Flat-field super-apochromatic long working distance microscope objective

Abstract

This invention provides a plan-field superapochromatic microscope objective with a long working distance. From the image side to the object side, along the optical axis, a first, second, third, and fourth optical element and an optical window are arranged sequentially. The first optical element is a lens group with positive optical power; the second optical element is a cemented meniscus lens or a biconcave lens with negative optical power; the third optical element is a biconvex lens with positive optical power; and the fourth optical element includes 1 to 3 cemented meniscus lenses with positive optical power. The optical window is a thick glass plate with two parallel surfaces. The working distance of this microscope objective can reach 15 mm (including the thickness of the observation window). The microscope objective designed in this invention provides good chromatic aberration correction in the wavelength range of 450 nm to 1100 nm and achieves diffraction-limited resolution within a central field of view of φ < 0.2 mm. It can be widely used for observing microscopic samples at the micrometer scale in extreme environments.

Description

Technical Field

[0001] This invention relates to the field of long working distance microscope objectives, specifically a plan superapochromatic long working distance microscope objective. Background Technology

[0002] Planar superachromatic long working distance microscope objectives are widely used in industries such as semiconductors, electronics, and metallurgy, especially in the observation of samples in extreme environments such as ultra-high vacuum and ultra-low temperature. In order to obtain extreme environmental conditions, the samples being observed are usually sealed in equipment with a glass observation window. The thickness of the glass observation window is usually around 5 mm, and the distance between the sample and the observation window can reach 10 mm. Existing products and technologies cannot meet the requirements for the observation of such samples. For example, the commercial planar superachromatic long working distance microscope objectives produced by companies such as Mitutoyo in Japan can only compensate for a maximum glass window thickness of 3.5 mm, and its maximum numerical aperture is NA = 0.5. Existing long working distance microscope objectives that have been disclosed (patent publication numbers CN110308548A and CN113031240A) cannot compensate for the glass observation window and use multiple cemented lenses, which leads to increased processing costs and assembly difficulties. Summary of the Invention

[0003] The purpose of this invention is to provide a plan-field superapochromatic long-working-distance microscope objective with a working distance greater than 15mm. It provides excellent chromatic aberration correction within the wavelength range of 450nm to 1100nm, achieving diffraction-limited resolution within a central field of view of φ<0.2mm. This solves the problem of existing optical windows with a thickness greater than 3.5mm being unable to compensate for aberrations. Furthermore, this microscope objective has a simple structure, employing only a single cemented doublet lens, facilitating fabrication and assembly. To solve the aforementioned technical problems, the technical solution adopted by this invention is as follows:

[0004] A plan-field superapochromatic long working distance microscope objective includes a first optical element, a second optical element, a third optical element, a fourth optical element, and an optical window arranged sequentially along the optical axis from the image side to the object side. The first optical element is a lens group with positive optical power, used to correct chromatic aberration and spherical aberration. The second optical element is a cemented meniscus lens or a biconcave lens with negative optical power, mainly used to correct spherical aberration and field curvature. The third optical element is a biconvex lens with positive optical power, mainly used to correct spherical aberration. The fourth optical element includes 1 to 3 cemented meniscus lenses with positive optical power, the convex surfaces of which all face the image side, and the concave surfaces face the object side, used to provide optical power. The optical window is a thick glass plate with two parallel surfaces, placed perpendicular to the optical axis, used to isolate the observed object.

[0005] The lens group of the first optical element consists of a cemented meniscus lens and a biconvex lens. The convex surface of the cemented meniscus lens faces the image side, and the concave surface faces the object side. Its refractive index and Abbe number satisfy 1.65 ≤ n d ≤ 1.75 and 50 ≤ v d ≤ 60; The surface with a smaller curvature radius of the biconvex lens faces the image side, and its refractive index and Abbe number satisfy 1.45 ≤ n d ≤ 1.55 and 77 < v d < 87;

[0006] The concave surface or the concave surface with a smaller curvature radius of the cemented meniscus lens or the biconcave lens of the second optical element faces the image side, and its refractive coefficient and Abbe number satisfy 1.69 ≤ n d ≤ 1.79 and 44 ≤ v d ≤ 54;

[0007] The biconvex lens of the third optical element is made of fluorite material CaF2. When the size of the lens φ ≤ 25.4, the surface with a smaller curvature radius faces the image side. When the size of the lens φ > 25.4, the surface with a larger curvature radius faces the image side;

[0008] Let the total focal length of the microscope objective be f, the focal length of the first optical element be f1, the focal length of the second optical element be f2, the focal length of the third optical element be f3, and the focal length of the fourth optical element be f4. Then they satisfy the conditions

[0009]

[0010] When the objective NA ≤ 0.4, the fourth optical element is a cemented meniscus lens;

[0011] When the objective 0.4 < NA ≤ 0.5, the fourth optical element consists of two cemented meniscus lenses placed in sequence along the optical axis from the image side to the object side. Let the focal lengths of the two cemented meniscus lenses placed in sequence along the optical axis from the image side to the object side be f 41 and f 42 , then they should satisfy the conditions: and f 42 < f 41 .

[0012] When the objective NA > 0.5, the fourth optical element consists of three cemented meniscus lenses. Let the focal lengths of the three cemented meniscus lenses placed in sequence along the optical axis from the image side to the object side be f 41 、f 42 and f 43 , then they should satisfy the conditions: and f 43 < f 42 < f 41.

[0013] As a further improvement to the above solution, the optical window material is quartz glass.

[0014] As a further improvement to the above scheme, the cemented doublet meniscus lenses of the fourth optical component are all made of fluorite material CaF2.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] This invention provides a plan-field superapochromatic long-working-distance microscope objective, capable of compensating for spherical aberration, coma, and chromatic aberration in quartz glass windows with thicknesses ranging from 3 mm to 10 mm within a wavelength range of 450 nm to 1100 nm, with a working distance greater than 15 mm. Furthermore, this invention employs only one cemented doublet lens and a maximum of five spherical lenses, resulting in a simple structure that facilitates fabrication and assembly. By adjusting and optimizing the number and size of lenses in the fourth optical component, the requirements for different numerical apertures and working distances for sample observation can be met. In summary, our design, through optimization, can achieve the requirements for large-field-of-view imaging, resolution, optical manipulation, and detection of samples in extreme environments with thick glass observation windows over a wide wavelength range. Figure 4-6 The images show the analysis diagrams of multicolor wave aberration, multicolor longitudinal spherical aberration, astigmatism, distortion, and modulation transfer function of the microscope objectives of the present invention when the optical window thickness is D = 5 mm, corresponding to NA = 0.4, 0.5, and 0.69, as well as the resolution test diagram under an 852 nm light field. The comparison shows that the microscope objectives designed according to the present invention provide good correction for chromatic aberration in the wavelength range of 450 nm to 1100 nm, and can achieve diffraction-limited resolution in the range of φ < 0.2 mm in the central field of view. Attached Figure Description

[0017] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, wherein:

[0018] Figure 1 This is a schematic diagram of the structure of the microscope objective of the present invention when NA=0.4 and the thickness of the compensated quartz window D=5mm;

[0019] Figure 2 This is a schematic diagram of the structure of the microscope objective of the present invention when NA=0.5 and the thickness of the compensated quartz window D=5mm;

[0020] Figure 3 This is a schematic diagram of the microscope objective of the present invention when NA = 0.69 and the thickness of the compensated quartz window D = 5 mm;

[0021] Figure 4The microscope objectives of this invention are analyzed for polychromatic aberration, polychromatic longitudinal spherical aberration, astigmatism, distortion, and modulation transfer function in the wavelength range of 450nm to 1100nm when NA=0.4 and the compensated quartz window thickness D=5mm, as well as the resolution test diagram under an 852nm light field.

[0022] Figure 5 The microscope objectives of this invention are analyzed in terms of polychromatic aberration, polychromatic longitudinal spherical aberration, astigmatism, distortion, and modulation transfer function in the wavelength range of 450nm to 1100nm when NA=0.5 and the thickness of the compensated quartz window D=5mm, as well as the resolution test diagram under an 852nm light field.

[0023] Figure 6 The diagrams show the analysis of polychromatic aberration, polychromatic longitudinal spherical aberration, astigmatism, distortion, and modulation transfer function of the microscope objective in the wavelength range of 450nm to 1100nm when NA=0.69 and the compensated quartz window thickness D=5mm, as well as the resolution test diagram under an 852nm light field. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] The surface curvature radius and thickness of each lens, as well as the distance between lenses, are determined based on the working distance of the microscope objective and the thickness of the optical window to be compensated. The thickness D of the optical window satisfies 3mm≤D≤10mm.

[0026] Example 1

[0027] like Figure 1 As shown, a plan-field superapochromatic long working distance microscope objective with a numerical aperture NA = 0.4 is composed of a first optical element 1, a second optical element 2, a third optical element 3, a fourth optical element 4, and an optical window 5, arranged sequentially from the image side to the object side. The first optical element 1 is a lens group with positive optical power, the second optical element 2 is a cemented meniscus lens with negative optical power, the third optical element 3 is a biconvex lens with positive optical power, the fourth optical element 4 is a positive cemented meniscus lens, and the optical window 5 is a thick glass plate with two parallel surfaces.

[0028] The lens group includes a cemented doublet meniscus lens and a biconvex lens. The convex surface of the cemented doublet meniscus lens faces the image side, and the concave surface faces the object side. It is made of crown glass with a refractive index and Abbe coefficient of n. d =1.697, v d=55.5, the biconvex lens with the smaller radius of curvature faces the image side, and crown glass material is selected, with a refractive index and Abbe coefficient of n. d =1.497, v d =81.6; The concave surface of the cemented doublet meniscus lens faces the image side, and it is made of flint glass with a refractive index and Abbe coefficient of n. d =1.743, v d =49.2; The biconvex lens is made of fluorite material CaF2, with a lens size φ=25.4mm and the same radius of curvature on both sides; The cemented doublet meniscus lens is made of fluorite material CaF2, with its convex surface facing the image side and its concave surface facing the object side; The optical window is a thick glass plate with two parallel sides, made of quartz glass, with a thickness D=5mm, and placed perpendicular to the optical axis;

[0029] Let the total focal length of the microscope objective be f, the focal length of the lens group be f1, the focal length of the lens be f2, the focal length of the lens be f3, and the focal length of the fourth element 4 be f4. Then they satisfy the condition...

[0030] Example 2

[0031] like Figure 2 As shown, a plan-field superapochromatic long working distance microscope objective with a numerical aperture NA = 0.5 is composed of a first optical element 1, a second optical element 2, a third optical element 3, a fourth optical element 4, and an optical window 5, arranged sequentially from the image side to the object side. The first optical element 1 is a lens group with positive optical power, the second optical element 2 is a cemented meniscus lens with negative optical power, the third optical element 3 is a biconvex lens with positive optical power, the fourth element 4 includes two cemented meniscus lenses with positive optical power, and the optical window 5 is a thick glass plate with two parallel surfaces.

[0032] The lens group includes a cemented doublet meniscus lens and a biconvex lens. The convex surface of the cemented doublet meniscus lens faces the image side, and the concave surface faces the object side. It is made of crown glass with a refractive index and Abbe coefficient of n. d =1.697, v d =55.5, the biconvex lens with the smaller radius of curvature faces the image side, and crown glass material is selected, with a refractive index and Abbe coefficient of n. d =1.497, v d =81.6; The concave surface of the cemented doublet meniscus lens faces the image side, and it is made of flint glass with a refractive index and Abbe coefficient of n. d =1.743, v d=49.2; The biconvex lens is made of fluorite material CaF2, with a lens size φ=25.4mm, and the side with the smaller radius of curvature on both sides faces the image side; The cemented doublet meniscus lens is also made of fluorite material CaF2, with its convex side facing the image side and its concave side facing the object side; The optical window is a thick glass plate with two parallel sides, made of quartz glass, with a thickness of D=5mm, and placed perpendicular to the optical axis;

[0033] Let the total focal length of the microscope objective be f, the focal length of the lens group be f1, the focal length of the lens be f2, the focal length of the lens be f3, and the focal length of the fourth element 4 be f4. Then they satisfy the following condition: Let the focal lengths of two cemented meniscus lenses placed sequentially along the optical axis from the image side to the object side be f. 41 and f 42 Then they should meet the following conditions:

[0034] Example 3

[0035] like Figure 3 As shown, a plan-field superapochromatic long working distance microscope objective with a numerical aperture NA = 0.69 is composed of a first optical element 1, a second optical element 2, a third optical element 3, a fourth optical element 4, and an optical window 5, arranged sequentially from the image side to the object side. The first optical element 1 is a lens group with positive optical power, the second optical element 2 is a biconcave lens with negative optical power, the third optical element 3 is a biconvex lens with positive optical power, the fourth element 4 includes three cemented meniscus lenses with positive optical power, and the optical window 5 is a thick glass plate with two parallel surfaces.

[0036] The lens group includes a cemented doublet meniscus lens and a biconvex lens. The convex surface of the cemented doublet meniscus lens faces the image side, and the concave surface faces the object side. It is made of crown glass with a refractive index and Abbe coefficient of n. d =1.697, v d =55.5, the biconvex lens with the smaller radius of curvature faces the image side, and crown glass material is selected, with a refractive index and Abbe coefficient of n. d =1.497, v d =81.6; The concave surface of the biconcave lens with the smaller radius of curvature faces the image side, and flint glass material is selected, with a refractive index and Abbe coefficient of n. d =1.743, v d=49.2; The biconvex lens is made of fluorite material CaF2, with a lens size φ=50.8mm, and the side with the smaller radius of curvature on both sides faces the image side; The cemented doublet meniscus lens is also made of fluorite material CaF2, with its convex side facing the image side and its concave side facing the object side; The optical window is a thick glass plate with two parallel sides, made of quartz glass, with a thickness D=5mm, and placed perpendicular to the optical axis;

[0037] Let the total focal length of the microscope objective be f, the focal length of the lens group be f1, the focal length of the lens be f2, the focal length of the lens be f3, and the focal length of the fourth element 4 be f4. Then they satisfy the following condition: Let the focal lengths of the cemented meniscus lenses placed sequentially along the optical axis from image side to object side be f. 41 f 42 and f 43 Then they should meet the following conditions:

[0038] Table 1 shows the optical parameters and characteristic data of a plan-field superapochromatic long working distance microscope objective with a median numerical aperture NA = 0.4 and a compensated quartz window thickness D = 5 mm, from left to right. It can be seen that the working distance of this microscope objective is greater than 25 mm (the distance from the rear surface of the cemented meniscus lens to the observed sample, including the 5 mm thick quartz observation window). The thickness and radius of curvature of each optical element are within a reasonable range, and the lens accuracy can be guaranteed by ordinary optical processing. At the same time, the optical lens types include one cemented doublet and three single lenses, which are simple in structure and easy to assemble, while greatly reducing the processing cost. Figure 4 This is an embodiment of the present invention, showing the analysis of polychromatic aberration, polychromatic longitudinal spherical aberration, astigmatism, distortion, and modulation transfer function in the wavelength range of 450nm to 1100nm at NA=0.4 and a compensated quartz window thickness D=5mm, as well as the resolution test diagram under an 852nm light field. The diagrams show that spherical aberration, chromatic aberration, astigmatism, and distortion are well controlled and corrected. The optical transfer function approaches the diffraction limit within an object size of φ=0.2mm, indicating that the microscope objective can perform high-resolution imaging and observation of the observed sample within this range. Using a standard USAF1951 resolution test target at 852nm, 8-5 sets of fringes can be clearly distinguished, indicating a resolution better than 1.2 micrometers.

[0039] Table 1

[0040]

[0041] To achieve a plan-field superachromatic long working distance microscope objective with a larger numerical aperture, this invention can be achieved by increasing the number of lenses in the fourth optical element, increasing the size of the lenses, and optimizing the spherical radius of curvature, lens thickness, and distance between lenses. Tables 2 and 3 show the optical parameters and characteristic data of a plan-field superachromatic long working distance microscope objective of this invention from left to right when compensating for a quartz window thickness D = 5 mm, NA = 0.5, and NA = 0.69.

[0042] Table 2

[0043]

[0044] Table 3

[0045]

[0046] As can be seen from Table 2, when the numerical aperture NA of the microscope objective is 0.5, the number of lenses in the fourth optical component increases to 2, the working distance is greater than 17 mm, and the curvature radius and thickness of each lens surface are also optimized accordingly. Figure 5 The figures show the analysis of polychromatic aberration, polychromatic longitudinal spherical aberration, astigmatism, distortion, and modulation transfer function in the wavelength range of 450nm to 1100nm, as well as the resolution test results under an 852nm light field. The figures demonstrate that spherical aberration, chromatic aberration, astigmatism, and distortion are well controlled and corrected, especially with a slight improvement in chromatic aberration compared to the embodiment with NA = 0.4. The optical transfer function approaches the diffraction limit within an object size of φ = 0.2mm. Using a standard USAF 1951 resolution test target at 852nm, nine sets of fringes can be clearly distinguished, indicating a resolution better than 1 micrometer.

[0047] As can be seen from Table 3, when the numerical aperture NA of the microscope objective is 0.69, the number of lenses in the fourth optical component increases to 3, the size of each element increases to 50.8 mm, the working distance of the microscope objective is greater than 16 mm, and the curvature radius and thickness of each lens surface are also optimized accordingly. Figure 6 Yes, the graphs show the analysis of polychromatic aberration, polychromatic longitudinal spherical aberration, astigmatism, distortion, and modulation transfer function in the wavelength range of 450nm to 1100nm, as well as the resolution test graph under an 852nm light field. The graphs show that spherical aberration, astigmatism, and distortion are still well controlled and corrected, and while chromatic aberration has decreased slightly, it is still well compensated. Due to the influence of chromatic aberration, the overall optical transfer function has also decreased, but it is close to the diffraction limit within an object size of φ = 0.2mm. Using a standard USAF 1951 resolution test target at 852nm, 9-3 sets of fringes can be clearly distinguished, indicating that its resolution is better than 0.8 micrometers.

[0048] For quartz glass observation windows of different thicknesses, this invention can maintain the basic structure of the lens group unchanged under different numerical aperture requirements, and correct various aberrations by optimizing the spherical curvature radius of the lens and the distance between the lenses;

[0049] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description, and all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0050] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A plan-field superapochromatic microscope objective with a long working distance, characterized in that: The system includes a first optical element (1), a second optical element (2), a third optical element (3), a fourth optical element (4), and an optical window (5) arranged sequentially along the optical axis from the image side to the object side. The first optical element (1) is a lens group with positive optical power; the second optical element (2) is a cemented meniscus lens or a biconcave lens with negative optical power; the third optical element (3) is a biconvex lens with positive optical power; the fourth optical element (4) includes 1 to 3 cemented meniscus lenses with positive optical power, the convex surfaces of which all face the image side and the concave surfaces face the object side; the optical window (5) is a thick glass plate with two parallel surfaces, placed perpendicular to the optical axis. The lens group of the first optical component (1) consists of a double cemented meniscus lens and a double convex lens, the convex surface of the double cemented meniscus lens faces the image side, the concave surface faces the object side, the refractive index and Abbe number thereof satisfy 1.65≤n d ≤1.75 and 50≤v d ≤60; the surface with small curvature radius of the double convex lens faces the image side, the refractive index and Abbe number thereof satisfy 1.45≤n d ≤1.55 and 77<v d <87; The concave surface or the concave surface with a small radius of curvature of the double-cemented meniscus lens or the double-concave lens of the second optical component (2) faces the image side, and the refractive index and Abbe number thereof satisfy 1.69 d ≤ 1.79 and 44 ≤ v d ≤ 54. The biconvex lens of the third optical component (3) is made of fluorite material CaF2. When the size of the lens φ≤25.4, the surface with the smaller radius of curvature faces the image side. When the size of the lens φ>25.4, the surface with the larger radius of curvature faces the image side. Let the total focal length of the microscope objective be f, the focal length of the first optical component (1) be f1, the focal length of the second optical component (2) be f2, the focal length of the third optical component (3) be f3, and the focal length of the fourth optical component (4) be f4. Then they satisfy the condition...

2. The plan-field superapochromatic long working distance microscope objective according to claim 1, characterized in that: When the objective lens NA≤0.4, the fourth optical component (4) is a cemented doublet meniscus lens.

3. The plan-field superapochromatic long working distance microscope objective according to claim 1, characterized in that: When the numerical aperture of the objective lens satisfies \(0.4 < NA \leq 0.5\), the fourth optical component (4) consists of two cemented meniscus lenses arranged along the optical axis from the image side to the object side in sequence. Let the focal lengths of the two cemented meniscus lenses arranged along the optical axis from the image side to the object side be \(f 41 and \(f 42 respectively. Then the following conditions should be satisfied between them: and \(f 42 < f 41 .

4. The plan-field superapochromatic long working distance microscope objective according to claim 1, characterized in that: When the objective lens NA>0.5, the fourth optical component (4) consists of three cemented meniscus lenses. Let the focal lengths of the three cemented meniscus lenses placed sequentially along the optical axis from the image side to the object side be f. 41 f 42 and f 43 Then they should meet the following conditions: And f 43 <f 42 <f 41 .

5. The plan-field superapochromatic long working distance microscope objective according to claim 1, characterized in that: The optical window (5) is made of quartz glass.

6. A plan-field superapochromatic long working distance microscope objective according to any one of claims 1-5, characterized in that: The doublet meniscus lenses of the fourth optical component (4) are all made of fluorite material CaF2.

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