A heterodyne digital holographic microscopic phase imaging device and measurement method based on a moving grating

Abstract

This invention discloses a heterodyne digital holographic microscopic phase imaging scheme based on a moving grating. The scheme includes an optical imaging device and a measurement method. The optical imaging device includes a light source system, an amplification system, a heterodyne interferometry system, and an image acquisition system. A light beam emitted from the light source penetrates the object under test, carrying the object's optical phase information. This information is amplified by the amplification system and then passed through a moving grating in the heterodyne microscopy system, splitting the incident light into multiple beams of different frequencies. An aperture is used to select the 0th and +1st order beams, which interfere on the camera's detection surface to generate continuous moving interference fringes. The measurement method mainly includes: optical path adjustment, image data acquisition, image processing, and phase-height calibration. This invention achieves higher reconstruction accuracy compared to traditional single-image reconstruction schemes, and features low cost and good stability, providing a new measurement scheme and approach for digital holographic microscopy.

Description

Technical Field

[0001] This invention relates to the field of digital holographic microscopy, specifically to a heterodyne digital holographic microscopy phase imaging device and measurement method based on a moving grating. Background Technology

[0002] Digital holographic microscopy is a microscopy technique that converts holographic images into visualized images through digital processing. It provides high-resolution, holographic, and three-dimensional microscopic images, enabling us to observe and study the structure and properties of tiny objects. Digital holographic microscopy can provide more comprehensive and accurate sample information, helping scientists gain a deeper understanding of the structure and behavior of objects; it allows for non-contact, non-destructive observation, protecting the integrity of the sample; it enables real-time observation and recording, facilitating data analysis and sharing; and it can be applied to multiple fields, such as biomedicine, materials science, and nanotechnology, promoting research and application development in these areas.

[0003] The steps involved in digital holographic microscopy generally include acquiring holographic images and image reconstruction. Single-image reconstruction methods include Fresnel reconstruction, convolution, and angular spectral methods. These algorithms can quickly calculate the phase information of the object from a single holographic image, but the reconstruction accuracy is limited by the single-image limitation. Multi-image phase-shifting methods introduce a known phase into the optical path to obtain multiple holograms with different phase-shift steps. Simple mathematical operations on these holograms can yield more accurate results, but precisely controlling the phase-shift step size is very difficult. Heterodyne digital holography based on continuous phase shifting adds multiple acousto-optic frequency shifters to the optical path, causing a frequency shift in the beam. Interference between two beams of different frequencies then obtains a series of holographic images that change over time, but the experimental costs are often very high.

[0004] The Doppler effect is a physical phenomenon that describes the change in the frequency or wavelength of a wave perceived by an observer when the light or sound source moves relative to the observer. The Doppler effect is widely used in astronomy to measure the velocity and distance of celestial bodies and to study the expansion of the universe and the motion of galaxies. In earth sciences, the Doppler effect can be used to measure the propagation speed of seismic waves and to detect underground structures; in medicine, it can be used for echocardiography and blood flow measurement. Studying the Doppler effect helps to understand the laws governing the motion of objects and wave propagation, expanding our knowledge of the universe, the Earth, and living organisms. Based on this characteristic, when a laser beam is incident on a moving transmission grating, each diffracted beam will undergo a frequency change of varying magnitude. Summary of the Invention

[0005] The purpose of this invention is to provide a heterodyne digital holographic microscopic phase imaging device and measurement method based on a moving grating. The solution comprises two parts: an optical imaging device and a measurement method. The optical imaging device includes a light source system, a magnification system, a heterodyne interferometry system, and an image acquisition system. The light source system uses a helium-neon laser. The magnification system consists of a sample to be tested, a sample stage, a microscope objective, and a convex lens. The heterodyne interferometry system consists of a convex lens, a grating, a linear translation stage, a stepper motor, and an aperture. The image acquisition system consists of a CMOS camera.

[0006] When using the above-mentioned optical imaging device, as shown in the attached Figure 1 As shown, the specific optical path of the magnification system is as follows: when the light beam passes through the sample to be tested placed on the sample stage, it is magnified a second time through the microscope objective and the convex lens. The sample stage needs to be placed at the focal point of the microscope objective, while the sample to be tested needs to be placed within half the field of view of the light beam. The light beam diverges after passing through the microscope objective and is then collimated after passing through the convex lens.

[0007] When using the above-mentioned optical imaging device, the specific optical path of the heterodyne interference system is as follows: after the beam passes through the amplification system, it first passes through a convex lens to converge the beam, and then passes through a grating to generate multiple diffracted beams. An aperture is placed at the focal plane to select the 0th and +1st order diffracted beams.

[0008] When using the above-mentioned optical imaging device, the grating is placed on a linear translation stage driven by a stepper motor, and the movement path of the linear translation stage is perpendicular to the optical axis of the beam. Due to the Doppler effect of the beam and the moving grating, the +1st order diffracted beam has a certain frequency change compared to the 0th order diffracted beam. The specific amount of change is determined by the following formula:

[0009]

[0010] Where, ω d V is the frequency change, v is the grating speed, and d is the grating constant.

[0011] When using the above-mentioned optical imaging device, the CMOS camera in the image acquisition system is placed last, and its position is adjusted so that the 0th and +1st order beams have half of their overlap area. The two beams interfere with each other, and a series of interferograms with the movement of fringes over time are obtained on the receiving surface of the CMOS camera.

[0012] The above measurement scheme mainly includes the following steps:

[0013] Step S1: Optical path adjustment. Place the sample to be tested on the sample stage, within half the field of view, and move the sample stage to a position near the focus. Turn on the light source system and, while changing the distance between the grating and the convex lens, observe the degree of separation between the 0th and +1st order diffraction beams, ensuring that there is a half-overlapping area between the 0th and +1st order diffraction beams on the detection surface of the CMOS camera. At this time, the overlapping part of the +1st order diffraction beam is the reference beam, and the 0th order diffraction beam is the beam to be tested. Adjust the aperture size so that only the 0th and +1st order beams pass through. Start the stepper motor, and use the belt to link with the linear moving platform. The grating moves at a constant speed, generating a stable frequency difference between the 0th and +1st order diffraction beams. By adjusting the exposure and integration time of the CMOS camera, high-contrast fringes can be observed, and the fringes move stably.

[0014] Step S2: Image data acquisition. Run the CMOS camera to capture video of stripe movement over a period of time. Here, the frame rate of the video acquisition must satisfy the Nyquist sampling theorem.

[0015] Step S3: Image processing, specifically, mathematical processing of the acquired multiple images to obtain the wrapped phase information of the object under test, phase unfolding using an unwrapping algorithm, and finally Zernike fitting to remove background phase to obtain phase information containing only the object under test.

[0016] Step S4: Phase-height calibration, using the phase-height mapping relationship to calculate the height information of the object to be measured.

[0017] In summary, the heterodyne digital holographic microscopic phase imaging device and measurement method based on a moving grating proposed in this invention have several significant advantages over traditional digital holographic microscopy schemes:

[0018] First, the reconstruction method using multiple images can achieve higher accuracy results than the traditional single-image reconstruction method. Second, for the first time, a motor-driven grating method is used to achieve heterodyne. The motor is a common commercial motor, which is more cost-effective than the method using multiple acousto-optic modulators. Finally, the optical imaging device structure is more compact than the traditional Mach-Zehnder optical path. The reference light and the probe light interfere in a quasi-common optical path manner, and the interference pattern has higher stability and anti-interference ability. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0020] The numbers on the map are:

[0021] 1—Light source system;

[0022] 2a—Sample to be tested; 2b—Sample stage; 2c—Microscope objective; 2d—Convex lens 1;

[0023] 3a—convex lens 2; 3b—grating; 3c—linear translation stage; 3d—stepper motor; 3e—aperture;

[0024] 4a—CMOS camera; 4b—Schematic diagram of two-beam interference. Detailed Implementation

[0025] As attached Figure 1 As shown, the specific optical path of the magnification system is as follows: when the light beam passes through the sample to be tested (2a) placed on the sample stage (2b), it is magnified twice by the microscope objective (2c) and the convex lens 1 (2d). The sample stage (2b) needs to be placed at the focal point of the microscope objective (2c), while the sample to be tested (2a) needs to be placed within half the field of view of the light beam. The light beam diverges after passing through the microscope objective (2c), and is collimated after passing through the convex lens 1 (2d).

[0026] When using the above-mentioned optical imaging device, the specific optical path of the heterodyne interference system is as follows: after the beam passes through the amplification system, it first passes through the convex lens 2 (3a) to converge the beam, and then passes through the grating (3b) to generate multiple diffracted beams. An aperture (3e) is placed at the focal plane to select the 0th and +1st order diffracted beams.

[0027] When using the above-mentioned optical imaging device, the grating (3b) is placed on a linear translation stage (3c) driven by a stepper motor (3d), and the moving path of the linear translation stage (3c) is perpendicular to the optical axis of the beam. Due to the Doppler effect between the beam and the moving grating (3b), the +1st order diffracted beam has a certain frequency change compared to the 0th order diffracted beam. The specific change is determined by the following formula:

[0028]

[0029] Where, ω d V is the frequency change, v is the grating speed, and d is the grating constant.

[0030] When the above-mentioned optical imaging device is used, the CMOS camera (4a) in the image acquisition system is placed last, and the position is adjusted so that the 0 and +1 order beams have half of the overlapping area. The two beams interfere to obtain a two-beam interference diagram (4b). A series of fringes moving over time are obtained on the receiving surface of the CMOS camera (4a).

[0031] The above measurement scheme mainly includes the following steps:

[0032] Step S1: Optical path adjustment. Place the sample to be tested (2a) on the sample stage (2b) and position it within half the field of view. Move the sample stage to a position near the focus. Turn on the light source system (1). While changing the distance between the grating (3b) and the convex lens 2 (3a), observe the degree of separation between the 0th and +1st order diffraction beams, and ensure that there is a half-overlapping area between the 0th and +1st order diffraction beams on the detection surface of the CMOS camera (4a). At this time, the overlapping part of the +1st order diffraction beam is the reference beam, and the 0th order diffraction beam is the beam to be tested. Adjust the aperture (3e) opening size so that only the 0th and +1st order beams pass through. Start the stepper motor (3d) and use the belt to link with the linear translation stage (3c). The grating (3b) moves at a constant speed, and a stable frequency difference is generated between the 0th and +1st order diffraction beams. By adjusting the exposure and integration time of the CMOS camera (4a), high-contrast fringes can be observed, and the fringes move stably.

[0033] Step S2: Image data acquisition. Run the CMOS camera (4a) to capture video of stripe movement over a period of time. Here, the frame rate of the video acquisition must satisfy the Nyquist sampling theorem.

[0034] Step S3: Image processing, specifically, mathematical processing of the acquired multiple images to obtain the encapsulation phase information of the sample to be tested (2a), phase unfolding using the unwrap algorithm, and finally using Zernike fitting to remove the background phase to obtain phase information containing only the sample to be tested (2a).

[0035] Step S4: Phase-height calibration, using the phase-height mapping relationship to calculate the height information of the sample (2a) to be tested.

[0036] The specific real-time mode of the present invention will be described in detail below with reference to examples and accompanying drawings:

[0037] refer to Figure 1 The specific implementation process is illustrated in the following example:

[0038] Before measurement, prepare the sample. The sample should be able to transmit light and be between a few micrometers and several hundred micrometers in size. Place the sample to be measured (2a) on the sample stage (2b) and position it within half of the field of view of the beam. Move the sample stage to a position near the focus. Turn on the light source system (1). While changing the distance between the grating (3b) and the convex lens 2 (3a), observe the degree of separation between the 0th and +1st order diffraction beams and ensure that there is a half beam overlap area between the 0th and +1st order diffraction beams on the detection surface of the CMOS camera (4a). At this point, the overlapping part of the +1st order diffracted beam is the reference beam, and the 0th order diffracted beam is the beam to be tested. The aperture size of the aperture (3e) is adjusted so that only the 0th and +1st order beams pass through. The stepper motor (3d) is started, and the grating (3b) moves at a constant speed by means of the belt and the linear translation stage (3c). A stable frequency difference is generated between the 0th and +1st order diffracted beams. By adjusting the exposure and integration time of the CMOS camera (4a), high-contrast fringes can be observed, and the fringes move stably. The driving software corresponding to the CMOS camera (4a) is run to capture a video of the fringes moving over a period of time. Here, the frame rate of the video acquisition must satisfy the Nyquist sampling law. The acquired images are processed. Specifically, the acquired multiple images are mathematically processed to obtain the encapsulation phase information of the sample to be tested (2a). The phase is unfolded using the unwrap algorithm. Finally, the background phase is removed by Zernike fitting to obtain the phase information containing only the sample to be tested (2a). Finally, the height information of the sample to be tested (2a) is calculated using the phase-height mapping relationship.

Claims

1. A heterodyne digital holographic microscopic phase imaging device based on a moving grating, characterized in that: The imaging device includes a light source system (1), an amplification system, a heterodyne interference system, and an image acquisition system. The light source system (1) is composed of a helium-neon laser. The amplification system consists of a sample to be tested (2a), a sample stage (2b), a microscope objective (2c), and a convex lens 1 (2d). The heterodyne interference system consists of a convex lens 2 (3a), a grating (3b), a linear translation stage (3c), a stepper motor (3d), and an aperture (3e). The image acquisition system consists of a CMOS camera (4a). After the beam passes through the amplification system, it first passes through the convex lens 2 (3a) to converge the beam, and then passes through the grating (3b) to generate multiple diffracted beams on the focusing plane. An aperture (3e) is placed to select the 0th and +1st order diffracted beams. The grating (3b) is placed on a linear translation stage (3c) driven by a stepper motor (3d), and the moving path of the linear translation stage (3c) is perpendicular to the optical axis of the beam. Due to the Doppler effect of the beam and the moving grating (3b), there is a certain difference in frequency between the +1st order diffracted beam and the 0th order diffracted beam. The CMOS camera (4a) is placed last and its position is adjusted so that the 0th and +1st order beams have half of the overlapping area. The 0th and +1st order beams interfere to obtain a schematic diagram of the interference between the two beams (4b). A series of fringes moving over time are obtained on the receiving surface of the CMOS camera (4a).

2. The heterodyne digital holographic microscopic phase imaging device based on a moving grating according to claim 1, characterized in that: The specific optical path of the amplification system is as follows: when the light beam penetrates the sample to be tested (2a) placed on the sample stage (2b), it is magnified a second time through the microscope objective (2c) and the convex lens 1 (2d). The sample stage (2b) needs to be placed at the focal point of the microscope objective (2c), while the sample to be tested (2a) needs to be placed within half the field of view of the light beam. After the light beam diverges through the microscope objective (2c), it is collimated after passing through the convex lens 1 (2d).

3. A heterodyne digital holographic microscopic phase imaging measurement method based on a moving grating, characterized in that: The imaging process, performed using the heterodyne digital holographic microscopic phase imaging device based on a moving grating as described in claim 1, includes the following steps: Step S1: Optical path adjustment. Place the sample to be tested (2a) on the sample stage (2b) and position it within half the field of view. Move the sample stage to a position near the focus. Turn on the light source system (1). While changing the distance between the grating (3b) and the convex lens 2 (3a), observe the degree of separation between the 0th and +1st order diffraction beams. Make sure there is a half-overlapping area between the 0th and +1st order diffraction beams on the detection surface of the CMOS camera (4a). At this time, the overlapping part of the +1st order diffraction beam is the reference beam, and the 0th order diffraction beam is the beam to be tested. Adjust the aperture (3e) opening size so that only the 0th and +1st order beams pass through. Start the stepper motor (3d). Use the belt to link with the linear translation stage (3c). The grating (3b) moves at a constant speed. A stable frequency difference is generated between the 0th and +1st order diffraction beams. By adjusting the exposure and integration time of the CMOS camera (4a), high-contrast fringes can be observed, and the fringes move stably. Step S2: Image data acquisition. Run the CMOS camera (4a) to capture video of stripe movement over a period of time. Here, the frame rate of the video acquisition must satisfy the Nyquist sampling theorem. Step S3: Image processing, specifically, mathematical processing of the acquired multiple images to obtain the encapsulation phase information of the sample to be tested (2a), phase unfolding using the unwrap algorithm, and finally using Zernike fitting to remove the background phase to obtain phase information containing only the sample to be tested (2a). Step S4: Phase-height calibration, using the phase-height mapping relationship to calculate the height information of the sample (2a) to be tested.

Images