A high repetition rate single photon three-dimensional imaging device and range extension method

Abstract

The present invention relates to the fields of photon detection and laser radar, specifically providing a high-repetition-rate single-photon three-dimensional imaging device and range extension method. The three-dimensional imaging device comprises: a laser, a first acousto-optic modulator, a fiber circulator, a fiber collimator, a piezoelectric ceramic galvanometer, a galvanometer driver, a beam expander telescope, a second acousto-optic modulator, a single-photon detector, a control unit, and a time-correlated single-photon counter. Compared to existing high-repetition-rate three-dimensional imaging, this invention avoids the problem of a short maximum measurable distance caused by the high laser repetition frequency. Furthermore, by using different laser pulse emission frequencies, combined with a proposed scanning imaging strategy, it can significantly improve the measurement distance and efficiency of three-dimensional imaging.

Description

Technical Field

[0001] The present invention relates to the fields of photon detection and laser radar, and in particular to a high-repetition-rate single-photon three-dimensional imaging device and a range extension method. Background Art

[0002] Long-range active optical imaging has garnered widespread attention in fields such as remote sensing, target identification, and space science research. Single-photon lidar (LIDAR), based on time-correlated single-photon counting (TCSPC), boasts single-photon-level detection sensitivity and picosecond-level temporal resolution, making it an effective technology for long-range, high-precision three-dimensional imaging. Single-photon LIDAR uses periodic laser pulses to illuminate the detection area and a time-resolved single-photon detector to detect the scattered photons. The detector records the time interval between each photon's arrival and the emission of the most recently transmitted pulse, thereby estimating the depth map of the detection area.

[0003] Currently, long range and high-efficiency imaging are mutually constrained. To obtain a longer range, a lower laser pulse repetition rate (1000Hz or less) is usually used. A lower repetition rate will greatly reduce the imaging efficiency. Increasing the laser repetition rate will limit the maximum detectable range of single-photon lidar. If the arrival time of a photon exceeds the duration of a single cycle of the laser pulse, the arrival time will be recorded based on the time of the most recently emitted pulse, resulting in distance aliasing. Currently, the industry often uses distance priors to compensate for the aliasing effect of measuring distance, but often in scenarios of ultra-long-range detection or moving target detection, it is not possible to obtain usable distance prior information. Summary of the Invention

[0004] Purpose of the Invention

[0005] Based on the above statement, the present invention proposes a high-repetition-rate single-photon three-dimensional imaging device and a range extension method. By changing the detection period of adjacent pixels, the method solves the distance ambiguity problem caused by the high laser repetition rate during early warning or moving target detection without distance prior conditions.

[0006] Based on the theory of time-correlated single-photon counting and the LiDAR equation, this paper designs a high-repetition-rate single-photon 3D imaging device and incorporates a scanning imaging strategy to achieve highly efficient photon imaging. This imaging strategy uses a laser emission repetition rate that is similar but different from that of surrounding pixels, and leverages the distance correlation between adjacent pixels to determine the true distance of the current imaging area. The single-photon 3D imaging device and imaging strategy proposed in this paper enable single-photon 3D scanning imaging without range ambiguity, even without requiring a priori distance information.

[0007] Specifically, the present invention provides a high-repetition-rate single-photon three-dimensional imaging device, which includes: a laser, a first acousto-optic modulator, a fiber circulator, a fiber collimator, a piezoelectric ceramic galvanometer, a galvanometer driver, a beam expander telescope, a second acousto-optic modulator, a single-photon detector, a control unit, and a time-correlated single-photon counter.

[0008] The laser is used to emit pulsed laser light with a predetermined wavelength and a predetermined repetition frequency.

[0009] The first acousto-optic modulator performs gate control on the pulsed laser, opening and closing synchronously with the laser pulse.

[0010] The fiber circulator is arranged at the output end of the first acousto-optic modulator, and has a first port, a second port and a third port. The first port of the fiber circulator serves as an input port to receive the modulated pulse laser, the second port serves as an output port connected to the fiber collimator, and the third port serves as an echo port.

[0011] The fiber collimator collimates the received pulse laser and inputs it into the piezoelectric ceramic galvanometer.

[0012] The control unit is in communication with the piezoelectric ceramic galvanometer mirror and is used to control the piezoelectric ceramic galvanometer mirror to scan and output in an array manner.

[0013] After receiving the polarized laser pulse, the beam expander telescope expands the laser pulse and irradiates the laser pulse to the imaging area. The photon signal reflected from the imaging area is collected by the beam expander telescope, passes through the piezoelectric ceramic galvanometer and the fiber collimator in sequence, and is focused into the second port of the fiber circulator. It is then emitted from the third port of the fiber circulator and transmitted to the second acousto-optic modulator. The second acousto-optic modulator gates the received signal and inputs it into the single-photon detector for photoelectric conversion. The time-correlated single-photon counter counts the photoelectrically converted signal.

[0014] The control unit controls the laser to adjust the laser pulse repetition frequency so that the laser pulse alternates between at least five different frequencies. Each time a scanning position is changed, the repetition frequency is switched to obtain a photon signal corresponding to the scanning position and the photon signal arrival time.

[0015] The equations are established based on the arrival time of the first echo signal corresponding to every five pixels:

[0016]

[0017] n1~n5 are the number of cycles experienced by the target echo under five different repetition frequencies; t1~t5 are the arrival times of the target echo within the most recently transmitted pulse cycle. Any value of n1~n5 is traversed to determine the n1~n5 value that minimizes the total error of each equation in the equation group, and the actual measured distance L is calculated based on the determined n1~n5.

[0018] Furthermore, the laser is a pulsed fiber laser with an emission wavelength of 1550 nm.

[0019] Furthermore, in the pulse transmission phase, the first acousto-optic modulator is turned on and the second acousto-optic modulator is turned off; in the echo reception phase, the first acousto-optic modulator is turned off and the second acousto-optic modulator is turned on.

[0020] Furthermore, the magnification of the beam expanding telescope is adjustable within the range of 5 to 10 times, the divergence angle of the collimated laser pulse is 100 to 200 μra, and the repetition frequencies of the laser pulses are 1000 kHz, 700 kHz, 635 kHz, 612 kHz, and 585 kHz, respectively.

[0021] Furthermore, the control unit is a SoC embedded processing platform.

[0022] In another aspect, the present invention provides a range extension method using the three-dimensional imaging device, characterized in that the method comprises:

[0023] (1) using a laser to generate at least five pulsed lasers with different repetition frequencies, wherein the pulsed lasers with different repetition frequencies are emitted alternately;

[0024] (2) performing gate control on the output pulse laser using a first acousto-optic modulator;

[0025] (3) The pulsed laser is input into a fiber circulator, and then input into a fiber collimator for collimation;

[0026] (4) collimating the received pulse laser and inputting it into the piezoelectric ceramic galvanometer mirror to change the emission direction of the pulse laser;

[0027] (5) expanding the output laser pulse so that it irradiates the imaging area and receiving the photon signal reflected back from the imaging area;

[0028] (6) using a beam expanding telescope to collect the reflected photon signals, and sending the photon signals to a piezoelectric ceramic galvanometer and a fiber collimator in sequence, and then focusing the photon signals by the fiber collimator and then entering the second port of the fiber circulator, and then transmitting the photon signals from the third port of the fiber circulator to the second acousto-optic modulator for gating conditions, and then inputting the photon signals into a single-photon detector for photoelectric conversion, and the time-correlated single-photon counter counting and time-measuring the photoelectrically converted signals;

[0029] (7) Repeating steps (1) to (6) using a piezoelectric ceramic galvanometer to change the orientation of the emitted light, scanning in an array form, changing the laser repetition frequency once each time a pixel is scanned to obtain the single photon count corresponding to each pixel position on the two-dimensional array, and obtaining the arrival time of the first echo signal received at the frequency after the pulse laser is emitted for each pixel;

[0030] (8) For each frequency variation period, a set of equations is established based on the arrival time of the first echo signal corresponding to the five pixel points:

[0031]

[0032] n1~n5 are the number of cycles experienced by the target echo under five different repetition frequencies; t1~t5 are the arrival time of the target echo within the most recent transmission pulse period. Any value of n1~n5 is traversed to determine the n1~n5 value that minimizes the error or variance of each equation in the equation group, and the actual measured distance L is calculated based on the determined n1~n5.

[0033] Furthermore, the method includes: for each scanning position, repeating five frequencies of pulse laser scanning, and solving the range measurement of the scanning position according to the equation group formed in step (8).

[0034] 5. Beneficial effects

[0035] The proposed solution enables high-repetition-rate, long-range single-photon 3D imaging. Compared to existing high-repetition-rate 3D imaging, this approach avoids the short maximum measurable distance associated with high laser repetition rates. It also enables 3D imaging detection over longer distances without requiring a priori distance measurements. Furthermore, compared to existing long-range 3D imaging systems, this solution utilizes a higher laser pulse emission frequency and alternating emission frequencies, combined with a proposed scanning imaging strategy, to significantly improve 3D imaging efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a block diagram of the high repetition rate single-photon three-dimensional imaging device provided by an embodiment of the present invention.

[0037] Figure 2 This is the scanning imaging strategy of an embodiment of the present invention, where each square represents a pixel, and A through E represent five different laser pulse emission frequencies. Each pixel and the surrounding eight pixels in the pixel block all use different laser emission frequencies for each row, column, and diagonal pixel. DETAILED DESCRIPTION

[0038] The present invention will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0040] The high repetition rate single photon three-dimensional imaging device of this embodiment is as follows: Figure 1 As shown, it includes: a laser 1, a first acousto-optic modulator 2, a fiber circulator 3, a fiber collimator 4, a piezoelectric ceramic galvanometer 5, a galvanometer driver 6, a beam expander telescope 7, a second acousto-optic modulator 8, a single-photon detector 9, a control unit 10, and a time-correlated single-photon counter 11.

[0041] The laser 1 in the high-repetition-rate single-photon three-dimensional imaging device of this embodiment is a 1550nm pulsed fiber laser. The laser 1 operates in an external trigger mode and can emit laser pulses under the triggering of the laser trigger pulse output by the control unit (SoC embedded processing platform 10).

[0042] Laser 1 and AOM 2 are connected via an optical fiber. The pulses output by laser 1 are modulated by AOM 2 and then output to the first port of a fiber circulator 3. In this embodiment, fiber circulator 3 includes three ports. The first port is used to receive input laser pulses. These laser pulses are output through the second port to a fiber collimator 4. Fiber collimator 4 shapes the optical signal propagating through the optical fiber into collimated light for spatial transmission, which is incident on the reflective surface of a piezoelectric ceramic galvanometer mirror 5.

[0043] The control signal output by the SoC embedded processing platform 10 of the piezoelectric ceramic galvanometer mirror 5 is analyzed by the galvanometer mirror driver 6 to control the deflection direction, accurately controlling the direction of the emitted collimated light. In this embodiment, the piezoelectric ceramic galvanometer mirror 5 controls the emitted light to scan in an array form.

[0044] The outgoing collimated light is further expanded by the beam expander 7 and then illuminates the imaging area. The photon signal reflected from the imaging area is collected by the beam expander 7, and then passes through the piezoelectric ceramic galvanometer 5 and the fiber collimator 4 in sequence, and is focused into the second port of the fiber circulator 3. It is then transmitted from the three-port to the second acousto-optic modulator 8. After being modulated by the second acousto-optic modulator 8, it enters the single-photon detector 9 for photoelectric conversion. The two acousto-optic modulators are used to isolate the noise of the fiber laser and further improve the pulse extinction ratio. Although the fiber laser emits pulses, there is also energy between the pulse intervals. The photon detector is extremely sensitive. Without isolation measures, the energy between the pulse intervals will cause the detector to saturate. The two acousto-optic modulators are equivalent to two doors that are only open when the pulse passes through and are closed at other times.

[0045] The control unit (SoC embedded processing platform 10 in this embodiment) precisely controls the timing of the first AOM 2 and the second AOM 8, so that the first AOM 2 is turned on during the pulse transmission phase and the second AOM is turned on during the reception phase, thereby suppressing the background noise and improving the isolation between the transmission channel and the reception channel.

[0046] The control unit (SoC embedded processing platform 10) simultaneously sends a gating signal to the single-photon detector 9 (the gating signal is only activated when the target photon returns, reducing the probability of background noise triggering the detector and causing dead time that affects target photon detection). It operates when the second acousto-optic modulator 8 is turned on to avoid detector saturation caused by unisolated background noise in the optical fiber. When triggering the laser to output an optical pulse, the SoC embedded processing platform 10 simultaneously outputs a start pulse to the start port of the time-correlated single-photon counter 11 as the starting signal for timing. The echo photon pulse after photoelectric conversion by the single-photon detector 9 is input into the stop port of the photon counter 11. Ultimately, the flight time of the photon echo for that cycle is recorded for further reconstruction of the three-dimensional image.

[0047] The imaging strategy of the present invention uses 5 different laser pulse repetition frequencies for scanning imaging, and switches the laser pulse repetition frequency after completing each pixel integration. Figure 2 As shown, each pixel uses a different laser emission frequency than the surrounding eight pixels in each row, column, and diagonal pixel block. The scanning imaging strategy employed in this invention assumes that the distances between targets imaged in adjacent pixels are similar. Specifically, assuming the distances between targets imaged in adjacent pixels are within the tolerance, the maximum imaging distance is determined by the least common multiple of the maximum ranges corresponding to the five laser repetition frequencies.

[0048] The laser used in the embodiment of the present invention is a pulsed fiber laser with an emission wavelength of 1550nm. The pulse repetition frequency of the laser can be controlled by an external trigger electric pulse, and its setting range is 500kHz to 1000kHz. The pulse width is 300 to 500ps, and the output fiber is single-mode.

[0049] The first AOM 2 and the second AOM 8 used in the embodiment of the present invention are used to isolate the background noise of the fiber laser. At the same time, during the pulse transmission phase, the first AOM 2 is turned on and the second AOM 8 is turned off; during the echo reception phase, the first AOM 2 is turned off and the second AOM 8 is turned on.

[0050] The focal length of the fiber collimator 6 used in the embodiment of the present invention is 18.07 mm, and its interface is FC / APC. The magnification of the end beam expander telescope is adjustable within the range of 5x to 10x, and the divergence angle of the collimated laser pulse is 100 to 200 μrad.

[0051] In the embodiment of the present invention, an optical fiber circulator 5 is used to realize the common optical path setting of transmission and reception, so as to maximize the transmission and reception efficiency.

[0052] The piezoelectric ceramic galvanometer mirror 5 used in the embodiment of the present invention is a two-dimensional scanning galvanometer mirror. The mirror plane is coated with a 1550 anti-reflection film, and light in other bands is only weakly reflected, which reduces the stray light noise in the detection and imaging process to a certain extent. The scanning step size is set to 100 μrad in the system.

[0053] The single-photon detector 9 used in the embodiment of the present invention is an InGaAs single-photon detector, which has a quantum efficiency of better than 20% in the 1550 nm band and a dark count rate of less than 2000 Hz.

[0054] The SoC embedded processing platform 11 employed in the embodiment of the present invention has five TTL digital outputs, two analog outputs, and one digital signal input. One output is used to trigger the laser to output laser pulses. This output has five pulse repetition rates: A: 1000kHz, B: 700kHz, C: 635kHz, D: 612kHz, and E: 585kHz. The laser emits corresponding laser pulses based on this pulse control signal. The second output is used to synchronize the acquisition start of the time-correlated single-photon counter 11. Signals from channels 3 to 5 are used to provide gating signals to the first acousto-optic modulator 2, the second acousto-optic modulator 8, and the single-photon detector 9. The two analog outputs are used to control the galvanometer driver 6, which drives the piezoelectric ceramic galvanometer 5 to rotate to a specified angle, thereby directing the light beam toward the target area. The light beam scans the target area in an array pattern. One digital signal input is used to receive information indicating that the piezoelectric ceramic galvanometer 5 has reached its specified position. Upon receiving this information, the SoC processing platform changes the repetition rate of the triggering laser pulse and begins scanning the next pixel.

[0055] According to the existing implementation method, if a single laser pulse emission frequency is used, the maximum measurable distance of the system without distance prior is Where c is the speed of light, f rep is a single pulse repetition frequency.

[0056] For the five repetition frequencies mentioned in the embodiment of the present invention, if only one repetition frequency is used, the maximum measurable distance is 256 m when the frequency is 585 kHz.

[0057] When five repetition frequencies are used, the maximum measurable distance is:

[0058]

[0059] is the lowest common multiple of the length of each period. Under this condition, the maximum detectable distance is greatly increased.

[0060] Therefore, in the embodiment of the present invention, it is assumed that 9 adjacent pixels (such as Figure 2 The distances of the detection targets corresponding to the detection targets relative to the imaging device are similar.

[0061] The actual target distance L is

[0062]

[0063] The equations are established based on the arrival time of the first echo signal corresponding to the five pixels obtained in each round of frequency conversion.

[0064]

[0065] Among them, n1~n5 are the number of cycles experienced by the target echo under five different repetition frequencies; t1~t5 are the arrival times of the target echo within the most recent transmission pulse cycle (the time difference between laser emission and echo reception).

[0066] n1~n5 are all natural numbers. Any value of n1~n5 is traversed. Each time a value is input, the other n values ​​can be calculated. After all n values ​​are calculated, the L values ​​obtained after substituting the 5 n values ​​in the above equation group and the average value of the L values ​​are calculated respectively. The n1~n5 values ​​that minimize the total error or total variance of each equation in the equation group are determined, and the actual measured distance L is calculated based on the determined n1~n5 (the average value of the L values ​​calculated after substituting n1~n5 is used as the final distance measurement value).

[0067] The actual distance of the target can be solved through multiple different repetition frequencies.

[0068] The proposed solution enables high-repetition-rate, long-range single-photon 3D imaging. Compared to existing high-repetition-rate 3D imaging, this approach avoids the short maximum measurable distance associated with high laser repetition rates. Furthermore, it enables 3D imaging detection over larger distances without requiring a priori distance measurements. Furthermore, compared to existing long-range 3D imaging systems, this solution utilizes a higher laser pulse frequency, combined with the proposed scanning imaging strategy, significantly improving 3D imaging efficiency.

[0069] It should be pointed out that, according to the needs of implementation, the various steps / components described in this application can be split into more steps / components, or two or more steps / components or partial operations of steps / components can be combined into new steps / components to achieve the purpose of the present invention.

[0070] Although the preferred embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments. The above-mentioned specific embodiments are merely illustrative and not restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms of specific changes without departing from the scope of protection of the invention and the claims. These all fall within the scope of protection of the present invention.

Claims

1. A high repetition rate single photon three-dimensional imaging device, characterized in that: The three-dimensional imaging device includes: a laser, a first acousto-optic modulator, a fiber circulator, a fiber collimator, a piezoelectric ceramic galvanometer, a galvanometer driver, a beam expander telescope, a second acousto-optic modulator, a single photon detector, a control unit, and a time-correlated single photon counter. The laser is used to emit pulsed laser light with a predetermined wavelength and a predetermined repetition frequency. The first acousto-optic modulator performs gate control on the pulsed laser, opening and closing synchronously with the laser pulse. The fiber circulator is arranged at the output end of the first acousto-optic modulator, and has a first port, a second port and a third port. The first port of the fiber circulator serves as an input port to receive the modulated pulse laser, the second port serves as an output port connected to the fiber collimator, and the third port serves as an echo port. The fiber collimator collimates the received pulse laser and inputs it into the piezoelectric ceramic galvanometer. The control unit is in communication with the piezoelectric ceramic galvanometer mirror and is used to control the piezoelectric ceramic galvanometer mirror to scan and output in an array manner. After receiving the polarized laser pulse, the beam expander telescope expands the laser pulse and irradiates the laser pulse to the imaging area. The photon signal reflected from the imaging area is collected by the beam expander telescope, passes through the piezoelectric ceramic galvanometer and the fiber collimator in sequence, and is focused into the second port of the fiber circulator. It is then emitted from the third port of the fiber circulator and transmitted to the second acousto-optic modulator. The second acousto-optic modulator gates the received signal and inputs it into the single-photon detector for photoelectric conversion. The time-correlated single-photon counter counts the photoelectrically converted signal. The control unit controls the laser to adjust the laser pulse repetition frequency so that the laser pulse alternates between at least five different frequencies. Each time a scanning position is changed, the repetition frequency is switched to obtain a photon signal corresponding to the scanning position and the photon signal arrival time. The equations are established based on the arrival time of the first echo signal corresponding to every five pixels: n1~n5 are the number of cycles experienced by the target echo under five different repetition frequencies; t1~t5 are the arrival times of the target echo within the most recently transmitted pulse cycle. Any value of n1~n5 is traversed to determine the n1~n5 value that minimizes the total error of each equation in the equation group, and the actual measured distance L is calculated based on the determined n1~n5.

2. The high repetition rate single photon three-dimensional imaging device according to claim 1, characterized in that: The laser is a pulsed fiber laser with an emission wavelength of 1550 nm.

3. The high repetition rate single photon three-dimensional imaging device according to claim 1, characterized in that: During the pulse transmission phase, the first AOM is turned on and the second AOM is turned off; during the echo reception phase, the first AOM is turned off and the second AOM is turned on.

4. The high repetition rate single photon three-dimensional imaging device according to claim 1, characterized in that: The magnification of the beam expanding telescope is adjustable within the range of 5 to 10 times, the divergence angle of the collimated laser pulse is 100 to 200 μra, and the repetition frequencies of the laser pulses are 1000 kHz, 700 kHz, 635 kHz, 612 kHz, and 585 kHz.

5. The high repetition rate single photon three-dimensional imaging device according to claim 1, characterized in that: The control unit is a SoC embedded processing platform.

6. A range extension method using the three-dimensional imaging device according to claim 1, characterized in that: The method comprises: (1) using a laser to generate at least five pulsed lasers with different repetition frequencies, wherein the pulsed lasers with different repetition frequencies are emitted alternately; (2) performing gate control on the output pulse laser using a first acousto-optic modulator; (3) The pulsed laser is input into a fiber circulator, and then input into a fiber collimator for collimation; (4) collimating the received pulse laser and inputting it into the piezoelectric ceramic galvanometer mirror to change the emission direction of the pulse laser; (5) expanding the output laser pulse so that it irradiates the imaging area and receiving the photon signal reflected back from the imaging area; (6) using a beam expanding telescope to collect the reflected photon signals, and sending the photon signals to a piezoelectric ceramic galvanometer and a fiber collimator in sequence, and then focusing the photon signals by the fiber collimator and then entering the second port of the fiber circulator, and then transmitting the photon signals from the third port of the fiber circulator to the second acousto-optic modulator for gating conditions, and then inputting the photon signals into a single-photon detector for photoelectric conversion, and the time-correlated single-photon counter counting and time-measuring the photoelectrically converted signals; (7) Repeat steps (1) to (6) and use the piezoelectric ceramic galvanometer to change the direction of the emitted light. Scanning is performed in an array format, and the laser repetition frequency is changed every time a pixel is scanned to obtain the single photon count corresponding to each pixel position on the two-dimensional array. The arrival time of the first echo signal received at that frequency after the pulsed laser is emitted for each pixel is obtained; (8) For each frequency variation period, a set of equations is established based on the arrival time of the first echo signal corresponding to the five pixel points: n1~n5 are the number of cycles experienced by the target echo under five different repetition frequencies; t1~t5 are the arrival time of the target echo within the most recent transmission pulse period. Any value of n1~n5 is traversed to determine the n1~n5 value that minimizes the error or variance of each equation in the equation group, and the actual measured distance L is calculated based on the determined n1~n5.

7. The range extension method according to claim 6, characterized in that: The method comprises: for each scanning position, repeating pulse laser scanning at five frequencies, and solving the range measurement of the scanning position according to the equation group formed in step (8).

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