Geiger mode single-photon focal plane crosstalk measurement device and method

Abstract

The invention belongs to the technical field of photon detection, and particularly relates to a Geiger mode single-photon focal plane crosstalk measurement device and method, and the device comprises a beam expander, a deformable mirror, a beam splitter BS1, a beam splitter BS2, an industrial personal computer, an alignment camera, an observation camera, and a photoelectric detector. According to the method, the wavefront sensor and the deformable mirror are combined to carry out real-time measurement feedback and correction on incident wavefront, a high-quality plane wave is formed, a standard small light spot close to the diffraction limit can be formed after the plane wave is converged through the focusing system, and accurate measurement of the crosstalk of the single-photon focal plane is achieved.

Description

Technical Field

[0001] This application belongs to the field of photon detection technology, specifically relating to a Geiger-mode single-photon focal plane crosstalk measurement device and method. Background Technology

[0002] A single-photon focal plane array (SPFA) is a photodetector array with one or more columns of single-photon detection units, signal processing circuitry, and packaging or necessary cooling devices, used for laser signal detection. It can be viewed as a two-dimensional matrix composed of n×m pixels. When using gated operation, the response to a photon within the gate time is the sum of responses to multiple photon events at specific moments. As a novel enhanced photon detection device, SPFA is mainly used in laser active imaging 3D radar, deep-space optical communication, and long-range laser ranging. However, since SPFA is an array device, its performance is inevitably affected by inter-pixel crosstalk. The presence of crosstalk reduces the spatial and temporal resolution of the SPFA, severely affecting the device's performance and imaging accuracy; excessive crosstalk can even cause the device to malfunction. With the rapid development of single-photon focal plane array devices in recent years, the array size is getting larger and larger, and the pixel size and spacing of the focal plane are getting smaller and smaller, making crosstalk more and more obvious. However, the testing technology has not kept up with the times. At present, only preliminary testing can be carried out for small-scale devices. As the scale of devices continues to increase, the existing testing methods cannot meet the testing requirements. Summary of the Invention

[0003] This invention proposes a Geiger-mode single-photon focal plane crosstalk measurement device and method. First, the laser beam emitted from the laser is collimated and expanded. Then, the wavefront of the incident beam is measured by a wavefront sensor, and the result is fed back to a deformable mirror. A computer-driven deformable mirror corrects the incident wavefront to generate a high-quality plane wave. The resulting parallel beam is then focused into a near-diffraction-limited small spot by a focusing objective, thus achieving single-photon focal plane crosstalk measurement. The technical solution is as follows:

[0004] A Geiger-mode single-photon focal plane crosstalk measurement device includes a beam expander, a deformable mirror, a beam splitter BS1, a beam splitter BS2, an industrial control computer, an alignment camera, an observation camera, an objective lens, and a photodetector.

[0005] After passing through a beam expander and a deformable mirror, the laser emission source is split by a beam splitter BS1. The reflected light is detected by a wavefront sensor, which monitors the wavefront of the incident beam and feeds the result back to the industrial control computer. The industrial control computer controls the deformable mirror controller to adjust the deformable mirror and correct the incident wavefront to generate a high-quality plane wave.

[0006] The alignment camera is used for beam alignment, and works in conjunction with the reticle on the back of the beam splitter BS1 to perform spot-target alignment and adjustment, thereby achieving optical path calibration of the focal plane of the single photon under test.

[0007] After passing through the objective lens and then through the beam splitter BS2, the transmitted light illuminates the target surface of the device under test. The light reflected back from the target surface is reflected again by BS2, and the light spot is collected by the observation camera.

[0008] The reflected light is incident on the photodetector with a front aperture aperture mounted on the displacement stage, so as to measure the intensity distribution and size of the detection spot.

[0009] Preferably, after passing through the beam expander, the laser is reflected by a deformable mirror at a 45° angle to the incident optical axis.

[0010] Preferably, the wavefront sensor collects the wavefront of the light beam, calculates the beam aberration coefficient through a convolutional neural network, controls the surface shape of the deformable mirror through the deformable mirror controller, corrects the aberrations in the optical path, and forms an adaptive optical path aberration correction closed loop.

[0011] A method for measuring crosstalk in Geiger mode single-photon focal plane arrays includes the following steps:

[0012] S1. Control the displacement stage at the bottom of the device under test to move the device upward a certain distance, and monitor the size of the light spot on the target surface through the observation camera. Stop when the light spot increases significantly.

[0013] S2. Control the displacement stage to move downward in equal steps, record the moving distance L, until the spot size first decreases and then increases again, and stop. In conjunction with the observation camera, collect the image of the spot focused by the objective lens on the target surface during each displacement, and transmit the image information to the control unit for sharpness evaluation.

[0014] S3. Calculate the focus position based on the sharpness evaluation, and control the shift stage to move to the calculated position C;

[0015] S4. Control the displacement stage to move upward from position C to L, and then move it at equal distances with a single displacement step of L / 4, until it reaches a position L below position C. Repeat the S2-S3 operation, and record the resulting focus position as C'.

[0016] S5. Calculate the difference between the height values ​​of position C and position C'. Determine whether the current image is in focus based on whether the difference is less than the positioning threshold. If it is, stop calculating the focus position and control the shift stage to move to C' and lock the Z-axis position. If it is not, repeat the above steps until the current position is determined to be the optimal focus position.

[0017] S6. After focusing the small light spot, measure the crosstalk of a single pixel on the single-photon focal plane.

[0018] Preferably, the calculation of image sharpness evaluation adopts an image gradient-based method, with the following steps:

[0019] The Sobel operator is used to extract the gradient values ​​in the horizontal and vertical directions respectively, and the sum of their squares is used as the evaluation function.

[0020] Let Sobel's convolution kernel G be... x G y :

[0021]

[0022] The corresponding operator matrix is:

[0023]

[0024] The gradient of image I at (x,y) is:

[0025]

[0026] The Tenengard value of this image is:

[0027]

[0028] Where n is the total number of pixels in the image.

[0029] Compared with the prior art, the beneficial effects of this application are as follows:

[0030] 1. This invention proposes a Geiger-mode single-photon focal plane crosstalk measurement device. It uses a combination of wavefront sensor and deformable mirror to correct the incident beam and form a high-quality parallel beam. Then, it is focused by a focusing system to form a standard small light spot close to the diffraction limit. By accurately locating the spot, the single-photon focal plane crosstalk can be measured.

[0031] 2. The present invention adds a feedback correction module for the wavefront sensor and deformable mirror, which can repeatedly correct the incident beam and form a micrometer-sized light spot through convergence. Compared with the existing technology, the size of the light spot is further reduced, which can accurately cover a single pixel of the single-photon focal plane.

[0032] 3. The present invention is based on the positioning and intensity distribution measurement method of small light spots. By calibrating the spot size and intensity distribution of the small light spots, the light power intensity after the pinhole aperture is measured. By aligning the camera with the small light spot and observing its position in real time, the position of a single pixel can be accurately located, thereby realizing the crosstalk measurement of single-photon focal plane pixels. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of this application;

[0034] Figure 2 Flowchart for locating a small light spot;

[0035] Figure 3 This is a longitudinal self-focusing positioning process.

[0036] In the diagram: 1-Beam expander, 2-Deformable mirror, 3-Deformable mirror controller, 4-Beam splitter BS1, 5-Alignment camera, 6-Wavefront sensor, 7-Objective lens, 8-Beam splitter BS2, 9-Observation camera, 10-Industrial control computer, 11-Displacement stage, 12-Photodetector. Detailed Implementation

[0037] The technical solution of this application will be described in detail below with reference to specific embodiments and accompanying drawings. It should be understood that the embodiments and specific features in the embodiments are detailed descriptions of the technical solution of this application, rather than limitations thereof. Specific technical features can be combined with each other.

[0038] This invention proposes a Geiger-mode single-photon focal plane crosstalk measurement device. First, the laser beam is collimated and expanded. Then, the wavefront of the incident beam is measured by a wavefront sensor, and the result is fed back to a deformable mirror. The computer drives the deformable mirror to correct the incident wavefront and generate a high-quality plane wave. The resulting parallel beam is focused into a small spot close to the diffraction limit by a focusing objective lens, thus realizing the crosstalk measurement of the single-photon focal plane.

[0039] A Geiger mode single-photon focal plane crosstalk measurement device, such as Figure 1As shown, the laser beam is collimated and expanded by beam expander 1, then reflected and modulated by deformable mirror 2 (at 45° to the incident optical axis). The beam is then split by beam splitter BS1. The reflected light is detected by wavefront sensor 6 to monitor the incident beam wavefront, while the transmitted light is incident on the focal plane of the single-photon detector array. The reflected light is received by wavefront sensor 6, and the result is fed back to industrial control computer 10. Industrial control computer 10 controls deformable mirror controller 3 to adjust deformable mirror 2, correcting the incident wavefront to generate a high-quality plane wave. Simultaneously, a reticle pattern is engraved on the back of beam splitter BS1 to cooperate with alignment camera 5 for spot-target alignment. The deformable mirror, deformable mirror controller, beam splitter BS1, and wavefront sensor constitute a beam quality control system. The wavefront sensor acquires the beam wavefront, calculates the beam aberration coefficients using a convolutional neural network (CNN), and controls the deformable mirror surface shape via the deformable mirror controller to correct aberrations in the optical path, forming an adaptive optical path aberration correction closed-loop circuit. After real-time correction of wavefront aberration, the beam passes through replaceable units that perform different measurement functions, and then through beam splitter BS2. The transmitted light then illuminates the target surface of the device under test. The light reflected back from the target surface is reflected again by beam splitter BS2 and collected by spot observation camera 9 to monitor the illumination of the probe spot on the target surface of the device under test. Alignment camera 5 is used for beam alignment system. It works with the reticle on the back of beam splitter BS1 to perform spot-target alignment and achieve optical path calibration of the focal plane of the single photon under test. The reflected light is incident on the photodetector 12 of the front aperture mounted on the high-precision displacement stage 11 to measure the intensity distribution and size of the probe spot.

[0040] Laser source: Lasers are used to provide a stable short-pulse light source for the device.

[0041] Beam expander: A beam expander (collimator) is used to expand and collimate the laser beam emitted from a laser, so that the laser beam is matched with the size of the system components.

[0042] Deformable mirror: The deformable mirror is used to correct the aberration of the beam emitted from the laser source. It works with the wavefront sensor to detect the wavefront aberration parameters of the beam, forming an adaptive optical path aberration correction closed loop, which corrects the beam aberration projected onto the single-photon focal plane of the test in real time.

[0043] Wavefront sensor: The wavefront sensor measures the wavefront aberration of the beam incident on the focal plane of the single photon under test, and feeds the result back to the deformable mirror to form an adaptive optical path aberration correction closed loop, which corrects the beam aberration projected on the focal plane of the single photon under test in real time.

[0044] Apochromatic long working distance near-infrared converging objective: The apochromatic long working distance near-infrared converging objective is used to converge a laser beam to generate a small light spot that illuminates the target surface of the detector under test, and is used to test the crosstalk of its individual pixels.

[0045] CCD Cameras: Two CCD cameras are required. One is used for the beam alignment system, which works with the reticle on the back of the beam splitter to align the light spot with the target surface and achieve the optical path calibration of the focal plane of the single photon under test. The other is used to acquire the image of the light spot projected by the objective lens onto the target surface under test.

[0046] Three-axis displacement stage: The entire testing system is assembled on a three-axis precision displacement stage, which is used for position movement and optical path correction during the testing process.

[0047] Photodetector: used for calibrating the spot size and intensity distribution in small spot measurement mode, and measuring the light power intensity after the pinhole aperture.

[0048] The aforementioned device can generate a near-diffraction-limited small light spot. To align this spot with the focal plane of the single-photon image being measured, precise positioning of the small light spot is required. This invention employs an image-based autofocus algorithm to accurately locate the small light spot, uses a sharpness function to evaluate the sharpness of the current image, and then uses a search algorithm to find a sharp image to complete focusing. The process is as follows: Figure 2 As shown, the longitudinal self-focusing positioning process is as follows: Figure 3 As shown.

[0049] The specific location process is as follows:

[0050] ① Control the displacement stage at the bottom of the device under test to move the device upward a certain distance, and monitor the size of the light spot on the target surface through the detection camera. Stop when the light spot increases significantly.

[0051] ② Control the displacement stage to move downward in equal steps, record the moving distance L, and stop when the spot size first decreases and then increases again. In conjunction with the detection camera, collect the image of the spot focused by the objective lens on the target surface during each displacement, and transmit the image information to the control unit for sharpness evaluation.

[0052] ③ Calculate the focus position based on the sharpness evaluation, and control the displacement stage to move to the calculated position C;

[0053] ④ Control the displacement stage to move upward from position C to L, and then move it at equal distances with a single displacement step of L / 4, until it reaches a position L below position C. Repeat Step 2 and Step 3, and record the resulting focus position as C'.

[0054] ⑤ Calculate the difference between the height values ​​of position C and position C'. Determine whether the current image is in focus based on whether the difference is less than the positioning threshold. If it is, stop calculating the focus position and control the shift stage to move to C' and lock the Z-axis position. If it is not, repeat the above steps until the current position is determined to be the optimal focus position.

[0055] ⑥ After focusing the small light spot, measure the crosstalk of individual pixels on the single-photon focal plane.

[0056] Image sharpness assessment employs a gradient-based method, where the sharpness function is based on the first derivative of the image. Since images are represented as matrices in a computer, derivatives exist between numerical sequences. When the difference between adjacent pixels is too large, it indicates a boundary between the background and the target image. Therefore, the gradient-based sharpness function assesses image sharpness by detecting changes in these derivatives.

[0057] This paper evaluates the image sharpness using the Tenengard image sharpness function, which calculates the gradient value of a point by the difference between adjacent points in the horizontal and vertical directions. It has good unimodality and high reliability. It uses the Sobel operator to extract the gradient values ​​in the horizontal and vertical directions respectively, and calculates the sum of their squares as the evaluation function.

[0058] Let Sobel's convolution kernel G be... x G y :

[0059]

[0060] The corresponding operator matrix is:

[0061]

[0062] The gradient of image I at (x,y) is:

[0063]

[0064] The Tenengard value of this image is:

[0065]

[0066] Where n is the total number of pixels in the image.

[0067] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A Geiger-mode single-photon focal plane crosstalk measurement device, characterized in that, Includes beam expanders, deformable mirrors, beam splitters BS1 and BS2, industrial control computers, alignment cameras, observation cameras, objective lenses, and photodetectors; After passing through a beam expander and a deformable mirror, the laser emission source is split by a beam splitter BS1. The reflected light is detected by a wavefront sensor, which monitors the wavefront of the incident beam and feeds the result back to the industrial control computer. The industrial control computer controls the deformable mirror controller to adjust the deformable mirror and correct the incident wavefront to generate a high-quality plane wave. The alignment camera is used for beam alignment, and works in conjunction with the reticle on the back of the beam splitter BS1 to perform spot-target alignment and adjustment, thereby achieving optical path calibration of the focal plane of the single photon under test. After passing through the objective lens and then through the beam splitter BS2, the transmitted light illuminates the target surface of the device under test. The light reflected back from the target surface is reflected again by BS2, and the light spot is collected by the observation camera. The reflected light is incident on the photodetector with a front aperture aperture mounted on the displacement stage, so as to measure the intensity distribution and size of the detection spot.

2. The Geiger-mode single-photon focal plane crosstalk measurement device according to claim 1, characterized in that, After passing through a beam expander, the laser beam is reflected by a deformable mirror at a 45° angle to the incident optical axis.

3. The Geiger-mode single-photon focal plane crosstalk measurement device according to claim 1, characterized in that, The wavefront sensor collects the wavefront of the light beam, calculates the beam aberration coefficient through a convolutional neural network, controls the shape of the deformable mirror through the deformable mirror controller, corrects the aberrations in the optical path, and forms an adaptive optical path aberration correction closed loop.

4. A method for measuring crosstalk in Geiger mode single-photon focal plane array, characterized in that, Includes the following steps: S1. Control the displacement stage at the bottom of the device under test to move the device upward a certain distance, and monitor the size of the light spot on the target surface through the observation camera. Stop when the light spot increases significantly. S2. Control the displacement stage to move downward in equal steps, record the moving distance L, until the spot size first decreases and then increases again, and stop. In conjunction with the observation camera, collect the image of the spot focused by the objective lens on the target surface during each displacement, and transmit the image information to the control unit for sharpness evaluation. S3. Calculate the focus position based on the sharpness evaluation, and control the shift stage to move to the calculated position C; S4. Control the displacement stage to move upward from position C to L, and then move it at equal distances with a single displacement step of L / 4, until it reaches a position L below position C. Repeat the S2-S3 operation, and record the resulting focus position as C'. S5. Calculate the difference between the height values ​​of position C and position C'. Determine whether the current image is in focus based on whether the difference is less than the positioning threshold. If it is, stop calculating the focus position and control the shift stage to move to C' and lock the Z-axis position. If it is not, repeat the above steps until the current position is determined to be the optimal focus position. S6. After focusing the small light spot, measure the crosstalk of a single pixel on the single-photon focal plane.

5. The Geiger mode single-photon focal plane crosstalk measurement method according to claim 4, characterized in that, The image sharpness assessment is calculated using an image gradient-based method, with the following steps: The Sobel operator is used to extract the gradient values ​​in the horizontal and vertical directions respectively, and the sum of their squares is used as the evaluation function. Let Sobel's convolution kernel G be... x G y : The corresponding operator matrix is: The gradient of image I at (x,y) is: The Tenengard value of this image is: Where n is the total number of pixels in the image.

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