Optical imaging system and method of resonant cavity

Abstract

The present application relates to a resonant cavity optical imaging system and method. The system includes: a light source; an emission modulator connected to the light source signal; the emission modulator includes an entrance pupil lens, an exit pupil lens, an electro-optical crystal, and a resonant cavity formed between the entrance and exit pupil lenses, with the electro-optical crystal disposed within the resonant cavity; an imaging module and a receiving demodulator connected to the front end of the imaging module; the receiving demodulator has the same configuration as the emission modulator; a microwave generator connected to the emission modulator and receiving demodulator signals, respectively; a processing module connected to the microwave generator signal; the processing module is also connected to the imaging module signal and is used to control the imaging module's frame synchronization signal so that the imaging module receives the demodulated signal within the effective time window of microwave optical modulation. The method is applied to the above-mentioned system. The present application can modulate the frequency of the emitted light to the GHz level, thereby reducing the ToF ranging accuracy to the millimeter level, thereby improving the ToF ranging accuracy.

Description

Technical Field

[0001] The present application relates to the technical field of optical imaging, and in particular to a resonant cavity optical imaging system and method. Background Art

[0002] Compared to traditional two-dimensional imaging, which requires obtaining image depth information, Time of Flight (ToF) 3D imaging technology calculates the distance to an object by measuring the time difference between the emission and reflection of a light pulse. It uses a laser or LED as a light source to emit light pulses of a specific wavelength toward the imaging target. The imaging module receives the reflected light pulses and converts them into electrical signals. The signal processing unit processes the signals received by the sensor, calculates the flight time of the light pulses, and converts them into depth information.

[0003] ToF technology enables long-distance measurement: ToF technology can measure objects at long distances, with a detection range ranging from a few centimeters to hundreds of meters. Because it uses global shutter imaging, ToF technology can quickly acquire three-dimensional information about a scene, making it suitable for dynamic scenes. It is highly adaptable to ambient light: ToF technology operates effectively in both low-light and bright-light environments, and is not susceptible to interference from ambient light.

[0004] Existing ToF technology can achieve medium-precision 3D imaging at long distances, but the accuracy is limited by the modulation frequency of structured light emission, which is usually several hundred megahertz, resulting in measurement accuracy only at the centimeter level, which needs to be improved. Summary of the Invention

[0005] In order to solve the problems existing in the above-mentioned prior art, the purpose of this application is to provide an optical imaging system and method for a resonant cavity. This application arranges an electro-optical crystal in the resonant cavity and makes the microwave signal frequency of the microwave generator be located at the fixed resonant frequency point of modulation and demodulation. Under the action of the microwave hotspot field intensity, the electro-optical crystal causes the refractive index of light to change periodically, playing the role of debugging the emitted light in the optical emission circuit. The frequency of the emitted light can be modulated to the GHz level, thereby improving the ToF ranging accuracy to the millimeter level, thereby improving the ToF ranging accuracy.

[0006] The optical imaging system of a resonant cavity described in the present application comprises:

[0007] light source;

[0008] an emission modulator, connected to the light source signal and configured to modulate the emission of the light source; the emission modulator comprises an entrance pupil lens, an exit pupil lens, an electro-optical crystal, and a resonant cavity formed between the entrance pupil lens and the exit pupil lens, wherein the electro-optical crystal is disposed within the resonant cavity;

[0009] An imaging module, and a receiving demodulator connected to the front end of the imaging module, the receiving demodulator is used to demodulate the received light to generate a demodulation signal, and the imaging module is used to form an image based on the demodulation signal; the receiving demodulator has the same configuration as the transmitting modulator;

[0010] A microwave generator, which is respectively connected to the transmitting modulator and the receiving demodulator signals;

[0011] a processing module, signal-connected to the microwave generator, for controlling the phase and amplitude of the microwave signal emitted by the microwave generator, and ensuring that the frequency of the microwave signal is located at a fixed resonant frequency of the transmitting modulator or the receiving demodulator; and further signal-connected to the imaging module, for controlling the frame synchronization signal of the imaging module, ensuring that the imaging module receives the demodulated signal within an effective time window of microwave optical modulation.

[0012] Preferably, the light source is a visible light source or an infrared light source.

[0013] Preferably, the electro-optical crystal is made of lithium niobate, lithium tantalate, or an electro-optical crystal with an electro-optic coefficient greater than 30 pm / V.

[0014] Preferably, the optical path length of the electro-optical crystal is greater than 2 mm.

[0015] Preferably, the imaging module includes a lens unit connected to the receiving demodulator signal, and an image sensor connected to the lens unit signal, the image sensor is connected to the processing module signal, the lens unit adopts a rod mirror imaging system, the rod mirror imaging system includes an electro-optical crystal component and a lens, the optical path length of the electro-optical crystal component is greater than 2 mm, and the lens is a glass lens or a plastic lens.

[0016] An optical imaging method of the present application, applied to the optical imaging system described above, comprises the following steps:

[0017] S01. At the time of the first frame image, set the phase difference between the resonant microwave phase of the light source and the resonant microwave phase of the imaging module to θ1, and let the light source emit light toward the imaging target. The imaging module receives the reflected light formed on the surface of the imaging target by the emitted light and forms an image, which is recorded as the first frame image.

[0018] S02. At the time of the second frame image, the phase difference between the resonant microwave phase of the light source and the resonant microwave phase of the imaging module is set to θ2, and the light source is caused to emit light toward the imaging target. The imaging module receives the reflected light formed on the surface of the imaging target by the emitted light and forms an image, which is recorded as the second frame image.

[0019] S03, repeating steps S01 and S02 to receive N consecutive frames of images with alternating phase differences;

[0020] S04. Define the transmitted light signal as:

[0021] I ′ =I0+I1cos(ωt),

[0022] The received optical signal is:

[0023]

[0024] Where I0 represents the intensity of the DC optical signal in the transmitted light, I1 represents the intensity of the modulated component in the transmitted light, and ωt represents the phase angle of the transmitted optical signal that changes with time. It represents the phase angle of the received light related to the depth of the imaging target, which is recorded as the depth phase information;

[0025] Use the demodulation formula K=K0+K1cos(ωt+θ ′ ) Demodulate the received light signal to obtain the depth phase information of the received light

[0026]

[0027] Wherein, K0 represents the DC gain part of the transmit modulator, K1 represents the AC gain part of the transmit modulator, θ ′ represents the initial phase of the demodulation voltage of the receiving demodulator, i represents the i-th frame image, i∈[1,N];

[0028] Based on the obtained depth phase information, the depth information of the image is calculated, and an imaging image with depth information is generated.

[0029] Preferably, in step S01 and step S02, when receiving the reflected light, the imaging module filters high-frequency signals in the reflected light and retains low-frequency signals related to the phase difference.

[0030] Preferably, when the depth is an integer multiple of the wavelength, an image algorithm is used to eliminate depth aliasing.

[0031] Preferably, the image algorithm is a phase unwrapping algorithm, a multiple differential method or a spatial constraint method.

[0032] Preferably, in step S01 , the phase difference between the resonant microwave phase of the light source and the resonant microwave phase of the imaging module is set to θ1=0.

[0033] The optical imaging system and method of a resonant cavity described in the present application have the advantage that, by placing an electro-optical crystal with a high electro-optical coefficient at the hotspot of the microwave field of the resonant cavity of the transmitting modulator and the receiving demodulator, and controlling the phase and amplitude of the microwave signal through a microwave generator to achieve the required modulation phase, and controlling the microwave generator to generate a microwave signal of a fixed frequency, the frequency of the microwave signal is locked to the fixed resonant frequency point of the transmitting modulator or the receiving demodulator, so that the signal input to the resonant cavity can form a resonant hotspot inside the resonant cavity, greatly improving the microwave electric field intensity in the hotspot area, forming sufficient field intensity to excite the electro-optical effect of the electro-optical crystal, and generating sufficient modulation to modulate the frequency of the emitted light to the GHz level, thereby accurately reducing the ToF ranging accuracy to the millimeter level, thereby improving the ToF ranging accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a structural block diagram of an optical imaging system of a resonant cavity described in this application;

[0035] Figure 2 is a schematic structural diagram of the transmit modulator described in this application;

[0036] Figure 3 This is a flowchart of the steps of an optical imaging method described in this application.

[0037] Explanation of the reference numerals: 1-light source, 2-transmitting modulator, 21-entrance pupil lens, 22-exit pupil lens, 23-electro-optic crystal, 24-resonant cavity, 25-microwave control circuit, 3-imaging module, 31-lens unit, 32-image sensor, 4-receiving demodulator, 5-microwave generator, 6-processing module. DETAILED DESCRIPTION

[0038] like Figure 1 As shown, the optical imaging system of a resonant cavity described in the present application includes:

[0039] Light source 1 is used to emit outgoing light, and generally adopts visible light source or infrared light source. When infrared light source is used, depth imaging of the imaging target can be completed. When visible light source is used, depth imaging and planar color imaging can be completed at the same time, and the point cloud and color fusion can be realized to facilitate the subsequent image processing.

[0040] The emission modulator 2 is connected to the light source 1 signal. Specifically, it is connected to the emission end of the light source 1 and is used to modulate the emission light of the light source 1. Figure 2As shown, the emission modulator 2 includes an entrance pupil lens 21, an exit pupil lens 22, an electro-optical crystal 23, and a resonant cavity 24 formed between the entrance pupil lens 21 and the exit pupil lens 22. The electro-optical crystal 23 is arranged in the resonant cavity 24, and the optical path direction is from the entrance pupil lens 21 to the exit pupil lens 22. The emission modulator 2 also includes a microwave control circuit 25, which is embedded in the middle of the emission modulator 2. When the microwave generator 5 generates a microwave signal of a fixed frequency and the frequency of the microwave signal is locked at the fixed resonant frequency point of the resonant cavity 24 of the emission modulator 2, the input resonant The signal of cavity 24 forms a resonant hotspot inside the resonant cavity 24, greatly increasing the microwave electric field intensity in the hotspot area. Under the action of the microwave hotspot field intensity, the electro-optical effect of the electro-optical crystal 23 is stimulated, causing the refractive index of the light to change periodically, playing the role of debugging the emitted light in the light emission circuit, and generating sufficient modulation, which can modulate the frequency of the emitted light to GHz. The modulation of GHz corresponds to a ToF ranging accuracy of up to millimeter level, so that the modulation frequency of this embodiment can be from 200MHz to 70GHz, which can be designed according to different application scenarios, and thus the depth measurement accuracy of the ToF system can be adjusted.

[0041] In a feasible embodiment, the electro-optical crystal 23 is made of lithium niobate material, lithium tantalate material, or an electro-optical crystal material with an electro-optic coefficient greater than 30 pm / V, such as polyvinyl carbazole (PVK), polymethyl acrylate (PMMA) or polyimide (PI), but this embodiment is not limited to this.

[0042] The optical imaging system also includes an imaging module 3 and a receiver-demodulator 4 connected to the front end of the imaging module 3. Receiver-demodulator 4 is used to demodulate received light and generate a demodulated signal. Specifically, receiver-demodulator 4 has the same configuration as the aforementioned transmit modulator 2. It includes an entrance lens, an exit lens, an electro-optical crystal, a resonant cavity formed between the entrance and exit lens elements, and a microwave control circuit embedded in the center of receiver-demodulator 4. The electro-optical crystal is located within the resonant cavity. The specific structure of receiver-demodulator 4 can be understood by referring to the structure of transmit modulator 2, and will not be further described here.

[0043] Imaging module 3 includes a lens unit 31 and an image sensor 32. Lens unit 31 receives the demodulated signal. To achieve sufficient demodulation depth, lens unit 31 requires a long optical path through the microwave resonant hotspot. To achieve both demodulation and imaging, a rod-mirror imaging system with a long optical path is employed. This system includes an electro-optical crystal component and a lens. The optical path length of the electro-optical crystal component is generally greater than 2 mm to ensure both modulation depth and imaging clarity for lens unit 31. The lens can be made of glass or plastic. Image sensor 32 forms an image based on the signal transmitted by lens unit 31.

[0044] The microwave generator 5 has its output connected to the microwave control circuits of the transmit modulator 2 and the receive demodulator 4, respectively, for sending microwave signals to the transmit modulator 2 and the receive demodulator 4, thereby controlling the resonant phase angles of the transmit modulator 2 and the receive demodulator 4.

[0045] The processing module 6 uses a processor with computing functions, such as an MCU. The processing module 6 is connected to the microwave generator 5 signal and is used to control the phase and amplitude of the microwave signal emitted by the microwave generator 5, so as to control the transmitting modulator 2 and the receiving demodulator 4 to reach the required resonant phase angle. The processing module 6 is also used to make the frequency of the microwave signal be located at the fixed resonant frequency point of the transmitting modulator 2 or the receiving demodulator 4, so that the signal input to the resonant cavity can form a resonant hotspot inside the resonant cavity, greatly improving the microwave electric field strength in the hotspot area, thereby forming sufficient field strength to excite the electro-optical effect of the electro-optical crystal and produce sufficient modulation.

[0046] In addition, the processing module 6 is also signal-connected to the imaging module 3, specifically to the image sensor 32 of the imaging module 3, for controlling the image sensor 32 to receive the signal of the lens unit 31 within the effective time window of microwave optical modulation to ensure effective imaging.

[0047] like Figure 3 As shown, this embodiment further provides an optical imaging method, which is applied to the above-mentioned optical imaging system and includes the following steps:

[0048] S01. At the time of the first frame image, the phase difference between the resonant microwave phase of light source 1 and the resonant microwave phase of imaging module 3 is set to θ1. Light source 1 is instructed to emit light toward an imaging target. Imaging module 3 receives reflected light formed on the surface of the imaging target by the emitted light and forms an image. This is recorded as the first frame image.

[0049] S02. At the time of the second frame image, the phase difference between the resonant microwave phase of light source 1 and the resonant microwave phase of imaging module 3 is set to θ2, and light source 1 is caused to emit light toward the imaging target. Imaging module 3 receives the reflected light formed on the surface of the imaging target by the emitted light and forms an image, which is recorded as the second frame image.

[0050] S03, repeating steps S01 and S02 to receive N consecutive frames of images with alternating phase differences;

[0051] S04. Define the transmitted light signal as:

[0052] I ′ =I0+I1cos(ωt),

[0053] The received optical signal is:

[0054]

[0055] Where I0 represents the intensity of the DC optical signal in the transmitted light, I1 represents the intensity of the modulated component in the transmitted light, and ωt represents the phase angle of the transmitted optical signal that changes with time. It represents the phase angle of the received light related to the depth of the imaging target, which is recorded as the depth phase information;

[0056] Use the demodulation formula K=K0+K1cos(ωt+θ ′ ) Demodulate the received light signal to obtain the depth phase information of the received light

[0057]

[0058] Wherein, K0 represents the DC gain part of the transmit modulator, K1 represents the AC gain part of the transmit modulator, θ ′ represents the initial phase of the demodulation voltage of the receiving demodulator, i represents the i-th frame image, i∈[1,N];

[0059] Based on the obtained depth phase information, the depth information of the image is calculated, and an imaging image with depth information is generated.

[0060] Exemplarily, the depth phase information is converted to depth information using the following formula:

[0061]

[0062] Where d represents the depth value of the image, represents the change in depth phase information, and λ represents the wavelength of the emitted light.

[0063] Specifically, in step S01 and step S02, when receiving the reflected light, the imaging module 3 filters the high-frequency signal in the reflected light and retains the low-frequency signal related to the phase difference. In a feasible embodiment, an analog low-pass filter, a digital low-pass filter, or a band-pass filter can be used to filter the high-frequency signal.

[0064] When the depth is an integer multiple of the wavelength, an image algorithm is used to eliminate depth aliasing. Specifically, the depth of an integer multiple of the wavelength refers to the depth position corresponding to when the distance the light wave propagates in the medium is an integer multiple of the wavelength of the light wave. For example, assuming that visible light is used (assuming the modulation wavelength is 500 microns), then at a depth of 1000 microns, the phase of the modulated light wave will repeat once. If in a 3D imaging system, the depths of two objects are 1000 microns and 2000 microns respectively, then their phase difference will be an integer multiple of 2π, resulting in phase aliasing, and the system may not be able to distinguish the actual depths of the two objects. In order to solve the aliasing problem caused by the depth of an integer multiple of the wavelength, image algorithms such as phase unwrapping algorithm, multiple differential method or spatial constraint method are used to deal with the aliasing problem.

[0065] In a specific embodiment, in step S01 , the phase difference between the resonant microwave phase of the light source 1 and the resonant microwave phase of the imaging module 3 is set to θ1=0 to facilitate subsequent calculations.

[0066] In the description of this application, it should be understood that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description. Unless otherwise specified, these directional words do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of this application.

[0067] Those skilled in the art can make various other corresponding changes and deformations based on the technical solutions and concepts described above, and all of these changes and deformations should fall within the scope of protection of the claims of this application.

Claims

1. An optical imaging system of a resonant cavity, characterized in that: include: light source; An emission modulator, connected to the light source signal and used to modulate the emission of the light source; the emission modulator includes an entrance pupil lens, an exit pupil lens, an electro-optical crystal, and a resonant cavity formed between the entrance pupil lens and the exit pupil lens, the electro-optical crystal is disposed in the resonant cavity, and the optical path direction is from the entrance pupil lens to the exit pupil lens; An imaging module, and a receiving demodulator connected to the front end of the imaging module, the receiving demodulator is used to demodulate the received light to generate a demodulation signal, and the imaging module is used to form an image based on the demodulation signal; the receiving demodulator has the same configuration as the transmitting modulator; A microwave generator, which is respectively connected to the transmitting modulator and the receiving demodulator signals; a processing module, signal-connected to the microwave generator, for controlling the phase and amplitude of the microwave signal emitted by the microwave generator, and ensuring that the frequency of the microwave signal is located at a fixed resonant frequency of the transmitting modulator or the receiving demodulator; and further signal-connected to the imaging module, for controlling the frame synchronization signal of the imaging module, ensuring that the imaging module receives the demodulated signal within an effective time window of microwave optical modulation.

2. The optical imaging system of the resonant cavity according to claim 1, characterized in that: The light source is a visible light source or an infrared light source.

3. The optical imaging system of the resonant cavity according to claim 1, characterized in that: The electro-optical crystal is made of lithium niobate, lithium tantalate, or an electro-optical crystal material with an electro-optic coefficient greater than 30 pm / V.

4. The optical imaging system of the resonant cavity according to claim 3, characterized in that: The optical path length of the electro-optical crystal is greater than 2 mm.

5. The optical imaging system of the resonant cavity according to claim 3 or 4, characterized in that: The imaging module includes a lens unit connected to the receiving demodulator signal, and an image sensor connected to the lens unit signal. The image sensor is connected to the processing module signal. The lens unit adopts a rod mirror imaging system. The rod mirror imaging system includes an electro-optical crystal component and a lens. The optical path length of the electro-optical crystal component is greater than 2 mm, and the lens is a glass lens or a plastic lens.

6. An optical imaging method, applied to the optical imaging system according to any one of claims 1 to 5, characterized in that: The following steps are involved: S01. At the time of the first frame image, set the phase difference between the resonant microwave phase of the light source and the resonant microwave phase of the imaging module to θ1, and let the light source emit light toward the imaging target. The imaging module receives the reflected light formed on the surface of the imaging target by the emitted light and forms an image, which is recorded as the first frame image. S02. At the time of the second frame image, the phase difference between the resonant microwave phase of the light source and the resonant microwave phase of the imaging module is set to θ2, and the light source is caused to emit light toward the imaging target. The imaging module receives the reflected light formed on the surface of the imaging target by the emitted light and forms an image, which is recorded as the second frame image. S03, repeating steps S01 and S02 to receive N consecutive frames of images with alternating phase differences; S04. Define the transmitted light signal as: I'=I0+I1cos(ωt), The received optical signal is: Where I0 represents the intensity of the DC optical signal in the transmitted light, I1 represents the intensity of the modulated component in the transmitted light, and ωt represents the phase angle of the transmitted optical signal that changes with time. It represents the phase angle of the received light related to the depth of the imaging target, which is recorded as the depth phase information; The received light signal is demodulated using the demodulation formula K=K0+K1cos(ωt+θ') to obtain the depth phase information of the received light. Wherein, K0 represents the DC gain part of the transmit modulator, K1 represents the AC gain part of the transmit modulator, θ' represents the initial phase of the demodulation voltage of the receive demodulator, i represents the i-th frame image, i∈[1,N]; Based on the obtained depth phase information, the depth information of the image is calculated, and an imaging image with depth information is generated.

7. The optical imaging method according to claim 6, characterized in that: In step S01 and step S02 , when receiving the reflected light, the imaging module filters the high-frequency signal in the reflected light and retains the low-frequency signal related to the phase difference.

8. The optical imaging method according to claim 6 or 7, characterized in that: When the depth is an integer multiple of the wavelength, an image algorithm is used to eliminate depth aliasing.

9. The optical imaging method according to claim 8, characterized in that: The image algorithm is a phase unwrapping algorithm, a multiple differential method or a spatial constraint method.

10. The optical imaging method according to claim 6, characterized in that: In step S01 , the phase difference between the resonant microwave phase of the light source and the resonant microwave phase of the imaging module is set to θ1=0.

Images