Single-photon lidar coaxial light path stray light suppression method

Abstract

The application discloses a single-photon laser radar coaxial light path stray light suppression method, belongs to the technical field of laser radar detection, and is used for stray light suppression and comprising the following steps: a main control module triggers a laser to emit linearly polarized pulse light beams; the linearly polarized pulse light beams are projected to an underwater target through a beam expander, a 45-degree open hole reflector and a two-dimensional scanning galvanometer; echoes are reflected to the 45-degree open hole reflector through the galvanometer; after converging through a focusing lens group and a narrow-band optical filter, the echoes penetrate a small aperture diaphragm; closed-loop attenuation control is performed on the echoes after penetration; and a two-dimensional depth map of the underwater detection target is obtained. Through the collection of light intensity voltage changes and the cooperation of an extreme value optimization algorithm, the motor is dynamically tracked and controlled to orthogonally intercept random flares, the water surface flare and internal stray light are suppressed to a very low level, the detection capability of the system for effective echoes is improved, and the stability of target depth information extraction and two-dimensional distance reconstruction is effectively improved.

Description

Technical Field

[0001] This invention discloses a method for suppressing stray light in the coaxial optical path of a single-photon lidar, belonging to the field of lidar detection technology. Background Technology

[0002] Airborne single-photon lidar is an important tool for shallow-sea topographic mapping, primarily relying on emitting 532nm laser pulses and receiving the time difference between surface and seabed echoes to invert water depth. The extremely high detection sensitivity of single-photon avalanche diodes (SPADs) significantly enhances the detection capability of very weak echoes, but also presents severe challenges during daytime operations due to strong ambient light interference. In complex sea conditions, surface waves not only alter the refraction and reflection direction of the laser beam entering the water, causing extremely unstable surface echo intensity, but also generate extremely strong diffuse reflection of sunlight in the same wavelength band. Once this strong background light, including that in the same wavelength band, is received by the receiving optical path, it instantly triggers the SPAD's physical saturation and dead time, inducing an avalanche accumulation effect. Under these conditions, the SPAD cannot capture subsequent weak seabed echoes, directly leading to random deviations in water depth and elevation calculations, and even causing complete loss of ranging data.

[0003] Existing technologies generally suffer from fundamental flaws, such as limited suppression capabilities or poor dynamic adaptability. Conventional narrowband filters combined with pinhole diaphragms can only remove cross-band and large-angle stray light, offering no interception capability for water surface reflections within the same 532nm band. Furthermore, solutions that directly add fixed-angle polarizers often fail to suppress or mistakenly block valid echoes due to the inability to adapt to the transient and random changes in the polarization state of water surface reflections within the same band under complex sea conditions. In addition, purely software-based solutions that attempt to optimize backend classification algorithms solely through wave feature extraction face the challenge of having no effective raw signals available for processing in extreme sea conditions where strong background light and water surface reflections within the same band cause avalanche stacking of the front-end SPAD device, rendering it ineffective. Summary of the Invention

[0004] The purpose of this invention is to provide a method for suppressing stray light in the coaxial optical path of a single-photon lidar, in order to solve the problem in the prior art where the detector is blinded by avalanche accumulation due to direct exposure to strong background light of the same band during high-reflectivity field operations such as underwater topographic mapping of airborne single-photon lidar.

[0005] A method for suppressing stray light in the coaxial optical path of a single-photon lidar includes:

[0006] S1. The main control module sends a synchronization trigger signal to drive the laser to emit a wavelength of... The linearly polarized pulse beam passes sequentially through a beam expander, a 45° aperture mirror, and a two-dimensional scanning mirror. The two-dimensional scanning mirror deflects the pulse beam and projects it onto the underwater detection target area.

[0007] S2. The lidar receives the echo signal from the underwater target. The echo signal is first reflected by the two-dimensional scanning galvanometer and then reflected on the 45° open-aperture mirror for optical path reception. The echo signal is input into the focusing lens group for convergence. The converged echo signal passes through a narrow-band filter to filter out non-laser bands and then passes through the small aperture placed at the back focal plane.

[0008] S3. Closed-loop attenuation control is performed on the echo signal penetrating the pinhole aperture to output an effective echo photon stream. The single-photon detector receives the effective echo photon stream, and after timestamp histogram analysis, echo time extraction and underwater refraction correction, a two-dimensional depth map of the underwater target is finally obtained. The closed-loop attenuation control includes photoelectric proportional sampling and transimpedance conversion, discrete extremum optimization and orthogonal pose driving, sliding window filtering and dead zone anti-shake.

[0009] S2 includes setting the physical aperture diameter of the pinhole aperture. Equivalent focal length of focusing lens group and light aperture Satisfy the diffraction-limited matching equation:

[0010] ;

[0011] In the formula, To compensate for the tolerance coefficient of light spot broadening caused by wave distortion at the water-air interface;

[0012] Instantaneous reception field of view Constraints .

[0013] S3 includes S3.1, photoelectric proportional sampling and transimpedance conversion, which includes setting a light intensity ratio threshold. The echo signal is controlled to pass through the non-polarizing beam splitter, separating the portion that accounts for a proportion of the total light intensity. The light beam enters the low-light sensor, and the low-light sensor outputs a transient photocurrent. To the transimpedance amplifier circuit:

[0014] ;

[0015] In the formula, To monitor the instantaneous power of the beam, Photoelectric responsivity, This is the dark current of the detector;

[0016] Transimpedance amplifier circuit execution The conversion calculation outputs an analog voltage signal representing the intensity of the current environmental polarization flare to the main control module. :

[0017] ;

[0018] In the formula, For feedback impedance, This is the temperature drift compensation coefficient. This is the reference bias voltage.

[0019] S3 includes S3.2, Discrete Extremum Optimization and Orthogonal Pose Driving, which includes the main beam separated by the non-polarizing beam splitter continuing to pass through a rotatable polarizing filter driven by a servo motor.

[0020] The main control module has High-speed analog-to-digital conversion is performed to obtain discrete voltage sequences. .

[0021] S3 includes, S3.3, where the current principal polarization vector angle of the flare is... The transmission axis vector angle of the polarization filter is Introducing Malus's law:

[0022] ;

[0023] In the formula, The intensity of light after passing through the polarizing filter. This represents the maximum light intensity transmitted when the transmission axis of the polarizing filter is aligned with the principal polarization direction of the flare. Index for discrete time points;

[0024] The main control module is close to With the goal of approaching 0, the discrete control law equations are solved and output to drive the servo motor.

[0025] ;

[0026] In the formula, The target angle of the polarization filter at the next moment. To dynamically track step size, For symbolic functions, for The analog voltage value collected by the main control module at any time.

[0027] S3 includes S3.4, calculating the angle deviation. :

[0028] ;

[0029] Will The proportional control is converted into the corresponding PWM duty cycle, and the PWM pulse is output to the servo motor, which drives the servo motor to rotate the polarization filter to the target angle. .

[0030] S3 includes S3.5, sliding window filtering and dead zone stabilization, which includes setting the stabilization dead zone threshold. ,when At this time, the main control module suspends control law calculation, pauses the control signal of the servo motor, and locks... Proceed to the next sampling cycle;

[0031] When continuous Next sampling period When the main control module is reset If the value is the maximum, the discrete control law equation is recalculated and output to drive the servo motor.

[0032] S3 includes, S3.6, if ,judge The direction;

[0033] like Flip The sign is determined, the discrete control law is solved and output, and then step S3.4 is executed;

[0034] like ,Keep The symbol is determined, the discrete control law is calculated and output, and then step S3.4 is executed.

[0035] S3 includes S3.7, which involves guiding the echo signal after discrete extremum optimization and orthogonal pose driving into the conical light shield, and continuously filtering the residual micro-angle stray light generated by Fresnel reflection of the internal optical elements on the inner wall. Secondary physical collision, inner wall coating absorption rate The matting film performs exponential energy dissipation:

[0036] ;

[0037] In the formula, The residual stray light energy at the final output The initial residual stray light energy incident inside the conical light shield;

[0038] The optical signal output from the conical light shield is used as the effective echo photon stream.

[0039] S3 includes S3.8, where a single-photon detector receives the final effective echo photon stream, and a data processing unit extracts timestamp data based on a time-correlated single-photon counting mechanism; the data processing unit constructs a time-of-flight histogram based on the timestamp data, extracts the echo center time after smoothing and filtering, subtracts the water surface echo time, and calculates the depth value by combining it with an underwater refraction correction model, and finally fills in to form a two-dimensional depth map of the underwater detection target.

[0040] Compared with existing technologies, this invention has the following advantages: It uses geometric beam splitting instead of a traditional polarization beam splitter, allowing the linear polarization characteristics of water surface flares to fully enter the echo path. In terms of algorithm control, by collecting light intensity and voltage changes and combining them with an extreme value optimization algorithm, the motor is controlled to dynamically track and orthogonally intercept flares with random directions. A hysteresis dead zone is added to the program to prevent frequent motor oscillations and overheating caused by minor water surface flickers. Furthermore, a light shield is added to the front of the detector to further reduce residual reflections from the internal lenses, improving the signal-to-noise ratio and system stability of the single-photon radar under complex, high-light sea conditions. Attached Figure Description

[0041] Figure 1 This is a system architecture diagram of the present invention;

[0042] Figure 2 This is a flowchart of the technology of this invention;

[0043] Figure 3 This is a flowchart of the adaptive polarization optimization and anti-jitter dead zone control of the present invention;

[0044] Figure 4 This is an equivalent block diagram of photoelectric signal processing and conversion of the present invention;

[0045] Figure 5 These are time histograms and SG filtering results without applying the method of this invention;

[0046] Figure 6 These are time histograms and SG filtering results obtained by applying the method of this invention;

[0047] Figure 7 It is a two-dimensional depth map that does not apply the method of this invention;

[0048] Figure 8 It is a two-dimensional depth map obtained by applying the method of the present invention;

[0049] In the diagram, 1-Laser; 2-Beam expander; 3-45° aperture mirror; 4-Two-dimensional scanning galvanometer; 5-Focusing lens group; 6-Narrowband filter; 7-Micron-level pinhole aperture; 8-Unpolarized beam splitter; 9-Low light sensor; 10-Transimpedance amplifier circuit; 11-Main control module; 12-Servo motor; 13-Rotable polarizing filter; 14-Conical light shield; 15-Single photon detector; 16-Data processing unit; 17-Underwater detection target. Detailed Implementation

[0050] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0051] A method for suppressing stray light in the coaxial optical path of a single-photon lidar includes:

[0052] S1. The main control module sends a synchronization trigger signal to drive the laser to emit a wavelength of... The linearly polarized pulse beam passes sequentially through a beam expander, a 45° aperture mirror, and a two-dimensional scanning mirror. The two-dimensional scanning mirror deflects the pulse beam and projects it onto the underwater detection target area.

[0053] S2. The lidar receives the echo signal from the underwater target. The echo signal is first reflected by the two-dimensional scanning galvanometer and then reflected on the 45° open-aperture mirror for optical path reception. The echo signal is input into the focusing lens group for convergence. The converged echo signal passes through a narrow-band filter to filter out non-laser bands and then passes through the small aperture placed at the back focal plane.

[0054] S3. Closed-loop attenuation control is performed on the echo signal penetrating the pinhole aperture to output an effective echo photon stream. The single-photon detector receives the effective echo photon stream, and after timestamp histogram analysis, echo time extraction and underwater refraction correction, a two-dimensional depth map of the underwater target is finally obtained. The closed-loop attenuation control includes photoelectric proportional sampling and transimpedance conversion, discrete extremum optimization and orthogonal pose driving, sliding window filtering and dead zone anti-shake.

[0055] S2 includes setting the physical aperture diameter of the pinhole aperture. Equivalent focal length of focusing lens group and light aperture Satisfy the diffraction-limited matching equation:

[0056] ;

[0057] In the formula, To compensate for the tolerance coefficient of light spot broadening caused by wave distortion at the water-air interface;

[0058] Instantaneous reception field of view Constraints .

[0059] S3 includes S3.1, photoelectric proportional sampling and transimpedance conversion, which includes setting a light intensity ratio threshold. The echo signal is controlled to pass through the non-polarizing beam splitter, separating the portion that accounts for a proportion of the total light intensity. The light beam enters the low-light sensor, and the low-light sensor outputs a transient photocurrent. To the transimpedance amplifier circuit:

[0060] ;

[0061] In the formula, To monitor the instantaneous power of the beam, Photoelectric responsivity, This is the dark current of the detector;

[0062] Transimpedance amplifier circuit execution The conversion calculation outputs an analog voltage signal representing the intensity of the current environmental polarization flare to the main control module. :

[0063] ;

[0064] In the formula, For feedback impedance, This is the temperature drift compensation coefficient. This is the reference bias voltage.

[0065] S3 includes S3.2, Discrete Extremum Optimization and Orthogonal Pose Driving, which includes the main beam separated by the non-polarizing beam splitter continuing to pass through a rotatable polarizing filter driven by a servo motor.

[0066] The main control module has High-speed analog-to-digital conversion is performed to obtain discrete voltage sequences. .

[0067] S3 includes, S3.3, where the current principal polarization vector angle of the flare is... The transmission axis vector angle of the polarization filter is Introducing Malus's law:

[0068] ;

[0069] In the formula, The intensity of light after passing through the polarizing filter. This represents the maximum light intensity transmitted when the transmission axis of the polarizing filter is aligned with the principal polarization direction of the flare. Index for discrete time points;

[0070] The main control module is close to With the goal of approaching 0, the discrete control law equations are solved and output to drive the servo motor.

[0071] ;

[0072] In the formula, The target angle of the polarization filter at the next moment. To dynamically track step size, For symbolic functions, for The analog voltage value collected by the main control module at any time.

[0073] S3 includes S3.4, calculating the angle deviation. :

[0074] ;

[0075] Will The proportional control is converted into the corresponding PWM duty cycle, and the PWM pulse is output to the servo motor, which drives the servo motor to rotate the polarization filter to the target angle. .

[0076] S3 includes S3.5, sliding window filtering and dead zone stabilization, which includes setting the stabilization dead zone threshold. ,when At this time, the main control module suspends control law calculation, pauses the control signal of the servo motor, and locks... Proceed to the next sampling cycle;

[0077] When continuous Next sampling period When the main control module is reset If the value is the maximum, the discrete control law equation is recalculated and output to drive the servo motor.

[0078] S3 includes, S3.6, if ,judge The direction;

[0079] like Flip The sign is determined, the discrete control law is solved and output, and then step S3.4 is executed;

[0080] like ,Keep The symbol is determined, the discrete control law is calculated and output, and then step S3.4 is executed.

[0081] S3 includes S3.7, which involves guiding the echo signal after discrete extremum optimization and orthogonal pose driving into the conical light shield, and continuously filtering the residual micro-angle stray light generated by Fresnel reflection of the internal optical elements on the inner wall. Secondary physical collision, inner wall coating absorption rate The matting film performs exponential energy dissipation:

[0082] ;

[0083] In the formula, The residual stray light energy at the final output The initial residual stray light energy incident inside the conical light shield;

[0084] The optical signal output from the conical light shield is used as the effective echo photon stream.

[0085] S3 includes S3.8, where a single-photon detector receives the final effective echo photon stream, and a data processing unit extracts timestamp data based on a time-correlated single-photon counting mechanism; the data processing unit constructs a time-of-flight histogram based on the timestamp data, extracts the echo center time after smoothing and filtering, subtracts the water surface echo time, and calculates the depth value by combining it with an underwater refraction correction model, and finally fills in to form a two-dimensional depth map of the underwater detection target.

[0086] The present invention calculates SNR and constructs a two-dimensional depth map of underwater targets, including reconstructing the equations of a single-photon radar system:

[0087] ;

[0088] In the formula, For signal-to-noise ratio, This represents the original effective echo photon count from the seabed. The original water surface flare and environmental background noise photon count, The overall transmittance of the system to water surface flares and environmental background noise is considered. For the inherent dark count of the single-photon detector 15, The overall transmittance of the system for effective underwater echo photons;

[0089] For each data cell corresponding to a spatial scan location, extract the timestamp data sequence for that location (i.e., the micro-time-of-flight sequence in the binary stream) and construct a time-of-flight histogram for that location. :

[0090] ;

[0091] in, Let be the Dirac function, representing that in Photon capture events at a given moment In the first In a single effective photon detection event, the microscopic flight time from the start of the laser emission pulse to the moment when the single-photon detector records the photon;

[0092] right A Savitzky-Golay (SG) filter is introduced for processing:

[0093] ;

[0094] Among them, the preset sliding window half width Set to 2. Indexed by current time The offset from the center, its value range is: arrive Integers within the range, let the order of the polynomial fitting be... Then, a set of normalized smoothing coefficients is calculated based on the least squares criterion. Without changing the depth sounding accuracy, polynomial fitting is used to suppress shot noise and improve the stability of strength calculation.

[0095] A small window is defined near the maxima of the smooth envelope, and the echo time is calculated using the centroid method. :

[0096] ;

[0097] Obtaining echo time Then, subtract the water surface echo time. Obtain the underwater two-way flight time;

[0098] An underwater refraction correction model is adopted, combined with the refractive index of water. (Approximately 1.33) and the incident angle of the scanning device Calculate depth D. Where c is the speed of light in a vacuum. This is a timestamp representing the time from the emission of a laser pulse to the reception of the echo from the water surface interface;

[0099] Read the external location synchronization index and convert the discrete depth values. By filling in the corresponding spatial pixel matrix one by one, a two-dimensional depth map of the underwater target is obtained.

[0100] This invention provides an embodiment of setting a stabilization dead zone threshold. The value is 20mV to 100mV, typically set to 50mV. The anti-shake dead zone threshold is determined comprehensively based on the output noise of the low-light sensor, circuit thermal noise, ADC quantization error, and the short-term fluctuation amplitude of sea surface flares, to avoid frequent reciprocating movements of the servo motor caused by minor disturbances. This embodiment of the invention sets a tolerance coefficient to compensate for the broadening of the light spot caused by wave distortion at the water-air interface. The range of values ​​is The sampling period is set in this embodiment of the invention. The value ranges from 2 to 5.

[0101] The following description, in conjunction with the accompanying drawings, further illustrates the system architecture of this invention. Figure 1As shown, it includes a laser 1, a beam expander 2, a 45° aperture mirror 3, a two-dimensional scanning galvanometer 4, a focusing lens group 5, a narrowband filter 6, a micron-sized aperture 7, a non-polarizing beam splitter 8, a weak light sensor 9, a transimpedance amplifier circuit 10, a main control module 11, a servo motor 12, a rotatable polarizing filter 13, a conical light shield 14, a single-photon detector 15, a data processing unit 16, and an underwater detection target 17.

[0102] The data flow process of this invention includes: laser 1 emits 532nm rays, which are sequentially projected onto the underwater detection target 17 region via beam expander 2, 45° aperture mirror 3, and two-dimensional scanning mirror 4; the diffuse reflection effective echo signal generated by the underwater detection target 17, and the specular reflection stray light generated by the water surface environment, mix to form a wide-beam echo signal. This wide-beam echo signal returns along the original emission optical path, is reflected by two-dimensional scanning mirror 4, and then illuminates the mirror surface of 45° aperture mirror 3, where the echo signal is reflected into the receiving optical path. No polarization intervention is applied during this process, maintaining the initial polarization state vectors of the effective light and polarized stray light in the echo signal. The echo signal is converged by focusing lens group 5, and the converged echo signal passes through narrowband filter 6, physically cutting off wavelength offsets exceeding [a certain value]. The full-spectrum ambient background radiation is transmitted through a pinhole aperture 7 positioned at the back focal plane. In the echo signal penetrating the pinhole aperture 7, the surface flare noise exhibits extremely strong linear polarization characteristics according to Fresnel's law, while the effective echo signal from the bottom of the water has degenerated into a random polarization state. The echo signal is controlled to pass through a non-polarizing beam splitter 8, and then split into two branches. The first branch, through the non-polarizing beam splitter 8, directly inputs the result into a rotatable polarizing filter 13. The second branch separates the light source that accounts for a proportion of the total light intensity. The light beam enters the low-light sensor 9; the low-light sensor 9 outputs a transient photocurrent. The transimpedance amplifier circuit 10 performs... The conversion calculation outputs an analog voltage signal representing the intensity of the current environmental polarization flare to the main control module 11. The main beam continues to pass through the rotatable polarizing filter 13 driven by the servo motor 12; the drive command formed by the discrete control law equation forces the motor to adjust the polarizing filter 13 in real time to lock it in place. The physical orthogonal orientation is used to dynamically cut off strong polarized flares. The echo signal, after orthogonal screening, is introduced into the conical light shield 14. This light shield forces the Fresnel refraction of micro-angle residual stray light on the inner surface of the system to continuously pass through its inner wall. Second-rate Physical collision. Relying on the absorption rate of the inner wall. The extinction film performs exponential energy dissipation, reducing residual internal reflection stray light energy to its initial value. This greatly reduces the lateral optical false triggering path of the single-photon detector 15 target surface; the single-photon detector 15 can receive an effective echo photon stream, and the data processing unit 16 extracts depth data based on the time-correlated single-photon counting (TCSPC) mechanism, in a strong flare direct radiation environment ( In this method, The number of terms is greatly reduced. This method ensures that the SNR is greater than the counting threshold from the physical level, effectively improving the avalanche accumulation blinding effect under the single-photon system.

[0103] The specific process of the method of this invention is as follows: Figure 2 As shown, the laser emits 532nm pulsed light, and then the two-dimensional galvanometer performs coaxial scanning; the echo beam from the underwater target is received by the aperture reflector, the narrowband filter and the pinhole aperture filter out non-wavelength light, after extreme value optimization, the servo motor drives the polarizer to orthogonality according to the extreme value optimization result, and then the light is input into the conical light shield to absorb the small-angle reflected light, the single-photon detector receives the echo, and the target depth is reconstructed in the data processing unit.

[0104] The adaptive polarization optimization and anti-jitter dead zone control process of this invention is as follows: Figure 3 As shown, the ADC acquires the transient voltage output from the low-light sensor and calculates the differential voltage between the previous and next time points. Based on the differential voltage, it determines whether the dead zone condition is met. If the dead zone condition is met, the optimization algorithm is suspended, the click drive command is frozen, the current polarization filter transmission axis angle is locked, and the process returns to the next sampling period. If the dead zone condition is not met, the gradient direction is determined. If the gradient is greater than 0, the click tracking step size is flipped, the discrete control law is output, and the DAC outputs a drive pulse to the micro servo motor. The motor mechanically deflects the polarizer, approximating the orthogonal angle of the flare. If the gradient direction is less than or equal to 0, the motor tracking step size is maintained, the discrete control law is output, and the DAC outputs a drive pulse to the micro servo motor. The motor mechanically deflects the polarizer, approximating the orthogonal angle of the flare.

[0105] The photoelectric signal processing and conversion process of this invention is as follows: Figure 4 As shown, the input source is the beam from the beam splitter. The light intensity is input to the low light sensor, which outputs a photocurrent. This photocurrent is then amplified by the transimpedance amplifier circuit and the analog voltage signal is input to the main control module. The main control module is an FPGA, which performs high-speed ADC analog-to-digital sampling, extreme value optimization, and generates PWM pulses. The PWM pulses are then input to the servo motor, which rotates the rotatable polarizing filter through mechanical torque.

[0106] To verify the effectiveness of this invention in single-photon timestamp data processing and deep reconstruction, a time histogram was constructed from the echoes, and smoothed using the Savitzky-Golay (SG) filtering method. Based on this, the peak position and center time of the echoes were extracted, and the two-dimensional distance map reconstruction was further completed.

[0107] Construct a time histogram, such as Figure 5 and Figure 6 As shown in the figure, the blue curve is the original time histogram, and the orange dashed line is the processed SG filtering result. It can be seen that without the method of this invention, although a clear main peak exists in the time histogram, the local peak shape is greatly affected by random fluctuations, and stray peaks and background undulations interfere with subsequent peak localization. However, after applying the method of this invention, the main peak position remains basically stable, the peak shape is smoother, and background undulations are suppressed, thereby improving the stability of echo peak extraction.

[0108] The extracted echo center time is mapped to distance information and filled into a two-dimensional pixel matrix according to the scan position to obtain a two-dimensional distance reconstruction map, such as... Figure 7 and Figure 8 As shown, without the application of the method of this invention, there are many discrete outliers in the background area, and the clarity of the target edge is relatively low. After applying the method of this invention, the outline of the target area is clearer, the background area is more uniform, the number of discrete outliers is significantly reduced, and the target areas corresponding to different distance layers can be more stably distinguished. This indicates that the method of this invention can improve the reconstruction quality and depth recovery stability of the two-dimensional distance map.

[0109] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for suppressing stray light in a single-photon lidar coaxial optical path, characterized in that, include: S1. The main control module sends a synchronization trigger signal to drive the laser to emit a wavelength of... The linearly polarized pulse beam passes sequentially through a beam expander, a 45° aperture mirror, and a two-dimensional scanning mirror. The two-dimensional scanning mirror deflects the pulse beam and projects it onto the underwater detection target area. S2. The lidar receives the echo signal from the underwater target. The echo signal is first reflected by the two-dimensional scanning galvanometer and then reflected on the 45° open-aperture mirror for optical path reception. The echo signal is input into the focusing lens group for convergence. The converged echo signal passes through a narrow-band filter to filter out non-laser bands and then passes through the small aperture placed at the back focal plane. S3. Closed-loop attenuation control is performed on the echo signal penetrating the pinhole aperture to output an effective echo photon stream. The single-photon detector receives the effective echo photon stream, and after timestamp histogram analysis, echo time extraction and underwater refraction correction, a two-dimensional depth map of the underwater detection target is finally obtained. The closed-loop attenuation control includes photoelectric proportional sampling and transimpedance conversion, discrete extremum optimization and orthogonal pose driving, sliding window filtering and dead zone anti-shake. S3 includes S3.1, photoelectric proportional sampling and transimpedance conversion, which includes setting a light intensity ratio threshold. The echo signal is controlled to pass through the non-polarizing beam splitter, separating the portion that accounts for a proportion of the total light intensity. The light beam enters the low-light sensor, and the low-light sensor outputs a transient photocurrent. To the transimpedance amplifier circuit: ； wherein Pbeam is the instantaneous power of the light beam, R is the photoelectric responsivity, I dark is the dark current of the detector; Crossed resistance amplification circuit executes Converts the calculation, outputs the analog voltage signal representing the current environmental polarization glint intensity to the master module : ； wherein is the feedback impedance, is a temperature drift compensation coefficient, is a reference bias voltage; S3 includes S3.2, Discrete Extremum Optimization and Orthogonal Pose Driving, which includes the main beam separated by the non-polarizing beam splitter continuing to pass through a rotatable polarizing filter driven by a servo motor. The master module is connected to the slave module through the serial communication interface The high-speed analog-digital conversion is performed to obtain a discrete voltage sequence ; S3 includes, S3.3, set the current flare main polarization vector angle as , the polarization filter transmission axis vector angle is , the Malus law is introduced: ; In the formula, The intensity of light after passing through the polarizing filter. This represents the maximum light intensity transmitted when the transmission axis of the polarizing filter is aligned with the principal polarization direction of the flare. Index for discrete time points; The master module approximates The master module approximates The master module approximates ； In the formula, is the target angle of the polarization filter at the next time, is the dynamic tracking step, is the sign function, is the analog voltage value collected by the time master module S3 comprises, S3.4, calculating an angular deviation : ； Will The proportional control is converted into the corresponding PWM duty cycle, and the PWM pulse is output to the servo motor, which drives the servo motor to rotate the polarization filter to the target angle. ; S3.5, Sliding window filtering and dead zone stabilization include setting the stabilization dead zone threshold. ,when At this time, the main control module suspends control law calculation, pauses the control signal of the servo motor, and locks... Proceed to the next sampling cycle; When consecutive Sub-sampling period The master module resets To the maximum, re-solve and output discrete control law equation to drive servo motor; S3 comprises, S3.6, if , determining the direction of; If , the sign of the term is reversed, the discrete control law is solved and output, and step S3.4 is performed. If , the sign of is maintained, the discrete control law is solved and output, and step S3.4 is performed.

2. The method of claim 1, wherein the method is a method of suppressing stray light in a single-photon lidar coaxial optical path. S2 comprises setting a physical aperture diameter of the pinhole aperture , an equivalent focal length of the focusing lens group and a clear aperture satisfies a diffraction limit matching equation: ； In the formula, is a tolerance coefficient to compensate for the spot broadening caused by the wave distortion at the water-air interface; Instantaneous reception field of view Constraints .

3. The method of claim 2, wherein the method is a method of suppressing stray light in a single-photon lidar coaxial optical path, characterized in that, S3 includes, S3.7, the echo signal after discrete extreme value optimization and orthogonal pose driving is introduced into the conical light shield, the residual micro-angle stray light generated by the Fresnel reflection of the internal optical element is continuously carried out on the inner wall Sub-physical collision, the inner wall is coated with an extinction film with an absorption rate Exponential energy dissipation: ； wherein is the residual stray light energy of the final output, is the initial residual stray light energy incident to the interior of the conical light shield; The optical signal output from the conical light shield is used as the effective echo photon stream.

4. The method of claim 3, wherein the method is a method of suppressing stray light in a single-photon lidar coaxial optical path, characterized in that, S3 includes S3.8, where a single-photon detector receives the final effective echo photon stream, and a data processing unit extracts timestamp data based on a time-correlated single-photon counting mechanism; the data processing unit constructs a time-of-flight histogram based on the timestamp data, extracts the echo center time after smoothing and filtering, subtracts the water surface echo time, and calculates the depth value by combining it with an underwater refraction correction model, and finally fills in to form a two-dimensional depth map of the underwater detection target.

Images