Laser radar gas concentration calibration method and system based on differential absorption

Abstract

The application provides a differential absorption based laser radar gas concentration calibration method and system, relates to the field of environmental monitoring, and solves the technical problem of low detection sensitivity leading to large concentration inversion error. The method comprises: chopping and beam combining processing of emitted dual-wavelength laser, proportional beam splitting into measurement light and reference light after amplification; the measurement light is emitted to the atmosphere, and the reference light enters the optical fiber delay line; the measurement light after atmospheric attenuation and the delayed reference light are combined, separated into four electric pulses and the number of photons is counted; the number of photons is normalized and preprocessed by filtering, the product of the absorption coefficient difference and the concentration is inverted based on the Beer-Lambert law; and finally, the gas concentration is calculated according to the pre-calibrated function relationship. The application is applied to the concentration detection process of methane and other gases, and can realize high-precision, long-distance real-time detection of gas concentration in industrial emission monitoring, atmospheric environment research and other scenes.

Description

Technical Field

[0001] The present invention belongs to the field of atmospheric environment monitoring, and specifically relates to a lidar gas concentration calibration method based on differential absorption. Background Art

[0002] In the field of atmospheric environmental monitoring, gas concentration detection technology based on the principle of differential absorption lidar (DIAL) is often used for real-time, high-precision measurement of gases such as methane. It can be applied in scenarios such as industrial emission monitoring, environmental safety warnings, and greenhouse gas source tracing. Traditional differential absorption lidars typically modulate the output of a semiconductor laser through temperature and current, and lock the wavelength to the absorption peak wavelength of the gas to be measured and a reference wavelength through frequency locking, while attempting to ensure constant power. However, in actual applications, slight fluctuations in laser power and wavelength can still lead to gas concentration inversion errors, resulting in reduced detection sensitivity and inaccurate gas concentration detection. Summary of the Invention

[0003] The present application provides a lidar gas concentration calibration method based on differential absorption, which solves the technical problem of insufficient gas detection accuracy in the prior art.

[0004] To achieve the above objectives, this application adopts the following technical solutions:

[0005] In a first aspect, a lidar gas concentration calibration method based on differential absorption is provided, comprising:

[0006] performing chopping processing and signal beam combining processing on the emitted first wavelength laser and second wavelength laser in sequence to obtain a first combined optical signal;

[0007] After amplifying and splitting the first combined optical signal, a measurement optical signal and a reference optical signal are obtained;

[0008] The measurement optical signal is emitted into the atmosphere, and the reference optical signal is input and output from the optical fiber delay line;

[0009] Combining the measurement optical signal attenuated by atmospheric transmission and the reference optical signal delayed by the optical fiber into a second combined optical signal;

[0010] Separating the second combined optical signal into four electrical pulses and counting the number of photons in each electrical pulse;

[0011] Normalizing and filtering the photon number statistics of each electrical pulse to obtain a first value;

[0012] The first value is input into a pre-calibrated functional relationship to determine the concentration of the gas to be measured, wherein the pre-calibrated functional relationship is a functional relationship between the number of photons and the concentration of the gas to be measured.

[0013] Based on the above technical solution, in the differential absorption-based lidar gas concentration calibration method provided in this application, by sequentially chopping and combining the first and second wavelength lasers, combined with the design of atmospheric transmission of the measurement light and fiber delay of the reference light, a dual-wavelength differential absorption + reference light real-time calibration system is constructed. The synchronous transmission of the measurement light and the reference light can effectively offset slow-varying interference such as laser power fluctuations and optical system losses. Then, the optical signals transmitted through different paths are combined into a second combined optical signal and separated into four electrical pulses. The number of photons is counted using a single-photon detector, realizing photon-level weak signal detection, greatly improving the system's adaptability to low-concentration gas and long-distance detection scenarios. The normalization and filtering operations in the preprocessing link can further eliminate random interference such as atmospheric turbulence and circuit noise. Combined with the physical model inversion of the Beer-Lambert law and the pre-calibrated functional relationship, a more accurate gas concentration can be obtained, providing a high-precision and high-robust solution for lidar gas concentration measurement.

[0014] Furthermore, the splitting ratio of the measurement light signal to the reference light signal is 1:9999, and the measurement light signal accounts for 99.99% and is used to detect the absorption attenuation of gases in the atmosphere. The reference light signal accounts for 0.01% and is used to calibrate the output power of the laser, which is used to emit a first wavelength laser and a second wavelength laser.

[0015] Furthermore, separating the second combined optical signal into four electrical pulses includes:

[0016] using a single-photon detector to convert the second combined optical signal into discrete electrical pulses;

[0017] The arrival times of discrete electrical pulses are recorded using a time-to-digital converter;

[0018] The discrete electrical pulses are separated into a first electrical pulse, a second electrical pulse, a third electrical pulse, and a fourth electrical pulse according to the arrival time.

[0019] Furthermore, the four electric pulses include a first electric pulse, a second electric pulse, a third electric pulse and a fourth electric pulse, wherein:

[0020] The first electrical pulse represents an echo pulse of a first wavelength in the measurement optical signal, the second electrical pulse represents an echo pulse of a second wavelength in the measurement optical signal, the third electrical pulse represents a direct pulse of a first wavelength in the reference optical signal, and the fourth electrical pulse represents a direct pulse of a second wavelength in the reference optical signal.

[0021] Furthermore, the relationship between the first wavelength laser and the gas to be measured is: the absorption peak wavelength of the first wavelength laser coincides with that of the gas to be measured, and the gas concentration is detected through the resonance absorption effect;

[0022] The relationship between the second wavelength laser and the gas to be measured is: the second wavelength is located at the wavelength position where the absorption cross section of the gas to be measured is zero, and is used to calibrate atmospheric background interference and instrument parameter fluctuations.

[0023] Furthermore, the pre-processing of the photon number statistics of each electrical pulse includes:

[0024] Calculating the ratio of the number of photons of the first electric pulse to the number of photons of the second electric pulse to obtain a first normalized result;

[0025] Calculating the ratio of the number of photons of the third electric pulse to the number of photons of the fourth electric pulse to obtain a second normalized result;

[0026] Calculate the ratio of the first normalized result to the second normalized result to obtain a third normalized result;

[0027] The third normalized result is filtered to obtain a statistical value of the number of photons in each electric pulse after preprocessing.

[0028] Furthermore, the functional relationship is obtained by pre-calibration through experiments, including:

[0029] pre-processing the number of photons of each electric pulse obtained from multiple groups of standard gases with known concentrations to obtain pre-processed values ​​of the multiple groups of standard gases;

[0030] The least squares method is used to fit the logarithmic values ​​A of multiple sets of known concentrations C and multiple sets of pre-processed values ​​of the standard gas to obtain a linear function: C=k×A+b; wherein, , N r (λ on ) represents the number of photons in the first electrical pulse, N t (λ off ) represents the number of photons in the fourth electrical pulse, N t (λ on ) represents the number of photons in the third electrical pulse, N r (λ off ) represents the number of photons in the second electrical pulse, and represents the first value, k represents the fitting slope, and b represents the fitting intercept.

[0031] Secondly, a lidar gas concentration calibration system based on differential absorption is provided, including:

[0032] The laser transmitter comprises a first laser transmitter and a second laser transmitter, wherein the first laser transmitter transmits a laser of a first wavelength for detecting the absorption effect of the gas to be measured, and the second laser transmitter transmits a laser of a second wavelength for calibrating the atmospheric background interference;

[0033] An acousto-optic modulator is configured to time-chop the first-wavelength laser and the second-wavelength laser to generate an alternating pulse sequence containing the first wavelength and the second wavelength;

[0034] A coupler includes a first coupler, a second coupler, and a third coupler;

[0035] The first coupler is configured to combine the alternating pulse sequence into a first combined optical signal,

[0036] The second coupler is configured to split the amplified optical signal into a reference optical signal and a measurement optical signal at a ratio of 1:9999;

[0037] The third coupler is configured to combine the reference optical signal transmitted through the optical fiber and the measurement optical signal transmitted through the atmosphere into a second combined optical signal;

[0038] A signal amplifier is configured to amplify the power of the first combined optical signal to obtain an amplified optical signal;

[0039] A single-photon detector is configured to convert the second combined optical signal into discrete electrical pulses;

[0040] A time-to-digital converter is configured to record the arrival time of the discrete electrical pulses;

[0041] A processing unit is configured to separate the discrete electrical pulses into a first electrical pulse, a second electrical pulse, a third electrical pulse, and a fourth electrical pulse according to the arrival time;

[0042] The number of photons in each electrical pulse is counted, and the concentration of the gas is calculated based on the pre-processing result of the number of photons in each electrical pulse and the function relationship between the number of photons and the concentration of the gas to be measured.

[0043] Further, the first laser emitter and the second laser emitter are connected in parallel to the first coupler, the first coupler, the signal amplifier, the second coupler, and a transmitting lens are connected in series, and the transmitting lens is configured to transmit the measurement optical signal output by the second coupler to the atmosphere.

[0044] Further, the second coupler is connected to the third coupler, and the reference optical signal output by the second coupler is transmitted to the third coupler through a delay optical fiber;

[0045] A receiving lens, the third coupler, the single-photon detector, and the time-to-digital converter are connected in series, and the receiving lens is configured to receive the measurement optical signal transmitted to the atmosphere and transmit the measurement optical signal to the third coupler.

[0046] In a third aspect, the present application provides a computer-readable storage medium, in which instructions are stored, and the computer-readable storage medium stores a computer program, which, when executed by a processor, implements the differential absorption-based lidar gas concentration calibration method as described in the first aspect.

[0047] The present application provides a lidar gas concentration calibration method and system based on differential absorption. By calibrating the laser output power in real time and using a reference light signal to directly monitor light source fluctuations, it can accurately correct the attenuation error caused by factors such as atmospheric transmission and optical loss, and effectively improve the accuracy of concentration calibration. The application of single-photon detection technology greatly reduces the interference of background noise on weak signals, and can achieve high-sensitivity detection at the photon level, which not only improves the signal reception efficiency, but also breaks through the traditional detection distance limitation and supports long-distance, low-concentration gas detection scenarios. In addition, the system continuously monitors and dynamically compensates for system parameter drift caused by environmental changes and equipment aging through periodic calibration combined with reference signal comparison, ensuring the stability and reliability of measurement results under long-term operation, and can provide high-precision and robust gas concentration detection solutions for industrial emission monitoring, atmospheric environment research and other fields.

[0048] It should be understood that the description of technical features, technical solutions, beneficial effects or similar language in this application does not imply that all features and advantages can be realized in any single embodiment. On the contrary, it is understood that the description of a feature or beneficial effect means that a specific technical feature, technical solution or beneficial effect is included in at least one embodiment. Therefore, the description of a technical feature, technical solution or beneficial effect in this specification does not necessarily refer to the same embodiment. Furthermore, the technical features, technical solutions and beneficial effects described in the present embodiment can also be combined in any appropriate manner. Those skilled in the art will understand that the embodiment can be implemented without one or more specific technical features, technical solutions or beneficial effects of a specific embodiment. In other embodiments, additional technical features and beneficial effects can also be identified in specific embodiments that do not embody all embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0050] Figure 1 This is a system architecture diagram of a differential absorption-based lidar gas concentration calibration system provided in an embodiment of the present application;

[0051] Figure 2 A schematic flow chart of a differential absorption-based lidar gas concentration calibration method provided in an embodiment of the present application;

[0052] Figure 3 A schematic flow chart of another differential absorption-based lidar gas concentration calibration method provided in an embodiment of the present application;

[0053] Figure 4 A schematic flow chart of another differential absorption-based lidar gas concentration calibration method provided in an embodiment of the present application;

[0054] Figure 5 A statistical graph of the number of pulse photons provided in an embodiment of the present application;

[0055] Figure 6 A statistical graph of the number of pulse photons after preprocessing provided in an embodiment of the present application;

[0056] Figure 7 A functional relationship diagram between the pre-calibrated logarithmic value A of the number of photons and the concentration C of the gas to be measured provided in an embodiment of the present application. DETAILED DESCRIPTION

[0057] In the description of this application, unless otherwise specified, " / " means "or", for example, A / B can mean A or B. "And / or" in this article is merely a description of the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, "at least one" means one or more, and "a plurality" means two or more. Words such as "first" and "second" do not limit the quantity and execution order, and words such as "first" and "second" do not limit them to be necessarily different.

[0058] It should be noted that, in this application, words such as "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described in this application as "exemplary" or "for example" should not be construed as being preferred or advantageous over other embodiments or designs. Rather, the use of words such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0059] The laser radar gas concentration calibration method based on differential absorption provided in the embodiment of the present application can be applied to Figure 1 In the lidar gas concentration calibration system based on differential absorption, as shown in Figure 1 As shown, the communication system includes: a laser transmitter, an acousto-optic modulator, a coupler, a signal amplifier, a single photon detector, a time-to-digital converter and a processing unit.

[0060] The laser emitter includes a first laser emitter and a second laser emitter, wherein the first laser emitter emits a first wavelength laser for detecting the absorption effect of the gas to be measured, and the second laser emitter emits a second wavelength laser for calibrating the atmospheric background interference;

[0061] an acousto-optic modulator, configured to perform time-sequential chopping of the first wavelength laser and the second wavelength laser to generate an alternating pulse sequence including the first wavelength and the second wavelength;

[0062] The coupler includes a first coupler, a second coupler, and a third coupler; wherein,

[0063] The first coupler is used to combine the alternating pulse sequence into a first combined optical signal,

[0064] The second coupler is used to split the amplified optical signal at a ratio of 1:9999 to obtain a reference optical signal and a measurement optical signal;

[0065] The third coupler is used to combine the reference optical signal after optical fiber delay and the measurement optical signal after atmospheric transmission attenuation to obtain a second combined optical signal;

[0066] a signal amplifier, configured to amplify the power of the first combined optical signal to obtain an amplified optical signal;

[0067] a single-photon detector, configured to convert the second combined optical signal into discrete electrical pulses;

[0068] a time-to-digital converter, which records the arrival times of discrete electrical pulses;

[0069] a processing unit, configured to separate the discrete electrical pulses into a first electrical pulse, a second electrical pulse, a third electrical pulse, and a fourth electrical pulse according to arrival times;

[0070] The number of photons of each electric pulse is counted, and the gas concentration is calculated based on the pre-processing result of the number of photons of each electric pulse and the pre-calibrated functional relationship between the number of photons and the concentration of the gas to be measured.

[0071] like Figure 1 As shown, in the gas concentration calibration system, the first laser emitter and the second laser emitter are connected in parallel to the first coupler, the first coupler, the signal amplifier, the second coupler and the transmitting lens are connected in series, and the transmitting lens is used to transmit the measurement light signal output by the second coupler to the atmosphere;

[0072] The second coupler is connected to the third coupler, and the reference optical signal output by the second coupler is transmitted to the third coupler via the delay optical fiber;

[0073] The receiving lens, the third coupler, the single photon detector and the time-to-digital converter are connected in series, and the receiving lens is used to receive the measurement light signal emitted into the atmosphere and transmit it to the third coupler.

[0074] To address the technical issues in the prior art where gas concentration measurement using lidar is susceptible to laser power fluctuations, atmospheric background interference, and system parameter drift, resulting in low measurement accuracy, weak long-range detection capabilities, and poor long-term monitoring stability, the present invention provides a lidar gas concentration calibration method based on differential absorption, which includes:

[0075] emitting a laser beam of a first wavelength and a laser beam of a second wavelength using a laser transmitter;

[0076] performing chopping processing and signal beam combining processing on the emitted first wavelength laser and second wavelength laser in sequence to obtain a first combined optical signal;

[0077] After amplifying and splitting the first combined optical signal, a measurement optical signal and a reference optical signal are obtained;

[0078] The measurement optical signal is emitted into the atmosphere, and the reference optical signal is input and output from the optical fiber delay line;

[0079] Combining the measurement optical signal attenuated by atmospheric transmission and the reference optical signal delayed by the optical fiber into a second combined optical signal;

[0080] Separating the second combined optical signal into four electrical pulses and counting the number of photons in each electrical pulse; preprocessing includes normalization and filtering;

[0081] The number of photons of each electric pulse is counted, and the gas concentration is calculated based on the pre-processing result of the number of photons of each electric pulse and the pre-calibrated functional relationship between the number of photons and the concentration of the gas to be measured.

[0082] In practical applications, traditional methods struggle to accurately invert gas concentrations due to factors such as unstable laser output power, optical signal attenuation caused by atmospheric turbulence and aerosol scattering, and parameter drift during device operation. Furthermore, detection distances are limited, and measurement results are prone to significant errors. This application effectively overcomes these issues through a dual-wavelength differential absorption mechanism, real-time power calibration, single-photon detection, and dynamic parameter compensation, improving the accuracy, sensitivity, and stability of gas concentration measurements.

[0083] like Figure 2 As shown, the lidar gas concentration calibration method based on differential absorption provided in the embodiment of the present application includes:

[0084] S1. Using a laser transmitter to emit a laser beam of a first wavelength and a laser beam of a second wavelength, the laser beams are combined into a first combined optical signal after chopping.

[0085] Among them, the first wavelength laser is used to produce resonant absorption with the gas to be measured (such as methane), and the gas concentration is reflected by the attenuation degree; the second wavelength laser is used as a reference beam to avoid the gas absorption effect to calibrate the atmospheric background interference; the first combined beam light signal is used to encode the two lasers into an alternating pulse sequence, which is convenient for subsequent timing differentiation and signal processing.

[0086] In some implementations, a continuous laser is time-chopped by an acousto-optic modulator (AOM) to convert it into a discrete signal with controllable pulse width and period.

[0087] It should be pointed out that the frequency of the chopping process needs to match the time window of the atmospheric echo signal to avoid signal overlap.

[0088] For example, the chopping period may be set to 1000 ns, so that the pulses of the first wavelength laser and the second wavelength laser appear alternately on the time axis.

[0089] S2. After amplifying the first combined optical signal, split it into a measurement optical signal and a reference optical signal according to a preset ratio.

[0090] Among them, the measurement light signal is used to be emitted into the atmosphere and transmitted through free space to obtain an attenuation signal containing gas absorption information; the reference light signal is used to be directly transmitted through the optical fiber delay line to calibrate the fluctuation of the laser output power in real time.

[0091] It's important to note that the specific setting of the preset ratio can affect the measurement signal-to-noise ratio and calibration accuracy: a high ratio of measurement light may result in insufficient reference light energy, while a low ratio may affect the signal strength of atmospheric detection. In this application, a 1:9999 splitting ratio is used, with fiber couplers used to achieve energy distribution between the measurement and reference light signals.

[0092] For example, when the detection distance is 5 km, 99.99% of the measurement light energy can ensure that the echo signal has a sufficient number of photons, and 0.01% of the reference light energy can still be effectively identified by the single-photon detector after a 1 km optical fiber delay.

[0093] S3. Combining the measurement optical signal after atmospheric attenuation and the reference optical signal after optical fiber delay into a second combined optical signal, and then converting it into four electrical pulses through a single-photon detector.

[0094] The second combined optical signal is used to align the measurement light carrying atmospheric absorption information with the reference light not affected by the atmosphere in time and space for synchronous detection.

[0095] In some implementations, two optical signals are combined using a coupler, and an optical fiber delay line is used to compensate for the atmospheric transmission delay of the measurement light, so that the two are synchronized in time at the detector end.

[0096] It should be pointed out that the accuracy of delay matching directly affects the accuracy of separation of the four electrical pulses, and the optical fiber length needs to be dynamically adjusted according to the detection distance.

[0097] For example, when the detection distance is 2 km, the atmospheric transmission delay is about 13.3 μs. Time synchronization is achieved by combining a 1 km optical fiber (delay of about 5 μs) with an additional delay line.

[0098] S4. Count the number of photons of the four electrical pulses, perform normalization and filtering processing in sequence, and obtain pre-processed values.

[0099] Normalization and filtering are applied to the photon count statistics (specifically, the time series of photon counts) to eliminate the impact of interfering factors such as system noise and power fluctuations on the data. Specifically, normalization, through multi-step ratio calculations, offsets systematic errors such as laser output power fluctuations, optical system losses, and detector quantum efficiency drift, making the photon count statistics more consistent with actual gas absorption effects. Filtering smoothes random noise in the photon count time series (such as signal jitter caused by atmospheric turbulence and circuit thermal noise), eliminating abnormal pulses and reducing data jitter to ensure the accuracy of the final concentration calculation.

[0100] In some implementations, a time-to-digital converter (TDC) can be used to mark the arrival time of the electrical pulses at the picosecond level, and combined with timing coding to separate the four signals; and the electrical pulses are divided into four time windows based on the timestamp: the first wavelength of the measurement light, the second wavelength of the measurement light, the first wavelength of the reference light, and the second wavelength of the reference light. The number of electrical pulses in each window is counted to obtain the number of photons in the four electrical pulses.

[0101] It should be pointed out that in this application, the principle of obtaining the number of photons by counting the number of electrical pulses is based on the quantum photoelectric effect of the single-photon detector and the system signal processing mechanism: the single-photon detector utilizes the photoelectric effect. When a single photon is incident, it will excite electron-hole pairs, which are converted into an electrical pulse through avalanche amplification or superconducting phase change. Under ideal circumstances, one photon triggers one electrical pulse, and the number of electrical pulses is linearly positively correlated with the number of incident photons. In practice, it is calibrated by the detector quantum efficiency (50%~90%), that is, the number of photons = the number of electrical pulses / detector quantum efficiency. Therefore, the number of photons can be obtained by counting the number of electrical pulses.

[0102] S5. Based on the pre-processed value and the pre-calibrated functional relationship between the number of photons and the concentration of the gas to be measured, the gas concentration is calculated.

[0103] The functional relationship is: C=kA+b; C represents the gas concentration, A represents the logarithm of the preprocessed value, k and b represent the calibrated slope and intercept, respectively.

[0104] In some implementations, pre-calibration of the linear function requires using multiple sets of standard gases with known concentrations, and performing least squares fitting after counting the number of photons.

[0105] Based on the above technical solution, in the differential absorption-based lidar gas concentration calibration method provided by this application, by real-time calibration of the laser output power and using the reference light signal to directly monitor the light source fluctuation, the attenuation error caused by factors such as atmospheric transmission and optical loss can be accurately corrected, effectively improving the accuracy of concentration calibration; the application of single-photon detection technology can greatly reduce the interference of background noise on weak signals and achieve high-sensitivity detection at the photon level, which not only improves the signal reception efficiency, but also breaks through the traditional detection distance limitation and supports long-distance, low-concentration gas detection scenarios; in addition, the system continuously monitors and dynamically compensates for system parameter drift caused by environmental changes and equipment aging through periodic calibration combined with reference signal comparison, which can ensure the stability and reliability of measurement results under long-term operation, and provides high-precision and robust gas concentration detection solutions for industrial emission monitoring, atmospheric environment research and other fields.

[0106] In a possible implementation of the embodiment of the present application, the above S1 can be specifically implemented by the following S101 and S102, which are specifically described below:

[0107] S101 , using an acousto-optic modulator (AOM) to perform time-sequential chopping on a first wavelength laser and a second wavelength laser to generate an alternating pulse sequence.

[0108] The purpose of chopping is to convert the continuous laser signal into discrete pulses, which facilitates the encoding and timing distinction of the two-wavelength lasers and lays the foundation for the subsequent separation and identification of the echo signals.

[0109] In some implementations, the AOM is driven by an electrical signal to alternately chop the two-wavelength laser light at a preset frequency to form a pulse train of the first wavelength and the second wavelength.

[0110] It should be pointed out that the chopping frequency needs to match the time window of the atmospheric echo to avoid the overlap of the first wavelength and the second wavelength pulses on the time axis, which will affect the accuracy of subsequent photon count statistics.

[0111] S102 , combining the chopped two-wavelength pulse light into a first combined optical signal through a first coupler (1:1).

[0112] The purpose of the beam combining operation is to combine the first wavelength λ on With the second wavelength λ off The pulse sequences are integrated into the same optical path for transmission, ensuring the spatial and temporal consistency of the two wavelength lasers, facilitating subsequent unified amplification and beam splitting processing.

[0113] In some implementations, a fiber-optic 1:1 coupler is used to combine the two pulsed lights into a single beam with an energy ratio of 50%:50%, ensuring energy balance between the two wavelength signals.

[0114] It should be pointed out that the beam combining efficiency needs to be higher than 95% to reduce light energy loss and avoid affecting the atmospheric penetration ability of subsequent measurement light and the power calibration accuracy of the reference light.

[0115] Based on the above technical solution, the timing coding of the two-wavelength laser is realized by AOM chopping, and the energy matching before beam combining and power amplification is completed by 1:1 coupler to ensure that the λ in the first combined optical signal is on and λ off The pulse time interval is clear and the energy is balanced, which provides a stable signal basis for the subsequent measurement light atmospheric detection and reference light power calibration after beam splitting, effectively improving the system's recognition accuracy and anti-interference ability for dual-wavelength signals.

[0116] S103: Input the first combined optical signal into a signal amplifier through 1:1 coupling to provide power support for subsequent beam splitting.

[0117] The purpose of coupling to the amplifier is to compensate for the energy loss in the chopping and combining process, so that the measurement optical signal has sufficient power to penetrate the atmosphere and return a valid echo, while ensuring that the power of the reference optical signal can be recognized by the single-photon detector.

[0118] In some implementations, a polarization-maintaining fiber is used to directly couple the combined optical signal to an optical fiber amplifier (EDFA), and optical power amplification is achieved through pump light excitation.

[0119] It should be pointed out that the gain of the amplifier needs to be dynamically adjusted according to the detection distance to avoid excessive power leading to detector saturation, or too low power leading to insufficient signal-to-noise ratio of the echo signal.

[0120] For example, when the detection distance is 5 km, the amplifier gain is set to 20 dB, which increases the peak power of the first combined optical signal from 10 mW to 1 W. After subsequent 99.99% splitting, the peak power of the measured optical signal is approximately 1000 mW, which meets the requirements of long-distance atmospheric transmission.

[0121] In a possible implementation of the embodiment of the present application, combined with Figure 2 and Figure 3 The above S2 can be implemented by the following S201, S202 and S203, which are described in detail below:

[0122] S201: Amplify the power of the first combined optical signal through a signal amplifier.

[0123] The purpose of power amplification is to compensate for the energy loss during the chopping and combining process, so that the measurement optical signal has sufficient power to penetrate the atmosphere and return a valid echo, while ensuring that the power of the reference optical signal can be recognized by the single-photon detector.

[0124] For example, when the detection distance is 5 km, the gain of the signal amplifier can be set to 20 dB, increasing the peak power of the first combined optical signal from 10 mW to 1 W, thereby providing sufficient energy for subsequent beam splitting.

[0125] S202: Perform beam splitting processing on the amplified optical signal through a second coupler.

[0126] Among them, the second coupler is a 1:9999 fiber optic coupler. Through beam splitting processing, the amplified optical signal is separated into a measurement optical signal (99.99%) and a reference optical signal (0.01%) according to a preset ratio. The former is used for atmospheric detection, and the latter is used for real-time calibration of laser power.

[0127] It should be pointed out that the beam splitting ratio directly affects the measurement signal-to-noise ratio and calibration accuracy: if the proportion of measurement light is too high, it will lead to insufficient reference light energy (which may be lower than the detector noise threshold), while if it is too low, it will weaken the signal strength of atmospheric detection. The ratio of 1:9999 has the best compatibility in long-distance detection.

[0128] For example, when the amplified optical signal power is 1 W, the measured optical signal power is 999.9 mW, and the reference optical signal power is 0.1 mW. After a 1 km optical fiber delay, the reference optical power attenuates to approximately 0.05 mW, which is still higher than the minimum detectable power of the single-photon detector (approximately 10 nW).

[0129] S203, transmitting the measurement light signal into the atmosphere through the transmitting lens, and receiving the measurement light signal after atmospheric attenuation through the receiving lens; at the same time, the reference light signal continues to be transmitted through the delay optical fiber.

[0130] The transmitting lens uses a large-aperture (e.g., 100mm) optical lens. Through collimation, the measurement light signal is focused into a beam with a divergence angle of less than 1mrad, ensuring energy concentration during long-distance atmospheric transmission. The receiving lens utilizes a highly sensitive parabolic reflector (diameter ≥ 200mm) in conjunction with a narrowband filter (bandwidth < 0.5nm) to suppress background light interference and enhance echo signal acquisition efficiency. The length of the delay fiber must be precisely calculated based on the detection distance. For example, at a detection distance of 5km, the atmospheric transmission delay is approximately 33.3μs. Using a 3km length of polarization-maintaining fiber (refractive index n=1.5, delay of approximately 15μs) combined with an 18.3μs electrical delay, the reference light signal and the measurement light echo arrive at the subsequent beam combining unit synchronously.

[0131] It should be pointed out that the transmission and receiving links of the measurement optical signal need to maintain optical alignment, and the field of view overlap between the transmitting lens and the receiving lens must be greater than 95% to ensure the effective collection of atmospheric echo signals; the delay fiber of the reference light must use low-loss single-mode fiber to reduce the impact of power attenuation on calibration accuracy.

[0132] For example, when the detection distance is 5 km, the energy of the measured optical signal attenuates by about 60 dB after transmission through the atmosphere, and the receiving lens can collect an echo power of about 1 nW, while the power of the reference optical signal after a 3 km optical fiber delay is about 0.03 mW, which meets the detection threshold of the single-photon detector.

[0133] Based on the above technical solution, through power compensation of the signal amplifier, precise beam splitting of the 1:9999 coupler, and the coordinated design of the transmit / receive link and the delay fiber, efficient atmospheric transmission of the measurement optical signal and real-time power calibration of the reference optical signal are achieved. This not only ensures the signal-to-noise ratio of the echo signal during long-distance detection, but also provides a temporal consistency basis for subsequent dual-wavelength signal beam combining detection and photon counting through reference optical delay synchronization technology.

[0134] In a possible implementation of the embodiment of the present application, combined with Figure 2 and Figure 4 The above S3 can be implemented through the following S301, S302 and S303, which are described in detail below:

[0135] S301 : Combine a measurement optical signal and a reference optical signal into a second combined optical signal through a third coupler (50:50 fiber coupler).

[0136] Beam combining involves temporally and spatially aligning the measurement light, which carries atmospheric absorption information, with a reference light, which is unaffected by the atmosphere, to facilitate simultaneous detection by single-photon detectors. The measurement light signal, after transmitting through the atmosphere, carries information about gas absorption attenuation, while the reference light signal, after being delayed by an optical fiber, serves as a power calibration benchmark. After beam combining, the gas concentration can be inferred by comparing the difference in photon counts between the two.

[0137] It's important to note that the time delays of the two optical signals must be precisely matched during beam combining to avoid signal overlap due to timing misalignment. The atmospheric transmission delay of the measurement light and the fiber delay of the reference light must be calculated using the formulas tatmosphere = 2D / C (D is the detection distance, c is the speed of light) and tfiber = nL / c (n is the fiber refractive index, L is the fiber length), ensuring that the time difference between the two reaching the detector is within ±1ns.

[0138] For example, when the detection distance D = 5 km, the atmospheric transmission delay is about 33.3 μs. The measurement light and the reference light are synchronized in time through a 3 km optical fiber (n = 1.5, a delay of about 15 μs) combined with an 18.3 μs electrical delay.

[0139] S302: Convert the second combined light signal into discrete electrical pulses using a single-photon detector.

[0140] Single-photon detectors convert light signals into electrical signals through the photoelectric effect, achieving highly sensitive detection at the photon level. Each incident photon triggers an electrical pulse, and the number of electrical pulses is linearly positively correlated with the number of photons, providing the basis for subsequent photon counting.

[0141] For example, when the number of photons in the second combined light signal is 100, the single-photon detector can output about 90 electrical pulses, and the original number of photons can be inferred after quantum efficiency calibration.

[0142] S303 , recording the arrival time of the discrete electrical pulses through a time-to-digital converter (TDC), and separating the discrete electrical pulses into four electrical pulses according to the timestamps.

[0143] The TDC is used to time-stamp the electrical pulses at the picosecond level. Combined with the chopping sequence, the pulses are divided into four signals: the first wavelength of measurement light, the second wavelength of measurement light, the first wavelength of reference light, and the second wavelength of reference light. Temporal resolution directly impacts the accuracy of separation of the four pulses and is crucial for achieving differential absorption measurement.

[0144] Based on this technical solution, a third coupler is used to combine the measurement and reference beams. Single-photon detectors and a time-dependent detector (TDC) are then used to convert and separate the optical signals into four electrical pulses. Time synchronization and high-sensitivity detection ensure the timing consistency and photon counting accuracy of the dual-wavelength signals, offsetting the effects of atmospheric transmission delays on the measurement results.

[0145] In a possible implementation of the embodiment of the present application, the above S4 can be specifically implemented by the following S401, S402 and S403, which are specifically described below:

[0146] S401 , using a time-to-digital converter (TDC) to time-mark the four electrical pulses and count the number of photons according to the time window.

[0147] Among them, TDC divides the electrical pulse into four time windows through picosecond time stamps, corresponding to the first wavelength of the measurement light (λ on ) echo, measuring light second wavelength (λ off ) echo, direct transmission of reference light at the first wavelength, and direct transmission of reference light at the second wavelength. The number of electrical pulses in each window is the statistical value of the number of photons in the optical signal of the corresponding path.

[0148] S402: Perform three-step normalization processing on the photon number statistics.

[0149] The purpose of normalization is to offset systematic errors such as laser power fluctuations and detector efficiency drift, including:

[0150] 1. First normalization: Calculate the ratio of the number of photons of the dual wavelengths of the measurement light, R1 = the number of photons in the first electrical pulse / the number of photons in the second electrical pulse, to eliminate the difference in optical path attenuation;

[0151] 2. Second normalization: Calculate the ratio of the number of photons of the reference light at two wavelengths, R2 = the number of photons of the third electrical pulse / the number of photons of the fourth electrical pulse, and calibrate the laser output power;

[0152] 3. Third normalization: Calculate the ratio of the dual ratios R3 = R1 / R2 to obtain a normalized result directly related to gas absorption.

[0153] like Figure 5 and Figure 6 As shown, 1 is the number of photons N of the first electrical pulse r (λ on ), 2 is the number of photons of the second electrical pulse N r (λ off ), 3 is the number of photons N of the third electric pulse t (λ on ), 4 is the number of photons N of the fourth electrical pulse t (λ off ), R1=1 / 2, R2=3 / 4, R3=(1 / 2) / (3 / 4).

[0154] It should be pointed out that outliers need to be eliminated during the normalization process: if the statistical value of the photon count of a certain channel is lower than the detector noise threshold (such as <10 electrical pulses), the data re-acquisition mechanism is triggered to avoid noise interference in the inversion results.

[0155] S403: Filter the normalized result to obtain a pre-processed value.

[0156] Filtering can use a 5-point sliding average algorithm to smooth random noise in the time series (such as jitter caused by atmospheric turbulence). Alternatively, wavelet transform filtering can be used to reduce low-frequency noise below 10 Hz while retaining the high-frequency characteristic signals of gas absorption.

[0157] The photon number statistics after normalization and filtering are shown in the figure below: Figure 6 As shown. Figure 6 It can be seen that the fluctuation of the number of photons after preprocessing is smoother, which can effectively reduce the interference of random noise on concentration calibration and make the calibration results more stable and accurate.

[0158] Based on the above technical solution, accurate quantification of the number of photons is achieved through TDC time marking and windowed statistics. Combined with three-step normalization and filtering processing, the interference of system noise and power fluctuations can be eliminated, making the inversion results of the Beer-Lambert law more consistent with the actual gas absorption characteristics and more accurate.

[0159] In a possible implementation of the embodiment of the present application, the above S5 specifically includes the following S501 to S504, which are described in detail below:

[0160] S501. Prepare multiple groups of standard gases with known concentrations and collect their photon count data.

[0161] The purpose of preparing a standard gas is to provide benchmark data for establishing a quantitative relationship between photon number and concentration. The standard gas must cover the typical concentration range of the target gas to ensure the universality of the subsequent fitting function.

[0162] In some implementations, a dynamic gas distribution method can be used to prepare five groups of standard gases with concentrations of 1000 ppm·m, 5000 ppm·m, 10000 ppm·m, 20000 ppm·m, and 30000 ppm·m, respectively. The measurement is repeated 10 times in each group of gas environments, and the statistical average of the number of photons is taken to reduce random errors.

[0163] It's important to note that gas mixing must be performed under a constant temperature and pressure environment to avoid the effects of temperature and pressure changes on concentration. For example, in a methane concentration calibration experiment, high-purity methane (99.999%) and dry air are mixed in a volumetric ratio, with a mass flow controller precisely controlling the ratio to produce a standard gas sample with a concentration of 5000 ppm·m.

[0164] S502 , preprocessing is performed based on the photon number data of the standard gas to obtain multiple groups of preprocessed values.

[0165] Among them, such as Figure 6 As shown, 1 is the number of photons N of the first electrical pulse r (λ on ), 2 is the number of photons of the second electrical pulse N r (λ off ), 3 is the number of photons N of the third electric pulse t (λ on ), 4 is the number of photons N of the fourth electrical pulse t (λ off ), the value after preprocessing is (1 / 2) / (3 / 4), that is, .

[0166] S503. Using the least squares method, multiple groups of known concentrations and multiple groups of pre-processed values ​​are fitted to obtain a linear function C=k×A+b.

[0167] Where k is the fitting slope and b is the intercept. The accuracy of the function fitting can be ensured by minimizing the mean square error (MSE) between the measured concentration and the fitted concentration.

[0168] For example, Figure 7 This is a fitting relationship diagram obtained by conducting a calibration experiment using methane gas. It shows the relationship between the methane gas concentration C and the logarithmic value A of the number of photons. It can be seen from the figure that the two conform to a linear function and direct proportional function relationship.

[0169] It should be noted that the concentration C can also be obtained by inversion calculation, specifically:

[0170] (1) According to the Beer-Lambert law, when laser radiation is transmitted in atmospheric gas, it attenuates according to formula (1):

[0171] (1);

[0172] Where λ represents the wavelength, P r (λ) represents the receiving power of the receiving lens when the wavelength is λ; P t (λ) represents the emission power of the emitting lens at wavelength λ; T(λ) represents the optical efficiency of the measurement system at wavelength λ, S(t) represents the fluctuation factor of the laser power with time t caused by the atmospheric turbulence effect, α(λ) represents the absorption coefficient of the gas to be measured at wavelength λ, K g (λ) represents the atmospheric extinction coefficient at wavelength λ, and L represents the transmission distance of the laser in the atmosphere;

[0173] (2) For the first wavelength λ with a small difference on and the second wavelength λ off , respectively, and substitute them into formula 1, and find the ratio to obtain:

[0174] (2);

[0175] Since the first wavelength λ on and the second wavelength λ off The phase difference is extremely small, so the optical efficiency of the measurement system is approximately equal, that is: ; Then, by transforming formula (2), the calculation formula for gas concentration can be obtained as follows:

[0176] (3);

[0177] The inverse of the absorption coefficient difference in formula (3) is 1 / [α(λ off )-α(λ on )] is defined as η, and the relationship between η and C is:

[0178] (4);

[0179] (4) Since the laser power P and the number of photons N satisfy: P = N × hv; where h represents the Planck constant and v represents the photon frequency, which are fixed parameters of the system; therefore, formula (4) can be transformed into:

[0180] (5)

[0181] Therefore, from formula (5), when setting a gas with a known concentration for inversion experiment, we can use the formula The value of η is calculated.

[0182] So, define Finally, the expression of concentration C is: C=η / L×A; at this time, the logarithm of concentration and photon number is also in direct proportion, which also reflects the correctness of the above fitting function.

[0183] It should be noted that P=N×hv is derived from Planck's formula E=hv and the power expression P=ΔE / Δt:

[0184] If N photons pass through in 1s, the total energy is: ΔE=N×the energy of a single photon E=N×hv;

[0185] Therefore, the power of the laser P = ΔE / Δt = Nhv / 1s = Nhv.

[0186] S504: Apply the calibrated linear function to solve the gas concentration.

[0187] The gas concentration can be calculated by substituting the pre-processed values ​​obtained during the detection process into the calibrated linear function formula.

[0188] Based on this technical solution, a precise mapping relationship between photon number and concentration was established through standard gas preparation and least-squares fitting. This calibration method ensures the reliability of the functional relationship by averaging multiple sets of data and correcting errors, significantly simplifying the calculation process for on-site measurements and providing a quantitative basis for scenarios such as real-time industrial emissions monitoring and atmospheric environmental early warning.

[0189] An embodiment of the present application also provides a computer-readable storage medium, comprising instructions, which, when executed on a computer, enables the computer to execute any of the above methods.

[0190] An embodiment of the present application also provides a computer program product comprising instructions, which, when executed on a computer, enables the computer to execute any of the above methods.

[0191] An embodiment of the present application also provides a chip, which includes a processor and an interface circuit, the interface circuit is coupled to the processor, the processor is used to run a computer program or instruction to implement the above method, and the interface circuit is used to communicate with other modules outside the chip.

[0192] In the above embodiments, all or part of the embodiments can be implemented using software, hardware, firmware, or any combination thereof. When implemented using a software program, all or part of the embodiments can be implemented in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the processes or functions described in the embodiments of this application are generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device such as a server or data center that includes one or more available media. Available media may be magnetic media (eg, floppy disks, hard disks, magnetic tapes), optical media (eg, DVDs), or semiconductor media (eg, solid state disks (SSDs)).

[0193] Although the present application is described herein in conjunction with various embodiments, in the process of implementing the claimed application, those skilled in the art may understand and implement other variations of the disclosed embodiments by reviewing the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple situations. A single processor or other unit may implement several functions listed in the claims. The fact that certain measures are recorded in different dependent claims does not mean that these measures cannot be combined to produce good results.

[0194] Although the present application has been described with reference to specific features and embodiments thereof, it is apparent that various modifications and combinations may be made thereto without departing from the spirit and scope of the present application. Accordingly, this specification and the drawings are merely illustrative of the present application as defined by the appended claims and are deemed to cover any and all modifications, variations, combinations or equivalents within the scope of the present application. Obviously, those skilled in the art may make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, the present application is intended to encompass such modifications and variations as fall within the scope of the claims of the present application and their equivalents.

Claims

1. A lidar gas concentration calibration method based on differential absorption, characterized in that: include: The emitted first wavelength laser and second wavelength laser are sequentially subjected to chopping processing and signal beam combining processing to obtain a first beam combined light signal; the first wavelength is the absorption peak wavelength of the gas to be measured, and the second wavelength is located at a wavelength position where the absorption cross section of the gas to be measured is zero; After amplifying and splitting the first combined optical signal, a measurement optical signal and a reference optical signal are obtained; The measurement optical signal is emitted into the atmosphere, and the reference optical signal is input into the optical fiber delay line; Combining the measurement optical signal attenuated by atmospheric transmission and the reference optical signal delayed by the optical fiber into a second combined optical signal; Separating the second combined optical signal into four electrical pulses and counting the number of photons in each electrical pulse; Normalizing and filtering the photon number statistics of each electrical pulse to obtain a first value; Inputting the first value into a pre-calibrated functional relationship to determine the concentration of the gas to be measured, wherein the pre-calibrated functional relationship is a functional relationship between the number of photons and the concentration of the gas to be measured; The step of separating the second combined optical signal into four electrical pulses includes: using a single-photon detector to convert the second combined optical signal into discrete electrical pulses; The arrival times of discrete electrical pulses are recorded using a time-to-digital converter; Separating the discrete electrical pulses into a first electrical pulse, a second electrical pulse, a third electrical pulse, and a fourth electrical pulse according to the arrival time; wherein the first electrical pulse represents an echo pulse of a first wavelength in the measurement optical signal, the second electrical pulse represents an echo pulse of a second wavelength in the measurement optical signal, the third electrical pulse represents a direct pulse of the first wavelength in the reference optical signal, and the fourth electrical pulse represents a direct pulse of the second wavelength in the reference optical signal; The normalizing and filtering of the photon number statistics of each electrical pulse includes: Calculating the ratio of the number of photons of the first electric pulse to the number of photons of the second electric pulse to obtain a first normalized result; Calculating the ratio of the number of photons of the third electric pulse to the number of photons of the fourth electric pulse to obtain a second normalized result; Calculate the ratio of the first normalized result to the second normalized result to obtain a third normalized result; The third normalized result is filtered to obtain a first value, where the first value represents a statistical value of the number of photons after preprocessing.

2. The laser radar gas concentration calibration method based on differential absorption according to claim 1 is characterized in that: The splitting ratio of the measurement light signal to the reference light signal is 1:9999, and the measurement light signal accounts for 99.99% and is used to detect the absorption attenuation of gases in the atmosphere. The reference light signal accounts for 0.01% and is used to calibrate the output power of a laser emitter, which is used to emit a first wavelength laser and a second wavelength laser.

3. The laser radar gas concentration calibration method based on differential absorption according to claim 1 is characterized in that: The pre-calibrated functional relationship is determined based on the following process: Obtain multiple groups of standard gases with known concentrations, and the number of photons of four electrical pulses corresponding to each group of standard gases with known concentrations; Normalizing and filtering the photon counts of the four electrical pulses corresponding to each group of standard gases of known concentration to obtain a second value; The gas concentrations of multiple groups of standard gases with known concentrations and the logarithm of the second value are fitted using the least squares method to obtain the pre-calibrated functional relationship.

4. The method according to claim 3, characterized in that The pre-calibrated function satisfies the following formula: C=k×A+b; in, , N r (λ on ) represents the number of photons in the first electrical pulse, N t (λ off ) represents the number of photons in the fourth electrical pulse, N t (λ on ) represents the number of photons in the third electrical pulse, N r (λ off ) represents the number of photons in the second electrical pulse, and represents the first value, k represents the fitting slope, and b represents the fitting intercept.

5. The laser radar gas concentration calibration system based on differential absorption is characterized by: include: Laser emitter, acousto-optic modulator, coupler, signal amplifier, single photon detector, time-to-digital converter and processing unit; the coupler includes a first coupler, a second coupler and a third coupler; The laser emitter includes a first laser emitter and a second laser emitter, wherein the first laser emitter emits laser light of a first wavelength, and the second laser emitter emits laser light of a second wavelength; the first wavelength is the absorption peak wavelength of the gas to be measured, and the second wavelength is located at a wavelength position where the absorption cross section of the gas to be measured is zero; The acousto-optic modulator comprises a first acousto-optic modulator and a second acousto-optic modulator, wherein the first acousto-optic modulator is used to perform time-sequential chopping on the first wavelength laser to generate a first modulated laser, and the second acousto-optic modulator is used to perform time-sequential chopping on the second wavelength laser to generate a second modulated laser; The first coupler is used to combine the first modulated laser light and the second modulated laser light into a first combined optical signal; The signal amplifier is configured to amplify the power of the first combined optical signal to obtain an amplified optical signal; The second coupler is used to perform beam splitting processing on the amplified optical signal to obtain a reference optical signal and a measurement optical signal; The third coupler is used to combine the reference optical signal after optical fiber delay and the measurement optical signal after atmospheric transmission attenuation to obtain a second combined optical signal; The single-photon detector is used to convert the second combined light signal into discrete electrical pulses; The time-to-digital converter is used to record the arrival time of discrete electrical pulses; The processing unit is configured to separate the discrete electrical pulses into a first electrical pulse, a second electrical pulse, a third electrical pulse, and a fourth electrical pulse according to the arrival time; The number of photons of each electric pulse is counted, and the gas concentration is calculated based on the pre-processing result of the number of photons of each electric pulse and the pre-calibrated functional relationship between the number of photons and the concentration of the gas to be measured.

6. The differential absorption-based lidar gas concentration calibration system according to claim 5, characterized in that: The first laser emitter and the second laser emitter are connected in parallel to a first coupler, the first coupler, the signal amplifier, the second coupler and a transmitting lens are connected in series, and the transmitting lens is used to transmit the measurement light signal output by the second coupler into the atmosphere.

7. The laser radar gas concentration calibration system based on differential absorption according to claim 6 is characterized in that: The second coupler is connected to the third coupler, and the reference optical signal output by the second coupler is transmitted to the third coupler via the delay optical fiber; The receiving lens, the third coupler, the single photon detector and the time-to-digital converter are connected in series. The receiving lens is used to receive the measurement light signal emitted into the atmosphere and transmit it to the third coupler.

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