A device and method for real-time large dynamic range Doppler velocimeters lidar

Abstract

This invention belongs to the field of lidar technology, specifically relating to a device and method for real-time large dynamic range Doppler velocimetry lidar. By employing wavelength division multiplexing, spectrum multiplexing, and closed-loop feedback modulation control, it simultaneously achieves velocity measurement in multiple line-of-sight directions. The advantages of this invention are that it avoids the problems of poor real-time performance caused by using rotating mirrors and optical switches for beam spatial scanning, and the difficulty of high-speed signal acquisition and processing caused by large velocity range measurements. Furthermore, it employs single-channel detection and data acquisition, saving on detection and acquisition hardware costs and power consumption.

Description

Technical Field

[0001] This invention belongs to the field of lidar technology, specifically relating to a device and method for real-time large dynamic range Doppler velocimetry lidar. Background Technology

[0002] Doppler velocimetry lidar is widely used in industrial processing and monitoring, meteorological research and monitoring, aerospace safety assurance, and other fields. It typically requires measuring velocities in two or more directions to deduce parameters such as triaxial velocity, wind speed, and wind direction. Taking wind-measuring lidar as an example, traditional methods use wedge prism rotation scanning and optical switches to scan the beam in space, measuring wind speeds in different line-of-sight directions and thus retrieving wind field information. However, in applications on high-speed moving platforms such as airborne platforms, or in environments with rapidly changing wind fields, the methods using rotating mirror scanning or optical switch scanning suffer from poor timeliness and cannot accurately acquire wind field information.

[0003] To achieve rapid wind field measurement, a scheme involving parallel signal generation, sampling, and computation can be adopted. However, this method is often bulky, power-consuming, and costly. Furthermore, for high-speed platform speed measurement, the Doppler frequency shift is often significant. Direct sampling would require high-speed AD converters above GHz for sampling and quantization, resulting in a large amount of data that needs processing. Data acquisition and processing lead to high power consumption and significant data processing difficulty.

[0004] Patent CN 107003411A discloses a lidar device that uses a multi-wavelength optical oscillator to generate multiple laser beams of different wavelengths, performs frequency shifting at different frequencies, and uses a wave splitter and combiner to guide the laser beams of different wavelengths to different line-of-sight directions, thus avoiding the problems caused by beam scanning. It can simultaneously measure wind speed in multiple line-of-sight directions. However, due to spectrum reuse, the Doppler frequency shift range of this system is small, making it difficult to achieve a large speed range measurement. Summary of the Invention

[0005] The purpose of this invention is to propose a device and method for real-time large dynamic range Doppler velocimeters lidar. By employing wavelength division multiplexing, spectrum multiplexing, and closed-loop feedback modulation control, it achieves velocity measurement in multiple line-of-sight directions, thereby retrieving the velocity. This avoids the problems of poor real-time performance caused by using rotating mirrors and optical switches for beam spatial scanning, as well as the difficulties in high-speed signal acquisition and processing caused by wind measurement on high-speed platforms.

[0006] The technical solution of the present invention:

[0007] To achieve the above-mentioned objective, according to a first aspect of the present invention, a real-time large dynamic range laser Doppler velocimetry lidar device is provided, the device comprising: a seed laser group 1, a first beam combiner 2, a beam splitter 3, a first wavelength division multiplexer 4, a modulator group 5, a second beam combiner 6, an optical fiber amplifier 7, a circulator 8, a second wavelength division multiplexer 9, and an optical lens group 10 connected in sequence by optical fibers.

[0008] The beam splitter 3 is also optically connected to the coupler 12 and the balanced photodetector 13 in sequence; the circulator 8 is optically connected to the coupler 12.

[0009] The output of the first wavelength division multiplexer 4 is connected to each modulator in the modulator group 5 in a one-to-one correspondence; the output of the second wavelength division multiplexer 9 is connected to each optical lens in the optical lens group 7 in a one-to-one correspondence.

[0010] The signal processing module 14 is electrically connected to the modulator group 5 and the balanced photodetector 13;

[0011] The seed laser group 1 contains at least two lasers that emit narrow linewidth lasers with a linewidth range of 100Hz to 1MHz. The laser wavelengths or frequency intervals emitted by different lasers meet the channel spacing requirements of the wavelength division multiplexer.

[0012] In one possible embodiment, the device further includes an optical attenuator 11, which is disposed between the beam splitter 3 and the coupler 12 and is connected to the beam splitter 3 and the coupler 12 respectively via optical fibers.

[0013] In one possible embodiment, the modulator group 5 includes at least two modulators; the modulators are acousto-optic modulators or electro-optic modulators.

[0014] In one possible embodiment, when the modulator is an electro-optic modulator, a phase modulator or an intensity modulator may be selected.

[0015] In one possible embodiment, the intensity modulator is selected as a Mach-Zehnder type or a cascaded, parallel, orthogonal or other modified structure based on the Mach-Zehnder type.

[0016] According to a second aspect of the present invention, a method for realizing real-time large dynamic range Doppler velocities is provided, employing the aforementioned real-time large dynamic range laser Doppler velocities lidar device, comprising the following steps:

[0017] Seed laser group 1 emits multiple laser beams, which are mixed into a single optical signal by first beam combiner 2, split into signal light and local oscillator light by beam splitter 3, and the local oscillator light enters coupler 12. The signal light is split into multiple laser beams of different wavelengths by first wavelength division multiplexer 4, which enter the corresponding modulators for frequency shifting and pulse modulation. The modulated signals are mixed into a single optical signal by second beam combiner 6, amplified by fiber amplifier 7, and then enter circulator 8. After being split into multiple laser signals of different wavelengths by second wavelength division multiplexer 9, they enter the corresponding optical lenses. The optical lenses emit the signal laser into the air and receive the backscattered signals from atmospheric aerosol particles. The backscattered signals received by each optical lens are combined by second wavelength division multiplexer 9 and mixed into a single scattered signal, which enters coupler 12 after passing through circulator 8. The signal undergoes heterodyne interference with the local oscillator light signal. The heterodyne interference signal is detected by balanced detector 13 and outputs an intermediate frequency electrical signal. Signal processing module 14 collects the intermediate frequency electrical signal and processes the collected data.

[0018] The signal processing module 14 controls multiple modulation signals loaded onto the modulator group 5, causing different wavelengths of optical signals to generate different frequency shifts. It also performs FFT operations on the acquired intermediate frequency signals to obtain the corresponding amplitude spectrum, which includes frequency signals corresponding to the number of modulation signals. By controlling the frequency of the modulation signals loaded onto the modulator group 5 through closed-loop control, the frequency values ​​of the frequency signals on the amplitude spectrum are stabilized at a fixed operating frequency point. At this time, based on the frequency information of multiple different laser line-of-sight Doppler velocities contained in the frequencies of the modulation signals loaded onto the modulator group 5, the Doppler frequency shifts of multiple different laser line-of-sight directions are calculated, thereby calculating the line-of-sight velocity magnitude. Furthermore, the velocity components, velocity magnitude, and direction information of the three axes can be retrieved.

[0019] In one possible embodiment, the closed-loop control process is as follows: Set the modulation frequency operating point F0j loaded onto the modulator group 5, j = 1, 2, 3...; set the proportional control coefficient K and the frequency offset judgment threshold Ft; require that the values ​​of F02-F01, F03-F02, and Fmax-F03 are greater than the frequency shift corresponding to the maximum relative velocity in each line-of-sight direction at the start of measurement, where Fmax is the maximum measurable frequency value of the acquisition card; set the initial modulation frequency of the modulator group Fj = F0j; acquire the heterodyne intermediate frequency electrical signal, and... Perform FFT calculation to obtain the signal spectrum, estimate the signal frequency fj, and calculate Δfj = F0j – fj. If |Δfj| < Ft, then the Doppler frequency shift fdj = F0j – Fj in each line-of-sight direction is calculated. The relative motion velocity in each laser line-of-sight direction is calculated based on fdj. If |Δfj| > Ft, calculate the frequency value Fj applied to the modulator = Fj + K × Δfj. Change the frequency value Fj applied to the modulator and re-estimate the signal frequency and calculate Δfj. Repeat the above process until |Δfj| < Ft.

[0020] In one possible embodiment, the relative motion velocity V of each laser line-of-sight direction j The calculation formula is Where λ is the laser wavelength.

[0021] In one possible embodiment, the proportional control coefficient K ranges from 0.01 to 0.5.

[0022] Advantages of this invention: This invention provides a device and method for Doppler velocimetry lidar suitable for real-time, wide-range speed measurement applications. By employing wavelength division multiplexing, spectrum multiplexing, and closed-loop feedback modulation control, it achieves speed measurement in multiple line-of-sight directions, thereby retrieving the velocity. The advantages of this invention are that it avoids the problems of poor real-time performance caused by using rotating mirrors and optical switches for beam spatial scanning, and the difficulties in high-speed signal acquisition and processing on airborne and other high-speed platforms for wind measurement. Furthermore, by using single-channel detection and data acquisition, it saves on detection and acquisition hardware costs and power consumption. This invention is suitable for wind measurement lidar applications on high-speed platforms with high requirements for real-time performance, size, power consumption, and cost. Attached image description:

[0023] Figure 1 This is a schematic diagram of the structure of a real-time large dynamic range laser Doppler velocimetry lidar device according to a preferred embodiment of the present invention.

[0024] Figure 2 This is a schematic diagram of the intermediate frequency signal spectrum acquired according to a preferred embodiment of the present invention.

[0025] Figure 3 This is a flowchart illustrating a closed-loop control scheme according to one embodiment of the present invention.

[0026] Figure 4 Flowchart of another embodiment of the closed-loop control scheme of the present invention

[0027] Explanation of reference numerals in the attached figures: 1 - Seed laser group, 2 - First beam combiner, 3 - Beam splitter, 4 - First wavelength division multiplexer, 5 - Modulator group, 6 - Second beam combiner, 7 - Fiber amplifier, 8 - Circulator, 9 - Second wavelength division multiplexer, 10 - Optical lens group, 11 - Optical attenuator, 12 - Coupler, 13 - Balanced photodetector module, 14 - Signal processing module; Detailed implementation method:

[0028] The present invention will now be described in further detail with reference to the accompanying drawings.

[0029] like Figure 1As shown, a real-time large dynamic range laser Doppler velocimetry lidar device is provided. The device includes: a seed laser group 1, a first beam combiner 2, a beam splitter 3, a first wavelength division multiplexer 4, a modulator group 5, a second beam combiner 6, an optical fiber amplifier 7, a circulator 8, a second wavelength division multiplexer 9, and an optical lens group 10 connected in sequence by optical fibers.

[0030] Beam splitter 3 is sequentially connected to optical attenuator 11 (optional), coupler 12, and balanced photodetector 10 via optical fiber; circulator 8 is connected to coupler 12 via optical fiber.

[0031] The output of the first wavelength division multiplexer 4 is connected to each modulator of the modulator group 5 in a one-to-one correspondence, and the output of the second wavelength division multiplexer 9 is connected to each optical lens of the optical lens group 7 in a one-to-one correspondence.

[0032] The signal processing module 14 is electrically connected to the modulator group 3 and the balanced photodetector 10;

[0033] The signal processing module 14 calculates the Doppler frequency shift and feeds it back to the modulator group 3, so that the intermediate frequency signal frequency estimated by the signal processing module 14 is always near the modulation operating point, thereby realizing closed-loop control of the measurement.

[0034] Seed laser group 1 contains at least two lasers emitting narrow-linewidth lasers, selectable from 100Hz to 1MHz. The wavelength or frequency spacing of the lasers emitted by different lasers should meet the channel spacing requirements of the wavelength division multiplexer. Seed laser group 1 generates multiple wavelength lasers, which, compared with the traditional single wavelength, can reduce the fiber Brillouin scattering threshold at the same optical power level, enabling higher power or pulse energy amplification.

[0035] Optionally, the modulator group 5 implements frequency shifting and pulse modulation functions, including at least two modulators; the modulators are acousto-optic modulators or electro-optic modulators. When the modulator is an electro-optic modulator, it can be a phase modulator or an intensity modulator. The intensity modulator can be a Mach-Zehnder type or a cascaded, parallel, orthogonal, or other modified structure based on the Mach-Zehnder type. The purpose of the phase or intensity modulator is to cause a certain change in the frequency of the seed laser.

[0036] On the other hand, a method for realizing real-time large dynamic range Doppler velocities lidar is provided, utilizing the measurement device described above, the method comprising:

[0037] Seed laser group 1 emits multiple laser beams, which are mixed into a single optical signal by a first beam combiner. The signal beam is then split into a signal beam and a local oscillator beam by a beam splitter 3. The local oscillator beam passes through an optical attenuator 11 and enters a coupler 12. The signal beam is split into multiple laser beams of different wavelengths by a first wavelength division multiplexer 4, which then enter corresponding modulators for frequency shifting and pulse modulation. The modulated signals are mixed into a single optical signal by a second beam combiner 6 and amplified by an fiber amplifier 7. The signal then enters a circulator and is split into multiple laser signals of different wavelengths by a second wavelength division multiplexer 9, which enter corresponding optical lenses. The optical lenses emit the signal laser into the air and receive backscattered signals from atmospheric aerosol particles. The backscattered signals received by each optical lens are combined by a second wavelength division multiplexer and mixed into a single scattered signal. This signal then enters a coupler 12 after passing through a circulator and undergoes heterodyne interference with the local oscillator beam signal. The heterodyne interference signal is detected by a balanced detector 13, which outputs an intermediate frequency electrical signal. The signal processing module 14 acquires the intermediate frequency electrical signal and processes the acquired data.

[0038] Taking the seed laser array emitting three laser beams of different wavelengths as an example, the signal processing module 14 controls the modulation signals loaded onto the modulator group 5 to be F1, F2, and F3, respectively, so that the optical signals of different wavelengths produce different frequency shifts. The acquired intermediate frequency signal is then subjected to an FFT operation to obtain its amplitude spectrum, which can be found in [reference needed]. Figure 2 Three signals with a certain frequency interval will appear on the amplitude spectrum. By controlling the frequency of the modulation signal loaded onto the modulator group through closed-loop control, the frequency values ​​of the three frequency signals on the spectrum can be stabilized at a fixed operating frequency point. At this time, the frequency of the modulation signal loaded onto the modulator group contains the frequency information of the Doppler velocity in three different laser line-of-sight directions. The Doppler frequency shift in the three different line-of-sight directions can be calculated, thereby calculating the velocity magnitude in the line-of-sight direction. Furthermore, the velocity components, velocity magnitude, and direction of the three axes can be retrieved.

[0039] The algorithm for achieving closed-loop control is as follows: (Refer to...) Figure 3Set the modulation frequency operating point F0j loaded onto modulator group 5, an appropriate proportional control coefficient K, and a frequency offset judgment threshold Ft. The values ​​of F02-F01, F03-F02, and Fmax-F03 must be greater than the frequency shift corresponding to the maximum relative velocity in each line-of-sight direction at the start of the measurement, where Fmax is the maximum measurable frequency value of the acquisition card. Set the initial modulation frequency of the modulator group to Fj = F0j, acquire the heterodyne intermediate frequency electrical signal, and perform FFT calculation to estimate the signal spectrum. Given the frequency fj, calculate Δfj=F0j–fj. If |Δfj|<Ft, then the Doppler frequency shift fdj=F0j–Fj in each line of sight direction. Based on fdj, the relative motion velocity Vj in each laser line of sight direction can be calculated. If |Δfj|>Ft, calculate the frequency value Fj=Fj+K×Δfj loaded onto the modulator. Change the frequency value Fj loaded onto the modulator and re-estimate the signal frequency and calculate Δfj. Repeat the above process until |Δfj|<Ft.

[0040] Another algorithm for achieving closed-loop control is: (refer to...) Figure 4 Estimate the frequency values ​​f1, f2, and f3 of each signal within each spectral range, and calculate the feedback quantity K×(F0j-fj), where j = 1, 2, and 3. Superimpose the feedback quantity as a negative feedback signal onto the modulator, so that the actual frequency of the modulated signal loaded by the modulator is F0j + ∑[K×(F0j-fj)]. After multiple feedbacks, the spectral estimate fj approaches the operating point frequency F0j, and the final Doppler frequency shift in each line of sight is the cumulative value of each feedback quantity fdj = ∑[K×(fj-F0j)]. Calculate the relative motion velocity Vj in each line of sight direction.

[0041] Relative velocity V in each line of sight j The calculation formula is Where λ is the laser wavelength. The inversion algorithm can obtain information such as the velocity components, magnitude, and direction of the three axes.

[0042] Optionally, the proportional control coefficient K can be set to 0.01–0.5.

Claims

1. A real-time large dynamic range laser Doppler velocimetry lidar device, characterized in that, The device comprises: a seed laser group (1), a first beam combiner (2), a beam splitter (3), a first wavelength division multiplexer (4), a modulator group (5), a second beam combiner (6), an optical fiber amplifier (7), a circulator (8), a second wavelength division multiplexer (9), and an optical lens group (10) connected in sequence by optical fibers. The beam splitter (3) is also connected to the coupler (12) and the balanced photodetector (13) in sequence by optical fiber; the circulator (8) is connected to the coupler (12) by optical fiber. The output of the first wavelength division multiplexer (4) is connected to each modulator in the modulator group (5) in a one-to-one correspondence; the output of the second wavelength division multiplexer (9) is connected to each optical lens in the optical lens group (10) in a one-to-one correspondence. The signal processing module (14) is electrically connected to the modulator group (5) and the balanced photodetector (13); The seed laser group (1) contains at least two lasers that emit narrow linewidth lasers with a linewidth range of 100Hz to 1MHz. The laser wavelengths or frequency intervals emitted by different lasers meet the channel spacing requirements of the wavelength division multiplexer. The seed laser array (1) emits multiple laser beams, which are then combined into a single optical signal by the first beam combiner (2). The signal beam is then split into a signal beam and a local oscillator beam by the beam splitter (3). The local oscillator beam enters the coupler (12), and the signal beam is split into multiple laser beams of different wavelengths by the first wavelength division multiplexer (4). These beams are then fed into corresponding modulators for frequency shifting and pulse modulation. The modulated signals are then combined into a single optical signal by the second beam combiner (6), amplified by the fiber amplifier (7), and subsequently amplified by the circulator (8). Finally, the signal is split into multiple wavelength beams by the second wavelength division multiplexer (9). Different laser signals enter the corresponding optical lenses respectively. The optical lenses emit the signal laser into the air and receive the backscattered signals of atmospheric aerosol particles. The backscattered signals received by each optical lens are combined into one scattering signal after being combined by the second wavelength division multiplexer (9). After passing through the circulator (8), it enters the coupler (12) and performs heterodyne interference with the local oscillator signal. The heterodyne interference signal is detected by the balanced photodetector (13) and outputs the intermediate frequency electrical signal. The signal processing module (14) collects the intermediate frequency electrical signal and processes the collected data.

2. The real-time large dynamic range laser Doppler velocimetry lidar device according to claim 1, characterized in that, The device also includes an optical attenuator (11), which is disposed between the beam splitter (3) and the coupler (12) and is connected to the beam splitter (3) and the coupler (12) respectively via optical fibers.

3. The real-time large dynamic range laser Doppler velocimetry lidar device according to claim 1, characterized in that, The modulator group (5) includes at least two modulators; the modulators are acousto-optic modulators or electro-optic modulators.

4. The real-time large dynamic range laser Doppler velocimetry lidar device according to claim 3, characterized in that, When the modulator is an electro-optic modulator, a phase modulator or an intensity modulator is selected.

5. The real-time large dynamic range laser Doppler velocimetry lidar device according to claim 4, characterized in that, The intensity modulator is selected from Mach-Zehnder type or cascaded, parallel, orthogonal or other modified structures based on Mach-Zehnder type.

6. A method for realizing real-time large dynamic range Doppler velocities, employing a real-time large dynamic range laser Doppler velocities lidar device as described in any one of claims 1-5, characterized in that, Includes the following steps: The seed laser array (1) emits multiple laser beams, which are then combined into a single optical signal by the first beam combiner (2). The signal beam is then split into a signal beam and a local oscillator beam by the beam splitter (3). The local oscillator beam enters the coupler (12), and the signal beam is split into multiple laser beams of different wavelengths by the first wavelength division multiplexer (4). These beams are then fed into corresponding modulators for frequency shifting and pulse modulation. The modulated signals are then combined into a single optical signal by the second beam combiner (6), amplified by the fiber amplifier (7), and subsequently amplified by the circulator (8). Finally, the signal is split into multiple wavelength beams by the second wavelength division multiplexer (9). Different laser signals enter the corresponding optical lenses respectively. The optical lenses emit the signal laser into the air and receive the backscattered signals of atmospheric aerosol particles. The backscattered signals received by each optical lens are combined into one scattering signal after being combined by the second wavelength division multiplexer (9). After passing through the circulator (8), it enters the coupler (12) and performs heterodyne interference with the local oscillator signal. The heterodyne interference signal is detected by the balanced photodetector (13) and outputs the intermediate frequency electrical signal. The signal processing module (14) collects the intermediate frequency electrical signal and processes the collected data. The signal processing module (14) controls multiple modulation signals loaded onto the modulator group 5 to generate different frequency shifts in optical signals of different wavelengths, and performs FFT operation on the acquired intermediate frequency signal to obtain the corresponding amplitude spectrum. The amplitude spectrum includes frequency signals corresponding to the number of modulation signals. By controlling the frequency of the modulation signals loaded onto the modulator group (5) through closed loop, the frequency value of the frequency signal on the amplitude spectrum is stabilized at a fixed frequency operating point. At this time, based on the frequency information of multiple different laser line-of-sight Doppler velocities contained in the modulation signal frequencies loaded onto the modulator group (5), the Doppler frequency shifts in multiple different laser line-of-sight directions are calculated, thereby calculating the line-of-sight velocity magnitude, and further retrieving the three-axis velocity components, velocity magnitude, and direction information.

7. The method for realizing real-time large dynamic Doppler velocities according to claim 6, characterized in that, The process for achieving closed-loop control is as follows: Set the modulation frequency operating point applied to modulator group 5. F0j Let j = 1, 2, 3..., and set the proportional control coefficient. K and frequency offset judgment threshold Ft ,Require F02-F01 , F03-F02, Fmax-F03 The value is greater than the frequency shift corresponding to the maximum relative velocity in each line-of-sight direction at the start of the measurement, where Fmax This is the maximum measurable frequency value of the data acquisition card; Set the initial modulation frequency of the modulator group. Fj=F0j The heterodyne intermediate frequency electrical signal is acquired, and the spectrum of the signal is calculated by FFT to estimate the frequency of the signal. fj Calculate Δ fj=F0j –fj If |Δ fj |< Ft The Doppler frequency shift in each line of sight direction fdj=F0j –Fj ,according to fdj Calculate the relative velocity of each laser line of sight; if |Δ fj |> Ft Calculate the frequency value applied to the modulator. Fj=Fj+K× Δ fj Change the frequency value applied to the modulator Fj And re-estimate the signal frequency and calculate Δ fj Repeat the above process until |Δ fj |< Ft until.

8. A method for realizing real-time large dynamic Doppler velocities according to claim 7, characterized in that, The relative motion speeds of the laser line of sight directions V j The calculation formula is , where λ is the laser wavelength.

9. A method for realizing real-time large dynamic Doppler velocities according to claim 7, characterized in that, The proportional control coefficient K The value range is 0.01-0.5.

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