Compact dual-wavelength common-path holographic microscope

Abstract

The application discloses a compact dual-wavelength common-path holographic microscope, which comprises two laser light sources, a collimating and expanding system, a polarizer, a first non-polarization beam splitter prism, a second non-polarization beam splitter prism, a plate-type beam splitter, a microscope objective and an image sensor. The laser beams emitted by the two laser light sources are modulated into vertically polarized laser beams and horizontally polarized laser beams through the collimating and expanding system and the polarizer respectively. The vertically polarized laser beams and the horizontally polarized laser beams are combined through the first non-polarization beam splitter prism and the second non-polarization beam splitter prism, and are separated into object light and reference light through the plate-type beam splitter. The object light irradiates a sample through the microscope objective. The reference light is reflected light of the plate-type beam splitter. The laser beams at two wavelengths are separated after passing through a polarization beam splitter prism. The reference light and the object light at each wavelength are interfered respectively, and a holographic image is recorded by the image sensor. The application has the characteristics of real-time rapidness, compact arrangement, high vibration resistance and high time stability. The reference light and the object light and the laser beams at two wavelengths are separated and angularly adjusted through simple optical elements and adjustment modes, and the application has obvious advantages for the characterization of the topography of a precise structure and high-precision dynamic imaging.

Description

Technical Field

[0001] This invention relates to the field of digital holographic imaging technology, specifically to a compact dual-wavelength common-path holographic microscope. Background Technology

[0002] Digital holography is an evolution of conventional holography, encompassing both recording and reconstruction steps. Unlike conventional holography, digital holography uses detectors such as CCDs and CMOS sensors instead of holographic plates to record and store holograms. Furthermore, digital holography reconstructs the amplitude and phase distribution of the object's optical field through numerical calculations.

[0003] Digital holographic microscopy combines digital holography and microscopic imaging techniques. It is a non-contact, high-quality, high-resolution phase imaging technique widely used to measure parameters such as cell thickness and refractive index, as well as the surface morphology of opaque structures. Digital holographic microscopy offers the following advantages:

[0004] (1) Only one image needs to be taken within the exposure time of the recording camera to obtain two pieces of information: amplitude distribution and phase distribution of the object under test;

[0005] (2) It has digital autofocus function and does not require manual mechanical focusing.

[0006] In digital holographic microscopy (DHM), when the optical length of the object under test (DUT) is greater than the wavelength of the light source, the phase distribution of the DUT will be trapped between [-π, π], a phenomenon known as phase wrapping. To obtain a continuous and accurate phase distribution, a phase unwrapping algorithm is needed to reconnect the truncated phases. However, the maximum measurable optical length between adjacent pixels of the DUT is limited to one wavelength. When the DUT does not meet this condition, such as for objects with abrupt changes in structure, the unwrapping algorithm will fail, leading to phase recovery errors. Therefore, single-wavelength holographic microscopy has limitations for measuring high-depth abrupt structures. Dual-wavelength holographic microscopy, as an extension of single-wavelength holographic microscopy, obtains an accurate phase distribution by acquiring phase maps of two wavelengths separately and subtracting them, thus expanding the longitudinal measurement range of DHM.

[0007] When dual-wavelength holographic microscopy is applied to the morphological measurement of precision structures, the phase retrieval process of the two wavelengths can amplify system noise. As a more stable and vibration-resistant optical path arrangement compared to conventional optical path systems, the combination of common-path systems and digital holography is an inevitable trend. However, in existing dual-wavelength common-path holographic microscopy optical path arrangements, the separation methods of reference light and object light and the optical elements used are relatively complex. Typically, pinhole elements are used for spatial filtering (KUMAR M, QUAN X, AWATSUJI Y, et al. Single-shot common-path off-axis dual-wavelength digital holographic microscopy[J / OL]. Applied Optics, 2020, 59(24):7144-7152.DOI:10.1364 / AO.395001.), or multiple reflective elements are used for beam separation (GUO R, WANG F. Compact and stable real-time dual-wavelength digital holographic microscopy with a long-working distance objective[J / OL]. Optics Express, 2017, 25(20):24512-24520.DOI:10.1364 / OE.25.024512; SHAN M, LIU L, ZHONG Z, et al. Single-shot dual-wavelength off-axis quasi-common-path digital holography using polarization-multiplexing[J / OL]. Optics Express, 2017, 25(21):26253-26261. DOI:10.1364 / OE.25.026253), which is not conducive to its widespread application. Summary of the Invention

[0008] The purpose of this invention is to provide a compact dual-wavelength common-path holographic microscope in which the reference light and object light, two wavelength beams, are separated and their angles adjusted through simple optical elements and adjustment methods, and the arrangement is compact and fast in real time.

[0009] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0010] A compact dual-wavelength common-path holographic microscope includes a first laser source with wavelength λ1, a second laser source with wavelength λ2, a first collimating and expanding beam system, a second collimating and expanding beam system, a first polarizer, a second polarizer, a first unpolarized beam splitter, a second unpolarized beam splitter, a plate beam splitter, a microscope objective, a polarizing beam splitter, and an image sensor. The optical path structure of the holographic microscope is as follows:

[0011] The laser beam emitted by the first laser source with a wavelength of λ1 is collimated and expanded by the first collimation and beam expanding system, and then modulated into a vertically polarized laser beam by the first polarizer.

[0012] The laser beam emitted by the second laser source with a wavelength of λ2 is collimated and expanded by the second collimation and beam expanding system, and then modulated into a horizontally polarized laser beam by the second polarizer.

[0013] The vertically polarized laser beam and the horizontally polarized laser beam are combined by the first and second unpolarized beam splitters, and then split into object light and reference light by the plate beam splitter.

[0014] The object beam is transmitted light that passes through a plate beam splitter and illuminates the sample through a microscope objective; the reference beam is reflected light that passes through a plate beam splitter; the laser beams at the two wavelengths are separated after passing through a polarizing beam splitter, and the reference beam and object beam at each wavelength interfere with each other, and the holographic image is recorded by an image sensor.

[0015] The laser source is used to emit a continuous laser beam as a detection source.

[0016] As an improvement of the present invention, the first laser source and the second laser source are single-longitudinal-mode lasers.

[0017] The combined wavelength of the two wavelengths is greater than the optical length of the sample under test.

[0018] The first and second unpolarized beam splitters are placed at 45° to the optical axis. The choice of 45° angle allows the light of the two wavelengths to be separated, and the direction of light propagation remains unchanged after passing through the polarized beam splitter.

[0019] The first and second unpolarized beam splitters both have a beam splitting ratio of 50:50 [R:T], thus ensuring that the beam intensities of the two wavelengths remain consistent.

[0020] As an improvement of the present invention, the plate beam splitter is mounted on a displacement stage for angle fine-tuning to control the off-axis angle; in order to match the beam intensity of the object beam and the reference beam and improve the interference effect, the beam splitting ratio of the plate beam splitter is set to 30:70 [R:T], and the beam splitting ratio of the polarizing beam splitter is set to 50:50 [R:T].

[0021] As an improvement of the present invention, the holographic microscope includes a sample stage for placing a sample, the sample stage being placed on the object-side focal plane of the microscope objective.

[0022] The holographic microscope includes a computer processing system for controlling an image sensor and processing the holographic images received by the image sensor. The holographic images include two holograms at two wavelengths, λ1 and λ2. The processing method is as follows:

[0023] The same sample regions of the two holograms were extracted by feature matching algorithm, and numerical reconstruction was performed to obtain the encapsulated phase distribution at two wavelengths.

[0024] After unwrapping the wrapped phase using a phase unwrapping algorithm, the system distortion contained in the phase distribution is removed using a phase distortion numerical compensation algorithm.

[0025] After obtaining the encapsulation phase at two wavelengths, the accurate phase distribution is solved using a dual-wavelength phase retrieval algorithm. Finally, the sample surface height is recovered using the linear relationship between the phase and the sample surface height.

[0026] As an improvement of the present invention, the phase unwrapping algorithm is a branch-cut phase unwrapping algorithm based on path tracking. It identifies positive and negative residual points and connects neighboring residual points to form the optimal branch tangent line, and determines the integration path that does not pass through this branch tangent line.

[0027] System distortion typically consists of skewness caused by the alignment of optical elements, the introduction of off-axis angles, tilt distortion caused by the skewness of the camera target surface, surface distortion caused by the microscope objective, and a small amount of higher-order distortion.

[0028] As an improvement of the present invention, the phase distortion numerical compensation algorithm is the ZPF compensation method, which accurately fits and removes the overall distortion of the system by matching the coefficients in the Zernike inequality.

[0029] As an improvement of the present invention, the dual-wavelength phase recovery algorithm is a dual-wavelength phase unwrapping algorithm based on linear programming.

[0030] Compared with existing technologies, the dual-wavelength common-path holographic microscope provided by this invention has the following advantages:

[0031] 1. This invention features the characteristics of in-situ, real-time, and fast holographic technology, while also possessing the advantages of high vibration resistance and high time stability of common-path optical path arrangement.

[0032] 2. In this invention, the separation and adjustment of the light beam does not require complex optical components and adjustment steps. The reference light and the object light, and the two wavelengths of the light beam, can be separated and their angles adjusted using simple optical components and adjustment methods.

[0033] 3. The optical path arrangement of the present invention is compact and easy to promote and apply; it has obvious advantages for the morphological characterization of precision structures and high-precision dynamic imaging. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the structure of the compact dual-wavelength common-path holographic microscope in an embodiment of the present invention;

[0035] The following are the labels in the diagram: 1. First laser source; 2. Second laser source; 3. First collimating and beam expanding system; 4. Second collimating and beam expanding system; 5. First polarizer; 6. Second polarizer; 7. First unpolarizing beam splitter; 8. Second unpolarizing beam splitter; 9. Plate beam splitter; 10. Microscope objective; 11. Sample stage; 12. Polarizing beam splitter; 13. Image sensor; 14. Computer processing system. Detailed Implementation

[0036] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples. The described embodiments are merely some examples of the present invention and are not intended to limit the scope of the invention. All other embodiments made by those skilled in the art based on the embodiments of the present invention without inventive effort are within the protection scope of the present invention.

[0037] like Figure 1 As shown, the compact dual-wavelength common-path holographic microscope provided in this embodiment includes a laser source, a collimation and beam expansion system, a polarizer, a non-polarizing beam splitter, a plate beam splitter 9, a microscope objective 10, a sample stage 11, a polarizing beam splitter 12, an image sensor 13, and a computer processing system 14. The laser source includes a first laser source 1 and a second laser source 2; the collimation and beam expansion system includes a first collimation and beam expansion system 3 and a second collimation and beam expansion system 4; the polarizer includes a first polarizer 5 and a second polarizer 6; the non-polarizing beam splitter includes a first non-polarizing beam splitter 7 and a second non-polarizing beam splitter 8; the second non-polarizing beam splitter 8, the plate beam splitter 9, and the microscope objective 10 are placed coaxially; the laser source 1 and laser source 2 are used to emit continuous laser light; the image sensor 13 is used to record off-axis holographic images generated by the interference of reference light and object light at two wavelengths; the computer processing system 14 is used to control the image sensor 13 and process the off-axis holographic images received by the image sensor 13.

[0038] It should be noted that the laser source used in this embodiment is a single-longitudinal-mode laser, which has the characteristics of high performance, continuous operation, and ultra-low noise, and is suitable for holographic microscopy imaging. The sample stage 11 is placed on the object-side focal plane of the microscope objective 10; the plate beam splitter 9 is mounted on a two-dimensional micro-displacement stage for fine-tuning of the angle to control the off-axis angle; the beam splitting ratio of the plate beam splitter 9 is 30:70 [R:T]; the beam splitting ratio of the first unpolarized beam splitter 7 and the second unpolarized beam splitter 8 is 50:50 [R:T].

[0039] The specific process of computer processing system 14 processing off-axis holographic images is as follows:

[0040] Image sensor 13 records off-axis holographic images at two wavelengths, and obtains the wrapping phase matrix of the object on the object plane through numerical reconstruction. Then, using a phase retrieval algorithm, including a phase unwrapping algorithm and a distortion numerical compensation algorithm, the wrapping phase matrices at two single wavelengths are obtained respectively. The surface height of the sample under test at a single wavelength is obtained through the linear relationship between surface height and phase distribution. The accurate height distribution of the sample under test is obtained through a dual-wavelength phase retrieval algorithm. The results are then saved and output. Specifically:

[0041] (1) Recording and numerical reconstruction of single-wavelength off-axis holographic images:

[0042] When recording and numerically reconstructing the above off-axis holographic image, it is assumed that two beams of light... (λ2>λ1) The sample to be tested is illuminated separately and recorded by an image sensor, forming off-axis holographic images at two wavelengths. The system's composite wavelength is... The object beam reflected from the sample and the reference beam interfere and propagate to the recording plane of the image sensor. The complex amplitude and intensity distribution of the total optical field on the recording plane of the image sensor can be expressed in the following forms:

[0043]

[0044] Where O(x,y) and R(x,y) are the complex amplitudes of the object beam and the reference beam, respectively, and α and β are the off-axis angles of the two beams k1 and k2 under the off-axis holographic optical path setting.

[0045] When performing numerical reconstruction using the angular spectrum method, the object beam field is reproduced using a reference beam. The complex amplitude distribution of the object beam field on the object plane can be expressed as:

[0046] O1'(x,y)=IFFT{FFT[R1(x,y)I1(x,y)]G V1 (f x ,f y )}

[0047] O2'(x,y)=IFFT{FFT[R2(x,y)I2(x,y)]GV2 (f x ,f y )}

[0048] Where IFFT represents the inverse Fourier transform, FFT represents the Fourier transform, and G... V1 and G V2 Let f represent the transfer functions of the angular diffraction at two wavelengths, respectively. x and f y This indicates the frequency of the light wave in the x and y directions.

[0049] After obtaining the complex amplitude distribution of the object light field, its intensity and phase distribution can be obtained by the following formula:

[0050] I i (x,y)=|O i '(x,y)| 2

[0051]

[0052] (2) Phase unwrapping and distortion numerical compensation for single-wavelength phase distribution:

[0053] The phase distribution obtained from the above calculation process The phase distribution is contained not only in the phase of the sample under test but also in the distortion of the system. Therefore, the phase distribution is bound between [-π, π]. Single-wavelength phase recovery is performed using a branch-cut phase unwrapping algorithm and a distortion numerical compensation method. The overall phase difference of the system is removed using a ZPF polynomial fitting numerical compensation method.

[0054]

[0055] Where angle represents the phase representation. It is a distortion of the system. It is a wrapper phase that has had system distortion removed.

[0056] (3) Solving for the surface height distribution of the sample under two single wavelengths:

[0057] After obtaining the wrapper phase of a single wavelength, the height distribution of that single wavelength needs to be solved using the linear relationship between surface height and phase. When both the illumination direction and the viewing direction are perpendicular to the phase zero plane, the relationship between object height and phase can be simplified as follows:

[0058]

[0059] (4) Dual-wavelength accurate phase recovery:

[0060] It should be noted that obtaining the accurate phase through single-wavelength phase subtraction will amplify the noise in the single-wavelength result, which is not conducive to the topographic characterization of precise structures. The dual-wavelength phase retrieval algorithm in this invention employs a linear programming-based method, and the specific execution method is as follows:

[0061] The height distribution obtained from two single-wavelength phase maps can typically be represented as:

[0062]

[0063] Here, m1 and m2 are the unknown non-negative integers at the pixel (x, y) under two wavelengths. Next, the least squares of the height difference is applied to search for the best integer matching pair of m1 and m2:

[0064]

[0065] After obtaining the best integer matching pair of m1 and m2, the accurate height result of the sample to be measured can be obtained by substituting them into the height calculation formula for any wavelength.

[0066] (5) Results output: The computer processing system saves and outputs the measurement results in the form of three-dimensional distribution data.

[0067] Although embodiments of the invention have been shown and described, those skilled in the art will recognize that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A compact dual-wavelength common-path holographic microscope, characterized in that, The holographic microscope includes a first laser source (1) with a wavelength of λ1, a second laser source (2) with a wavelength of λ2, a first collimating beam expanding system (3), a second collimating beam expanding system (4), a first polarizer (5), a second polarizer (6), a first unpolarized beam splitter (7), a second unpolarized beam splitter (8), a plate beam splitter (9), a microscope objective (10), a polarizing beam splitter (12), and an image sensor (13). The optical path structure of the holographic microscope is as follows: The laser beam emitted by the first laser source (1) with a wavelength of λ1 is collimated and expanded by the first collimation and beam expansion system (3), and then modulated into a vertically polarized laser beam by the first polarizer (5). The laser beam emitted by the second laser source (2) with a wavelength of λ2 is collimated and expanded by the second collimation and beam expansion system (4), and then modulated into a horizontally polarized laser beam by the second polarizer (6). The vertically polarized laser beam and the horizontally polarized laser beam are combined by the first unpolarized beam splitter (7) and the second unpolarized beam splitter (8), and then split into object light and reference light by the plate beam splitter (9); The object light is transmitted light that passes through the plate beam splitter (9) and illuminates the sample through the microscope objective (10); the reference light is reflected light that passes through the plate beam splitter (9); the laser beams at the two wavelengths are separated after passing through the polarizing beam splitter (12), and the reference light and object light at each wavelength interfere with each other, and the holographic image is recorded by the image sensor (13).

2. The compact dual-wavelength common-path holographic microscope according to claim 1, characterized in that, The first laser source (1) and the second laser source (2) are single-longitudinal-mode lasers.

3. The compact dual-wavelength common-path holographic microscope according to claim 1, characterized in that, The combined wavelength of the two wavelengths is greater than the optical length of the sample under test.

4. The compact dual-wavelength common-path holographic microscope according to claim 1, characterized in that, The first unpolarized beam splitter (7) and the second unpolarized beam splitter (8) are placed at 45° to the optical axis, and the beam splitting ratio of the first unpolarized beam splitter (7) and the second unpolarized beam splitter (8) is 50:50 [R:T].

5. The compact dual-wavelength common-path holographic microscope according to claim 1, characterized in that, The plate beam splitter (9) is mounted on the displacement stage for angle fine-tuning to control the off-axis angle. The beam splitting ratio of the plate beam splitter (9) is 30:70 [R:T]; the beam splitting ratio of the polarizing beam splitter (12) is 50:50 [R:T].

6. The compact dual-wavelength common-path holographic microscope according to claim 1, characterized in that, The holographic microscope includes a sample stage (11) for placing a sample, the sample stage (11) being placed on the object-side focal plane of the microscope objective (10).

7. The compact dual-wavelength common-path holographic microscope according to any one of claims 1-6, characterized in that, The holographic microscope includes a computer processing system (14) for controlling the image sensor (13) and processing the holographic images received by the image sensor (13). The holographic images include two holograms at two wavelengths, λ1 and λ2. The processing method is as follows: The same sample regions of the two holograms were extracted by feature matching algorithm, and numerical reconstruction was performed to obtain the encapsulated phase distribution at two wavelengths. After unwrapping the wrapped phase using a phase unwrapping algorithm, the system distortion contained in the phase distribution is removed using a phase distortion numerical compensation algorithm. After obtaining the encapsulation phase at two wavelengths, the accurate phase distribution is solved using a dual-wavelength phase retrieval algorithm. Finally, the sample surface height is recovered using the linear relationship between the phase and the sample surface height.

8. The compact dual-wavelength common-path holographic microscope according to claim 7, characterized in that, The phase unwrapping algorithm is a branch-cut phase unwrapping algorithm based on path tracing. It identifies positive and negative residual points and connects neighboring residual points to form the optimal branch tangent line, thus determining the integration path that does not pass through this branch tangent line.

9. The compact dual-wavelength common-path holographic microscope according to claim 7, characterized in that, The phase distortion numerical compensation algorithm is the ZPF compensation method, which accurately fits and removes the overall distortion of the system by matching the coefficients in the Zernike inequality.

10. The compact dual-wavelength common-path holographic microscope according to claim 7, characterized in that, The dual-wavelength phase recovery algorithm is a dual-wavelength phase unwrapping algorithm based on linear programming.

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