Objective lens back focal plane alignment method and device

Abstract

The present invention provides an objective lens back focal plane alignment method and device, the alignment method comprises the following steps: deploying a light source, a slit, an objective lens, a lens, a diffuse scattering screen and a camera; selecting a standard calibration image for comparison according to the type of the objective lens; turning on the light source, introducing the light of the light source horizontally into the slit, passing through the objective lens after diffraction through the slit and generating interference at the back focal plane of the objective lens to form an interference fringe image, and the lens projects the interference fringe image onto the diffuse scattering screen; moving one or more of the objective lens, the lens or the diffuse scattering screen along the optical axis direction, and acquiring the image on the diffuse scattering screen through the camera at the same time; comparing the image acquired by the camera with the standard calibration image, and when the similarity between the two is greater than or equal to a set value, determining that the interference fringe image on the back focal plane of the objective lens is projected onto the diffuse scattering screen, and completing the alignment of the back focal plane of the objective lens. In this way, the back focal plane of the objective lens is quickly and accurately aligned.

Description

Technical Field

[0001] The invention belongs to the technical field of optical detection and analysis, and in particular relates to a method and a device for aligning the back focal plane of an objective lens. Background Art

[0002] Photonic crystals and metamaterials are a class of optical materials with periodic structures, which are characterized by the ability to control the propagation and properties of light by adjusting their periodic structures. Since their introduction in the 1990s, the study of photonic crystals has attracted widespread attention and has shown great potential in the fields of photonics and optoelectronics. In recent years, with the development of nanotechnology and the continuous advancement of preparation technology, the research on photonic crystals has made significant progress, successfully realizing the construction of optical waveguides, photonic crystal lasers, photonic crystal fibers and other devices, and has made important breakthroughs in the fields of optical communications, sensing, photonic computing, etc.

[0003] The angle-resolved spectroscopy measurement system is an important device for characterizing photonic crystals and metamaterials, and is used to obtain the band structure of photonic crystals and metamaterials. At present, angle-resolved spectroscopy measurement devices based on Fourier transform have been developed on the market. Compared with traditional angle-resolved spectroscopy measurement devices with rotating detectors or sample stages, they have higher detection efficiency and smaller volume, but their detection precision and accuracy strictly depend on the alignment of the rear focal plane of the objective lens, and there are problems such as difficulty in debugging and calibration.

[0004] Therefore, a method and device for aligning the back focal plane of an objective lens are needed to solve or at least partially solve the above problems. Summary of the invention

[0005] The invention provides a method and a device for aligning the back focal plane of an objective lens, which can realize rapid and accurate alignment of the back focal plane of the objective lens.

[0006] To achieve the above object, the present invention provides the following technical solutions.

[0007] The present invention provides a method for aligning the back focal plane of an objective lens, comprising the following steps:

[0008] Arrange a light source, a slit, an objective lens, a lens, a diffuse scattering screen and a camera, wherein the light source, the slit, the objective lens, the lens and the diffuse scattering screen are sequentially arranged on the same optical axis, and the camera is arranged tilted relative to the diffuse scattering screen to capture an image on the diffuse scattering screen;

[0009] selecting a standard calibration image for comparison according to the type of the objective lens;

[0010] Turning on the light source, introducing the light from the light source horizontally into the slit, diffracting through the slit, passing through the objective lens and generating interference at the rear focal plane of the objective lens to form an interference fringe image, and the lens projects the interference fringe image onto the diffuse scattering screen;

[0011] Moving one or more of the objective lens, the lens or the diffuse scattering screen along the optical axis, and acquiring an image on the diffuse scattering screen through the camera;

[0012] The image acquired by the camera is compared with the standard calibration image. When the similarity between the two is greater than or equal to a set value, it is determined that the interference fringe image on the rear focal plane of the objective lens is projected onto the diffuse scattering screen, thereby completing the alignment of the rear focal plane of the objective lens.

[0013] Optionally, comparing the image acquired by the camera with the standard calibration image comprises the following steps: performing grayscale analysis on the image acquired by the camera to generate an image grayscale matrix; performing grayscale analysis on the standard calibration image to generate a standard grayscale matrix; calculating a correlation coefficient between the image grayscale matrix and the standard grayscale matrix; when the calculated correlation coefficient is greater than or equal to a set value, determining that the similarity between the image acquired by the camera and the standard calibration image is greater than or equal to the set value, and completing the alignment of the rear focal plane of the objective lens.

[0014] Optionally, the following steps are included: moving the objective lens or the lens along the optical axis, observing the image on the diffuse scattering screen through the camera, until a clear interference fringe image appears on the diffuse scattering screen, stopping moving the objective lens or the lens, and acquiring the image on the diffuse scattering screen through the camera.

[0015] Optionally, the method comprises the following steps: moving the diffuse scattering screen along the optical axis, observing the image on the diffuse scattering screen through the camera until a clear interference fringe image appears on the diffuse scattering screen, stopping moving the diffuse scattering screen, and acquiring the image on the diffuse scattering screen through the camera.

[0016] Optionally, after completing the alignment of the rear focal plane of the objective lens, the following steps are also included: slightly moving the objective lens, the lens or an element of the diffuse scattering screen along the optical axis direction; checking the image acquired by the camera, if the stripe width of the image increases or the stripe image becomes blurred, then confirming that the position before the movement is the alignment position of the rear focal plane of the objective lens; returning the slightly moved element to the position before the movement.

[0017] Based on the same concept, the present invention also provides an objective lens back focal plane alignment device, comprising a light source, a slit, an objective lens, a lens, a diffuse scattering screen and a camera; the light source, the slit, the objective lens, the lens and the diffuse scattering screen are arranged on the same optical axis, and the camera is arranged tilted relative to the diffuse scattering screen;

[0018] The light source is used to provide a monochromatic and parallel laser beam; the slit is used to diffract the laser beam;

[0019] The objective lens is used as a two-dimensional optical Fourier transform device to focus the diffracted laser light onto the rear focal plane of the objective lens, so that different diffracted light rays meet and interfere with each other, thereby forming an interference fringe image of alternating light and dark on the rear focal plane of the objective lens;

[0020] The lens is used as an imaging element to project the image of the rear focal plane of the objective lens into the diffuse scattering screen;

[0021] The camera is used to capture images in the diffuse scattering screen, and to determine whether the rear focal plane of the objective lens is aligned based on the comparison between the captured images and the standard calibration image.

[0022] Optionally, the objective lens is a combination of one or more of a microscope objective lens, a spherical lens, and an aspherical lens; and the standard calibration image is an interference fringe image of an objective lens of the same type that has been calibrated and taken according to the type of the objective lens.

[0023] Optionally, the light source is a monochromatic laser light source.

[0024] Optionally, the lens is a convex lens, and the lens may be one lens or a lens group consisting of a plurality of lenses.

[0025] Optionally, the diffuse scattering screen is a small hole, a diaphragm or a slit capable of diffuse scattering.

[0026] Compared with the prior art, the technical solution of the embodiment of the present invention has beneficial effects.

[0027] For example, the present invention deploys a light source, a slit, an objective lens, a lens and a diffuse scattering screen which are arranged in sequence on the same optical axis, and deploys a camera to shoot an image on the diffuse scattering screen; adjusts the objective lens, the lens or the diffuse scattering screen, and compares the image on the diffuse scattering screen acquired by the camera with the standard calibration image to confirm whether the alignment of the rear focal plane of the objective lens is completed, and the alignment of the rear focal plane of the objective lens is completed quickly and accurately. The calibration method is independent of parameters such as the working distance, numerical aperture and clear aperture of the objective lens, and has flexible applicability; it can be directly embedded in a variety of angular resolution spectral systems or devices, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a flow chart of a method for aligning the rear focal plane of an objective lens in an embodiment of the present invention;

[0029] Figure 2 2 is a schematic diagram of the structure of the objective lens rear focal plane alignment device in an embodiment of the present invention;

[0030] Figure 3 is a theoretical imaging diagram of the light source at the slit in an embodiment of the present invention;

[0031] Figure 4 is a theoretical image diagram of a light source on a diffuse scattering screen in an embodiment of the present invention;

[0032] Figure 5 is an image on a diffuse scattering screen acquired by a camera when the rear focal plane of the objective lens in the embodiment of the present invention is aligned;

[0033] Figure 6a is a white light transmission spectrum of a photonic crystal sample at a transmission angle of 1° measured by an angle-resolved spectroscopy measurement system when the objective lens back focal plane alignment method in an embodiment of the present invention is applied to the angle-resolved spectroscopy measurement system;

[0034] Figure 6b It is the white light transmission spectrum of the photonic crystal under the condition of 1° transmission angle calculated theoretically.

[0035] Description of reference numerals:

[0036] 1. Light source; 2. Slit; 3. Objective lens; 4. Lens; 5. Diffuse scattering screen; 6. Camera. DETAILED DESCRIPTION

[0037] In order to make the purpose, features and beneficial effects of the present invention more obvious and understandable, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. It is understood that the specific embodiments described below are only used to explain the present invention, rather than to limit the present invention. In addition, the same or similar reference numerals may be used in the figures to refer to the same or similar elements in different embodiments, and the description of the same or similar elements in different embodiments and the description of the prior art elements, features, effects, etc. may also be omitted.

[0038] Reference Figures 1 to 5 , an embodiment of the present invention provides a method and device for aligning the back focal plane of an objective lens.

[0039] See also Figure 1 and Figure 2 , a method for aligning the back focal plane of an objective lens comprises the following steps:

[0040] S101: deploying a light source 1, a slit 2, an objective lens 3, a lens 4, a diffuse scattering screen 5 and a camera 6. The light source 1, the slit 2, the objective lens 3, the lens 4 and the diffuse scattering screen 5 are sequentially arranged on the same optical axis. The camera 6 is arranged tilted relative to the diffuse scattering screen 5 to capture an image on the diffuse scattering screen 5.

[0041] S102: selecting a standard calibration image for comparison according to the type of objective lens 3;

[0042] S103: Turn on the light source 1, and introduce the light of the light source 1 horizontally into the slit 2. After being diffracted by the slit 2, the light passes through the objective lens 3 and generates interference at the back focal plane of the objective lens 3 to form an interference fringe image. The lens 4 projects the interference fringe image onto the diffuse scattering screen 5.

[0043] S104: moving one or more of the objective lens 3, the lens 4 or the diffuse scattering screen 5 along the optical axis direction, and acquiring an image on the diffuse scattering screen 5 through the camera 6;

[0044] S105: Compare the image acquired by the camera 6 with the standard calibration image. When the similarity between the two is greater than or equal to the set value, it is determined that the interference fringe image on the rear focal plane of the objective lens 3 is projected onto the diffuse scattering screen 5, and the alignment of the rear focal plane of the objective lens is completed.

[0045] In some embodiments, the image acquired by the camera 6 is compared with the standard calibration image, including the following steps: grayscale analysis of the image acquired by the camera 6 to generate an image grayscale matrix; grayscale analysis of the standard calibration image to generate a standard grayscale matrix; correlation coefficient calculation between the image grayscale matrix and the standard grayscale matrix; when the calculated correlation coefficient is greater than or equal to a set value, it is determined that the similarity between the image acquired by the camera 6 and the standard calibration image is greater than or equal to the set value, and the alignment of the rear focal plane of the objective lens is completed.

[0046] The function of lens 4 is to project the interference fringe image formed on the rear focal plane of the objective lens 3 onto the diffuse scattering screen 5, and it can be a combination of one or more lenses 4; interference fringes are observed in the diffuse scattering screen 5, indicating that the interference fringe image formed on the rear focal plane of the objective lens is projected onto the diffuse scattering screen 5 to complete the alignment.

[0047] In some embodiments, the following steps are included: moving the objective lens 3 or the lens 4 along the optical axis, observing the image on the diffuse scattering screen 5 through the camera 6 until a clear interference fringe image appears on the diffuse scattering screen 5, stopping moving the objective lens 3 or the lens 4, and acquiring the image on the diffuse scattering screen 5 through the camera 6.

[0048] In some embodiments, the following steps are included: moving the diffuse scattering screen 5 along the optical axis direction, observing the image on the diffuse scattering screen 5 through the camera 6 until a clear interference fringe image appears on the diffuse scattering screen 5, stopping moving the diffuse scattering screen 5, and acquiring the image on the diffuse scattering screen 5 through the camera 6.

[0049] In some embodiments, if the optical elements are placed in a lens barrel, it is relatively difficult to adjust the lens 4 and the diffuse scattering screen 5; in this case, calibration is performed by moving the objective lens 3.

[0050] In some embodiments, when an angle-resolved spectrum is collected using a spectrometer equipped with a surface array CCD, the diffuse scattering screen 5 is usually a slit; since the slit is usually connected to the spectrometer, it is not convenient to move, and the focal plane of the objective lens 3 usually coincides with the sample plane; at this time, calibration is performed by moving the lens 4.

[0051] In some embodiments, such as when using a fiber-coupled spectrometer to collect angle-resolved spectra, the diffuse scattering screen 5 is usually a small hole; the focal plane of the objective lens 3 usually coincides with the sample plane, and moving the lens 4 may change the image size, thereby affecting the angular resolution; at this time, calibration is performed by moving the small hole, i.e., the diffuse scattering screen 5.

[0052] In some embodiments, after completing the alignment of the rear focal plane of the objective lens 3, the following steps are also included: slightly moving an element of the objective lens 3, the lens 4 or the diffuse scattering screen 5 along the optical axis direction; checking the image acquired by the camera 6, if the stripe width of the image increases or the stripe image becomes blurred, then confirming that the position before the movement is the alignment position of the rear focal plane of the objective lens 3; returning the slightly moved element to the position before the movement.

[0053] See also Figure 2 The present invention provides an objective lens back focal plane alignment device, comprising a light source 1, a slit 2, an objective lens 3, a lens 4, a diffuse scattering screen 5 and a camera 6. The light source 1, the slit 2, the objective lens 3, the lens 4 and the diffuse scattering screen 5 are sequentially arranged on the same optical axis, and the camera 6 is inclined relative to the diffuse scattering screen 5;

[0054] The light source 1 is used to provide a monochromatic and parallel laser beam; the slit 2 is used to diffract the laser beam;

[0055] The objective lens 3 is used as a two-dimensional optical Fourier transform device to focus the diffracted laser light onto the rear focal plane of the objective lens 3, so that different diffracted light rays meet and interfere with each other, thereby forming an interference fringe image of alternating light and dark on the rear focal plane of the objective lens 3;

[0056] The lens 4 is used as an imaging element to project the image of the rear focal plane of the objective lens 3 onto the diffuse scattering screen 5;

[0057] The camera 6 is used to capture the image in the diffuse scattering screen 5 and to determine whether the rear focal plane of the objective lens 3 is aligned based on the comparison between the captured image and the standard calibration image.

[0058] In some embodiments, the objective lens 3 is a combination of one or more of a microscope objective lens, a spherical lens, and an aspherical lens; the standard calibration image is an interference fringe image of an objective lens 3 of the same type that has been calibrated and taken according to the type of the objective lens 3.

[0059] In some embodiments, the light source 1 is a monochromatic laser light source, and the laser wavelength provided by the light source 1 is greater than or equal to 600 nm, which is conducive to generating clear and fine interference fringe patterns.

[0060] In some embodiments, the lens 4 is a convex lens, and the lens 4 may be a single lens or a lens group consisting of a plurality of lenses.

[0061] In some embodiments, the diffuse scattering screen 5 is a small hole, a diaphragm or a slit that can perform diffuse scattering.

[0062] In some embodiments, the width of the slit 2 is less than or equal to 50 micrometers, which is conducive to generating clear and fine interference fringe patterns.

[0063] In some embodiments, the camera 6 is equipped with a zoom lens, which magnifies the image in the diffuse scattering screen 5 by 10 times for observation, which is conducive to clearly observing the changes of the image in the diffuse scattering screen 5.

[0064] See also Figure 3 and Figure 4 , slit 2 is set vertically, and the monochromatic and parallel laser emitted by light source 1 passes through slit 2. Since slit 2 is very narrow in the horizontal direction and the length of the vertical image is the same as the laser diameter, a Figure 3 The long strip light spot with small width shown in the figure.

[0065] Consider the discrete Fourier transform formula:

[0066]

[0067] Where x and y are real space coordinates, M and N are the width and height of the image, and u and v are momentum space coordinates.

[0068] Using the discrete Fourier transform formula Figure 3 The pattern in is Fourier transformed to obtain its image in momentum space, that is, the theoretical imaging pattern at the rear focal plane of objective lens 3, as shown in Figure 4 As shown, it is a horizontal arrangement of light and dark stripes.

[0069] According to the Fraunhofer diffraction principle, when the projections of the diffuse scattering screen 5 and the rear focal plane of the objective lens 3 do not overlap, there will be laser light in the black area between the stripes, which will cause the stripe clarity to decrease and the stripe width to become wider. Therefore, the imaging condition in the camera 6 can be used as a criterion to achieve rapid alignment of the rear focal plane of the objective lens 3.

[0070] Figure 5 This is an image obtained by the camera 6 after calibration in this embodiment. Obvious and clear light and dark stripes can be observed, and the arrangement direction of the stripes is parallel to the width direction of the slit 2.

[0071] See also Figure 5 In a specific implementation, the objective lens back focal plane alignment method in the embodiment of the present invention is applied to the angle-resolved spectroscopy measurement system, and a pinhole is selected as the diffuse scattering screen 5; the back focal plane of the objective lens 3 is accurately corrected, and the white light transmission spectrum of the photonic crystal sample under the transmission angle of 1° is measured by the angle-resolved spectroscopy measurement system aligned with the objective lens back focal plane. The results are as follows: Figure 6a As shown, the horizontal axis represents the wavelength, and the vertical axis represents the transmission rate; Figure 6bThe theoretical calculation results of the white light transmission spectrum of the photonic crystal sample under the condition of 1° transmission angle are shown, with the horizontal axis representing the wavelength and the vertical axis representing the transmittance. The high degree of agreement between the two not only verifies the accuracy of the angle-resolved spectroscopy measurement experimental method, but also confirms the feasibility of the objective lens back focal plane alignment method and device in the embodiment of the present invention in practical applications.

[0072] In summary, the present invention deploys a light source 1, a slit 2, an objective lens 3, a lens 4 and a diffuse scattering screen 5 which are arranged in sequence on the same optical axis, and deploys a camera 6 to shoot an image on the diffuse scattering screen 5; adjusts the objective lens 3, the lens 4 or the diffuse scattering screen 5, and compares the image on the diffuse scattering screen 5 obtained by the camera 6 with the standard calibration image to confirm whether the alignment of the rear focal plane of the objective lens 3 is completed, and the alignment of the rear focal plane of the objective lens 3 is completed quickly and accurately. The calibration method is independent of parameters such as the working distance, numerical aperture and clear aperture of the objective lens 3, and has flexible applicability; it can be directly embedded in a variety of angular resolution spectral systems or devices, and has broad application prospects.

[0073] Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even when only a single embodiment is described with respect to specific features. The feature examples provided in the present disclosure are intended to be illustrative rather than limiting, unless otherwise stated. In specific implementations, the technical features of one or more dependent claims may be combined with the technical features of the independent claim, based on actual needs and where technically feasible, and may be derived from the technical features of the corresponding independent claim in any appropriate manner rather than simply through the specific combination listed in the claims.

Claims

1. A method for aligning the back focal plane of an objective lens, characterized in that: The steps include: Arrange a light source, a slit, an objective lens, a lens, a diffuse scattering screen and a camera, wherein the light source, the slit, the objective lens, the lens and the diffuse scattering screen are arranged on the same optical axis in sequence, the camera is arranged obliquely relative to the diffuse scattering screen, and is located on the same side of the diffuse scattering screen as the light source, the slit, the objective lens and the lens, so as to shoot an image on the diffuse scattering screen, the lens is a convex lens, and the slit has a width of less than or equal to 50 microns and is arranged vertically; The objective lens is a two-dimensional optical Fourier transform device and satisfies the discrete Fourier transform formula: Where x and y are real space coordinates, M and N are image width and height, and u and v are momentum space coordinates. A standard calibration image for comparison is selected according to the type of objective lens; Turning on the light source, introducing the light of the light source horizontally into the slit, diffracting through the slit, passing through the objective lens and interfering at the rear focal plane of the objective lens to form an interference fringe image, and the lens projects the interference fringe image onto the diffuse scattering screen; Moving one or more of the objective lens, the lens or the diffuse scattering screen along the optical axis, and acquiring an image on the diffuse scattering screen through the camera; Comparing the image acquired by the camera with the standard calibration image comprises the following steps: performing grayscale analysis on the image acquired by the camera to generate an image grayscale matrix; Perform grayscale analysis on the standard calibration image to generate a standard grayscale matrix; calculate the correlation coefficient between the image grayscale matrix and the standard grayscale matrix; When the calculated correlation coefficient is greater than or equal to a set value, it is determined that the similarity between the image acquired by the camera and the standard calibration image is greater than or equal to the set value, and it is determined that the interference fringe image on the rear focal plane of the objective lens is projected onto the diffuse scattering screen, completing the alignment of the rear focal plane of the objective lens.

2. The objective lens back focal plane alignment method according to claim 1, characterized in that: The method comprises the following steps: moving the objective lens or the lens along the optical axis, observing the image on the diffuse scattering screen through the camera, until a clear interference fringe image appears on the diffuse scattering screen, stopping moving the objective lens or the lens, and acquiring the image on the diffuse scattering screen through the camera.

3. The objective lens back focal plane alignment method according to claim 1, characterized in that: The method comprises the following steps: moving the diffuse scattering screen along the optical axis, observing the image on the diffuse scattering screen through the camera, until a clear interference fringe image appears on the diffuse scattering screen, stopping moving the diffuse scattering screen, and acquiring the image on the diffuse scattering screen through the camera.

4. The objective lens back focal plane alignment method according to claim 1, characterized in that: After the alignment of the rear focal plane of the objective lens is completed, the following steps are also included: slightly moving the objective lens, the lens or an element of the diffuse scattering screen along the optical axis direction; checking the image acquired by the camera, if the stripe width of the image increases or the stripe image becomes blurred, then confirming that the position before the movement is the alignment position of the rear focal plane of the objective lens; returning the slightly moved element to the position before the movement.

5. An objective lens back focal plane alignment device, used to implement the objective lens back focal plane alignment method according to any one of claims 1 to 4, characterized in that: The invention comprises a light source, a slit, an objective lens, a lens, a diffuse scattering screen and a camera; the light source, the slit, the objective lens, the lens and the diffuse scattering screen are arranged on the same optical axis, and the camera is arranged tilted relative to the diffuse scattering screen; the light source is used to provide a monochromatic and parallel laser beam; the slit is used to diffract the laser beam; the objective lens is used as a two-dimensional optical Fourier transform device to focus the diffracted laser beam onto the rear focal plane of the objective lens, so that different diffracted light rays meet and interfere with each other, thereby forming an interference fringe image of alternating light and dark on the rear focal plane of the objective lens; The lens is used as an imaging element to project the image of the rear focal plane of the objective lens onto the diffuse scattering screen; the camera is used to capture the image in the diffuse scattering screen, and to determine whether the rear focal plane of the objective lens is aligned based on the comparison between the captured image and the standard calibration image.

6. The objective lens back focal plane alignment device according to claim 5, characterized in that: The objective lens is a combination of one or more of a microscope objective lens, a spherical lens, and an aspherical lens; the standard calibration image is an interference fringe image of an objective lens of the same type that has been calibrated and is taken according to the type of the objective lens.

7. The objective lens back focal plane alignment device according to claim 5, characterized in that: The light source is a monochromatic laser light source.

8. The objective lens back focal plane alignment device according to claim 5, characterized in that: The lens is one lens or a lens group consisting of a plurality of lenses.

9. The objective lens back focal plane alignment device according to claim 5, characterized in that: The diffuse scattering screen is a small hole, a diaphragm or a slit that can perform diffuse scattering.

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