Method and system for improving signal-to-noise ratio of wind measurement laser radar with pseudo-random code phase modulation

Abstract

The present application belongs to the technical field of laser radar measurement, and provides a pseudo-random code phase modulation wind measurement laser radar signal-to-noise ratio improving method and system, the method comprising: generating a pseudo-random code sequence, BPSK phase modulating a continuous wave seed laser to convert into an encoded pulse signal; transmitting the encoded pulse signal to the atmosphere, receiving a back wave and mixing with a local oscillator light; multiplying the coherent beat frequency signal with the same pseudo-random code sequence and integrating, so that the back wave signal of the matching distance gate obtains coherent accumulation gain in the correlation output, and the non-matching signal energy is expanded to a noise base, realizing correlation demodulation; performing spectrum analysis on the demodulated signal, and inverting the wind speed. The present application significantly improves the average power and processing gain under the premise of not improving the peak power through high duty cycle encoding pulse and pseudo-random code correlation demodulation, thereby improving the signal-to-noise ratio and detection distance.

Description

Technical Field

[0001] This invention relates to the field of lidar measurement technology, specifically to a method and system for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation. This method is suitable for achieving high signal-to-noise ratio and long-distance wind field detection under conditions of limited peak power (such as human eye safety limitations, flammable and explosive environments, and fiber nonlinear effects). Background Technology

[0002] Due to their high precision and high spatiotemporal resolution, lidar has been widely used in wind field detection, meteorological monitoring, and aviation safety. Current wind-measuring lidar primarily employs pulsed coherent detection, with some using continuous wave coherent detection.

[0003] Existing pulsed coherent wind lidar acquires echo signals by emitting high-energy short pulses. To improve the signal-to-noise ratio for long-range detection, it is usually necessary to increase the peak power of the emitted laser. However, increasing the peak power faces two limitations: first, nonlinear effects and device damage. Excessively high peak power can easily induce nonlinear effects such as stimulated Brillouin scattering in fiber amplifiers, limiting power output and potentially causing permanent damage to optical components; second, human eye safety limitations. Lidar is often used in outdoor environments and must comply with human eye safety standards (such as IEC 60825-1).

[0004] Furthermore, there is an irreconcilable contradiction between range resolution and signal-to-noise ratio in pulse radars that do not employ pseudo-random code phase modulation: to improve range resolution, the pulse width needs to be compressed, but this reduces signal energy; to improve the signal-to-noise ratio, the pulse width needs to be increased, but this sacrifices range resolution. Simultaneously, pulse radars without pseudo-random code phase modulation have a low duty cycle (typically less than 1%), meaning the system is idle most of the time, resulting in low average power and insufficient signal utilization.

[0005] To address the aforementioned issues, Chinese invention patent CN117452434A discloses a low-blind-zone lidar wind measurement method. This method combines pseudo-random phase coding with coherent reception and employs multiple changes in the sub-pulse interval to improve measurement accuracy and reduce blind zone. However, the pseudo-random phase coding in this scheme suffers from insufficient sidelobe suppression during related demodulation, limiting its ability to detect weak signals. Furthermore, this scheme requires repeated measurements and changes in the sub-pulse interval to obtain complete wind field information, resulting in long measurement times, poor real-time performance, and an inability to fundamentally solve the signal-to-noise ratio improvement problem under peak power limitations. Chinese invention patent CN109541636B discloses a blind-zone-free, high-range-resolution lidar wind measurement method. This method uses a wide-pulse, low-peak-power system and an algorithm to remove the influence of end-face signals to achieve blind-zone-free detection. However, this scheme relies on backend algorithm processing, and fiber optic reflection in a coaxial transceiver structure still leads to receiver channel saturation. The algorithm is complex and its real-time performance is limited. Additionally, the improvement in range resolution under the wide-pulse system remains limited. This invention achieves a balance between real-time performance and sidelobe-free operation by proposing hardware pulse modulation and a specific coding algorithm, overcoming the shortcomings of existing technologies that either have poor real-time performance or cannot completely eliminate sidelobe interference. Furthermore, none of the aforementioned existing technologies address the application of complementary sequence pairs in wind-measuring lidar, nor do they disclose a specific technical solution for combining coherent detection and correlation demodulation under a high duty cycle coded pulse system.

[0006] Therefore, there is an urgent need for a wind-measuring lidar solution that can effectively improve the signal-to-noise ratio and detection range under conditions of limited peak power. Summary of the Invention

[0007] This invention provides a method and system for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation, in order to solve technical problems in the prior art such as limited peak power, contradiction between range resolution and signal-to-noise ratio, and low average power.

[0008] To address the aforementioned technical problems, this invention provides a method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation, comprising: Generate a set of pseudo-random code sequences; Based on the pseudo-random code sequence, the continuous wave seed laser is subjected to binary phase shift keying (BPSK) phase modulation to generate a phase-modulated optical signal. The phase-modulated optical signal is converted into a coded pulse signal. The duty cycle of the coded pulse signal is higher than that of the pulse radar without pseudo-random code phase modulation, and the repetition frequency is greater than 10kHz. The coded pulse signal is transmitted into the atmosphere; The echo signal backscattered from the atmosphere is received and coherently mixed with the local oscillator light to obtain a coherent beat frequency signal. Using the same pseudo-random code sequence as the transmitter, the coherent beat frequency signal is multiplied by the pseudo-random code sequence and then integrated to coherently accumulate the energy of the echo signal of the matched range gate and expand the energy of the echo signal of the unmatched range gate into a noise floor, thus obtaining the correlated demodulated signal. Spectral analysis was performed on the demodulated signal to calculate the Doppler frequency shift and invert the wind speed.

[0009] Furthermore, the continuous wave seed laser is subjected to binary phase shift keying (BPSK) phase modulation, including: when the pseudo-random code is at the first logic level, the phase of the optical carrier is shifted by π; when the pseudo-random code is at the second logic level, the phase remains unchanged; and the pseudo-random code sequence is an M-sequence, and the symbols of the M-sequence are arranged continuously with no gap between adjacent symbols.

[0010] Furthermore, the coded pulse signal is chopped by an acousto-optic modulator to the phase-modulated optical signal. The repetition frequency and duty cycle of the acousto-optic modulator output are configured such that, when the peak power is limited, the average power is greater than the average power of the pulse radar without pseudo-random code phase modulation.

[0011] Furthermore, the pseudo-random code sequence is a pair of complementary sequences. When a pair of complementary sequences is used, the pair consists of a first sequence and a second sequence, and the sum of the autocorrelation functions of the first sequence and the second sequence is a unit impulse function.

[0012] Furthermore, the code length of the pseudo-random code sequence is 2. n -1, where n is an integer from 7 to 11, and the phase modulation code rate is from 100 kHz to 1 MHz.

[0013] Furthermore, spectral analysis is performed on the demodulated signal, including: determining the number of intercept points based on the sampling rate of the wind lidar system and the preset distance resolution; zero-padding the intercepted signal points to make the data length after zero-padding the preset length; and performing a fast Fourier transform on the zero-padding data to obtain the power spectrum.

[0014] Furthermore, the method employs an off-axis transceiver optical system, wherein the transmitting telescope and the receiving telescope are independently configured.

[0015] Furthermore, the demodulated signal is windowed using one of the following methods: Hanning window, Hamming window, or Blackman window, to suppress spectral leakage and improve the detection capability of weak signals.

[0016] A pseudo-random code phase modulation wind lidar system includes: A pseudo-random code generator is used to generate pseudo-random code sequences. Continuous wave laser, used to generate continuous wave seed laser; An electro-optic phase modulator, connected to a pseudo-random code generator and a continuous wave laser, is used to perform binary phase shift keying (BPSK) phase modulation on a continuous wave seed laser according to a pseudo-random code sequence to generate a phase-modulated optical signal. An acousto-optic modulator is used to convert phase-modulated optical signals into coded pulse signals. The duty cycle of the coded pulse signals is higher than that of pulse radars that do not use pseudo-random code phase modulation, and the repetition frequency is greater than 10kHz. Fiber optic amplifiers are used to amplify coded pulse signals; A transmitting telescope, used to transmit amplified coded pulse signals into the atmosphere; A receiving telescope used to receive echo signals backscattered from the atmosphere; An optical mixer is used to coherently mix the echo signal with the local oscillator light and output a coherent beat frequency signal. A balanced detector is used to convert coherent beat frequency signals into electrical signals; The correlation demodulator is used to multiply the electrical signal with the pseudo-random code sequence and then integrate it, so that the signal energy of the matched distance gate is coherently accumulated and the signal energy of the unmatched distance gate is expanded into the noise floor. The spectrum analysis module is used to perform a fast Fourier transform on the demodulated signal to obtain the power spectrum and calculate the Doppler frequency shift to retrieve the wind speed.

[0017] Furthermore, it also includes an off-axis transceiver optical system, in which the transmitting telescope and the receiving telescope are set up independently; It also includes a windowing processing module, which is used to window the demodulated signal to suppress spectral leakage.

[0018] Compared with existing technologies, this invention has the following advantages: By using pseudo-random code phase modulation and high duty cycle encoded pulses, the average power is significantly increased under peak power constraints, thereby significantly improving the signal-to-noise ratio and detection range; by utilizing correlation demodulation to obtain coding gain, the contradiction between range resolution and signal-to-noise ratio is resolved, enabling long-range detection with high range resolution; the use of complementary sequence pairs can completely eliminate autocorrelation sidelobes, with a sidelobe suppression ratio greater than 20dB, improving wind measurement accuracy and weak signal detection capability; windowing processing further suppresses sidelobe levels, enhancing weak signal detection capability, and is particularly suitable for low aerosol concentration or long-range wind field detection. Compared with existing pulse coding schemes using Gold codes, this invention uses M-sequences with continuously arranged symbols, reducing the gain non-uniformity of the fiber amplifier for different distance gate echoes by more than 50% at the same peak power.

[0019] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the written description and drawings.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 A schematic diagram illustrating the steps of a method for improving the signal-to-noise ratio of a wind-measuring lidar using pseudo-random code phase modulation. Figure 2 This is a schematic diagram illustrating the steps of a method for performing spectrum analysis on a demodulated signal. Figure 3 This is a schematic diagram of a wind-measuring lidar signal-to-noise ratio enhancement system based on pseudo-random code phase modulation. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of this invention.

[0023] Example 1: Basic BPSK encoded pulse scheme. For example... Figure 1As shown, in the method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation, the M-sequence symbols are arranged continuously with no gap between adjacent symbols. The pseudo-random code generator uses a field-programmable gate array to generate the M-sequence, and a continuous-wave laser outputs a 1550nm narrow-linewidth seed laser. An electro-optic phase modulator receives the pseudo-random code and performs binary phase-shift keying (BPSK) modulation: when the symbol is "1", the optical carrier phase is shifted by π; when the symbol is "0", the phase remains unchanged. The modulated optical signal enters an acousto-optic modulator, which chops the continuous-phase modulated light into a high-repetition-frequency, high-duty-cycle coded pulse signal. This pulse is amplified by an fiber optic amplifier and then emitted into the atmosphere through a transmitting telescope. Atmospheric backscattered echoes are received by a receiving telescope and coherently beat with the local oscillator light (a beam split from laser 2) in an optical mixer. The resulting signal is converted into an electrical signal by a balanced detector. A correlation demodulator multiplies the received signal with a locally identical pseudo-random code sequence and integrates the result. When the local code aligns with the echo signal code, the product integration achieves coherent accumulation, outputting a narrowband peak signal; otherwise, the output is a broadband noise floor. Subsequently, a spectrum analysis module performs a Fast Fourier Transform (4096 points, integration time 100ms) on the correlated demodulated signal to obtain the power spectrum, extract the Doppler shift, and retrieve the wind speed.

[0024] The correlation demodulation in this embodiment is essentially a discrete cross-correlation operation, mathematically expressed as: ,in, For the received coherent beat frequency signal, y It is a local pseudo-random code sequence. The delay offset is n, where n represents the symbol number and L is the length of the pseudo-random code sequence. When the value is 0, it means that the local pseudo-random code sequence is perfectly aligned with the modulation code sequence in the received signal. In this case, the product of x(n) and y(n) is multiplied with the same sign in each symbol period, and the integral result is... Reaching the maximum value corresponds to the effective accumulation of signal energy; while when When the value is not 0, the local code and the received code are misaligned, and the product of x(n) and y(n+θ) will cancel out in different code periods, resulting in an integral result. This results in a lower noise floor, which is achieved by scanning with a delayed offset θ and searching for... The peaking process enables precise capture of the arrival time of the target echo signal, thereby achieving selective extraction of atmospheric scattered echoes at specific distances and effectively suppressing noise and stray light interference at non-target distances. This is one of the core mechanisms by which pseudo-random code phase modulation technology improves the signal-to-noise ratio of wind-measuring lidar. By selecting pseudo-random codes with good autocorrelation characteristics (such as M-sequences), the contrast between the peak signal and the noise floor can be further enhanced, enabling accurate detection of target Doppler shift information even under weak echo conditions.

[0025] The working principle of this embodiment is as follows: Continuous laser light is converted into a pulse signal with specific coding characteristics through pseudo-random code phase modulation. The autocorrelation of the pseudo-random code is utilized to separate the signal from noise at the receiving end through correlation demodulation. Specifically, when the local pseudo-random code sequence is precisely aligned with the coded sequence in the echo signal, the integral result after multiplication will achieve coherent accumulation due to code element synchronization, outputting a narrowband peak signal, effectively highlighting the useful signal. When the code sequences are not aligned, the product integral output presents a broadband noise floor, thereby significantly suppressing background noise and non-target scattering interference. The M-sequence has sharp autocorrelation characteristics: the autocorrelation function R(τ) is equal to 0 when τ=0. For the sequence length, it is approximately -1 when |τ|≥1; therefore, when the received signal is perfectly aligned with the local code, the integral output after multiplication is at its maximum value (signal energy is concentrated); as long as it is offset by one symbol width, the integral value rapidly decays to near zero (the unmatched signal and noise are broadened), this process is called spread spectrum despreading or correlation demodulation; it should be noted that the essence of correlation demodulation is coherent accumulation: through matched filtering, the signal energy dispersed in multiple symbols is coherently superimposed, which improves the signal-to-noise ratio of the target echo. At the same time, the signal after coherent accumulation exhibits narrowband characteristics in the frequency domain (bandwidth is approximately 1 / T, where T is the code length), which facilitates the subsequent FFT extraction of Doppler frequency shift. Therefore, the subsequent Fast Fourier Transform can effectively extract the Doppler frequency shift of the narrowband signal, while the interference of the broadband noise floor on the spectrum analysis is significantly reduced; by performing spectrum analysis on the correlated demodulated signal to extract the Doppler frequency shift information, high-precision inversion of atmospheric wind speed is finally achieved. This scheme utilizes the coding gain of pseudo-random codes to improve the signal-to-noise ratio of the system under the same transmission power conditions, thereby enhancing the wind-measuring lidar's ability to detect weak echo signals.

[0026] In this embodiment, the pulse radar without pseudo-random code phase modulation is a traditional pulse radar with a repetition frequency ≤10kHz and a duty cycle <1%. Under the same peak power (human eye safety threshold), compared with traditional pulse radar, the average power is increased and the echo signal energy is significantly enhanced. Utilizing the sharp autocorrelation characteristics of the M-sequence, coherent accumulation is achieved through correlation demodulation, concentrating the signal energy into a narrowband peak value. Unmatched signals and noise are broadened to a broadband substrate, effectively suppressing noise interference and thus significantly improving the signal-to-noise ratio of the wind-measuring lidar. After processing by the spectrum analysis module, Doppler frequency shift information can be extracted more accurately, improving the accuracy and reliability of wind speed inversion, especially suitable for atmospheric detection environments with low signal-to-noise ratios.

[0027] Example 2: Complementary sequence pair scheme. The pseudo-random code generator uses complementary sequence pairs or M-sequences. When using complementary sequence pairs, the complementary sequence pair consists of two sequences A and B of equal length. During transmission, the first pulse is modulated and transmitted using sequence A. After a pulse repetition period, the second pulse is modulated and transmitted using sequence B. The receiver performs correlation demodulation using A and B respectively to obtain two correlation results, which are then added together. In addition, after correlation demodulation and before the fast Fourier transform, windowing processing (Hanning window, Hamming window, or Blackman window) is added to further suppress residual sidelobes.

[0028] The technical working principle and effects of this embodiment are as follows: The complementary sequence pair satisfies the following: the sum of the autocorrelation functions of A and B is a unit impulse function, that is, except for zero delay, all sidelobes are completely canceled. This makes it possible to effectively eliminate sidelobe interference and significantly improve the signal-to-noise ratio when the two results are added after correlation demodulation. Compared with the M sequence, the complementary sequence pair has a natural advantage in suppressing sidelobes, especially in multi-target environments or in the presence of strong clutter interference, and can more clearly distinguish target signals. The introduction of windowing processing further smooths the correlation results on the basis of the complementary sequence pair suppressing sidelobes. By selecting an appropriate window function (such as the Hanning window, which can effectively reduce the sidelobe level, the Hamming window, which achieves a good balance between the main lobe width and sidelobe suppression, and the Blackman window, which has a stronger sidelobe suppression capability), the residual sidelobe energy can be further attenuated, avoiding the adverse effects of sidelobe leakage on subsequent fast Fourier transform analysis, thereby improving the accuracy and reliability of wind speed measurement.

[0029] Example 3: Off-axis transceiver optical system. This example employs an off-axis transceiver optical system, where the transmitting and receiving telescopes are independently configured. The transmitting telescope emits coded pulse signals into the atmosphere, while the receiving telescope collects atmospheric backscattered echoes from a different angle. The off-axis structure prevents reflected light from the fiber end face in the transmitting optical path from directly entering the receiving channel, thus eliminating the blind zone problem inherent in continuous wave systems. The system design is simpler and more reliable. The optical axes of the transmitting and receiving telescopes can remain parallel or intersect at a small angle to optimize the transmit-receive overlap factor at different detection distances. Other parameters in this example are the same as in Example 1.

[0030] The working principle of this embodiment is as follows: because the transmitting and receiving optical paths are separated, the strong light during transmission will not directly couple to the receiving detector, and the receiving channel does not need to be shut down during transmission, allowing continuous reception of echo signals. Even when using high duty cycle coded pulses, saturation will not occur due to fiber reflection. Therefore, this solution can perform blind-zone-free detection over the entire distance range (from several meters to several kilometers), making it particularly suitable for scenarios requiring continuous observation of near-field wind fields.

[0031] The advantages of this embodiment are: the off-axis system avoids fiber optic reflection interference found in coaxial systems, eliminates the need for complex blind zone avoidance designs, simplifies system debugging, and ensures high reliability. By combining pseudo-random code phase modulation and high duty cycle coded pulses, long-distance, high signal-to-noise ratio wind field detection can be achieved under peak power-limited conditions.

[0032] Example 4: Combination scheme of pseudo-random code length. This example involves the selection of the pseudo-random code length and code rate. The code length of the pseudo-random code sequence is 2. n -1, where n is an integer from 7 to 11; the code rate for phase modulation is from 100 kHz to 1 MHz. In practical applications, the code length and code rate can be selected independently according to system requirements.

[0033] Code length L=2 n -1 determines the processing gain of the correlation demodulation, and the gain is... dB. The gain is approximately 21 dB when n=7 (L=127), approximately 27.1 dB when n=9 (L=511), and approximately 33.1 dB when n=11 (L=2047). Higher n values ​​result in greater processing gain and stronger extraction capabilities for weak signals, but also increase the data length for cross-correlation calculations, reducing processing efficiency and causing a decrease in real-time performance due to longer sequence periods. Experiments have shown that n=9 (code length 511) achieves a good balance between gain and real-time performance, making it suitable for most wind measurement scenarios; n=7 is suitable for scenarios with high real-time requirements and short detection distances; and n=11 is suitable for scenarios with extremely high detection distance requirements and acceptable low data update rates.

[0034] The code rate R (in kHz or MHz) determines the symbol width τ = 1 / R, and the theoretical distance resolution ΔR = c·τ / 2 = c / (2R). A higher code rate results in higher distance resolution, but also higher requirements for detector bandwidth and ADC sampling rate, increasing hardware costs. At a code rate of 100kHz, the theoretical distance resolution is approximately 1500m; at 500kHz, it's approximately 300m; and at 1MHz, it's approximately 150m. In practical systems, effective resolution can be further improved through sampling and interpolation algorithms. Experiments have shown that a code rate of 500kHz achieves a good balance between distance resolution (up to 60m) and hardware cost; 100kHz is suitable for cost-sensitive applications; 1MHz is suitable for short-range wind measurement scenarios with extremely high distance resolution requirements; however, at a code rate of 80kHz, the distance resolution > 2000m does not meet practical requirements; and at a code rate of 1.2MHz, the detector bandwidth is insufficient, resulting in a 3dB decrease in signal-to-noise ratio.

[0035] In this embodiment, n=9 (code length 511) and code rate 500kHz are the preferred combination. Under the condition of peak power 10μJ, this combination achieves a measured detection distance of 500 meters, a signal-to-noise ratio improvement of approximately 12dB, a distance resolution of 60m, and a moderate computational load. Other combinations of n values ​​(7, 8, 10, 11) and code rates (100kHz, 200kHz, 1MHz, etc.) can be selected according to the actual scenario requirements, and all fall within the protection scope of this invention.

[0036] The technical advantage of this embodiment is that by providing a flexible selection range of code length n=7-11 and code rate 100kHz-1MHz, the performance of the wind-measuring lidar is optimized and flexibly adapted to different application scenarios. Users can independently select the code length and code rate according to indicators such as detection distance, signal-to-noise ratio requirements, hardware cost, and real-time performance, thereby obtaining the optimal wind-measuring performance.

[0037] Example 5: Perform spectrum analysis on the demodulated signal, such as... Figure 2 As shown, the specific steps include: Based on the actual sampling rate of the wind-measuring lidar system and the preset distance resolution, combined with the speed of light, the corresponding time resolution is calculated, thereby obtaining the number of sampling points corresponding to a single distance gate; this value serves as the original signal truncation length, ensuring that the spectrum analysis has time-domain support that strictly matches the distance resolution; zero-padding is performed on the truncated signal point sequence to extend its total length to the preset length of the fast Fourier transform. Zero padding is only performed at the end of the sequence, without changing the physical information of the signal, but improving the spectrum interpolation density and frequency resolution visualization accuracy; a fast Fourier transform is performed on the zero-padding signal sequence to obtain the final power spectrum, which is used for Doppler frequency shift identification and wind speed inversion.

[0038] The working principle and technical effects of this embodiment are as follows: By accurately calculating the number of sampling points per distance gate, the time-domain window of the spectrum analysis is tightly coupled with the distance resolution, avoiding the distance-frequency confusion problem caused by improper data truncation, and providing an accurate time-domain-frequency domain correspondence for subsequent wind speed inversion; the zero-fill operation effectively improves the frequency resolution and graphical display effect of the power spectrum without introducing false signals, making the peak characteristics of the Doppler frequency shift clearer and facilitating the accurate extraction of wind speed information; the fast Fourier transform realizes the efficient conversion of the signal from the time domain to the frequency domain, providing technical support for real-time wind speed monitoring; this spectrum analysis scheme significantly improves the identification accuracy and reliability of Doppler frequency shift in weak echo signals by wind-measuring lidar, thereby improving the accuracy and stability of wind speed measurement.

[0039] Example 6: As Figure 3As shown, the wind-measuring lidar system with pseudo-random code phase modulation includes: a pseudo-random code generator implemented using a field-programmable gate array (FPGA), configurable to generate M-sequences, or Gold sequences or complementary sequence pairs (as other implementations) as required by design, outputting TTL-level pseudo-random codes; a continuous-wave laser, a narrow-linewidth (<10kHz) fiber laser with an output wavelength of 1550nm and a power of 10-50mW; an electro-optic phase modulator with a bandwidth ≥1GHz and a half-wave voltage ≤5V, receiving the pseudo-random code drive signal for BPSK modulation, shifting the optical carrier phase by π when the pseudo-random code is at the first logic level, and maintaining the phase unchanged when it is at the second logic level; an acousto-optic modulator to chop the continuous-phase modulated light into coded pulses, and simultaneously providing a frequency shift (typically 80MHz) to distinguish stray light; and an erbium-doped fiber amplifier to amplify the average pulse power to the required level (e.g., 10μJ single-wave power). The transmitting telescope is a beam-expanding and collimating optical system. In this embodiment, the beam-expanding magnification of the transmitting telescope is 10 times, and the aperture is 50mm. It is used to collimate and expand the amplified pulse before transmitting it into the atmosphere. The receiving telescope is used to collect atmospheric backscattered echoes. It can be shared with the transmitting telescope (through a circulator) or separate. In this embodiment, the aperture of the receiving telescope is 100mm, and the field of view is 1mrad. It is used to efficiently collect atmospheric backscattered echoes. The optical mixer is a 2×2 fiber coupler. It coherently mixes the echo signal with the local oscillator. In this embodiment, the splitting ratio of the optical mixer is 50:50, and the insertion loss is less than 1dB to ensure coherent mixing efficiency. The balanced detector consists of two matched photodiodes with differential outputs. It is used to suppress common-mode noise and improve the signal-to-noise ratio. In this embodiment, the bandwidth of the balanced detector is 350MHz, and the common-mode rejection ratio is greater than 25dB. It can effectively suppress laser intensity noise and common-mode interference. The correlation demodulator is a digital signal processing module (FPGA) that employs a multi-channel parallel pipeline architecture to achieve real-time correlation demodulation of all distance gates. In this embodiment, the correlation demodulator has a 16-bit processing bit and a core clock frequency of 200MHz. The system sampling rate is 400MHz (corresponding to a sampling interval of 2.5ns). Since the pseudo-random code length is a preset value, to complete the sliding correlation calculation for all distance gates within each pulse cycle, the correlation demodulator instantiates multiple parallel correlation operation units within the FPGA. Each operation unit is assigned a different delay offset and performs multiplication and accumulation operations simultaneously. All correlation operation units share the input sampled data stream and the local pseudo-random code sequence. Data buffering and pipeline scheduling are implemented using the block RAM (BRAM) within the FPGA, ensuring that the calculation delay of each channel is less than one sampling cycle. By pipeline cascading and timing constraints on the outputs of each channel, the correlation values ​​of all distance gates can be obtained after each sampling point, thereby completing the demodulation of all distance gates within the pulse repetition cycle. The local pseudo-random code sequence is pre-stored in the read-only memory inside the FPGA. Each relevant arithmetic unit uses the sampling clock as a synchronization signal to continuously slide and multiply the sampled data point by point and accumulate the results. The accumulation period is consistent with the code length. This parallel architecture fully utilizes the parallel processing capability of the FPGA, enabling real-time demodulation of the full range gate within each pulse period, without the need for serial calculation of each range gate.

[0040] The spectrum analysis module obtains the power spectrum by performing a fast Fourier transform on the correlated demodulated signal and calculates the Doppler frequency shift to invert the wind speed. In this embodiment, the FFT points of the spectrum analysis module are 4096 points and the integration time is 100ms, which is consistent with the parameters in Embodiment 1. With this setting, a data update rate of about 10Hz can be obtained, which meets the requirements of real-time wind field detection. The system also includes an off-axis transceiver optical system, with the transmitting telescope and the receiving telescope set independently. It also includes a windowing processing module (used to window the correlated demodulated signal to suppress spectrum leakage).

[0041] The technical working principle of this embodiment is as follows: a pseudo-random code generator generates a pseudo-random code sequence; a continuous wave laser outputs a seed laser; an electro-optic phase modulator performs BPSK phase modulation on the seed laser according to the pseudo-random code; an acousto-optic modulator chops the phase-modulated light into a high duty cycle coded pulse; an optical fiber amplifier increases the average power of the pulse; a transmitting telescope directs the pulse into the atmosphere; a receiving telescope collects the backscattered echo; an optical mixer and a balanced detector achieve coherent detection, converting the optical signal into an electrical signal; a correlation demodulator performs matched filtering on the electrical signal to achieve distance resolution; and a spectrum analysis module performs FFT on the demodulated signal to extract the Doppler frequency shift and invert the wind speed.

[0042] This system significantly improves the signal-to-noise ratio and range resolution of wind-measuring lidar by combining pseudo-random code phase modulation with coherent detection technology. The flexible configuration of the pseudo-random code (M-sequence, Gold sequence, or complementary sequence pairs) optimizes coding characteristics according to different detection requirements, with complementary sequence pairs particularly effective in suppressing sidelobe interference. The high bandwidth (≥1GHz) and low half-wave voltage (≤5V) of the electro-optic phase modulator ensure accurate and high-speed modulation of the laser phase, laying the foundation for generating high-quality coded pulses. The acousto-optic modulator provides a fixed frequency shift while achieving pulse chopping, effectively distinguishing atmospheric echoes from stray light within the system and reducing background noise. The differential output design of the balanced detector maximizes the suppression of common-mode noise, further improving the received signal strength. The signal-to-noise ratio (SNR) of the signal is improved. The core function of the correlation demodulator is to compress the echo signal energy of a specific distance gate in the time domain and concentrate it in the frequency domain through matched filtering, while the interference signal energy of the non-matched distance is dispersed into the noise floor, thereby achieving high range resolution while improving the SNR. The spectrum analysis module accurately extracts Doppler frequency shift information through FFT and finally inverts the atmospheric wind speed. The off-axis transceiver optical system can avoid fiber reflection interference in the transmitting optical path, reduce the risk of receiving channel saturation, and simplify system debugging. The windowing processing module can effectively suppress the spectrum leakage phenomenon in the FFT process and improve the accuracy of wind speed measurement. The overall system design is compact, and the modules work together to achieve high-precision and high-sensitivity detection of wind fields in complex atmospheric environments.

[0043] The technical effect of this embodiment is that the system can use any one or more combinations of embodiments 1-4 according to design requirements to achieve high signal-to-noise ratio, long distance, and high precision wind field detection under peak power limited conditions (such as human eye safety and flammable and explosive environments).

[0044] The wind-measuring lidar method and system provided by this invention can effectively improve the signal-to-noise ratio and detection distance in outdoor environments with strict human eye safety restrictions (cities, airports, highways), flammable and explosive hazardous gas environments (mines, chemical plants), and high-power systems where peak power is limited by fiber nonlinearity.

[0045] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims and their equivalents, this invention is also intended to include these modifications and variations.

Claims

1. A method for improving the signal-to-noise ratio of wind-measuring lidar based on pseudo-random code phase modulation, characterized in that, include: Generate a set of pseudo-random code sequences; Based on the pseudo-random code sequence, the continuous wave seed laser is subjected to binary phase shift keying (BPSK) phase modulation to generate a phase-modulated optical signal; The phase-modulated optical signal is converted into a coded pulse signal. The duty cycle of the coded pulse signal is higher than that of the pulse radar without pseudo-random code phase modulation, and the repetition frequency is greater than 10kHz. The coded pulse signal is transmitted into the atmosphere; The echo signal backscattered from the atmosphere is received and coherently mixed with the local oscillator light to obtain a coherent beat frequency signal. Using the same pseudo-random code sequence as the transmitter, the coherent beat frequency signal is multiplied by the pseudo-random code sequence and then integrated to coherently accumulate the energy of the echo signal of the matched range gate and expand the energy of the echo signal of the unmatched range gate into a noise floor, thus obtaining the correlated demodulated signal. Spectral analysis was performed on the demodulated signal to calculate the Doppler frequency shift and invert the wind speed.

2. The method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation according to claim 1, characterized in that, The continuous wave seed laser is subjected to binary phase shift keying (BPSK) phase modulation, including: when the pseudo-random code is at the first logic level, the phase of the optical carrier is shifted by π; when the pseudo-random code is at the second logic level, the phase remains unchanged; and the pseudo-random code sequence is an M-sequence, and the symbols of the M-sequence are arranged continuously with no gap between adjacent symbols.

3. The method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation according to claim 1, characterized in that, The coded pulse signal is chopped by an acousto-optic modulator to chop the phase-modulated optical signal. The repetition frequency and duty cycle of the acousto-optic modulator output are configured such that, when the peak power is limited, the average power is greater than the average power of the pulse radar without pseudo-random code phase modulation.

4. The method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation according to claim 1, characterized in that, The pseudo-random code sequence is a pair of complementary sequences. When a pair of complementary sequences is used, the pair consists of a first sequence and a second sequence. The sum of the autocorrelation functions of the first sequence and the second sequence is the unit impulse function.

5. The method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation according to claim 1, characterized in that, The code length of the pseudo-random code sequence is 2. n -1, where n is an integer from 7 to 11, and the phase modulation code rate is from 100 kHz to 1 MHz.

6. The method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation according to claim 1, characterized in that, The demodulated signal is subjected to spectrum analysis, including: determining the number of intercept points based on the sampling rate of the wind lidar system and the preset distance resolution; zero-padding the intercepted signal points to make the length of the zero-padding data the preset length; and performing a fast Fourier transform on the zero-padding data to obtain the power spectrum.

7. The method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation according to claim 1, characterized in that, The method employs an off-axis transceiver optical system, in which the transmitting telescope and the receiving telescope are set up independently.

8. The method for improving the signal-to-noise ratio of wind-measuring lidar using pseudo-random code phase modulation according to claim 4, characterized in that, The demodulated signal is windowed using one of the following methods: Hanning window, Hamming window, or Blackman window, to suppress spectral leakage and improve the detection capability of weak signals.

9. A signal-to-noise ratio enhancement system for wind-measuring lidar based on pseudo-random code phase modulation, characterized in that, include: A pseudo-random code generator is used to generate pseudo-random code sequences. Continuous wave laser, used to generate continuous wave seed laser; An electro-optic phase modulator, connected to a pseudo-random code generator and a continuous wave laser, is used to perform binary phase shift keying (BPSK) phase modulation on a continuous wave seed laser according to a pseudo-random code sequence to generate a phase-modulated optical signal. An acousto-optic modulator is used to convert phase-modulated optical signals into coded pulse signals. The duty cycle of the coded pulse signals is higher than that of pulse radars that do not use pseudo-random code phase modulation, and the repetition frequency is greater than 10kHz. Fiber optic amplifiers are used to amplify coded pulse signals; A transmitting telescope, used to transmit amplified coded pulse signals into the atmosphere; A receiving telescope used to receive echo signals backscattered from the atmosphere; An optical mixer is used to coherently mix the echo signal with the local oscillator light and output a coherent beat frequency signal. A balanced detector is used to convert coherent beat frequency signals into electrical signals; The correlation demodulator is used to multiply the electrical signal with the pseudo-random code sequence and then integrate it, so that the signal energy of the matched distance gate is coherently accumulated and the signal energy of the unmatched distance gate is expanded into the noise floor. The spectrum analysis module is used to perform a fast Fourier transform on the demodulated signal to obtain the power spectrum and calculate the Doppler frequency shift to retrieve the wind speed.

10. The signal-to-noise ratio enhancement system for wind-measuring lidar with pseudo-random code phase modulation according to claim 9, characterized in that, It also includes an off-axis transceiver optical system, in which the transmitting telescope and the receiving telescope are set up independently; It also includes a windowing processing module, which is used to window the demodulated signal to suppress spectral leakage.

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