Optical imaging system and method of resonant cavity

Abstract

The invention relates to an optical imaging system and method of a resonant cavity. The system comprises a light source; the emission modulator is in signal connection with 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, and the electro-optical crystal is arranged in the resonant cavity; the receiving demodulator is connected to the front end of the imaging module; the receiving demodulator and the transmitting modulator have the same configuration; the microwave generator is respectively in signal connection with the transmitting modulator and the receiving demodulator; the processing module is in signal connection with the microwave generator; and the processing module is also in signal connection with the imaging module and is used for controlling a frame synchronization signal of the imaging module and enabling the imaging module to receive a demodulation signal in an effective time window of microwave optical modulation. The method is applied to the system. According to the invention, the frequency of the emitted light can be modulated to the GHz level, so that the ToF ranging precision can be accurate to the millimeter level, and the ToF ranging precision can be improved.

Description

Technical Field

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

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

[0003] ToF technology can measure at long distances: ToF technology can measure objects at long distances, with a detection range ranging from a few centimeters to hundreds of meters. Due to global shutter imaging, ToF technology can quickly obtain three-dimensional information of the scene and is suitable for dynamic scenes. Strong adaptability to ambient light: ToF technology can work normally in both dark and bright light environments and is not easily disturbed by 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, so that the measurement accuracy can only reach 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 of a resonant cavity. This application sets an electro-optical crystal in the resonant cavity, and makes the microwave signal frequency of the microwave generator located at the fixed resonant frequency point of modulation and demodulation, so that the electro-optical crystal makes the refractive index of light change periodically under the action of the microwave hot spot field intensity, plays the role of debugging the emitted light in the optical emission circuit, and can modulate the frequency of the emitted light to the GHz level, thereby accurately increasing the ToF ranging accuracy to the millimeter level, which can improve 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, which is connected to the light source signal and is 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, and the electro-optical crystal is arranged in 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 according to 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 is connected to the microwave generator signal and is used to control the phase and amplitude of the microwave signal emitted by the microwave generator and make the frequency of the microwave signal be located at the fixed resonant frequency of the transmitting modulator or the receiving demodulator; the processing module is also connected to the imaging module signal and is used to control the frame synchronization signal of the imaging module and make the imaging module receive the demodulated signal in the 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-optic crystal is made of lithium niobate, lithium tantalate, or an electro-optic 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 as described above, comprises the following steps:

[0017] S01. At the time of the first 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 θ1, and the light source is allowed to emit light to the imaging target, and the imaging module receives the reflected light formed by the emitted light on the surface of the imaging target 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, the light source is allowed to emit light to the imaging target, and the imaging module receives the reflected light formed by the emitted light on the surface of the imaging target 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 emitted light, I1 represents the intensity of the modulated component in the emitted light, and ωt represents the phase angle of the emitted optical signal changing with time. It represents the phase angle in 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 ′ ) demodulates 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 difference 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 advantages that, by setting an electro-optical crystal with a high electro-optical coefficient at the hot spot 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, so that the frequency of the microwave signal is locked at the fixed resonant frequency point of the transmitting modulator or the receiving demodulator, the signal input into the resonant cavity can form a resonant hot spot inside the resonant cavity, greatly improving the field strength of the microwave electric field in the hot spot area, forming a sufficient field strength to excite the electro-optical effect of the electro-optical crystal, and generating a sufficient modulation degree to modulate the frequency of the emitted light to the GHz level, thereby accurately improving the ToF ranging accuracy to the millimeter level, and improving the ToF ranging accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0036] Figure 3 It is a flow chart 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 a visible light source or an infrared light source is used. When an infrared light source is used, depth imaging of the imaging target can be completed, and when a visible light source is used, depth imaging and planar color imaging can be completed simultaneously, and the point cloud and color fusion can be realized to facilitate 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 the direction 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, which greatly increases 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 degree, and the frequency of the emitted light can be modulated 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 receiving demodulator 4 connected to the front end of the imaging module 3. The receiving demodulator 4 is used to demodulate the received light to generate a demodulation signal. Specifically, the receiving demodulator 4 has the same configuration as the aforementioned transmitting modulator 2. It includes an entrance pupil lens, an exit pupil lens, an electro-optical crystal, a resonant cavity formed between the entrance pupil lens and the exit pupil lens, and a microwave control circuit embedded in the middle of the receiving demodulator 4. The electro-optical crystal is arranged in the resonant cavity. The specific structure of the receiving demodulator 4 can be understood by referring to the structure of the aforementioned transmitting modulator 2, and will not be repeated here.

[0043] The imaging module 3 includes a lens unit 31 and an image sensor 32. The lens unit 31 is used to receive demodulation signals. In order to form a sufficient demodulation signal, the lens unit 31 needs to have a long optical path through the microwave resonance hotspot area. In order to simultaneously realize the demodulation and imaging functions, a rod mirror imaging system with a long optical path is used. 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 generally above 2 mm to ensure that the lens unit 31 has a modulation depth and imaging clarity at the same time. The lens can be a glass lens or a plastic lens. The image sensor 32 forms an image based on the signal transmitted by the lens unit 31.

[0044] The microwave generator 5 has its output ends connected to the microwave control circuits of the transmitting modulator 2 and the receiving demodulator 4, respectively, for sending microwave signals to the transmitting modulator 2 and the receiving demodulator 4, thereby controlling the resonant phase angles of the transmitting modulator 2 and the receiving 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 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, so as to form a sufficient field strength to excite the electro-optical effect of the electro-optical crystal and produce a sufficient modulation degree.

[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 also provides an optical imaging method, which is applied to the above optical imaging system and includes the following steps:

[0048] S01, at the moment of the first frame image, 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, the light source 1 emits the emission light to the imaging target, and the imaging module 3 receives the reflected light formed by the emission light on the surface of the imaging target and forms an image, which is recorded as the first frame image;

[0049] S02, at the moment of the second frame image, 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 θ2, the light source 1 is allowed to emit light to the imaging target, and the imaging module 3 receives the reflected light formed by the emitted light on the surface of the imaging target 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 emitted light, I1 represents the intensity of the modulated component in the emitted light, and ωt represents the phase angle of the emitted optical signal changing with time. It represents the phase angle in 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 ′ ) demodulates 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 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 be repeated 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 integer multiples of the wavelength, image algorithms such as phase unwrapping algorithms, multiple extra difference methods, or spatial constraint methods 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 the present application, it should be understood that the directions or positional relationships indicated by directional words such as "front, back, up, down, left, right", "lateral, vertical, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the drawings, and are only for the convenience of describing the present 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, and therefore cannot be understood as limiting the scope of protection of the present application.

[0067] For those skilled in the art, various other corresponding changes and deformations can be made according to the technical solutions and concepts described above, and all of these changes and deformations should fall within the protection scope 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, which is connected to the light source signal and is 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, and the electro-optical crystal is arranged in the resonant cavity; 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 according to 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 is connected to the microwave generator signal and is used to control the phase and amplitude of the microwave signal emitted by the microwave generator and make the frequency of the microwave signal be located at the fixed resonant frequency of the transmitting modulator or the receiving demodulator; the processing module is also connected to the imaging module signal and is used to control the frame synchronization signal of the imaging module and make the imaging module receive the demodulated signal in the 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-optic crystal is made of lithium niobate, lithium tantalate, or an electro-optic 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 2mm, 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, 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, and the light source is allowed to emit light to the imaging target, and the imaging module receives the reflected light formed by the emitted light on the surface of the imaging target 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, the light source is allowed to emit light to the imaging target, and the imaging module receives the reflected light formed by the emitted light on the surface of the imaging target 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 emitted light, I1 represents the intensity of the modulated component in the emitted light, and ωt represents the phase angle of the emitted optical signal changing with time. It represents the phase angle in the received light related to the depth of the imaging target, which is recorded as the depth phase information; Use the demodulation formula K = K0 + K1cos(ωt + θ ′ ) demodulates the received light signal 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 receiving 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 difference method or a space 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.

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