All-solid-state high-resolution optical three-dimensional imaging system and method based on coherent detection

Abstract

The application discloses a kind of full solid-state high-resolution optical three-dimensional imaging system and method based on coherent detection.The application uses electro-optic modulator to load sweep frequency microwave signal to laser, and builds space light bridge, so that signal light and reference light have 2 times the optical path difference of ranging distance, simultaneously using space light bridge, obtain coherent pattern with 90 degree phase difference on 4 CCDs, then based on coherent image, eliminate the direct current term and phase of signal light and reference light, finally calculate the distance of the measured object by calculating the overlapping area of two signals (signal light and reference light).Since the application can exclude phase interference and background light interference, it has stronger anti-interference ability compared with single-CCD imaging.Compared with traditional laser radar, it has high measurement accuracy, long measurement distance and strong anti-interference.

Description

Technical Field

[0001] This invention relates to the field of intelligent sensor technology, specifically to an all-solid-state high-resolution optical three-dimensional imaging system and method based on coherent detection. Background Technology

[0002] With the rapid development of advanced scientific and technological fields such as autonomous driving and artificial intelligence, fast and high-precision distance measurement has become a key technology supporting related applications. Compared with traditional methods such as visual ranging and millimeter-wave radar ranging, lidar has the advantages of high range and angular resolution, and can accurately perceive the distance and shape of objects, making it the most promising technological solution paradigm. Currently, lidar is mainly divided into two categories: Time-of-Flight (ToF) lidar and Frequency Modulated Continuous Wave (FMCW) lidar. Time-of-Flight lidar has a simple principle but low ranging accuracy; FMCW lidar has high accuracy, but due to the limitation of laser linewidth, the coherence length of direct-modulation FMCW lidar is short, making it unable to detect targets at long distances. In addition, the imaging quality of traditional lidar is greatly affected by mechanical vibration. Therefore, researching all-solid-state lidar with high accuracy, long measurement range, and anti-interference capabilities is key to achieving fast and high-precision distance measurement.

[0003] Currently, all-solid-state lidar mainly includes flash lidar and phased array (OPA) frequency-modulated continuous wave lidar. Flash lidar technology is based on the time-of-flight method, using optical rotators to convert the return time into light polarization and other information, further converting it into light intensity. This intensity information is then directly acquired via a CCD through optical imaging, and the distance to the object corresponding to each pixel of the CCD is calculated. This method is simple in principle, can simultaneously image all positions within the target area, and is insensitive to vibration, but requires prior information and its measurement accuracy remains limited. Phased array frequency-modulated continuous wave lidar scans the target object by controlling the relative phase difference between emitted beams. This scheme offers high ranging accuracy and small device size, but it generates sidelobes during control, affecting the control of the beam direction. Furthermore, phased array frequency-modulated continuous wave lidar generally has low output power, thus limiting the measurement distance. However, all-solid-state lidar with high precision, long-distance measurement capabilities and strong anti-interference capabilities has not yet been reported. Summary of the Invention

[0004] In view of this, the present invention provides an all-solid-state high-resolution optical three-dimensional imaging system and method based on coherent detection. It employs an electro-optic modulator to load a swept-frequency microwave signal onto light, and uses a spatial optical bridge to ensure that the signal light received by the four CCDs has different phase differences with the reference light. The distance to the object under test is calculated by determining the overlap area of ​​the two signals. Compared with traditional lidar, it offers higher measurement accuracy, longer measurement distance, and stronger anti-interference capability.

[0005] The all-solid-state high-resolution optical three-dimensional imaging system based on coherent detection of the present invention includes: a laser, a microwave source, a first polarization controller, an electro-optic modulator, an optical coupler, a second polarization controller, a first collimator, a first convex lens, a third polarization controller, a second collimator, a second convex lens, a first polarizer, a quarter-wave plate, an unpolarized beam splitter cube, a first polarized beam splitter cube, a second polarized beam splitter cube, a third convex lens, a second polarizer, a first CCD, a second CCD, a third CCD, and a fourth CCD;

[0006] The system comprises a laser emitting laser light; a microwave source generating a modulation signal; the laser light entering an electro-optic modulator via a first polarization controller, modulated by the electro-optic modulator based on the modulation signal, and then split into two beams by an optical coupler. One beam is a signal beam, which passes sequentially through a second polarization controller, a first collimating lens, and a first convex lens to illuminate the object under test; the other beam is a reference beam, which passes sequentially through a third polarization controller, a second collimating lens, a second convex lens, a first polarizer, and a quarter-wave plate to illuminate a non-polarized beam splitter cube. The non-polarized beam splitter cube splits the light into two paths: one path is split by a second polarization beam splitter cube and imaged on the first and second CCDs respectively; the other path is split by the first polarization beam splitter cube and imaged on the third and fourth CCDs respectively. The signal beam, after being reflected by the object under test, passes through the third convex lens and the second polarizer cube to illuminate the non-polarized beam splitter cube, and after being split by the non-polarized beam splitter cube, the second polarization beam splitter cube, and the first polarization beam splitter cube, imaged on the first, second, third, and fourth CCDs respectively.

[0007] The four CCDs have a 90° phase difference, the signal light and the reference light have equal intensity on the four CCDs, and the optical path difference between the signal light and the reference light is twice the distance to the object under test.

[0008] Preferably, the laser is a narrow linewidth laser.

[0009] Preferably, a pinhole filter is provided between the second convex lens and the quarter-wave plate.

[0010] Preferably, the modulation signal is a linear chirped signal.

[0011] Preferably, the electro-optic modulator is a Mach-Zehnder intensity modulator.

[0012] Ideally, the voltage of the microwave source is much smaller than the half-wave voltage of the electro-optic modulator.

[0013] This invention also provides an all-solid-state high-resolution optical three-dimensional imaging method based on coherent detection. Imaging is performed using the aforementioned system to obtain the output currents of four CCDs. After merging the output currents of the four CCDs and eliminating the DC and phase terms, the signal i is obtained.sig (t); for i sig (t) Perform a Fast Fourier Transform to obtain the spectral peak value f peak Then the distance d between the object to be tested and the laser is:

[0014]

[0015] Where c is the speed of light, T is the sweep time of the modulation signal, and B is the sweep bandwidth of the modulation signal.

[0016] Ideally, after imaging with the system, the images from the four CCDs should be calibrated to eliminate translation and rotation before merging.

[0017] Beneficial effects:

[0018] (1) This invention uses an electro-optic modulator to load a swept-frequency microwave signal onto a laser and constructs a spatial optical bridge, resulting in an optical path difference between the signal light and the reference light that is twice the ranging distance. Simultaneously, using the spatial optical bridge, coherent patterns with a 90° phase difference are obtained on four CCDs. Then, based on the coherent image, the DC term and phase of the signal light and reference light are eliminated. Finally, the distance to the object under test is calculated by calculating the overlapping area of ​​the two signals (signal light and reference light). Because this invention can eliminate phase interference and background light interference, it has stronger anti-interference capabilities compared to single-CCD imaging. Compared with traditional lidar, it has higher measurement accuracy, longer measurement distance, and is more resistant to interference.

[0019] (2) This invention employs external intensity modulation of the laser and loads a linear chirped signal, resulting in a longer coherence length compared to internal modulation. Furthermore, the use of a narrow-linewidth laser further extends the coherence length, ensuring that the system's measurement distance is not limited by the coherence length.

[0020] (3) The present invention uses a Mach-Zehnder intensity modulator to perform external intensity modulation on the laser. This method has better modulation linearity than direct-modulated lasers, so that the measurement resolution is not limited by the modulation linearity.

[0021] (4) The four CCDs share the same convex lens for imaging, which reduces the complexity of calibration between CCDs. Attached Figure Description

[0022] Figure 1 This is a structural block diagram of the all-solid-state lidar three-dimensional imaging system of the present invention.

[0023] Among them, 1-narrow linewidth laser, 2-first polarization controller, 3-fiber electro-optic modulator, 4-90:10 optical coupler, 5-second polarization controller, 6-first collimator, 7-first convex lens, 8-controller, 9-microwave source, 10-third polarization controller, 11-second collimator, 12-second convex lens, 13-pinhole filter, 14-first polarizer, 15-quarter-wave plate, 16-third convex lens, 17-second polarizer, 18-first polarization beam splitter cube, 19-unpolarization beam splitter cube, 20-second polarization beam splitter cube, 21-first CCD, 22-second CCD, 23-third CCD, 24-fourth CCD, 25-object under test.

[0024] Figure 2 This is the image of the imaging result. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0026] This invention provides an all-solid-state high-resolution optical three-dimensional imaging system based on coherent detection, such as... Figure 1 As shown, the system includes: a laser 1, a first polarization controller 2, a fiber electro-optic modulator 3, an optical coupler 4, a second polarization controller 5, a first collimator 6, a first convex lens 7, a controller 8, a microwave source 9, a third polarization controller 10, a second collimator 11, a second convex lens 12, a pinhole filter 13, a third convex lens 16, a first CCD 21, a second CCD 22, a third CCD 23, a fourth CCD 24, an object under test 25, and a spatial optical bridge. The spatial optical bridge includes a first polarizer 14, a quarter-wave plate 15, a second polarizer 17, a first polarization beam splitter 18, a non-polarization beam splitter 19, and a second polarization beam splitter 20.

[0027] like Figure 1 As shown, the laser 1 is connected to the first polarization controller 2 via optical fiber. The first polarization controller 2 is connected to the input end of the electro-optic modulator 3 via optical fiber. The output end of the electro-optic modulator 3 is connected to the input end of the optical coupler 4 via optical fiber. The two output ends of the optical coupler 4 are connected to the second polarization controller 5 and the third polarization controller 10 via optical fiber, respectively. The second polarization controller 5 is connected to the first collimator 6, and the third polarization controller 10 is connected to the second collimator 11 via optical fiber.

[0028] The output light of the first collimator 6 is received by the first convex lens 7, the output light of the first convex lens 7 is received by the object under test 25, the reflected light of the object under test 25 is received by the third convex lens 16, the output light of the third convex lens 16 is received by the second polarizer 17, and the output light of the second polarizer 17 is received by the first receiving surface of the unpolarized beam splitter 19.

[0029] The output light of the second collimator 11 is received by the second convex lens 12, the output light of the second convex lens is received by the pinhole filter 13, the output light of the vacuum filter 13 is received by the first polarizer 14, the output light of the first polarizer 14 is received by the quarter-wave plate 15, and the output light of the quarter-wave plate 15 is received by the second receiving surface of the unpolarized beam splitter 19.

[0030] The output light of the unpolarized beam splitter 19 is split into two paths, which are received by the first polarized beam splitter 18 and the second polarized beam splitter 20, respectively. The output light of the first polarized beam splitter 18 is split into two paths, which are received by the third CCD 23 and the fourth CCD 24, respectively. The output light of the second polarized beam splitter 20 is split into two paths, which are received by the first CCD 21 and the second CCD 22, respectively.

[0031] The first CCD 21, the second CCD 22, the third CCD 23, and the fourth CCD 24 are connected to the controller 8. The controller 8 is connected to the laser 1 and the microwave source 9. The microwave source 9 is connected to the electro-optic modulator 3.

[0032] Laser 1 generates an optical signal, which, after its polarization direction is adjusted by the first polarization controller 2, is emitted to the electro-optic modulator 3 for intensity modulation. The modulation signal originates from the microwave source 9. The modulated light then enters the optical coupler 4 and is split into signal light and reference light. In this embodiment, laser 1 uses a narrow-linewidth laser, emitting a single-frequency optical signal at frequency f. Narrow-linewidth lasers have a longer coherence distance, allowing for measurement at greater distances. The modulation signal output by the microwave source 9 is a linearly chirped signal, offering advantages such as high resolution, strong anti-interference capability, and high precision. In this embodiment, the electro-optic modulator 3 uses a Mach-Zehnder intensity modulator, which, compared to traditional FMCW lidar, is unaffected by the Doppler effect caused by atmospheric disturbances in its ranging error. The voltage of the microwave source 9 is much lower than the half-wave voltage of the electro-optic modulator 3, negligible influencing higher-order sidebands. In this embodiment, the optical coupler 4 splits the input light into signal light and reference light in a 90:10 ratio.

[0033] The signal light, after its polarization direction is adjusted by the second polarization controller 5, is sent to the first collimator 6 and coupled into space via optical fiber. The signal light coupled into space is expanded by the first convex lens 7, covering the object under test 25. The light reflected from the object under test is converged by the third convex lens 16 and sent to the second polarizer 17 in the spatial optical bridge. Simultaneously, the reference light, after its polarization direction is adjusted by the third polarization controller 10, is sent to the second collimator 11 and coupled into the spatial light. The reference light is then focused by the second convex lens 12 at the center of the pinhole in the pinhole filter 13 to achieve spatial filtering. The pinhole filter 13 filters out high-frequency components of the light, eliminates interference fringes, and improves beam quality. The filtered reference light is divergent; after its polarization direction is adjusted by the first polarizer 14, it is sent to a quarter-wave plate and then enters the spatial optical bridge. After passing through the spatial optical bridge, the signal light and reference light are each split into four beams and output from four outlets to the first CCD 21, the second CCD 22, the third CCD 23, and the fourth CCD 24. In this system, the signal light is focused onto the CCD, while the reference light is uniformly distributed across the four CCDs without being focused. A quarter-wave plate is used to convert linearly polarized light into circularly polarized light, creating a 90° phase difference between the horizontal and vertical light, which is used to generate a 90° phase difference on each of the four CCDs.

[0034] Assume the output frequency of microwave source 9 is f m The single-frequency sinusoidal signal, the modulation index of electro-optic modulator 3 is M, and the CCD responsivity is The signal light arriving at the CCD has the same intensity as the reference light, with a total intensity of I0. The optical path difference is ΔL, the speed of light is c, and J1(·) is a first-order Bessel function. When the modulation index is small, the output currents of the first, second, third, and fourth CCDs are respectively:

[0035]

[0036]

[0037]

[0038]

[0039] Through calculation, we can obtain the signal:

[0040]

[0041] Equation (5) is the signal obtained by combining the outputs of 4 CCDs. Compared with equations (1)-(4), this equation does not contain DC or AC terms, and can eliminate the influence of changes in optical path difference caused by changes in air refractive index or system vibration on the system.

[0042] In practical applications, the microwave source 9 outputs a linear frequency modulated signal, i.e., f in equation (5). m It can be represented as:

[0043]

[0044] Wherein, the starting frequency is f0, the sweep bandwidth is B, the time is t, and the sweep time is T.

[0045] Substituting equation (6) into equation (5), we get:

[0046]

[0047] Where, τ C τ is a constant C =f0T / B.

[0048] It can also be expressed as:

[0049]

[0050] Performing a Fast Fourier Transform (FFT) on equation (8) yields:

[0051]

[0052] From the properties of the sinc function, the optical path difference can be derived from the spectral peak value f. peak The calculation shows that at this time:

[0053]

[0054] When the optical path difference is twice the distance d of the object being measured:

[0055]

[0056] In practice, after the four CCDs acquire images, all the images need to be calibrated first, and then equation (5) is obtained through simple calculation. The two-dimensional matrix is ​​obtained by calculation, and the three-dimensional matrix can be obtained by the change of the signal over time. The fast Fourier transform of this three-dimensional matrix can be used to obtain the spectral peak position of each pixel, as shown in equation (9). Then, the distance of each pixel to the object under test is calculated using equation (11).

[0057] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A fully solid-state high-resolution optical three-dimensional imaging system based on coherent detection, characterized in that, include: Laser (1), microwave source (9), first polarization controller (2), electro-optic modulator (3), optical coupler (4), second polarization controller (5), first collimator (6), first convex lens (7), third polarization controller (10), second collimator (11), second convex lens (12), first polarizer (14), quarter wave plate (15), unpolarized beam splitter cube (19), first polarized beam splitter cube (18), second polarized beam splitter cube (20), third convex lens (16), second polarizer (17), first CCD (21), second CCD (22), third CCD (23) and fourth CCD (24); In this system, a laser (1) emits a laser beam; a microwave source (9) generates a modulation signal; the laser beam enters an electro-optic modulator (3) via a first polarization controller (2), and after being modulated by the electro-optic modulator (3) based on the modulation signal, it is split into two beams by an optical coupler (4). One beam is a signal beam, which passes through a second polarization controller (5), a first collimating lens (6), and a first convex lens (7) in sequence, and illuminates the object under test (25); the other beam is a reference beam, which passes through a third polarization controller (10), a second collimating lens (11), a second convex lens (12), a first polarizer (14), and a quarter-wave plate (15) in sequence, and illuminates a non-polarized beam splitter cube (19). The signal light is split into two paths. One path is split by the second polarization beam splitter (20) and then imaged on the first CCD (21) and the second CCD (22), respectively. The other path is split by the first polarization beam splitter (18) and then imaged on the third CCD (23) and the fourth CCD (24), respectively. The signal light is reflected by the object under test (25) and then irradiated onto the non-polarization beam splitter (19) by the third convex lens (16) and the second polarizer (17). After being split by the non-polarization beam splitter (19), the second polarization beam splitter (20), and the first polarization beam splitter (18), the signal light is imaged on the first CCD (21), the second CCD (22), the third CCD (23), and the fourth CCD (24), respectively. The four CCDs have a 90° phase difference, the signal light and the reference light have equal intensity on the four CCDs, and the optical path difference between the signal light and the reference light is twice the distance to the object under test.

2. The system as described in claim 1, characterized in that, The laser (1) is a narrow linewidth laser.

3. The system as described in claim 1, characterized in that, A pinhole filter (13) is provided between the second convex lens (12) and the quarter-wave plate (15).

4. The system as described in claim 1, characterized in that, The modulation signal is a linear chirped signal.

5. The system as described in claim 1, characterized in that, The electro-optic modulator is a Mach-Zehnder intensity modulator.

6. The system as described in claim 1, 4, or 5, characterized in that, The voltage of the microwave source (9) is much smaller than the half-wave voltage of the electro-optic modulator (3).

7. A method for all-solid-state high-resolution optical three-dimensional imaging based on coherent detection, characterized in that, Imaging is performed using the system described in any one of claims 1 to 6 to obtain the output currents of the four CCDs; after combining the output currents of the four CCDs and eliminating the DC and phase terms, a signal is obtained. ;right Perform a Fast Fourier Transform to obtain the spectral peaks. Then the distance d between the object to be tested (25) and the laser (1) is: Where c is the speed of light, T is the sweep time of the modulation signal, and B is the sweep bandwidth of the modulation signal.

8. The method as described in claim 7, characterized in that, After imaging using the system, the images from the four CCDs are first calibrated to eliminate translation and rotation before being merged.

Images