High-resolution flash lidar imaging system

Abstract

A high-resolution flash laser radar imaging system includes: a modulated light generator, an optical transmitting mechanism, an optical receiving mechanism and a signal acquisition processor, wherein: the laser generator emits laser light, which generates a periodic laser pulse train after passing through the modulator, and the laser pulse is converted into a space laser pulse by the optical transmitting mechanism and emitted to the free space, and the laser pulse reflected by the target passes through the optical receiving mechanism to generate an electrical signal, and the signal is processed by the signal acquisition processor. The present invention is based on the direct time of flight (dTOF) of a single-photon detection array, adopts a linear wavelength scanning laser, and uses two virtual phase arrays (VIPA) to perform two-dimensional light splitting of laser pulses of different wavelengths to achieve an increase in imaging pixels.

Description

Technical Field

[0001] The present invention relates to a technology in the field of laser radar, in particular to a flash laser radar imaging system with a resolution of 75urad. Background Art

[0002] The pixels of existing flash lidar systems are determined by the pixels of the detection array, and it is very expensive to obtain a multi-pixel single-photon detection array. In order to increase the imaging pixels of flash lidar, a beam scanning method is generally used. However, general mechanical scanning will lead to a decrease in system stability, while solid-state beam scanning solutions based on micro-motor galvanometers and optical phased arrays face the problems of only being able to scan in one dimension and having a large limit on laser power. Summary of the invention

[0003] In view of the defects of the prior art that the number of pixels of three-dimensional imaging is limited by the number of timers and the light intensity information and the light propagation time information are difficult to synchronize, the present invention proposes a high-resolution flash lidar imaging system, specifically a direct time-of-flight (dTOF) flash lidar imaging system based on a single-photon detection array, which adopts a linear wavelength scanning laser and uses two virtual phase arrays (VIPA) to perform two-dimensional spectral separation of laser pulses of different wavelengths to achieve an increase in imaging pixels.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a high-resolution flash laser radar imaging system, comprising: a modulated light generator, an optical transmitting mechanism, an optical receiving mechanism and a signal acquisition processor, wherein: a laser generator emits laser light, which generates a periodic laser pulse train after passing through a modulator, and the laser pulse is converted into a space laser pulse through the optical transmitting mechanism and emitted to free space, and the laser pulse reflected back by the target generates an electrical signal through the optical receiving mechanism, and the signal is processed by the signal acquisition processor.

[0006] The modulated light generator comprises: a laser generator and a modulator, wherein: the laser generator emits a linear wavelength scanning laser.

[0007] The optical transmitting mechanism comprises: a first cylindrical lens, a first virtual phase array, a second cylindrical lens and a second virtual phase array, wherein: the two virtual phase arrays are placed vertically to each other and the materials of the two surfaces are the same.

[0008] The optical receiving mechanism comprises: a focusing lens, a microlens array and a single photon detection array which are arranged in sequence.

[0009] The signal acquisition processor comprises: a data acquisition card and a data processor which are connected to each other.

[0010] The present invention relates to an imaging method based on the above system, comprising the following steps:

[0011] Step 1: Turn on the modulated light generator to generate periodic laser pulses.

[0012] Step 2: The laser pulse passes through an optical launch mechanism consisting of two VIPAs placed perpendicular to each other, generating a two-dimensional laser dot array that can be scanned with varying wavelengths.

[0013] Step 3: The laser pulse dot array reaches the target surface and is scattered by the target surface. The optical receiving mechanism receives the laser pulse dot array scattered by the target surface and generates a photon counting histogram.

[0014] Step 4: The signal acquisition processor obtains the laser pulse propagation time of each pixel based on the photon counting histogram generated in step 3 and calculates the distance value of the target corresponding to each pixel to obtain a lower resolution depth map.

[0015] Step 5: Adjust the emission laser wavelength of the laser generator in the modulated light generator and repeat steps 2 to 4 to obtain a lower resolution depth map at the wavelength at a single location, and then repeat steps 2 to 4 under the entire target area to obtain a lower resolution depth map at multiple wavelengths covered by the laser dot array.

[0016] Step 6: Combine the lower resolution depth maps at multiple wavelengths to obtain a high-resolution depth map.

[0017] Technical Effects

[0018] The present invention uses two VIPAs placed perpendicular to each other and operates at an inclined angle of 75urad to generate a two-dimensional laser dot array that can be scanned with wavelength changes. The two-dimensional laser dot array can be scanned in both the pitch angle and the azimuth angle by only using the change of the laser wavelength, and has high stability, low energy consumption and fast scanning speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the structure of the present invention;

[0020] Figure 2 Schematic diagram of the working principle of the microlens array and the single photon detection array;

[0021] Figure 3 Schematic diagram of the working principle of the optical receiving mechanism

[0022] Figure 4 Schematic diagram of VIPA interference principle;

[0023] Figure 5 It is a simulation schematic diagram of the relationship between different emission angles and their corresponding interference levels;

[0024] Figure 6 It is a schematic diagram of the simulation of the pixel sensitivity angle range and the dispersion angle of different wavelengths;

[0025] Figure 7 A schematic diagram of the simulation of the coupling relationship between all 80 pixels and each interference level of VIPA;

[0026] Figure 8 This is a schematic diagram of the working principle of the two-dimensional spectroscopic module;

[0027] Fig. 9 It is a simulation schematic diagram of each interference level scanning in two dimensions of pitch angle and azimuth angle;

[0028] Fig.10 Schematic diagram of the laser dot matrix generated corresponding to 400 wavelength scanning points;

[0029] Fig.11 Schematic diagram of the electrical pulse generated by a single pixel receiving a laser pulse;

[0030] Fig.12 A schematic diagram of the result after preprocessing the electric pulse signal generated by a single pixel;

[0031] Fig.13 It is a schematic diagram of an application example;

[0032] In the figure: a linear wavelength scanning laser 1, an optical modulator 2, a beam expander 3, a first cylindrical lens 4, a first virtual phase array 5, a second cylindrical lens 6, a second virtual phase array 7, a focusing lens 8, a microlens array 9, a single photon detection array 10, and a data processor 11. DETAILED DESCRIPTION

[0033] like Figure 1As shown, this embodiment includes: an optical transmitting mechanism and an optical receiving mechanism, wherein: the optical transmitting mechanism includes: a linear wavelength scanning laser 1, an optical modulator 2, a beam expander 3, a first cylindrical lens 4, a first virtual phase array 5, a second cylindrical lens 6, and a second virtual phase array 7 arranged in sequence; the optical receiving mechanism includes: a focusing lens 8, a microlens array 9, a single photon detection array 10 and a data processor 11 arranged in sequence, wherein: the linear wavelength scanning laser 1 is connected to the optical modulator 2, the output of the optical modulator 2 generates a wider parallel light beam through the beam expander 3, the first cylindrical lens 4 focuses and couples the wide laser beam into the first virtual phase array 5, the output laser of the first virtual phase array 5 is coupled into the second virtual phase array 7 through the second cylindrical lens 6, the output laser of the second virtual phase array 7 is emitted to the free space, the laser pulse reflected by the target returns along the original direction, reaches the microlens array 9 through the focusing lens 8, the microlens array 9 further focuses the incident light to the single photon detection array 10, and the single photon detection array 10 is connected to the data processor 11.

[0034] like Figure 2 As shown, the single-photon detection array 10 has a pixel count of 60×80, wherein the area of ​​each pixel is 30μm×30μm, the photosensitive area is 12.88μm×23μm, and the filling factor is 32.9%. In order to improve the filling factor, a microlens array 9 with the same number of elements of 60×80 and the same size of each element of 30μm×30μm is placed on the single-photon detection array 10, wherein the focal length f1 of each microlens is the same as the spacing between the microlens array 9 and the single-photon detection array 10.

[0035] like Figure 3 As shown, the focal length f2 of the focusing lens 8 is the same as the distance between it and the plane of the microlens array 9, and satisfies f2tan(θ s )≥40×30μm+r2, where r2 is the radius of the focusing lens 8, 40×30 μm represents the distance between the central axis of the focusing lens 8 and the edge of the microlens array 9; so that light focused onto the plane of the microlens array 9 from any direction can be focused onto the corresponding single photon array 10 pixel.

[0036] The working mode of the optical receiving mechanism is the same as that of the photoelectric detection array with a filling factor of 100%. The central angle of the photosensitive area corresponding to the jth pixel is where y j=(40-j+0.5)30μm, j=1,2,...,80. The linear wavelength scanning laser 1 in the optical transmitting mechanism emits continuous laser light at a wavelength scanning interval of 0.03nm and a scanning speed of 10kHz in the range of 850nm to 855.4nm. The optical modulator 2 modulates the continuous laser light with an electrical signal having a period of 64ns and a duty cycle of 7.8% to generate a laser pulse train with a pulse repetition period of 64ns and a pulse width of 5ns. The laser pulse is collimated by the beam expander 3 and enters the first cylindrical lens 4, and then is focused and coupled into the first virtual phase array 5.

[0037] like Figure 4 As shown, the placement direction of the first virtual phase array 5 forms an angle θ of 28 degrees with the incident light. i , its thickness t=1.686mm, its refractive index n=1.452469 at 852.11nm, the first virtual phase array 5 is composed of two surfaces with different reflectivity, wherein the reflectivity R of the first surface is 100%, and the reflectivity r of the second surface is 95%, the light beam will be partially transmitted while being reflected by the second surface, and there is a fixed phase difference between the multiple outgoing light beams and they interfere with each other to form fringes, when the intensity distribution of the incident light beam satisfies the Gaussian distribution, then its intensity is about the outgoing pitch angle θ e The distribution of is:

[0038] in:

[0039] λ is the wavelength, which appears as periodic fringes. The total number of emitted interference fringes is related to the envelope determined by the width of the beam after collimation by the beam expander 3 and the focal length of the first cylindrical lens 4. The peak intensity emission direction of the m-th order interference fringe changes with the wavelength, that is, in Incident light of different wavelengths will be differentiated in the pitch angle direction.

[0040] In order to achieve that each column of pixels corresponds to a pitch angle and each row of pixels corresponds to a direction angle, the optical receiving mechanism is set to correspond to each pixel on the vertical axis and the pitch angle direction θ e One-to-one correspondence means that the angular spacing of the photosensitive range of the jth and j+1th pixels should be the same as the angular spacing of the emission angles of the mth and m+1th interference fringes, respectively, that is, |θ j+1 -θ j |=|θ m+1 (λ)-θ m (λ)|, where j = 1, 2, ..., 80, the interference order m and its corresponding emission direction θ e satisfy That is Figure 5 As shown. The interference order m is θ eThe quadratic function of |θ j+1 -θ j |=|θ m+1 (λ)-θ m (λ)|The changes on both sides of the equation are not synchronized and cannot be strictly satisfied. In practical applications, only the case when j = 40 is considered. At this time, the center of the photosensitive range is located at Substituting the above value into in Substituting the parameters into the equation, we get f2=41.9mm.

[0041] As the laser wavelength changes, the emission direction of the interference fringes will move. The working mode and the photosensitive angle range corresponding to the j=1,...,39,40,41,42,...,80th pixels are as follows: Figure 6 As shown, each pixel corresponds to an interference level. The total intensity distribution of each interference level is enveloped by the intensity distribution of the incident beam. Decision, such as Figure 7 shown.

[0042] like Figure 6 As shown in FIG. 1 , as the laser wavelength increases, the pitch angle of the interference fringes of the same interference level decreases, but the photosensitivity range corresponding to each pixel remains unchanged. When the laser wavelength further increases, the interference fringes of the lower level will appear in the photosensitivity range corresponding to the pixel column. The specific form is that when the light frequency (wavelength) changes to an integer multiple of the free spectrum range (FSR), the corresponding pitch angle of each pixel is the same. Therefore, the output light of the first virtual phase array 5 is focused by the second cylindrical lens 6 into the second virtual phase array 7 placed laterally to realize the distinction of incident light of different wavelengths in azimuth. The working method is as follows: Figure 8 As shown. The calculation formula of FSR is:

[0043] Compared with the first virtual phase array 5, the second virtual phase array 7 has a tilt angle of 35°. Due to the different tilt angles, the FSR of the second virtual phase array 7 is different from that of the first phase array 5. Therefore, as the wavelength of the emitted laser changes, the interference level can be scanned in two dimensions, the pitch angle and the azimuth angle. Fig. 9 As shown, each grid represents the sensitive angle range of each pixel of the single photon detection array 10. When the linear wavelength scanning laser 1 scans at a wavelength interval of 0.03 nm in the range of 850 nm to 855.4 nm, the scanning dot matrix formed is as follows: Fig.10 As shown in the figure. Theoretically, the larger the wavelength scanning range, the smaller the wavelength scanning interval, the denser the scanning dot matrix, and the higher the pixel increase of the high-resolution depth image finally obtained. The echo laser pulse generated by the target reflection returns along the original direction and is generated by the optical receiving mechanism as shown in the figure. Fig.11 The electrical signal generated by the corresponding single pixel receiving the photon is transmitted to the data processor 11 for processing, and the moment of each data transmission should be synchronized with the moment when the linear wavelength scanning laser 1 scans to the next wavelength.

[0044] In order to prevent large ranging errors caused by the target being too far away and the echo energy being too low, a threshold needs to be imposed on the photon count value. When the photon count value is less than the threshold, it is considered that there is no echo and it is reset to zero. The higher the threshold value, the higher the noise tolerance of the system, but the higher the sensitivity requirement for the single photon detection array 10. Because the photon counts outside the echo pulse are generally caused by dark counts introduced by the high temperature of the single photon detection array 10 device, they generally appear as shot noise and can be eliminated using a one-dimensional median filter. In order to eliminate the shot noise as much as possible without affecting the pulse waveform, the median filter order 2N can be made proportional to the pulse width. If Fig.11 The signal in is set as r(n), n = 1, 2, ..., M, where M is 1024 (the total number of sampling points for the entire detection period of 64ns with a sampling interval of 62.5ps), and the one-dimensional median filtering can be expressed as m(n) = med(r(nN), ..., r(n), ..., r(n+N)), where med() represents the median function. The signal R(n) obtained after the above processing is as follows Fig.12 As shown, the arrival time t of the echo pulse laser can be obtained through autocorrelation processing and peak discrimination. According to the formula d=c×t / 2, the distance between the field of view (FOV) and the detection device can be obtained. The data transmitted by the single-photon detection array 10 to the data processor 11 each time includes the similarity generated by each pixel receiving the photon. Fig.11 By performing the above processing on the electrical signal data of each pixel, a lower-resolution three-dimensional depth map can be generated, and a high-resolution three-dimensional depth image can be obtained by combining the lower-resolution three-dimensional depth maps generated by multiple different laser wavelengths.

[0045] like Fig.10 As shown in the figure, the distribution of the dot matrix is ​​not uniform. In practical applications, when it is stored directly, additional space is required to store the pitch angle and azimuth angle corresponding to each point. The problem can be solved by converting it into a depth image with uniform pixel distribution through inverse bilinear interpolation, and the number of pixels in the depth image is close to the number of points in the original dot matrix.

[0046] After specific practical experiments, a VIPA with a length of 20 mm, a thickness of 1.686 mm, a refractive index of 1.452469 at 852.11 nm, and a front and rear surface reflectivity of 95% and 100% respectively was arranged at an inclined angle of 40 degrees, and a laser with a wavelength of 850 nm was coupled in. An interference lattice with an angular width of 75urad can be obtained, and changing the wavelength can make the interference lattice scan simultaneously, such as Fig.13 shown.

[0047] Compared with the prior art, the present invention improves the imaging resolution of the flash laser radar by two orders of magnitude and uses a microlens array to compensate for the lower fill factor. At the same time, beam scanning can be achieved by simply changing the laser wavelength, and has higher stability, lower power consumption and faster scanning speed. The present invention utilizes a combination of a linear wavelength scanning laser and two virtual phase arrays to achieve two-dimensional scanning of the outgoing laser pulses, and realizes the imaging pixel expansion of the flash laser radar based on the single-photon detection array. Compared with the prior art, the present invention uses a relatively simple, easy-to-integrate all-solid-state structure to achieve a high-multiple expansion of imaging pixels for the flash laser radar based on the single-photon detection array.

[0048] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A high-resolution flash laser radar imaging system, characterized in that: include: A modulated light generator, an optical transmitting mechanism, an optical receiving mechanism and a signal acquisition processor, wherein: a laser generator emits laser light, which generates a periodic laser pulse train after passing through a modulator, and the laser pulse is converted into a space laser pulse by an optical transmitting mechanism and emitted into free space, and the periodic laser pulse train reflected by the target generates an electrical signal through an optical receiving mechanism, and the signal is processed by a signal acquisition processor; The optical transmitting mechanism comprises: a first cylindrical lens, a first virtual phase array, a second cylindrical lens and a second virtual phase array, wherein: the two virtual phase arrays are placed vertically to each other and the materials of the two surfaces are the same; The modulated light generator comprises: a laser generator and a light modulator, wherein: the laser generator emits a linear wavelength scanning laser; The optical receiving mechanism comprises: a focusing lens, a microlens array and a single photon detection array arranged in sequence; The signal acquisition processor comprises: a data acquisition card and a data processor connected to each other; The placement direction of the first virtual phase array forms an angle θ with the incident light i , the light beam will be partially transmitted while being reflected by the second surface, and there is a fixed phase difference between the multiple outgoing light beams and they interfere with each other to form fringes. When the intensity distribution of the incident light beam satisfies the Gaussian distribution, its intensity is related to the outgoing pitch angle θ e The distribution of is: in: λ is the wavelength, which appears as periodic fringes. The total number of emitted interference fringes is related to the envelope determined by the width of the beam after collimation by the beam expander and the focal length of the first cylindrical lens. The peak intensity emission direction of the m-th order interference fringe changes with the wavelength, that is, in Incident light of different wavelengths will be differentiated in the pitch angle direction.

2. The high-resolution flash laser radar imaging system according to claim 1, characterized in that: The first virtual phase array is composed of two surfaces with different reflectivity, wherein the reflectivity R of the first surface is 100%, and the reflectivity r of the second surface is 95%. Its thickness is t=1.686 mm, and its refractive index n=1.452469 at 852.11 nm.

3. The high-resolution flash laser radar imaging system according to claim 1, characterized in that: The placement direction of the first virtual phase array forms an angle of 28° with the incident light.

4. The high-resolution flash laser radar imaging system according to claim 1, characterized in that: The pixels of the optical receiving mechanism on the vertical axis and the pitch angle direction θ e One-to-one correspondence means that the angular spacing of the photosensitive range of the jth and j+1th pixels should be the same as the angular spacing of the emission angles of the mth and m+1th interference fringes, respectively, that is, |θ j+1 -θ j |=|θ m+1 (λ)-θ m (λ)|, where j = 1, 2, ..., 80, the interference order m and its corresponding emission direction θ e satisfy The interference order m is θ e The quadratic function of |θ j+1 -θ j |=|θ m+1 (λ)-θ m (λ)|The changes on both sides of the equation are not synchronized and cannot be strictly satisfied. In practical applications, only the case when j = 40 is considered. At this time, the center of the photosensitive range is located at Substituting the above value into in Substituting the parameters into the equation, we get f2=41.9mm.

5. An imaging method according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1: Turn on the modulated light generator to generate periodic laser pulses; Step 2: The laser pulse passes through an optical transmission mechanism consisting of two virtual phase arrays placed perpendicular to each other, generating a two-dimensional laser dot array that can be scanned with wavelength changes; Step 3: The laser pulse dot matrix reaches the target surface and is scattered by the target surface. The optical receiving mechanism receives the laser pulse dot matrix scattered by the target surface and generates a photon counting histogram. Step 4: The signal acquisition processor obtains the laser pulse propagation time of each pixel according to the photon counting histogram generated in step 3 and calculates the distance value of each pixel corresponding to the target to obtain a lower resolution depth map; Step 5: Adjust the emission laser wavelength of the laser generator in the modulated light generator and repeat steps 2 to 4 to obtain a lower resolution depth map at the wavelength at a single location, and then repeat steps 2 to 4 in the entire target area to obtain lower resolution depth maps at multiple wavelengths covered by the laser dot array; Step 6: Combine the lower resolution depth maps at multiple wavelengths to obtain a high-resolution depth map.

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