A method and device for intermediate frequency correction in a solid-state coherent laser wind lidar

Abstract

The application discloses a method and an implementation device for correcting the center frequency of a signal in real time in a full-solid-state remote coherent laser wind-radar, so as to eliminate the adverse effect of the center frequency jitter of a laser pulse on the identification and detection of a return signal. The application optimizes a detection device and a signal processing procedure on the basis of the full-solid-state remote coherent laser wind-radar, so as to correct the center frequency of each single-frequency pulse return signal in real time, improve the signal-to-noise ratio, and thus improve the detection distance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of all-solid-state laser and laser radar detecting atmosphere, in particular to a control method and device for eliminating the influence of non-coherent accumulation effect caused by the center frequency jitter of laser pulse emitted by all-solid-state coherent laser wind lidar. BACKGROUND

[0002] It is very important to master accurate atmospheric wind field information in the fields of aviation safety, meteorological disaster warning, wind power generation, etc. Coherent laser wind lidar has become one of the main means for detecting atmospheric wind field due to its good real-time performance, high precision, long detection distance, etc. The laser system as the main body of the coherent laser wind lidar determines the detection performance of the lidar.

[0003] From the wavelength of the emitted laser, the eye-safe band coherent laser wind lidar has become the main research direction at present due to its high atmospheric transmittance, good eye safety, compact structure, etc. Most of the coherent laser wind lidars reported at home and abroad at present are 1.5 μm band all-fiber system, which has stable structure and the pulse center frequency does not jitter. However, due to the low damage threshold of optical fiber and the influence of nonlinear effects, the output single-frequency laser energy is low, the detection distance is short, and the range resolution is poor. The coherent laser wind lidar based on all-solid-state single-frequency pulsed laser has high output single-pulse energy and can realize long-distance wind field detection. The all-solid-state single-frequency laser using injection locking technology is an ideal light source for long-range coherent laser wind lidar due to its compact structure, high precision, etc.

[0004] The essence of injection locking technology is frequency pulling technology, which controls the slave laser cavity length to match the seed light frequency. However, due to environmental interference, control mode, etc., the cavity length will have a certain jitter every cycle, so that the output pulse center frequency will change. In the coherent laser wind lidar, the return signal of a single pulse is very weak and difficult to identify, so the frequency spectrum signals of multiple pulses are usually accumulated to improve the signal-to-noise ratio and improve the detection distance. However, due to the jitter of the laser pulse center frequency, the signal will be broadened during frequency spectrum accumulation, thereby restricting the detection capability.

[0005] At present, there are few studies on the influence of the center frequency jitter of the single-frequency pulsed laser of the all-solid-state coherent laser wind lidar on the signal-to-noise ratio of the return signal, and how to eliminate the adverse effects of the intermediate frequency jitter on signal detection has not been reported. SUMMARY

[0006] The present application aims to provide an intermediate frequency correction method and device for the all-solid-state coherent laser wind lidar, so as to achieve the purpose of improving the signal-to-noise ratio and improving the detection distance without changing the structure of the laser system.

[0007] Therefore, the first aspect of the present application provides a full solid-state coherent laser wind radar intermediate frequency correction method, the method comprising:

[0008] a. A step of generating a narrow linewidth single frequency seed laser;

[0009] b. A step of realizing single frequency seed laser frequency shift amplification by controlling the slave laser cavity length to obtain single frequency pulsed laser output;

[0010] c. A step of monitoring the center frequency of the transmitted laser pulse and the locking probability;

[0011] d. A step of receiving the echo signal after the laser pulse irradiates the atmospheric scattering;

[0012] e. A step of realizing real-time correction of the center frequency according to the monitoring signal and the echo signal;

[0013] Characterized in that a small amount of single frequency pulsed laser of the slave laser is mixed with a part of the seed laser, and the center frequency information of each pulse of the laser can be obtained through the photodetector. The aerosol backscattering signal is received by the telescope system and mixed with the local oscillator light, and the spectrum of each pulse of the echo signal is moved and corrected according to the value of the center frequency of the transmitted pulse, and finally the N corrected echo signal spectra are accumulated and averaged.

[0014] Further, the center frequency υ of each pulse of the laser is obtained by fast Fourier transform (FFT) of the time domain signal.

[0015] Further, the seed laser is injected into the slave laser after frequency shift by the Q-switch crystal, and the frequency shift frequency is υ0, that is, the pulse center frequency jitter value is υ-υ0 relative to the frequency shift frequency υ.

[0016] Further, the spectrum data of each range bin of the echo signal is obtained by analog-to-digital conversion, time domain windowing and fast Fourier transform of the photodetector, and the intermediate frequencies of the signals of m range bins are respectively υ1, υ2, …, υ m .

[0017] Further, the intermediate frequencies of the signals of m range bins are respectively moved by υ-υ0, that is, the intermediate frequencies of the signals of each range bin are respectively υ1-(υ-υ0), υ2-(υ-υ0), …, υ m -(υ-υ0).

[0018] Further, the N pulse frequency domain signals are respectively corrected and accumulated and averaged in the frequency spectrum to improve the signal-to-noise ratio.

[0019] Further, the wind speed value in each range bin in the time period is:

[0020]

[0021] wherein v is a radial wind speed, is the center frequency of the mth accumulation average, and λ is the laser emission wavelength.

[0022] In a second aspect, the present application provides a kind of solid-state coherent laser wind-radar intermediate frequency correction device, the device uses the intermediate frequency correction method described above.The device includes single frequency continuous laser, pulse driven laser, optical transceiver system, intermediate frequency correction and wind speed inversion system.

[0023] Single frequency continuous laser is used to generate narrow linewidth single frequency continuous laser, a part is injected into driven laser as seed light, and another part is used as local light to heterodyne detection with echo signal.

[0024] Pulse driven laser is used to generate large energy single frequency pulse laser, and single frequency seed laser is injected into driven laser after frequency shifting through frequency shifting crystal, and large energy single frequency pulse laser output is realized through injection locking control circuit and Q switch.

[0025] Optical transceiver system includes first polarization beam splitter prism, pre-beam expander system, second polarization beam splitter prism, quarter wave plate, telescope system, first coupler, beam splitter, first 2x2 fiber coupler, first balanced detector, second coupler, second 2x2 fiber coupler and second balanced detector.

[0026] The narrow linewidth single frequency continuous laser is divided into first light beam and second light beam, the first light beam is used as seed laser, and the second light beam is divided into third light beam and fourth light beam through fiber beam splitter. The third light beam is used as local light to beat with echo signal; the fourth light beam is beat with a small amount of single frequency pulse laser obtained by polarization beam splitter prism, and the center frequency of each transmission pulse is monitored.

[0027] The single frequency pulse laser is used to emit large energy single frequency pulse laser, and most of the output laser is expanded by pre-beam expander system after passing through first polarization beam splitter prism, and the laser after beam expansion passes through second polarization beam splitter prism and quarter wave plate, and is further expanded and compressed by telescope system to emit into the atmosphere, and the laser irradiated on moving aerosol particles will produce Doppler shift, and the aerosol backscattering light is received by telescope system again, and then passes through quarter wave plate and first polarization beam splitter prism to be received by first coupler.

[0028] The first 2x2 fiber coupler is used to mix the second light beam with echo signal; and the second 2x2 coupler is used to mix the third light beam with a small amount of pulse laser obtained by first polarization beam splitter prism.

[0029] The balance detector is used for converting the beat frequency signal into a photoelectric current, and an output electric signal is connected with a frequency correction and wind speed inversion module, so as to perform analog-digital conversion, central frequency correction and wind speed inversion;

[0030] The intermediate frequency correction and wind speed inversion system comprises a high-speed ADC, a programmable logic gate circuit FPGA and an engineering machine. The high-speed ADC converts the output analog signal of the detector into a digital signal. The programmable logic gate circuit FPGA is used for realizing intermediate frequency monitoring, fast Fourier transform, intermediate frequency correction and spectrum cumulative average of each transmitting pulse. The obtained spectrum signal is sent to the industrial computer through a gigabit Ethernet port, and is used for inverting and displaying three-dimensional real-time wind field information.

[0031] The present application has at least the following advantages:

[0032] (1) The intermediate frequency correction method of the all-solid-state coherent laser wind-radar provided by the present application takes the frequency shift crystal frequency as a reference, monitors the intermediate frequency jitter information of each pulse through the balance detector, and thus corrects the frequency of the spectrum signal of each pulse echo signal in all distance bins. The signal-to-noise ratio can be improved by spectrum accumulation, so as to improve the detection distance.

[0033] (2) The intermediate frequency correction method of the all-solid-state coherent laser wind-radar provided by the present application can not only obtain the intermediate frequency jitter information of each transmitting laser pulse through the balance detector, but also monitor the output energy of the single-frequency pulse laser according to the signal amplitude of the detector, so as to facilitate integration and increase the reliability of the system.

[0034] (3) The intermediate frequency correction device of the all-solid-state coherent laser wind-radar provided by the present application can realize real-time correction of the pulse intermediate frequency through the high-speed ADC and the FPGA, and obtain the denoised spectrum information in real time, without requiring high frequency stability of the laser, thereby reducing the complexity of the system.

[0035] (4) The present application adopts single-frequency continuous laser, which is divided into seed light and local oscillator light through a coupler, so as to realize the functions of injection locking, pulse intermediate frequency monitoring and echo detection, and has simple structure and high reliability. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the examples of the present application, the drawings needed in the following embodiment description will be briefly introduced. Obviously, the drawings in the following description are only examples of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0037] Figure 1 A schematic diagram of the intermediate frequency correction method and device of the all-solid-state coherent laser wind-radar provided by the present application is shown in the following figure.

[0038] Figure 2 The statistical result of the center frequency of the pulse emitted by the single-frequency pulsed laser in one embodiment of the application is provided;

[0039] Figure 3 The intermediate frequency correction flowchart of the all-solid-state coherent laser wind-radar provided by the embodiment of the application is provided;

[0040] Figure 4 The echo signal acquisition and processing timing diagram provided by the embodiment of the application is provided;

[0041] Figure 5 The cumulative average result diagram after the intermediate frequency correction and the intermediate frequency correction provided by the embodiment of the application are provided.

[0042] The mark explanation is as follows:

[0043] 1-narrow linewidth single-frequency continuous laser; 2-single-frequency pulsed laser; 3-first polarization beam splitter prism; 4-pre-expansion beam system; 5-second polarization beam splitter prism; 6-quarter wave plate; 7-Galilean telescope system; 8-first coupler; 9-beam splitter; 10-first 2*2 fiber coupler; 11-first balanced detector; 12-second coupler; 13-second 2*2 fiber coupler; 14-second balanced detector; and 15-intermediate frequency correction and wind speed inversion system. DETAILED DESCRIPTION

[0044] The core of the application is to provide an intermediate frequency correction method and device of an all-solid-state coherent laser wind-radar. The control method and device are used for eliminating the influence of the center frequency jitter of the laser pulse emitted by a laser on the non-coherent accumulation effect. The single-pulse energy of the all-solid-state single-frequency laser is large, and the wind field can be detected at a long distance. However, the jitter of the intermediate frequency of the emitted pulse is inevitable, which greatly limits the improvement of the signal-to-noise ratio and the improvement of the detection distance. The intermediate frequency correction method and device of the all-solid-state coherent laser wind-radar provided by the application corrects the echo signal spectrum according to the intermediate frequency jitter information of the emitted laser pulse, improves the signal-to-noise ratio, and thus improves the detection distance.

[0045] Referring to the accompanying drawings, Figure 1 the specific method of the application is as follows:

[0046] An intermediate frequency correction device of an all-solid-state coherent laser wind-radar, which comprises a narrow linewidth single-frequency continuous laser 1, a single-frequency pulsed laser 2, a first polarization beam splitter prism 3, a pre-expansion beam system 4, a second polarization beam splitter prism 5, a quarter wave plate 6, a Galilean telescope system 7, a first coupler 8, a beam splitter 9, a first 2*2 fiber coupler 10, a first balanced detector 11, a second coupler 12, a second 2*2 fiber coupler 13, a second balanced detector 14, and an intermediate frequency correction and wind speed inversion system 15. Among them,

[0047] A narrow linewidth single frequency continuous laser 1 is used to generate single frequency, narrow linewidth, high beam quality continuous laser; preferably, the single frequency continuous laser is a monolithic non-planar ring cavity, which is structurally stable, and outputs single frequency laser with narrow linewidth and low noise.

[0048] A single frequency pulsed laser 2 is used to generate single frequency, high energy, high beam quality pulsed laser; the single frequency seed light is injected into the driven laser after frequency shifting, and the cavity length of the driven laser is controlled by injection locking technology to match the frequency of the seed light. However, due to environmental interference and other reasons, the cavity length of each cycle will have a certain jitter, so that the center frequency of the output pulse will change. Figure 2 The center frequency statistical results of the single frequency pulsed laser provided in the embodiment show that the minimum value of the center frequency of 1000 pulses is 68.49 MHz, the maximum value is 72.03 MHz, and the root mean square error is 1.98%.

[0049] A first polarization beam splitter prism 3 is used to extract a small amount of single frequency pulsed laser to obtain the center frequency of each pulse signal;

[0050] A pre-expansion system 4 is used for beam expansion and shaping of the laser beam to adapt to the entrance aperture of the telescope system;

[0051] A second polarization beam splitter prism 5 and a quarter-wave plate 6 are used for separation of the transmitted laser and the received echo signal;

[0052] A Galileo telescope system 7 including an aspheric negative lens 16 and an aspheric positive lens 17 is used for further expansion of the laser beam, compression of the divergence angle, and emission into the atmosphere;

[0053] A first coupler 8 is used to couple the echo signal into an optical fiber;

[0054] A beam splitter 9 is used to divide the single frequency continuous laser into two beams, which are used as the local oscillator light of the monitoring signal and the echo signal, respectively;

[0055] A first 2×2 fiber coupler 10 is used for mixing of the echo signal and the local oscillator light;

[0056] A first balanced detector 11 is used for photoelectric conversion of the echo mixing signal;

[0057] A second coupler 12 is used to couple the monitoring weak pulse signal into an optical fiber;

[0058] A second 2×2 fiber coupler 13 is used for mixing of the monitoring signal and the local oscillator light;

[0059] A second balanced detector 14 is used for photoelectric conversion of the monitoring mixing signal;

[0060] The intermediate frequency correction and wind speed inversion system performs frequency correction on the signal of each distance bin of the echo signal according to the intermediate frequency information of the transmitted pulse, and then performs spectrum accumulation averaging. The data result is sent to the host computer to realize real-time inversion of the wind field.

[0061] attached Figure 3 The intermediate frequency correction flowchart of the all-solid-state coherent laser wind radar corresponding to the embodiment of the present application.

[0062] Specifically, first, narrow linewidth single frequency continuous laser is generated, single frequency pulsed laser is obtained through injection locking technology, after passing through a polarization beam splitter prism 5, it is shaped and expanded through a pre-expansion system 4, after passing through a transmitting-receiving isolation system, it is further expanded and compressed by a Galileo telescope 7, and then is emitted to the atmosphere. The atmospheric backscattered light is received by the telescope system, and then passes through a quarter-wave plate 6 and a polarization beam splitter prism 5 again, and then is received and coupled into a single-mode polarization maintaining optical fiber by an aspheric coupler 8, and is mixed with one of the local light in a 2*2 optical fiber coupler 10, and is converted into photocurrent by a balanced detector 11 for collection and processing. Preferably, in order to eliminate the intermediate frequency jitter of the transmitted laser pulse and remove the out-of-lock pulse signal, the wind speed precision is improved, the signal-to-noise ratio is increased, and a monitoring signal of the intermediate frequency of the transmitted laser pulse is added. The coupler 12 is used to receive a small amount of pulsed laser, which is mixed with another local light in a 2*2 optical fiber coupler 13 through a single-mode polarization maintaining optical fiber, and then is optoelectronically converted by a balanced detector 14, and then the intermediate frequency monitoring of the laser pulse and the real-time correction of the center frequency of the echo signal are realized by FPGA.

[0063] In the specific implementation, the intermediate frequency correction step of the radar echo signal mainly includes:

[0064] Step 1: Transmitting laser pulse intermediate frequency monitoring. After the monitoring light mixing signal is analog-to-digital converted, fast Fourier transform and peak value identification are performed by FPGA. The center frequency of the single frequency laser pulse is υ, the frequency shift amount of the frequency shifter is υ0, and then the center frequency jitter value υ' of the transmitted laser pulse is υ-υ0.

[0065] Step 2: Echo signal collection and processing. Referring to Figure 4 For the echo signal collection and processing timing, the FPGA is connected with a high-speed high-precision ADC to collect and process the echo beat frequency signal in real time. The collection starts after a period of time from the rising edge of the TTL trigger signal. First, the window function is used to divide the echo signal into distance bins. Preferably, the distance bin is increased as much as possible under the premise of meeting the distance resolution, so as to improve the spectrum resolution. After windowing, the fast Fourier transform is performed on the signal of each distance bin, so as to obtain the frequency domain signal of each distance bin.

[0066] Step 3: Echo signal intermediate frequency correction. After the center frequencies υ1, υ2, …, υ mAfter determining the center frequency jitter value υ′ of the pulse, the center frequency of each range library is corrected, that is, the intermediate frequency of each range library is shifted to υ1-υ′, υ2-υ′, ..., υ′ respectively. m -υ′. Furthermore, due to limitations in the sampling rate of the acquisition board and the distance to the library width, let its spectral resolution be υ. s That is, the movement frequency corresponding to each point moved is υ. s Therefore, the number of points that need to be moved for each distance library is round[(υ1-υ′) / υ]. s ]、round[(υ2-υ′) / υ s ]、…、round[(υ m -υ′) / υ s ], where round(x) represents rounding x to the nearest integer. Preferably, the higher the spectral resolution, the more accurate the intermediate frequency correction.

[0067] Step 4: Accumulation Denoising and Wind Speed ​​Inversion Display. The echo signal from a coherent laser wind radar is a weak signal with a very low signal-to-noise ratio (SNR), thus requiring denoising processing. Frequency-domain incoherent accumulation can effectively improve the SNR of the echo signal. The FPGA corrects and stores the intermediate frequency (IF) signals of N pulses, and then accumulates and averages the echo signals of the N pulses. See [link / reference] Figure 5 The cumulative averaging results without IF correction and with IF correction show a greater improvement in signal-to-noise ratio (SNR) compared to the uncorrected signal, due to the elimination of pulse IF jitter. The reduced spectral width and improved SNR make the signal appear "slender" and easier to distinguish from noisy environments, thus increasing detection range. Furthermore, the FPGA transmits the denoised data after spectral cumulative averaging to an industrial computer via a gigabit Ethernet port. The industrial computer then performs wind speed inversion and 3D synthesis, enabling real-time display of the wind field.

[0068] In summary, this invention provides an intermediate frequency correction method and apparatus for all-solid-state coherent laser wind radar. By monitoring the center frequency of the emitted laser pulse, the intermediate frequency of the signal in each range library is corrected during echo signal data processing, which can improve the signal-to-noise ratio and increase the detection range without changing the laser structure.

[0069] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for intermediate frequency correction in a solid-state coherent laser wind lidar, characterized in that The method comprises the following steps: a. generating narrow linewidth single frequency seed laser; b. realizing single frequency seed laser frequency shift amplification and obtaining single frequency pulse laser output by controlling the slave laser cavity length; c. mixing a small amount of emitted pulse laser with part of the seed laser, monitoring the center frequency υ of each pulse in real time through a balanced detector, a high-speed ADC and an FPGA, and calculating the jitter value υ' = υ-υ0 relative to the frequency shift reference υ0; d. receiving the echo signal after the laser pulse irradiates the atmospheric scattering; e. realizing real-time correction of the center frequency according to the monitoring signal and the echo signal: • divide m distance banks and get each bank spectrum center frequency υ1, υ2,..., υm m • the signal intermediate frequency of each of the m distance bins is shifted by υ - υ0, i.e. the signal intermediate frequency of each distance bin is υ1- (υ - υ0), υ2- (υ - υ0),..., υm- (υ - υ0) m -(υ - υ0) Incoherent cumulative average of the corrected spectrum of N pulses f. Inverting the wind speed based on the cumulative average result: where v is the radial wind speed and λ is the laser wavelength, is the center frequency accumulated after the mth distance bin.

2. The wind finding radar intermediate frequency correction method of claim 1, wherein: The narrow linewidth single frequency seed laser is divided into three paths, the first path is used as the seed light to inject into the slave laser after frequency shift, the second path is used as the local light to mix with the echo signal, and the third path is mixed with the pulse monitoring light, and the mixing operation is realized through a 2*2 fiber coupler.

Images