A system, method and apparatus for frequency discrimination curve calibration

Abstract

This application discloses a system, method, and apparatus for frequency discrimination curve calibration. Frequency locking technology is employed to stably lock the output laser frequency of the seed laser onto the absorption peak of the atomic / molecular absorption cell, effectively solving the problem of seed laser frequency drift. Simultaneously, two acousto-optic modulators are used to perform precise extracavity passive frequency modulation of the seed laser, shifting the laser frequency from the atomic / molecular absorption peak position to the preset zero-velocity frequency point and the edge frequency point of the linear operating region of the optical frequency discriminator. During calibration, the telescope is vertically pointed towards the zenith, providing atmospheric echoes with zero Doppler shift as a reference. At this time, the optical frequency discriminator measures and records the transmittance at the preset zero-velocity frequency point at the center of the linear operating region and at the edge frequency point. Based on the linear assumption of the frequency discrimination curve, a new frequency discrimination curve can be derived.

Description

Technical Field

[0001] This application relates to the field of laser remote sensing technology, and in particular to a system, method and apparatus for frequency discrimination curve calibration. Background Technology

[0002] In the field of laser remote sensing of wind fields, direct detection technology based on edge detection mainly relies on optical frequency discriminators to extract Doppler frequency shift information, and then infer the line-of-sight wind speed. An optical frequency discriminator is an optical device with steep monotonic transmittance characteristics, and the calibration of its monotonic transmittance characteristic curve (frequency discriminant curve) is an important part of the lidar system.

[0003] Traditional methods employ tunable lasers and high-precision spectrometers for offline, non-real-time calibration of frequency discrimination curves. However, this method is time-consuming and labor-intensive, and the calibration results are unstable, easily leading to measurement system errors. Therefore, improving the accuracy and stability of frequency discrimination curve calibration has become an urgent problem to be solved. Summary of the Invention

[0004] To address the aforementioned issues, this application provides a system, method, and apparatus for frequency discrimination curve calibration, thereby improving the accuracy and stability of frequency discrimination curve calibration.

[0005] This application discloses a frequency discrimination curve calibration system, including: a first optical path, a second optical path, a third optical path, a fourth optical path, a fifth optical path, a first photodetector, a second photodetector, a third photodetector, and a frequency locking module;

[0006] The first optical path is emitted from the seed laser, reflected by the first beam splitter to the first mirror, and then sequentially passes through the electro-optic modulator, the atomic / molecular absorption cell, and is incident on the first photodetector.

[0007] The first photodetector is used to receive the first laser signal obtained by the atomic / molecular absorption cell from the first optical path, and to obtain the first photoelectric signal based on the first laser signal;

[0008] The frequency locking module is used to receive the first photoelectric signal and generate a control signal so that the frequency of the laser output by the seed laser is locked to the absorption peak of the atomic / molecular absorption cell.

[0009] The second optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the first acousto-optic modulator, coupler, laser amplifier, telescope, and second reflector to reach the second beam splitter, and is reflected by the second beam splitter to the second photodetector.

[0010] The third optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the second acousto-optic modulator, the coupler, the laser amplifier, the telescope, and the second reflector, reaches the second beam splitter, and is reflected by the second beam splitter to the second photodetector.

[0011] The second photodetector is used to receive a first atmospheric echo signal obtained by the second optical path and a second atmospheric echo signal obtained by the third optical path, and to obtain a second photoelectric signal based on the first atmospheric echo signal and a third photoelectric signal based on the second atmospheric echo signal.

[0012] The fourth optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the first acousto-optic modulator, the coupler, the laser amplifier, the telescope, and the second reflector, reaches the second beam splitter, and passes through the second beam splitter to the optical frequency discriminator, and is then input to the third photodetector.

[0013] The fifth optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the second acousto-optic modulator, the coupler, the laser amplifier, the telescope, and the second reflector, reaches the second beam splitter, and passes through the second beam splitter to the optical frequency discriminator, and is then input to the third photodetector.

[0014] The third photodetector is used to receive the third atmospheric echo signal obtained by the fourth optical path and the fourth atmospheric echo signal obtained by the fifth optical path, and to obtain the fourth photoelectric signal based on the third atmospheric echo signal and the fifth photoelectric signal based on the fourth atmospheric echo signal.

[0015] Optionally, the seed laser is a tunable single-frequency continuous wave laser, and the frequency of the laser output by the seed laser is tuned by the control signal.

[0016] Optionally, the atomic / molecular absorption cell is filled with a low-concentration, high-purity gas, the atomic / molecular energy level structure of which provides absorption peaks within the operating wavelength range of the lidar system.

[0017] Optionally, the electronically controlled optical switch is a gating switch with one input terminal and two output terminals.

[0018] Optional,

[0019] The first acousto-optic modulator is used to bias the incident seed laser in the second optical path and the fourth optical path with a first frequency shift and chop it into a pulse output.

[0020] The second acousto-optic modulator is used to bias the incident seed laser in the third and fifth optical paths with a second frequency shift and chop it into a pulse output.

[0021] Optionally, the telescope's pointing is electrically adjustable.

[0022] Optionally, the optical frequency discriminator has a monotonically linear frequency discrimination curve characteristic within the operating frequency range of the lidar system.

[0023] Optional,

[0024] The first frequency shift is the frequency shift between the initial zero-velocity frequency point at the center of the linear frequency discrimination curve of the optical frequency discriminator and the absorption peak of the atomic / molecular absorption cell.

[0025] The second frequency shift is the frequency shift between the frequency point at the edge of the linear frequency discrimination curve of the optical frequency discriminator and the absorption peak of the atomic / molecular absorption cell.

[0026] Based on the aforementioned frequency discrimination curve calibration system, this application also discloses a frequency discrimination curve calibration method applied to the aforementioned system, the method comprising:

[0027] Control the telescope to point vertically towards the zenith;

[0028] Based on the first optical path, the second optical path, the third optical path, the fourth optical path, and the fifth optical path, the first photoelectric signal, the second photoelectric signal, the third photoelectric signal, the fourth photoelectric signal, and the fifth photoelectric signal are obtained;

[0029] Obtain a first ratio between the fourth photoelectric signal and the second photoelectric signal, and obtain a second ratio between the fifth photoelectric signal and the third photoelectric signal;

[0030] Based on the first ratio and the second ratio, the frequency discrimination curve is calculated to complete the calibration.

[0031] Based on the above-mentioned frequency discrimination curve calibration system, this application also discloses a frequency discrimination curve calibration device, including: a control unit, an acquisition unit, a calculation unit, and a calibration unit;

[0032] The control unit is used to control the telescope to point vertically toward the zenith;

[0033] The acquisition unit is used to obtain a first photoelectric signal, a second photoelectric signal, a third photoelectric signal, a fourth photoelectric signal, and a fifth photoelectric signal based on the first optical path, the second optical path, the third optical path, the fourth optical path, and the fifth optical path;

[0034] The calculation unit is used to obtain a first ratio of the fourth photoelectric signal to the second photoelectric signal, and to obtain a second ratio of the fifth photoelectric signal to the third photoelectric signal;

[0035] The calibration unit is used to calculate the frequency discrimination curve based on the first ratio and the second ratio to complete the calibration.

[0036] This application discloses a system, method, and apparatus for frequency discrimination curve calibration. Frequency locking technology is employed to stably lock the output laser frequency of the seed laser onto the absorption peak of the atomic / molecular absorption cell, effectively solving the problem of seed laser frequency drift. Simultaneously, two acousto-optic modulators are used to perform precise extracavity passive frequency modulation of the seed laser, shifting the laser frequency from the atomic / molecular absorption peak position to the preset zero-velocity frequency point and the edge frequency point of the linear operating region of the optical frequency discriminator. During calibration, the telescope is vertically pointed towards the zenith, providing atmospheric echoes with zero Doppler shift as a reference. At this time, the optical frequency discriminator measures and records the transmittance at the preset zero-velocity frequency point at the center of the linear operating region and at the edge frequency point. Based on the linear assumption of the frequency discrimination curve, a new frequency discrimination curve can be derived. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0038] Figure 1a This is a schematic diagram of the structure of a frequency discrimination curve calibration system disclosed in an embodiment of this application;

[0039] Figure 1b This is a diagram showing the positional relationship of the frequency shift points used for frequency discrimination curve calibration as disclosed in the embodiments of this application;

[0040] Figure 2 This is a flowchart illustrating a frequency discrimination curve calibration method disclosed in an embodiment of this application;

[0041] Figure 3 This is a schematic diagram illustrating the measurement error caused by variations in the linear frequency discrimination curve disclosed in the embodiments of this application. Detailed Implementation

[0042] In order to ensure that the system, method and apparatus for frequency discrimination curve calibration described in this application can be accurately understood, a brief overview of the basic knowledge in this field is provided first.

[0043] In the field of laser remote sensing, the horizontal axis of the frequency discrimination curve of an optical frequency discriminator represents the frequency or wavelength of the laser, while the vertical axis is determined by the ratio of the signal amplitudes measured by the photodetectors located at the input and output ports of the discriminator. Typically, plotting a frequency discrimination curve requires setting up a dedicated calibration optical path, using a tunable laser and a high-precision spectrometer. The laser's output frequency is gradually adjusted, and the current laser frequency or wavelength is recorded (as the horizontal axis data of the frequency discrimination curve). The signal amplitudes detected by the two photodetectors are also collected and stored (the ratio of these amplitudes is the vertical axis data of the frequency discrimination curve). This calibration process involves adjusting the optical path and using additional equipment, and is an offline and non-real-time calibration method.

[0044] Subsequently, during the system detection phase, the lidar calculates the Doppler frequency shift based on the transmittance of atmospheric echo signal light through the optical frequency discriminator, combined with pre-saved frequency discrimination curve data, thereby retrieving atmospheric wind field information.

[0045] The aforementioned methods are time-consuming and labor-intensive. It is desirable for the effective detection time between calibrations of a lidar system to be as long as possible, meaning the validity period of a single calibration should be sufficiently long. However, in reality, various factors can lead to calibration failure, thereby introducing measurement system errors. For example, the laser may experience frequency drift during non-calibration periods, causing the transmittance of the laser through the optical frequency discriminator to no longer equal the zero-velocity value during calibration. This is equivalent to a horizontal shift of the zero-velocity operating point on the frequency discrimination curve; this type of error is called summation error. Summation error can be corrected by monitoring the frequency change of the laser emitted and subtracting the laser's frequency shift from the uncorrected Doppler shift.

[0046] However, another type of error exists, such as changes in the detection performance of the photodetectors at both ends of the optical frequency discriminator (e.g., device aging, unstable gain control voltage, etc.) and changes in the performance of the beam splitter before the input port of the frequency discriminator (e.g., mechanical stress release, changes in the splitting ratio caused by changes in ambient temperature, etc.). These factors cause changes in the transmittance ratio of the optical frequency discriminator to the incident light intensity for the same frequency, i.e., changes in the slope of the frequency discrimination curve. This type of error is called multiplicative error and lacks effective direct monitoring methods. Furthermore, changes in the optical cavity temperature of the optical frequency discriminator can simultaneously introduce both summative and multiplicative errors, making them even more difficult to directly monitor and correct.

[0047] To address the aforementioned problems, this application aims to resolve the issues of frequent, time-consuming, labor-intensive, and costly offline calibration methods for frequency discrimination curves in direct-detection wind lidar, as well as the inability to effectively monitor and correct the system's frequency discrimination performance during the calibration interval. Based on the linear assumption of the frequency discrimination curve, this application proposes a low-cost, convenient, and accurate online calibration method for real-time frequency discrimination curves.

[0048] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0049] Example 1: This application discloses a system for frequency discrimination curve calibration.

[0050] For details, please refer to Figure 1a The frequency discrimination curve calibration system disclosed in this embodiment includes: a first optical path, a second optical path, a third optical path, a fourth optical path, a fifth optical path, a first photodetector 601, a second photodetector 602, a third photodetector 603, and a frequency locking module 7.

[0051] In the system described in this embodiment, the first optical path is emitted from the seed laser 1, and a portion of the laser beam is reflected by the first beam splitter 201 to the first reflector 301. The reflected beam then passes sequentially through the electro-optic modulator 4 and the atomic / molecular absorption cell 5, and finally enters the first photodetector 601.

[0052] As an alternative method, the seed laser 1 is a tunable single-frequency continuous-wave laser, the frequency of which is tuned by a control signal. The electro-optic modulator 4 is a Pockel cell, driven by an external sinusoidal signal, which modulates the phase of the electric field of the incident laser beam.

[0053] As an alternative method, the atomic / molecular absorption cell 5 is filled with a high-purity, low-pressure gas whose atomic / molecular energy level structure provides a stable absorption peak within the laser's operating wavelength range for frequency locking. For example, an iodine vapor absorption cell provides a frequency-locking absorption peak for the frequency-second harmonic 532nm green light of a common Nd:YAG solid-state laser. Specifically, such as... Figure 1b As shown, the transmittance curve of atomic / molecular absorption cell 5 shows a dip in transmittance caused by gas molecule absorption.

[0054] In the system described in this embodiment, the first photodetector 601 is used to receive the first laser signal obtained from the first optical path by the atomic / molecular absorption cell 5, and to obtain a first photoelectric signal based on the first laser signal. The frequency locking module 7 is used to receive the first photoelectric signal and generate a control signal. This control signal acts on the seed laser 1 so that the frequency of its output laser is locked to a certain absorption peak of the atomic / molecular absorption cell 5. (Specifically, as shown in the example...) Figure 1b (ν0 frequency point)

[0055] In the system described in this embodiment, the second optical path originates from the seed laser 1, passes through the first beam splitter 201, transmits a portion of the laser light to the electro-optical switch 8, and then enters the first acousto-optic modulator 901. A frequency-shifted low-energy pulsed light is output from the first acousto-optic modulator 901, enters the coupler 10, then enters the laser amplifier 11 for amplification, and is subsequently emitted into the atmosphere by the telescope 12 for atmospheric remote sensing, obtaining the first atmospheric echo signal. The light continues to transmit, passing through the second reflector 302 to the second beam splitter 202, where a portion of the light is reflected to the second photodetector 602.

[0056] As an alternative approach, the electro-optical switch 8 is a gating switch, which is a micromechanical fiber optic device with one input and two outputs. An external electrical signal can quickly control the gating switch state of the device.

[0057] In the system described in this embodiment, the third optical path originates from the seed laser 1, passes through the first beam splitter 201, transmits a portion of the laser light to the electro-optical switch 8, and then enters the second acousto-optic modulator 902. The second acousto-optic modulator 902 outputs a frequency-shifted low-energy pulse light, which passes through the coupler 10 and the laser amplifier 11, and reaches the telescope 12. The telescope 12 then emits the pulse light into the atmosphere for atmospheric remote sensing, obtaining the second atmospheric echo signal. The signal continues to be transmitted, passes through the second reflector 302 to the second beam splitter 202, and is reflected by the second beam splitter 202 to the second photodetector 602.

[0058] In the system described in this embodiment, the second photodetector 602 is used to receive the first atmospheric echo signal obtained from the second optical path and the second atmospheric echo signal obtained from the third optical path. It then obtains the second photoelectric signal based on the first atmospheric echo signal and the third photoelectric signal based on the second atmospheric echo signal. The first atmospheric echo signal corresponds to the second photoelectric signal, and the second atmospheric echo signal corresponds to the third photoelectric signal.

[0059] In the system described in this embodiment, the fourth optical path originates from the seed laser 1, passes through the first beam splitter 201, transmits a portion of the laser light to the electro-optical switch 8, and then sequentially passes through the first acousto-optic modulator 901, coupler 10, laser amplifier 11, and telescope 12 for atmospheric remote sensing. The third atmospheric echo signal collected by the telescope 12 continues to be transmitted through the second reflector 302, reaches the second beam splitter 202, and then transmits a portion of the laser light through the second beam splitter 202 to the optical frequency discriminator 13. After passing through the optical frequency discriminator 13, the light is input to the third photodetector 603.

[0060] In the system described in this embodiment, the fifth optical path originates from the seed laser 1, passes through the first beam splitter 201, is transmitted to the electro-optical switch 8, and then sequentially passes through the second acousto-optic modulator 902, coupler 10, laser amplifier 11, and telescope 12 for atmospheric remote sensing. The fourth atmospheric echo signal collected by the telescope 12 continues to be transmitted through the second reflector 302, reaches the second beam splitter 202, and a portion of the laser light is transmitted through the second beam splitter 202 to the optical frequency discriminator 13. After passing through the optical frequency discriminator 13, the light is input to the third photodetector 603.

[0061] In the system described in this embodiment, the third photodetector 603 is used to receive the third atmospheric echo signal obtained from the fourth optical path and the fourth atmospheric echo signal obtained from the fifth optical path. It then obtains a fourth photoelectric signal based on the third atmospheric echo signal and a fifth photoelectric signal based on the fourth atmospheric echo signal. The third atmospheric echo signal corresponds to the fourth photoelectric signal, and the fourth atmospheric echo signal corresponds to the fifth photoelectric signal.

[0062] As an alternative method, the optical frequency discriminator 13 exhibits a monotonically linear frequency discrimination curve characteristic within the operating frequency range of the lidar system. Specifically, as... Figure 1b The linear operating range is shown in the figure. The frequency discrimination curve involved in edge techniques for direct wind measurement is monotonic, increasing or decreasing monotonically with increasing incident laser frequency. Thus, the incident light frequency change can be calculated from the measured transmittance change, thereby obtaining the Doppler frequency shift. Figure 1b The description only depicts the monotonically increasing case; similarly, the monotonically decreasing case is handled similarly in practice, and this embodiment does not impose limitations here. The frequency discrimination curve may exhibit nonlinearity over a wider frequency range; however, in actual operation, the selected operating frequency range is usually within its linear segment, in which case the frequency discrimination curve can be approximated as a sloping straight line. The method described in this embodiment is based on the linear assumption of the frequency discrimination curve and therefore is not applicable to the nonlinear region of the frequency discrimination curve.

[0063] As an optional method, the first acousto-optic modulator 901 biases the incident continuous-wave seed laser frequency with a first frequency shift and chops it into a pulse output. The second acousto-optic modulator 902 biases the incident continuous-wave laser frequency with a second frequency shift and chops it into a pulse output. The first frequency shift is the frequency shift between the initial zero-velocity frequency point at the center of the linear frequency discrimination curve of the optical frequency discriminator 13 and the absorption peak of the atomic / molecular absorption cell 5. The second frequency shift is the frequency shift between the frequency point at the edge of the linear frequency discrimination curve of the optical frequency discriminator 13 and the absorption peak of the atomic / molecular absorption cell 5. Specifically, the relative positions of the first frequency shift Δν1 and the second frequency shift Δν2 are as follows: Figure 1b As shown.

[0064] The system described in this embodiment aims to address the shortcomings of current offline calibration methods for frequency discrimination curves in direct-detection wind lidar. These methods require periodic repetition, are time-consuming and labor-intensive, and have high maintenance costs. Furthermore, they lack real-time monitoring and adjustment of the system's frequency discrimination performance between calibrations. Therefore, this embodiment proposes a low-cost, efficient, and convenient real-time online calibration method based on the linear assumption of the frequency discrimination curve. This method uses frequency locking technology to stably lock the laser output onto the atomic or molecular absorption lines of the absorption cell, solving the frequency drift problem. Simultaneously, two acousto-optic modulators are used to precisely perform external passive frequency modulation, enabling the laser frequency to accurately move from the absorption peak to the preset zero-velocity frequency point and the linear operating region boundary of the system's optical frequency discriminator. During calibration, the system telescope is vertically aligned with the zenith to acquire atmospheric echoes with zero Doppler shift as a reference. The optical frequency discriminator measures and records the transmittance at these two frequency points, thereby calculating a new frequency discrimination curve.

[0065] Based on the aforementioned frequency discrimination curve calibration system, this embodiment discloses a corresponding frequency discrimination curve calibration method applied to the above system. Please refer to [link to relevant documentation]. Figure 2 The method for frequency discrimination curve calibration includes:

[0066] S1: Control the telescope to point vertically towards the zenith.

[0067] In the method described in this embodiment, when the telescope is pointed vertically at the zenith, since the wind field component in the direction perpendicular to the zenith is almost zero, the laser beam is emitted vertically upwards, and its atmospheric echo signal light has zero Doppler frequency shift. Such echo signal light can be used to calibrate the frequency discrimination characteristic curve of an optical frequency discriminator. The telescope's pointing is electrically controlled. The method described in this embodiment using a single telescope is for ease of understanding only. In actual work, there are many different implementation methods, such as using three fixed-pointing telescopes, one pointing vertically at the zenith for calibration, and the other two tilted at a certain zenith angle orthogonally pointing for horizontal wind field measurement, with the emitted laser beam alternating in the three directions, etc. This embodiment does not specifically limit the number or type of telescopes; anything that can complete the method described in this embodiment is acceptable.

[0068] S2: Based on the first optical path, the second optical path, the third optical path, the fourth optical path, and the fifth optical path, the first photoelectric signal, the second photoelectric signal, the third photoelectric signal, the fourth photoelectric signal, and the fifth photoelectric signal are obtained.

[0069] S3: Obtain the first ratio of the fourth photoelectric signal to the second photoelectric signal, and obtain the second ratio of the fifth photoelectric signal to the third photoelectric signal.

[0070] In the method described in this embodiment, the ratio of the amplitudes of the photoelectric signals detected by the two photodetectors is the vertical axis data of the frequency discrimination curve.

[0071] S4: Calculate the frequency discrimination curve based on the first ratio and the second ratio to complete the calibration.

[0072] In the method described in this embodiment, the electrically controlled telescope changes its direction, for example, tilting it to point due north or due south at a non-zero zenith angle. The second and fourth photoelectric signals are then obtained again based on the second and fourth optical paths. The ratio of the fourth photoelectric signal to the second photoelectric signal is calculated. Based on this ratio and the frequency discrimination curve obtained in step S4, the current Doppler frequency shift is obtained, and the line-of-sight wind speed in the tilted pointing direction is obtained according to the Doppler velocity measurement principle.

[0073] Accordingly, the electrically controlled telescope changes its direction, for example, tilting it to point due west or due east at a non-zero zenith angle. The second and fourth photoelectric signals are then obtained again based on the second and fourth optical paths. Similarly, the ratio of the fourth photoelectric signal to the second photoelectric signal is calculated again. Based on this ratio and the frequency discrimination curve obtained in step S4, the current Doppler frequency shift is obtained, and the line-of-sight wind speed in the tilted direction is obtained according to the Doppler velocimetry principle.

[0074] The horizontal wind speed can then be obtained based on the line-of-sight wind speeds from due north (or due south) and due west (or due east). By repeating these steps, the real-time online calibration of the frequency discrimination curve and the wind field measurement are completed.

[0075] As an optional method, Figure 3 This is a schematic diagram illustrating the measurement error caused by variations in the linear frequency discrimination curve disclosed in an embodiment of this application. Figure 3 This more clearly illustrates the error in system measurement caused by not performing frequency discrimination curve calibration, and the principle of linear frequency discrimination curve calibration involved in this application. Figure 3 The example only depicts the case where the frequency discrimination curve monotonically increases with increasing incident laser frequency. In practical applications, the case of monotonically decreasing curves is similar, and this embodiment does not impose limitations on it. Figure 3 As shown, the straight line The frequency discrimination curve represents the linear operating region during the last calibration. Point A is the initial zero-velocity operating point, located in the central region of the linear operating region of the frequency discrimination curve, while point B is located at the edge of the linear operating region.

[0076] To determine the abscissa frequencies of points A and B, the entire lidar system needs to use the traditional offline calibration method of the frequency discrimination curve to record image measurements over a wider range. Then, the linear operating range, the zero-velocity frequency ν1, and the edge frequency ν2 are determined, thus obtaining the first frequency shift Δν1 = ν1 - ν0 and the second frequency shift Δν2 = ν2 - ν0. The frequency shift of the acousto-optic modulator is set by an external RF driver and will not change. The absorption peak frequency ν0 is determined by the energy level structure of the atomic or molecular frequency standard used, and the laser frequency is locked to it without drift. The vertical component of the atmospheric wind field is very small. If stratospheric atmospheric echoes are chosen for calibration, the vertical component of the wind speed can be considered zero. In this case, the echo signal has no Doppler frequency shift, and the echo light frequency is equal to the emitted laser frequency.

[0077] In the method described in this embodiment, the electro-optical switch first selects the first acousto-optic modulator, locking the frequency of the vertically emitted laser at the zero-velocity frequency point ν1, so the frequency of the atmospheric echo signal light is also ν1. Let the amplitude of the second photoelectric signal be S at this time. A1 The amplitude of the fourth photoelectric signal is S A2 Then the ordinate of point A on the frequency discrimination curve is T. A =S A2 / S A1 Then, the electro-optical switch selects the second acousto-optic modulator, locking the frequency of the vertically emitted laser at the edge frequency ν2. The frequency of the atmospheric echo signal is also ν2. Let the amplitude of the third photoelectric signal at this time be S. B1 The amplitude of the fifth photoelectric signal is S B2 Then the ordinate of point B on the frequency discrimination curve is T. B =S B2 / S B1 Based on the ordinates and abscissas of points A and B, the equation of the frequency discrimination curve is obtained as follows:

[0078]

[0079] At this point, the calibration of the frequency discrimination curve is completed. Subsequently, the lidar system enters the detection state, calculates the transmittance based on the signal amplitudes of the two photodetectors, and then calculates the echo signal frequency according to formula (1) to obtain the Doppler frequency and inversely determine the line-of-sight wind speed.

[0080] In the method described in this embodiment, the performance of devices such as the optical frequency discriminator, two photodetectors, and the second beam splitter changes over time. When the frequency discrimination curve is not updated, the calculated Doppler frequency shift will produce errors. Figure 3 As shown, This represents the current actual frequency discrimination curve after the system has been running for a period of time. The zero velocity point has changed from point A to point A', and the corresponding transmittance should be changed from T... A Transform into TA However, if the frequency discrimination curve is not updated and the old frequency discrimination curve is still used to calculate the frequency shift, then the transmittance T will be... A The corresponding atmospheric echo frequency will be incorrectly identified as the working point A, and the corresponding atmospheric echo frequency will be incorrectly calculated as ν1', which will result in an error of ν1'-ν1.

[0081] The method described in this embodiment first uses a traditional offline calibration method to determine the linear operating frequency range and the frequency shift of the two acousto-optic modulators. Then, an electrically controlled optical switch enables flexible switching between the system calibration and detection states, thereby achieving real-time online calibration of the linear frequency discrimination curve. In this method, the laser frequency emitted by the laser is locked to the absorption peak of gas molecules, effectively solving the long-term frequency drift problem of the laser. By switching the telescope pointing and the state of the electrically controlled optical switch, the system can seamlessly switch between detection and calibration modes, updating the frequency discrimination curve in real time. Using this real-time frequency discrimination curve for wind field inversion can significantly reduce measurement errors caused by the instability of the optical frequency discriminator, thereby improving measurement accuracy.

[0082] Based on the frequency discrimination curve calibration system disclosed in the above embodiments, this embodiment discloses a frequency discrimination curve calibration device, including: a control unit, an acquisition unit, a calculation unit, and a calibration unit;

[0083] The control unit is used to control the telescope to point vertically toward the zenith;

[0084] The acquisition unit is used to obtain a first photoelectric signal, a second photoelectric signal, a third photoelectric signal, a fourth photoelectric signal, and a fifth photoelectric signal based on the first optical path, the second optical path, the third optical path, the fourth optical path, and the fifth optical path;

[0085] The calculation unit is used to obtain a first ratio of the fourth photoelectric signal to the second photoelectric signal, and to obtain a second ratio of the fifth photoelectric signal to the third photoelectric signal;

[0086] The calibration unit is used to calculate the frequency discrimination curve based on the first ratio and the second ratio to complete the calibration.

[0087] The embodiments in this specification are described in a progressive manner. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant details can be found in the method section.

[0088] It should also be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0089] The steps of the methods or algorithms described in conjunction with the embodiments disclosed herein can be implemented directly by hardware, a software module executed by a processor, or a combination of both. The software module can be located in random access memory (RAM), main memory, read-only memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, registers, hard disk, removable disk, CD-ROM, or any other form of storage medium known in the art.

[0090] The features described in the embodiments of this specification can be substituted for or combined with each other, so that those skilled in the art can implement or use this application.

[0091] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A system for frequency discrimination curve calibration, characterized in that, include: The optical path consists of a first optical path, a second optical path, a third optical path, a fourth optical path, a fifth optical path, a first photodetector, a second photodetector, a third photodetector, and a frequency locking module. The first optical path is emitted from the seed laser, reflected by the first beam splitter to the first mirror, and then sequentially passes through the electro-optic modulator and the atomic / molecular absorption cell before being incident on the first photodetector. The first photodetector is used to receive the first laser signal obtained by the atomic / molecular absorption cell from the first optical path, and to obtain the first photoelectric signal based on the first laser signal; The frequency locking module is used to receive the first photoelectric signal and generate a control signal so that the frequency of the laser output by the seed laser is locked to the absorption peak of the atomic / molecular absorption cell. The second optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the first acousto-optic modulator, coupler, laser amplifier, telescope, and second reflector to reach the second beam splitter, and is reflected by the second beam splitter to the second photodetector. The third optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the second acousto-optic modulator, the coupler, the laser amplifier, the telescope, and the second reflector, reaches the second beam splitter, and is reflected by the second beam splitter to the second photodetector. The second photodetector is used to receive a first atmospheric echo signal obtained by the second optical path and a second atmospheric echo signal obtained by the third optical path, and to obtain a second photoelectric signal based on the first atmospheric echo signal and a third photoelectric signal based on the second atmospheric echo signal. The fourth optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the first acousto-optic modulator, the coupler, the laser amplifier, the telescope, and the second reflector, reaches the second beam splitter, and passes through the second beam splitter to the optical frequency discriminator, and is then input to the third photodetector. The fifth optical path originates from the seed laser, passes through the first beam splitter to the electro-optical switch, then sequentially passes through the second acousto-optic modulator, the coupler, the laser amplifier, the telescope, and the second reflector, reaches the second beam splitter, and passes through the second beam splitter to the optical frequency discriminator, and is then input to the third photodetector. The third photodetector is used to receive the third atmospheric echo signal obtained by the fourth optical path and the fourth atmospheric echo signal obtained by the fifth optical path, and to obtain the fourth photoelectric signal based on the third atmospheric echo signal and the fifth photoelectric signal based on the fourth atmospheric echo signal. The first acousto-optic modulator is used to bias the incident seed laser in the second optical path and the fourth optical path with a first frequency shift and chop it into a pulse output. The second acousto-optic modulator is used to bias the incident seed laser in the third optical path and the fifth optical path with a second frequency shift and chop it into a pulse output. The first frequency shift is the frequency shift between the initial zero-velocity frequency point at the center of the linear frequency discrimination curve of the optical frequency discriminator and the absorption peak of the atomic / molecular absorption cell. The second frequency shift is the frequency shift between the frequency point at the edge of the linear frequency discrimination curve of the optical frequency discriminator and the absorption peak of the atomic / molecular absorption cell.

2. The system according to claim 1, characterized in that, The seed laser is a tunable single-frequency continuous wave laser, and the frequency of the laser output by the seed laser is tuned by the control signal.

3. The system according to claim 1, characterized in that, The atomic / molecular absorption cell is filled with a low-concentration, high-purity gas, the atomic / molecular energy level structure of which provides absorption peaks within the operating wavelength range of the lidar system.

4. The system according to claim 1, characterized in that, The electro-optical switch is a gating switch with one input terminal and two output terminals.

5. The system according to claim 1, characterized in that, The telescope's pointing is electrically adjustable.

6. The system according to claim 1, characterized in that, The optical frequency discriminator exhibits a monotonically linear frequency discrimination curve characteristic within the operating frequency range of the lidar system.

7. A method for frequency discrimination curve calibration, characterized in that, Applied to the system according to any one of claims 1-6, the method comprises: Control the telescope to point vertically towards the zenith; Based on the first optical path, the second optical path, the third optical path, the fourth optical path, and the fifth optical path, the first photoelectric signal, the second photoelectric signal, the third photoelectric signal, the fourth photoelectric signal, and the fifth photoelectric signal are obtained; Obtain a first ratio between the fourth photoelectric signal and the second photoelectric signal, and obtain a second ratio between the fifth photoelectric signal and the third photoelectric signal; Based on the first ratio and the second ratio, the frequency discrimination curve is calculated to complete the calibration.

8. A device for frequency discrimination curve calibration, characterized in that, The device, applicable to the system according to any one of claims 1-6, comprises: a control unit, an acquisition unit, a calculation unit, and a calibration unit; The control unit is used to control the telescope to point vertically toward the zenith; The acquisition unit is used to obtain a first photoelectric signal, a second photoelectric signal, a third photoelectric signal, a fourth photoelectric signal, and a fifth photoelectric signal based on the first optical path, the second optical path, the third optical path, the fourth optical path, and the fifth optical path; The calculation unit is used to obtain a first ratio of the fourth photoelectric signal to the second photoelectric signal, and to obtain a second ratio of the fifth photoelectric signal to the third photoelectric signal; The calibration unit is used to calculate the frequency discrimination curve based on the first ratio and the second ratio to complete the calibration.

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