Calibration method of multi-longitudinal mode Doppler lidar based on dual FP interferometer

Abstract

The present invention discloses a multi-longitudinal mode Doppler laser radar calibration method based on dual F-P interferometers. The cavity length of the dual F-P interferometer is adjustable within a large range, and has two tuning modes: coarse adjustment and fine adjustment, which are controlled by an F-P interferometer controller; the calibration light source is multi-longitudinal mode, quasi-continuous backscattered light after the emitted laser pulse is incident on a long bare optical fiber. During calibration, a method of alternating coarse and fine scanning of the F-P interferometer cavity length is adopted. The present invention can accurately and conveniently realize the calibration of a multi-longitudinal mode Doppler laser radar system based on a dual F-P interferometer without changing the laser radar optical path structure, including the laser longitudinal mode spacing Λ and the free spectrum spacing ν of the F-P interferometer. FSR Matching calibration and dual F‑P interferometer transmittance curve calibration, between Λ and ν FSR Under the premise of matching, the optimal dual F-P interferometer transmittance calibration curve is obtained, so that the detection performance of the multi-longitudinal mode Doppler lidar system can be optimized.

Description

Technical Field

[0001] The invention relates to a multi-longitudinal mode laser radar calibration method, and in particular to a multi-longitudinal mode Doppler laser radar calibration method based on a dual FP interferometer. Background Art

[0002] At present, the Doppler laser radar, whether it adopts the coherent detection system or the direct detection system, uses a narrow linewidth, stable frequency single longitudinal mode laser as the emission source. This makes the existing Doppler laser radar system have relatively strict requirements on the working environment. Once the environmental conditions are not met, the detection performance of the system may be affected, or even unable to work normally, which seriously restricts the industrial application and airborne and satellite applications of Doppler laser radar. If the Doppler laser radar can use a multi-longitudinal mode laser as the emission light source, it can not only reduce the system cost, reduce the system volume, improve the system stability and environmental adaptability, but also improve the system measurement accuracy. Therefore, trying to use a multi-longitudinal mode laser as the emission source of Doppler laser radar is one of the current research frontiers. In 2008, Gong Mali et al. conducted a preliminary analysis and study on the feasibility of using a multi-longitudinal mode laser as the emission source of the FP interferometer double edge Doppler laser radar, and proposed that in order to realize Doppler measurement based on a multi-longitudinal mode laser source and a FP interferometer frequency detector, the cavity length of the multi-longitudinal mode laser resonator and the cavity length of the FP interferometer need to meet a matching relationship. After in-depth theoretical research, the applicant found that in a multi-longitudinal mode Doppler laser radar system based on a dual FP interferometer, all longitudinal modes of the multi-longitudinal mode laser can be efficiently used for measurement, that is, the single measurement effect of a multi-longitudinal mode laser source is the same as the multiple measurement effect of a single longitudinal mode laser source, and the longitudinal mode spacing Λ of the multi-longitudinal mode laser is equal to the free spectrum spacing ν of the FP interferometer. FSR Strict matching is required, that is, the matching condition Λ=pν′ must be met FSR , p = 1, 2, .... However, the actual Λ and ν FSR There may be mismatch, that is, there is a matching error Δν FSR =|Λ-ν FSR |. As the matching error increases, the line width of the transmittance curve of the dual FP interferometer increases, the peak value decreases, and the wind speed measurement sensitivity decreases. This is an error that only multi-longitudinal mode laser radar has, which will cause the wind speed measurement error to increase, or even make it impossible to measure. Therefore, before measuring the wind field, it is necessary to calibrate the multi-longitudinal mode Doppler laser radar so that the wind speed measurement error between Λ and ν is greater than that between ν and Λ. FSR Under the condition of perfect matching, the transmittance calibration curve of the dual FP interferometer is measured. Through investigation, it is found that the method for calibrating the multi-longitudinal mode Doppler lidar system based on dual FP interferometer has not been reported yet. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a multi-longitudinal mode Doppler laser radar calibration method based on a dual FP interferometer, which can be used for calibrating a multi-longitudinal mode Doppler laser radar system based on a dual FP interferometer.

[0004] The technical solution adopted by the present invention to solve its technical problem is:

[0005] A multi-longitudinal mode Doppler lidar calibration optical path based on dual FP interferometer Figure 1 As shown, the dual FP interferometer is tunable, its cavity length can be adjusted within a wide range, and there are two tuning modes, coarse and fine, which are controlled by the FP interferometer controller. After the multi-longitudinal mode laser emits a pulse laser with a center frequency of v0, it is coupled into a 100m long bare optical fiber by one branch of the 1×2 fiber coupler, and its quasi-continuous backscattered light is output from the other branch of the 1×2 fiber coupler and collimated into an approximately parallel beam by the collimator. A small part of the parallel beam is first split and received by the photodetector E; most of the parallel light is evenly split and incident on FP interferometer 1 and FP interferometer 2, and the transmitted light is received by photodetector 1 and photodetector 2 respectively. When the cavity length of the dual FP interferometer is fixed, the transmittance value of the multi-longitudinal mode laser with a center frequency of v0 incident on FP interferometer 1 and FP interferometer 2 can be obtained by using the ratio of the received signals of photodetector 1 and photodetector 2 to the received signal of photodetector E and the splitting ratio of the two beam splitters.

[0006] The radar system calibration using the above calibration optical path is achieved by the following method:

[0007] 1. Increase or decrease the cavity length of the dual FP interferometer (coarse scan) with a step size of Δdμm (the step size can be increased depending on the situation), and the corresponding free spectrum spacing v of the FP interferometer FSR MHz decreases or increases by 2Δdν FSR 2 / c MHz, where c is the speed of light. After the cavity length changes by Δdμm, a multi-longitudinal mode laser is used to scan the cavity length of the double FP interferometer in the range of ±λnm (λ is the wavelength of the incident laser), and the scanning step is δdnm (a change in the cavity length by δdnm is equivalent to a change in the center frequency of the emitted laser by δv0 = 2ν FSR δd / λ MHz, scanning the cavity length with a step size of δdnm is equivalent to continuously changing the center frequency of the emitted laser with a step size of δv0), and the transmittance data sequence of FP interferometers 1 and 2 corresponding to the number of steps of the cavity length scanning is obtained. The data sequence is fitted using the nonlinear least squares method to obtain the transmittance curves of FP interferometers 1 and 2;

[0008] 2. Compare the peak values ​​of the transmittance curves before and after the cavity length changes by Δdμm, and select the direction in which the peak value gradually increases as the direction in which the cavity length changes;

[0009] 3. Change the direction according to the cavity length determined in step 2, scan the cavity length of the dual FP interferometer with a step length of Δdμm (coarse scan), one step per scan (the number of steps is counted as m), and then fine scan the cavity length with a step length of δdnm to obtain a set of transmittance data sequences of FP interferometers 1 and 2 corresponding to the number of fine scanning steps of the cavity length (such as Figure 2 shown);

[0010] 4. For the multiple data sets obtained in step 3, the peak value T of each transmittance curve is obtained by nonlinear least squares fitting. p , and then obtain a series of dual FP interferometer cavity length change steps m-transmittance peak T p Data pairs (such as Figure 3 As shown in Figure 2), the data pair is fitted nonlinearly to determine the cavity length step number corresponding to the maximum peak, and then the cavity length of the dual FP interferometer is adjusted to the corresponding position. At this time, the relationship between Λ and ν is realized. FSR Matching calibration;

[0011] 5. Stop the coarse scan of the dual FP interferometer cavity length, and finely scan the dual FP interferometer cavity length again with a step size of δv0 (or a smaller step size) to obtain the transmittance data sequence of FP interferometers 1 and 2 corresponding to the number of cavity length fine scan steps, and then use the nonlinear least squares method to fit the data sequence to obtain the transmittance calibration curve of FP interferometers 1 and 2.

[0012] Principle of the Invention

[0013] The transmittance function of the parallel monochromatic light with a frequency of ν incident on the i-th FP interferometer is:

[0014]

[0015] Among them: i=1, 2; η i =T p，i (1-R e，i ) / (1+R e，i ) is the average transmittance of the i-th FP interferometer, T p，i and R e，i are the peak transmittance and effective reflectivity of the slab of the i-th FP interferometer, ν i and ν FSR are the center frequency and free spectrum spacing of the i-th FP interferometer respectively.

[0016] Each single longitudinal mode spectrum line in the multi-longitudinal mode can be approximated by a Gaussian line shape, except that the intensity of each longitudinal mode is modulated by the gain curve of the laser medium. Therefore, the spectrum function of the multi-longitudinal mode emission laser is:

[0017]

[0018] Where: Δv l =δv / (4ln2) 1 / 2 is the spectral width of a single longitudinal mode laser spectrum at 1 / e height, δv is the half-height spectral width of a single longitudinal mode of the emitted laser; ν0 is the center frequency of the emitted laser; q is the longitudinal mode number with reference to the selected center frequency ν0 (the center frequency q is 0); Λ is the longitudinal mode interval; C q is the relative intensity of each spectral line (the relative intensity at ν0 is set to 1).

[0019] After being collimated by the collimator, the transmittances of the emitted laser with a total divergence angle of 2θ0 incident on the dual FP interferometer are

[0020]

[0021] Substituting (1) to (2) into (3) and integrating, we get:

[0022] T il (ν0)=η i (1+2σ il ) (4)In the formula

[0023]

[0024] where ν′ FSR =2ν FSR / (1+cosθ0). When the condition Λ=pν′ is satisfied FSR , when p=1, 2, ..., the above formula can be simplified to

[0025]

[0026] This formula is completely consistent with the result obtained using a single longitudinal mode light source.

[0027] For the vacuum cavity FP interferometer, ν FSR =c / 2d, then

[0028]

[0029] Therefore, Δν FSR = -2Δdν FSR 2 / c, that is, the cavity length changes by Δd, and the free spectrum spacing of the FP interferometer changes by Δν FSR .

[0030] In addition, according to equations (4) and (5), when the center frequency ν0 increases by δν0, its transmittance is T il (ν0+δν0, d), let T il (ν0+δν0,d)=T il (ν0, d+δd), then we have

[0031]

[0032] Therefore, δν0=-ν0δd / d=-2ν FSR δd / λ, that is, the cavity length of the FP interferometer is reduced by δd, which is equivalent to the incident light frequency increasing by δν0. The scanning of the incident light frequency can be equivalent to the scanning of the FP interferometer cavity length.

[0033] Figure 2 The relationship curve between the number of cavity length fine scanning steps multiplied by the step length and the transmittance of FP interferometers 1 and 2 for different matching errors obtained by simulation. It can be seen that the smaller the frequency matching error, the larger the transmittance peak; when the frequency is completely matched, the peak reaches the maximum. Figure 3 The relationship curve between the number of steps of cavity length rough scan multiplied by the step length and the peak transmittance of FP interferometers 1 and 2 can be used to determine the cavity length of FP interferometer when the frequency is completely matched. The number of steps of cavity length rough scan multiplied by the step length is converted into scanning frequency, and then the frequency value corresponding to the peak of the curve is deducted to obtain the corresponding relationship between the number of steps of cavity length rough scan multiplied by the step length and the matching error. Figure 3 It can also represent the curve of the peak transmittance of FP interferometer 1 or 2 changing with the matching error.

[0034] Due to the adoption of the above technical solution, the advantages and positive effects of the present invention are: without changing the laser radar optical path structure, it is possible to accurately and conveniently realize the calibration of the multi-longitudinal mode Doppler laser radar system based on the dual FP interferometer, including the laser longitudinal mode spacing Λ and the free spectrum spacing ν of the FP interferometer. FSR Matching calibration and dual FP interferometer transmittance curve calibration, in Λ and ν FSR Under the premise of matching, the optimal dual FP interferometer transmittance calibration curve is obtained, so that the detection performance of the lidar system can be optimized. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is an optical path diagram for implementing the present invention.

[0036] Figure 2 This is a relationship curve between the number of cavity length fine scanning steps multiplied by the step length and the transmittance of FP interferometers 1 and 2 when different matching errors occur during the implementation of the present invention.

[0037] Figure 3 It is a relationship curve between the number of cavity length rough scanning steps multiplied by the step length and the transmittance peak value of FP interferometers 1 and 2 during the implementation of the present invention. DETAILED DESCRIPTION

[0038] The specific implementation method of the present invention is as follows:

[0039] 1. Utilize Figure 1The calibration optical path shown in the figure is used to roughly scan the cavity length of the dual FP interferometer with a step size of Δdμm. The corresponding free spectrum spacing ν of the FP interferometer is FSR MHz decreases or increases by 2Δdν FSR 2 / c MHz, where c is the speed of light. When the cavity length of the double FP interferometer changes by Δd μm, a multi-longitudinal mode laser is used to scan the cavity length of the double FP interferometer in a step size of δdnm within the range of ±λnm (λ is the wavelength of the incident laser) (equivalent to the cavity length being unchanged, and the center frequency of the emitted laser being scanned in steps of δv0=2ν FSR δd / λMHz fine scan), and the transmittance data sequence of FP interferometers 1 and 2 corresponding to the number of cavity length fine scan steps is obtained. The data sequence is fitted using the nonlinear least squares method to obtain the transmittance curves of FP interferometers 1 and 2. The peak values ​​of the transmittance curves before and after the cavity length changes by Δdμm are compared, and the direction in which the peak value gradually increases is selected as the direction in which the cavity length changes;

[0040] 2. Roughly scan the cavity length of the dual FP interferometers along the determined direction with a step length of Δd μm. After each rough scan step, finely scan the cavity length with a step length of δdnm to obtain a set of transmittance data sequences of FP interferometers 1 and 2 corresponding to the number of fine scan steps of the cavity length. Each set of data sequences corresponds to the number of rough scan steps one by one.

[0041] 3. Use nonlinear least squares method to fit the measured multiple data sets to obtain the peak value T of each transmittance curve. p , and then obtain a series of dual FP interferometer cavity length rough scanning steps and transmittance peak T p The data pairs are obtained by performing nonlinear fitting on the series of data pairs to determine the number of steps of the rough scanning cavity length corresponding to the maximum peak value;

[0042] 4. Adjust the cavity length of the dual FP interferometer to the position corresponding to the maximum peak value to complete the relationship between Λ and ν FSR Matching calibration, stop the coarse scanning of the cavity length, and scan the cavity length of the dual FP interferometer again with a step size of δv0 to obtain the transmittance data sequence of FP interferometers 1 and 2 corresponding to the number of cavity length fine scanning steps. Then use the nonlinear least squares method to fit the data sequence to obtain the transmittance calibration curve of FP interferometers 1 and 2.

Claims

1. A multi-longitudinal mode Doppler laser radar calibration method based on dual FP interferometer; its characteristics are: The calibration optical path is constructed without changing the laser radar optical path structure. The dual FP interferometer is tunable, and its cavity length can be adjusted in a large range. It has two tuning modes, coarse tuning and fine tuning, which are controlled by the FP interferometer controller. The calibration light source is the multi-longitudinal mode, quasi-continuous backscattered light after the laser pulse is incident on the long bare optical fiber. First, the cavity length of the dual FP interferometer is roughly scanned with a step length of Δd μm, and the corresponding free spectrum spacing v of the FP interferometer is FSR MHz decrease or increase 2Δdv FSR 2 / c MHz, where c is the speed of light; when the cavity length of the double FP interferometer changes by Δd μm, a multi-longitudinal mode laser is used within the range of ±λnm, where λ is the wavelength of the incident laser, and then the cavity length of the double FP interferometer is scanned with a step length of δd nm, which is equivalent to keeping the cavity length unchanged, and the center frequency of the emitted laser is scanned with a step length of δv0=2v FSR δd / λMHz fine scanning is performed to obtain the transmittance data sequence of FP interferometers 1 and 2 corresponding to the number of cavity length fine scanning steps; the data sequence is fitted by nonlinear least squares method to obtain the transmittance curves of FP interferometers 1 and 2, and the heights of the transmittance curves before and after the cavity length changes by Δd μm are compared. The direction in which the peak value gradually increases is selected as the direction of cavity length change; then, the cavity length of the dual FP interferometer is roughly scanned along the determined direction with a step length of Δd μm. After each rough scanning step, the cavity length is finely scanned with a step length of δd nm to obtain a set of transmittance data sequences of FP interferometers 1 and 2 corresponding to the number of cavity length fine scanning steps, and each set of data sequences corresponds to the number of rough scanning steps one by one; secondly, the measured multiple sets of data sequences are fitted by nonlinear least squares method to obtain the peak value T of each transmittance curve p , and then obtain a series of dual FP interferometer cavity length rough scanning steps and transmittance peak T p The data pairs are obtained by nonlinear fitting, and the number of steps of the rough scanning cavity length corresponding to the maximum peak is determined; finally, the cavity length of the dual FP interferometer is adjusted to the position corresponding to the maximum peak, and the relationship between Λ and v is completed. FSR Matching calibration, stop the rough scanning of the cavity length, and scan the cavity length of the dual FP interferometers again with a step length of δv0 to obtain the transmittance data sequence of FP interferometers 1 and 2 corresponding to the number of cavity length fine scanning steps, and then use the nonlinear least squares method to fit the data sequence to obtain the transmittance calibration curve of FP interferometers 1 and 2; The calibration optical path includes: a pulse laser with a center frequency of v0 emitted by a multi-longitudinal mode laser is coupled into a 100m long bare optical fiber through one branch of a 1×2 optical fiber coupler, and its quasi-continuous backscattered light is output from the other branch of the 1×2 optical fiber coupler and collimated into an approximately parallel light beam by a collimator; a small part of the parallel light beam is first split and received by a photodetector E; most of the parallel light is evenly split and then incident on FP interferometer 1 and FP interferometer 2, and the transmitted light is received by photodetector 1 and photodetector 2 respectively.

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