Spatial light modulator multi-focus adaptive generation method based on camera feedback

Abstract

The invention discloses a spatial light modulator multi-focus adaptive generation method based on camera feedback. The device for realizing the method comprises a laser light source, a light beam expanding and energy adjusting module, a spatial light modulator, a focusing / imaging optical system, a camera for collecting a focal plane, and a control calculation unit connected with the spatial light modulator and the camera. After the control calculation unit loads the initial phase hologram, the camera is triggered to collect a multi-focus image, focus positioning and intensity measurement are completed, a multi-focus uniformity index is calculated, a correction factor is generated, target amplitude is updated through weight constraint, a new phase hologram is generated through iteration, and the new phase hologram is written back to the spatial light modulator. After the intensity uniformity reaches the standard, the uniformity of the full width at half maximum of the light spot is used as secondary discrimination, so that a stable homogenization effect is kept under the conditions of system aberration, non-ideal device, environment drift and the like, and the method is suitable for femtosecond two-photon polymerization micro-nano processing, parallel laser writing, optical tweezers and the like.

Description

Technical Field

[0001] This invention relates to the fields of computational holography and laser micro / nano fabrication technology, and in particular to a multi-focus uniformity iterative optimization device and method based on camera measurement feedback. Background Technology

[0002] Using spatial light modulators (SLMs) to load phase holograms to generate multifocal arrays can extend single-point serial processing to multi-point parallel processing, offering significant efficiency advantages in scenarios such as femtosecond two-photon polymerization, direct writing of micro / nano structures, optical tweezers, and parallel optical stimulation. Existing multifocal generation typically relies on Gerchberg-Saxton (GS)-like algorithms or their improvements to optimize the phase hologram in the computational domain, assuming ideal optical systems and consistent device responses. However, in real-world systems, factors such as non-ideal SLM pixel responses, optical path aberrations, amplitude inhomogeneity, scattering, and drift can cause deviations in the intensity distribution of the multifocal points on the focal plane from the calculated results. This manifests as large energy differences and poor stability among the focal points, thus affecting processing consistency and the process window. Therefore, a scheme is urgently needed that can iteratively correct the phase hologram using actual measurement results, enabling adaptive uniformity of multifocal energy distribution in real-world systems, and possessing characteristics such as ease of implementation, scalability, and robustness to noise and drift. Summary of the Invention

[0003] In view of this, the purpose of the present invention is to provide a multifocal uniformity iterative optimization device and method based on camera measurement feedback, so as to at least partially solve the problems in the prior art that multifocal energy is difficult to be uniform in real systems and is sensitive to non-ideal factors of the system.

[0004] To achieve the above objectives, the present invention provides a multi-focus homogenization iterative optimization device, comprising: a light source incident optical path system, a spatial light modulator, a focusing / imaging optical module, a camera module, a control computing unit, and a sample processing system.

[0005] The incident optical path system of the light source includes a laser, an attenuator, and a beam expander. The attenuator is used to attenuate the energy of the laser beam emitted from the laser, and the beam expander is used to expand the attenuated beam.

[0006] Spatial light modulators are used to load phase holograms to generate multifocal light fields.

[0007] The camera module is used to acquire images of the multifocal light field on the focal plane or equivalent image plane.

[0008] The control calculation unit is used to calculate the multifocal uniformity based on the camera measurement results and iteratively update the phase hologram to make the intensity distribution of the multifocals tend to be uniform. Preferably, the control calculation unit can perform iterative optimization according to the following process: (1) trigger the camera to acquire the current multifocal image and convert it into a grayscale image; (2) locate the peak position in the grayscale image that is consistent with the number of focal points, and determine the ROI region of each focal point by combining the nominal coordinates and the camera coordinate mapping; (3) extract the focal intensity in each ROI, and the intensity extraction can be carried out by Gaussian weighted summation or energy aperture integration, etc.; (4) calculate the uniformity index based on the intensity. Preferred (5) Generate correction factors / weighting factors based on the deviation of the relative mean (or target intensity) of each focal intensity, and update the target amplitude constraint accordingly; (6) Iteratively generate a new phase hologram based on the updated target amplitude constraint and load it into the spatial light modulator, repeating the above process until U reaches the threshold. After the intensity uniformity meets the standard, the full width at half maximum (FWHM) uniformity of each focal point can be further calculated and used as a secondary discrimination condition.

[0009] Furthermore, the control computing unit can perform single-point sequential calibration: by sequentially generating single-focus holograms, the actual positions of each nominal focus are obtained in the camera image, thereby establishing a mapping relationship between nominal coordinates and camera coordinates, which is then used for ROI tracking in subsequent iterations to improve the stability of multi-focus localization.

[0010] Furthermore, the control calculation unit can perform segmented exposure reduction (or gain reduction) verification after the uniformity is met: each time the exposure / gain is reduced only once and the uniformity is immediately retested. If it is not met, the exposure / gain is maintained and iterated until it is met again, thereby reducing the risk of saturation and improving the credibility of the evaluation.

[0011] The sample processing system may include a 4f system, a high numerical aperture objective lens, and a high-precision three-dimensional electrically controlled displacement stage; wherein the 4f system is used to block the zero-order light generated by the spatial light modulator, the high numerical aperture objective lens is used to perform Fourier transform on the phase hologram and form a focal array on the focal plane, and the displacement stage is used to carry the sample and realize position adjustment or scanning processing.

[0012] Based on the above technical solutions, the present invention has at least one of the following beneficial effects: (1) Using actual camera measurements as feedback, it can compensate for the deviation caused by system aberrations and device non-ideals, and realize the adaptive uniformity of multi-focus energy in the real system; (2) It does not rely on additional wavefront sensors, the hardware implementation is simple, and it is compatible with different light sources, different focusing systems and different focal number configurations; (3) Through strategies such as saturation detection, segmented exposure reduction verification and full width at half maximum (FWHM) secondary discrimination, the robustness of uniformity evaluation and the consistency of final focus quality are improved. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the optical path structure of the device of the present invention;

[0014] Figure 2 This is a schematic flowchart of the method of the present invention;

[0015] Figure 3 This is a schematic diagram comparing phase holograms and multifocal focal plane images, including a comparison of unoptimized and optimized focal distributions.

[0016] Explanation of reference numerals in the attached figures

[0017] 1. Laser; 2. First mirror; 3. Second mirror; 4. Attenuator; 5. Beam expander optical assembly; 6. Third mirror; 7. Spatial light modulator; 8. First lens; 9. Aperture; 10. Fourth mirror; 11. Fifth mirror; 12. Second lens; 13. Dichroic mirror; 14. 100× objective lens; 15. Sample; 16. Displacement stage; 17. CCD camera; 18. Computer (controlling CCD and SLM). Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described below with reference to the accompanying drawings. It should be understood that the following embodiments are only for explaining the invention and are not intended to limit the scope of protection of the invention.

[0019] like Figure 1 As shown, in an example device implementation, the multi-focus homogenization iterative optimization device of the present invention includes a laser 1, a mirror 2, a mirror 3, an attenuator 4, a beam expander optical assembly 5, a mirror 6, a spatial light modulator 7, a first lens 8, an aperture 9, a fourth mirror 10, a fifth mirror 11, a second lens 12, a dichroic mirror 13, an objective lens 14, a sample 15, a displacement stage 16, a CCD camera 17, and a computer 18. The laser beam output from the laser 1 is guided by mirrors 2 and 3, then its energy is adjusted by the attenuator 4, and expanded by the beam expander optical assembly 5. The expanded beam is then incident on the spatial light modulator 7 via mirror 6. The spatial light modulator 7 is preferably a reflective phase modulation device used to load a phase hologram.

[0020] The modulated light beam passes sequentially through the first lens 8 and the aperture stop 9 to form a spatial filter / aperture limit, and is guided to the dichroic mirror 13 and the objective lens 14 via a relay optical path composed of the fourth reflecting mirror 10, the fifth reflecting mirror 11, and the second lens 12. The objective lens 14 performs a Fourier transform on the phase hologram, forming a multifocal array on the focal plane of the sample 15 to achieve parallel illumination or processing. The dichroic mirror 13 is used to couple the intensity distribution of the focal plane (or equivalent image plane) to the CCD camera 17, which transmits the acquired image to the computer 18. The computer 18 is communicatively connected to the spatial light modulator 7 and the CCD camera 17, and is used to perform hologram generation, image acquisition triggering, positioning measurement, and uniformization iterative optimization.

[0021] like Figure 2 As shown, in an example implementation, the multi-focus homogenization iterative optimization method of the present invention can be performed according to the following steps: S1: In computer 18, set the nominal coordinates (or array parameters) of the target focus array and the initial target amplitude, and set the uniformity threshold. and maximum number of iterations ; S2: Based on the initial target amplitude, an initial phase hologram is generated using the GS algorithm or its improved algorithm, and the phase hologram is loaded into the spatial light modulator 7;

[0022] S3: Trigger the CCD camera 17 to acquire a multifocal image of the focal plane or equivalent image plane, and convert the image into a grayscale image;

[0023] S4: Perform focus localization and intensity measurement in the grayscale image, calculate the uniformity index U, and generate a correction factor.

[0024] S5: Update the target amplitude / weight constraint based on the correction factor and re-iterate to generate the phase hologram. Return to step S2 and repeat until U reaches the threshold or the termination condition is met.

[0025] In a preferred implementation of step S4, focus localization can employ a "global peak finding + local ROI confirmation" approach: first, peak candidates matching the number of focus points are identified across the entire grayscale image; then, based on the nominal coordinates-camera coordinate mapping (if established), a corresponding ROI is assigned to each focus point, and the brightest pixel within the ROI is further searched as a local peak. The ROI can be set as a fixed window covering the focus energy, or the ROI center can be adaptively updated according to the drift situation.

[0026] In a preferred implementation of step S4, the intensity of each focal point A Gaussian weighted method can be used for calculation: Centered on local peak values, multiply the grayscale values ​​within the ROI by a two-dimensional Gaussian kernel and sum them. Alternatively, an energy aperture integration method can be used to accumulate pixel grayscale values ​​within a given radius. Subsequent statistics and Calculate the uniformity of intensity .

[0027] In a preferred implementation of step S5, a correction factor can be generated based on the measured intensity, and the target amplitude constraint can be updated. For example, let the measured intensity of the i-th focus be... The mean intensity of all focal points is By introducing a small constant ε to avoid division by zero, and with the update exponent being α, the correction factor can be taken as... The updated target amplitude is )^α; ( ), and for { Normalization is performed to maintain the total energy constraint. To reduce noise-induced jumps, [the following can be done]: or Introduce a smoothing term or an exponential moving average.

[0028] In one alternative implementation, when the intensity uniformity U reaches a preset threshold Then, computer 18 can trigger a segmented reduction verification of camera exposure time: the exposure is reduced only once each time and the uniformity is immediately retested; if the retested uniformity is lower than the threshold, the exposure parameter is maintained and steps S2-S5 are continued until the standard is met again, and then the next reduction verification is performed to reduce the risk of saturation and improve the credibility of the assessment.

[0029] In an optional implementation, once the intensity uniformity U meets the standard, the full width at half maximum (FWHM) of each focal spot can be further calculated and its uniformity evaluated. For example, an intensity profile can be calculated along the x and y directions within each focal spot's ROI to obtain the corresponding FWHM, or an equivalent FWHM can be obtained by fitting a two-dimensional light spot; the FWHM uniformity can then be calculated accordingly. .when When the intensity is below the threshold, the iteration can continue by adjusting the update index α, the smoothing coefficient, or introducing morphology-related constraints to avoid the situation of "uniform intensity but large differences in focal morphology".

[0030] like Figure 3 As shown, Figure 3 (a) is a schematic diagram of a phase hologram; Figure 3 (b) is a schematic diagram of a multifocal image without measurement feedback optimization, in which the intensity of each focal point is different; Figure 3 (c) is a schematic diagram of a multifocal image after iterative optimization based on the measurement feedback of the present invention. The difference in intensity of each focal point is significantly reduced, and the focal point distribution is more consistent, thereby improving the energy consistency and stability in parallel processing or parallel lighting scenarios.

[0031] The parameters, thresholds, and update rules described in the above embodiments are all examples. Those skilled in the art can make various equivalent modifications and substitutions to the focus positioning method, intensity measurement method, uniformity index, update strategy, and calibration model without departing from the spirit of the present invention, and all such modifications and substitutions should fall within the protection scope of the present invention.

Claims

1. A spatial light modulator multi-focus adaptive generation device based on camera feedback, characterized in that, include: The system comprises a light source incident optical path system, a spatial light modulator, a focusing / imaging optical module, a camera module, a control and computing unit, and a sample processing system. The light source incident optical path system includes a laser, an attenuator, and a beam expander. The attenuator attenuates the energy of the laser beam, and the beam expander expands the attenuated beam. The spatial light modulator loads a phase hologram to generate a multifocal light field. The camera module acquires images of the multifocal light field on the focal plane or equivalent image plane. The control and computing unit calculates the multifocal uniformity based on camera measurements and iteratively updates the phase hologram to make the intensity distribution of the multifocal points more uniform. The sample processing system utilizes the focal array for illumination, manipulation, or processing.

2. The spatial light modulator multi-focus adaptive generation device based on camera feedback according to claim 1, characterized in that, The light source incident optical path system also includes at least one reflector for guiding the laser beam emitted from the laser to the beam expander and guiding the expanded beam to the spatial light modulator.

3. The spatial light modulator multi-focus adaptive generation device based on camera feedback according to claim 1, characterized in that, The phase hologram is generated by the GS algorithm, an improved GS algorithm, or a rotation angle optimization algorithm; or it is generated by calculating and custom-fabricating corresponding phase modulation elements based on the principle of grating diffraction. The control and computing unit includes an image processing module, which is used to preprocess the images acquired by the camera and complete multifocal localization and intensity extraction. The preprocessing includes: background subtraction, noise filtering, dynamic range compression and / or ROI cropping. The indices for multifocal uniformity include the ratio of the minimum to the maximum intensity of each focal point.

4. The spatial light modulator multi-focus adaptive generation device based on camera feedback according to claim 1, characterized in that, The control calculation unit updates the phase hologram in one of the following ways: generating a correction factor based on the inverse proportional correction of the measured intensity, and updating the target amplitude or weight constraint accordingly; and normalizing the target amplitude of each focal point after the update to maintain the total energy constraint.

5. The spatial light modulator multi-focus adaptive generation device based on camera feedback according to claim 1, characterized in that, The device also includes an exposure / gain control module, which is used to adaptively adjust the camera exposure time and gain according to the saturation ratio or peak intensity during the iteration process, or to perform segmented exposure reduction verification after the uniformity is achieved to check the uniformity robustness.

6. The spatial light modulator multi-focus adaptive generation device based on camera feedback according to claim 1, characterized in that, The control calculation unit further measures the full width at half maximum (FWHM) of each focal point and calculates the uniformity of the FWHM. When the intensity uniformity meets the threshold, the uniformity of half-height and full width is used as a secondary discrimination condition to determine whether to continue iterating or to modify the target constraint. The control computing unit is configured to perform a calibration process: by sequentially generating single-focus or multi-focus holograms, the actual position of the corresponding focus is obtained in the camera image, thereby establishing a mapping relationship between the nominal focus coordinates and the camera coordinates, which is then used for ROI automatic tracking and robust localization in subsequent iterations.

7. A method for adaptive multi-focus generation of spatial light modulator based on camera feedback to implement the apparatus as described in any one of claims 1-6, characterized in that, Includes the following steps: Acquire the nominal coordinates of the target at multiple focal points and the initial target amplitude; Generate an initial phase hologram and load it into the spatial light modulator; trigger the camera to acquire multifocal images and perform focus positioning and intensity measurement; calculate the uniformity index and generate a correction factor; update the target amplitude / weight constraint based on the correction factor and iteratively generate a new phase hologram; repeat the acquisition-calculation-update process until the uniformity reaches the preset threshold or the iteration termination condition is met.

8. The method according to claim 7, characterized in that, The focus localization is achieved through the following method: peak search within the ROI based on mapping relationships; and adaptive updating of the ROI center when focus drift is detected.

9. The method according to claim 7, characterized in that, The method further includes: after uniformity reaches a first threshold, reducing the camera exposure time or gain and re-measuring uniformity; if uniformity is lower than a second threshold, continuing the iteration under the current exposure / gain parameters until the threshold is met again.

10. The method according to claim 7, characterized in that, The method is applicable to any of the following applications: femtosecond two-photon polymerization, laser micro / nano fabrication, parallel exposure, or optical tweezers, and the image acquired by the camera is a focal plane intensity distribution or an intensity distribution equivalent to that on the camera via an imaging system.

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