical elements in the polarization lidar system. Studying the influence of the two optical elements on the depolarization ratio of atmospheric backscatter signals can be used for reference when the system calibration methods are designed. In this paper, the depolarization ratio of dust and cirrus clouds at two wavelengths is calculated by the Stokes-Miller matrix method after the depolarization ratio signal passes through the dichroic mirror and polarization beam splitter. The results show that the dichroic mirror has some influence on the depolarization ratio of the signal, and the transmission channel has more influence than the reflection channel. The polarization beam splitter will produce greater depolarization error for aerosols with a smaller depolarization ratio. The simulation results under the cascade of two optical elements show that the influence on the depolarization ratio is approximately the accumulation of the independent action of optical elements. In the analysis of the cascade case, the influence of the polarization beam splitter on the depolarization ratio is the largest. However, due to the addition of the dichroic mirror, the depolarization ratio error will increase by about 5% compared with the effect of the polarization beam splitter alone, and the error will be further increased when there is a polarization error angle during the installation of optical elements. The analytical method used in this paper can be used to calculate the detection accuracy of the depolarization ratio of the polarization lidar system and improve the design of the system calibration method. The above analysis method can also be used to evaluate the performance of the polarization lidar system with optical elements cascaded at more wavelengths.