A method for correcting polarization mismatch angle in a polarization lidar

Abstract

This invention discloses a method for correcting polarization mismatch angle in a polarization lidar, comprising the following steps: (1) constructing a polarization lidar system and inserting a half-wave plate in front of the receiving optical path of the system; under normal operating conditions, recording the main polarization channel data of the polarization beam splitter in the system, denoted as I0; (2) rotating the half-wave plate by 22.5°, keeping other conditions unchanged, and recording the main polarization channel data in this state, denoted as I1; (3) in the same direction as step (2), rotating the half-wave plate again by 22.5°, keeping other conditions unchanged, and recording the main polarization channel data in this state, denoted as I2; (4) rotating the half-wave plate in the opposite direction by 45° to restore it to its original state, and calculating the polarization mismatch angle α; (5) rotating the half-wave plate in the opposite direction to complete the correction of the polarization mismatch angle. Using this invention, the polarization detection error problem caused by polarization mismatch angle in polarization lidar can be solved.

Description

Technical Field

[0001] This invention relates to the field of atmospheric environment remote sensing, and in particular to a method for correcting polarization mismatch angle of a polarization lidar. Background Technology

[0002] Atmospheric aerosols are small solid or liquid particles suspended in the air, with a radius of 10⁻⁶. -2 -10 -8 Between 1000 and 1000 cm. With the continuous development of history and the advent of the industrial age, the extensive use of fossil fuels has led to significant changes in the global climate. Haze, sandstorms, typhoons, global warming, and other abnormal environmental phenomena are increasingly affecting everyone's lives. For the sake of nature and the future of humanity, the importance and urgency of studying the global atmosphere are growing daily.

[0003] Lidar is an active remote sensing instrument that can detect atmospheric vertical profile information, offering advantages such as high spatiotemporal resolution and all-weather observation. Among atmospheric lidar, polarized Mie scattering lidar is widely used in atmospheric aerosol remote sensing due to its simple structure and low cost. The depolarization ratio of aerosols detected by polarized lidar can be used for various atmospheric studies, including aerosol type identification and microphysical property inversion.

[0004] However, aerosol type identification and microphysical property inversion both rely on the accurate detection of the depolarization ratio. Commonly used polarization lidars consist of a high-polarization emission source and a polarization beam splitter system. By adding a rotatable half-wave plate or directly rotating the receiving system, the main polarization direction of the emission system is made consistent with that of the receiving system. However, in actual assembly and adjustment, there is inevitably a certain angle between the main polarization directions of the two subsystems, which is usually called the polarization mismatch angle.

[0005] The presence of this mismatch angle causes interference between the main polarization channel signal and the depolarization channel signal of the polarization lidar, resulting in detection errors. During polarization gain calibration, this mismatch angle leads to incorrect calibration results, ultimately affecting the detection results. Due to the uncertainty of atmospheric conditions, this mismatch angle is difficult to correct by directly observing the lidar echo signal intensity.

[0006] Therefore, it is urgent to design a mismatch angle correction method to solve the polarization detection error problem caused by polarization mismatch angle in polarization lidar. Summary of the Invention

[0007] This invention provides a method for correcting polarization mismatch angle in a polarization lidar. The method involves inserting a half-wave plate in front of the receiving optical path, rotating the half-wave plate and recording the signal amplitude after rotation, calculating the polarization mismatch angle, and finally correcting the polarization mismatch angle.

[0008] A method for correcting polarization mismatch angle in a polarization lidar includes the following steps:

[0009] (1) Build a polarization lidar system and insert a half-wave plate in front of the receiving optical path of the system; under normal working conditions of the system, record the main polarization channel data of the polarization beam splitter in the system and denote it as I0;

[0010] (2) Rotate the half-wave plate by 22.5°, keeping other conditions unchanged, and record the main polarization channel data in this state, denoted as I1;

[0011] (3) In the same direction as step (2), rotate the half-wave plate again by 22.5°, keeping other conditions unchanged, and record the main polarization channel data in this state, denoted as I2;

[0012] (4) Rotate the half-wave plate in the opposite direction by 45° to restore it to its original shape, and calculate the polarization mismatch angle.

[0013] (5) Rotate the half-wave plate in the opposite direction by an angle Complete the correction of polarization mismatch angle.

[0014] Before correction, there is a mismatch angle α between the main polarization direction of the receiving system and the polarization direction of the emitted laser, resulting in polarization crosstalk in the system. The method of this invention can conveniently and accurately correct the polarization mismatch angle.

[0015] Furthermore, the polarization lidar system constructed in step (1) is as follows:

[0016] The laser emits a beam that enters the atmosphere perpendicularly. It interacts with the atmosphere, and the backscattered light is received by the telescope. After passing through the aperture stop, collimating eyepiece, half-wave plate, and mirror, the beam is split into two directions by the polarization beam splitter: the main polarization direction and the depolarization direction. The main polarization direction is consistent with the polarization direction of the emitted laser, while the depolarization direction is perpendicular to the polarization direction of the emitted laser. The two beams are detected by the main polarization and depolarization channel detectors, respectively, and converted into electrical signals, which are then recorded by the data acquisition card.

[0017] Preferably, the correction is performed under stable weather conditions, without rain, snow, or strong winds.

[0018] Preferably, when recording the main polarization channel data, a 5-15 minute detection is performed, and the detection results over a distance of 3-5 kilometers during this time are averaged.

[0019] In step (4), the specific process for calculating the polarization mismatch angle is as follows:

[0020] The original position of the main polarization channel signal strength is

[0021] I0 = I in T T[1+D T [(1-d)cos2α] (1)

[0022] After the first rotation of the half-wave plate

[0023]

[0024] The signal strength of the main polarization channel is

[0025] I1=I in T T [1-D T [(1-d)sin2α] (3)

[0026] After the second rotation of the half-wave plate

[0027]

[0028] The signal strength of the main polarization channel is

[0029] I2=I in T T [1-D T [(1-d)cos2α] (5)

[0030] The polarization mismatch angle is calculated using the combined formulas (1), (3), and (5).

[0031] In the formula, I in The constant is affected by laser energy, telescope aperture, and atmospheric scattering coefficient; T T D represents the transmittance of the transmission channel. T The parameters are affected by the polarizing beam splitter. For an ideal polarizing beam splitter D... T =1; d characterizes the depolarization ability of the scattering particle. It represents the ratio of the total intensity of the scattered unpolarized light to the incident fully polarized light. The value of d ranges from 0 to 1. When d = 0, it means that no depolarization has occurred. When d = 1, it means that all the incident ray-polarized light has become unpolarized light.

[0032] In this invention, the recorded main polarization channel data is the signal amplitude. The recorded detection data is not limited to the main polarization channel; the depolarization channel can also be recorded. In this case, the mismatch angle calculation method in step (4) needs to be adjusted accordingly.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. Existing correction methods can generally only compress the polarization mismatch angle to ±2°. This method is not affected by atmospheric aerosols or imperfections in polarization beam splitters, and theoretically can compress the polarization mismatch angle to 0°.

[0035] 2. Existing correction methods require rotating the half-wave plate more than 3 times, performing curve fitting, and then calculating the mismatch angle. This method selects a specific angle and only requires 2 rotations to obtain the accurate mismatch angle. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the polarization lidar system in this invention;

[0037] In the diagram: 1. Laser; 2. Telescope; 3. Aperture; 4. Collimating eyepiece; 5. Half-wave plate; 6. Mirror; 7. Polarizing beam splitter; 8. Main polarization channel detector; 9. Depolarization channel detector; 10. Data acquisition card.

[0038] Figure 2 This is a flowchart of a polarization mismatch angle correction method for a polarization lidar according to the present invention. Detailed Implementation

[0039] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.

[0040] like Figure 1 The diagram shows the structure of a polarization lidar system. Linearly polarized laser light is emitted by laser 1, interacts with the atmosphere, and is received by telescope 2. After passing through aperture 3 and collimating eyepiece 4, the polarization direction is adjusted by half-wave plate 5. Then, it is reflected by mirror 6 and passed through polarization beam splitter 7, which separates it into main polarization direction and depolarization direction. These directions are then received and detected by main polarization channel detector 8 and depolarization channel detector 9, respectively, and converted into light intensity signals, which are recorded by data acquisition card 10.

[0041] Before lidar detection, it is necessary to ensure that the polarization direction of the emitted laser is completely consistent with the main polarization direction of the beam splitter in the receiving module. This is crucial for separating the depolarized and non-depolarized light into the corresponding detection channels. Otherwise, crosstalk between the two channels will occur, ultimately affecting the accuracy of atmospheric optical property detection. This invention achieves precise calibration of the polarization mismatch angle by repeatedly rotating the half-wave plate 5 and calculating the magnitude of the polarization mismatch angle based on the change in the received signal intensity.

[0042] In this embodiment, laser 1 is a polarized laser with a wavelength of 532.27nm, such as an Nd:YAG pulsed laser, with a single pulse energy of 300mJ, a repetition frequency of 10Hz, and a polarization extinction ratio of 1000:1.

[0043] Telescope 2 can be a general-purpose telescope such as a reflecting or catadioptric telescope, such as the Celestron C11-A XLT 280mm Cassegrain telescope.

[0044] The aperture 3 can be made with a 1mm aperture diameter, such as the SM1D12C from Thorlabs, USA.

[0045] Collimating eyepiece 4 can use a 100mm focal length, such as Beijing Daheng Company's GCL-010111.

[0046] The half-wave plate 5 has a wavelength of 532nm and is installed in an electrically rotating frame, such as the PCC50 model from Newport Corporation in the United States.

[0047] Reflector 6 is a standard 532nm wavelength reflector, such as Beijing Daheng Company's GCC-101044.

[0048] The polarizing beam splitter 7 is a common polarizing beam splitter, with S-polarization reflection and P-polarization transmission, such as the PBS252 beam splitter from Thorlabs, Inc.

[0049] High-sensitivity photodiodes, photomultiplier tubes, or charge-coupled devices (CCDs) can be selected for the main polarization channel detector 8 and the depolarization channel detector 9, such as the R6358 photomultiplier tube from Hamamatsu Corporation of Japan.

[0050] Data acquisition card 10 can use the PCI8504 data acquisition system from Beijing Altair Technology Co., Ltd.

[0051] like Figure 2 As shown below, each step will be explained in detail:

[0052] Before the experiment, suitable weather conditions should be selected for calibration. This method has no requirements on aerosol load, but stable atmospheric conditions should be selected as much as possible, and the atmospheric state should be kept stable during the experiment. The laser emitted by laser 1 of the polarization lidar enters the atmosphere perpendicularly and interacts with the atmosphere. The backscattered light is received by telescope 2 and passes through aperture 3, collimating eyepiece 4, half-wave plate 5, and reflecting mirror 6 in sequence. It is then split into two directions, the main polarization direction and the depolarization direction, by polarization beam splitter 7. The main polarization direction is consistent with the polarization direction of the emitted laser, and the depolarization direction is perpendicular to the polarization direction of the emitted laser. The two beams are detected by main polarization channel detector 8 and depolarization channel detector 9, respectively, and converted into electrical signals, which are recorded by data acquisition card 10.

[0053] Step 1: Record the amplitude of the main polarization channel under normal operating conditions of the lidar, and denote it as I0;

[0054] Step 2: Rotate the half-wave plate by 22.5°, keeping other conditions unchanged, and record the amplitude of the main polarization channel in this state, denoted as I1;

[0055] Step 3: In the same direction as in Step 2, rotate the half-wave plate again by 22.5°, keeping other conditions unchanged, and record the amplitude of the main polarization channel in this state, denoted as I2;

[0056] Step 4: Rotate the half-wave plate 45° in the opposite direction to restore it to its original shape. The polarization mismatch angle at this point can be calculated. Rotate the half-wave plate in reverse by an angle Complete polarization mismatch angle correction.

[0057] In this example, the polarized laser emitted by the polarized laser, after passing through a Glan prism (typically with an extinction ratio of 20000:1), can be considered as an ideal linearly polarized laser. Choosing the linear polarization direction as the reference plane, the Stokes vector of the emitted laser can be expressed as , where I L This indicates the laser energy intensity.

[0058]

[0059] The parameter 'd' is used to characterize the depolarization capability of scattering particles; it represents the ratio of the total intensity of the scattered unpolarized light to the total intensity of the incident fully polarized light. The value of 'd' ranges from 0 to 1. When d = 0, no depolarization occurs; when d = 1, all incident ray-polarized light becomes unpolarized. For macroscopically isotropic, randomly oriented particles in the atmosphere, their Mueller matrix is...

[0060]

[0061] Where B is the atmospheric backscattering coefficient scalar.

[0062] The linearly polarized light emitted by the laser is scattered by atmospheric particles and received by the telescope. The Stokes vector of the received light is represented as follows:

[0063]

[0064] Where I in It is a constant, affected by laser energy, telescope aperture and atmospheric scattering coefficient.

[0065] Due to the incomplete alignment of the half-wave plate and assembly errors, there is a mismatch angle α between the polarization direction of the emitted laser and the main polarization channel of the polarization beam splitter. This is equivalent to the emitted laser beam being rotated by an angle α when it is received by the polarization beam splitter.

[0066]

[0067] In this example, the transmission channel is chosen as the main polarization direction. For the transmission channel of the polarizing beam splitter, the Mueller matrix is ​​expressed as follows:

[0068]

[0069] Where TT D represents the transmittance of the transmission channel. T Z T c T and Z T s T Due to the imperfections of the prism, for an ideal polarizing beam splitter D... T =1,Z T c T =0, Z T s T =0. The original position main polarization channel signal strength can be obtained as:

[0070] I0 = I in T T [1+D T (1-d)cos2α]. (11)

[0071] After the first rotation of the half-wave plate

[0072]

[0073] The signal strength of the main polarization channel is

[0074] I1=I in T T [1-D T (1-d)sin2α]. (13)

[0075] After the second rotation of the half-wave plate

[0076]

[0077] The signal strength of the main polarization channel is

[0078] I2=I in T T [1-D T (1-d)cos2α]. (15)

[0079] The polarization mismatch angle can be calculated using the combined formulas (11)(13(15)).

[0080] The aforementioned data recording method typically involves conducting a 10-minute probe and averaging the probe results over a distance of 3-5 kilometers within that 10-minute period to determine the signal strength under these conditions.

[0081] During the above half-wave plate rotation process, clockwise or counterclockwise rotation is not strictly required. It is sufficient that the rotation direction of the half-wave plate is the same in steps 2 and 3, and the rotation direction of the half-wave plate is opposite in steps 2 and 4.

[0082] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for correcting polarization mismatch angle in a polarization lidar, characterized in that, Includes the following steps: (1) Build a polarization lidar system and insert a half-wave plate in front of the receiving optical path of the system; under normal operating conditions of the system, record the main polarization channel data of the polarization beam splitter in the system and denote it as I0; (2) Rotate the half-wave plate by 22.5°, keeping other conditions unchanged, and record the main polarization channel data in this state, denoted as I1; (3) In the same direction as step (2), rotate the half-wave plate again by 22.5°, keeping other conditions unchanged, and record the main polarization channel data in this state, denoted as I2; (4) Rotate the half-wave plate in the opposite direction by 45° to restore it to its original shape, and calculate the polarization mismatch angle. (5) Rotate the half-wave plate in the opposite direction by an angle Complete the correction of polarization mismatch angle.

2. The polarization mismatch angle correction method for polarization lidar according to claim 1, characterized in that, In step (1), the polarization lidar system constructed is as follows: The laser emits a beam that enters the atmosphere perpendicularly. It interacts with the atmosphere, and the backscattered light is received by the telescope. After passing through the aperture stop, collimating eyepiece, half-wave plate, and mirror, the beam is split into two directions by the polarization beam splitter: the main polarization direction and the depolarization direction. The main polarization direction is consistent with the polarization direction of the emitted laser, while the depolarization direction is perpendicular to the polarization direction of the emitted laser. The two beams are detected by the main polarization and depolarization channel detectors, respectively, and converted into electrical signals, which are then recorded by the data acquisition card.

3. The polarization mismatch angle correction method for polarization lidar according to claim 1, characterized in that, Choose stable weather conditions for correction, with no rain, snow, or strong winds.

4. The polarization mismatch angle correction method for polarization lidar according to claim 1, characterized in that, In step (4), the specific process for calculating the polarization mismatch angle is as follows: The original position of the main polarization channel signal strength is I0=I in T T [1+D T (1-d)cos2α] (1) After the first rotation of the half-wave plate The signal strength of the main polarization channel is I1=I in T T [1-D T (1-d)sin2α] (3) After the second rotation of the half-wave plate The signal strength of the main polarization channel is I2=I in T T [1-D T (1-d)cos2α] (5) The polarization mismatch angle is calculated using the combined formulas (1), (3), and (5). In the formula, I in The constant is affected by laser energy, telescope aperture, and atmospheric scattering coefficient; T T D represents the transmittance of the transmission channel. T The parameters are affected by the polarizing beam splitter. For an ideal polarizing beam splitter D... T =1; d characterizes the depolarization ability of the scattering particle. It represents the ratio of the total intensity of the scattered unpolarized light to the incident fully polarized light. The value of d ranges from 0 to 1. When d = 0, it means that no depolarization has occurred. When d = 1, it means that all the incident ray-polarized light has become unpolarized light.

5. The polarization mismatch angle correction method for polarization lidar according to claim 1, characterized in that, When recording the main polarization channel data, a 5-15 minute probe is performed, and the probe results over a distance of 3-5 kilometers during this time are averaged.

6. The polarization mismatch angle correction method for polarization lidar according to claim 1, characterized in that, The recorded main polarization channel data is the signal amplitude.

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