High-repetition-frequency single-photon three-dimensional imaging device and ranging extension method

Abstract

The invention relates to the field of light quantum detection and laser radar, and particularly provides a high-repetition-frequency single-photon three-dimensional imaging device and a ranging extension method. The three-dimensional imaging device comprises a laser, a first acousto-optic modulator, an optical fiber circulator, an optical fiber collimator, a piezoelectric ceramic galvanometer, a galvanometer drive, a beam expanding telescope, a second acousto-optic modulator, a single photon detector, a control unit and a time correlation single photon counter. Compared with existing high-repetition-frequency three-dimensional imaging, the problem that the maximum measurable distance is short due to high laser repetition frequency is solved, different laser pulse emission frequencies are used in cooperation with a proposed scanning imaging strategy, and the measurement distance and the measurement efficiency of three-dimensional imaging can be greatly improved.

Description

Technical Field

[0001] The invention relates to the field 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 received widespread attention in the fields of remote sensing, target recognition, and space science research. Single-photon LiDAR based on time-correlated single-photon counting technology has single-photon-level detection sensitivity and picosecond-level time resolution, and has become an effective technical means for long-distance high-precision three-dimensional imaging. Single-photon LiDAR uses periodic laser pulses to illuminate the detection area and uses time-resolved single-photon detectors to detect the scattered photons. The detector records the time interval between the time when each photon arrives at the detector and the time when the most recently transmitted pulse is emitted, and estimates the depth map of the detection area from this.

[0003] Currently, long range and high-efficiency imaging restrict each other. In order to obtain a longer range, a lower laser pulse repetition rate (1000Hz or less) is usually used. The 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 the 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. At present, the industry often uses distance prior to compensate for the aliasing effect of measuring distance, but often in the scenarios of ultra-long-range detection or moving target detection, it is impossible to obtain available distance prior information. Summary of the invention

[0004] Purpose of the Invention

[0005] Based on the above description, 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 problem of distance ambiguity caused by excessively high laser repetition rate during early warning or moving target detection without distance prior is solved.

[0006] The present invention starts from the time-correlated single-photon counting theory and the laser radar equation, designs a high-repetition-rate single-photon three-dimensional imaging device, and sets a scanning imaging strategy for this device to achieve efficient photon imaging. In this imaging strategy, each pixel and the surrounding pixels adopt a similar but different laser emission repetition frequency, and use the distance correlation between adjacent pixels to determine the true distance of the current imaging area. Through the single-photon three-dimensional imaging device and imaging strategy proposed by the present invention, single-photon three-dimensional scanning imaging without distance ambiguity can be achieved without distance prior.

[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, an optical fiber circulator, an optical 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 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 optical fiber collimator collimates the received pulse laser and inputs it into the piezoelectric ceramic galvanometer.

[0012] The control unit is in communication connection 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 it to the imaging area. The photon signal reflected from the imaging area is collected by the beam expander telescope, and then passes through the piezoelectric ceramic galvanometer and the optical fiber collimator in sequence, and is focused into the second port of the optical fiber circulator, and then emitted from the third port of the optical fiber circulator 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 is alternately changed at at least five different frequencies, and the repetition frequency is switched every time the scanning position is changed to obtain the 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] n 1 ~n 5 is the number of cycles experienced by the target echo under five different repetition frequencies; t 1 ~t5 is the arrival time of the target echo within the latest transmitted pulse cycle, for n 1 ~n 5 Traverse any value in and determine the n that minimizes the total error of each equation in the system of equations. 1 ~n 5 value, based on the determined n 1 ~n 5 Calculate the actual measured distance L.

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

[0019] Furthermore, in the pulse transmission stage, the first AOM is turned on and the second AOM is turned off; in the echo reception stage, the first AOM is turned off and the second AOM 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 pulse lasers with different repetition frequencies, wherein the pulse lasers with different repetition frequencies are emitted alternately;

[0024] (2) For the output pulse laser, gate adjustment is performed using the first acousto-optic modulator;

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

[0026] (4) collimating the received pulse laser and inputting it into the piezoelectric ceramic galvanometer 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 turn, and then entering the second port of the fiber circulator after being focused by the fiber collimator, and then being emitted from the third port of the fiber circulator to the second acousto-optic modulator for gating conditions, and then being input into a single-photon detector for photoelectric conversion, and the time-correlated single-photon counter counting and time-measuring the signals after the photoelectric conversion;

[0029] (7) Repeating steps (1) to (6) using a piezoelectric ceramic galvanometer to change the orientation of the emitted light and scan in an array form, changing the laser repetition frequency once each time a pixel is scanned to obtain a 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] n 1 ~n 5 is the number of cycles experienced by the target echo under five different repetition frequencies; t 1 ~t 5 is the time between the arrival of the target echo within the latest transmission pulse period, for n 1 ~n 5 Traverse any value in and determine the n that minimizes the error or variance of each equation in the system of equations. 1 ~n 5 value, based on the determined n 1 ~n 5 Calculate the actual measured distance L.

[0033] Furthermore, the method comprises: for each scanning position, repeating pulsed laser scanning at five frequencies, and solving the range measurement of the scanning position according to the set of equations formed in step (8).

[0034] 5. Beneficial effects

[0035] The scheme proposed by the present invention can realize single-photon three-dimensional imaging with high repetition rate and large range. Compared with the existing high repetition rate three-dimensional imaging, it avoids the problem of short maximum measurable distance caused by high laser repetition frequency, and can realize three-dimensional imaging detection of larger distance without distance prior. Compared with the existing long-range three-dimensional imaging system, this scheme uses a higher laser pulse emission frequency and a plurality of alternating emission modes of different frequencies, combined with the proposed scanning imaging strategy, which can greatly improve the efficiency of three-dimensional imaging. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0037] Figure 2 This is the scanning imaging strategy of the embodiment of the present invention, where each square represents a pixel, and A to E represent five different laser pulse emission frequencies. The laser emission frequencies adopted by each pixel and each row, column and three diagonal pixels in the pixel block composed of the surrounding 8 pixels are all different. DETAILED DESCRIPTION

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

[0039] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below 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 invention and are not used 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 works 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] The laser 1 is connected to the acousto-optic modulator 2 by an optical fiber. The pulse output by the laser 1 is modulated by the acousto-optic modulator 2 and then output to the first port of the optical fiber circulator 3. In this embodiment, the optical fiber circulator 3 includes three ports. The first port is used to receive the input laser pulse. The laser pulse is output from the second port to the optical fiber collimator 4. The optical fiber collimator 4 shapes the optical signal propagated by the optical fiber into collimated light for spatial transmission and is incident on the reflection surface of the piezoelectric ceramic galvanometer 5.

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

[0044] The outgoing collimated light is further expanded by the beam expander telescope 7 and then illuminates the imaging area. The photon signal reflected from the imaging area is collected by the beam expander telescope 7, and then passes through the piezoelectric ceramic galvanometer 5 and the fiber collimator 4 in sequence, and then is focused into the second port of the fiber circulator 3, and is transmitted from the three ports to the second acousto-optic modulator 8, and then is modulated by the second acousto-optic modulator 8 and 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, and the photon detector is extremely sensitive. If no isolation measures are taken, the energy between the pulse intervals will cause the detector to saturate. The two acousto-optic modulators are equivalent to two doors, which are only opened when the pulse passes through, and are not opened at other times.

[0045] The control unit (SoC embedded processing platform 10 in this embodiment) accurately controls the timing of the first AOM 2 and the second AOM 8, so that the first AOM 2 is turned on in the pulse transmission stage and the second AOM is turned on in the receiving stage, so as to suppress the background noise and improve the isolation between the transmission channel and the receiving 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 turned on when the target photon returns, reducing the probability that the background noise triggers the detector to cause dead time and affects the detection of the target photon), and works when the second acousto-optic modulator 8 is turned on to avoid detector saturation caused by the background noise that is not isolated in the optical fiber. While triggering the laser to output an optical pulse, the SoC embedded processing platform 10 outputs a start pulse to the start port of the time-correlated single-photon counter 11 as a timing start signal. The echo photon pulse after the photoelectric conversion of the single-photon detector 9 is input to the stop port of the photon counter 11. Finally, the photon echo flight time of this cycle is recorded for further reconstruction of the three-dimensional image later.

[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 point integration, such as Figure 2 As shown, the laser emission frequency adopted by each pixel and each row, column and three diagonal pixels in the pixel block composed of the surrounding 8 pixels is different. The scanning imaging strategy in the present invention is based on the assumption that the distances of the imaging targets of adjacent pixels are similar, that is, assuming that the distances between the imaging targets in adjacent pixels are within the allowable error range, 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 pulse 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, in the pulse emission stage, the first AOM 2 is turned on and the second AOM 8 is turned off; in the echo reception stage, 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 5 to 10 times, and the divergence angle of the laser pulse after collimation is 100 to 200 μrad.

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

[0052] The piezoelectric ceramic galvanometer 5 used in the embodiment of the present invention is a two-dimensional scanning galvanometer. The plane of the reflector is coated with a 1550 anti-reflection film. 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 100μrad in the system.

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

[0054] The SoC embedded processing platform 11 used in the embodiment of the present invention has a total of 5 TTL digital outputs, 2 analog outputs, and 1 digital signal input. One of them is used to trigger the laser to output laser pulses. The output pulses have 5 repetition frequencies, namely A: 1000kHz, B: 700kHz, C: 635KHz, D: 612KHz, and E: 585kHz. The laser emits corresponding laser pulses according to the pulse control signal. The second channel is used to synchronize the acquisition start of the time-correlated single photon counter 11. The third to fifth channels of signals 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 to drive the piezoelectric ceramic galvanometer 5 to rotate to a specified angle, thereby directing the light beam to the target area, and the light beam is scanned in an array manner in the target area. One digital signal input is used to receive the in-place information after the piezoelectric ceramic galvanometer 5 rotates to the specified position. After receiving the in-place information, the SoC processing platform changes the repetition frequency of the trigger laser pulse and starts scanning the next pixel.

[0055] According to the existing implementation, 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] With regard to the five repetition frequencies mentioned in the embodiment of the present invention, if only one repetition frequency is used, when the frequency is 585 kHz, the maximum measurable distance is 256 m.

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

[0058]

[0059] is the lowest common multiple of each cycle length. 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 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, n 1 ~n 5 is the number of cycles experienced by the target echo under five different repetition frequencies; t 1 ~t 5 It is the arrival time of the target echo within the most recent transmission pulse cycle (the time difference between laser emission and echo reception).

[0066] n 1 ~n 5 are all natural numbers, for n 1 ~n 5 Each time a value is input, other n values ​​can be calculated. After all n values ​​are calculated, the L values ​​obtained by substituting the 5 n values ​​in the above equations and the average value of L are calculated to determine the n that minimizes the total error or total variance of each equation in the equation group. 1 ~n 5 value, based on the determined n1 ~n 5 Calculate the actual measured distance L(n 1 ~n 5 The average value of the L value calculated after substitution is taken as the final distance measurement value).

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

[0068] The scheme proposed by the present invention can realize single-photon three-dimensional imaging with high repetition rate and large range. Compared with the existing high repetition rate three-dimensional imaging, it avoids the problem of short maximum measurable distance due to high laser repetition frequency, and can realize three-dimensional imaging detection at a larger distance without distance prior. Compared with the existing long-range three-dimensional imaging system, this scheme uses a higher laser pulse emission frequency, combined with the proposed scanning imaging strategy, which can greatly improve the efficiency of three-dimensional imaging.

[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, and 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, which 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, all of which 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 comprises: a laser, a first acousto-optic modulator, an optical fiber circulator, an optical 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 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 optical fiber collimator collimates the received pulse laser and inputs it into the piezoelectric ceramic galvanometer. The control unit is in communication connection 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 it to the imaging area. The photon signal reflected from the imaging area is collected by the beam expander telescope, and then passes through the piezoelectric ceramic galvanometer and the optical fiber collimator in sequence, and is focused into the second port of the optical fiber circulator, and then emitted from the third port of the optical fiber circulator 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 is alternately changed at at least five different frequencies, and the repetition frequency is switched every time the scanning position is changed to obtain the 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 recent transmission pulse cycle. Any value of n1~n5 is traversed to determine the n1~n5 that minimizes the sum error of each equation in the equation group. n5 value, calculates the actual measured distance L 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 pulse 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: In the pulse transmission stage, the first AOM is turned on and the second AOM is turned off; in the echo reception stage, 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 respectively.

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 pulse lasers with different repetition frequencies, wherein the pulse lasers with different repetition frequencies are emitted alternately; (2) performing gate control on the output pulse laser using the first acousto-optic modulator; (3) The pulsed laser is input into a fiber circulator, and then the pulsed laser is input into a fiber collimator for collimation; (4) collimating the received pulse laser and inputting it into the piezoelectric ceramic galvanometer 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 turn, and then entering the second port of the fiber circulator after being focused by the fiber collimator, and then being emitted from the third port of the fiber circulator to the second acousto-optic modulator for gating conditions, and then being input into a single-photon detector for photoelectric conversion, and the time-correlated single-photon counter counting and time-measuring the signals after the photoelectric conversion; (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 form, and the repetition frequency of the laser is changed once each pixel is scanned to obtain the single photon count corresponding to each pixel position on the two-dimensional array, and the arrival time of the first echo signal received at the frequency after the pulse 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 cycle. Any value of n1~n5 is traversed to determine the n1~n5 values ​​that minimize 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 with five frequencies, and solving the range measurement of the scanning position according to the equation group formed in step (8).

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