Single-photon three-dimensional imaging system based on double-optical wedge scanning and registration method

Abstract

The invention discloses a single-photon three-dimensional imaging system based on double-optical-wedge scanning and a registration method. According to the single-photon three-dimensional imaging system based on double-optical-wedge scanning, multi-beam emission illumination based on Dammann gratings is adopted, so that the energy utilization efficiency can be effectively improved, and the detection distance is increased; by adopting the area array single-photon detector, high-sensitivity detection of extremely weak long-distance target reflected light signals can be realized; an imaging view field can be expanded under the condition that image rotation is not introduced by adopting a double-optical-wedge scanning mode; internal reflection can be effectively inhibited and a close-range detection blind area can be eliminated by adopting sub-aperture common-path coupling; a prior-based rapid reconstruction algorithm is adopted, so that the three-dimensional image reconstruction speed of double-optical-wedge scanning can be effectively improved; by adopting an off-axis focal plane method, high-efficiency coupling registration of emitting laser beams and receiving the field of view of the area array detector can be realized.

Description

Technical Field

[0001] The invention relates to the technical field of laser three-dimensional imaging, and in particular to a single-photon three-dimensional imaging system and a registration method based on double-optical wedge scanning. Background Art

[0002] Three-dimensional imaging has many applications, including: machine vision, security monitoring, and autonomous driving. At present, the means of long-distance obstacle detection include radar, visual cameras, and three-dimensional laser radar. Radar is mainly used for long-distance target detection and tracking. There is a certain blind spot in the closer range, and the detection ability of small targets is insufficient; the advantage of visual cameras is that they can obtain target contour features, but they are easily affected by ambient lighting conditions and have insufficient ranging capabilities; the advantage of three-dimensional laser radar is that it has the characteristics of radar waves and optics. It can not only obtain the distance and direction information of the target, but also obtain the three-dimensional contour information of the target itself, which is used for target detection and identification. It can better make up for the shortcomings of radar and visual camera obstacle detection technology. Due to its better three-dimensional scene perception ability, it has been widely used in various platforms.

[0003] Linear detection lidar distinguishes echo signals from noise by setting the threshold level in the echo detection circuit. However, due to the low gain of the linear detector, in order to ensure the signal-to-noise ratio of the detection, the echo signal needs to contain at least hundreds of photons, which places high requirements on the energy of the emitted laser single pulse and the transceiver efficiency of the optical system. In the application of large field of view lidar, it is difficult to detect long-distance targets. Compared with linear detection lidar, single-photon lidar uses a highly sensitive single-photon detector and a high repetition rate and low pulse energy detection mechanism, which is of great significance to improving the system's range.

[0004] Single-photon LiDAR can be divided into two detection modes: single-point scanning and array imaging. Single-point scanning single-photon LiDAR is limited by the imaging mode of single-point scanning, and the speed of three-dimensional imaging is slow, which is not suitable for large field of view and fast imaging applications. In order to further improve the speed of single-photon imaging and expand the imaging field of view and resolution, it is necessary to use single-photon imaging technology research of array single-photon detection devices. Gm-APD array is a type of array single-photon detection device. At present, the preparation technology of large-scale Gm-APD array in China is not yet mature. The maximum pixel size of the developed Gm-APD array is 64×64, which is difficult to meet the detection needs of large field of view and high resolution. Therefore, it is necessary to combine scanning imaging technology and point cloud image processing to achieve extended imaging field of view and resolution.

[0005] On the other hand, for laser radars using array detectors, flood illumination is a common method for transmitting optical systems. However, since flood illumination requires the emission beam to illuminate the entire field of view, it has strict requirements on laser emission energy and low laser energy utilization. If the fill factor of the array detector is not high, it is also necessary to cooperate with the entire array of microlenses to receive laser echoes. Therefore, in order to further improve the utilization efficiency of laser pulse energy, it is necessary to study multi-beam imaging solutions based on pulse beam splitting.

[0006] At present, single-photon three-dimensional imaging devices based on dual wedge scanning and multi-beam illumination usually use a common aperture method to couple the light-transmitting and light-transmitting paths (such as MIT Lincoln Laboratory, China Aerospace Science and Industry Corporation, etc.), and use the intermediate image plane conjugation to ensure the strict alignment of the transmitting and receiving fields of view. However, this alignment method is very difficult and it is difficult to achieve a high precision matching efficiency. In addition, the internal reflection problem caused by the common aperture will also increase the blind area of ​​close-range detection, thereby limiting the working range of the system. Therefore, a more efficient and easy-to-implement system structure is needed to construct a single-photon three-dimensional imaging device and achieve precise matching of the light-transmitting and light-transmitting paths. Summary of the invention

[0007] The present invention provides a system, control, system adjustment and image processing method for a detection system for long-distance targets that overcomes the above problems or at least partially solves the above problems.

[0008] The present invention discloses a single-photon three-dimensional imaging system based on dual-optical wedge scanning, comprising a single-photon three-dimensional imaging device, a transceiver coupling registration device and an image processing system, wherein the single-photon three-dimensional imaging device comprises a multi-beam transmitting module, a sub-aperture common optical path coupling module, a dual-optical wedge scanning module and a synchronous control module;

[0009] The multi-beam transmitting module is used to generate a multi-beam laser beam for illumination, and includes a beam expansion unit and a beam splitting unit, which are used to control the divergence angle of the laser beam and generate a laser beam array of required size and specification;

[0010] The sub-aperture common optical path coupling module is used to realize sub-aperture receiving and transmitting optical path coupling for the projection and detection of the laser light signal, so as to expand the scanning field of view through the dual optical wedge scanning module.

[0011] The dual optical wedge scanning module is used for transmitting and receiving the transmitted and received laser optical signals on a common optical path. The dual optical wedge scanning module projects a two-dimensional laser beam to a target area at a set speed and angle, and receives a diffuse reflection echo signal of the target.

[0012] The synchronization control module is used to synchronously control the signals of each channel, and includes a scanning control unit, a laser output unit and a detection unit, which are respectively used to provide a wedge scanning control signal for the dual wedge scanning module and provide a synchronization control trigger signal for the multi-beam transmitting module and the detector;

[0013] The image processing system is used to transform the photon flight time data and recover the three-dimensional information of the target from the extremely weak photon counting data at a long distance through a reconstruction algorithm based on sub-pixel scanning and computational imaging;

[0014] The transceiver coupling alignment device comprises an off-axis parabolic mirror, a focal plane detector and a target, and is used for alignment to achieve a one-to-one correspondence between the emitted two-dimensional laser beam and the array detector pixel.

[0015] Furthermore, the multi-beam transmission module includes a beam expansion unit and a beam splitting unit;

[0016] The beam expansion unit is used to control the laser divergence angle;

[0017] The beam splitting unit includes a Dammann grating and a reflector group, and is used to generate an illumination laser beam;

[0018] The divergence angle, for the multi-beam emission mode of the single-photon three-dimensional imaging device, the duty cycle of the illuminating laser beam is determined by the laser divergence angle and the separation angle of the beam splitting system. Due to the limitation of the flight time of long-distance photons, the duty cycle determines the maximum distance of long-distance single-photon detection;

[0019] The laser beam is a series of beams with the same beam properties generated after the expanded laser beam passes through the Dammann grating, and is projected to the aperture common optical path coupling module through a reflector group and coupled to the receiving optical path.

[0020] Furthermore, the sub-aperture common optical path coupling module couples the laser beam emitted by the multi-beam transmitting module and the receiving optical path of the system together through a horseshoe reflector;

[0021] The split-aperture coupling optical path uses a horseshoe reflector as a system splitting element to couple the transmitting optical path and the receiving optical path. The horseshoe reflector is located at the center of the aperture in front of the receiving optical path. For long-distance imaging, this can be approximately considered as a parallel optical path, resulting in a light energy loss and image quality degradation that is approximately equal to the ratio of the reflector diameter to the receiving aperture.

[0022] The receiving light path uses a zoom lens to collect the received light energy.

[0023] Furthermore, the synchronization control module includes a scanning control unit, a laser output and a detector synchronization control unit;

[0024] The scanning control unit is used to control the scanning optical wedge to deflect and project the incident two-dimensional laser beam at a specified angle and speed;

[0025] The laser output and detector synchronization control unit is used to send control signals to the laser and the array detector respectively to trigger their orderly operation: the pulse laser sends a synchronization signal to the array detector while emitting a laser pulse, and synchronously generates a START timing start pulse signal; the START signal generates a distance gate signal EN as the starting signal of the high-frequency clock counter TDC in each pixel after a fixed delay; when the laser pulse echo signal returns and reaches the photosensitive surface of the array detector, the photosensitive pixel will generate a Geiger avalanche signal, and the threshold identification circuit generates a voltage pulse STOP to stop the high-frequency clock counter TDC of the pixel after detecting the avalanche signal, and then outputs the TDC count value of each pixel in series after the EN signal ends.

[0026] Furthermore, the processing of the photon counting data by an image processing system includes:

[0027] Pre-calculate the point cloud coordinates corresponding to each scanning transient according to the system parameters and generate a transient-point cloud space mapping table. Then, quickly rasterize the point cloud data by looking up the table according to the current input photon counting data to quickly obtain the transient image of the spatiotemporal distribution.

[0028] According to the spatial correlation of noise distribution, background noise photons are deleted through spatial neighborhood surround filtering.

[0029] Solve the optimization function through convex optimization global minimization to obtain the penalty maximum likelihood estimate of the optimization function;

[0030] The three-dimensional image of the target area is reconstructed using the target distance estimation value.

[0031] A registration method for a single-photon three-dimensional imaging system based on dual-optical wedge scanning is also provided, the registration method comprising:

[0032] Step 401, the transceiver coupling alignment device controls the divergence angle of the beam expansion system through the off-axis parabolic mirror and the focal plane detector;

[0033] Step 402, calibrating an infinite laser beam distribution target through an off-axis parabolic mirror;

[0034] Step 403, aligning the field of view of the zoom lens of the receiving optical path by matching the off-axis parabolic mirror and the target, and adjusting the focal length of the zoom lens so that the field of view of the array detector and the target overlap;

[0035] Step 404: Repeat fine-tuning the focal length of the Dammann grating and the zoom lens to ensure one-to-one correspondence between the array detector pixels and the two-dimensional laser beam.

[0036] Furthermore, in step 401, the following steps are also included:

[0037] Only the beam expander group is installed, and the beam splitting system and the split-aperture common optical path coupling module are not installed;

[0038] Turn on the laser, and the single beam after passing through the beam expander group will irradiate the off-axis parabolic mirror, and then converge at the focal plane of the off-axis parabolic mirror;

[0039] Install an attenuation plate and a focal plane detector at the focal plane of the off-axis parabolic mirror to ensure that the photosensitive surface of the focal plane detector coincides with the focal plane of the off-axis parabolic mirror;

[0040] Use a focal plane detector to measure the size of the focused light spot, and adjust the intervals between the lenses of the beam expander group so that the focused light spot is smaller than the specified value, that is, the divergence angle is controlled to be smaller than the required value.

[0041] Furthermore, in step 402, the following steps are also included:

[0042] A Dammann grating and a split-aperture common optical path coupling module are installed behind the beam expander group, but a double wedge scanning module is not installed;

[0043] Turn on the laser, and the multi-beams after the beam expander and Dammann grating are irradiated to the off-axis parabolic mirror, and then converge into a two-dimensional laser dot matrix at the focal plane of the off-axis parabolic mirror;

[0044] Use the Dammann grating to adjust the direction of the two-dimensional laser dot array so that it is parallel to the crosshairs at the center of the target, calibrate the positions of the four corners and the center beam of the two-dimensional laser dot array, and calibrate the target to a grid of alternating light and dark. It should be noted that the center position of the beam should correspond to the pixel center of the detector array, so the position of the corresponding pixel edge also needs to be calculated based on the center of the beam;

[0045] Make the distribution of the two-dimensional laser dot array and the target grid distribution strictly consistent.

[0046] Furthermore, in step 403, the following steps are also included:

[0047] Turn off the laser;

[0048] Use an infrared light source to illuminate the calibrated target. Since the array detector is located at the focal plane of the transmitting mirror group and the target is located at the focal plane of the off-axis parabola, the array detector can also clearly image the target, and its effective distance is equivalent to infinity; adjust the position of the array detector so that the crosshairs at the center of the target and the array detector basically coincide;

[0049] Adjust the focal length of the zoom lens so that the four corners of the array detector's field of view roughly coincide with the four corners of the target.

[0050] Furthermore, in step 404, the following steps are also included:

[0051] Use an infrared light source to illuminate the calibrated target. At this time, a grid of alternating light and dark can be observed using an array detector.

[0052] Adjust the direction of the Dammann grating, the position of the array detector and the focal length of the zoom lens again, so that the grid brightness observed by the array detector is uniform, and there should be no grid with grayscale between light and dark;

[0053] Turn off the infrared light source and turn on the laser, and the image uniformity observed by the array detector should remain unchanged.

[0054] The beneficial effects achieved by the present invention are:

[0055] The present invention provides a system, control, system assembly and image processing method for a detection system for a long-distance target, which can be applied to rapid, high-resolution three-dimensional reconstruction of the long-distance target. The multi-beam emission illumination based on Dammann grating can effectively improve the energy utilization efficiency and increase the detection distance; the surface array single-photon detector can realize high-sensitivity detection of extremely weak reflected light signals of long-distance targets; the dual-wedge scanning method can expand the imaging field of view without introducing image rotation; the divided-aperture common optical path coupling can effectively suppress internal reflection and eliminate the blind area of ​​close-range detection; the off-axis focal plane method can realize high-efficiency coupling and registration of the emission laser beam and the receiving surface array detector field of view; the variable focal length receiving system and the non-conjugate optical path can greatly simplify the assembly difficulty and improve the coupling efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 A schematic diagram of the principle of a single-photon three-dimensional imaging system based on double wedge scanning provided by the present invention;

[0057] Figure 2 A flow chart of a single-photon three-dimensional imaging method based on double-wedge scanning provided by the present invention.

[0058] Figure 3 A schematic diagram of the principle of a multi-beam emission and array detector pixel precise registration method provided by the present invention.

[0059] Figure 4 A flow chart of a multi-beam emission and array detector pixel precise registration method provided by the present invention.

[0060] Figure 5 A schematic diagram of the structure of a single-photon three-dimensional imaging system based on double-wedge scanning provided by the present invention. DETAILED DESCRIPTION

[0061] The present invention will be further described below in conjunction with specific embodiments, and the advantages and features of the present invention will become clearer as the description proceeds. However, these embodiments are exemplary only and do not constitute any limitation to the scope of the present invention. It should be understood by those skilled in the art that the details and forms of the technical solution of the present invention may be modified or replaced without departing from the spirit and scope of the present invention, but these modifications and replacements all fall within the scope of protection of the present invention.

[0062] Figure 1 The present invention provides a single-photon three-dimensional imaging system based on dual-wedge scanning, which includes a single-photon three-dimensional imaging device and an image processing system, wherein the single-photon three-dimensional imaging device includes a multi-beam emission module, a sub-aperture common optical path coupling module, a dual-wedge scanning module and a synchronization control module.

[0063] The multi-beam transmitting module is used to generate multi-beam laser beams for illumination, and includes a beam expansion unit and a beam splitting unit.

[0064] The beam expansion unit is used to control the laser divergence angle; the beam splitting unit includes a Dammann grating and a reflector group, which is used to generate an illuminating laser beam; the divergence angle, for the multi-beam emission mode of the single-photon three-dimensional imaging device, the duty cycle of the illuminating laser beam is jointly determined by the laser divergence angle and the separation angle of the beam splitting system. Due to the limitation of the flight time of long-distance photons, the duty cycle determines the maximum distance of long-distance single-photon detection; the laser beam is a series of beams with the same beam properties generated after the expanded laser beam passes through the Dammann grating, and is projected through the reflector group to the sub-aperture common optical path coupling module and coupled to the receiving optical path. Compared with the conventional common-aperture reflective optical path, the split-aperture beam-splitting projection system does not require focusing mirrors and emitting mirrors for secondary beam expansion, avoiding the generation of focused laser dot arrays on the intermediate conjugate image plane, preventing air ionization caused by excessive energy density (long-distance imaging usually requires higher laser energy), and improving the safety of the system; at the same time, it avoids the increase in laser beam diameter and compression of the separation angle caused by secondary beam expansion, reduces the distance required for beam separation, and thereby reduces the close-range blind area of ​​detection.

[0065] In this embodiment, a laser is used to generate a laser beam with high peak energy and narrow pulse width, providing a better active light source for long-distance ranging. The single pulse energy is 0.5 mJ, the repetition frequency is 10 kHz, and the beam quality is 1.5; the beam expansion system collimates and expands the laser beam so that the divergence angle of the laser beam matches the beam splitting angle of the Dammann grating to meet the matching requirements with the duty cycle of the array detector. The filling factor of the Gm-APD array detector (CEC) is 60%, the beam splitting angle of the Dammann grating (Holo / Or) is 0.025°, and the specification is 10×10 mm. In order to prevent adjacent laser beam spots from overlapping and being smaller than the detector pixel field of view during long-distance detection, a telescope system with a beam expansion ratio of 10 is used for beam expansion, and a laser beam with a divergence angle of 0.158 mrad and a beam diameter of 10 mm is output; the beam splitting system based on the Dammann grating generates a 64×64 far-field projection laser beam.

[0066] The divided-aperture common optical path coupling module is used for projecting and detecting the laser light signal to realize divided-aperture common optical path transmission, reception and scanning.

[0067] The split-aperture coupling optical path uses a horseshoe reflector as the system's beam splitter to couple the transmitting optical path and the receiving optical path. It is located at the center of the receiving optical path aperture. For long-distance imaging, this can be approximately considered as a parallel optical path, and the resulting light energy loss and image quality degradation are approximately equal to the ratio of the reflector diameter to the receiving aperture. Compared with the conventional hollow reflector placed in the converging optical path behind the receiving optical path aperture, which results in light energy loss and image quality degradation far greater than the aperture diameter, the use of a front horseshoe reflector can greatly improve image quality and the efficiency of the receiving optical path. On the other hand, compared with the conventional common-aperture system, the split-aperture coupling optical path avoids the internal reflection problem caused by the shared mirror group of the transmitting and receiving optical paths, thereby reducing the short-range blind area of ​​detection.

[0068] In the receiving optical path, a zoom lens is used to collect the received light energy. Although the zoom lens has higher complexity and relatively lower stability than the fixed focal length lens used in the conventional common aperture transceiver system, it greatly simplifies the alignment process of the reflected laser beam and the array detector pixel, and makes it easier to obtain a higher coupling efficiency.

[0069] The dual wedge scanning module is used to project a two-dimensional laser beam to a target area at a set speed and angle, and receive diffuse reflection echo signals from the target.

[0070] In this embodiment, the transceiver optical system uses a horseshoe reflector to couple the transmitting optical path and the receiving optical path. Since the transmitting and receiving fields of view are completely matched, the instantaneous field of view angle corresponding to a single pixel is 0.025°, the focal length of the projection objective lens is 114.5mm, and the instantaneous field of view angle of each imaging is 1.6°; the two-dimensional laser beam is projected to the target surface through a dual wedge scanning module. After the emitted laser beam echo signal is diffusely reflected by the target surface, the energy is gathered by the receiving optical system and finally projected to the photosensitive surface of the Gm-APD array detector, thereby obtaining the round-trip flight time of the laser beam and calculating the distance information of the target.

[0071] Among them, the synchronization control module is used to synchronize the signals of each channel, including the scanning control unit, the laser output unit and the detection unit, which are respectively used to provide the optical wedge scanning control signal for the common aperture scanning transceiver module and the synchronization control trigger signal for the multi-beam emission illumination module and the detector; the pulse laser sends a synchronization signal to the Gm-APD array detector while emitting a laser pulse, and synchronously generates a START timing start pulse signal; the START signal generates a distance gate signal EN as the start signal of the high-frequency clock counter TDC in each pixel after a fixed delay; when the laser pulse echo signal returns to the photosensitive surface of the Gm-APD array detector, the photosensitive pixel will generate a Geiger avalanche signal, and the threshold discrimination circuit will generate a voltage pulse STOP to stop the high-frequency clock counter TDC of the pixel after detecting the avalanche signal, and then serially output the TDC count value of each pixel after the EN signal ends. The TDC counter is 12bit, with a time resolution of 1.25ns. The gate is set to a maximum of 5.12μs cutoff, and its distance detection range is 768m.

[0072] Among them, the image processing system is used to transform the photon flight time data, and restore the three-dimensional information of the target from the extremely weak photon counting data at a long distance through a reconstruction algorithm based on sub-pixel scanning and computational imaging. The photon counting data is processed by the image processing system, including: 201. Acquire a transient image of the spatiotemporal distribution based on the current input photon counting data; 202. Eliminate background noise photons through preprocessing; 203. Solve the optimization function through convex optimization global minimization to obtain the penalty maximum likelihood estimate of the optimization function; 204. Reconstruct a three-dimensional image of the target area. Its process is as follows: Figure 2 shown.

[0073] Due to the scale limitation of the current GM-APD array detector, its resolution can only be about 64×64, which makes it difficult to directly realize high-resolution imaging applications. Therefore, we use the dual wedge scanning method to expand the imaging field of view and use sub-pixel scanning to improve the resolution of three-dimensional imaging. However, due to the complexity of the dual wedge scanning trajectory, if the spatial coordinates of each point cloud are calculated according to the position pointed by the instantaneous scanning trajectory, it will require a large amount of computing resources and a long reconstruction time, which is difficult to meet the real-time requirements of the system. Therefore, we use a fast reconstruction algorithm based on a priori to improve the three-dimensional image reconstruction speed of dual wedge scanning. When the system parameters are known, the point cloud coordinates corresponding to each scanning transient are pre-calculated and the transient-point cloud space mapping table is generated. Then, the point cloud data is quickly rasterized by looking up the table during actual data acquisition, so as to quickly obtain the transient image of spatiotemporal distribution; then, the spatial correlation of the noise distribution is used to delete the background noise photons through the spatial neighborhood surround filtering process; then, the optimization function is solved by convex optimization global minimization to obtain the penalty maximum likelihood estimate of the optimization function; finally, the three-dimensional image of the target area is reconstructed by three-dimensional deconvolution.

[0074] Figure 3 The invention provides a multi-beam emission and array detector pixel precise registration system, which realizes one-to-one correspondence between emission of two-dimensional laser beams and array detector pixels through off-axis parabolic mirrors, focal plane detectors and targets.

[0075] The divergence angle of the laser beam expansion system is first controlled by an axis adjustment system consisting of an off-axis parabolic mirror and a focal plane detector; then the focal plane detector is replaced with a target to align the emitted two-dimensional laser beam with the array detector pixels. (Only the target is drawn in the figure, and the focal plane detector is in the same position, that is, the focal plane of the off-axis parabolic mirror)

[0076] like Figure 4 As shown, the transceiver coupling registration device realizes one-to-one correspondence between the emitted two-dimensional laser beam and the array detector pixel through an off-axis parabolic mirror, a focal plane detector and a target, and includes:

[0077] 401. Controlling the divergence angle of the beam expansion system through an off-axis parabolic mirror and a focal plane detector;

[0078] Only the beam expander group is installed, and the beam splitting system and the split-aperture common optical path coupling module are not installed;

[0079] Turn on the laser, and the single beam after passing through the beam expander group will irradiate the off-axis parabolic mirror, and then converge at the focal plane of the off-axis parabolic mirror;

[0080] Install an attenuation plate and a focal plane detector at the focal plane of the off-axis parabolic mirror to ensure that the photosensitive surface of the focal plane detector coincides with the focal plane of the off-axis parabolic mirror;

[0081] Use a focal plane detector to measure the size of the focused light spot, and adjust the intervals between the lenses of the beam expander group so that the focused light spot is smaller than the specified value, that is, the divergence angle is controlled to be smaller than the required value.

[0082] 402. Calibrate the infinite laser beam distribution target;

[0083] A Dammann grating and a split-aperture common optical path coupling module are installed behind the beam expander group, but a double wedge scanning module is not installed;

[0084] Turn on the laser, and the multi-beams after the beam expander and Dammann grating are irradiated to the off-axis parabolic mirror, and then converge into a two-dimensional laser dot matrix at the focal plane of the off-axis parabolic mirror;

[0085] Use the Dammann grating to adjust the direction of the two-dimensional laser dot array so that it is parallel to the crosshairs at the center of the target, calibrate the positions of the four corners and the center beam of the two-dimensional laser dot array, and calibrate the target to a grid of alternating light and dark (the number of grids is an integer fraction of the number of pixels in the array detector, usually set to half). It should be noted that the center position of the beam should correspond to the pixel center of the detector array, so the position of the corresponding pixel edge also needs to be calculated based on the center of the beam;

[0086] At this time, the distribution of the two-dimensional laser dot array is strictly consistent with the target grid distribution.

[0087] 403. Adjust the focal length of the zoom lens to control the field of view;

[0088] Turn off the laser;

[0089] Use an infrared light source to illuminate the calibrated target. Since the array detector is located at the focal plane of the transmitting mirror group and the target is located at the focal plane of the off-axis parabola, the array detector can also clearly image the target, and its effective distance is equivalent to infinity; adjust the position of the array detector so that the crosshairs at the center of the target and the array detector basically coincide;

[0090] Adjust the focal length of the zoom lens so that the four corners of the array detector's field of view roughly coincide with the four corners of the target.

[0091] 404. Adjust the one-to-one correspondence between the pixels and the laser dot matrix.

[0092] Use an infrared light source to illuminate the calibrated target. At this time, a grid of alternating light and dark can be observed using an array detector.

[0093] Adjust the direction of the Dammann grating (i.e., adjust the rotation), the position of the array detector (i.e., adjust the translation), and the focal length of the zoom lens (i.e., adjust the zoom) again, so that the grid brightness observed by the array detector is uniform, and there should be no grid with grayscale between light and dark;

[0094] Turn off the infrared light source and turn on the laser, and the image uniformity observed by the array detector should remain unchanged.

[0095] It is understandable that the laser 3D imaging systems currently published often use a divided-aperture single-point scanning method, which has a slow scanning speed, low resolution, and problems with transmitter-receiver registration. The smaller number of single-photon 3D imaging devices that use array detectors use a common-aperture transmission and reception imaging method, which is limited by the close-range detection blind spot caused by internal emission, and the huge difficulties in achieving transmitter-receiver matching using intermediate image plane conjugation, making it difficult to achieve good results.

[0096] According to the technical problem to be solved by the present invention, a single-photon three-dimensional imaging system and a registration method based on double optical wedges are provided by the present invention. The system simplifies the transmission and reception matching by adopting aperture common optical path multi-beam transmission illumination and zoom lens to realize array detector pixel registration, and proposes a corresponding registration method, thereby realizing the precise matching of multi-beam illumination and array detection in long-distance single-photon three-dimensional imaging, greatly improving energy efficiency, suppressing close-range detection blind spots, and obtaining a larger detection range and higher imaging quality.

[0097] The above are only specific steps of the present invention and do not constitute any limitation to the protection scope of the present invention; any technical solutions formed by equivalent transformation or equivalent replacement shall fall within the protection scope of the present invention; some parts not elaborated in detail in the present invention belong to the known technologies of those skilled in the art.

Claims

1. A single-photon three-dimensional imaging system based on double wedge scanning, characterized in that: It includes a single-photon three-dimensional imaging device, a transceiver coupling registration device and an image processing system, wherein the single-photon three-dimensional imaging device includes a multi-beam transmitting module, a sub-aperture common optical path coupling module, a dual optical wedge scanning module and a synchronous control module; The multi-beam transmitting module is used to generate a multi-beam laser beam for illumination, and includes a beam expansion unit and a beam splitting unit, which are used to control the divergence angle of the laser beam and generate a laser beam array of required size and specification; The sub-aperture common optical path coupling module is used to realize sub-aperture receiving and transmitting optical path coupling for the projection and detection of the laser light signal, so as to expand the scanning field of view through the dual optical wedge scanning module. The dual optical wedge scanning module is used for transmitting and receiving the transmitted and received laser optical signals on a common optical path. The dual optical wedge scanning module projects a two-dimensional laser beam to a target area at a set speed and angle, and receives a diffuse reflection echo signal of the target. The synchronization control module is used to synchronously control the signals of each channel, and includes a scanning control unit, a laser output unit and a detection unit, which are respectively used to provide a wedge scanning control signal for the dual wedge scanning module and provide a synchronization control trigger signal for the multi-beam transmitting module and the detector; The image processing system is used to transform the photon flight time data and recover the three-dimensional information of the target from the extremely weak photon counting data at a long distance through a reconstruction algorithm based on sub-pixel scanning and computational imaging; The transceiver coupling alignment device comprises an off-axis parabolic mirror, a focal plane detector and a target, and is used for alignment to achieve a one-to-one correspondence between the emitted two-dimensional laser beam and the array detector pixel.

2. The single-photon three-dimensional imaging system based on double wedge scanning according to claim 1, characterized in that: The multi-beam transmission module includes a beam expansion unit and a beam splitting unit; The beam expansion unit is used to control the laser divergence angle; The beam splitting unit includes a Dammann grating and a reflector group, and is used to generate an illumination laser beam; The divergence angle, for the multi-beam emission mode of the single-photon three-dimensional imaging device, the duty cycle of the illuminating laser beam is determined by the laser divergence angle and the separation angle of the beam splitting system. Due to the limitation of the flight time of long-distance photons, the duty cycle determines the maximum distance of long-distance single-photon detection; The laser beam is a series of beams with the same beam properties generated after the expanded laser beam passes through the Dammann grating, and is projected to the aperture common optical path coupling module through a reflector group and coupled to the receiving optical path.

3. The single-photon three-dimensional imaging system based on double wedge scanning according to claim 1, characterized in that: The sub-aperture common optical path coupling module couples the laser beam emitted by the multi-beam emission module and the receiving optical path of the system together through a horseshoe reflector; The split-aperture coupling optical path uses a horseshoe reflector as a system splitting element to couple the transmitting optical path and the receiving optical path. The horseshoe reflector is located at the center of the aperture in front of the receiving optical path. For long-distance imaging, this can be approximately considered as a parallel optical path, resulting in a light energy loss and image quality degradation that is approximately equal to the ratio of the reflector diameter to the receiving aperture. The receiving light path uses a zoom lens to collect the received light energy.

4. The single-photon three-dimensional imaging system based on double wedge scanning according to claim 1, characterized in that: The synchronization control module includes a scanning control unit, a laser output and a detector synchronization control unit; The scanning control unit is used to control the scanning optical wedge to deflect and project the incident two-dimensional laser beam at a specified angle and speed; The laser output and detector synchronization control unit is used to send control signals to the laser and the array detector respectively to trigger them to work in an orderly manner: the pulse laser sends a synchronization signal to the array detector while emitting a laser pulse, and synchronously generates a START timing start pulse signal; the START signal generates a distance gate signal EN after a fixed delay as the start signal of the high-frequency clock counter TDC in each pixel; When the laser pulse echo signal returns to the photosensitive surface of the array detector, the photosensitive pixel will generate a Geiger avalanche signal. After the threshold discrimination circuit detects the avalanche signal, it generates a voltage pulse STOP to stop the high-frequency clock counter TDC of the pixel, and then serially outputs the TDC count value of each pixel after the EN signal ends.

5. The single-photon three-dimensional imaging system based on double wedge scanning according to claim 1, characterized in that: The processing of the photon counting data by an image processing system includes: Pre-calculate the point cloud coordinates corresponding to each scanning transient according to the system parameters and generate a transient-point cloud space mapping table. Then, quickly rasterize the point cloud data by looking up the table according to the current input photon counting data to quickly obtain the transient image of the spatiotemporal distribution. According to the spatial correlation of noise distribution, background noise photons are deleted through spatial neighborhood surround filtering. Solve the optimization function through convex optimization global minimization to obtain the penalty maximum likelihood estimate of the optimization function; The three-dimensional image of the target area is reconstructed using the target distance estimation value.

6. A registration method for a single-photon three-dimensional imaging system based on dual wedge scanning according to any one of claims 1 to 5, characterized in that: The registration method comprises: Step 401, the transceiver coupling alignment device controls the divergence angle of the beam expansion system through the off-axis parabolic mirror and the focal plane detector; Step 402, calibrating an infinite laser beam distribution target through an off-axis parabolic mirror; Step 403, aligning the field of view of the zoom lens of the receiving optical path by matching the off-axis parabolic mirror and the target, and adjusting the focal length of the zoom lens so that the field of view of the array detector and the target overlap; Step 404: Repeat fine-tuning the focal length of the Dammann grating and the zoom lens to ensure one-to-one correspondence between the array detector pixels and the two-dimensional laser beam.

7. The registration method according to claim 6, characterized in that: In step 401, the following steps are also included: Only the beam expander group is installed, and the beam splitting system and the split-aperture common optical path coupling module are not installed; Turn on the laser, and the single beam after passing through the beam expander group will irradiate the off-axis parabolic mirror, and then converge at the focal plane of the off-axis parabolic mirror; Install an attenuation plate and a focal plane detector at the focal plane of the off-axis parabolic mirror to ensure that the photosensitive surface of the focal plane detector coincides with the focal plane of the off-axis parabolic mirror; Use a focal plane detector to measure the size of the focused light spot, and adjust the intervals between the lenses of the beam expander group so that the focused light spot is smaller than the specified value, that is, the divergence angle is controlled to be smaller than the required value.

8. The registration method according to claim 6, characterized in that: In step 402, the following steps are also included: A Dammann grating and a split-aperture common optical path coupling module are installed behind the beam expander group, but a double wedge scanning module is not installed; Turn on the laser, and the multi-beams after the beam expander and Dammann grating are irradiated to the off-axis parabolic mirror, and then converge into a two-dimensional laser dot matrix at the focal plane of the off-axis parabolic mirror; Use the Dammann grating to adjust the direction of the two-dimensional laser dot array so that it is parallel to the crosshairs at the center of the target, calibrate the positions of the four corners and the center beam of the two-dimensional laser dot array, and calibrate the target to a grid of alternating light and dark. It should be noted that the center position of the beam should correspond to the pixel center of the detector array, so the position of the corresponding pixel edge also needs to be calculated based on the center of the beam; Make the distribution of the two-dimensional laser dot array and the target grid distribution strictly consistent.

9. The registration method according to claim 6, characterized in that: In step 403, the following steps are also included: Turn off the laser; Use an infrared light source to illuminate the calibrated target. Since the array detector is located at the focal plane of the transmitting mirror group and the target is located at the focal plane of the off-axis parabola, the array detector can also clearly image the target, and its effective distance is equivalent to infinity; adjust the position of the array detector so that the crosshairs at the center of the target and the array detector basically coincide; Adjust the focal length of the zoom lens so that the four corners of the array detector's field of view roughly coincide with the four corners of the target.

10. The registration method according to claim 6, characterized in that: In step 404, the following steps are also included: Use an infrared light source to illuminate the calibrated target. At this time, an array detector can be used to observe a grid of alternating light and dark. Adjust the direction of the Dammann grating, the position of the array detector and the focal length of the zoom lens again, so that the grid brightness observed by the array detector is uniform, and there should be no grid with grayscale between light and dark; Turn off the infrared light source and turn on the laser, and the image uniformity observed by the array detector should remain unchanged.

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