Single-photon laser radar imaging system and method for underwater target detection

Abstract

The invention discloses a single-photon laser radar imaging system and method for underwater target detection, and belongs to the technical field of detection imaging. The imaging system comprises an optical system and a control system, and the optical system comprises a transmitting unit, a receiving unit, a single-photon detector and a scanning unit. An optical path of the optical system adopts a transmitting-receiving combined optical path structure, the transmitting unit transmits an optical signal, the scanning unit receives the transmitted optical signal and a control signal and reflects an echo optical signal, the echo optical signal generated by the optical signal transmitted by the transmitting unit is received by the receiving unit, the echo optical signal is converged to the single-photon detector, and the single-photon detector is used for detecting the single-photon. The single-photon detector performs photoelectric conversion; the control system comprises an FPGA control board, and the FPGA control board is connected with a communication module, a gating signal module and a scanning signal module. The system is compact in structure and reasonable in design, detection of weak optical signals can be realized by using a reasonable optical path structure and the single-photon detector, and the farthest detection distance is increased.

Description

Technical Field

[0001] The present invention belongs to the field of detection imaging technology, and more specifically, relates to a single-photon laser radar imaging system and method for underwater target detection. Background Art

[0002] In the prior art, laser radar uses scanning imaging to achieve three-dimensional detection of targets through two-dimensional scanning, which has a series of advantages such as strong anti-interference ability, high angular resolution, high lateral (vertical) resolution, etc. Galvanometer scanning is currently the most mature two-dimensional beam scanning method, but the control data of most galvanometer laser scanning systems is completely generated by software and sent out after the single-chip timer interrupt. The system CPU resource occupancy rate is high and due to the non-real-time nature of the operating system, there will be problems such as uneven scanning.

[0003] Moreover, compared with detection on the water, underwater optical detection scenarios have more serious backscattering. The energy of the noise light signal is stronger than the target signal, and the time it enters the receiving optical path is earlier than the target echo signal. These noises need to be filtered out through gating to ensure the imaging quality of the target area. After receiving the synchronous light signal, the gated signal module outputs a high-level signal after a set delay time to achieve gating control of the single-photon detector. In most lidar solutions, a pulse delayer / signal generator is used as a gated signal module, but there are problems such as large size, separate power supply, and difficulty in integration into a waterproof cabin, and the parameters of the gated signal cannot be adjusted according to real-time conditions.

[0004] Therefore, it is necessary to develop a new system to improve the problems existing in the prior art, such as redundancy of control modules, uncontrollable propagation and response time of electrical signals between modules, and unevenness of the microcontroller scanning scheme. Summary of the invention

[0005] The present invention aims at solving the technical problems existing in the prior art and provides a single-photon laser radar imaging system and method for underwater target detection.

[0006] In order to solve the above technical problems, the present invention first provides a single-photon laser radar imaging system for underwater target detection, including an optical system and a control system, wherein the optical system includes a transmitting unit, a receiving unit, and also includes a single-photon detector and a scanning unit;

[0007] The optical path of the optical system adopts a combined transmitting and receiving optical path structure. The transmitting unit transmits an optical signal to the scanning unit, the scanning unit receives the transmitted optical signal and the control signal, and reflects the echo optical signal. After the echo optical signal generated by the optical signal transmitted by the transmitting unit is received by the receiving unit, the echo optical signal converges to the single photon detector, and the single photon detector performs photoelectric conversion.

[0008] The control system comprises an FPGA control board, on which are connected a communication module for receiving instructions, a gate signal module for controlling a single-photon detector, a trigger / synchronization signal module and a scanning signal module for generating a scanning signal.

[0009] Preferably, the transmitting unit and the receiving unit of the optical system are coaxially arranged in the optical path, the end of the transmitting unit is connected to the output end of the laser, and the output end of the laser is the spatial laser output end.

[0010] Preferably, the transmitting unit includes a plurality of reflectors and a beam expansion device, and the plurality of reflectors cooperate with each other to realize light path propagation;

[0011] The optical signal part of the transmitting unit reaches the scanning unit, and after reflection, the echo optical signal enters the receiving unit. The receiving unit includes a reflector, a lens and a multimode optical fiber. The echo optical signal is transmitted to the single-photon detector through the multimode optical fiber.

[0012] Preferably, the optical system further comprises an apertured reflector, the receiving unit comprises a reflector coated with a dielectric film, and a filter is arranged at the front end of the incident direction of the lens;

[0013] After being reflected by the scanning unit, the received light is separated from the outgoing light path by an apertured reflector, then coupled to the multimode optical fiber through a filter and a lens, and transmitted to the target area of ​​the single-photon detector through the multimode optical fiber.

[0014] Preferably, it also includes a host computer, a laser, and a TCSPC. The host computer sends control instructions to the FPGA control board through the serial port, and the TCSPC collects different echo light signals of the outgoing pulse in turn; the control system receives instructions from the host computer and controls the working status of the TCSPC, the single-photon detector, the laser, and the scanning unit.

[0015] Preferably, the control system further comprises a trigger-synchronization signal module, which is used to make the laser generate laser pulses of a specific frequency, cycle length, and duty cycle, and receive a synchronous light output signal of the laser;

[0016] The trigger-synchronization signal module generates the high-precision time reference signal required by TCSPC.

[0017] Preferably, after the FPGA control board receives the host computer instruction, the communication module decodes the instruction into four signals of synchronization, scanning, gating and triggering;

[0018] The synchronization signal includes frame and pixel. The synchronization signal is generated by a counter. The counter is connected to the TCSPC. The synchronization signal indicates the start and end of the image frame scanning and the start and end of the single pixel scanning in the complete imaging process.

[0019] The trigger signal includes a laser, the trigger signal is generated by a counter, and the trigger signal is connected to an external trigger port of the laser, so that the laser generates laser pulses of a specific number, period and duty cycle within a single pixel sub-period;

[0020] The scanning signal is generated by a counter and connected to the scanning unit. By controlling the deflection angle range, step accuracy and interval time of the scanning unit, the laser pulse traverses the pixel points of the target area in the expected scanning manner;

[0021] The gate signal is divided into two stages: coarse delay and fine delay. The coarse delay function is completed by the counter, and the fine delay function is implemented by the IDELAYE3 primitive.

[0022] Preferably, the scanning unit comprises a scanning galvanometer, and the scanning galvanometer is used to receive the emitted light signal.

[0023] The present invention also provides a single-photon laser radar imaging method for underwater target detection, the method comprising the following steps:

[0024] S1, after receiving the command from the host computer, the control system sends out multiple signals at the same time;

[0025] S2, the laser emits a laser pulse, which is expanded and shaped by the transmitting unit and propagates toward the target pixel in a longitudinal straight line; while the laser pulse is emitted, the laser sends an optical synchronization signal to the TSCPC and FPGA control board, which serves as the start time of the photon flight time (TOF) and the trigger mark of the gate signal respectively;

[0026] S3, after the FPGA control board receives the light synchronization signal, after the shielding time required by the preset shielding distance, the single photon detector is activated;

[0027] S4. After the emitted laser pulse encounters the target object, a laser echo is generated. The echo photon is received by the probe of the single-photon detector after being received through the receiving optical path. The received optical signal is then converted into an electrical signal and fed to the TCSPC input port. The TCSPC obtains the time difference between the received echo photon and the emitted optical signal through the internal circuit, and the time difference is used as the photon flight time.

[0028] S5. Calculate the distance of the target pixel point, and then enter the scanning process of the next pixel point until the imaging detection of the target area is completed.

[0029] Preferably, after receiving the host computer command, the control system sends out three signals, namely a pulse / synchronization signal, a scanning signal, and a gate signal;

[0030] The pulse / synchronization signal indicates the start and end of the TCSPC module image frame scanning and the start and end of the single pixel point scanning, and controls the laser to emit laser pulses according to the set frequency, period, and duty cycle;

[0031] The scanning signal controls the deflection of the scanning galvanometer of the scanning unit to emit the emitted light to the target pixel point;

[0032] The gating signal is used to control the high-speed opening and closing of the single-photon detector in the gating mode.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The present invention has a compact structure and reasonable design. The optical system adopts a light path structure with a combined transmitter and receiver, which is convenient for debugging the light path and does not have a detection blind spot in the system. The single-photon detector can receive the collected target echo light signal and perform photoelectric conversion to obtain an electrical signal, thereby having a higher detection efficiency, being able to detect weak light signals, and increasing the maximum detection distance.

[0035] Moreover, the control system of the present invention is provided with an FPGA control board, which can coordinate the working modes of various modules through the FPGA control board in conjunction with the control system. Parameter configuration is convenient and fast, and modules can respond quickly without the need for other conversion circuits, thereby ensuring the real-time and high efficiency of the system's execution of instructions. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0037] Figure 1 This is a schematic diagram of the structure of a prototype of a laser radar imaging system according to an embodiment of the present invention;

[0038] Figure 2 A schematic diagram of the working principle of the laser radar imaging system according to an embodiment of the present invention;

[0039] Figure 3 A schematic diagram of a synchronization and trigger signal generating module of a control system according to an embodiment of the present invention;

[0040] Figure 4 Schematic diagram of a scanning signal generating module of a control system according to an embodiment of the present invention;

[0041] Figure 5 Schematic diagram of a gating signal generating module of a control system according to an embodiment of the present invention;

[0042] Figure 6 This is a schematic diagram of the scanning imaging working principle of the laser radar system according to an embodiment of the present invention;

[0043] Figure 7 This is a schematic diagram of the arrangement of the aluminum sheet bracket of the laser radar imaging system according to an embodiment of the present invention;

[0044] Figure 8 This is an underwater imaging test diagram of the laser radar imaging system according to an embodiment of the present invention.

[0045] Symbols in the figure:

[0046] 1. Optical system; 2. Control system; 3. Single photon detector; 4. FPGA control board; 5. Host computer; 6. Laser; 7. TCSPC; 8. Power module; 9. Signal adapter card; 10. USB extender; 11. Laser driver; 12. Laser harness; 13. Laser bracket; 14. Reflector; 15. Right-angle adjustment bracket; 16. X-axis adjuster; 17. Y-axis adjuster; 18. Z-axis adjuster; 19. Beam expansion device; 20. Support rod; 21. Visible sleeve; 22. Y-axis differential adjuster; 23. Open-hole reflector; 24. Scanning galvanometer. DETAILED DESCRIPTION

[0047] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the following is a further detailed description of a single-photon laser radar imaging system and method for underwater target detection provided by the present application in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0048] See also Figure 1 , Figure 2 , an embodiment of the present invention provides a single-photon laser radar imaging system for underwater target detection, including an optical system 1 and a control system 2, the optical system 1 includes a transmitting unit, a receiving unit, and also includes a single-photon detector 3 and a scanning unit;

[0049] The optical path of the optical system 1 adopts a combined transmitting and receiving optical path structure. The transmitting unit transmits an optical signal to the scanning unit, the scanning unit receives the transmitted optical signal and the control signal, and reflects the echo optical signal. After the echo optical signal generated by the optical signal transmitted by the transmitting unit is received by the receiving unit, the echo optical signal converges to the single photon detector 3, and the single photon detector 3 performs photoelectric conversion.

[0050] The control system 2 includes an FPGA control board 4 , to which are connected a communication module for receiving instructions, a gating signal module for controlling the single-photon detector 3 , a trigger / synchronization signal module, and a scanning signal module for generating a scanning signal.

[0051] The present invention has a compact structure and a reasonable design. The optical system 1 adopts a light path structure in which both transmitting and receiving are combined, which is convenient for debugging the light path and the system does not have a detection blind spot. The single-photon detector 3 can receive the collected target echo light signal and perform photoelectric conversion to obtain an electrical signal, thereby having a higher detection efficiency, being able to detect weak light signals, and improving the maximum detection distance.

[0052] Moreover, the control system 2 of the present invention is provided with an FPGA control board 4. Through the FPGA control board 4 and the control system 2, the working modes of various modules can be coordinated, parameter configuration is convenient and fast, and modules can respond quickly without the need for other conversion circuits, thereby ensuring the real-time and high efficiency of the system's execution of instructions.

[0053] In this embodiment, the laser imaging system also includes a host computer 5, a laser 6, and a TCSPC7. The host computer 5 sends control instructions to the FPGA control board 4, and the TCSPC7 collects different echo light signals of the outgoing pulse in turn. The control system 2 receives instructions from the host computer 5, controls the working status of the TCSPC7, the single-photon detector 3, the laser 6, and the scanning unit, and completes the detection and imaging task of the target area.

[0054] Specifically, the host computer 5 sends the control instructions and preset parameters of the software to the FPGA control board 4 through the serial port via the USB to serial port cable, so that the laser radar system executes the instructions according to the specified working mode and preset parameters to complete the detection and imaging task;

[0055] Laser 6, responsible for the delivery of the laser light source of the entire system;

[0056] A single photon detector (SPAD) 3 is used to receive the collected target echo light signal and perform photoelectric conversion to obtain an electrical signal;

[0057] TCSPC (Time Correlated Single Photon Counter) 7, as a data acquisition and preprocessing unit of the system, is used to collect the electrical signal of the single photon detector 3, and generate a histogram of the cumulative number of echo photons over time in the entire sampling time according to the difference between the arrival time of the synchronization signal of the synchronization signal module and the echo light signal in multiple sub-periods;

[0058] In addition, TCSPC7 usually uses a high repetition rate pulsed laser as a light source. This light source can generate periodic light pulses and can receive and detect photons in these light pulses. Whenever the detector detects a photon, it outputs a pulse signal. Combined with high-precision time measurement equipment, it can measure and record the time difference between the detector output pulse and the light source pulse.

[0059] Single-photon counting laser radar is a new type of high-sensitivity, high-time precision laser radar. It is the product of the combination of single-photon detection technology and TCSPC data processing methods. It can make up for the shortcomings of traditional linear laser radars that are not suitable for underwater target detection scenarios with long distances, low reflectivity targets, strong attenuation transmission media, or laser power limitations. Single-photon counting laser radars can respond to echo signal light as low as a single photon through single-photon detectors. Applying it to the field of underwater target detection will greatly improve the detection accuracy and working distance of the system.

[0060] Specifically, Figure 1 A schematic diagram of the structure of a single-photon lidar imaging system provided in an embodiment of the present invention. In this embodiment, the bottom of the system is supported by a support plate, the support plate is a square plate structure, other system devices are arranged on the support plate, the upper end of the support plate is connected to a control system 2 and a power module 8, the control system main module is connected to a host computer and an FPGA control board 4, a signal adapter sub-card 9 is plugged into the FPGA control board 4, a USB extender 10 and a laser driver 11 are connected to the control system main module, the laser driver 11 is connected to the laser 6 via a laser harness 12, and the laser 6 is supported and arranged on the support plate via a laser bracket 13.

[0061] Furthermore, in the present embodiment, the output end of the laser 6 is a spatial laser output end, the output end of the laser 6 is connected to the end of the emitting unit, the light source output end of the laser 6 is connected to the reflector 14 of the emitting unit of the optical system, a right-angle adjustment frame 15 is provided on the periphery of the reflector 14, a coordinate adjuster is connected to the right-angle adjustment frame 15, and the coordinate adjuster includes an X-axis adjuster 16, a Y-axis adjuster 17 and a Z-axis adjuster 18; a beam expander 19 is provided on the output light path of the reflector 14, the other end of the beam expander 19 is connected to the receiving unit of the optical system, one end of the receiving unit is connected to a visible sleeve 21 through a support rod 20, the optical system equipment is provided in the visible sleeve 21, and the visible sleeve 21 is connected to a Y-axis differential adjuster 22.

[0062] The various devices of the scanning imaging system of the present invention are highly integrated, the overall size of the device is small, and it is easy to integrate, and each module can be flexibly selected and reasonably set in relative position and adaptively adjusted according to actual use requirements.

[0063] Furthermore, in this embodiment, the laser 6 is a pulsed laser, the laser band of the laser 6 is 532nm, and it has a minimum attenuation coefficient underwater, which is suitable for detecting underwater targets.

[0064] Moreover, the laser 6 selected by the system of this embodiment is a MCC-532-5 laser, with a repetition rate of 5kHz, a single pulse energy of 30μJ, a laser wavelength of 532J, a wavelength of 532nm, and a pulse width of 600ps. This wavelength is green visible light, which is in the weak absorption window of underwater transmission and is the most ideal wavelength for underwater transmission of lasers; and the smaller pulse width also avoids the situation where the detector moves toward the wave tail when it is triggered due to the attenuation of energy during long-term transmission, thereby ensuring the measurement accuracy.

[0065] In this embodiment, the transmitting unit and the receiving unit of the optical system 1 are coaxially arranged. By making the transmitting and receiving light paths coaxial, the optical path can be easily debugged and the system will not have a detection blind spot.

[0066] like Figure 2 As shown, the transmitting unit includes multiple reflectors and a beam expander 19, and the multiple reflectors cooperate with each other to realize optical path propagation; the optical signal part of the transmitting unit reaches the scanning unit, and after reflection, the echo optical signal enters the receiving unit, and the receiving unit includes a reflector, a lens and a multi-mode optical fiber. The echo optical signal is transmitted to the single-photon detector through the multi-mode optical fiber.

[0067] Specifically, the emitting unit is provided with an aperture, a first reflector, a beam expander, a second reflector, an aperture, and an aperture reflector 23 in sequence from front to back along the emitting light path, and the output end of the aperture reflector 23 is connected to the scanning unit.

[0068] In this embodiment, the output end of the laser 6 is a spatial laser output end, which has the advantages of small output energy loss and high efficiency, but the output laser is 6mrad@1 / e 2 The divergence angle is slightly larger, and the beam diameter increases approximately linearly with the increase of transmission distance. The spot diameter generated in the target area is too large, resulting in poor spatial resolution of the system. Since the output beam is a Gaussian beam, the product of its divergence angle and beam diameter is a constant. Therefore, by applying a beam expander to laser focusing, the divergence angle of the output light beam is reduced by increasing the beam diameter, thereby improving the spatial resolution of the system.

[0069] Furthermore, the beam expander provided in this embodiment is an adjustable divergence fixed-magnification beam expander, the beam expansion magnification of the beam expander is fixed at 20 times, and after beam expansion, the divergence angle of 6mrad can be reduced to 0.3mrad.

[0070] The core parameters of the optical system launch unit are shown in Table 1.

[0071] Table 1 Core parameters of the optical system launch unit

[0072] parameter Numeric Band 532nm Pulse Width 600ps Repetition rate 5kHz Single pulse energy 30μJ Divergence angle 0.3mrad

[0073] Specifically, the receiving unit is provided with an aperture reflector 23, a filter, an aperture, a lens, and a multi-mode optical fiber in sequence from front to back along the receiving optical path. The end of the aperture reflector is connected to the scanning unit. After the received light is reflected by the scanning unit, it is separated from the outgoing light path by the aperture reflector 23, and then coupled to the multi-mode optical fiber through the filter and the lens, and transmitted to the target area of ​​the single-photon detector through the multi-mode optical fiber. Finally, the echo light signal converges on the detection head of the single-photon detector.

[0074] In this embodiment, by setting a perforated reflector 23 in the optical system 1, the emission light path and the receiving light path can be made non-interfering with each other. The minimum perforation on the reflector ensures that the outgoing light can pass through its central circular hole without obstruction while also improving the receiving efficiency of the system.

[0075] Furthermore, this embodiment adopts a coaxial structure of a 45° aperture reflector, the reflector diameter is 2 inches, the center aperture diameter is 10 mm, and a 532nm dielectric film is added to make the 532nm wavelength laser reflectivity above 99.5%. By selecting a suitable beam expansion ratio to ensure that the outgoing light can pass through the circular hole in the center of the aperture reflector without obstruction, a smaller aperture can also improve the receiving efficiency of the system.

[0076] like Figure 2 As shown, with the scanning galvanometer 24 as the entrance, the outgoing light beam is diffusely reflected on the surface of the object and then partially returns to the system receiving window to reach the scanning galvanometer and enter the receiving light path.

[0077] Moreover, the reflector provided in the receiving unit is a reflector coated with a dielectric film. The use of a reflector coated with a dielectric film is insensitive to the polarization state of the incident light. Therefore, the return light can reach the filter and the coupling lens through the reflector with an extremely high reflectance ratio, and finally reach the detector through the multimode optical fiber.

[0078] In this embodiment, the lens used in the optical system 1 is a plano-convex mirror with a focal length of 100 mm, the reflector is a 1-inch laser line reflector, and a narrow-band filter with a central wavelength of 532 nm is provided at the front end of the incident direction of the lens to remove noise light signals.

[0079] Furthermore, after the received light is reflected by the scanning galvanometer 24, it is separated from the outgoing light path by the open reflector 23, coupled to the multimode optical fiber through the filter and the plano-convex mirror, and transmitted to the target area of ​​the single-photon detector through the multimode optical fiber. In addition, this embodiment uses the Sorebo LA1050-A plano-convex mirror with a focal length of 100 mm and a diameter of 2 inches. The AR coating for the visible light region of 350-700 nm can further improve the transmission efficiency.

[0080] The numerical aperture of the optical fiber should be slightly larger than that of the convex lens. After calculation, a multimode optical fiber with a numerical aperture of 0.22, a core diameter of 105μm, and a length of 1.0m was customized. The entire optical fiber is optimized for transmission in the visible light band and has a lower loss of about 20dB / Km when transmitting visible light with a wavelength of 532nm. The final calculated field of view FOV is 1.05mrad, and the output light has a full-angle divergence of 0.3mrad, which can completely cover the light spot while avoiding excessive background light noise.

[0081] The core parameters of the optical system receiving unit are shown in Table 2.

[0082] Table 2 Core parameters of optical system receiving optical path

[0083] parameter Numeric Fineness 100ps Input optical power <100mW Focusing lens NA 0.14 Multimode fiber NA 0.22 Field of view FOV 1.05mrad focal length 100mm

[0084] In this embodiment, the optical system 1 uses multimode optical fiber for optical signal transmission. Compared with single-mode optical fiber, multimode optical fiber has the advantages of higher coupling efficiency, easier-to-calculate numerical aperture and lower cost in short-distance transmission, and is more suitable for directly transmitting echo photons that only carry time information. Moreover, the numerical aperture and core diameter of multimode optical fiber are larger than those of single-mode optical fiber, so it is easy to couple more echo light, and the loss is smaller than that of single-mode optical fiber.

[0085] In this embodiment, the scanning unit includes a scanning galvanometer 24, which is used to receive the transmitted light signal. By adjusting the deflection angle of the scanning galvanometer 24, the laser pulse is emitted to the corresponding pixel point of the target imaging area and the echo light signal of the target area is reflected.

[0086] The scanning galvanometer 24 controls the deflection of the light beam in the X-axis and Y-axis directions respectively through two reflectors, and the control system independently controls the movement of the two reflectors to control the direction of the emitted laser. The scanning galvanometer has a small moment of inertia and can quickly respond to signals to deflect the light beam to a specified position. The scanning range, scanning accuracy and scanning speed can be set by the control system.

[0087] Furthermore, in this embodiment, ideally, all photons coupled into the optical fiber come from the system receiving window, and the system internal and ambient light noise cannot reach the target surface of the detector. For this reason, this embodiment adopts a fully enclosed cage system, rather than a traditional open desktop optical system, which can shield ambient light and achieve a more compact structure. Therefore, all structures adopted by the optical system 1 are cage structures, such as the optical system adopts a 30mm cage right-angle adjustment frame, a cage translation mounting frame Y-axis differential adjuster, a cage 2-inch visual sleeve, and a 6mm cage support rod.

[0088] Furthermore, in this embodiment, the single photon detector 3 has a dark count rate of 100 cps, a saturation count rate of 37 Mcps, and a circular target area with a diameter of 180 μm. Each photon detected by the single photon detector 3 generates a TTL level pulse (full height half width of about 10 ns), and the detection efficiency of the incident light with a wavelength of 532 nm is about 50%. The silicon avalanche photodiode constituting the target area adopts a thermoelectric cooling temperature control method, and the minimum dead time is shortened to 25 ns, which improves the linearity and timing resolution while also improving the dynamic range of the module.

[0089] The TCSPC module collects different echo signals of sequentially emitted pulses. Affected by factors such as backscattering of water impurities, system measurement accuracy, sampling jitter, and randomness of echo signals, the echo signals of different pulses have certain differences in distribution. Therefore, it is necessary to accumulate and count the echo signals of several emitted pulses to obtain a histogram.

[0090] The module used in this embodiment has 4 independent parallel acquisition channels, the minimum time measurement unit is 1 ps, the root mean square of the measured jitter is less than 10 ps, ​​and the maximum acquisition frequency is 475 MHz.

[0091] Furthermore, in this embodiment, the control system 2 includes a communication module, a gating signal module, a scanning signal module and a trigger-synchronization signal module.

[0092] Specifically, the communication module is used to receive commands from the host computer, execute different working modes, configure preset parameters to the control registers and transmit important register values ​​back to the host computer for real-time monitoring;

[0093] A gate signal module, used to control the high-speed opening and closing of the single-photon detector 3 in a gated mode;

[0094] The scanning signal module is used to generate a scanning signal that complies with the XY2-100 protocol, so that the scanning galvanometer completes the scanning task according to the range and time (space) interval indicated by the preset parameters;

[0095] The trigger-synchronization signal module is used to make the laser generate laser pulses with specific frequency, cycle length, and duty cycle, and receive the laser synchronization light output signal; at the same time, the trigger-synchronization signal module generates the high-precision time reference signal required by TCSPC.

[0096] In this embodiment, after the FPGA control board 4 receives the command from the host computer 5, the serial communication submodule of the FPGA control board 4 decodes the command into four signals of synchronization, scanning, gating and triggering. This module is used to achieve high-reliability control stream transmission. The CRC8 check mechanism is adopted according to the designed frame format length. The module structure is as follows Figure 3 shown.

[0097] Specifically, the connection relationship and functions between the various signals of the system and each module are described as follows.

[0098] like Figure 4 As shown, the synchronization signals include frame and pixel. The synchronization signals are generated by a counter driven by a 500MHz clock with an adjustment accuracy of 2ns. The counter is connected to the TCSPC. The synchronization signals are divided into two paths. The synchronization signals respectively indicate the start and end of the image frame scanning and the start and end of the single pixel scanning in a complete imaging process, which serves as the basis for the time-correlated single-photon counter to switch the working state.

[0099] The trigger signal includes a laser, which is generated by a counter driven by a 500MHz clock with an adjustment accuracy of 2ns. The trigger signal is connected to the external trigger port of the laser to enable the laser to generate laser pulses of a specific number, period and duty cycle within a single pixel sub-period.

[0100] like Figure 5 As shown, the scanning signal is generated by a counter driven by a 500MHz clock according to the XY2-100 protocol, with an adjustment accuracy of 2ns. The scanning signal generating module is connected to the scanning galvanometer. By controlling the deflection angle range, step accuracy and interval time of the scanning galvanometer, the laser pulse traverses the pixel points of the target area in the expected scanning manner.

[0101] In this embodiment, the control system uses a Ruilei two-axis galvanometer as the beam scanning mechanism, model SS-IIE, with an optical deflection angle of ±22.5° and an optical resolution of 12μrad, corresponding to a spot displacement of 0.6mm at a distance of 50 meters. The core parameters of the scanning unit are shown in Table 3.

[0102] Table 3 Scanning unit core parameters

[0103] parameter Numeric journey ±0.393rad Angular resolution 12μrad Accuracy 2μrad Running speed 25rad / s Response time 0.9ms

[0104] The schematic diagram of the gate signal generation module embodiment is as follows Figure 6 As shown in the figure, the gating signal is divided into two stages: coarse delay and fine delay. The coarse delay function is completed by a counter driven by a 500MHz clock with a step accuracy of 2ns. The fine delay function is implemented by the IDELAYE3 primitive with a step accuracy of 4-10ps. The combination of coarse and fine delay can easily and quickly adjust the imaging distance range of the lidar.

[0105] Moreover, the gating signal is connected to the input port of the single-photon detector to control the on and off of the detector and filter out the echo light signal that is not within the expected distance range.

[0106] In this embodiment, the echo light signal is the echo signal.

[0107] Furthermore, if Figure 6 As shown, the present invention also provides an embodiment of a single-photon lidar imaging method for underwater target detection, and the specific embodiment is described as follows.

[0108] Embodiment 1

[0109] The imaging process of the LiDAR system includes the following steps:

[0110] S1, after receiving the command from the host computer, the control system sends out multiple signals at the same time;

[0111] S2, the laser emits a laser pulse, which is expanded and shaped by the transmitting unit and propagates toward the target pixel in a longitudinal straight line; while the laser pulse is emitted, the laser sends an optical synchronization signal to the TSCPC and FPGA control board, which serves as the start time of the photon flight time (TOF) and the trigger mark of the gate signal respectively;

[0112] S3, after the FPGA control board receives the light synchronization signal, after the shielding time required by the preset shielding distance, the single photon detector is activated;

[0113] S4. After the emitted laser pulse encounters the target object, a laser echo is generated. The echo photon is received by the probe of the single-photon detector after being received through the receiving optical path. The received optical signal is then converted into an electrical signal and fed to the TCSPC input port. The TCSPC obtains the time difference between the received echo photon and the emitted optical signal through the internal circuit, and the time difference is used as the photon flight time.

[0114] S5. Calculate the distance of the target pixel point, and then enter the scanning process of the next pixel point until the imaging detection of the target area is completed.

[0115] Embodiment 2

[0116] The imaging process of the LiDAR system includes the following steps:

[0117] S1, after receiving the command from the host computer, the control system sends out three signals at the same time, namely pulse / synchronization signal, scanning signal and gate signal;

[0118] S11, the pulse / synchronization signal indicates the start and end of TCSPC image frame scanning and the start and end of single pixel point scanning, and controls the laser to emit laser pulses according to the set frequency, period, and duty cycle;

[0119] S12, the scanning signal controls the deflection of the scanning galvanometer to emit the emitted light to the target pixel point;

[0120] S13, the gate signal is used to control the high-speed opening and closing of the single-photon detector in the gated mode.

[0121] S2. The laser emits a laser pulse, which is expanded and shaped by the transmitting unit and propagates toward the target pixel in a longitudinal straight line. Each time a laser pulse is emitted, the laser sends an optical synchronization signal to the TSCPC and FPGA control board, which serve as the start time of the photon flight time (TOF) and the trigger mark of the gating signal respectively.

[0122] S3. After the FPGA control board receives the light synchronization signal, it enables the single-photon detector after the shielding time required for the preset shielding distance, that is, activates and enables the single-photon detector to work. The time difference × the flying speed of light in water is the shielding distance, which prevents the echo photons of impurities in the water from affecting the imaging results.

[0123] S4. After the laser pulse is emitted, it generates a laser echo when it encounters the target object. The echo photon is received by the probe of the single-photon detector after being received through the receiving optical path. The received optical signal is then converted into an electrical signal and fed to the TCSPC input port. The TCSPC obtains the time difference between the received echo photon and the outgoing optical signal through the internal circuit, and the time difference is used as the photon flight time.

[0124] S5. The photon count histogram of a single pixel requires multiple sub-cycles to construct. The photon flight time (TOF) is used as the horizontal axis, and it is filled into the corresponding time grid. The count is increased by one, and the highest peak of the histogram is considered to be the flight time to the real target. The distance to the target pixel is calculated; then the scanning process of the next pixel is entered until the imaging detection of the target area is completed.

[0125] In order to verify the actual effect of the system scanning imaging, an underwater imaging test was carried out. Figure 7 , Figure 8 This is the underwater imaging test result of the lidar system. Figure 7 The area displays square aluminum brackets of different heights. Figure 8 The area in the figure is the scanning imaging result diagram, and different distances are represented by pixels of different color domains. From the analysis of the figure, it can be seen that the single-photon radar system of the present invention has an underwater resolution of within 10 cm.

[0126] The present invention proposes a single-photon laser radar imaging system and method for underwater target detection. Compared with the existing laser radar scanning imaging system, by using FPGA to design a laser radar control system, using a single-photon counting detection method and a laser radar control system for underwater target detection, the working mode of each module can be coordinated to ensure the real-time execution of the system instructions. The system of the present invention uses a single-photon detector to collect the weak light signal reflected by the target, so it can detect at a longer distance; at the same time, the system integrates the electrical signal flow control system into the FPGA board, solving the problems of the existing system using a signal generator to realize the gating function, as a laser trigger signal source, using a circuit board for signal conversion and synchronization between modules, and using a single-chip microcomputer to realize the scanning solution. The problems such as large volume, uncontrollable delay, and inconvenient parameter adjustment.

[0127] The present invention has a compact structure, reasonable design, small size, easy integration, convenient parameter adjustment, and strong real-time stability. The optical system adopts a combined transmitting and receiving optical path structure, which is convenient for debugging the optical path and the system will not have a detection blind spot. The single-photon detector can receive the collected target echo light signal and perform photoelectric conversion to obtain an electrical signal, thereby having a higher detection efficiency, being able to detect weak light signals, and increasing the maximum detection distance;

[0128] Moreover, the control system of the present invention is provided with an FPGA control board, which can coordinate the working modes of various modules through the FPGA control board and the control system, and the parameter configuration is convenient and fast, and the modules can respond quickly without other conversion circuits, which can ensure the real-time and high efficiency of the system execution instructions. The present invention can achieve scanning imaging within 10cm spatial resolution at a distance of 50m in offshore waters, which is of great significance for application fields such as marine resource detection and surveying and mapping.

[0129] In the description of the present invention, it is necessary to understand that the orientation or positional relationship indicated by terms such as "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application.

[0130] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0131] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A single-photon laser radar imaging system for underwater target detection, comprising an optical system and a control system, wherein the optical system comprises a transmitting unit and a receiving unit, and is characterized in that: Also included are a single photon detector and a scanning unit; The optical path of the optical system adopts a transmitting and receiving combined optical path structure, the transmitting unit transmits an optical signal to the scanning unit, the scanning unit receives the transmitted optical signal and the control signal, and reflects the echo optical signal, the echo optical signal generated by the optical signal transmitted by the transmitting unit is received by the receiving unit, and then the echo optical signal converges to the single photon detector, and the single photon detector performs photoelectric conversion; The control system includes an FPGA control board, to which a communication module for receiving instructions, a gating signal module for controlling the single-photon detector, a trigger / synchronization signal module, and a scanning signal module for generating a scanning signal are connected.

2. A single-photon laser radar imaging system for underwater target detection according to claim 1, characterized in that: The transmitting unit of the optical system is coaxially arranged with the receiving unit optical path, and the end of the transmitting unit is connected to the output end of the laser, and the output end of the laser is the spatial laser output end.

3. The single-photon laser radar imaging system for underwater target detection according to claim 1, characterized in that: The transmitting unit includes a plurality of reflectors and a beam expander, and the plurality of reflectors cooperate with each other to realize light path propagation; The optical signal part of the transmitting unit reaches the scanning unit, and after reflection, the echo optical signal enters the receiving unit, which includes a reflector, a lens and a multimode optical fiber. The echo optical signal is transmitted to the single-photon detector through the multimode optical fiber.

4. The single-photon laser radar imaging system for underwater target detection according to claim 3, characterized in that: The optical system further comprises an aperture reflector, the receiving unit comprises a reflector coated with a dielectric film, and a filter is arranged at the front end of the incident direction of the lens; After being reflected by the scanning unit, the received light is separated from the outgoing light path by the aperture reflector, and then coupled to the multimode optical fiber through the filter and lens, and transmitted to the target area of ​​the single-photon detector through the multimode optical fiber.

5. The single-photon laser radar imaging system for underwater target detection according to claim 1, characterized in that: It also includes a host computer, a laser, and a TCSPC. The host computer sends control instructions to the FPGA control board through a serial port, and the TCSPC collects different echo light signals of the outgoing pulse in sequence; the control system receives instructions from the host computer and controls the working states of the TCSPC, the single-photon detector, the laser, and the scanning unit.

6. The single-photon laser radar imaging system for underwater target detection according to claim 5, characterized in that: The control system further comprises a trigger-synchronization signal module, which is used to make the laser generate laser pulses with a specific frequency, cycle length, and duty cycle, and receive a laser synchronous light output signal; The trigger-synchronization signal module generates the high-precision time reference signal required by TCSPC.

7. The single-photon laser radar imaging system for underwater target detection according to claim 5, characterized in that: After the FPGA control board receives the host computer command, the communication module decodes the command into four signals of synchronization, scanning, gating and triggering; The synchronization signal includes frame and pixel. The synchronization signal is generated by a counter. The counter is connected to the TCSPC. The synchronization signal indicates the start and end of image frame scanning and the start and end of single pixel scanning in the complete imaging process. The trigger signal includes a laser, the trigger signal is generated by a counter, and the trigger signal is connected to an external trigger port of the laser, so that the laser generates laser pulses of a specific number, period and duty cycle within a single pixel sub-period; The scanning signal is generated by a counter and connected to the scanning unit, and the laser pulse traverses the pixel points of the target area in an expected scanning manner by controlling the range of the deflection angle, the stepping accuracy and the interval time of the scanning unit; The gate control signal is divided into two stages: coarse delay and fine delay. The coarse delay function is completed by a counter, and the fine delay function is implemented by the IDELAYE3 primitive.

8. The single-photon laser radar imaging system for underwater target detection according to claim 1, characterized in that: The scanning unit includes a scanning galvanometer, and the scanning galvanometer is used to receive the emission light signal.

9. A single-photon laser radar imaging method for underwater target detection, characterized in that: The method comprises the following steps: S1, after receiving the command from the host computer, the control system sends out multiple signals at the same time; S2, the laser emits a laser pulse, which is expanded and shaped by the transmitting unit and propagates toward the target pixel in a longitudinal straight line; while the laser pulse is emitted, the laser sends an optical synchronization signal to the TSCPC and FPGA control board, which serves as the start time of the photon flight time (TOF) and the trigger mark of the gate signal respectively; S3, after the FPGA control board receives the light synchronization signal, after the shielding time required by the preset shielding distance, the single photon detector is activated; S4. After the emitted laser pulse encounters the target object, a laser echo is generated. The echo photon is received by the probe of the single-photon detector after being received through the receiving optical path. The received optical signal is then converted into an electrical signal and fed to the TCSPC input port. The TCSPC obtains the time difference between the received echo photon and the emitted optical signal through the internal circuit, and the time difference is used as the photon flight time. S5. Calculate the distance of the target pixel point, and then enter the scanning process of the next pixel point until the imaging detection of the target area is completed.

10. The single-photon laser radar imaging method for underwater target detection according to claim 9, characterized in that: After receiving the command from the host computer, the control system sends out three signals, namely, a pulse / synchronization signal, a scanning signal, and a gate signal; The pulse / synchronization signal indicates the start and end of the TCSPC module image frame scanning and the start and end of the single pixel point scanning, and controls the laser to emit laser pulses according to the set frequency, period, and duty cycle; The scanning signal controls the deflection of the scanning galvanometer of the scanning unit to emit the emitted light to the target pixel point; The gating signal is used to control the high-speed opening and closing of the single-photon detector in the gating mode.

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