Liquid Crystal Filter-Based LiDAR Chopper Device and Method for Atmospheric Wind and Temperature Detection

Abstract

This invention discloses a liquid crystal filter-based lidar chopping device for atmospheric wind and temperature detection, comprising a digitally controlled voltage output module, a liquid crystal module, a light guide module, a timing control module, a polarization beam splitter module, a frequency discriminator module, a reference channel photodetector, a frequency discriminator channel photodetector, a photon counting and acquisition card, a computer, a pulsed laser, a telescope, and a collimation module. This invention also discloses a liquid crystal filter-based lidar chopping method for atmospheric wind and temperature detection. This invention solves the problem of atmospheric wind and temperature detection being limited by the dynamic range of the photodetector; the received signal attenuation amplitude can be adaptively adjusted in real time, better adapting to weather changes. Furthermore, although liquid crystal filtering introduces some signal loss, wind and temperature detection typically uses the ratio of the reference channel and frequency discriminator channel signals for inversion, so the final result is independent of the liquid crystal's transmittance, thus avoiding the negative impact of liquid crystal filtering.

Description

Technical Field

[0001] This invention relates to atmospheric wind and temperature detection lidar echo signal processing technology, specifically to an atmospheric wind and temperature detection lidar chopping device based on liquid crystal filters, and also to an atmospheric wind and temperature detection lidar chopping method, suitable for the accurate detection of echo signals from atmospheric wind and temperature lidar. Background Technology

[0002] Atmospheric wind and temperature detection lidar receives echo light signals through a telescope and sends them to a signal processing unit for data inversion to obtain atmospheric wind and temperature information. The detection altitude ranges from near the ground to 110 km in the upper atmosphere. Within this nearly 100 km atmospheric range, the atmospheric density varies by more than six orders of magnitude, and the intensity range of the received echo light signals far exceeds the dynamic range of a photoelectric sensor (PMT, 3-4 orders of magnitude). Atmospheric wind and temperature detection lidar typically uses a high-power pulsed laser for transmission and a large-aperture telescope for reception, employing a high-sensitivity photoelectric sensor to improve the signal-to-noise ratio. However, the high laser power and high-sensitivity photoelectric sensor often lead to severe saturation or even blinding of the photoelectric sensor due to strong low-altitude echo signals, generating strong light-induced noise, which significantly affects the results of upper-altitude atmospheric detection. Therefore, to achieve accurate detection of the upper atmosphere, the interference problem of strong low-altitude echo signals must first be solved.

[0003] To solve this problem, the following methods are commonly used.

[0004] One approach is to use an off-axis receiver, which employs a very small receiving field of view to prevent low-altitude Mie-scattered echo signals from entering the telescope's field of view. However, this method introduces the problem that high and low altitudes cannot be simultaneously addressed, and that precise adjustments are required for both transmission and reception. (G. Baumgarten, Doppler Rayleigh / Mie / Raman lidar for wind and temperature measurements in the middle atmosphere up to 80km[J], Atmos MeasTech, 2010, 3(6): 1509-1518.)

[0005] Secondly, mechanical chopping is used. Mechanical chopping utilizes a high-speed rotating perforated chopper plate to control the on / off state of the received optical signal, allowing the echo light signal within the selected effective spatial range to pass normally through the chopper plate slot and be incident on the photodetector; while echoes from low altitudes and other spatial ranges are blocked by the opaque part of the chopper plate and cannot be incident on the photodetector, thus effectively suppressing stray light. Optically, it cuts off the strong background light at low altitudes while retaining the echo light at high altitudes, effectively achieving precise detection of the upper atmosphere. However, mechanical chopping also has the disadvantages of not being able to simultaneously cover high and low altitudes and the influence of the chopper opening and closing transition zone. (Lin Xin, Yang Yong, Cheng Xuewu, Guan Sai, Wang Jihong, Li Faquan, Liu Linmei, Song Shalei, Chen Zhenwei, Li Yajuan. Application of Mechanical Chopping in Atmospheric Detection Lidar [J]. Chinese Journal of Lasers, 2013, (8): 217-222.)

[0006] For the reasons mentioned above, there is an urgent need in the field of atmospheric wind and temperature detection lidar receiver echo signal processing technology for a new type of chopping device and method that can handle both high and low altitudes without affecting signal reception. Summary of the Invention

[0007] The purpose of this invention is to address the problems existing in the prior art by providing an atmospheric wind temperature detection lidar chopping device based on liquid crystal filtering, and also to provide an atmospheric wind temperature detection lidar chopping method based on liquid crystal filtering.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] The atmospheric wind and temperature detection lidar chopper device based on liquid crystal filtering includes a telescope. After receiving the echo signal, the signal passes through a collimation module and a light guide module, then through a liquid crystal module and enters a polarization beam splitter. The echo signal is split into horizontally polarized light and vertically polarized light by the polarization beam splitter. The vertically polarized light serves as a reference signal and enters a reference channel photodetector. The reference channel photodetector converts the reference signal into a reference electrical signal, which is then input into a photon counting acquisition card. The horizontally polarized light passes through a frequency discriminator and a frequency discriminator channel photodetector, and is then converted into a frequency-discriminated electrical signal, which is input into the photon counting acquisition card. The reference electrical signal and the frequency-discriminated electrical signal are then collected by the photon counting acquisition card to obtain the echo acquisition signal, which is then input into a computer.

[0010] The atmospheric wind and temperature detection lidar chopper device based on liquid crystal filter also includes a timing control module. The timing control module controls the pulsed laser, the digitally controlled voltage output module, and the photon counting and acquisition card to perform synchronous triggering. At the rising edge of the high level of the trigger signal, the pulsed laser sends a pulsed laser, the digitally controlled voltage output module outputs the corresponding driving voltage to the TFT switching device of the liquid crystal module according to the transmittance calculated by the computer, and the photon counting and acquisition card collects the echo acquisition signal.

[0011] As described above, the computer stores the laser echo intensity function g(x), calculates the liquid crystal transmittance function y(x) based on the laser echo intensity function g(x), calculates the liquid crystal voltage driving function f(x) based on the liquid crystal transmittance function y(x), and outputs the driving voltage based on the liquid crystal voltage driving function f(x).

[0012] As mentioned above, the liquid crystal transmittance function y(x) is based on the following formula:

[0013] When x ≤ 30km, y(x) = 0;

[0014] When x > 30km and g(x) / 2 ≥ Rmax, y(x) = 1.8Rmax / g(x);

[0015] When x > 30km and g(x) / 2 < Rmax, y(x) = Tmax;

[0016] Where x is the altitude, Rmax is the maximum detection value of the photodetector, and Tmax is the maximum transmittance of the liquid crystal module.

[0017] As mentioned above, the liquid crystal voltage driving function f(x) is based on the following formula:

[0018] When y(x) = 0, f(x) = 0;

[0019] When y(x) = Tmax, f(x) = 5;

[0020] When y(x) > 0 and y(x) < Tmax, f(x) = 5*y(x).

[0021] As described above, the computer updates the laser echo intensity function g(x) based on the received echo acquisition signal.

[0022] A chopping method for atmospheric wind and temperature detection lidar based on liquid crystal filtering includes the following steps:

[0023] Step 1: The computer obtains the laser echo intensity function g(x);

[0024] Step 2: The computer calculates the liquid crystal transmittance function y(x) based on the laser echo intensity function g(x);

[0025] Step 3: The computer calculates the liquid crystal voltage driving function f(x) based on the liquid crystal transmittance function y(x);

[0026] Step 4: Output the driving voltage based on the liquid crystal voltage driving function f(x);

[0027] The timing control module controls the pulsed laser, the numerical control voltage output module, and the photon counting and acquisition card to perform synchronous triggering. At the rising edge of the high level of the trigger signal, the pulsed laser sends a pulsed laser, the numerical control voltage output module outputs the corresponding driving voltage to the TFT switching device of the LCD module according to the transmittance calculated by the computer, and the photon counting and acquisition card collects the echo acquisition signal.

[0028] Step 5: The echo signal received by the telescope enters the liquid crystal module through the collimation module and the light guide device. The liquid crystal module attenuates the echo signal and then it enters the polarization beam splitter. The echo signal is split into horizontally polarized light and vertically polarized light by the polarization beam splitter. The vertically polarized light is used as a reference signal and enters the reference channel photodetector. The reference channel photodetector converts the reference signal into a reference electrical signal, which is then input into the photon counting acquisition card. The horizontally polarized light passes through the frequency discriminator and the frequency discriminator in sequence and is converted into a frequency discriminator electrical signal. The frequency discriminator electrical signal is then input into the photon counting acquisition card. The reference electrical signal and the frequency discriminator electrical signal are then collected by the photon counting acquisition card to obtain the echo acquisition signal, which is then input into the computer.

[0029] As mentioned above, the liquid crystal transmittance function y(x) is based on the following formula:

[0030] When x ≤ 30km, y(x) = 0;

[0031] When x > 30km and g(x) / 2 ≥ Rmax, y(x) = 1.8Rmax / g(x);

[0032] When x > 30km and g(x) / 2 < Rmax, y(x) = Tmax;

[0033] Where x is the altitude, Rmax is the maximum detection value of the photodetector, and Tmax is the maximum transmittance of the liquid crystal module.

[0034] The liquid crystal voltage driving function f(x) is based on the following formula:

[0035] When y(x) = 0, f(x) = 0;

[0036] When y(x) = Tmax, f(x) = 5;

[0037] When y(x) > 0 and y(x) < Tmax, f(x) = 5*y(x);

[0038] As described above, step 5 also includes: the computer updates the laser echo intensity function g(x) based on the received echo acquisition signal and feeds it back to step 2.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] This invention solves the problem that atmospheric wind and temperature detection is limited by the dynamic range of photoelectric detectors, making it difficult to achieve a single system compatible with both high and low altitude detection. The attenuation of the received signal can be adaptively adjusted in real time, better adapting to weather changes. Furthermore, although liquid crystal filtering introduces some signal loss, wind and temperature detection typically uses the ratio of the reference channel and the frequency discrimination channel signals for inversion. Therefore, the final result is independent of the liquid crystal's transmittance, thus avoiding the negative impact of liquid crystal filtering. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the structure of the device of the present invention.

[0042] Figure 2 This is a schematic diagram showing how the echo signal strength changes with altitude. Detailed Implementation

[0043] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to implementation examples. It should be understood that the implementation examples described herein are for illustration and explanation only and are not intended to limit the present invention.

[0044] Example 1:

[0045] Liquid crystal filter-based lidar chopper for atmospheric wind and temperature detection, such as Figure 1 As shown, it includes a digitally controlled voltage output module 4, an LCD module 3, a light guide module 2, a timing control module 1, a polarization beam splitting module 5, a frequency discriminator module 6, a reference channel photodetector 7, a frequency discriminator channel photodetector 8, a photon counting and acquisition card 9, a computer 10, a pulsed laser, a telescope, and a collimation module.

[0046] After receiving the echo signal, the telescope passes through the collimation module and the light guide module, then through the liquid crystal module and into the polarization beam splitter. The echo signal is split into horizontally polarized light and vertically polarized light by the polarization beam splitter, which then enter the frequency discrimination channel photodetector and the reference channel photodetector, respectively. The vertically polarized light serves as the reference signal and enters the reference channel photodetector, which converts the reference signal into a reference electrical signal. The reference electrical signal is input into the photon counting acquisition card. The horizontally polarized light passes through the frequency discriminator and the frequency discrimination channel photodetector in sequence, and is then converted into a frequency discriminant electrical signal. The frequency discriminant electrical signal is input into the photon counting acquisition card. The reference electrical signal and the frequency discriminant electrical signal are then collected by the photon counting acquisition card to obtain the echo acquisition signal, which is then input into the computer.

[0047] Timing control module 1 generates a 30Hz trigger signal to control the pulsed laser, digitally controlled voltage output module, and photon counting and acquisition card for synchronous triggering. When the high-level rising edge of the trigger signal arrives, the pulsed laser emits a pulsed laser; the digitally controlled voltage output module outputs a corresponding driving voltage to the TFT switching device of the liquid crystal module based on the transmittance calculated by the computer, thereby controlling the transmittance of the liquid crystal module; and the photon counting and acquisition card acquires the echo acquisition signal.

[0048] The computer 10 stores a laser echo intensity function g(x). The computer calculates the liquid crystal transmittance function y(x) based on the laser echo intensity function g(x), and calculates the liquid crystal voltage driving function f(x) based on the liquid crystal transmittance function y(x). The computer outputs a driving voltage based on the liquid crystal voltage driving function f(x).

[0049] The transmittance function y(x) of liquid crystal is based on the following formula:

[0050] When x ≤ 30km, y(x) = 0;

[0051] When x > 30km and g(x) / 2 ≥ Rmax, y(x) = 1.8Rmax / g(x);

[0052] When x > 30km and g(x) / 2 < Rmax, y(x) = Tmax.

[0053] Where x is the altitude, Rmax is the maximum detection value of the photodetector, and Tmax is the maximum transmittance of the liquid crystal module.

[0054] The liquid crystal voltage driving function f(x) is based on the following formula:

[0055] When y(x) = 0, f(x) = 0;

[0056] When y(x) = Tmax, f(x) = 5;

[0057] When y(x) > 0 and y(x) < Tmax, f(x) = 5*y(x).

[0058] The computer 10 automatically updates the laser echo intensity function g(x) in real time based on the received echo acquisition signal, so that the received echo signal is always within the detection range of the photodetector.

[0059] Example 2

[0060] A chopping method for atmospheric wind and temperature detection lidar based on liquid crystal filtering, such as... Figure 1 As shown, it includes the following steps:

[0061] Step 1: The computer acquires the laser echo intensity function g(x), where x is the altitude. The laser echo intensity function g(x) is the curve showing the variation of pulsed laser echo signal intensity with altitude at different altitudes. The laser echo intensity function satisfies the atmospheric lidar equation and is approximately an exponential function, such as... Figure 1 As shown, the echo signal gradually enters the telescope's receiving field of view and becomes stronger. When it fully enters the field of view, the echo signal is the strongest, and it decays exponentially with increasing distance.

[0062] Step 2: The computer retrieves the laser echo intensity function g(x) to obtain the echo signal intensity at different altitudes. Combined with the detection range of the photodetector, the approximate attenuation value of the echo signal is calculated. According to the working principle of the atmospheric wind-temperature lidar, the echo signal is split into horizontally polarized light and vertically polarized light by a polarizing beam splitter, which then enter the frequency discrimination channel photodetector and the reference channel photodetector, respectively. Assume the detection maximum value of the photodetector is Rmax. If g(x) / 2 exceeds Rmax, g(x) is attenuated, reducing the peak value of the echo signal by half to 90% of Rmax.

[0063] Considering that the detection range of the atmospheric wind and temperature detection lidar is 30–110 km, severe low-altitude saturation signals will occur below 30 km, requiring attenuation as much as possible; at this point, the transmittance should be 0.

[0064] The transmittance function of the liquid crystal is calculated accordingly.

[0065] When x ≤ 30km, y(x) = 0;

[0066] When x > 30km and g(x) / 2 ≥ Rmax

[0067] y(x)=1-(g(x) / 2-0.9Rmax) / (g(x) / 2)=1.8Rmax / g(x);

[0068] When x > 30 km and g(x) / 2 < Rmax, y(x) = Tmax. (Tmax is the maximum transmittance)

[0069] Considering that the introduction of attenuation would lead to distortion in wind temperature inversion, however, in the data inversion of atmospheric wind temperature detection lidar, only the ratio of the reference channel signal strength Sref to the frequency discrimination channel signal strength Sfd is needed. When the reference signal and the frequency discrimination signal attenuate at the same amplitude, the signal attenuation is canceled out and does not affect the final measurement result.

[0070] Step 3: After obtaining the liquid crystal transmittance function y(x), the computer converts the liquid crystal transmittance function y(x) into the corresponding liquid crystal voltage driving function f(x), that is, converts the transmittance into the driving voltage. The driving voltage varies between 0V and 5V, and the voltage driving function is calculated accordingly.

[0071] When y(x) = 0, f(x) = 0;

[0072] When y(x) = Tmax, f(x) = 5;

[0073] When y(x) > 0 and y(x) < Tmax, f(x) = 5*y(x).

[0074] Step 4: The timing control module generates a 30Hz trigger signal to synchronously trigger the pulsed laser, the digitally controlled voltage output module, and the photon counting and acquisition card. The arrival time of the high-level rising edge of the trigger signal is recorded as T(0). When the pulsed laser receives the high-level trigger signal, it sends a pulsed laser. The time t for the pulsed laser to reach a certain height h and be received by the telescope is 2h / c (where c is the speed of light in air). Therefore, the height h is t*c / 2. The liquid crystal voltage control function f(x) can then be transformed into a function of t, f(t*c / 2). The digitally controlled voltage output module outputs the corresponding voltage to the TFT switching device of the liquid crystal module based on f(t*c / 2), thereby controlling the transmittance of the liquid crystal module. The dependent variable of the corresponding liquid crystal voltage driving function is transformed from altitude to time, as shown below.

[0075] When y(x) = 0, f(t*c / 2) = 0;

[0076] When y(x) = Tmax, f(t*c / 2) = 5;

[0077] When y(x) > 0 and y(x) < Tmax, f(t*c / 2) = 5*y(x).

[0078] To achieve f(t*c / 2), a numerically controlled voltage output module is used. This module consists of a high-precision DAC and a driving circuit. The high-precision DAC receives control voltage commands from the computer and converts them into voltage. The driving circuit then applies the voltage to the TFT switching device of the LCD module.

[0079] Step 5: The echo signal received by the telescope enters the liquid crystal module through the collimation module and light guide device. After the liquid crystal module attenuates the echo signal, it is split by a polarizing beam splitter. The vertically polarized light, as a reference signal, enters the reference channel photodetector and is converted into a reference electrical signal, which is then input into the photon counting acquisition card. The horizontally polarized light enters the frequency discriminator for frequency discrimination and then enters the frequency discrimination channel photodetector, where it is converted into a frequency-discriminated electrical signal, which is also input into the photon counting acquisition card. The reference electrical signal and the frequency-discriminated electrical signal are then collected by the photon counting acquisition card to obtain the echo acquisition signal. After the photon counting acquisition card collects the echo acquisition signal, it is input into the computer. The computer automatically updates the laser echo intensity function g(x) in real time based on the collected echo acquisition signal, ensuring that the received echo signal is always within the detection range of the photodetector. Then, return to step 2.

[0080] In actual observations, the intensity of the echo acquisition signal not only decreases with altitude but is also affected by weather conditions. When there are many obstructions such as clouds in the sky, the echo acquisition signal will become weaker. Through the above control methods, the intensity of the echo signal can be adjusted in real time.

[0081] This invention enables simultaneous high-altitude and low-altitude detection by an atmospheric wind and temperature detection lidar. The filtering and chopping amplitude is continuously adjustable, and the attenuation caused by filtering and chopping does not negatively impact the test results. Furthermore, the received signal attenuation can be adaptively adjusted to better adapt to changes in weather conditions.

[0082] It should be noted that the specific embodiments described in this invention are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains can make various modifications or additions to the described specific embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A chopper device for atmospheric wind temperature sounding lidar based on liquid crystal filter, comprising a telescope, characterized in that, After receiving the echo signal, the telescope passes through a collimation module and a light guide module, then through a liquid crystal module and enters a polarizing beam splitter. The echo signal is split into horizontally polarized light and vertically polarized light by the polarizing beam splitter. The vertically polarized light serves as a reference signal and enters a reference channel photodetector. The reference channel photodetector converts the reference signal into a reference electrical signal, which is then input into a photon counting and acquisition card. The horizontally polarized light passes through a frequency discriminator and a frequency discriminator photodetector, and is then converted into a frequency-discriminated electrical signal, which is input into the photon counting and acquisition card. The reference electrical signal and the frequency-discriminated electrical signal are then collected by the photon counting and acquisition card to obtain the echo acquisition signal, which is then input into a computer. It also includes a timing control module, which controls the pulsed laser, the digitally controlled voltage output module, and the photon counting and acquisition card to perform synchronous triggering. On the rising edge of the high level of the trigger signal, the pulsed laser emits a pulsed laser, the digitally controlled voltage output module outputs the corresponding driving voltage to the TFT switching device of the LCD module according to the transmittance calculated by the computer, and the photon counting and acquisition card collects the echo acquisition signal. The computer stores a laser echo intensity function g(x). Based on g(x), the computer calculates the liquid crystal transmittance function y(x), and then calculates the liquid crystal voltage driving function f(x) based on y(x). Finally, it outputs a driving voltage based on the liquid crystal voltage driving function f(x). The liquid crystal transmittance function y(x) is based on the following formula: When x ≤ 30km, y(x) = 0; When x > 30km and g(x) / 2 ≥ Rmax, y(x) = 1.8Rmax / g(x); When x > 30km and g(x) / 2 < Rmax, y(x) = Tmax; Where x is the altitude, Rmax is the maximum detection value of the photodetector, and Tmax is the maximum transmittance of the liquid crystal module. The liquid crystal voltage driving function f(x) is based on the following formula: When y(x)=0, f(x)=0; When y(x) = Tmax, f(x) = 5; When y(x) > 0 and y(x) < Tmax, f(x) = 5 * y(x). The computer updates the laser echo intensity function g(x) based on the received echo acquisition signal.

2. A chopper method for atmospheric wind temperature detection laser radar based on liquid crystal filter, characterized in that, Includes the following steps: Step 1: The computer obtains the laser echo intensity function g(x); Step 2: The computer calculates the liquid crystal transmittance function y(x) based on the laser echo intensity function g(x); Step 3: The computer calculates the liquid crystal voltage driving function f(x) based on the liquid crystal transmittance function y(x); Step 4: Output the driving voltage based on the liquid crystal voltage driving function f(x); The timing control module controls the pulsed laser, the numerical control voltage output module, and the photon counting and acquisition card to perform synchronous triggering. At the rising edge of the high level of the trigger signal, the pulsed laser sends a pulsed laser, the numerical control voltage output module outputs the corresponding driving voltage to the TFT switching device of the LCD module according to the transmittance calculated by the computer, and the photon counting and acquisition card collects the echo acquisition signal. Step 5: The echo signal received by the telescope enters the liquid crystal module through the collimation module and light guide device. The liquid crystal module attenuates the echo signal before it enters the polarization beam splitter. The echo signal is split into horizontally polarized light and vertically polarized light by the polarization beam splitter. The vertically polarized light serves as a reference signal and enters the reference channel photodetector. The reference channel photodetector converts the reference signal into a reference electrical signal, which is then input into the photon counting acquisition card. The horizontally polarized light passes through the frequency discriminator and the frequency discriminator photodetector in sequence, and is then converted into a frequency-discriminated electrical signal. This frequency-discriminated electrical signal is input into the photon counting acquisition card. The reference electrical signal and the frequency-discriminated electrical signal are then acquired by the photon counting acquisition card to obtain the echo acquisition signal, which is then input into the computer. The liquid crystal transmittance function y(x) is based on the following formula: When x ≤ 30km, y(x) = 0; When x > 30km and g(x) / 2 ≥ Rmax, y(x) = 1.8Rmax / g(x); When x > 30km and g(x) / 2 < Rmax, y(x) = Tmax; Where x is the altitude, Rmax is the maximum detection value of the photodetector, and Tmax is the maximum transmittance of the liquid crystal module. The liquid crystal voltage driving function f(x) is based on the following formula: When y(x)=0, f(x)=0; When y(x) = Tmax, f(x) = 5; When y(x) > 0 and y(x) < Tmax, f(x) = 5 * y(x); Step 5 further includes: the computer updates the laser echo intensity function g(x) based on the received echo acquisition signal and feeds it back to step 2.

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