Objective lens and optical apparatus

Abstract

The embodiment of the invention provides an objective lens and optical equipment. The optical device includes a light source and an objective lens. The objective lens comprises a front lens group and a rear lens group, and the front lens group and the rear lens group are sequentially arranged from the object side to the image side; the front lens group has positive focal power, and the rear lens group has negative focal power; the front lens group comprises a plurality of first lenses, at least one of the plurality of first lenses is provided with a diffraction surface, the diffraction surface faces the rear lens group, and the diffraction surface is used for emitting diffraction light; the rear lens group comprises a plurality of second lenses, and two opposite surfaces of each second lens are used for refracting light. According to the embodiment of the invention, the combination of the refraction and diffraction mixed lens and the refraction lens is adopted, so that athermalization and achromatism of the optical system can be realized. Compared with an electromechanical active athermalization method and a mechanical passive athermalization method, the scheme of the embodiment can reduce the weight, the size and the structural complexity of an instrument, meanwhile, the aiming error caused by shaking of the optical axis is avoided, and the stability of a system is improved.

Description

Technical Field

[0001] The present application relates to the technical field of optical instruments, and in particular to an objective lens and an optical device. Background Art

[0002] In precision optical systems, temperature changes can cause changes in the curvature, thickness and spacing of optical elements, and also cause changes in the refractive index of the optical element base material and the medium in which it is located. In addition, temperature changes can also cause the thermal expansion and contraction of the optical system shell. All of the above will lead to a sharp decline in the imaging quality of the optical system. The adverse effect of temperature changes on the imaging quality of the optical system is called thermal difference. In order to ensure the imaging quality, the optical system needs to be designed to eliminate thermal differences.

[0003] Traditional methods for athermalizing optical systems are mainly electromechanical active and mechanical passive athermalization. These methods increase the structural complexity, volume and weight of the instrument, and require high precision in system assembly and adjustment, which can easily cause optical axis shaking and lead to aiming errors. Summary of the invention

[0004] The embodiment of the present application provides an objective lens and an optical device, which realizes athermalization by combining a refractive-diffractive hybrid lens with a refractive lens. This optical passive athermalization method reduces the complexity, volume and weight of the instrument and enhances the stability of the system.

[0005] In a first aspect, an embodiment of the present application provides an objective lens, comprising a front lens group and a rear lens group, wherein the front lens group and the rear lens group are arranged in sequence from the object side to the image side. The front lens group has a positive focal length, and the rear lens group has a negative focal length. The front lens group comprises a plurality of first lenses, at least one of the first lenses having a diffraction surface, the diffraction surface facing the rear lens group, and the diffraction surface is used to emit diffracted light. The rear lens group comprises a plurality of second lenses, and the two opposite surfaces of each second lens are used to refract light.

[0006] In an embodiment of the present application, the first lens with a diffractive surface can be referred to as a refractive-diffractive hybrid lens. The change in the normalized optical focal length of the lens as the temperature changes is called a thermal constant, which directly affects the thermal difference. The thermal constant of the refractive lens is related to the thermal expansion coefficient of the material and the refractive index of the lens itself; while the thermal constant of the refractive-diffractive hybrid lens has nothing to do with the refractive index of the lens itself, but is only related to the thermal expansion coefficient of the material. Compared with the refractive lens, the refractive-diffractive hybrid lens reduces a factor that affects the thermal difference, which is beneficial to reducing the thermal difference. Therefore, by introducing a refractive-diffractive hybrid lens, the optical system can be athermalized, thereby improving the imaging quality.

[0007] In the embodiment of the present application, by adopting the optical passive athermalization method, no special structure or material is needed to be added, thereby reducing the volume and weight of the objective lens, and reducing the cost and complexity of the objective lens. Moreover, compared with the traditional active electromechanical athermalization and passive mechanical athermalization, the solution of this embodiment avoids the deviation of the aiming axis caused by the relative movement of the lens group, thereby reducing additional aiming errors and improving the stability of the objective lens.

[0008] In an implementation of the first aspect, the first lens having a diffractive surface is a multi-step refractive-diffractive hybrid lens, wherein the multi-step refractive-diffractive hybrid lens is an optical component that controls the phase of incident light through a step structure on its surface.

[0009] In this implementation, a multi-step refractive-diffractive hybrid lens is formed by etching or manufacturing a plurality of steps on the lens surface. Compared with a smooth and continuous ring zone, this structure is easier to manufacture.

[0010] In an implementation manner of the first aspect, the number of steps of the multi-step refractive-diffractive hybrid lens is greater than or equal to 4.

[0011] In this implementation, the more steps there are, the higher the light energy conversion efficiency of the refractive-diffractive hybrid lens. For example, the theoretical light energy conversion efficiency of a 4-step refractive-diffractive hybrid lens is 81%, the theoretical light energy conversion efficiency of an 8-step refractive-diffractive hybrid lens is 95%, and the theoretical light energy conversion efficiency of a 16-step refractive-diffractive hybrid lens is 99%. By limiting the number of steps of a multi-step refractive-diffractive hybrid lens to be greater than or equal to 4, the light energy conversion efficiency of the refractive-diffractive hybrid lens can be higher.

[0012] In an implementation manner of the first aspect, the multi-step refractive-diffractive hybrid lens is a harmonic diffraction device with a harmonic diffraction order greater than or equal to 2.

[0013] In this implementation, the height and gap width of the harmonic diffraction device are increased compared to the traditional refractive-diffractive hybrid lens, which is convenient for processing.

[0014] In an implementation of the first aspect, the objective lens further comprises a lens barrel, and the front lens group and the rear lens group are both arranged in the lens barrel. h , the focal length of the i-th lens in the objective lens φ i And the thermal constant of the i-th lens satisfies the following relationship: Among them, φα h It can characterize the thermal difference caused by heating of the lens barrel, φ i γ i The thermal difference of the i-th lens can be characterized, and the first lens and the second lens are collectively referred to as lenses.

[0015] In this implementation, by making the φα of the lens barrel structure h With all lenses φ i γ i The sum of the thermal differences of the lens barrel structure and all the lenses can be made zero, thus completely eliminating the thermal difference of the optical system. In other words, the thermal difference of the lens barrel can be compensated by designing the lenses. In actual products, the thermal difference of the optical system can be minimized to meet the need for thermal elimination.

[0016] In an implementation of the first aspect, in the front lens group, the material of the first lens with the diffractive surface is calcium fluoride, and the material of the remaining first lenses is fused quartz; and the material of all the second lenses in the rear lens group is fused quartz.

[0017] In this implementation, the thermal expansion coefficient of calcium fluoride is much larger than that of molten quartz, which makes the thermal constant of calcium fluoride larger and has better thermal difference compensation ability. Therefore, the use of calcium fluoride to make a refractive-diffractive hybrid lens is conducive to eliminating thermal differences. However, the cost of calcium fluoride is high and the processing difficulty is greater, so a smaller number of calcium fluoride refractive-diffractive hybrid lenses are usually selected during design to save costs and improve processing efficiency. The thermal expansion coefficient of molten quartz is small, which results in its small thermal constant and poor thermal compensation ability. Therefore, molten quartz is not suitable as a material for refractive-diffractive hybrid lenses. However, the process of molten quartz in manufacturing refractive lenses is mature, the cost is low and it is easy to process, so molten quartz can be used to make a large number of refractive lenses. Therefore, through the above-mentioned material combination of calcium fluoride and molten quartz, thermal differences can be better eliminated.

[0018] In an implementation of the first aspect, the first lens with the largest aperture in the front lens group has a diffraction surface, wherein the aperture refers to the maximum diameter of the lens that can receive light.

[0019] In this implementation, by using the first lens with the largest aperture as the refractive-diffractive hybrid lens, the incident angle of the refractive-diffractive hybrid lens can be reduced, which is beneficial to the realization of the diffraction function.

[0020] In an implementation of the first aspect, the total focal power φ of the objective lens, the focal power φi of the i-th lens in the objective lens, and the Abbe number v of the i-th lens i Satisfies the following relationship: Where λ is the wavelength of light, It can characterize the chromatic aberration of optical systems. The chromatic aberration of the i-th lens can be characterized.

[0021] In this implementation, by making all lenses The sum of the chromatic aberrations of all lenses is 0, which can make the sum of the chromatic aberrations of all lenses zero, thus achieving achromatism of the optical system.

[0022] In an implementation of the first aspect, the RMS wave aberration of the objective lens satisfies the following relationship: RMS wave aberration ≤ 0.07λ, where λ is the wavelength of light.

[0023] In this implementation, RMS wave aberration is used as a comprehensive indicator of the athermal and achromatic effects. By reducing the RMS wave aberration, the wavefront distortion can be effectively reduced, thereby improving the resolution of the optical system, reducing thermal and chromatic aberrations, and ultimately improving the imaging quality.

[0024] In an implementation of the first aspect, the numerical aperture NA of the objective lens is ≥ 0.88. The numerical aperture is a dimensionless number of an optical system, which is used to measure the angular range of light that the system can collect. The numerical aperture is directly related to the resolution. The larger the numerical aperture, the higher the resolution.

[0025] In this implementation, by limiting the numerical aperture of the objective lens as above, a higher resolution can be achieved, so that the device can distinguish smaller details.

[0026] In an implementation of the first aspect, from the object side to the image side, the apertures of the plurality of first lenses in the front lens group tend to increase, and the apertures of the plurality of second lenses in the rear lens group tend to decrease. It is understood that the increasing trend may include a continuous increase; or may include an overall increasing trend, but in certain local intervals, the aperture value may remain unchanged or decrease. The decreasing trend may include a continuous decrease; or may include an overall decreasing trend, but in certain local intervals, the aperture value may remain unchanged or increase.

[0027] In this implementation, the aperture variation characteristics of this lens can meet the optical design requirements of the objective lens, and can achieve a larger numerical aperture and smaller RMS wave aberration, thereby achieving higher imaging quality.

[0028] In an implementation manner of the first aspect, the number of first lenses is at least four, and / or the number of second lenses is at least four.

[0029] In this embodiment, by using a larger number of lenses, it is beneficial to better correct thermal aberration and chromatic aberration and improve imaging quality.

[0030] In an implementation of the first aspect, the objective lens operates under deep ultraviolet light, wherein deep ultraviolet light refers to light with a wavelength less than or equal to 350 nm.

[0031] In this implementation, by limiting the application scenario of the objective lens to the deep ultraviolet band, the imaging quality and the performance of the optical system can be improved, so that the objective lens can meet high-precision and high-efficiency optical operations.

[0032] In a second aspect, an embodiment of the present application provides an optical device, including a light source and the objective lens. The optical device includes but is not limited to a semiconductor detection device and a high-resolution microscope. The position of the light source can be determined according to product requirements.

[0033] The optical device of the embodiment of the present application can effectively eliminate thermal differences and chromatic aberrations of the optical system, thereby improving imaging quality, reducing the complexity, volume and weight of the optical device, and improving the stability of the optical device. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic diagram of the side view structure of an objective lens in an embodiment;

[0035] Figure 2 yes Figure 1 A schematic diagram of the side view structure of the optical components in the objective lens;

[0036] Figure 3 is a schematic diagram of the three-dimensional structure of a refractive-diffractive hybrid lens in an embodiment;

[0037] Figure 4 yes Figure 3 Schematic diagram of the interface of the refractive-diffractive hybrid lens;

[0038] Figure 5 yes Figure 3 Another interface schematic diagram of the refractive-diffractive hybrid lens;

[0039] Figure 6 yes Figure 3 Another interface schematic diagram of the refractive-diffractive hybrid lens;

[0040] Figure 7 It is a schematic diagram comparing the traditional refractive-diffractive hybrid lens and the harmonic diffractive device;

[0041] Figure 8 The imaging point diagrams at different imaging positions are shown;

[0042] Fig. 9 The RMS wave aberration at different imaging positions is illustrated.

[0043] Reference numerals

[0044] 1-Objective lens;

[0045] 11-lens barrel; 12-optical components;

[0046] G1-first lens group; G2-second lens group;

[0047] L1~L8-second lens; L9~L14-first lens;

[0048] L1a-surface; L1b-surface; L10a-surface; L10b-surface;

[0049] D1-first diffraction surface; D2-second diffraction surface;

[0050] S1-the first step; S2-the second step; S3-the third step; S4-the fourth step. DETAILED DESCRIPTION

[0051] For ease of understanding, the English abbreviations and related technical terms involved in the embodiments of the present application are explained and described below.

[0052] Focal power is equal to the difference between the convergence of the image-side beam and the convergence of the object-side beam. It characterizes the ability of an optical system to deflect light.

[0053] A lens or lens group with positive optical power has a positive focal length and has the effect of converging light.

[0054] A lens or lens group with negative optical power has a negative focal length and has the effect of diverging light.

[0055] Focal length, also known as focal length, is a measure of the convergence or divergence of light in an optical system. It refers to the vertical distance from the optical center of a lens or lens group to the focal plane when an infinitely distant scene forms a clear image on the focal plane through a lens or lens group. From a practical point of view, it can be understood as the distance from the center of the lens to the plane when the object is infinitely far away. For a fixed-focus lens, the position of its optical center is fixed; for an optical lens, the change of the optical center of the lens brings about a change in the focal length of the lens.

[0056] The object side is bounded by the lens. The side where the object is located is called the object side, and the surface of the lens close to the object side is called the object side.

[0057] The image side is bounded by the lens. The side where the image of the object is located is called the image side, and the surface of the lens close to the image side is called the image side.

[0058] The aperture diaphragm is a device used to control the amount of light that passes through the lens and enters the photosensitive surface inside the camera body. It is usually inside the lens.

[0059] The imaging plane is located on the image side of all lenses in the optical lens, and is the plane on which the image is formed after the light passes through each lens in the optical lens in sequence.

[0060] The optical axis is an axis that passes vertically through the center of the lens. The optical axis of the lens is the axis that passes through the center of each lens of the lens. When light parallel to the optical axis enters a convex lens, the ideal convex lens should have all the light rays converge at one point behind the lens. This point where all the light rays converge is the focus. The direction of the optical axis is the extension direction of the optical axis.

[0061] Focus is the point where parallel light rays converge after being refracted by a lens or lens group.

[0062] The image focal plane, also called the back focal plane or the second focal plane, is a plane passing through the image focus (also called the back focus or the second focus) and perpendicular to the optical axis of the system.

[0063] The Abbe number, or dispersion coefficient, is the difference ratio of the refractive index of an optical material at different wavelengths, representing the degree of dispersion of the material.

[0064] Aberration, the paraxial region of the optical system has the properties of an ideal optical system. The paraxial light emitted from a point on the object intersects the image plane at one point (also known as the paraxial image point). However, the light that actually passes through different apertures of the lens is difficult to intersect perfectly at one point, but has a certain deviation from the position of the paraxial image point. These differences are collectively called aberrations.

[0065] Thermal aberration, also known as thermal difference, is the adverse effect of temperature change on the imaging effect of the lens. In order to make the lens have stable imaging quality within a certain temperature range, certain compensation measures need to be taken to ensure the imaging quality. This design is called athermalization.

[0066] Thermal constant: The change in the normalized optical focal length of an optical element as the temperature changes is called the thermal constant, which is related to the thermal difference.

[0067] Refractive index is an important physical quantity that describes the change in the propagation speed of light in different media. It is defined as the ratio of the propagation speed of light in a vacuum to the propagation speed of light in the medium.

[0068] Chromatic aberration is a common aberration in optical imaging. It is mainly caused by the different refractive indices of light of different wavelengths in the lens, which leads to imaging distortion. Light of different wavelengths cannot be focused on the same point.

[0069] Root mean square wavefront error, also known as RMS wavefront error, is an important parameter used to quantify wavefront aberration in optical systems. RMS wavefront error describes the degree of deviation between the actual wavefront and the ideal wavefront and is one of the important indicators for evaluating the imaging quality of optical systems.

[0070] Harmonic diffraction device is an optical element that uses the diffraction principle and harmonic characteristics to perform optical modulation or signal processing. This type of device usually produces a specific diffraction pattern when the incident light passes through the device by designing a specific optical structure or pattern, thereby achieving light regulation.

[0071] A diffractive optical element (DOE) is a device that uses the principle of diffraction optics to modulate the amplitude or phase of incident light. It is generally used in the fields of beam convergence, beam shaping, laser beam splitting, and generation of various diffraction patterns.

[0072] A multi-step refractive-diffractive hybrid lens, also known as a multi-step DOE, is an optical component that controls the phase of incident light through the step structure on its surface.

[0073] The refractive-diffractive hybrid lens efficiency, also known as DOE efficiency, refers to the efficiency of the refractive-diffractive hybrid lens in converting incident light into a specific diffraction pattern or beam. Specifically, it refers to how much of the incident light energy is effectively converted into the desired diffraction beam or pattern.

[0074] Numerical aperture (NA) is a dimensionless number for an optical system that measures the angular range of light that the system can collect. Numerical aperture is directly related to resolution. A higher numerical aperture can improve the resolution of a microscope, making it possible to resolve smaller details.

[0075] Deep ultraviolet (DUV) refers to light with a wavelength less than or equal to 350nm.

[0076] The embodiment of the present application provides an optical device, which includes but is not limited to semiconductor detection equipment and high-resolution microscopes. The optical device may include a light source and an objective lens. Exemplarily, the light source can emit deep ultraviolet light, so the optical device of the embodiment of the present application can be applied to an optical environment in the deep ultraviolet band. In another embodiment, the band of light emitted by the light source is not limited, and the optical device can be applied to any suitable band.

[0077] In the embodiment of the present application, the position of the light source can be determined according to product requirements.

[0078] Exemplarily, the light source can be located on the object side of the objective lens, that is, on the same side as the sample to be tested. For example, in a semiconductor device or high-resolution microscope for bright field detection, the light source can be placed on the side of the sample away from the objective lens, and the light emitted by the light source directly illuminates the sample. After the light passes through the sample, it will enter the objective lens. The objective lens collects the light passing through the sample to form a bright image, so that the features and defects of the sample are clearly visible on the bright background. For example, in dark field detection, the light source is located on the same side of the sample to be tested and forms an angle with the objective lens. The light emitted by the light source illuminates the object at an oblique angle, and only scattered light (such as defects on the surface of the sample or light scattered by particles) can enter the objective lens. The objective lens mainly collects these scattered lights, while the background remains dark, so the defects and particles of the sample are more obvious on the dark background.

[0079] For example, in some special bright field detection semiconductor devices or high-resolution microscopes, the light source can be set on the image side of the objective lens, that is, on the side of the objective lens away from the sample to be measured. In this case, the optical device can also be equipped with a catadioptric lens, through which the light of the light source can be reflected or refracted onto the sample. This configuration can be used for certain specific imaging techniques, such as reflection microscopy or special fluorescence microscopy.

[0080] In the embodiment of the present application, the optical device may further include other optical elements, for example, it may also include structures such as a cylindrical mirror and an eyepiece. The cylindrical mirror may be located on the image side of the objective lens, and is used to focus light or change the shape of the light beam, and is used in conjunction with the objective lens to guide the light to the eyepiece so that the final image can be seen by the observer. The eyepiece may be located on the image side of the cylindrical mirror, and may be used to magnify the image formed by the objective lens so that the observer can see the details more clearly. Exemplarily, the optical device may further include an image sensor, so that the light passing through the objective lens is directly imaged on the image sensor, and the observer can observe through a display screen.

[0081] It can be understood that the structure of the optical device described above is merely an illustrative example, and the present application does not limit the specific structure of the optical device.

[0082] Figure 1 is a schematic diagram of the side view structure of the objective lens 1 in an embodiment, Figure 2 for Figure 1 Schematic diagram of the side view structure of the optical component 12.

[0083] like Figure 1 As shown, the objective lens 1 may include a lens barrel 11 and an optical assembly 12. The lens barrel 11 may be a component assembled from multiple components, or may be a single component. Exemplarily, the lens barrel 11 may be roughly cylindrical, or may be other shapes that meet the requirements. It is understood that Figure 1The lens barrel 11 shown is only a schematic diagram, and the embodiment of the present application does not limit the structure of the lens barrel 11.

[0084] The optical assembly 12 may be located in the lens barrel 11. In this embodiment, the lens barrel 11 may protect the optical assembly 12 from external factors such as physical damage and dust. At the same time, the lens barrel 11 may also effectively control the light entering the lens, reduce unnecessary scattered light and glare, thereby improving the contrast and clarity of the image. In another embodiment, according to product requirements, the objective lens 1 may not include the lens barrel 11, but may use other structures to protect the optical assembly 12.

[0085] like Figure 2 As shown, the optical assembly 12 may include a front lens group G1 and a rear lens group G2, wherein the front lens group G1 and the rear lens group G2 may be arranged in sequence along the direction from the object side to the image side of the optical assembly 12. In this embodiment, the front lens group G1 and the rear lens group G2 may each include a plurality of lenses, and the lens of the front lens group G1 may be referred to as a first lens, and the lens of the rear lens group G2 may be referred to as a second lens.

[0086] Exemplarily, the front lens group G1 may include at least 4 first lenses, such as 6 lenses including the first lens L9, the first lens L10, the first lens L11, the first lens L12, the first lens L13 and the first lens L14, and these first lenses are arranged in sequence from the image side to the object side. Exemplarily, the rear lens group G2 may include at least 4 second lenses, such as 8 lenses including the second lens L1, the second lens L2, the second lens L3, the second lens L4, the second lens L5, the second lens L6, the second lens L7 and the second lens L8, and these second lenses are arranged in sequence from the image side to the object side. By making the front lens group G1 and the rear lens group G2 both have multiple lenses, it is beneficial to better correct thermal aberration and chromatic aberration and improve the imaging quality. In another embodiment, the number of the first lens and the second lens may not be limited in this way.

[0087] Exemplarily, both the front lens group G1 and the rear lens group G2 can be a close-contact thin lens group, which is an optical system composed of multiple thin lenses tightly fitted together. In optics, when the ratio of the thickness of a lens to its spherical curvature radius can be ignored, the lens is called a thin lens. When two or more such thin lenses are tightly fitted and their main optical axes coincide, a close-contact thin lens group is formed. The close-contact thin lens group has the characteristics of compact structure, which can effectively reduce the thickness of the objective lens, reduce the amount of material used, and thus reduce the weight of the objective lens. In addition, this design can also reduce the absorption of light by the material, ensuring efficient use of light.

[0088] The aperture in this embodiment refers to the maximum diameter of the lens that can receive light, that is, Figure 2The maximum diameter of the first lens or the second lens in the vertical direction. Figure 2 As shown, from the object side to the image side, the aperture of the first lens in the front lens group G1 may increase, and the aperture of the second lens in the rear lens group G2 may decrease. It can be understood that the increasing trend may include a continuous increase; or an overall increasing trend, but in some local intervals, the aperture value may remain unchanged or decrease. The decreasing trend may include a continuous decrease; or an overall decreasing trend, but in some local intervals, the aperture value may remain unchanged or increase.

[0089] In this embodiment, the aperture variation characteristics of the lens can meet the optical design requirements of the objective lens, and can achieve a larger numerical aperture and a smaller RMS wave aberration, thereby achieving higher imaging quality. In another embodiment, the aperture variation of the lens may not be limited in this way.

[0090] In this embodiment, the front lens group G1 may have a positive focal length, which may be used to receive light reflected from an object and gather the light. Each first lens may be a positive lens. The rear lens group G2 may have a negative focal length, which may be used to correct aberrations. Each second lens in the rear lens group G2 may be a negative lens, or a portion of the second lenses may be negative lenses and another portion of the second lenses may be positive lenses. For example, the second lens L1, the second lens L2, the second lens L3, the second lens L5, and the second lens L8 may be negative lenses, and the second lens L4, the second lens L6, and the second lens L7 may be positive lenses.

[0091] like Figure 2 As shown, the first lens L9 with the largest aperture in the front lens group G1 may have a diffractive surface. For the convenience of description, the first lens L9 with a diffractive surface may be referred to as a refractive-diffractive hybrid lens. The object side surface of the first lens L9 is a refractive surface, and the image side surface of the first lens L9 is a first diffractive surface D1. It can also be understood that the first diffractive surface D1 faces the rear lens group G2.

[0092] In this embodiment, by using the first lens L9 with the largest aperture as the refractive-diffractive hybrid lens, the incident angle of the refractive-diffractive hybrid lens can be reduced, which is beneficial to the realization of the diffractive function. This design principle will be further explained below. In another embodiment, the position of the refractive-diffractive hybrid lens is not limited in this way, that is, the refractive-diffractive hybrid lens can be any first lens in the front lens group G1.

[0093] like Figure 2 As shown, the first lens L11 can also be a refractive-diffractive hybrid lens, the object side surface of the first lens L11 is a refractive surface, and the image side surface of the first lens L11 is a second diffractive surface D2, which can also be understood as the second diffractive surface D2 facing the rear lens group G2. The second diffractive surface D2 can be used to emit diffracted light.

[0094] In this embodiment, by providing two diffraction surfaces, the period of each diffraction surface can be increased, which can meet the design requirements and facilitate processing, as will be described below. In another embodiment, the number of diffraction surfaces is not limited, and the position of each diffraction surface in the front lens group G1 is not limited to Figure 2 Position shown.

[0095] like Figure 2 As shown, the first lens L10, the first lens L12, the first lens L13 and the first lens L14 can all be refractive lenses, and the two opposite surfaces of each refractive lens can be used to refract light. For example, the surface L10a and the surface L10b of the first lens L10 can both refract light.

[0096] like Figure 2 As shown, the second lenses in the rear lens group G2 can all be refractive lenses, that is, the second lenses L1 to L8 are all refractive lenses, and the two opposite surfaces of each second lens can be used to refract light. For example, the surface L1a and the surface L1b of the second lens L1 can both refract light.

[0097] In this embodiment, the materials of the refractive-diffractive hybrid lens and the refractive lens can be determined as needed. Figure 2 The material of the first lens L9 and the first lens L11 in the embodiment is calcium fluoride, and the refractive lens (such as Figure 2 The material of the second lenses L1 to L8, the first lens L10, the first lens L12, the first lens L13 and the first lens L14 in the optical system is fused silica. The linear expansion coefficient (a kind of thermal expansion coefficient) of calcium fluoride is much larger than that of fused silica, so it can be combined with the refractive lens to achieve athermalization. The above design principle will be further explained below.

[0098] It is understandable that the lens parameters of the optical component 12 can be determined as needed. The following is an exemplary list of lens parameters of the optical component 12 in a table. Table 1 is Figure 2 The optical component 12 shown is a lens parameter, which includes surface number, surface shape, radius of curvature, thickness, material and aperture.

[0099] Each lens surface is numbered in sequence from the image side to the object side, including the first virtual surface, the two opposite surfaces of each lens, and the second virtual surface. The surface numbers are corresponded to the two opposite surfaces of each lens (for example, surface L1b of the second lens L1 is surface 1, surface L1a is surface 2, and so on). Among them, the surface type of surface 17 (first diffraction surface D1, i.e., the image side surface of the first lens L9) and surface 21 (second diffraction surface D2, i.e., the image side surface of the first lens L11) is BINARY_2, i.e., the first lens L9 and the first lens L11 are refractive-diffractive hybrid lenses. The surface type of the first virtual surface, surfaces 1 to 16, surfaces 18 to 20, surfaces 22 to 24, and the second virtual surface is a standard spherical surface, i.e., the second lens L1 to L8, the first lens L10, the first lens L12, the first lens L13, and the first lens L14 are refractive lenses.

[0100] The thickness corresponding to each surface is the gap between the surface and the next adjacent surface. For example, the thickness 7.999977 corresponding to surface number 1 is the gap between surface 1 and surface 2; the thickness 13.54439 corresponding to surface number 2 is the gap between surface 2 and surface 3, that is, the gap between the second lens L1 and the second lens L2, and so on.

[0101] The material corresponding to each surface is the material between the surface and the next adjacent surface (including the two surfaces themselves), for example, the material corresponding to surface number 1 is fused silica, which means that the material between surface 1 and surface 2 is fused silica, that is, the material of the second lens L1 is fused silica; the material corresponding to surface number 2 is blank, which means that the material between surface 2 and surface 3 is the medium between the second lens L1 and the second lens L2, for example, it can be air, and so on. According to Table 1, the material of the first lens L9 and the first lens L11 is calcium fluoride, that is, the material of the refractive-diffractive hybrid lens is calcium fluoride; the material of the second lenses L1 to L8, the first lens L10, the first lens L12, the first lens L13 and the first lens L14 is fused silica, that is, the material of the refractive lens is fused silica.

[0102] The aperture of each surface refers to the effective diameter of the lens, that is, the maximum diameter of the lens that can receive light, such as Figure 2 The aperture of each lens surface can be the maximum diameter in the vertical direction. Figure 2 As can be seen from Table 1, there are two diffraction surfaces in this embodiment, namely surface 17 and surface 21.

[0103] It is understandable that the larger the lens surface diameter, the smaller the incident angle of light, which is more conducive to the realization of the diffraction function. The diffraction surface is located in the front lens group G1 and is set on the lens with a larger diameter (the diameters of the first diffraction surface D1 and the second diffraction surface D2 are 114.3284 and 109.0519 respectively).

[0104] This is conducive to the realization of diffraction, thereby achieving better imaging effects.

[0105]

[0106]

[0107] Table 1 Lens parameters of optical component 12

[0108] In this embodiment, the numerical aperture NA of the objective lens 1 is greater than or equal to 0.88. According to optical principles, the larger the numerical aperture, the higher the resolution. By limiting the numerical aperture of the optical system as above, high resolution can be achieved, so that the device can distinguish smaller details. In another embodiment, the numerical aperture of the objective lens is not limited in this way.

[0109] Figure 3 is a schematic diagram of a three-dimensional structure of a refractive-diffractive hybrid lens in an embodiment, Figure 4 for Figure 3 Schematic diagram of the interface of the refractive-diffractive hybrid lens.

[0110] like Figure 3 and Figure 4 As shown in FIG. 1 , the refractive-diffractive hybrid lens can be a centrosymmetric variable-period diffraction grating, and the shape of the refractive-diffractive hybrid lens can be determined as needed. For example, the refractive-diffractive hybrid lens is in the shape of a disk as a whole, and its thickness or height is d. The diffractive surface (such as Figure 3 The center of the upper surface of the median refractive-diffractive hybrid lens is roughly a part of a sphere or a part of an ellipsoid, and multiple rings are formed from the center to the edge. From the center to the edge of the refractive-diffractive hybrid lens, the gap between adjacent rings gradually decreases (such as Figure 3 From r1 to r n It can also be understood that the period of the diffraction grating decreases gradually from the center to the edge.

[0111] Figure 4 The horizontal axis corresponds to the radial position of the refractive-diffractive hybrid lens, and the unit is mm, where the zero point corresponds to the center of the refractive-diffractive hybrid lens; the vertical axis represents the phase, and the unit is radian. Figure 4 As shown in the figure, the phase changes periodically from the center to the edge, and the period gradually decreases. According to optical principles, the height of the refractive-diffractive hybrid lens is positively correlated with the phase, so Figure 4 It also shows that the annular zone height of the refractive-diffractive hybrid lens remains unchanged, and the annular zone width changes periodically from the center to the edge, and the period gradually decreases.

[0112] Figure 5 for Figure 3 Another interface diagram of the refractive-diffractive hybrid lens, Figure 6 for Figure 3 Schematic diagram of another interface of the refractive-diffractive hybrid lens.

[0113] In this embodiment, the refractive-diffractive hybrid lens may be a multi-step DOE, and the number of steps is greater than or equal to 4. A multi-step DOE is an optical element having multiple steps of different heights or phases. Figure 5 As shown, the refractive-diffractive hybrid lens can be a 4-step DOE, including 4 steps (such as Figure 5 The first step S1, the second step S2, the third step S3, and the fourth step S4 are shown in FIG. Each step may correspond to a different phase value. For example, the phase value of the first step S1 may be 0, and the phase value of the fourth step S4 may be 2π. The phases or heights of the steps of the same order remain consistent. The adjacent four steps from high to low may be referred to as a group of steps, and the width of each group of steps gradually decreases from the center to the edge of the refractive-diffractive hybrid lens.

[0114] like Figure 5 As shown, by etching or manufacturing multiple steps on the lens surface, a multi-step DOE is formed. Figure 4 The smooth and continuous annular band shown in the figure is easier to manufacture. The more steps there are, the higher the light energy conversion efficiency of the refractive-diffractive hybrid lens, that is, the higher the DOE efficiency. For example, the theoretical DOE efficiency of a 4-step refractive-diffractive hybrid lens is 81%, the theoretical DOE efficiency of an 8-step refractive-diffractive hybrid lens is 95%, and the theoretical DOE efficiency of a 16-step refractive-diffractive hybrid lens is 99%.

[0115] like Figure 6 As shown, in another embodiment, the number of steps of the refractive-diffractive hybrid lens may be 2, including a first step S1 and a second step S2. The theoretical DOE efficiency of a 2-step DOE is 40%.

[0116] In another embodiment, the refractive-diffractive hybrid lens may be Figure 4 Continuous loop as shown or other structure as required.

[0117] Figure 7 FIG. 1 is a schematic diagram comparing a conventional refractive-diffractive hybrid lens and a harmonic diffractive device. In this embodiment, the refractive-diffractive hybrid lens may use a harmonic diffractive device. Figure 7 As shown, the height of the conventional refractive-diffractive hybrid lens is d0 (d0 = 2π), and the maximum period (the gap between the highest point at the center and the first highest point near the center) is r 01 ; The height of the harmonic diffraction device is D0 (D0 = N·2π), and the maximum period is R 01 . Where D0 = N·d0, R 01 ≈N·r 01, where N is the harmonic diffraction order, and the harmonic diffraction order in this embodiment is not less than 2. Compared with the traditional refractive-diffractive hybrid lens, the harmonic diffraction device has an increased height and gap width, which is conducive to processing.

[0118] Understandably, Figure 7 The shapes of the conventional refractive-diffractive hybrid lens and the harmonious diffractive element shown are only examples, and the present embodiment does not limit the shapes of the refractive-diffractive hybrid lens. Figure 7 The size relationship between the traditional refractive-diffractive hybrid lens and the harmonious diffractive element is only a schematic representation and does not represent the actual size relationship.

[0119] In another embodiment, in order to meet product requirements, the refractive-diffractive hybrid lens may be a traditional refractive-diffractive hybrid lens.

[0120] The structural features of the objective lens 1 are introduced above, and the working principle of the objective lens 1 will be described in detail below.

[0121] The change in the normalized focal length of a lens with temperature is called the thermal constant, which can be expressed as γ. The calculation formulas for the thermal constants of the refractive lens and the refractive-diffractive hybrid lens in the optical system are:

[0122]

[0123] In the formula, γ i,r is the thermal constant of the i-th refractive lens, γ i,d is the thermal constant of the i-th refractive-diffractive hybrid lens, φ i is the focal length of the i-th lens, and the thermal expansion coefficient of the material is α g Indicates that n i is the refractive index of the space medium (the medium between the lenses, such as air or nitrogen, etc.), n is the refractive index of the ith lens, T is the temperature, and abs refers to the absolute value of the value in the brackets.

[0124] From formula (1), we know that the thermal constant of the refractive lens is related to the thermal expansion coefficient of the material and the refractive index of the lens itself; from formula (2), we know that the thermal constant of the refractive-diffractive hybrid lens is independent of the refractive index of the lens itself, but only related to the thermal expansion coefficient of the material. The thermal constant directly affects the thermal aberration, so compared with the refractive lens, the refractive-diffractive hybrid lens reduces a factor that affects the thermal aberration, which is beneficial to reduce the thermal difference.

[0125] The only commonly used lens materials in the deep ultraviolet band are calcium fluoride and fused quartz. The thermal expansion coefficient of calcium fluoride is much larger than that of fused quartz, which makes the thermal constant of calcium fluoride larger and has better thermal difference compensation ability. Therefore, using calcium fluoride to make a refractive-diffractive hybrid lens is conducive to eliminating thermal differences. However, calcium fluoride is expensive and difficult to process, so a small number of calcium fluoride refractive-diffractive hybrid lenses are usually selected during design to save costs and improve processing efficiency.

[0126] The thermal expansion coefficient of fused quartz is small, which results in a small thermal constant and poor thermal compensation capability. Therefore, fused quartz is not suitable as a material for refractive-diffractive hybrid lenses. However, fused quartz has a mature process for manufacturing refractive lenses, is low in cost and easy to process, so fused quartz can be used to make a large number of refractive lenses.

[0127] In this embodiment, the thermal expansion of the lens barrel 11 may also cause the optical system to generate large thermal aberrations. Therefore, the designs of the lens barrel 11 and the optical assembly 12 may be comprehensively considered to compensate for the thermal difference and achieve athermalization of the optical system.

[0128] For example, the total focal length φ of the objective lens 1 and the thermal expansion coefficient α of the material of the lens barrel 11 are h , the focal power φ of the i-th lens in objective lens 1 i And the thermal constant of the i-th lens can satisfy the following relationship:

[0129]

[0130] Among them, φα h It can characterize the thermal difference caused by the heating of the lens barrel 11, φ i γ i The thermal difference of the i-th lens can be characterized, and the first lens and the second lens are collectively referred to as lenses. From formula (3), it can be seen that by making the φα of the lens barrel structure h With all lenses φ i γ i The sum of the thermal differences of the lens barrel 11 is zero, which can make the sum of the thermal differences of the lens barrel structure and all the lenses zero, thereby completely eliminating the thermal difference of the optical system. In other words, the thermal difference of the lens barrel 11 can be compensated by designing the optical component 12. In actual products, the thermal difference of the optical system can be minimized to meet the need for eliminating thermal differences.

[0131] The above describes the principle of athermalization of the optical lens 12 of this embodiment. In this embodiment, while eliminating thermal differences in the optical system, chromatic aberration of the optical system can also be eliminated. This is described below.

[0132] The chromatic aberration of an optical system is directly related to the Abbe number of the optical material, which can be represented by v. The Abbe number calculation formulas for refractive lenses and refractive-diffractive hybrid lenses are:

[0133]

[0134] In the formula, υ r is the Abbe number of the refractive lens, υ d is the Abbe number of the diffractive lens, n is the refractive index of the refracting lens, λ is the wavelength of light, and λ1, λ2, and λ3 are equal to 486.1nm, 587.6nm, and 656.3nm, respectively.

[0135] According to formula (4), the Abbe number of the refractive lens is related to both the material and the wavelength of light, and is a positive value; the Abbe number of the refractive-diffractive hybrid lens is independent of the material and only related to the wavelength. The calculated wavelength of the refractive-diffractive hybrid lens is a negative value (-3.452). The Abbe number of the refractive-diffractive hybrid lens is independent of the material, so there is no need to consider chromatic aberration when considering the material selection of the refractive-diffractive hybrid lens. In addition, the Abbe number of the refractive-diffractive hybrid lens has an opposite sign to that of the refractive lens, so it is beneficial to achromatism.

[0136] Exemplarily, the total focal length φ of the objective lens, the focal length φ of the i-th lens in the objective lens i and the Abbe number υ of the i-th lens i Satisfies the following relationship:

[0137]

[0138] Where λ is the wavelength of light, It can characterize the chromatic aberration of optical systems. It can be used to characterize the chromatic aberration of the i-th lens. From formula (6), we can see that by making the The sum of the chromatic aberrations of all lenses is 0, which can make the sum of the chromatic aberrations of all lenses zero, thus achieving achromatism of the optical system.

[0139] In summary, in this embodiment, by combining the refractive-diffractive hybrid lens with the refractive lens, and the combination of calcium fluoride and fused quartz, the optical system can simultaneously satisfy formula (3) and formula (6), thereby achieving the dual goals of eliminating thermal difference and chromatic aberration. It can be understood that the above-mentioned thermal difference elimination and chromatic aberration elimination schemes are two independent schemes, and the objective lens 1 of the embodiment of the present application can adopt any one or both of these schemes. In addition, the refractive-diffractive lens and the refractive lens are respectively made of different materials, which is only an example and not a limitation of the embodiment of the present application.

[0140] The comprehensive technical effect of this embodiment on the athermalization and chromatic aberration of the optical system is described below.

[0141] In the embodiment of the present application, within the design wavelength range (266.01±0.01nm), the comprehensive RMS wave aberration of the whole field of view is ≤0.07λ, and within the range of 20±2°C, the comprehensive RMS wave aberration ≤0.07λ can be achieved by adjusting the back intercept (the distance between the objective lens and the object).

[0142] Figure 8 The imaging point diagram at different imaging positions is shown. Fig. 9 The RMS wave aberration at different imaging positions is illustrated.

[0143] like Figure 8 As shown in the figure, IMA represents the virtual surface of imaging, and IMA: 0.000mm, IMA: 0.100mm, IMA: 0.200mm, IMA: 0.300mm, IMA: 0.400mm, IMA: 0.500mm and IMA: 0.600mm represent imaging rings at 7 different positions. The root mean square radius (RMS radius) and geometric radius (GEO radius) of three wavelengths (265.91nm, 266.01nm and 266.11nm) at different positions are calculated. The calculation results show that the root mean square radius values ​​of the imaging rings at 7 different positions are 2.047, 2.047, 2.046, 2.044, 2.042, 2.039 and 2.037, and the geometric radius values ​​are 3.890, 4.014, 4.088, 4.145, 4.233, 4.330 and 4.460, respectively. The smaller the radius, the better the imaging quality. Within the design wavelength range, the root mean square radius values ​​and geometric radius values ​​of the imaging rings at 7 different positions are both small, indicating that the objective lens of this embodiment has a high imaging quality.

[0144] like Fig. 9 As shown, the abscissa represents the field radius on the object side, in mm; the ordinate represents the RMS wave aberration in units of wavelength λ. Fig. 9 The four curves in the figure represent the RMS wave aberration at wavelengths of 265.91nm, 266.01nm and 266.11nm, as well as the average values ​​of the three wavelengths. Fig. 9 It can be seen that under different field of view radii, within the design wavelength range, the optical system of this embodiment satisfies RMS wave aberration ≤ 0.07λ, thus ensuring higher resolution and good imaging quality. In another embodiment, the range of RMS wave aberration can be limited as needed.

[0145] It can be understood that the designed wavelength range and temperature range of the embodiment of the present application are only examples, and the wavelength range and temperature range can actually be adjusted according to the design requirements of the objective lens 1.

[0146] The solution of this embodiment adopts the method of refractive-diffractive hybrid lens, utilizes the unique thermal constant and Abbe number of the refractive-diffractive hybrid lens mentioned above, and combines the material properties of calcium fluoride and fused quartz to achieve athermalization and achromatization of the optical system, thereby improving the imaging quality.

[0147] This embodiment adopts an optical passive athermalization method, without adding special structures or materials, thereby reducing the volume and weight of the objective lens 1, and reducing the cost and complexity of the objective lens 1. Moreover, compared with the traditional active electromechanical athermalization and passive mechanical athermalization, this embodiment avoids the offset of the aiming axis caused by the relative movement of the lens group, thereby reducing additional aiming errors and improving the system's fault tolerance, anti-interference ability, and ability to adapt to changes.

[0148] For ease of understanding, the relevant technical terms involved in the embodiments of the present application are explained and described below.

[0149] In the description of the embodiments of the present application, unless otherwise specified, "plurality" refers to two or more.

[0150] The terms "first", "second", etc. are used for descriptive purposes only and should not be understood to imply or suggest relative importance or implicitly indicate the number of technical features indicated. Features qualified as "first" or "second" may explicitly or implicitly include one or more of the features.

[0151] The directional terms mentioned in the embodiments of the present application, such as "upper", "lower", "front", "back", "left", "right", "inner", "outer", "side", "top", "bottom", etc., are only reference directions of the drawings. The directional terms are for better and clearer explanation and understanding of the embodiments of the present application, and do not explicitly or implicitly indicate that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, etc., and therefore cannot be understood as limiting the embodiments of the present application.

[0152] In the description of the embodiments of the present application, unless otherwise specified, "and / or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone.

[0153] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. An objective lens, characterized in that: It comprises a front lens group and a rear lens group, and the front lens group and the rear lens group are arranged in sequence from the object side to the image side; the front lens group has a positive optical power, and the rear lens group has a negative optical power; The front lens group includes a plurality of first lenses, at least one of the plurality of first lenses has a diffraction surface, the diffraction surface faces the rear lens group, and the diffraction surface is used to emit diffracted light; The rear lens group includes a plurality of second lenses, and two opposite surfaces of each of the second lenses are used for refracting light.

2. The objective lens according to claim 1, characterized in that: The first lens having the diffractive surface is a multi-step refractive diffractive hybrid lens.

3. The objective lens according to claim 2, characterized in that: The number of steps of the multi-step refractive-diffractive hybrid lens is greater than or equal to 4.

4. The objective lens according to claim 2 or 3, characterized in that: The multi-step refractive-diffractive hybrid lens is a harmonic diffraction device with a harmonic diffraction order greater than or equal to 2.

5. The objective lens according to any one of claims 1 to 3, characterized in that: The objective lens also includes a lens barrel, and the front lens group and the rear lens group are both arranged in the lens barrel; The total focal length φ of the objective lens, the thermal expansion coefficient α of the material of the lens barrel h , the focal length φ of the i-th lens in the objective lens i And the thermal constant of the i-th lens satisfies the following relationship:

6. The objective lens according to any one of claims 1 to 3, characterized in that: In the front lens group, the material of the first lens having the diffraction surface is calcium fluoride, and the material of the remaining first lenses is fused quartz; The material of all the second lenses in the rear lens group is fused quartz.

7. The objective lens according to any one of claims 1 to 3, characterized in that: The first lens with the largest aperture in the front lens group has the diffraction surface.

8. The objective lens according to any one of claims 1 to 3, characterized in that: The total focal power φ of the objective lens, the focal power φ of the i-th lens in the objective lens i and the Abbe number v of the i-th lens i Satisfies the following relationship: Here, λ is the wavelength of light.

9. The objective lens according to any one of claims 1 to 3, characterized in that: The RMS wave aberration of the objective lens satisfies the following relationship: RMS wave aberration ≤ 0.07λ, where λ is the wavelength of light.

10. The objective lens according to any one of claims 1 to 3, characterized in that: The numerical aperture NA of the objective lens is ≥0.

88.

11. The objective lens according to any one of claims 1 to 3, characterized in that: From the object side to the image side, the apertures of the plurality of first lenses tend to increase; The apertures of the plurality of second lenses tend to decrease from the object side to the image side.

12. The objective lens according to any one of claims 1 to 3, characterized in that: The number of the first lenses is at least 4, and / or the number of the second lenses is at least 4.

13. The objective lens according to any one of claims 1 to 3, characterized in that: The objective lens operates under deep ultraviolet light.

14. An optical device, characterized in that: It comprises a light source and the objective lens as claimed in any one of claims 1 to 13.

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