Rapid scanning method of photon counting type laser radar based on illumination light spot modulation

Abstract

The invention discloses a rapid scanning method of a photon counting type laser radar based on illumination light spot modulation, and belongs to the technical field of laser radars. The amplification factor of an object image is changed by adopting optical fiber defocusing or lens zooming, the size of a light spot of illumination on a target is modulated, the probability density of illumination on the target is changed, and the change of the detection probability is realized. With the detection probability as a constraint condition, the size of a target illumination light spot is established according to a geometrical optical imaging relation, a laser radar echo signal equation is used to determine the number of echo photons, the detection probability and the false alarm rate are established according to Poisson statistical distribution, accumulation time is determined according to threshold method target identification, and a full-chain physical relation of the turntable scanning speed is obtained. Therefore, the scanning strategy optimization of the whole space is realized, and the three-dimensional rapid scanning of the space target is realized under the condition of ensuring the constant detection probability. The size of an illumination light spot is modulated by using optical fiber defocusing or lens zooming, and the scanning speed and the detection probability can be considered in the whole three-dimensional space range.

Description

Technical Field

[0001] This invention belongs to the field of lidar technology, specifically relating to a fast scanning method for a photon-counting lidar based on illumination spot modulation. Background Technology

[0002] Photon-counting lidar has the technical advantages of long detection range and high sensitivity, and has important application value in the detection of small targets at low altitudes over long distances.

[0003] Figure 1 It is a traditional single-photon lidar based on a fixed-focus lens, in Figure 1 In the process, a synchronization signal generator produces a synchronization TTL signal. One path externally triggers the fiber pulse laser, outputting a nanosecond-level pulse signal. This pulse signal is then emitted through a zoom lens to illuminate the target. The other TTL signal is sent to the TDC (Transmitter Control Center) to begin recording the laser's emission time. After the target illumination signal passes through a narrowband filter, the receiving zoom lens, fiber filter, and single-photon detector generate TTL photoelectric pulses. The time difference between these pulses and the background TTL signal is calculated. Based on the laser's round-trip time, the target distance is recorded. Combined with the turntable's own pitch and azimuth, the target's three-dimensional information is obtained.

[0004] However, current fiber optic lidar systems are generally designed optimally for specific targets at specific distances, determining the optimal focal length and aperture. However, in practice, a fixed focal length results in a uniform field of view across all points in the airspace, typically requiring scanning at a constant speed. This approach suffers from significant echo signal redundancy at close range and low detection probability due to weak target echo signals at long range, failing to meet the high detection probability and rapid scanning requirements across the entire three-dimensional space.

[0005] To address the challenge of current photon-counting lidars failing to simultaneously achieve high scanning speed and short reconstruction cycle at close range, and high detection probability and low false alarm rate at long range, this invention proposes an adaptive laser emission spot spatial modulation technique. This technique can increase the emission spot size at close range to improve scanning speed, and decrease the spot size at long range to improve detection probability. Summary of the Invention

[0006] To address the aforementioned issues, this invention provides a rapid scanning method for photon-counting lidar based on illumination spot modulation. By modulating the emission field of view under constraints of detection probability and false alarm rate, the method optimizes the illumination spot size and accumulated pulse time at different spatial distances, guiding the selection of scanning strategies at different distances. This technique can significantly improve scanning speed at close range and significantly improve detection probability at long range, achieving performance improvement without changing the complexity of the system.

[0007] The specific technical solution is as follows: a fast scanning method for a photon-counting lidar based on illumination spot modulation, the method comprising:

[0008] Step 1: Based on the target size, reflectivity, laser energy, and receiver aperture, establish the functional relationship between the number of echo photons and the distance and spot size;

[0009] Step 2: With the lidar off, measure the background noise at different distances; or calculate the background noise theoretically by combining the calculated sky background radiance with parameters such as the system's receiving field of view, optical efficiency, and filter spectral transmittance.

[0010] Step 3: Based on the number of photons in the single-pulse echo and the background noise, establish the relationship between the single-detection probability and the accumulated photon count;

[0011] Step 4: Calculate the minimum number of accumulated pulses under different probability conditions based on the number of accumulated photons, and calculate the target dwell time in combination with the laser repetition rate, thereby obtaining the radar scanning speed at different distances;

[0012] Step 5: Calculate the spot diameter and scanning speed at different distances based on the echo photon count, background count, single detector probability, threshold, accumulated photon count, and laser repetition rate. Set the defocus distance and optimize the scanning strategy for each region to achieve the best scanning scheme.

[0013] The beneficial effects of this invention are as follows:

[0014] This invention first employs fiber optic defocusing or lens zooming to balance and optimize scanning speed and detection probability while considering the constraints of detection probability and false alarm rate. It combines the technical advantages of fast scanning speed at close range, high detection probability at long range, and strong noise anti-interference capability, providing a feasible solution for further improvement of photon counting lidar in the field of rapid imaging. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a single-photon lidar based on a fixed-focus lens in the prior art;

[0016] Figure 2 This is a schematic diagram of a fast scanning system based on lens zoom.

[0017] Figure 3 This is a schematic diagram of a fast scanning system based on fiber optic zoom.

[0018] Figure 4 This is a flowchart of a fast scanning method for a photon-counting lidar based on illumination spot modulation according to the present invention. Detailed Implementation

[0019] 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 embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above objectives, this invention adopts the following technical solution.

[0020] like Figure 2 As shown, this invention provides a photon-counting fast scanning lidar based on light spot modulation, employing lens zoom, and comprising: a synchronization signal generator, a laser, a receiving and transmitting fiber array, a receiving and transmitting zoom lens, a fiber filter, a detector, a time-to-digital converter (TDC), and a data processing module.

[0021] like Figure 3 As shown, this invention also provides a photon-counting fast scanning lidar based on spot modulation, employing fiber optic zoom. It comprises: a synchronization signal generator, a laser, a fiber optic array, receiving and transmitting lenses, linear displacement stages, fiber optic filters, a detector, a time-to-digital converter (TDC), and a data processing module. The two linear displacement stages are located at one end of the receiving and transmitting lenses, respectively.

[0022] A synchronization signal generator produces a synchronization TTL signal. One path externally triggers the laser, outputting a nanosecond-level pulse signal. This pulse is then transmitted through a zoom lens or linear displacement stage and its transmitting lens to illuminate the target. The other TTL signal is sent to the TDC (Transmitter Control Center) to begin recording the laser's emission time. The illuminated target signal is then filtered through a narrowband filter, received by a zoom lens or linear displacement stage and its receiving lens, along with a fiber optic filter and detector. The time difference between this signal and the background TTL signal is calculated. Based on the laser's round-trip time, the target distance is recorded. Combined with the stage's pitch and azimuth measurements, the target's three-dimensional information is obtained.

[0023] Furthermore, the fiber array can be a single fiber, a linear array, or a surface array.

[0024] Furthermore, the illumination spot modulation employs either the same zoom technology or a fiber array electric displacement stage defocusing technology to achieve illumination spot sizes at different distances.

[0025] Furthermore, it is also possible to use a transmitter with zoom and a receiver with fixed focus; a transmitter with fixed focus and a receiver with zoom; or both can be zoomed simultaneously.

[0026] Furthermore, a method can be employed where the transmitter defocuses while the receiver remains focused. Both the transmitter and receiver defocus simultaneously.

[0027] Furthermore, the detector is a single-photon detector, and the laser is an externally triggered fiber pulse laser.

[0028] Based on the above-mentioned photon-counting fast scanning lidar based on illumination spot modulation, this invention provides a fast scanning method for a photon-counting lidar based on illumination spot modulation, such as... Figure 4 As shown, it includes the following steps:

[0029] Step 1: Based on the target size, reflectivity, laser energy, and receiver aperture, establish the functional relationship between the number of echo photons and the distance and spot size (corresponding zoom and defocus amounts):

[0030] (1)

[0031] in, For the number of echo photons, At the speed of light, For single-pulse laser energy, For target reflectivity, Atmospheric transmittance, To improve the efficiency of the optical system, To improve the efficiency of the receiving optical system, Let D be the target cross-sectional area, and D be the lens receiving aperture. R is the laser illumination area, R is the target distance, and h is Planck's constant. This refers to the quantum efficiency of a single-photon detector. This is the center wavelength of the laser.

[0032] The target distance R for imaging the fiber optic end face by the zoom lens at different distances is:

[0033] (2)

[0034] The corresponding magnification is:

[0035] (3)

[0036] Therefore, the diameter of the magnified imaging spot is:

[0037] (4)

[0038] The image spot is then:

[0039] (5)

[0040] in, Indicates the intrinsic focal length. The focal length after zooming. The diameter of the optical fiber, and β represents the magnification.

[0041] The method for calculating the imaging spot after fiber defocusing is similar to that described above and will not be repeated here. The imaging spot at different distances is as follows:

[0042] (6)

[0043] in, This indicates the defocusing amount of the fiber optic array.

[0044] Step 2: With the lidar turned off, measure the background noise at different distances. Or the sky background radiance calculated using atmospheric transport software (such as MODTRAN). Combining system receiving field of view and optical efficiency and the spectral transmittance of the filter Each parameter, theoretically calculated background noise:

[0045] (7)

[0046] in, 's' represents the radiance of the sky background; 's' represents the area of ​​the optical fiber.

[0047] Step 3: Based on the echo photon count and background noise under single-pulse laser, establish the relationship between the single-detection probability and the photon count threshold.

[0048] (8)

[0049] in, This represents the probability of a single detection. The cumulative detection probability over multiple periods is:

[0050] (9)

[0051] in, Set a threshold for photon counting, where x is the accumulated photon count and k is the index number. The detection probability of a target is calculated using multi-pulse accumulation, where n is the number of detections. Considering fluctuations in the number of accumulated echo photons, x is generally... big .

[0052] Step 4: Calculate the minimum number of pulses under different probability conditions based on the accumulated photon count x. By combining the laser repetition rate f, the dwell time of the target is calculated, thereby obtaining the radar scanning speed at different distances.

[0053] The cumulative time of m laser pulses on the target for:

[0054] (10)

[0055] f represents the laser repetition rate, or the dwell time of the laser-illuminated target. That is, the size of the scanning angle of the light spot itself ( The time required is determined by the radar's rotational angular velocity. The decision is as follows:

[0056] (11)

[0057] in, The diameter of the light spot. Let be the radar's rotational angular velocity. According to the illumination requirements, to ensure that all pulses are accumulated, the dwell time must be no less than the accumulation time, which needs to satisfy:

[0058] (12)

[0059] To achieve the fastest scanning speed, we take = To find the limiting case, substitute formulas (10) and (11) into formula (12) and solve. The maximum scan speed can be derived from this, therefore the target scan speed is:

[0060] (13)

[0061] When the lens is zoomed, its bokeh is When the lens is out of focus, its light spot is... .

[0062] Step 5: Calculate the spot diameter and scanning speed at different distances based on the echo photon count, background count, single detector probability, threshold, accumulated photon count, and laser repetition rate. Set the defocus distance and optimize the scanning strategy for different regions to ensure the best scanning scheme while taking into account the detection probability.

[0063] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A fast scanning method for a photon-counting lidar based on illumination spot modulation, characterized in that, The method includes: Step 1: Based on the target size, reflectivity, laser energy, and receiver aperture, establish the functional relationship between the number of echo photons and the distance and spot size. The spot size corresponds to the zoom and defocus amounts. Step 2: With the lidar off, measure the background noise at different distances; or calculate the background noise theoretically by combining the calculated sky background radiance with the system's receiving field of view, optical efficiency, and filter spectral transmittance. Step 3: Based on the number of photons in the single-pulse echo and the background noise, establish the relationship between the single-detection probability and the accumulated photon count; Step 4: Calculate the minimum number of accumulated pulses under different probability conditions based on the number of accumulated photons, and calculate the target dwell time in combination with the laser repetition rate, thereby obtaining the radar scanning speed at different distances; Step 5: Calculate the spot diameter and scanning speed at different distances based on the echo photon count, background count, single detector probability, threshold, accumulated photon count, and laser repetition rate. Set the defocus distance and optimize the scanning strategy for each region to achieve the best scanning scheme.

2. The rapid scanning method for a photon-counting lidar based on illumination spot modulation according to claim 1, characterized in that, In step 1, the functional relationship between the number of echo photons and the distance and spot size is as follows: （1） in, For the number of echo photons, At the speed of light, For single-pulse laser energy, For target reflectivity, Atmospheric transmittance, To improve the efficiency of the optical system, To improve the efficiency of the receiving optical system, Let D be the target cross-sectional area, and D be the lens receiving aperture. R is the laser illumination area, R is the target distance, and h is Planck's constant. This refers to the quantum efficiency of a single-photon detector. The center wavelength of the laser; The target distance R for imaging the fiber end face by the zoom lens at different distances is: （2） The corresponding magnification is: （3） Therefore, the diameter of the magnified imaging spot is: （4） The image spot is then: （5） in Indicates the intrinsic focal length. The focal length after zooming. The diameter of the optical fiber, and β represents the magnification.

3. The rapid scanning method for a photon-counting lidar based on illumination spot modulation according to claim 2, characterized in that, The imaging spot at different distances after the optical fiber defocuses is as follows: （6） in, This indicates the defocusing amount of the fiber optic array.

4. The rapid scanning method for a photon-counting lidar based on illumination spot modulation according to claim 2, characterized in that, In step 2, the formula for calculating background noise is: （7） in, λ represents the background radiance of the sky; s represents the area of ​​the optical fiber; η(λ) represents the spectral quantum efficiency of the single-photon detector; and λ1 and λ2 represent the lower and upper limits of the filter bandwidth, respectively.

5. A fast scanning method for a photon-counting lidar based on illumination spot modulation according to claim 4, characterized in that, In step 3, the relationship between the single detection probability and the photon number threshold is as follows: （8） in, The probability of a single detection is given; the cumulative detection probability over multiple periods is: （9） in, Set a threshold for photon counting, where x is the accumulated photon count and k is the index number. The probability of detecting a target is denoted by multi-pulse accumulation, where n is the number of detections.

6. The rapid scanning method for a photon-counting lidar based on illumination spot modulation according to claim 5, characterized in that, In step 4, the cumulative time of m laser pulses on the target is... for: （10） f represents the laser repetition rate, or the dwell time of the laser-illuminated target. That is, the time required for the light spot to scan its own angular size, which is determined by the radar's rotational angular velocity. The decision is as follows: （11） in, The diameter of the light spot. The radar's rotational angular velocity; the dwell time is not less than the accumulated time, satisfying: （12） Pick = To find the limiting case, substitute formulas (10) and (11) into formula (12) and solve. The maximum scan speed is derived, therefore the target scan speed is: （13） When the lens is zoomed, its bokeh is When the lens is out of focus, its light spot is .

7. A fast scanning method for a photon-counting lidar based on illumination spot modulation according to claim 1, characterized in that, Photon-counting fast-scanning lidar employs lens zoom and comprises: a synchronization signal generator, a laser, receiving and transmitting fiber optic arrays, receiving and transmitting zoom lenses, fiber optic filters, a detector, a time-to-digital converter (TDC), and a data processing module; or, The photon counting type fast scanning lidar uses fiber optic zoom and consists of: a synchronization signal generator, a laser, a fiber optic array, receiving and transmitting lenses, a linear displacement stage, a fiber optic filter, a detector, a time-to-digital converter (TDC), and a data processing module; the two linear displacement stages are located at one end of the receiving and transmitting lenses, respectively. A synchronization signal generator produces a synchronization TTL signal. One path externally triggers the laser, outputting a nanosecond-level pulse signal. This pulse signal is then transmitted through a zoom lens or linear displacement stage and a transmitting lens to illuminate the target. The other TTL signal is sent to the TDC to start recording the laser's emission time. The illuminated target signal is then filtered through a narrowband filter, received by a zoom lens or linear displacement stage and a receiving lens, a fiber optic filter, and a detector. The detector generates TTL photoelectric pulses, calculates the time difference with the background TTL signal, records the target distance based on the laser's round-trip time, and combines this with the turntable's own pitch and azimuth to obtain the target's three-dimensional information.

8. A fast scanning method for a photon-counting lidar based on illumination spot modulation according to claim 7, characterized in that, Fiber optic arrays can be single-fiber, linear, or area arrays.

9. A fast scanning method for a photon-counting lidar based on illumination spot modulation according to claim 7, characterized in that, The detector uses a single-photon detector.

10. A fast scanning method for a photon-counting lidar based on illumination spot modulation according to claim 7, characterized in that, The laser is an externally triggered fiber pulse laser.

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