A fast structured light digital holographic measurement system and method

Abstract

The present invention discloses a fast structured light digital holographic measurement system and method, which relates to the field of micro-nano detection technology; in the present invention, structured light after polarization modulation is used to irradiate an object to be measured and then becomes object light. The object light and the reference light are combined and incident on a polarization beam splitter element. After passing through the polarization beam splitter element, multiple structured light digital holograms are formed. The structured light digital holograms are collected by a photodetector, and the photodetector can achieve synchronous collection of two structured light holograms with fixed phase shifts in a single exposure; the measurement method of the present invention introduces spatial phase shift during the measurement process, eliminates the time phase shift step, doubles the overall collection efficiency, and improves the real-time performance of the measurement.

Description

Technical Field

[0001] The present invention relates to the technical field of micro-nano detection, and particularly to a fast structured light digital holographic measurement system and method. Background Art

[0002] With the continuous development of technology, micro-nano structures with various functions have emerged continuously. In order to pursue more excellent performance, micro-nano structures are continuously developing towards finer and more complex directions. As an optical measurement method, digital holography eliminates the scanning step compared with other measurement methods, can realize the rapid construction of surface shape information, and has the advantages of wide measurement range, high resolution, strong robustness, fast reconstruction speed, etc., and has great potential in the field of micro-nano structure measurement. According to the Abbe imaging theory, the lateral resolution of digital holography is limited, and its highest imaging resolution can only reach 1 / 2 wavelength. How to break through the diffraction limit and improve the resolution of the digital holographic system has always been one of the difficult problems concerned in the detection field.

[0003] With the in-depth research, researchers found that the structured light illumination super-resolution technology has good compatibility with the digital holographic microscope. In order to improve the measurement resolution of digital holography, researchers introduced the structured light illumination super-resolution technology into the digital holographic measurement optical path, forming a new structured light digital holographic microscopy scheme. However, to ensure the demodulation of super-resolution information and isotropy, this scheme needs to collect multiple structured light digital holograms with phase shifts in different directions. For example, the Chinese invention patent with the application publication number CN 109709786 A discloses "a super-resolution digital holographic imaging system and imaging method", which irradiates the object to be measured with structured light in different directions, and realizes higher-resolution imaging of the object by demodulating the collected digital holograms; however, this method needs to collect structured light digital holograms multiple times, and the collection efficiency is low.

[0004] In recent years, many researchers have proposed different measurement schemes to reduce or even eliminate phase shift and improve the subsequent demodulation algorithm to increase the time resolution of the measurement system. However, in the process of realizing phase shift elimination in the existing schemes, the aliased spectrum is demodulated in an iterative form, which cannot guarantee the real-time nature of the demodulation process, and the device is very complex, which limits the application of the structured light digital holographic technology in many micro-nano measurement fields such as particle field measurement, fluid field measurement, and crystal preparation monitoring. Summary of the Invention

[0005] One of the objectives of the present invention is at least to provide a fast structured light digital holographic measurement system and method for overcoming the problems existing in the above-mentioned prior art. By using polarization multiplexing technology and a polarization beam splitter element, spatial phase shift is introduced during the measurement process. Multiple structured light digital holograms with phase shift differences can be collected by a single exposure of the same photodetector, improving the image acquisition efficiency and measurement real-time performance.

[0006] To achieve the above objective, the technical solutions adopted by the present invention include the following aspects.

[0007] A fast structured light digital holographic measurement system includes: a light source, a second microscopic objective lens, a third non-polarizing beam splitter element, a polarization beam splitter element, a photodetector, and a computer;

[0008] The light source is configured to generate measurement light and reference light;

[0009] The measurement light is configured to be split into a 45° linearly polarized light beam and a circularly polarized light beam. After the 45° linearly polarized light interferes with the circularly polarized light to form structured light, the structured light illuminates the sample to be measured and then forms object light through the second microscopic objective lens. The object light is incident on the third non-polarizing beam splitter element;

[0010] The reference light is configured to form 45° linearly polarized light. The reference light is incident on the third non-polarizing beam splitter element. After being combined with the object light, a combined light beam is formed. The combined light beam is split by the polarization beam splitter element to form S light and P light. The S light and P light are incident on the photodetector;

[0011] The computer is connected to the photodetector and is configured to demodulate the super-resolution information of the structured light digital hologram.

[0012] Preferably, it further includes a polarizer, an expanding and collimating lens, a first non-polarizing beam splitter element, a second non-polarizing beam splitter element, a spatial light modulator, a focusing lens, a filter, a first microscopic objective lens, a second microscopic objective lens, a Tube lens, and a reflector;

[0013] The incident light generated by the light source passes through the polarizer to generate linearly polarized light. The linearly polarized light is expanded by the expanding and collimating lens and then incident on the first non-polarizing beam splitter element, and is divided into perpendicular reference light and measurement light by the first non-polarizing beam splitter element;

[0014] The reference light is reflected by the reflector and then incident on the third non-polarizing beam splitter element;

[0015] The measurement light is incident on the spatial light modulator through the second non-polarizing beam splitter element. After being modulated by the spatial light modulator, multi-level diffracted beams are generated. The diffracted beams are reflected back into the second non-polarizing beam splitter element, and then reflected by the second non-polarizing beam splitter element to the focusing lens, the filter, and the first microscope objective to form structured light. After the structured light illuminates the sample to be measured, object light is formed through the second microscope objective and the Tube lens. The object light is incident on the third non-polarizing beam splitter element.

[0016] Preferably, the spatial light modulator is a reflective phase-type spatial light modulator, and a binary diffraction grating is loaded therein.

[0017] Preferably, the cross-sectional shape of the filter is a regular hexagon, and a small hole array arranged in a regular hexagon is provided thereon. The aperture of the small holes is 0.5 - 2 mm.

[0018] Preferably, the included angle between the filter and the x-axis is 45°.

[0019] Preferably, the first non-polarizing beam splitter element, the second non-polarizing beam splitter element, and the third non-polarizing beam splitter element are all non-polarizing beam splitting prisms, beam splitting lenses, or beam splitting flat plates.

[0020] Preferably, a circularly polarized thin film adapted to the shape of the filter is provided on the surface of the filter. The circularly polarized thin film divides the filter into two equal parts, and the transmittance of the circularly polarized thin film is greater than or equal to 99.9%.

[0021] Preferably, the polarization beam splitter element is a polarization beam splitting prism or a Wollaston prism.

[0022] Preferably, the photodetector is a area array color camera, a line array color camera, a area array black and white camera, or a line array black and white camera, and the photosensitive element type of the photodetector is CMOS or CCD.

[0023] A fast structured light digital holographic measurement method, which uses the above-mentioned fast structured light digital holographic measurement system, includes the following steps:

[0024] Step 1: Collect the structured light digital hologram;

[0025] Step 2: Perform holographic reconstruction on the collected structured light digital hologram;

[0026] Step 3: Perform super-resolution demodulation on the complex amplitude information of the two structured light fields obtained by holographic reconstruction;

[0027] Step 4: Load phase gratings in different directions on the spatial light modulator SLM, and repeat Steps 1 to 3 to obtain isotropic three-dimensional super-resolution results.

[0028] In summary, due to the adoption of the above technical solutions, the present invention has at least the following beneficial effects:

[0029] The present invention uses a composite structured light formed by the interference of 45° linearly polarized light and circularly polarized light to illuminate the object to be measured. After the structured light illuminates the object to be measured, object light is formed. The object light and the 45° linearly polarized reference light are combined by a non-polarizing beam splitter element. The combined light is divided into two beams of light with a fixed phase difference of by a polarization beam splitter element. Two structured light digital holograms can be collected by a single exposure of the same photodetector, improving the image acquisition efficiency and the real-time performance of the measurement.

[0030] By loading binary diffraction gratings with different angles in the spatial light modulator, structured light digital holograms in multiple directions can be collected, and two structured light digital holograms can be collected each time. By demodulating these two structured light digital holograms, an isotropic three-dimensional super-resolution result can be obtained.

[0031] In the measurement optical path of the present invention, the multi-order diffracted light beams modulated by the spatial light modulator are divided into linearly polarized light and circularly polarized light by a filter, and a spatial phase shift is introduced by a polarization beam splitter element. The system structure is simpler, and the application field of the fast structured light digital holographic measurement system can be expanded. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is the schematic diagram of the principle of fast structured light digital holographic measurement of an exemplary embodiment of the present invention.

[0033] Figure 2 is the schematic diagram of the structure of the fast structured light digital holographic measurement system of an exemplary embodiment of the present invention.

[0034] Figure 3 is the diffraction grating diagram with different directions loaded on the spatial light modulator.

[0035] Figure 4 is the schematic diagram of the super-resolution demodulation algorithm flow of an exemplary embodiment of the present invention.

[0036] Reference numerals in the figures: 1 - He-Ne laser, 2 - polarizer, 3 - beam expander and collimator, 4 - first non-polarizing beam splitter element, 5 - second non-polarizing beam splitter element, 6 - spatial light modulator, 7 - focusing lens, 8 - filter, 9 - first microscopic objective lens, 10 - sample to be measured, 11 - second microscopic objective lens, 12 - Tube lens, 13 - mirror, 14 - third non-polarizing beam splitter element, 15 - polarization beam splitter element, 16 - photodetector. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments, so as to make the object, technical solution and advantages of the present invention more clear and understandable. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0038] As Figure 1 shown, the principle of the fast structured light digital holographic measurement of the exemplary embodiment of the present invention is as follows: A 45° linearly polarized light beam interferes with a circularly polarized light beam to form structured light for illuminating the sample 10 to be measured. The object light is generated by the second microscope objective 11 and is incident normally on the third non-polarizing beam splitter 14; another 45° linearly polarized light beam is incident on the third non-polarizing beam splitter 14 as the reference light. After the object light and the reference light are combined, they are incident on the polarization beam splitter 15, where they are split into S light and P light. The S light and the P light are respectively coherently superposed to generate two structured light digital holograms with a fixed phase shift difference, which are received by the photodetector 16. By demodulating the received digital holograms, three-dimensional super-resolution imaging can be achieved.

[0039] The present invention uses a composite structured light formed by the interference of 45° linearly polarized light and circularly polarized light to illuminate the object to be measured, and combines the polarization multiplexing technology to introduce spatial phase shift during the measurement process, improving the real-time performance of the structured light digital holographic measurement; the object light, the S light and the P light of the reference light are separated by the polarization beam splitter 15, so that the S light and the P light of the two interfere respectively, thereby forming two structured light digital holograms with a phase difference. The two structured light digital holograms are received by the same photodetector. Therefore, two holograms can be collected by a single exposure, greatly improving the image acquisition efficiency and the real-time performance of the measurement.

[0040] As Figure 2As shown in the figure, the structure of the fast structured light digital holographic measurement system according to an exemplary embodiment of the present invention includes: a He-Ne laser 1, a polarizer 2 and an expanding and collimating mirror 3 are coaxially arranged in the optical path direction of the He-Ne laser 1. The He-Ne laser 1 generates incident light, and the incident light generates a 45° linearly polarized wide beam after passing through the polarizer 2 and the expanding and collimating mirror 3. The linearly polarized light beam is incident on the first non-polarizing beam splitter 4 and is divided into a reference light and a measurement light that are perpendicular to each other by the first non-polarizing beam splitter 4; a reflecting mirror 13 and a third non-polarizing beam splitter 14 are sequentially arranged along the reference optical path direction. Along the reference optical path direction, the reference light is reflected by the reflecting mirror 13 and then incident on the third non-polarizing beam splitter 14; a second non-polarizing beam splitter 5, a spatial light modulator 6, a focusing lens 7, a filter 8, a first microscope objective 9, a sample to be measured 10, a second microscope objective 11, and a Tube lens 12 are sequentially arranged along the measurement optical path direction; along the measurement optical path direction, the measurement light is incident on the spatial light modulator 6 through the second non-polarizing beam splitter 5, and a multi-order diffracted beam is generated after being modulated by the spatial light modulator 6. The diffracted beam is reflected by the second non-polarizing beam splitter 5 and then enters a 4f system composed of the focusing lens 7 and the first microscope objective 9. A filter 8 is arranged at the Fourier plane of the 4f system. Only the ±1 order structured light remains after the multi-order diffracted light passes through the 4f system. After the structured light irradiates the sample to be measured 10, the generated image is magnified by the second microscope objective 11 and then forms an object light after passing through the Tube lens 12; the object light is incident on the third non-polarizing beam splitter 14 and is combined with the reference light; a polarization beam splitter 15 and a photodetector 16 are sequentially arranged along the combined beam optical path direction. The combined beam is divided into an S light and a P light by the polarization beam splitter 15, and the S light and the P light interfere respectively, and two structured light digital holograms with a phase shift of π / 2 are collected by the photodetector 16; the photodetector 16 is connected to a computer (not shown in the figure), and the computer demodulates the super-resolution information of the structured light digital hologram through a phase shift algorithm, reconstructs the object light wave, and realizes three-dimensional super-resolution imaging.

[0041] In the above optical path, the He-Ne laser 1 generates incident light with a wavelength of 632.8 nm. The incident light can also be of other wavelengths, and the incident light can also be generated by other lasers, or can be generated by an incoherent light source such as a mercury lamp or an LED lamp. The angle between the polarizer 2 and the x-axis (horizontal direction) is 45°, and the expanding and collimating mirror 3 has an expanding effect on the light spot of not less than 15 times.

[0042] The spatial light modulator 6 is a reflective phase-type spatial light modulator, and a binary diffraction grating is loaded therein. The grating period T is 100-200 μm, and the grating loading angle is adjustable. Refer to Figure 3, the loading angles of the grating are 0°, 60°, and 120° respectively; the spatial light modulator 6 is used to modulate the measurement light. By loading diffraction gratings in different directions, structured light in different directions is generated, so that multiple structured light digital holograms with phase shift amounts in different directions can be collected, ensuring the isotropy of super-resolution information. In addition to being generated by the spatial light modulator, structured light in different directions can also be generated by physical gratings, biprisms, etc.

[0043] The cross-sectional shape of the filter 8 is a regular hexagon, and a small hole array arranged in a regular hexagon is provided thereon. The aperture of the small hole is 0.5 - 2 mm. A circular polarization film adapted to the shape of the filter is provided on the surface of the filter 8. The circular polarization film divides the filter 8 into two equal parts. The transmittance of the circular polarization film is greater than or equal to 99.9%, and the circular polarization film is used to convert linearly polarized light into circularly polarized light.

[0044] The first non-polarizing beam splitter element 4, the second non-polarizing beam splitter element 5, and the third non-polarizing beam splitter element 14 are preferably non-polarizing beam splitting prisms, and can also be optical elements such as beam splitting lenses and beam splitting flat plates that can achieve non-polarizing beam splitting; the polarizing beam splitter element 15 is preferably a polarizing beam splitting prism, and can also be an optical element such as a Wollaston prism that can achieve polarizing beam splitting.

[0045] The photodetector 16 can be a area array color camera, a line array color camera, a area array black and white camera, or a line array black and white camera, and the photosensitive element type of the camera is CMOS or CCD.

[0046] Through the above-mentioned fast structured light digital holographic measurement system, two structured light digital holographic illumination diagrams with a fixed phase shift difference can be obtained simultaneously. The computer demodulates the super-resolution information through a structured light super-resolution demodulation algorithm based on phase shift, reconstructs the object light wave, and three-dimensional super-resolution imaging can be achieved, improving the real-time performance of the measurement.

[0047] In the fast structured light digital holographic measurement system of the present invention, in addition to using the Figure 1 , Figure 2 shown transmission method to form the object light, it can also be formed by a reflection method.

[0048] The fast structured light digital holographic measurement method of the present invention includes the following steps:

[0049] Step 1: Collect structured light digital holograms; assuming that the two interfering light beams of the structured light optical field are plane waves, the light beams can be expressed as:

[0050]

[0051]

[0052] Among them, O1 is a 45° linearly polarized light, O2 is a right-handed circularly polarized light, A is the amplitude of the light beam, f is the spatial frequency, and (x, y) is the coordinate vector in the two-dimensional space. Further, the S component and P component of the object light can be expressed as:

[0053]

[0054]

[0055] Among them, is the phase information of the sample to be measured. Assuming that the reference light is a linearly polarized plane wave at 45° to the horizontal direction, it can be expressed as:

[0056]

[0057] Among them, is the carrier frequency information of the reference light, B is the amplitude of the reference light. After the object light and the reference light pass through the polarization beam splitter element, the P light and S light parts are coherently superposed respectively to generate a structured light digital holographic light field with a fixed phase difference of :

[0058]

[0059]

[0060] Step 2: Perform holographic reconstruction on the acquired structured light digital hologram and complete numerical reconstruction using the angular spectrum method:

[0061]

[0062] In the above formula, FFT and IFFT respectively represent the fast Fourier transform and its inverse transform, d represents the diffraction propagation distance of the object light wave, f x and f y are the spatial frequencies in the x and y directions respectively. In order to remove the zero-order term and the twin term, an appropriate spectral window W(f x , f y ) is used to achieve filtering in the spatial frequency domain. By using the spectral translation function S(f x , f y ) to translate the filtered spectrum to the center position, the first-order aberration of the image can be eliminated. For the high-order aberrations generated by various optical lenses, a calibration method based on the conjugate reference hologram is used to remove the influence of phase distortion. Among them, the compensation term can be constructed by recording a blank hologram without a sample.

[0063] Step 3: Perform super-resolution demodulation on the complex amplitude information of the two structured light fields obtained by holographic reconstruction, and the process is as Figure 4As shown: Since the two spectral information are mutually aliased, to completely restore the spectral information, two separated spectral parts can be obtained through the transformation equation:

[0064]

[0065] Then, shift the spectra IGL(f) and IGH(f) back to their original positions, and superimpose and fuse the spectral information at the original positions to expand the frequency domain information.

[0066] Step 4: Load phase gratings in different directions on the spatial light modulator SLM, and repeat Steps 1 to 3 to obtain isotropic three-dimensional super-resolution results.

[0067] As described above, it is only a detailed description of the specific implementation manner of the present invention, rather than a limitation on the present invention. Various substitutions, variations, and improvements made by those skilled in the relevant technical fields without departing from the principle and scope of the present invention shall be included within the protection scope of the present invention.

Claims

1. A fast structured light digital holographic measurement system, characterized in that, Including: a light source, a second microscope objective lens (11), a third non-polarizing beam splitter (14), a polarizing beam splitter (15), a photodetector (16), and a computer; the light source is configured to generate a measurement light and a reference light; the measurement light is configured to be split into a 45° linearly polarized light beam and a circularly polarized light beam. After the 45° linearly polarized light interferes with the circularly polarized light to form a structured light, after the structured light illuminates the sample to be measured (10), an object light is formed through the second microscope objective lens (11), and the object light is incident on the third non-polarizing beam splitter (14); the reference light is configured to form a 45° linearly polarized light, the reference light is incident on the third non-polarizing beam splitter (14), and after being combined with the object light, a combined light is formed; the combined light is split by the polarizing beam splitter (15) to form an S light and a P light, and the S light and the P light are incident on the photodetector (16); the computer is connected to the photodetector (16) and is configured to demodulate the super-resolution information of the structured light digital hologram.

2. The fast structured light digital holographic measurement system according to claim 1, characterized in that, It further includes a polarizer (2), a beam expander and collimator (3), a first non-polarizing beam splitter (4), a second non-polarizing beam splitter (5), a spatial light modulator (6), a focusing lens (7), a filter (8), a first microscope objective lens (9), a second microscope objective lens (11), a Tube lens (12), and a reflector (13); the incident light generated by the light source generates a linearly polarized light through the polarizer (2), and after being expanded by the beam expander and collimator (3), it is incident on the first non-polarizing beam splitter (4), and is divided into a mutually perpendicular reference light and measurement light by the first non-polarizing beam splitter (4); the reference light is reflected by the reflector (13) and then incident on the third non-polarizing beam splitter (14); the measurement light is incident on the spatial light modulator (6) through the second non-polarizing beam splitter (5), and after being modulated by the spatial light modulator (6), multi-level diffracted light beams are generated. The diffracted light beams are reflected back into the second non-polarizing beam splitter (5), and after being reflected by the second non-polarizing beam splitter (5), they are incident on the focusing lens (7), the filter (8), and the first microscope objective lens (9) to form a structured light. After the structured light illuminates the sample to be measured (10), an object light is formed through the second microscope objective lens (11) and the Tube lens (12), and the object light is incident on the third non-polarizing beam splitter (14).

3. The fast structured light digital holographic measurement system according to claim 2, characterized in that, The spatial light modulator (6) is a reflective phase-type spatial light modulator, and a binary diffraction grating is loaded therein.

4. The fast structured light digital holographic measurement system according to claim 2, characterized in that, The cross-sectional shape of the filter (8) is a regular hexagon, and a small hole array arranged in a regular hexagon is provided thereon, and the aperture of the small holes is 0.5 - 2 mm.

5. The fast structured light digital holographic measurement system according to claim 2, characterized in that, The included angle between the filter and the x-axis is 45°.

6. The fast structured light digital holographic measurement system according to claim 2, characterized in that, The first non-polarizing beam splitter (4), the second non-polarizing beam splitter (5), and the third non-polarizing beam splitter (14) are all non-polarizing beam splitting prisms, beam splitting lenses, or beam splitting flat plates.

7. The fast structured light digital holographic measurement system according to claim 2, characterized in that, A circularly polarized thin film adapted to the shape of the filter is provided on the surface of the filter (8), and the circularly polarized thin film divides the filter (8) into two equal parts, and the light transmittance of the circularly polarized thin film is greater than or equal to 99.9%.

8. The fast structured light digital holographic measurement system according to any one of claims 1 to 7, characterized in that, The polarization beam splitting element (15) is a polarization beam splitting prism or a Wollaston prism.

9. The fast structured light digital holographic measurement system according to any one of claims 1 to 7, characterized in that, The photodetector (16) is an area array color camera, a linear array color camera, an area array black and white camera or a linear array black and white camera, and the photosensitive element type of the photodetector (16) is CMOS or CCD.

10. A fast structured light digital holographic measurement method, characterized in that, Using the fast structured light digital holographic measurement system according to any one of claims 1 to 9, the fast structured light digital holographic measurement method includes the following steps: Step 1: Acquire a structured light digital hologram; Step 2: Perform holographic reconstruction on the acquired structured light digital hologram; Step 3: Perform super-resolution demodulation on the complex amplitude information of the two structured light fields obtained by holographic reconstruction; Step 4: Load phase gratings in different directions on the spatial light modulator SLM, and repeat steps 1 to 3 to obtain an isotropic three-dimensional super-resolution result.

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