Time sequence scanning atmosphere laser radar based on continuous wave laser

Abstract

The invention discloses a time sequence scanning atmosphere laser radar based on a continuous wave laser. The device comprises a laser emission module which is used for outputting continuous wave laser, carrying out square wave modulation on the continuous wave laser and then emitting the continuous wave laser to the atmosphere; the signal detection module is used for receiving an echo signal after interaction of the laser and the atmosphere and converting the echo signal into an electric signal; and the signal processing module is connected with the signal detection module and is used for carrying out inversion processing on the electric signal based on a continuous wave radar equation to obtain three-dimensional space distribution information of the atmospheric parameters. By utilizing the narrow linewidth characteristic of the continuous wave laser, the absorption line of the atmospheric greenhouse gas molecules can be accurately matched, and the detection precision of the atmospheric greenhouse gas concentration is remarkably improved; according to the invention, through a time domain differential signal processing technology, high-precision distance resolution capability is realized.

Description

Technical Field

[0001] This invention belongs to the field of atmospheric environment monitoring technology, specifically relating to a time-series scanning atmospheric lidar based on a continuous wave laser, which is suitable for monitoring the three-dimensional spatial distribution of air pollutants and greenhouse gases. Background Technology

[0002] In recent years, rapid global industrialization and urbanization have led to increasingly prominent air pollution problems due to the emission of various greenhouse gases and pollutants. Lidar, as a long-range, non-contact atmospheric environmental monitoring technology, has begun to be used for the observation of various atmospheric particles and pollutants. Existing atmospheric lidar technologies mainly include pulsed lidar and imaging lidar, but they still have some limitations.

[0003] Pulsed atmospheric lidar system:

[0004] Pulsed atmospheric lidar utilizes time-of-flight ranging to obtain distance information based on the time difference between the transmitted and received pulses. The concentration of the atmospheric medium is then determined by retrieving the pulse power from the received pulse. Key technological limitations mainly lie in the following two aspects:

[0005] Detection accuracy is limited by the linewidth of the laser source: the spectral linewidth of the pulsed laser The pulse width Δt is constrained by the Fourier transform limit (time-frequency uncertainty relationship) ΔνΔt≥0.44. For example, at a wavelength of 1572 nm, the theoretical minimum linewidth is Δν=44 MHz, while the linewidth obtained using seed laser injection technology is approximately 100 MHz or higher. Therefore, under these conditions, it is difficult to meet the requirement of narrow linewidth absorption spectra (on the order of tens of GHz) for various gases (such as CO2, CH4, etc.) over a wide wavelength range, directly affecting the improvement of the measurement accuracy of different gas concentrations using this type of pulsed laser absorption method.

[0006] The overall cost of using and maintaining radar systems: Pulsed laser source systems lack advantages in cost, size, and energy consumption. Although 1064nm Nd:YAG pulsed lasers have been developing towards miniaturization and economy in recent years, they still need to rely on nonlinear frequency doubling, optical parametric oscillation, and other means to achieve the visible light and communication bands, which greatly increases the system cost and size.

[0007] Imaging lidar system:

[0008] Imaging lidar, exemplified by Sham imaging lidar, employs a continuous-wave laser source in conjunction with a tilting-image CCD / CMOS detector, achieving range resolution through geometric optics. Compared to pulse lidar, which has undergone more than half a century of development, Sham imaging lidar was first proposed in 2015. Currently, imaging lidar, combined with differential absorption technology, can detect particulate matter or gas molecules such as aerosols, nitrogen dioxide (NO2), and carbon dioxide (CO2). The main advantage of this technology is its ability to use narrow-linewidth semiconductor lasers (LDs), directly outputting MHz-level narrow linewidths, providing a better foundation for accurately matching gas absorption spectra.

[0009] However, imaging lidar also has many shortcomings. For example, spatial resolution degradation: due to the Rayleigh diffraction limit, spatial resolution decreases exponentially with distance, affecting the accuracy of mid- to long-range detection to some extent. And pixel aliasing errors: pixel aliasing errors caused by fine-tuning errors in the Sham angle, as well as systematic errors due to temperature effects, prevent the system from meeting the requirements of applications in complex environments. Summary of the Invention

[0010] The present invention aims to provide a time-series scanning atmospheric lidar based on a continuous wave laser, which can use a continuous wave laser as a radar light source and achieve high-precision uniform spatial resolution detection, thus overcoming the limitations of the two atmospheric lidar technologies mentioned above.

[0011] A time-series scanning atmospheric lidar based on a continuous-wave laser, comprising:

[0012] A laser emitting module is used to output continuous wave laser and modulate the continuous wave laser with a square wave before emitting it into the atmosphere;

[0013] The signal detection module is used to receive the echo signal after the laser interacts with the atmosphere and convert it into an electrical signal;

[0014] A signal processing module, connected to the signal detection module, is used to process and invert the echo signal, thereby obtaining the three-dimensional spatial distribution information of specific atmospheric parameters;

[0015] In a preferred example, the laser emitting module includes:

[0016] Continuous wave lasers of various wavelengths, powers, and linewidths, along with their matching laser amplifiers, are used to output continuous wave lasers.

[0017] A high-precision square wave signal source and a matching acousto-optic modulator are used to modulate the output laser power using square waves.

[0018] A beam expander or telescope system used to project modulated laser light into the atmosphere.

[0019] In a preferred embodiment, the signal detection module includes:

[0020] A telescope receiving system is used to receive backscattered signals generated by laser light irradiating atmospheric molecules or particles over the maximum range.

[0021] The optical signal transfer system uses a folding optical path or fiber optic system to ultimately project the optical signal received by the telescope onto the photodetector.

[0022] An optical signal receiving system includes a photodetector for receiving weak echo signal light and amplifying the signal;

[0023] The aforementioned reflective optical path guides the echo signal light to the photosensitive element of the photodetector through a combination of reflectors and lenses; the aforementioned optical fiber system guides the echo signal light to the photosensitive element of the photodetector through an optical fiber collimating lens and optical fiber; the aforementioned photodetector includes one or more silicon avalanche amplified photodetectors and indium gallium arsenide avalanche amplified photodetectors, used to convert photons of the echo signal into current signals.

[0024] In a preferred example, the signal processing module includes:

[0025] The multi-stage amplifier circuit is used to amplify the detected electrical signal step by step to increase the signal amplitude; the lock-in amplifier uses phase-sensitive detection to extract weak signals and effectively suppress environmental and system noise.

[0026] The FPGA's digital acquisition card is used to achieve precise synchronization control and high-speed signal acquisition during the laser modulation process;

[0027] The host computer software, deployed in the control computer, serves as the core data processing and control platform for the entire lidar system. It implements a series of key functions through integrated dedicated algorithm modules and optimized program code. These include signal filtering, power compensation, distance calculation, and concentration inversion.

[0028] The distance calculation function includes a visual overlap function calibration module, which is used to calibrate the overlap area of ​​the laser emission and reception fields of view through simulation and experiment to determine the starting point of the effective signal and effective detection distance; a position calibration function module, which is used to perform distance and position inversion on the time of the echo signal; and a signal distortion compensation module, which is used to correct the signal distortion caused by the modulation rising or falling edge through the waveform convolution model to achieve an accurate starting point position.

[0029] The concentration inversion function differentiates the echo signal to obtain the intensity of the light signal scattered by atmospheric molecules (or particles) at different laser irradiation distances.

[0030] In a preferred example, a three-optical-path structure is adopted:

[0031] The frequency-stabilized optical path is used to lock the laser center frequency to the target gas absorption line through the gas absorption cell and PID feedback.

[0032] The power compensation optical path is used to monitor the temporal fluctuations of laser emission power in real time and to perform power normalization compensation during data inversion.

[0033] The signal optical path is used to complete the external field emission of laser and to receive and detect atmospheric echo signals.

[0034] In a preferred example, differential absorption spectroscopy is used to achieve quantitative detection of the concentration of particles or atmospheric molecules in the atmosphere, with a distance resolution better than 6 meters.

[0035] The atmospheric particles include PM2.5 particles, PM10 particles, aerosol particles, etc.

[0036] The atmospheric molecules include CO2, O2, NO2, etc.

[0037] The beneficial effects of this invention are:

[0038] While maintaining the narrow linewidth advantage of continuous-wave lasers, this invention achieves high-precision range resolution, significantly improving gas detection accuracy. Existing pulsed lidars, limited by the time-frequency uncertainty principle (ΔνΔt≥0.44), typically have a linewidth greater than 100 MHz in the 1572 nm band, making it difficult to match the narrow absorption spectra of gases like CO2 and CH4, which are on the order of tens of GHz. This invention, however, uses a continuous-wave laser as the light source, converting it into a time-domain encoded signal through square-wave modulation. Then, a high-speed ADC (sampling rate ≥500 MS / s) synchronized with an FPGA is used to acquire the echo signal and perform time-domain differentiation processing, converting the time information into range information. This scheme retains the MHz-level narrow linewidth characteristic of continuous-wave lasers, enabling precise matching with gas absorption spectra, while achieving a range resolution better than 6 meters through time-domain analysis. This qualitatively improves the concentration inversion accuracy of differential absorption spectroscopy (DIAL) in three-dimensional space, resolving the fundamental contradiction between spectral and spatial resolution in pulsed lidars.

[0039] This invention overcomes the range-resolution limitations of continuous wave radar by employing time-domain differential signal processing technology, achieving uniform and high-precision detection over medium to long distances. Traditional imaging SAM radar suffers from an exponential decrease in spatial resolution with distance due to the Rayleigh diffraction limit, and is also affected by pixel aliasing and temperature drift. This invention treats square wave modulation as a superposition of infinitely narrow pulse sequences, establishing a differential form of the radar equation. Theoretically, this method is not constrained by the diffraction limit. Combined with nanosecond-level (<2 ns) synchronization control implemented using FPGA, it can maintain a constant 6-meter range resolution within a 0-15 km detection range, avoiding the resolution attenuation problem of imaging radar, where resolution decreases at closer ranges and decreases at farther ranges.

[0040] Employing a three-path parallel structure for collaborative real-time calibration significantly enhances the system's environmental adaptability and long-term operational reliability. Addressing the inherent defects of power fluctuations and frequency drift in continuous-wave lasers, this invention innovatively integrates a frequency stabilization optical path, a power compensation optical path, and a signal optical path: the frequency stabilization optical path uses a gas absorption cell and PID feedback to control the laser center frequency locking accuracy within 1 pm (24-hour drift <1 pm), ensuring the wavelength is locked to the center of the target gas absorption line; the power compensation optical path monitors emission power fluctuations in real time and generates normalization coefficients, dynamically compensating during data inversion to eliminate concentration calculation errors caused by light source instability; the signal optical path is dedicated to atmospheric detection. Based on existing mature optical and electronic components, the system structure is simplified, reducing system usage and maintenance costs. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments are briefly described below.

[0042] Figure 1 This invention provides a time-series scanning atmospheric lidar map based on a continuous-wave laser;

[0043] Figure 2 This invention treats the square wave modulation of continuous light as a superposition of infinitely narrow pulses;

[0044] Figure 3 It is the theoretical curve of the echo-scattered signal intensity changing with time. Detailed Implementation

[0045] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0046] like Figure 1 As shown, this application provides a time-series scanning atmospheric lidar based on a continuous-wave laser for three-dimensional CO2 concentration monitoring, including:

[0047] The laser emission module preferably uses an external cavity compressed semiconductor laser, model PL-DFB-1572-A-1-SA-14BF, as the laser source. This laser has a center wavelength of 1572 nm and a spectral width of 2... The output power is 10 This is to ensure the narrow linewidth and high frequency stability of the laser.

[0048] The output of the external cavity compressed semiconductor laser is then amplified by an erbium-doped fiber amplifier of model EDFA-LB-3-1-PA, which can amplify the laser power to a maximum of 3W to meet the transmission power requirements for long-distance atmospheric detection.

[0049] The signal processing module is electrically connected to the signal detection module. Its main function is to receive and process the electrical signal, perform inversion calculations on the electrical signal based on the continuous wave radar equation, and finally obtain the three-dimensional spatial distribution information of atmospheric parameters.

[0050] Furthermore, the core of the signal processing module is a high-speed data acquisition and processing unit, which preferably employs a high-speed data acquisition card precisely controlled by a field-programmable gate array (FPGA). For example, the high-speed data acquisition card can be the QT7135DC-FMC model, which embeds a high-performance high-speed analog-to-digital converter (ADC). The sampling rate of the ADC is preferably not less than 500 MS / s, and it has a 16-bit high resolution to ensure high-fidelity and high-precision digital conversion of the received analog electrical signals. The FPGA unit in the data acquisition card plays a crucial timing control role in the entire system. It can precisely synchronize the modulation timing of the laser emission module with the data acquisition timing of the high-speed data acquisition card, ensuring a synchronization accuracy better than 2 nanoseconds (<2 ns), thereby achieving precise capture and processing of weak echo signals.

[0051] At the system operation level, the signal processing module uses host computer software (such as LabVIEW) to achieve real-time triggering and management of the entire detection process. This process encompasses the transmission of laser modulation commands, real-time reception of echo signals, digital storage of acquired signals, implementation of efficient digital filtering (e.g., based on bandpass filters), and final inversion calculations to obtain atmospheric parameters such as greenhouse gas concentrations. This design significantly improves the system's timing accuracy, data processing efficiency, and final detection reliability.

[0052] Furthermore, lidar inversion obtains the intensity of light signals scattered by atmospheric molecules (or particles) at different laser irradiation distances by differentiating the time-varying echo signals received by the signal detection module; the intensity of scattered light signals at different laser irradiation distances can be directly used to calculate the concentration of specific atmospheric molecules or particles along the laser irradiation path.

[0053] The atmospheric lidar in this embodiment adopts a three-optical-path structure:

[0054] The reference optical path (i.e., the frequency-stabilized optical path) is used to connect the gas absorption cell and the lock-in amplifier to perform PID frequency stabilization and wavelength locking, suppressing drift. The frequency-stabilized optical path achieves laser frequency locking through a PID feedback control circuit, with a frequency stabilization accuracy better than 1 pm and a 24-hour drift of less than 1 pm.

[0055] The power compensation optical path uses a lock-in amplifier to monitor the power and integrates its compensation coefficient into the inversion algorithm.

[0056] The signal optical route uses an acousto-optic modulator and a signal generator to generate a square wave, which then enters the Newtonian telescope through a beam expander and is reflected by the secondary mirror and emitted into the atmosphere. The optical system of the receiving module is a Newtonian telescope with multiple narrowband filters and aspherical lenses.

[0057] In this embodiment, the system optical path and circuit links sequentially include: 1572nm narrow-linewidth continuous light emitted by the external cavity compressed semiconductor laser 1 is first modulated by a 5 kHz square wave through the acousto-optic modulator 2; then the laser is split into three beams by beam splitters 3 and 4: one beam enters the frequency-stabilized optical path composed of a gas absorption cell 5, a photodiode 6, a lock-in amplifier 7, and a PID feedback control circuit 8 to lock the laser frequency in real time; another beam enters the power compensation optical path composed of a photodiode 10 and a subsequent lock-in amplifier to monitor and compensate for power fluctuations; the main beam is reflected by the secondary mirror 12 of the Newtonian telescope 13 after passing through the beam expander 11, and is emitted into the atmosphere through the primary mirror 14. The backscattered echo is received by the secondary mirror 15 of the same telescope, passes through the narrow-band interference filter 16 to filter out background light, and is finally focused onto the avalanche photodiode 17 to achieve photon-to-current conversion. The signal generator 18 synchronously drives the acousto-optic modulator and the FPGA-based digital acquisition card 19 to complete high-speed ADC sampling, phase-locked demodulation and real-time inversion. The processing results are uploaded to the computer 20 for display and storage. The laser driver 9 provides low-noise current bias for the external cavity semiconductor laser, thus forming a complete continuous wave time-series scanning atmospheric lidar system.

[0058] Furthermore, the square wave modulation involves applying a square wave modulation signal to a continuous-wave laser, causing the laser's output power to change synchronously according to the square wave signal. The square wave signal has a duty cycle ≤ 50%. The high-value time of the square wave signal determines the theoretical range detection limit of the lidar. The square wave modulation of continuous light can be considered as an infinitely narrow pulse. The superposition, see Figure 2 Then, the differential form is introduced, in Integrating within the time interval (start time of the square wave signal) yields new information about the intensity of the echo-scattered signal. Radar equations:

[0059]

[0060] in, : Emitted laser pulse energy / pulse power; System optical efficiency; : Effective area of ​​the receiving telescope; Detection range; End time; Start time; Infinitely narrow pulse; : Volume backscattering coefficient (atmospheric molecules + aerosols). :distance The total extinction coefficient at that location.

[0061] The lidar system disclosed in this embodiment is based on an improved continuous wave atmospheric lidar architecture to achieve high-sensitivity, high-resolution atmospheric parameter detection.

[0062] At the signal transmitting end, the lidar uses an external cavity compressed semiconductor laser (model PL-DFB-1572-A-1-8A-14BF) as its light source, which features narrow linewidth and high frequency stability. To accurately monitor the laser output state and achieve active feedback control, the transmitting end is equipped with a reference optical path (including a frequency stabilization optical path and a power compensation optical path). This path is used to monitor the laser power in real time and stabilize the laser wavelength, effectively suppressing the impact of wavelength fluctuations on measurement accuracy. Specifically, a portion of the emitted beam is reflected by a beam splitter and received by a photodiode. The resulting electrical signal is transmitted to the control computer to compensate for power drift in the laser output in real time, ensuring the stability of the emitted power.

[0063] The signal processing module integrates a high-speed data acquisition card controlled by an FPGA. The core of this acquisition card lies in its precise timing control capability, enabling simultaneous triggering of laser modulation and echo signal reception. Combining the principles of lock-in amplification technology and based on the signal transmission frequency, the processing module employs a Fourier transform algorithm to accurately extract the echo signal corresponding to the modulation frequency. This series of processing steps effectively suppresses background noise interference and significantly improves signal sensitivity. The acquisition card has a sampling rate of 500 MS / s; based on this sampling rate, this embodiment achieves a distance resolution of 6 meters. This performance is significantly superior to the approximately 40-meter distance resolution typically achieved by traditional SAM imaging lidar, demonstrating the significant technical advantages and innovation of this invention.

[0064] Furthermore, this application embodiment also quantizes the visual overlap function using ZEMAX simulation. (Visual overlap function between the emitted laser beam and the receiver's angle of reception), experimentally calibrating the blind zone boundary to accurately pinpoint the starting point of the echo signal.

[0065] Figure 3 The curves showing the echo-scattered signal versus time, calculated based on the aforementioned fundamental equations, are presented. Taking a 1572 nm continuous-wave laser as an example, assuming the laser's average power... System efficiency receiver area The length of the light modulation pulse is At altitude Place, , Telescope primary mirror diameter secondary mirror diameter Field of view Laser beam waist radius divergence angle .

[0066] The working process of this application embodiment:

[0067] System initialization phase: The external cavity compressed semiconductor laser is preheated to a stable operating temperature (15 ± 0.1 ℃), and the laser frequency is calibrated using a wavelength meter to lock it at the center of the target gas absorption line. Subsequently, the PID frequency stabilization control system is activated to achieve laser frequency stabilization; at the same time, the operating point of the acousto-optic modulator is calibrated, and the modulation depth and output power are optimized to ensure that the system is in optimal working condition.

[0068] Data acquisition phase: The FPGA controls the signal generator to output a square wave modulated signal and simultaneously triggers the analog-to-digital converter (ADC) to start signal acquisition. The system continuously acquires echo signals, with a single acquisition duration of 0–200 seconds. (Based on actual settings), simultaneously record reference optical path monitoring data for subsequent system drift correction;

[0069] Signal processing stage: Digital filtering removes high and low frequency noise and baseline drift; time-domain differentiation extracts range-resolved signals; scattering intensity at each range point is inverted according to radar equations; greenhouse gas concentration distribution is calculated.

[0070] Data output stage: Generate distance-concentration profile, output key parameters (boundary layer height, column concentration, etc.), and store raw data and visualization results.

[0071] In summary, this application proposes an innovative continuous wave lidar system that combines a continuous wave laser with time-domain signal analysis technology to achieve high-precision three-dimensional monitoring of atmospheric pollutants. This system avoids the spectral matching problem of traditional pulse lidar and the high system complexity and low resolution of imaging lidar.

[0072] It should be noted that the above content merely illustrates the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. For those skilled in the art, various improvements and modifications can be made without departing from the principle of the present invention, and all such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A time-series scanning atmospheric lidar based on a continuous-wave laser, characterized in that, include: A laser emitting module is used to output continuous wave laser and modulate the continuous wave laser with a square wave before emitting it into the atmosphere; The signal detection module is used to receive the echo signal after the laser interacts with the atmosphere and convert it into an electrical signal; The signal processing module, connected to the signal detection module, is used to perform inversion processing on the electrical signal based on the continuous wave radar equation to obtain the three-dimensional spatial distribution information of atmospheric parameters. The continuous wave radar equation is established by treating square wave modulation as a superposition of infinitely narrow pulses and constructing a time-domain continuous echo signal model by combining Mie scattering and Rayleigh scattering characteristics, so as to achieve synchronous analysis of range and atmospheric parameters.

2. The time-series scanning atmospheric lidar based on a continuous-wave laser according to claim 1, characterized in that, The laser emitting module includes: External cavity compressed semiconductor laser, used to output narrow linewidth continuous wave laser; An erbium-doped fiber amplifier, connected to the laser, is used to amplify the power of the laser. An acousto-optic modulator is used to modulate the amplified laser beam with a square wave. Beam expander and telescope system used to transmit modulated laser light into the atmosphere.

3. A time-series scanning atmospheric lidar based on a continuous-wave laser according to claim 1 or above, characterized in that, A three-path differential structure is adopted: The frequency-stabilized optical path is used to lock the laser center frequency to the target gas absorption line through the gas absorption cell and PID feedback. A power compensation optical path is used to monitor laser power fluctuations in real time and compensate for them in the inversion algorithm. The signal optical path is used to complete laser emission and atmospheric echo signal reception.

4. A time-series scanning atmospheric lidar based on a continuous-wave laser according to claim 3, characterized in that, The frequency-stabilized optical path achieves laser frequency locking through a PID feedback control circuit, with a frequency stabilization accuracy better than 1 pm and a 24-hour drift of less than 1 pm.

5. A time-series scanning atmospheric lidar based on a continuous-wave laser according to claim 1, characterized in that, The signal detection module includes: Avalanche photodiodes are used to convert echo photons into current signals; Multistage amplifier circuits and lock-in amplifiers are used to extract weak signals and suppress noise.

6. A time-series scanning atmospheric lidar based on a continuous-wave laser according to claim 1 or 5, characterized in that, The signal processing module includes: An FPGA-based digital acquisition card is used for synchronous control of laser modulation and signal acquisition. The host computer software is used to implement signal filtering, power compensation, distance calculation, and concentration inversion.

7. A time-series scanning atmospheric lidar based on a continuous-wave laser according to claim 6, characterized in that, Also includes: The visual overlap function calibration module is used to calibrate the overlap area of ​​the laser emission and reception fields of view through simulation and experiment, and to determine the effective signal starting point; The signal distortion compensation module is used to correct signal distortion caused by the rising or falling edge of modulation through a waveform convolution model.

8. A time-series scanning atmospheric lidar based on a continuous-wave laser according to claim 1, characterized in that, The differential absorption spectroscopy technique enables the quantitative detection of greenhouse gas concentrations in the atmosphere, and has the capability of a distance resolution better than 3 meters and a detection limit in the ppm range.

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