A confocal optical path pixel difference-based focal plane detection method and device

Abstract

This invention discloses a focal plane detection method based on pixel difference in a confocal optical path. This method utilizes two image sensors positioned at the front and back focal points in a confocal differential optical path. By differentiating the defocused spot shapes acquired by the two image sensors and combining this with post-processing, real-time detection, tracking, and compensation of the focal plane are achieved. A focal plane detection device based on pixel difference in a confocal optical path is also disclosed. This device includes a light source, a filtering and collimating device, a half-wave plate, a polarizing beam splitter, a quarter-wave plate, an objective lens, and a sample, arranged sequentially along the central optical axis of the optical system. A converging lens, a beam splitter, and two image sensors are also arranged coaxially with the polarizing beam splitter. Compared with other existing focal plane detection methods, this invention has the advantages of simple device design, strong anti-interference capability, and the ability to achieve high-precision focal plane tracking, positioning, and compensation.

Description

Technical Field

[0001] This invention relates to the field of optical precision testing technology, and in particular to a focal plane detection method and apparatus based on confocal optical path pixel difference, which can be used for focal plane tracking compensation in optical systems. Background Technology

[0002] Confocal microscopy, with its unique slicing capabilities, enables axial scanning and 3D imaging of sample planes and has been widely applied in optical processing, engineering physics, biomedical engineering, and precision measurement. However, traditional confocal techniques still have some shortcomings, such as low signal-to-noise ratio, poor anti-interference ability, and measurement accuracy easily affected by light source intensity, ambient lighting, and the reflective properties of the measured surface. Furthermore, the inherent mechanical instability and environmental vibrations during measurement inevitably introduce axial drift of the object's position, resulting in poor linearity and the absence of an absolute zero point. Therefore, in the increasingly mature field of micro- and nanostructure detection, traditional confocal microscopy can no longer meet the high-resolution requirements of contemporary scientific research for optical measurements.

[0003] Compared to traditional confocal lasers, differential confocal lasers differentiate the signals from two detectors, offering advantages such as strong focus discrimination, high sensitivity, strong anti-interference capability, and absolute zero-point accuracy, enabling bipolar absolute tracking measurements. Currently, most laser differential confocal scanning imaging optical paths commonly employ the energy detection method, where the confocal beam passes through pinholes before being received by detectors (photomultiplier tubes). The two detector pinholes are axially offset and symmetrically placed relative to the focal point of the converging lens. The position of the objective lens is controlled by piezoelectric ceramic drive. When the sample surface is at the focal plane, the diffraction spot intensity extrema obtained by the confocal beam through the two pinholes are equal, resulting in zero differential signal and precise focusing on the sample surface. However, the intensity maxima and defocusing amount in the pinhole diffraction model exhibit a nonlinear relationship, making this method heavily reliant on the accuracy of the pinhole placement. Furthermore, the entire optical path system suffers from drawbacks such as a narrow linear range and difficulty in adjustment. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a focal plane detection method based on confocal optical path pixel difference. This method belongs to a type of differential confocal detection technology and possesses the advantages of strong focus discrimination capability, high sensitivity, and strong anti-interference capability of differential confocal technology. Furthermore, it avoids the nonlinearity problem of diffraction intensity extreme value changes by eliminating the need for pinholes. An image sensor (CCD) is used as the detector, and focusing is achieved by differentially analyzing the shape of the light spot on the detector. In addition, this method allows for flexible setting of system optical parameters according to the required focusing accuracy, enabling both large-area coarse focusing and small-area fine focusing. This invention also designs a focal plane detection device based on confocal optical path pixel difference to implement this method.

[0005] The specific technical solution of the present invention is as follows: A focal plane detection method based on confocal optical path pixel difference involves a parallel light beam illuminating the sample surface through an objective lens. After being reflected by the sample surface, the parallel light beam passes through the objective lens again. The light beam passing through the objective lens is then focused by a converging lens and received by two image sensors, CCD1 and CCD2. The position of the sample plane was changed multiple times, and the radius of the light spot r1 received by image sensor CCD1 and the radius of the light spot r2 received by image sensor CCD2 were measured each time. The values ​​were then substituted into formula (1) to calculate the defocus amount. The optical path length L between the converging lens and the objective lens; and the difference between r1 and r2 to obtain the differential signal. The value; based on the obtained multiple sets of r1, r2, and Data, for and By fitting the data, the difference signal is obtained. With defocus The correspondence; (1) In the formula, The effective focal length of the objective lens is... M is the focal length of the converging lens, and M is the defocusing amount. The maximum value of r0 is the spot radius incident on the principal plane of the objective lens; The sample is controlled to move to a specific position on the sample plane obtained based on the defocus amount, so as to adjust the defocus amount and achieve focus compensation.

[0006] Furthermore, M is a manually selected value, and the defocus amount... The range is If the spot size on CCD1 and CCD2 is smaller than the target size of the CCD, then continue with the next steps; otherwise, select M again.

[0007] Furthermore, CCD1 and CCD2 are located on both sides of the focal point of the converging lens, and both CCD1 and CCD2 can receive the light beam focused by the converging lens.

[0008] Furthermore, when the focal point of the objective lens is located between the sample and the objective lens, the defocusing amount... The value is positive; when the focal point of the objective lens is located on the sample surface, the defocusing amount is positive. The value is 0; otherwise, the defocus amount is... It is negative.

[0009] Further, adjust the defocus amount. The optical path is 0. At this point, a four-focal-length optical path is obtained, and the differential signal approaches 0.

[0010] A focal plane detection device based on confocal optical path pixel difference for implementing the aforementioned focal plane detection method based on confocal optical path pixel difference includes: a light source, a filtering and collimating device, a half-wave plate, a polarizing beam splitter, a quarter-wave plate, an objective lens, a sample, a converging lens, a beam splitter, an image sensor one, and an image sensor two. The light source, filtering and collimating device, half-wave plate, polarizing beam splitter, quarter-wave plate, objective lens, and sample are arranged coaxially in sequence; the polarizing beam splitter, converging lens, and beam splitter are arranged coaxially in sequence, and this axis is perpendicular to the optical axis of the objective lens. The light source is used to emit light, the filtering and collimating device is used to convert the initial light into a parallel flat-top beam, the half-wave plate and polarizing beam splitter are used to adjust the light intensity of the parallel flat-top beam and convert it into horizontally linearly polarized light; the quarter-wave plate is used to change the polarization direction of the horizontally linearly polarized light so that the resulting vertically linearly polarized light can undergo total internal reflection at the polarizing beam splitter; the converging lens is used to focus the beam, and the beam splitter divides the beam into two paths, which are received by image sensor one and image sensor two, respectively.

[0011] Furthermore, it also includes a three-dimensional high-precision stage, on which the sample is fixedly mounted. The three-dimensional high-precision stage drives the sample to perform three-dimensional movement to adjust the defocus amount and achieve focus compensation.

[0012] Furthermore, the objective lens is selected with a numerical aperture greater than 0.95 and a magnification greater than 50 to obtain higher focusing sensitivity.

[0013] Furthermore, the radius of the light spot captured by image sensor one and image sensor two is the full width at half maximum (FWHM) of the light spot intensity.

[0014] Furthermore, due to the diffraction of light during the propagation of the parallel flat-top beam, the differential signal is processed using an image processing algorithm to optimize the spot signals obtained by image sensor one and image sensor two.

[0015] The beneficial effects of this invention are: (1) Although the optical path used in the method of the present invention is also a confocal optical path, the use of pinholes is avoided during detection, which reduces the influence of light diffraction. The device structure is simple and the debugging is more convenient. (2) The method of the present invention fully combines the high sensitivity advantage of confocal focusing and the wide range of astigmatic focusing, and can increase the focusing range without sacrificing sensitivity; (3) The optical path used in this invention does not employ complex optical field modulation devices, such as diffractive optical elements or spatial light modulators, which greatly improves the stability and anti-interference capability of the system. (4) The present invention can flexibly select the optical parameters of the objective lens according to the focusing accuracy required by the system, and can achieve coarse focusing over a large range or fine focusing over a small range. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the focal plane detection device based on confocal optical path pixel difference according to the present invention.

[0017] Figure 2 This is a flowchart of the focal plane detection method based on confocal optical path pixel difference according to the present invention.

[0018] Figure 3 The optical path diagrams for calculating the image sensor position parameters u1 and u2 based on the focusing range ±M required by the system design are shown. (a) is the optical path diagram for calculating parameter u2 when the defocus amount is +M, and (b) is the optical path diagram for calculating parameter u2 when the defocus amount is -M.

[0019] Figure 4 These are optical path diagrams of the defocus state after determining the positions of the two image sensors CCD, where (a) is the optical path diagram of the positive defocus state and (b) is the optical path diagram of the negative defocus state.

[0020] Figure 5 This is a schematic diagram of the light spots and differential signals detected by the two image sensors (CCD) after the position is determined.

[0021] Figure 6 Based on the information provided in this embodiment regarding the spot radii r1, r2, and differential signal With defocus Relationship diagram.

[0022] In the figure, there are: light source 1, condenser lens 2, pinhole 3, collimating lens 4, half-wave plate 5, polarizing beam splitter prism 6, quarter-wave plate 7, objective lens 8, sample 9, three-dimensional high-precision stage 10, converging lens 11, beam splitter prism 12, image sensor one 13, and image sensor two 14. Detailed Implementation

[0023] The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. The objectives and effects of the present invention will become clearer as a result. The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0024] like Figure 1The diagram shown is a schematic of the focal plane detection device based on confocal optical path pixel difference according to the present invention. It includes: a light source 1, a condenser lens 2, a pinhole 3, a collimating lens 4, a half-wave plate 5, a polarizing beam splitter 6, a quarter-wave plate 7, an objective lens 8, a sample 9, a three-dimensional high-precision stage 10, a converging lens 11, a beam splitter 12, an image sensor 13, and an image sensor 2 14. Both image sensors are CCD sensors; image sensor 13 is CCD1, and image sensor 2 14 is CCD2.

[0025] The light source 1, condenser lens 2, pinhole 3, collimating lens 4, half-wave plate 5, polarizing beam splitter 6, quarter-wave plate 7, objective lens 8, and sample 9 are arranged coaxially, with sample 9 fixedly mounted on the three-dimensional high-precision stage 10. The polarizing beam splitter 6, converging lens 11, and beam splitter 12 are arranged coaxially, with this axis perpendicular to the optical axis of objective lens 8. Image sensor 13 and image sensor 14 are respectively arranged on the two beam paths after passing through beam splitter 12. In this embodiment, the three-dimensional high-precision stage 10 is made of piezoelectric ceramic (PZT).

[0026] Objective lens 8 is selected with a numerical aperture greater than 0.95 and a magnification greater than 50 to obtain higher focusing sensitivity. The radius of the light spot captured by image sensor 13 and image sensor 24 is the full width at half maximum (FWHM) of the light spot intensity in the experiment.

[0027] Light emitted from light source 1 passes sequentially through condenser lens 2, pinhole 3, and collimating lens 4, undergoing preprocessing (i.e., pinhole filtering and collimation) to obtain a parallel flat-top beam. This flat-top beam then passes sequentially through half-wave plate 5 and polarizing beam splitter 6 to obtain horizontally polarized light. The intensity of this horizontally polarized light can be adjusted by rotating half-wave plate 5. The horizontally polarized light then passes sequentially through quarter-wave plate 7 and objective lens 8 to reach the surface of sample 9. After surface reflection, it returns along the same path and passes sequentially through objective lens 8 and quarter-wave plate 7 to obtain vertically polarized light. This vertically polarized light is completely reflected by polarizing beam splitter 6, focused by converging lens 11, and then split by beam splitter prism 12. The two split beams are then emitted onto the receiving target surfaces of image sensor 13 and image sensor 14, respectively, and received by image sensor 13 and image sensor 14.

[0028] like Figure 2 As shown, the focal plane detection method based on confocal optical path pixel difference of the present invention is implemented using the above-mentioned focal plane detection device based on confocal optical path pixel difference. The specific steps are as follows: Step 1: Determine the exact locations of the two image sensors.

[0029] like Figure 3 As shown, arrange as follows Figure 1 After assembling the optical system as shown, let the distance between the focal point of objective lens 8 and sample 9 be the defocusing amount. When the focal point of objective lens 8 is above sample 9, the defocusing amount Positive; when the focal point of objective lens 8 is below sample 9, the defocusing amount is... If negative, design defocus amount The range is .

[0030] Step 2: Determine the specific locations of the two image sensors.

[0031] Let the optical path length between objective lens 8 and converging lens 11 be The distances between the image sensor 13, the image sensor 2 14 and the image-side focal point of the converging lens 11 are respectively (After burning) and (Focal front); The radius of the light spot incident on the principal plane of objective lens 8 is The radius of the light spot incident on the principal plane of the converging lens 11 is The radius of the defocused spot received by image sensor 13 is The radius of the defocused spot received by image sensor 214 is Among them, L, , , , One is a known quantity, and the rest are unknown quantities.

[0032] The relationship between the focal length of objective lens 8 and converging lens 11 and the object distance and image distance is as follows: (1) In the formula, Let be the effective focal length of objective lens 8, which is a known quantity; The object distance of objective lens 8 is... Let be the image distance of objective lens 8, and be an unknown quantity. Let be the focal length of converging lens 11, which is a known quantity; The object distance of converging lens 11, The image distance of converging lens 11 is denoted as , which is an unknown quantity.

[0033] like Figure 3 As shown in (a), when the defocus measurement is taken as the maximum positive defocus, that is At this time, image sensor 13 should be located at the front focal point where light passes through converging lens 11. The radius of the light spot received by image sensor 13 at this time is... The object distance of objective lens 8 According to geometric relationships, we can obtain: (2) In the formula, , Combining equations (1) and (2), we obtain the parameters. and The relational expression is: (3) Similarly, such as Figure 3 As shown in (a), when the defocus measurement is taken as the maximum negative defocus, that is At that time, the geometric relationship between the parameters is as follows: (4) In the formula, , Combining equations (1) and (4), the parameters are obtained. and The relational expression is: (5) The optical path between objective lens 8 and converging lens 11 It approaches the sum of the focal lengths of objective lens 8 and converging lens 11. At that time, the position parameters of image sensor 13 and image sensor 14 satisfy At this point, objective lens 8 and converging lens 11 form a four-focal-length optical path structure. The positions of image sensor 13 and image sensor 24 are symmetrical about the image-side focal point of converging lens 11.

[0034] Step 3: If the spot size on CCD1 and CCD2 is smaller than the CCD target size, proceed to Step 4; otherwise, repeat Step 1 and Step 2.

[0035] Step 4: Solve for the relationship between the differential signal and the defocus amount.

[0036] like Figure 4 As shown, after determining the specific positions of image sensor 13 and image sensor 14, the defocus amount can be used to... Based on the simplified optical path diagram within the range, and according to formulas (1) to (5), the spot radii received by image sensor 13 and image sensor 214 are obtained. and With defocus The relationship between them satisfies the following formula: (6) Substituting formulas (3) and (5) into formula (6), we can simplify to obtain... and They are respectively: (7) The differential signal can then be obtained. With defocus The relational expression is as follows: (8) When the focal point of objective lens 8 is located above the plane of sample 9, the defocusing amount satisfy The radius of the light spot on the image sensor 13 Larger than the spot radius on image sensor 214 At this time, the differential signal It is a positive value; when the focal point of objective lens 8 is below the plane of sample 9, the defocusing amount is... satisfy The radius of the light spot on the image sensor 13 Smaller than the spot radius on image sensor 214 At this time, the differential signal It is a negative value; when the focal point of objective lens 8 is located on the plane of sample 9, the defocusing amount is... satisfy At this time, the differential signal It can be simplified to: (9) At this time, the differential signal The positive or negative sign depends on the optical path length between objective lens 8 and converging lens 11. The sum of the focal lengths of objective lens 8 and converging lens 11 The size. If Then the differential signal If it is a positive value; Then the differential signal It is zero; if Then the differential signal It is a negative value.

[0037] Change the position of the sample plane (satisfying) The light spot radius was detected respectively. and Substituting into formula (7), the defocus amount is calculated. and optical path The value, and for and To do the work, one gets The value of . Record multiple experimental data to obtain multiple sets. , , and Data, for and By fitting the data, the difference signal is obtained. With defocus The correspondence.

[0038] In practical applications, the defocus amount is obtained based on the relationship between the differential signal and the defocus amount, and then the specific position of the sample plane is obtained based on the defocus amount. The defocus amount is adjusted using the three-dimensional high-precision workpiece stage 10 to achieve focus compensation.

[0039] To facilitate the calibration of the entire optical system, the differential signal... To get as close to zero as possible, the device typically uses a four-focal-length optical path, i.e. .

[0040] During the propagation of a parallel flat-top beam, due to the unavoidable diffraction of light, the processing of the differential signal includes, but is not limited to, using image processing algorithms to optimize the spot signals obtained by image sensor 13 and image sensor 24.

[0041] The present invention will now be described with reference to specific embodiments.

[0042] Example 1 Light source 1 is a laser source with a radius of 1 mm and a wavelength of 632.8 nm. After being filtered and collimated by a pinhole, the radius is obtained. The collimated flat-top beam; the half-wave plate 5 is a 1-inch zero-order half-wave plate WPQ-6328-2M; the polarizing beam splitter prism 6 is a 1-inch polarizing beam splitter cube PBS251; the quarter-wave plate 7 is a 1-inch zero-order quarter-wave plate WPQ-6328-4M; the effective focal length of the objective lens 8 is... Converging lens 11 selects a focal length The plano-convex lens; the beam splitter prism 12 is a non-polarizing beam splitter cubic prism MBS1445-A; both image sensor 13 and image sensor 14 are MV-CE200-11UM models. The defocus range is: .

[0043] like Figure 5 The image shows the light spots and corresponding differential signals detected by image sensor 13 and image sensor 14 under different defocus conditions. .

[0044] like Figure 6 As shown in (a), this is the spot radius calculated based on known parameters. and With defocus Correspondence curves; such as Figure 6 As shown in (b), this is the differential signal calculated based on known parameters. With defocus The corresponding curves show that the differential signal In the defocus range of the system design In-focus and defocus amount The differential signal exhibits a linear correlation within a defocus range of ±5μm. It changed by 2mm. From Figure 6 The data provided shows that there are 2,000 pixel changes in the light spot pixels within the 10μm focus detection range, meaning that a single pixel corresponds to a sensitivity of 5nm. Simulation data proves that the present invention can achieve high-precision focus plane tracking, positioning, and compensation.

[0045] It will be understood by those skilled in the art that the above descriptions are merely preferred examples of the invention and are not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. All modifications and equivalent substitutions made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A confocal optical path pixel difference based focal plane detection method, characterized in that, A parallel light beam is irradiated onto the sample surface through an objective lens. After being reflected by the sample surface, the parallel light beam passes through the objective lens again. The light beam passing through the objective lens is then focused by a converging lens and received by two image sensors, CCD1 and CCD2. The position of the sample plane is changed many times, and the spot radius r1 received by the image sensor CCD1 and the spot radius r2 received by the image sensor CCD2 are measured each time, which are substituted into formula (1) respectively to calculate the defocus amount , the optical path L between the converging lens and the objective lens; and the difference between r1 and r2 is obtained to obtain the value of the difference signal According to the obtained groups of r1, r2, and data, the and are fitted to obtain the corresponding relationship between the difference signal and the defocus amount . （1） In the formula, The effective focal length of the objective lens is... M is the focal length of the converging lens, and M is the defocusing amount. The maximum value of r0 is the spot radius incident on the principal plane of the objective lens; The sample is controlled to move to a specific position on the sample plane obtained based on the defocus amount, so as to adjust the defocus amount and achieve focus compensation.

2. The focal plane detection method based on confocal optical path pixel difference according to claim 1, characterized in that, M is a manually selected value, and the defocus amount The range is If the spot size on CCD1 and CCD2 is smaller than the target size of the CCD, then continue with the next steps; otherwise, select M again.

3. The focal plane detection method based on confocal optical path pixel difference according to claim 1, characterized in that, The CCD1 and CCD2 are located on either side of the focal point of the converging lens, and both CCD1 and CCD2 can receive the light beam focused by the converging lens.

4. The focal plane detection method based on confocal optical path pixel difference according to claim 1, characterized in that, When the focal point of the objective lens is located between the sample and the objective lens, the defocusing amount The value is positive; when the focal point of the objective lens is located on the sample surface, the defocusing amount is positive. The value is 0; otherwise, the defocus amount is... It is negative.

5. The focal plane detection method based on confocal optical path pixel difference according to claim 1, characterized in that, Adjust defocus amount The optical path is 0. At this point, a four-focal-length optical path is obtained, and the differential signal approaches 0.

6. A focal plane detection device based on confocal optical path pixel difference for implementing the focal plane detection method based on confocal optical path pixel difference as described in any one of claims 1 to 5, characterized in that, include: Light source (1), filtering collimation device, half-wave plate (5), polarizing beam splitter (6), quarter-wave plate (7), objective lens (8), sample (9), converging lens (11), beam splitter (12), image sensor one (13), image sensor two (14); The light source (1), the filtering and collimating device, the half-wave plate (5), the polarizing beam splitter (6), the quarter-wave plate (7), the objective lens (8), and the sample (9) are arranged coaxially in sequence; the polarizing beam splitter (6), the converging lens (11), and the beam splitter (12) are arranged coaxially in sequence, and the axis is perpendicular to the optical axis of the objective lens (8); The light source (1) is used to emit light, the filtering and collimating device is used to convert the initial light into a parallel flat-top beam, the half-wave plate (5) and the polarizing beam splitter (6) are used to adjust the light intensity of the parallel flat-top beam and convert the parallel flat-top beam into horizontally polarized light; the quarter-wave plate (7) is used to change the polarization direction of the horizontally polarized light so that the resulting vertically polarized light can undergo total internal reflection in the polarizing beam splitter (6); the converging lens (11) is used to focus the beam, and the beam splitter (12) splits the beam into two paths, which are received by the image sensor one (13) and the image sensor two (14) respectively.

7. The focal plane detection device based on confocal optical path pixel difference according to claim 6, characterized in that, It also includes a three-dimensional high-precision stage (10), on which the sample (9) is fixedly mounted. The three-dimensional high-precision stage (10) drives the sample (9) to perform three-dimensional motion to adjust the defocus amount and achieve focus compensation.

8. The focal plane detection device based on confocal optical path pixel difference according to claim 6, characterized in that, The objective lens (8) is selected with a numerical aperture greater than 0.95 and a magnification greater than 50 to obtain higher focusing sensitivity.

9. The focal plane detection device based on confocal optical path pixel difference according to claim 6, characterized in that, The radius of the light spot captured by the image sensor one (13) and the image sensor two (14) is the full width at half maximum (FWHM) of the light spot intensity.

10. The focal plane detection device based on confocal optical path pixel difference according to claim 6, characterized in that, During the propagation of the parallel flat-top beam, due to the diffraction of light, the differential signal is processed using an image processing algorithm to optimize the spot signals obtained by the image sensor one (13) and the image sensor two (14).

Images