An imaging method and an imaging device

Abstract

The present invention discloses an imaging method and an imaging device, relating to the field of imaging. The imaging method includes: irradiating a pulsed laser onto a target to be measured that rotates according to a time sequence, and obtaining the reflected laser of the target to be measured at multiple rotation angles; sequentially performing single-photon detection on the reflected laser to determine the flight time of target photons in the reflected laser; using the distribution results of the flight times obtained at multiple rotation angles as observation data; iteratively performing the following operations until the value of the optimization objective function meets a preset condition, to obtain a three-dimensional image of the target to be measured: determining the probability of measuring the observation data when the three-dimensional image of the target to be measured is the predicted three-dimensional image according to the rotation matrix of the target to be measured, the predicted three-dimensional image of the target to be measured, and the observation data; determining the value of the optimization objective function according to the above probability and the predicted three-dimensional image; and changing the predicted three-dimensional image according to the value of the optimization objective function.

Description

Technical Field

[0001] The present invention relates to the field of imaging, and particularly to an imaging method and an imaging device. Background Art

[0002] Long-distance imaging technology has important application values in the fields of target recognition, high-precision imaging, etc. However, it faces serious challenges in ultra-long-distance imaging (such as interstellar imaging or high-precision satellite-to-ground imaging). This is because traditional long-distance imaging technology is limited by the diffraction limit, and its lateral resolution decreases significantly with the increase of the imaging distance, resulting in the inability to achieve high-resolution imaging in ultra-long-distance scenarios.

[0003] Laser reflection tomography technology is a new method that breaks through the limitations of traditional long-distance imaging technology. It reconstructs the three-dimensional information of the detected target through one-dimensional distance detection of multi-angle pulsed lasers, getting rid of the dependence of the lateral resolution on the system aperture. This method only depends on the longitudinal resolution of the system, that is, the time resolution. However, current laser reflection tomography technology usually uses traditional photodetectors, and the time resolution of such detectors is usually on the order of nanoseconds, which is difficult to meet the requirements of high-resolution imaging at ultra-long distances. In addition, with the increase of the imaging distance, the optical signal carrying the information of the detected target will be significantly attenuated, further limiting the imaging accuracy of the system. Summary of the Invention

[0004] In view of this, the present invention provides an imaging method and an imaging device.

[0005] As the first aspect of the present invention, the imaging method includes:

[0006] Making a pulsed laser irradiate a to-be-detected target that rotates according to a time sequence, and obtaining the reflected laser of the to-be-detected target at multiple rotation angles respectively;

[0007] Performing single-photon detection on the reflected laser to determine the distribution result of the flight time of target photons in the reflected laser;

[0008] Statistically analyzing the distribution results of the flight time obtained at multiple rotation angles into a photon counting time histogram; the photon counting time histogram constitutes the observation data;

[0009] Iteratively performing the following operations until the value of the optimization objective function meets a preset condition, and determining the predicted three-dimensional image of the to-be-detected target when the value of the optimization objective function meets the preset condition as the three-dimensional image of the to-be-detected target:

[0010] Based on the Poisson likelihood model, determining the probability of measuring the observation data when the three-dimensional image of the to-be-detected target is the predicted three-dimensional image according to the rotation matrix of the to-be-detected target, the predicted three-dimensional image of the to-be-detected target, and the observation data;

[0011] Determine the value of the optimization objective function according to the above probabilities and the predicted three-dimensional image;

[0012] Change the predicted three-dimensional image according to the value of the optimization objective function.

[0013] According to an embodiment of the present invention, determining the value of the optimization objective function according to the above probabilities and the predicted three-dimensional image includes:

[0014] Perform negative logarithm processing on the above probabilities to obtain a first processing result;

[0015] Perform total variation regularization processing on the observed data to obtain a second processing result;

[0016] Determine the value of the optimization objective function according to the sum of the first processing result and the second processing result.

[0017] According to an embodiment of the present invention, changing the predicted three-dimensional image of the target to be measured for the optimization objective function includes:

[0018] Calculate the gradient of the optimization objective function and change the predicted three-dimensional image along the direction of gradient descent.

[0019] According to an embodiment of the present invention, based on the Poisson likelihood model, determine the probability of measuring the observed data when the three-dimensional image of the target to be measured is the predicted three-dimensional image according to the rotation matrix of the target to be measured, the predicted three-dimensional image of the target to be measured, and the observed data, including:

[0020] Determine the ideal distribution result of the photon numbers of the predicted three-dimensional image at multiple rotation angles according to the rotation matrix of the target to be measured and the predicted three-dimensional image of the target to be measured;

[0021] Based on the Poisson likelihood model, determine the probability of measuring the observed data when the three-dimensional image of the target to be measured is the predicted three-dimensional image according to the ideal distribution results of the photon numbers at multiple rotation angles and the observed data.

[0022] According to an embodiment of the present invention, the preset condition is that the change amount of the value of the optimization objective function is less than a preset threshold value.

[0023] As a second aspect of the present invention, there is also provided an imaging device for implementing the above imaging method, and the imaging device includes:

[0024] A laser module, adapted to emit pulsed laser light, and irradiate the pulsed laser light onto the target to be measured that rotates according to a time sequence, so as to obtain the reflected laser light of the target to be measured at multiple rotation angles;

[0025] A single-photon detector, adapted to perform single-photon detection on the reflected laser light;

[0026] A time-to-digital converter is applicable to determine the distribution result of the flight time of target photons detected by a single-photon detector in reflected laser, and statistically count the distribution results of the flight time obtained at multiple rotation angles into a photon counting time histogram; the photon counting time histogram constitutes the observation data;

[0027] A processing module is applicable to iteratively perform the following operations until the value of the optimization objective function meets a preset condition, and determine the predicted three-dimensional image of the target to be measured when the value of the optimization objective function meets the preset condition as the three-dimensional image of the target to be measured:

[0028] Based on the Poisson likelihood model, determine the probability of measuring the observation data when the three-dimensional image of the target to be measured is the predicted three-dimensional image according to the rotation matrix of the target to be measured, the predicted three-dimensional image of the target to be measured, and the observation data;

[0029] Determine the value of the optimization objective function according to the probability and the predicted three-dimensional image;

[0030] Change the predicted three-dimensional image according to the value of the optimization objective function.

[0031] According to an embodiment of the present invention, the imaging device further includes:

[0032] A transmission module is applicable to collimate the pulsed laser output by the laser module so that the collimated pulsed laser irradiates the target to be measured; and is applicable to collimate the reflected laser so that the collimated reflected laser enters the single-photon detector.

[0033] The transmission module includes:

[0034] An optical circulator is applicable to transmit the pulsed laser and is applicable to transmit the received reflected laser to the single-photon detector;

[0035] A collimator is applicable to collimate the pulsed laser from the optical circulator and make the collimated pulsed laser irradiate the target to be measured, and is applicable to collimate the reflected laser from the target to be measured and transmit it to the optical circulator.

[0036] According to an embodiment of the present invention, the laser module includes:

[0037] A signal generator is applicable to send control signals in sequence;

[0038] A laser is applicable to generate pulsed laser according to the control signal;

[0039] According to an embodiment of the present invention, the above imaging device further includes:

[0040] A first optical switch allows the pulsed laser to enter the target to be measured in the open state and prohibits the reflected laser from entering the laser module in the closed state;

[0041] A second optical switch that, when open, transmits the reflected laser to the single-photon detector and, when closed, prohibits the pulsed laser from entering the single-photon detector;

[0042] Wherein, during the emission stage of the pulsed laser, the first optical switch is in the open state and the second optical switch is in the closed state; during the reception stage of the reflected laser, the first optical switch is in the closed state and the second optical switch is in the open state;

[0043] The signal generator is also adapted to control the opening and closing of the first optical switch and the opening and closing of the second optical switch.

[0044] According to an embodiment of the present invention, the time-to-digital converter is adapted to determine the emission time of the pulsed laser and, after the pulsed laser is emitted, determine the time when the target photon of the reflected laser is received by the single-photon detector based on the electrical signal emitted by the single-photon detector, so as to calculate the flight time of the target photon.

[0045] According to an embodiment of the present invention, the signal generator is also adapted to emit a timing signal while emitting a control signal, so that the time-to-digital converter determines the emission time of each pulsed laser according to the timing signal.

[0046] Embodiments of the present invention utilize single-photon detection technology to obtain the distribution of the flight times of target photons at different rotation angles. For the Poisson noise problem in single-photon detection, embodiments of the present invention utilize a Poisson likelihood model and an optimization objective function to achieve the fusion of observed data and a predicted three-dimensional image. By iteratively optimizing the predicted three-dimensional image, it gradually approaches the three-dimensional image of the target to be measured, thereby organically combining single-photon detection and laser reflection tomography technology to achieve high-precision three-dimensional imaging of a rotating target to be measured.

[0047] Performing single-photon detection on the reflected laser and simultaneously detecting the flight time of the target photon in the reflected laser realizes the combination of single-photon detection technology and time-gating technology. The achievable time resolution is in the picosecond order of magnitude, overcoming the problem of low time resolution in existing laser reflection tomography technology when detecting reflected laser light, and enabling higher imaging accuracy. In addition, single-photon detection has very high sensitivity. Even when the reflected light signal decays to an extremely low level (even close to the single-photon level) after long-distance transmission, single-photon detection of the reflected laser can still effectively capture these weak light signals. Therefore, performing single-photon detection on the reflected laser in sequence can avoid the problem of poor imaging accuracy caused by long-distance transmission of the reflected light signal in existing laser reflection tomography technology.

[0048] According to an embodiment of the present invention, in ultra-long-distance imaging, since the transmission distance of the reflected laser is relatively long, the number of reflected photons that can be detected is very limited and follows a Poisson distribution. The Poisson likelihood model can accurately describe the data statistical characteristics in this low photon count situation, thereby more precisely reflecting the relationship between the observed data and the true target to be measured. The Poisson likelihood model can fully consider the Poisson noise characteristics in the observed data, thereby effectively suppressing the interference of Poisson noise during the processing. Total Variation (TV) regularization constrains the reconstruction result by introducing the sparsity prior of the image, that is, assuming that the image is sparse in the gradient domain, which can make the reconstructed image smoother and help suppress artifacts and noise during the imaging process, making the image have higher clarity and accuracy. The convex optimization process (i.e., the iterative process) has the uniqueness of the global optimal solution, which means that the global optimal reconstruction result can be obtained by solving the optimization objective function through the convex optimization method. Performing Poisson likelihood modeling on the observed data, total variation regularization on the predicted three-dimensional image, and convex optimization on the optimization objective function are to have anti-noise performance, suppress artifacts, maintain edge features, ensure the global optimal solution, and improve the algorithm stability and efficiency in ultra-long-distance single-photon reflection tomography. These processing methods act together on the imaging process, helping to achieve high-precision three-dimensional imaging over ultra-long distances. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 FIG. shows a flowchart of an imaging method provided according to an embodiment of the present invention;

[0050] Figure 2 FIG. shows a schematic diagram of an imaging device provided according to an embodiment of the present invention;

[0051] Figure 3 FIG. shows a comparison diagram of multiple images obtained by imaging a target to be measured using multiple methods according to an embodiment of the present invention.

[0052] DESCRIPTION OF REFERENCE NUMERALS

[0053] 1 Laser module;

[0054] 11 Signal generator;

[0055] 12 Laser;

[0056] 2 Single-photon detector;

[0057] 3 Time-to-digital converter;

[0058] 4 Transmission module;

[0059] 41 Circulator;

[0060] 42 Collimator;

[0061] 5 First optical switch;

[0062] 6 Second optical switch. Detailed implementation manners

[0063] In the process of implementing the present invention, it is found that single-photon detection technology has gradually attracted attention in the field of high-precision imaging. Single-photon detection technology is usually used in combination with Time-Correlated Single Photon Counting (TCSPC) technology. When the two are combined, it not only has single-photon-level detection sensitivity but also can achieve picosecond (ps)-level time resolution. However, directly applying single-photon detection technology to laser reflection tomography technology still faces some problems. For example, since the signals detected by single-photon detection have statistical characteristics, the process of image reconstruction using the signals detected by single-photon detection is easily affected by Poisson noise. In this case, traditional image reconstruction algorithms based on backprojection are prone to artifacts, thus failing to meet the requirements of high-precision imaging.

[0064] To make the objectives, technical solutions, and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to specific embodiments and the accompanying drawings.

[0065] The terms used herein are only for describing specific embodiments and are not intended to limit the present invention. The terms "including", "comprising", etc. used herein indicate the presence of the described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0066] All terms used herein, including technical and scientific terms, have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0067] In the case of using expressions such as "at least one of A, B, and C, etc.", generally, it should be interpreted according to the meaning commonly understood by those skilled in the art. For example, "a system having at least one of A, B, and C" should include, but not be limited to, a system having only A, only B, only C, having A and B, having A and C, having B and C, and / or having A, B, and C. In the case of using expressions such as "at least one of A, B, or C, etc.", generally, it should be interpreted according to the meaning commonly understood by those skilled in the art. For example, "a system having at least one of A, B, or C" should include, but not be limited to, a system having only A, only B, only C, having A and B, having A and C, having B and C, and / or having A, B, and C.

[0068] It should also be noted that the directional terms mentioned in the embodiments, such as "up", "down", "front", "back", "left", "right", etc., are only references to the directions in the accompanying drawings and are not used to limit the protection scope of the present invention. Throughout the accompanying drawings, the same elements are represented by the same or similar reference numerals. When it may cause confusion in the understanding of the present invention, the conventional structures or configurations will be omitted.

[0069] In the embodiments of the present invention, in aspects such as the collection, update, analysis, processing, use, transmission, provision, invention, storage, etc. of the data involved (for example, including but not limited to user personal information), they all comply with the provisions of relevant laws and regulations, are used for legal purposes, and do not violate public order and good customs. In particular, necessary measures are taken for user personal information to prevent illegal access to user personal information data and to maintain the security of user personal information and network security.

[0070] Figure 1 The flowchart of an imaging method provided according to an embodiment of the present invention is shown.

[0071] As Figure 1 shown, the imaging method includes operations S1 to S7.

[0072] In operation S1, a pulsed laser is irradiated onto the target to be measured that is rotating according to a time sequence, and the reflected laser of the target to be measured at each of a plurality of rotation angles is obtained. Among them, after the pulsed laser is incident on the target to be measured, it is reflected by the target to be measured, and the target to be measured can be, for example, a satellite or the earth, etc.

[0073] In operation S2, single-photon detection is sequentially performed on the reflected laser to determine the distribution result of the flight time of the target photons in the reflected laser. The target photons are the photons detected in the reflected laser.

[0074] In operation S3, the distribution results of the flight time obtained at a plurality of rotation angles are statistically processed into a photon count time histogram; among them, the photon count time histogram constitutes the observation data.

[0075] In operation S4, the following operations are iteratively executed until the value of the optimization objective function meets a preset condition, and the predicted three-dimensional image of the target to be measured when the value of the optimization objective function meets the preset condition is determined as the three-dimensional image of the target to be measured:

[0076] In operation S5, based on the Poisson likelihood model, according to the rotation matrix of the target to be measured, the predicted three-dimensional image of the target to be measured, and the observation data, the probability of measuring the observation data when the three-dimensional image of the target to be measured is the predicted three-dimensional image is determined;

[0077] In operation S6, the value of the optimization objective function is determined according to the above probability and the predicted three-dimensional image;

[0078] In operation S7, the predicted three-dimensional image is changed according to the value of the optimization objective function.

[0079] In the embodiment of the present invention, the irradiation range of the pulsed laser can cover a partial area of the target to be measured (i.e., the surface facing the pulsed laser), and the irradiated part is the detection area of the pulsed laser. Therefore, when the target to be measured rotates, the detection area changes, but there is overlap between different detection areas. As the target to be measured rotates, each position on the target to be measured can be detected by the pulsed laser at different rotation angles. The observation data in the embodiment of the present invention contains the detection information of any position on the target to be measured at different rotation angles.

[0080] According to the embodiment of the present invention, for each rotation angle of the target to be measured, multiple pulsed lasers irradiate the target to be measured at this rotation angle. This is because single-photon detection has high sensitivity and high time-resolution ability, and multiple measurements are required to improve the statistical accuracy of the data.

[0081] The principle of laser reflection tomography is to analyze the reflection characteristics of laser on the surface or inside of an object, and combine multi-angle measurement and computer algorithms for reconstruction to achieve high-resolution imaging of the object. The embodiment of the present invention uses single-photon detection technology to obtain the distribution of the flight time of target photons at different rotation angles. Aiming at the Poisson noise problem in single-photon detection, the embodiment of the present invention uses a Poisson likelihood model and an optimization objective function to realize the fusion of observation data and the predicted three-dimensional image. By iteratively optimizing the predicted three-dimensional image, it gradually approaches the three-dimensional image of the target to be measured, thereby organically combining single-photon detection and laser reflection tomography technology and achieving high-precision three-dimensional imaging of the rotating target to be measured.

[0082] According to the embodiment of the present invention, single-photon detection is performed on the reflected laser, and at the same time, the flight time of the target photons in the reflected laser is detected, realizing the combination of single-photon detection technology and time-gating technology. The achievable time resolution is in the picosecond order of magnitude, overcoming the problem of low time resolution in the existing laser reflection tomography technology when detecting the reflected laser, and enabling higher imaging accuracy. In addition, single-photon detection has very high sensitivity. Even when the reflected light signal attenuates to an extremely low level (even close to the single-photon level) after long-distance transmission, these weak light signals can still be effectively captured by performing single-photon detection on the reflected laser. Therefore, performing single-photon detection on the reflected laser in sequence can avoid the problem of poor imaging accuracy caused by the long-distance transmission of the reflected light signal in the existing laser reflection tomography technology.

[0083] According to an embodiment of the present invention, operations S4 to S7 are the Poisson noise convex optimization reconstruction algorithm provided by the embodiment of the present invention. In ultra-long-distance imaging, since the transmission distance of the reflected laser is relatively long, the number of reflected photons that can be detected is very limited and follows a Poisson distribution. The Poisson likelihood model can accurately describe the data statistical characteristics in this low photon count situation, thus more precisely reflecting the relationship between the observed data and the true target to be measured. The Poisson likelihood model can fully consider the Poisson noise characteristics in the observed data, thereby effectively suppressing the interference of Poisson noise during the processing. The total variation (TV) regularization constrains the reconstruction result by introducing the sparsity prior of the image, that is, assuming that the image is sparse in the gradient domain, which can make the reconstructed image smoother, and helps to suppress artifacts and noise during the imaging process, improving the clarity and accuracy of the image. The Poisson noise convex optimization reconstruction algorithm has the uniqueness of the global optimal solution, which means that the global optimal reconstruction result can be obtained by processing and solving the optimization objective function through the Poisson noise convex optimization reconstruction algorithm. Using the Poisson likelihood model to process the observed data, performing total variation regularization on the predicted three-dimensional image, and performing convex optimization on the optimization objective function are for having anti-noise performance, suppressing artifacts, maintaining edge features, ensuring the global optimal solution, and improving the stability and efficiency of the algorithm in ultra-long-distance single-photon reflection tomography. These processing methods act together on the imaging process, contributing to achieving high-precision three-dimensional imaging at ultra-long distances. According to an embodiment of the present invention, based on the Poisson likelihood model, when determining the probability of measuring the observed data when the three-dimensional image of the target to be measured is the predicted three-dimensional image according to the rotation matrix of the target to be measured, the predicted three-dimensional image of the target to be measured, and the observed data, it includes: operations S51 to S52.

[0084] In operation S51, according to the rotation matrix of the target to be measured and the predicted three-dimensional image of the target to be measured, determine the ideal distribution result of the number of photons at each of multiple rotation angles of the predicted three-dimensional image. When the irradiation range of the pulsed laser can cover one side surface of the target to be measured facing the pulsed laser, the ideal distribution result of the number of photons at each of multiple rotation angles is expressed as formula (1).

[0085] (1).

[0086] Where is the rotation matrix of the target to be measured, representing the transformation of the target to be measured at different rotation angles; is the predicted three-dimensional image of the target to be measured, representing the three-dimensional spatial coordinates of different positions of the target to be measured; is the lateral distribution of the pulsed laser (generally Gaussian light); is the time-domain waveform of the pulsed laser, is the speed of light, Represents an integration space that includes all regions where the target to be measured is irradiated by the pulsed laser. Represents the predicted three-dimensional image After passing through the rotation matrix The result of the transformation.

[0087] In operation S52, based on the Poisson likelihood model, according to the ideal distribution results and the observed data of the photon numbers at multiple rotation angles, determine the probability of being able to measure the observed data when the three-dimensional image of the target to be measured is the predicted three-dimensional image, as specifically shown in Equation (2).

[0088] (2).

[0089] Where Represents the probability of being able to measure the observed data when the three-dimensional image of the target to be measured is the predicted three-dimensional image, Is the background noise, Is the quantum efficiency of the detector .

[0090] Based on the maximum likelihood estimation, establish the optimization objective function. The optimization objective function is expressed as Equation (3).

[0091] (3).

[0092] Where Represents the optimization objective function, Represents that the three-dimensional image of the target to be measured is the predicted three-dimensional image When, it is possible to measure the observed data Probability of, Represents different rotation angles of the target to be measured, Represents the time of flight, Represents the three-dimensional coordinates of different positions of the target to be measured, Represents the total variation regularization term of the predicted three-dimensional image, used to retain the edges of the predicted three-dimensional image and avoid edge blurring. TV represents the total variation, Represents the regularization parameter.

[0093] According to Equation (3), it can be seen that in operation S6, determine the value of the optimization objective function according to the above probability and the predicted three-dimensional image, including operations S61 to S63.

[0094] In operation S61, perform a negative logarithm processing on the probability to obtain the first processing result.

[0095] In operation S62, perform a total variation regularization processing on the observed data to obtain the second processing result;

[0096] In operation S63, determine the value of the optimization objective function based on the sum of the first processing result and the second processing result.

[0097] According to an embodiment of the present invention, when changing the predicted three-dimensional image, non-negativity constraints and total variation regularization are applied to ensure that the updated predicted three-dimensional image satisfies the non-negativity constraints (i.e., the updated predicted three-dimensional image ≥0). If there are negative values in the updated predicted three-dimensional image , set them to zero. In addition, total variation regularization needs to be applied to the updated predicted three-dimensional image to further suppress noise and maintain the sparsity of the image.

[0098] Figure 2 Shows a schematic diagram of an imaging device provided according to an embodiment of the present invention.

[0099] As Figure 2 shown, the imaging device includes: a laser module 1, a single-photon detector 2, a time-to-digital converter 3, and a processing module (not shown in the figure).

[0100] The laser module 1 is adapted to emit pulsed laser light, so that the pulsed laser light is irradiated onto the target to be measured that rotates according to a time sequence, so as to obtain the reflected laser light of the target to be measured at multiple rotation angles. The single-photon detector 2 is adapted to receive the reflected laser light and perform single-photon detection on the reflected laser light. The time-to-digital converter 3 is adapted to determine the flight time of all target photons detected by the single-photon detector 2 in the reflected laser light, and statistically analyze the distribution results of all flight times obtained at multiple rotation angles into a photon counting time histogram, that is, the photon counting time histogram is the statistical distribution of all flight times obtained at multiple rotation angles, and the photon counting time histogram constitutes the observed data. The processing module is adapted to iteratively perform the following operations until the value of the optimization objective function meets a preset condition, and determine the predicted three-dimensional image of the target to be measured when the value of the optimization objective function meets the preset condition as the three-dimensional image of the target to be measured: Based on the Poisson likelihood model, according to the rotation matrix of the target to be measured, the predicted three-dimensional image of the target to be measured, and the observed data, determine the probability of measuring the observed data when the three-dimensional image of the target to be measured is the predicted three-dimensional image; determine the value of the optimization objective function according to the above probability and the predicted three-dimensional image.

[0101] According to an embodiment of the present invention, the single-photon detector 2 and the time-to-digital converter 3 are combined. Through the sensitivity of the single-photon detector 2 at the single-photon level and the picosecond-level time resolution of the time-to-digital converter 3, the problems of the traditional technology in terms of time resolution and signal attenuation are solved, and the ultra-long-distance imaging ability is improved. In addition, the present invention combines the Poisson noise convex optimization reconstruction algorithm to effectively suppress artifacts, so that the obtained three-dimensional image has high accuracy and robustness.

[0102] According to an embodiment of the present invention, the laser module 1 includes: a signal generator 11 and a laser 12. The signal generator 11 is adapted to emit control signals in a time sequence. The laser 12 is adapted to generate pulsed laser according to the control signals.

[0103] According to an embodiment of the present invention, the imaging device further includes: a transmission module 4. The transmission module 4 is adapted to collimate the pulsed laser so that the pulsed laser is incident on the target to be measured; and is adapted to collimate the reflected laser so that the reflected laser enters the single-photon detector.

[0104] According to an embodiment of the present invention, the imaging device adopts a coaxial transceiver design, that is, the pulsed laser is emitted from the transmission module 4, and the reflected laser is received by the transmission module 4. This design can greatly reduce the system volume, improve the integration degree, and meet the requirements of portability.

[0105] According to an embodiment of the present invention, the transmission module 4 includes: a circulator 41 and a collimator 42.

[0106] The circulator 41 is adapted to transmit the pulsed laser, and is adapted to transmit the received reflected laser to the single-photon detector 2. The collimator 42 is adapted to collimate the pulsed laser from the circulator and make the collimated pulsed laser incident on the target to be measured, and is adapted to collimate the reflected laser from the target to be measured and transmit it to the circulator 41.

[0107] According to an embodiment of the present invention, the signal generator 11 is further used to emit a timing signal while emitting the control signal, so that the time-to-digital converter determines the emission time of the pulsed laser according to the timing signal.

[0108] According to an embodiment of the present invention, the imaging device further includes: a first optical switch 5 and a second optical switch 6. The first optical switch 5 allows the pulsed laser to be incident on the target to be measured in the open state, and prohibits the reflected laser from entering the laser module 1 in the closed state. The second optical switch 6 is used to transmit the reflected laser to the single-photon detector 2 in the open state, and prohibits the pulsed laser from entering the single-photon detector 2 in the closed state. Among them, the first optical switch 5 and the second optical switch 6 are configured to be alternately opened. During the pulsed laser emission stage, the first optical switch 5 is in the open state and the second optical switch 6 is in the closed state, so that the pulsed laser can be emitted smoothly. During the receiving stage of the single-photon detector 2 for the reflected laser (from the reflection of the pulsed laser by the target to be measured until the reflected laser is received by the single-photon detector 2), the first optical switch 5 is closed and the second optical switch 6 is open, so that the reflected laser can be detected by the single-photon detector 2.

[0109] The signal generator 11 is also applicable to emit a timing signal while emitting a control signal, so that the time-to-digital converter 3 determines the emission time of the pulsed laser of the laser according to the timing signal. The signal generator 11 is also applicable to control the opening and closing of the first optical switch 5 and the opening and closing of the second optical switch 6. While the signal generator emits a control signal, the signal generator 11 also generates a high-level signal for the first optical switch 5 to enable the first optical switch 5, so that the pulsed laser can smoothly exit through the collimator. In the stage of receiving the reflected laser, the signal generator 11 generates a high-level signal for the second optical switch 6 to enable the second optical switch 6, so that the echo energy (reflected laser) can be smoothly detected by the single-photon detector 2, and at the same time, a synchronization signal is generated for the time-to-digital converter for signal statistics.

[0110] In the embodiment of the present invention, the two optical switches adopt a time-division control for transmitting and receiving, that is, half of the time of the imaging device according to the embodiment of the present invention is used for the emission of the pulsed laser (at this time, the first optical switch is open and the second optical switch is closed), and the other half of the time is used for the reception of the reflected signal (at this time, the second optical switch is open and the first optical switch is closed). This design avoids the noise interference caused by the reflection of the local optical signal.

[0111] The single-photon detector according to the embodiment of the present invention can be, for example, a miniaturized free-running InGaAs / InP single-photon avalanche diode.

[0112] The following combines Figure 2 , and lists specific embodiments to detail the imaging method provided by the embodiment of the present invention.

[0113] According to an embodiment of the present invention, taking an imaging distance of 6 km as an example, the target to be measured is fixed on a turntable 6 km away from the imaging device. The turntable rotates by 1 degree each time, causing the target to rotate on its own axis. The pulsed laser is incident on the target to be measured in sequence by using the laser module 1 and the first optical switch 5. After passing through the transmission module 4, multiple pulsed lasers are incident on the target to be measured and then reflected by the target. The reflected laser is transmitted to the single-photon detector 2 through the transmission module 4 and the second optical switch 6 in sequence, completing the 360-degree all-round data acquisition of the target to be measured. According to the target distance between the imaging device and the target to be measured, the time for the pulsed laser to reach the target to be measured can be estimated. For example, when the target distance is 6 km, it can be estimated that the time for the pulsed laser to reach the target to be measured is 40 microseconds. Therefore, both the pulsed laser emission time and the reflected laser reception time can be set to 40 microseconds. According to the estimated time, the signal generator 11 is used to accurately control the opening and closing times of the first optical switch 5 and the second optical switch 6 to ensure the accurate acquisition of the reflected laser. The single-photon detector 2 can be set to have a time interval (bin) width of 1 picosecond to obtain high-precision time resolution. Each acquisition lasts for 1 second, and the laser repetition frequency of the pulse is 1 MHz, ensuring real-time data update and high sampling rate. The time-to-digital converter is used to determine the flight time of the target photons in the reflected laser and count them into a photon counting time histogram. After the processing module processes the observation data obtained from the reflected laser at multiple rotation angles by using the Poisson noise convex optimization reconstruction algorithm, high-precision three-dimensional reconstruction of the target to be measured is achieved. Finally, through algorithm optimization and reconstruction, a three-dimensional image of the target to be measured is obtained. This three-dimensional image has a three-dimensional prominent effect and can accurately display the spatial contour of the target to be measured, providing high-quality imaging data for the identification and detection of the target to be measured.

[0114] Figure 3 The contrast results of multiple images obtained by imaging the target to be measured using multiple methods are shown.

[0115] As Figure 3 shown, part (a) is an image obtained by closely photographing the target to be measured using a visible light camera, and this image is the imaging target that the imaging method of the embodiment of the present invention aims to achieve. Part (b) is a schematic diagram of the observation data obtained according to the embodiment of the present invention. Part (c) is a three-dimensional image of the target to be measured obtained by using a traditional imaging method (traditional laser reflection tomography algorithm). Part (d) is a three-dimensional image of the target to be measured obtained by using the imaging method provided by the embodiment of the present invention. The scales of part (a) and part (d) are both 10 cm. It can be seen from part (c) and part (d) that the imaging method provided by the embodiment of the present invention can accurately obtain a three-dimensional image of the target to be measured and achieve the reconstruction of the target to be measured.

[0116] The imaging method provided by the embodiments of the present invention exhibits remarkable anti-noise performance when processing photon counting data under Poisson noise background, can achieve high-precision imaging under complex noise conditions, and can adapt to parameter adjustment under different signal-to-noise ratio conditions.

[0117] The imaging method provided by the embodiments of the present invention utilizes single-photon detection technology and combines with Poisson noise convex optimization reconstruction algorithm, which can effectively overcome the attenuation of reflected signals and the influence of noise during ultra-long-distance imaging, and significantly improve the resolution imaging ability at ultra-long distances.

[0118] The imaging device provided by the embodiments of the present invention adopts a coaxial transceiver single-photon laser reflection tomography system configuration, which not only reduces the system volume, but also improves the system integration, and reduces the hardware complexity and cost.

[0119] The embodiments of the present invention have been described above. However, these embodiments are for illustrative purposes only and not for limiting the scope of the present invention. Although the embodiments are described separately above, this does not mean that the measures in each embodiment cannot be used advantageously in combination. The scope of the present invention is defined by the appended claims and their equivalents. Without departing from the scope of the present invention, those skilled in the art can make various substitutions and modifications, and these substitutions and modifications should all fall within the scope of the present invention.

Claims

1. An imaging method, characterized in that, Including: Irradiating a pulsed laser onto a to-be-measured target that is rotating according to a time sequence, and obtaining the reflected laser of the to-be-measured target at multiple rotation angles respectively; Performing single-photon detection on the reflected laser to determine the distribution result of the flight time of target photons in the reflected laser; Statistically processing the distribution results of the flight time obtained at the multiple rotation angles into a photon counting time histogram; the photon counting time histogram constitutes the observation data; Iteratively perform the following operations until the value of the optimization objective function meets a preset condition, and determine the predicted three-dimensional image of the to-be-measured target when the value of the optimization objective function meets the preset condition as the three-dimensional image of the to-be-measured target: Based on the Poisson likelihood model, according to the rotation matrix of the to-be-measured target, the predicted three-dimensional image of the to-be-measured target, and the observation data, determine the probability of measuring the observation data when the three-dimensional image of the to-be-measured target is the predicted three-dimensional image; Determine the value of the optimization objective function according to the probability and the predicted three-dimensional image; Change the predicted three-dimensional image according to the value of the optimization objective function.

2. The imaging method according to claim 1, wherein Determining the value of the optimization objective function according to the probability and the predicted three-dimensional image includes: Performing negative logarithm processing on the probability to obtain a first processing result; Performing total variation regularization processing on the observation data to obtain a second processing result; Determine the value of the optimization objective function according to the sum of the first processing result and the second processing result.

3. The imaging method according to claim 1, wherein, Changing the predicted three-dimensional image includes: Calculating the gradient of the optimization objective function, and changing the predicted three-dimensional image along the direction of gradient descent.

4. The imaging method according to claim 1, characterized in that, Based on the Poisson likelihood model, according to the rotation matrix of the to-be-measured target, the predicted three-dimensional image of the to-be-measured target, and the observation data, determining the probability of measuring the observation data when the three-dimensional image of the to-be-measured target is the predicted three-dimensional image includes: According to the rotation matrix of the to-be-measured target and the predicted three-dimensional image of the to-be-measured target, determine the ideal distribution result of the number of photons of the predicted three-dimensional image at the multiple rotation angles respectively; Based on the Poisson likelihood model, according to the ideal distribution result of the number of photons at the multiple rotation angles respectively and the observation data, determine the probability of measuring the observation data when the three-dimensional image of the to-be-measured target is the predicted three-dimensional image.

5. The imaging method according to claim 2, wherein The preset condition is that the change amount of the value of the optimization objective function is less than a preset threshold.

6. An imaging device for implementing the imaging method according to any one of claims 1 to 5, characterized in that, The imaging device includes: A laser module, suitable for emitting a pulsed laser, irradiating the pulsed laser onto a to-be-measured target that is rotating according to a time sequence, so as to obtain the reflected laser of the to-be-measured target at multiple rotation angles respectively; A single-photon detector, suitable for performing single-photon detection on the reflected laser; A time-to-digital converter, suitable for determining the distribution result of the flight time of target photons detected by the single-photon detector in the reflected laser, and statistically processing the distribution results of the flight time obtained at multiple rotation angles into a photon counting time histogram; the photon counting time histogram constitutes the observation data; The processing module is adapted to iteratively perform the following operations until the value of the optimization objective function meets a preset condition, and determine the predicted three-dimensional image of the target to be measured when the value of the optimization objective function meets the preset condition as the three-dimensional image of the target to be measured: Based on the Poisson likelihood model, determine the probability of measuring the observed data when the three-dimensional image of the target to be measured is the predicted three-dimensional image according to the rotation matrix of the target to be measured, the predicted three-dimensional image of the target to be measured, and the observed data; Determine the value of the optimization objective function according to the probability and the predicted three-dimensional image; change the predicted three-dimensional image according to the value of the optimization objective function.

7. The imaging device according to claim 6, wherein The imaging device further includes: A transmission module, adapted to collimate the pulsed laser output by the laser module so that the collimated pulsed laser irradiates the target to be measured; and adapted to collimate the reflected laser so that the collimated reflected laser enters the single-photon detector; The transmission module includes: A circulator, adapted to transmit the pulsed laser and to transmit the received collimated reflected laser to the single-photon detector; A collimator, adapted to collimate the pulsed laser from the circulator and make the collimated pulsed laser irradiate the target to be measured, and to collimate the reflected laser from the target to be measured and transmit it to the circulator.

8. The imaging device according to claim 6, characterized in that, The laser module includes: A signal generator, adapted to issue control signals in sequence; A laser, adapted to generate the pulsed laser according to the control signal.

9. The imaging device according to claim 8, wherein The imaging device further includes: A first optical switch, which allows the pulsed laser to enter the target to be measured in the open state and prohibits the reflected laser from entering the laser module in the closed state; A second optical switch, which transmits the reflected laser to the single-photon detector in the open state and prohibits the pulsed laser from entering the single-photon detector in the closed state; Wherein, in the emission stage of the pulsed laser, the first optical switch is in the open state and the second optical switch is in the closed state; in the reception stage of the reflected laser, the first optical switch is in the closed state and the second optical switch is in the open state; The signal generator is further adapted to control the opening and closing of the first optical switch and the opening and closing of the second optical switch.

10. The imaging device according to claim 9, characterized in that, The time-to-digital converter is further adapted to determine the emission time of the pulsed laser, and determine the time when the single-photon detector receives the target photon of the reflected laser according to the electrical signal emitted by the single-photon detector after the pulsed laser is emitted, so as to calculate the flight time of the target photon; The signal generator is further adapted to issue a timing signal while issuing the control signal, so that the time-to-digital converter determines the emission time of the pulsed laser according to the timing signal.

Images