Objective lens used for being coupled with image transmitting optical fiber

Abstract

The invention discloses an objective lens used for being coupled with an image transmitting optical fiber, which comprises a first lens, a second lens, a third lens, a fourth lens and a cover glass which are sequentially arranged from an object plane to an image plane, and the third lens is a doublet lens. The first lens and the second lens correct aberrations such as coma, astigmatism and curvature of field, remain small chromatic aberration, and accumulate large positive spherical aberration. The third lens further corrects coma, astigmatism and curvature of field, also provides a large negative spherical aberration, and compensates for the spherical aberration accumulated by the first lens and the second lens. And the fourth lens focuses the light beam, so that various residual aberrations of the left lens can be almost completely corrected. The cover glass can be used for protecting the end face of the image transmitting optical fiber in a subsequent light path and blocking scattered light of a left light path. Through the cooperation of the lenses, not only can an image transmitting optical fiber bundle be matched, flat field and achromatism be realized, but also telecentric effect is realized, so that the coupling efficiency with the image transmitting optical fiber is remarkably improved, and crosstalk is reduced.

Description

Technical Field

[0001] The present invention belongs to the field of biomedical imaging, and more specifically, relates to an objective lens for coupling with an image transmission optical fiber. Background Art

[0002] Telecentricity is a feature required by many biomedical imaging systems, such as confocal microscopy imaging, microscope imaging, and optical coherence tomography imaging. Telecentricity can make objects at different depths have the same magnification, thereby obtaining a more accurate 3D image. In scenarios such as confocal microscopy imaging and two-photon microscopy imaging, the objective lens for coupling with an image transmission optical fiber usually requires telecentricity, which ensures that each point on the tissue is illuminated at the same angle, and the light cone collected by the objective lens is the same for each point. In the scenario of image transmission through an image transmission optical fiber, the telecentricity on the image transmission optical fiber side of the imaging lens group enables the illumination light emerging from the image transmission optical fiber to be effectively focused on the tissue, and the light backscattered from the tissue can be efficiently coupled into the optical fiber.

[0003] Although the objective lens for coupling with an image transmission optical fiber provided in the Chinese patent with the publication number CN110927955B can correct chromatic aberration of a specific wavelength for a probe-type confocal microendoscope, match the numerical aperture with the image transmission optical fiber, and design a sufficient working distance, it does not consider the influence of different illumination light cones on the coupling efficiency at different fields of view. Summary of the Invention

[0004] In view of the above defects or improvement requirements of the prior art, the present invention provides an objective lens for coupling with an image transmission optical fiber, aiming to solve the coupling efficiency problem between the objective lens and the image transmission optical fiber in biomedical imaging.

[0005] To achieve the above object, according to one aspect of the present invention, there is provided an objective lens for coupling with an image transmission optical fiber, including a first lens, a second lens, a third lens, a fourth lens, and a cover glass sequentially arranged from the object plane to the image plane. The third lens is a doublet lens. Both surfaces of the first lens bulge towards the image plane. One surface of the second lens close to the object plane bulges towards the object plane, and one surface close to the image plane bulges towards the image plane. One surface of the third lens close to the object plane bulges towards the object plane, the middle cemented surface bulges towards the image plane, and one surface close to the image plane bulges towards the object plane. Both surfaces of the fourth lens bulge towards the object plane. Both surfaces of the cover glass are flat.

[0006] With the above technical solution, the first lens is used to bend the incident light beam, and the second lens further bends and begins to converge the light beam inward. At this time, the first lens and the second lens correct aberrations such as coma, astigmatism, and field curvature, leaving a small amount of chromatic aberration and accumulating a large positive spherical aberration. The third lens further converges the light beam, not only further correcting coma, astigmatism, and field curvature, but also providing a large negative spherical aberration. This can compensate for the spherical aberration accumulated by the first lens and the second lens. The fourth lens focuses the light beam and can almost completely correct various aberrations remaining in the left lenses. During the coupling process, the side of the cover glass away from the fourth lens faces the image transmission optical fiber. Therefore, the cover glass can be used to protect the end face of the image transmission optical fiber in the subsequent optical path and can also block the scattered light in the left optical path.

[0007] Further, the focal length of the objective lens is f, and f = 9 mm.

[0008] Further, the entrance pupil diameter of the objective lens is 9 mm.

[0009] Further, the radius of curvature of the surface of the first lens close to the object surface is r11, the radius of curvature of the surface close to the image surface is r12, the radius of curvature of the surface of the second lens close to the object surface is r21, the radius of curvature of the surface close to the image surface is r22, the focal length of the first lens is f1, and the focal length of the second lens is f2, satisfying:

[0010] Further, the clear aperture radius of the surface of the third lens close to the object surface is R31, the clear aperture radius of the intermediate cemented surface is R32, the clear aperture radius of the surface close to the image surface is R33, the central thickness of the third lens is t3, the clear aperture radius of the surface of the fourth lens close to the object surface is R41, the clear aperture radius of the surface close to the image surface is R42, and the central thickness of the fourth lens is t4, satisfying:

[0011] Further, the numerical aperture of the objective lens is equal to the numerical aperture of the image transmission optical fiber.

[0012] Further, the parfocal distance of the objective lens is 45 mm.

[0013] Further, the maximum clear aperture radius of the objective lens is 8 mm.

[0014] Further, the radius of curvature of the surface of the first lens close to the object surface is -8.016 mm, the clear aperture radius is 4.62 mm, the radius of curvature of the surface close to the image surface is -14.077 mm, the clear aperture radius is 7.61 mm, the material of the first lens is H-ZLAF92, the central thickness is 10.359 mm, and the central thickness between the adjacent surfaces of the first lens and the second lens is 5.000 mm;

[0015] The radius of curvature of the surface of the second lens close to the object plane is 142.037 mm, the clear aperture radius is 7.63 mm, the radius of curvature of the surface close to the image plane is -44.713 mm, the clear aperture radius is 7.58 mm, the material of the second lens is H-ZLAF68C, the central thickness is 2.947 mm, and the central thickness between the two adjacent surfaces of the second lens and the third lens is 2.000 mm;

[0016] The radius of curvature of the surface of the third lens close to the object plane is 14.792 mm, the clear aperture radius is 6.98 mm, the radius of curvature of the cemented surface in the middle is -21.763 mm, the clear aperture radius is 5.17 mm, the radius of curvature of the surface close to the image plane is 20.295 mm, the clear aperture radius is 4.62 mm. The third lens is composed of two lenses cemented together. The lens close to the object plane is made of H-ZPK5 with a central thickness of 8.852 mm, and the lens close to the image plane is made of H-ZF88 with a central thickness of 2.000 mm. The central thickness between the two adjacent surfaces of the third lens and the fourth lens is 5.536 mm;

[0017] The radius of curvature of the surface of the fourth lens close to the object plane is 7.176 mm, the clear aperture radius is 3.92 mm, the radius of curvature of the surface close to the image plane is 20.972 mm, the clear aperture radius is 2.88 mm, the material of the fourth lens is H-ZLAF68C, the central thickness is 3.922 mm, and the central thickness between the two adjacent surfaces of the fourth lens and the cover glass is 4.216 mm;

[0018] The material of the cover glass is H-K8, the central thickness is 0.170 mm, the clear aperture radius of the surface close to the object plane is 0.56 mm, and the clear aperture radius of the surface close to the image plane is 0.50 mm.

[0019] Further, the radius of curvature of the surface of the first lens close to the object plane is -7.985 mm, the clear aperture radius is 4.62 mm, the radius of curvature of the surface close to the image plane is -14.042 mm, the clear aperture radius is 7.60 mm, the material of the first lens is H-ZLAF92, the central thickness is 10.293 mm, and the central thickness between the two adjacent surfaces of the first lens and the second lens is 5.000 mm;

[0020] The radius of curvature of the surface of the second lens close to the object plane is 187.368 mm, the clear aperture radius is 7.64 mm, the radius of curvature of the surface close to the image plane is -42.303 mm, the clear aperture radius is 7.60 mm, the material of the second lens is H-ZLAF68C, the central thickness is 2.845 mm, and the central thickness between the two adjacent surfaces of the second lens and the third lens is 2.000 mm;

[0021] The radius of curvature of the surface of the third lens close to the object plane is 14.823 mm, the clear aperture radius is 7.01 mm, the radius of curvature of the cemented surface in the middle is -21.981 mm, the clear aperture radius is 5.21 mm, the radius of curvature of the surface of the third lens close to the image plane is 20.987 mm, the clear aperture radius is 4.67 mm. The third lens is composed of two lenses cemented together. The material of the lens close to the object plane is H-ZPK5, and the central thickness is 8.887 mm. The material of the lens close to the image plane is H-ZF88, and the central thickness is 2.000 mm. The central thickness between the two adjacent surfaces of the third lens and the fourth lens is 5.644 mm;

[0022] The radius of curvature of the surface of the fourth lens close to the object plane is 7.179 mm, the clear aperture radius is 3.93 mm, the radius of curvature of the surface of the fourth lens close to the image plane is 20.242 mm, the clear aperture radius is 2.87 mm. The material of the fourth lens is H-ZLAF68C, and the central thickness is 3.959 mm. The central thickness between the two adjacent surfaces of the fourth lens and the cover glass is 4.204 mm;

[0023] The material of the cover glass is H-K8, and the central thickness is 0.170 mm. The clear aperture radius of the surface of the cover glass close to the object plane is 0.56 mm, and the clear aperture radius of the surface of the cover glass close to the image plane is 0.50 mm.

[0024] The objective lens provided by the present invention, through the cooperation between the lenses, can not only match the image transmission fiber bundle, achieve a flat field and achromatism, but also achieve telecentricity, thereby significantly improving the coupling efficiency with the image transmission fiber and reducing crosstalk. Brief Description of the Drawings

[0025] Figure 1 is a schematic structural diagram of the objective lens for coupling with an image transmission fiber according to the present invention;

[0026] Figure 2 is a typical optical layout diagram for imaging using an image transmission fiber;

[0027] Figure 3 is a schematic diagram of the light coupling efficiency when the objective lens for coupling with an image transmission fiber is not telecentric;

[0028] Figure 4 is a schematic diagram of the coupling efficiency when the field curvature of the objective lens for coupling with an image transmission fiber is not corrected;

[0029] Figure 5 is a schematic diagram of the chief ray angle of different fields of view in Example 1;

[0030] Figure 6 is a diagram of the root mean square radius of the blur spot of different fields of view in Example 1;

[0031] Figure 7 is the field curvature and distortion diagram of the first embodiment;

[0032] Figure 8 is the chromatic focal shift curve diagram of the first embodiment;

[0033] Figure 9 is the structural schematic diagram of the second embodiment;

[0034] Figure 10 is the chief ray angle schematic diagram of different fields of view of the second embodiment;

[0035] Figure 11 is the RMS radius diagram of the blur spot of different fields of view of the second embodiment;

[0036] Figure 12 is the field curvature and distortion diagram of the second embodiment;

[0037] Figure 13 is the chromatic focal shift curve diagram of the second embodiment.

[0038] In the figure, L1 is the first lens; L2 is the second lens; L3 is the third lens; L4 is the fourth lens; L5 is the cover glass; 10 is the objective lens; 20 is the image transmission optical fiber; 201 is the end face of the image transmission optical fiber; 30 is the imaging lens group. Detailed implementation manners

[0039] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0040] As Figure 1 shown, the present invention provides an objective lens for coupling with an image transmission optical fiber, which includes a first lens L1, a second lens L2, a third lens L3, a fourth lens L4 and a cover glass L5 arranged in sequence from the object plane to the image plane. The third lens L3 is a doublet lens. Both surfaces of the first lens L1 bulge towards the image plane. One surface of the second lens L2 close to the object plane bulges towards the object plane, and one surface close to the image plane bulges towards the image plane. One surface of the third lens L3 close to the object plane bulges towards the object plane, the middle cemented surface bulges towards the image plane, and one surface close to the image plane bulges towards the object plane. Both surfaces of the fourth lens L4 bulge towards the object plane. Both surfaces of the cover glass L5 are flat.

[0041] The first lens L1 is used to bend the incident light beam. In actual application scenarios, for example, in a probe-type confocal microendoscope, the light beam bent by the first lens L1 is the scanning light beam transmitted by the scanning system of the endoscope. The second lens L2 further bends and begins to converge the light beam inward. At this time, the first lens L1 and the second lens L2 correct aberrations such as coma, astigmatism, and field curvature, leaving a small amount of chromatic aberration and accumulating a large positive spherical aberration. The third lens L3 is a doublet lens, which further converges the light beam, not only further correcting coma, astigmatism, and field curvature, but also providing a large negative spherical aberration. This can compensate for the spherical aberration accumulated by the first lens and the second lens. The fourth lens L4 focuses the light beam and can almost completely correct various aberrations remaining in the left lenses. During the coupling process, the side of the cover glass L5 away from the fourth lens L4 faces the image transmission optical fiber. Therefore, the cover glass L5 can be used to protect the end face of the image transmission optical fiber in the subsequent optical path and can also block the scattered light in the left optical path.

[0042] Further, the entrance pupil diameter of the objective lens 10 is 9 mm. The designed wavelengths of the objective lens 10 are 0.488 μm, 0.515 μm, and 0.55 μm. These three wavelengths are selected as typical wavelengths for fluorescence imaging. The design of the objective lens 10 meets the imaging requirements at these 3 typical wavelengths, that is, it can meet the requirements of fluorescence imaging.

[0043] Further, the focal length of the objective lens 10 is f, and f = 9 mm.

[0044] Further, the radius of curvature of the surface of the first lens L1 close to the object plane is r11, and the radius of curvature of the surface close to the image plane is r12. The radius of curvature of the surface of the second lens L2 close to the object plane is r21, and the radius of curvature of the surface close to the image plane is r22. The focal length of the first lens L1 is f1, and the focal length of the second lens L2 is f2, satisfying:

[0045]

[0046] The clear aperture radius of the surface of the third lens L3 close to the object plane is R31, the clear aperture radius of the middle cemented surface is R32, and the clear aperture radius of the surface close to the image plane is R33. The central thickness of the third lens L3 is t3. The clear aperture radius of the surface of the fourth lens L4 close to the object plane is R41, and the clear aperture radius of the surface close to the image plane is R42. The central thickness of the fourth lens L4 is t4, satisfying:

[0047] The lenses with the above parameter combinations can not only better match the image transmission optical fiber bundle, achieve a flat field and achromatism, but also achieve telecentricity, thereby significantly improving the coupling efficiency with the image transmission optical fiber and reducing crosstalk.

[0048] Further, the numerical aperture of the objective lens 10 is 0.4. In the scenario of biomedical imaging endoscopes, the numerical aperture of the image transmission optical fiber used is usually 0.4. If the numerical aperture of the objective lens 10 is also 0.4, the crosstalk between the cores of the image transmission optical fiber can be minimized to the greatest extent.

[0049] Further, the parfocal distance of the objective lens 10 is 45 mm. It can be consistent with the product rules of most commercially available microscope objective lens brands, such as those of companies like Zeiss, Leica, and Olympus, facilitating replacement and comparative testing.

[0050] Further, the maximum semi-aperture of the objective lens 10 is 8 mm. This can leave sufficient redundancy for the design of the structural components, enabling the final outer diameter of the finished product to be the same as that of most manufacturers, thus facilitating replacement and comparative testing with objective lenses from other manufacturers.

[0051] As Figure 2 shown, it is a typical optical layout diagram for imaging using the image transmission optical fiber 20. The objective lens 10 for coupling with the image transmission optical fiber 20 forms a line target OA on the surface of the image transmission optical fiber 20. Then, the image transmission optical fiber 20 samples the linear target OA. The light from each sampling point propagates along the image transmission optical fiber 20 and exits from the other end of the image transmission optical fiber 20. The image O'A' of the line target OA serves as the virtual image of the subsequent imaging lens group 30. In fiber imaging systems such as probe-type confocal microscopy endoscopes, the "imaging lens group 30" is located at the distal end of the image transmission optical fiber 20; the objective lens 10 for coupling with the image transmission optical fiber 20 is located at the proximal end of the image transmission optical fiber 20. In traditional endoscopes with a lens relay system, the imaging lens group can be designed to compensate for the residual aberrations accumulated by the objective lens 10 and the relay system. In these imaging scenarios, the imaging lens group 30 can only clearly image the proximal end of the image transmission optical fiber 20 and cannot correct the image blurring formed by the objective lens 10 at the distal end of the image transmission optical fiber 20 and transmitted by the image transmission optical fiber 20. Therefore, the objective lens 10 should provide high imaging quality.

[0052] If the objective lens 10 is telecentric at the distal end of the image transmission optical fiber 20, then the central ray of the light beam exiting from the image transmission optical fiber 20 will also be perpendicular to the surface of the image transmission optical fiber 20. Therefore, the imaging lens group 30 also needs to be telecentric at the distal end of the image transmission optical fiber 20 in order to use as much light beam from the image transmission optical fiber 20 as possible. Otherwise, the light beam from the off-axis point may not be able to enter the imaging lens group 30.

[0053] As Figure 3As shown in the figure, if the objective lens 10 for coupling is not telecentric, even if the numerical aperture of the objective lens 10 in the space of the image transmission optical fiber 20 is smaller than that of the image transmission optical fiber 20, some or all of the light beams from off-axis object points will not be coupled into the image transmission optical fiber 20 and transmitted through the image transmission optical fiber 20. This is because the incident angles of some off-axis light are greater than the acceptance angle of the image transmission optical fiber 20. Only the part filled with hatching in the light rays from the off-axis object points is within the acceptance angle of the image transmission optical fiber 20, and then it can be transmitted through the image transmission optical fiber 20. Therefore, compared with the on-axis points, fewer light beams from off-axis points are coupled into the image transmission optical fiber. At this time, the coupling efficiency is not optimal. Therefore, the field curvature of the objective lens 10 coupled with the image transmission optical fiber 20 must be telecentric. This application optimizes the objective lens 10 coupled with the image transmission optical fiber 20 precisely for this point.

[0054] As Figure 4 shown, if the field curvature of the objective lens 10 coupled with the image transmission optical fiber 20 is not corrected, there will be obvious field curvature. This indicates that the aberration is well corrected on the curved image plane - that is, there is a sufficiently small spot size. Since the end face 201 of the image transmission optical fiber after grinding and polishing is a plane, only the area where the vertex of the curved image plane in the figure contacts the end face 201 of the image transmission optical fiber can obtain better coupling efficiency. By adjusting the distance between the objective lens 10 and the end face 201 of the image transmission optical fiber, better coupling efficiency can be achieved at other parts that coincide with the end face 201 of the image transmission optical fiber, but only a few parts can be fully coupled all the time. When the field curvature is fully corrected, the objective lens 10 coupled with the image transmission optical fiber 20 has an image plane close to a plane, and most areas within the depth of focus coincide with the end face 201 of the image transmission optical fiber. At this time, the best coupling effect can be achieved.

[0055] Embodiment 1

[0056] Figure 1 As a specific embodiment of the present invention, the lens parameters of the objective lens 10 are shown in Table 1:

[0057] Table 1

[0058]

[0059] In this embodiment, surface number 0, which is the 0th surface of the entire optical path, represents the object surface; surface number 1 is the 1st surface of the entire optical path, representing the surface of the first lens L1 close to the object surface; surface number 2 is the 2nd surface of the entire optical path, representing the side of the first lens L1 close to the image surface; surface number 3 represents the surface of the second lens L2 close to the object surface, surface number 4 represents the surface of the second lens L2 close to the image surface; surface number 5 represents the surface of the third lens L3 close to the object surface, surface number 6 represents the cemented surface in the middle of the third lens L3, surface number 7 represents the surface of the third lens L3 close to the image surface; surface number 8 represents the surface of the fourth lens L4 close to the object surface, surface number 9 represents the surface of the fourth lens L4 close to the image surface; surface number 10 represents the surface of the cover glass L5 close to the object surface, surface number 11 represents the surface of the cover glass L5 close to the image surface; surface number 12 represents the image surface.

[0060] The radius of curvature of the surface of the first lens L1 close to the object surface is -8.016 mm, and the clear aperture radius is 4.62 mm. The radius of curvature of the surface of the first lens L1 close to the image surface is -14.077 mm, and the clear aperture radius is 7.61 mm. The material of the first lens L1 is H-ZLAF92, the central thickness is 10.359 mm, and the central thickness between the adjacent two surfaces of the first lens L1 and the second lens L2 is 5.000 mm.

[0061] The radius of curvature of the surface of the second lens L2 close to the object surface is 142.037 mm, and the clear aperture radius is 7.63 mm. The radius of curvature of the surface of the second lens L2 close to the image surface is -44.713 mm, and the clear aperture radius is 7.58 mm. The material of the second lens L2 is H-ZLAF68C, the central thickness is 2.947 mm, and the central thickness between the adjacent two surfaces of the second lens L2 and the third lens L3 is 2.000 mm.

[0062] The radius of curvature of the surface of the third lens L3 close to the object surface is 14.792 mm, and the clear aperture radius is 6.98 mm. The radius of curvature of the middle cemented surface is -21.763 mm, and the clear aperture radius is 5.17 mm. The radius of curvature of the surface of the third lens L3 close to the image surface is 20.295 mm, and the clear aperture radius is 4.62 mm. The third lens L3 is composed of two lenses cemented together. The material of the lens close to the object surface is H-ZPK5, the central thickness is 8.852 mm, and the material of the lens close to the image surface is H-ZF88, the central thickness is 2.000 mm. The central thickness between the adjacent two surfaces of the third lens L3 and the fourth lens L4 is 5.536 mm. [[ID=]]

[0063] The radius of curvature of one side of the fourth lens L4 close to the object surface is 7.176 mm, and the clear aperture radius is 3.92 mm. The radius of curvature of the other side close to the image surface is 20.972 mm, and the clear aperture radius is 2.88 mm. The material of the fourth lens L4 is H-ZLAF68C, and the central thickness is 3.922 mm. The central thickness between the two adjacent sides of the fourth lens L4 and the cover glass is 4.216 mm.

[0064] The cover glass L5 is a standard #1.5 cover glass with a refractive index of 1.52. The material is H-K8, and the central thickness is 0.170 mm. The clear aperture radius of the side of the cover glass L5 close to the object surface is 0.56 mm, and the clear aperture radius of the side close to the image surface is 0.50 mm.

[0065] The focal lengths f1 of the first lens L1, f2 of the second lens L2, f3 of the third lens L3, and f4 of the fourth lens L4 are: -133.14 mm, 38.37 mm, -113.97 mm, and 10.77 mm respectively. The overall optical length TTL of the entire objective lens 10 is 45 mm.

[0066] r11 = -8.016 mm, r12 = -14.077 mm, r21 = 142.037 mm, r22 = -44.713 mm, f1 = -133.14 mm, f2 = 38.37 mm, R31 = 6.98 mm, R32 = 5.17 mm, R33 = 4.62 mm, t3 = 8.852 mm + 2.000 mm = 10.852 mm, R41 = 3.92 mm, R42 = 2.88 mm, t4 = 3.922 mm,

[0067]

[0068] Figure 5 The chief ray angles of different fields of view of this embodiment are shown. It can be seen from the figure that the chief ray angles within the entire field of view are all less than 0.1°, indicating good telecentric characteristics. In this way, the chief rays of different fields of view are not only almost parallel to the axes of the respective cores, but also have almost the same fiber incident cone angles, which can maximize the coupling efficiency between the objective lens 10 and the image transmission fiber.

[0069] Figure 6 The root mean square radius of the spot diagrams of different fields of view of this embodiment is shown. It can be seen from the figure that the root mean square radius of the spot diagrams within the entire field of view is less than 0.65 μm of the diffraction limit and also less than the core diameter of 2 μm of the commonly used image transmission fiber bundle, which can maximize the coupling efficiency.

[0070] Figure 7Shows the field curvature and distortion diagram of this embodiment. As can be seen from the figure, the design wavelengths of this embodiment are 0.488μm, 0.515μm, and 0.55μm, and the astigmatism and field curvature within the entire field of view are fully constrained and corrected. The field curvature within the entire field of view is less than 3μm, which can be perfectly matched with the end face of the image transmission optical fiber after grinding and polishing, maximizing the coupling efficiency with the fiber bundle. The astigmatism of each wavelength is less than 3μm, also maximizing the coupling efficiency of each wavelength within the broad-spectrum fluorescence band. The distortion within the entire field of view is less than 1%, which is less than the distortion that can be detected by the visual perception of the average human eye, and can maximally ensure that the shape of the imaging target is not distorted.

[0071] Figure 8 Shows the chromatic focal shift curve of this embodiment. As can be seen from the curve, the chromatic focal shift within the design wavelength range is constrained within 2.5μm, approaching the 2.194μm of the diffraction limit. This further indicates that the chromatic aberration of the objective lens 10 in this embodiment is fully corrected, maximizing the coupling efficiency with the fiber bundle.

[0072] Embodiment Two

[0073] Figure 9 As a specific embodiment of the present invention, the parameters of each lens of the objective lens 10 are shown in Table 2:

[0074] Table 2

[0075]

[0076] In this embodiment, surface number 0, which is the 0th surface of the entire optical path, represents the object surface; surface number 1 is the 1st surface of the entire optical path, representing the side of the first lens L1 close to the object surface; surface number 2 is the 2nd surface of the entire optical path, representing the side of the first lens L1 close to the image surface; surface number 3 represents the side of the second lens L2 close to the object surface, surface number 4 represents the side of the second lens L2 close to the image surface; surface number 5 represents the side of the third lens L3 close to the object surface, surface number 6 represents the cemented surface in the middle of the third lens L3, surface number 7 represents the side of the third lens L3 close to the image surface; surface number 8 represents the side of the fourth lens L4 close to the object surface, surface number 9 represents the side of the fourth lens L4 close to the image surface; surface number 10 represents the side of the cover glass L5 close to the object surface, surface number 11 represents the side of the cover glass L5 close to the image surface; surface number 12 represents the image surface.

[0077] The radius of curvature of the side of the first lens L1 close to the object surface is -7.985mm, the clear aperture radius is 4.62mm, and the radius of curvature of the side close to the image surface is -14.042mm, the clear aperture radius is 7.60mm. The material of the first lens L1 is H-ZLAF92, the central thickness is 10.293mm, and the central thickness between the two adjacent sides of the first lens L1 and the second lens L2 is 5.000mm.

[0078] The radius of curvature of the surface of the second lens L2 close to the object plane is 187.368 mm, the clear aperture radius is 7.64 mm, the radius of curvature of the surface close to the image plane is -42.303 mm, and the clear aperture radius is 7.60 mm. The material of the second lens L2 is H-ZLAF68C, the central thickness is 2.845 mm, and the central thickness between the two adjacent surfaces of the second lens L2 and the third lens L3 is 2.000 mm.

[0079] The radius of curvature of the surface of the third lens L3 close to the object plane is 14.823 mm, the clear aperture radius is 7.01 mm, the radius of curvature of the cemented surface in the middle is -21.981 mm, the clear aperture radius is 5.21 mm, and the radius of curvature of the surface close to the image plane is 20.987 mm, the clear aperture radius is 4.67 mm. The third lens L3 is composed of two lenses cemented together. The material of the lens close to the object plane is H-ZPK5, the central thickness is 8.887 mm, the material of the lens close to the image plane is H-ZF88, the central thickness is 2.000 mm, and the central thickness between the two adjacent surfaces of the third lens L3 and the fourth lens L4 is 5.644 mm.

[0080] The radius of curvature of the surface of the fourth lens L4 close to the object plane is 7.179 mm, the clear aperture radius is 3.93 mm, the radius of curvature of the surface close to the image plane is 20.242 mm, and the clear aperture radius is 2.87 mm. The material of the fourth lens L4 is H-ZLAF68C, the central thickness is 3.959 mm, and the central thickness between the two adjacent surfaces of the fourth lens L4 and the cover glass L5 is 4.204 mm.

[0081] The cover glass L5 is a standard #1.5 cover glass with a refractive index of 1.52, the material is H-K8, and the central thickness is 0.170 mm. The clear aperture radius of the surface of the cover glass L5 close to the object plane is 0.56 mm, and the clear aperture radius of the surface close to the image plane is 0.50 mm.

[0082] The focal lengths f1 of the first lens L1, f2 of the second lens L2, f3 of the third lens L3, and f4 of the fourth lens L4 are: -127.70 mm, 38.88 mm, -133.06 mm, 10.90 mm respectively. The overall optical length TTL of the entire objective lens 10 is 45 mm.

[0083] r11 = -7.985 mm, r12 = -14.042 mm, r21 = 187.368 mm, r22 = -42.303 mm, f1 = -127.70 mm, f2 = 38.88 mm, R31 = 7.01 mm, R32 = 5.21 mm, R33 = 4.67 mm, t3 = 8.887 mm + 2.000 mm = 10.887 mm, R41 = 3.93 mm, R42 = 2.87 mm, t4 = 3.959 mm,

[0084]

[0085] Figure 10 The chief ray angles of different fields of view of this embodiment are shown. It can be seen from the figure that the chief ray angles within the entire field of view are all less than 0.1°, indicating good telecentric characteristics. In this way, the chief rays of different fields of view are not only almost parallel to the axes of the respective optical fibers, but also have almost the same fiber incident cone angles, which can maximize the coupling efficiency between the objective lens 10 and the image transmission optical fiber.

[0086] Figure 11 The root mean square radius of the spot size of different fields of view of this embodiment is shown. It can be seen from the figure that the root mean square radius of the spot size within the entire field of view is less than 0.65 μm of the diffraction limit and also less than the core diameter of 2 μm of the commonly used image transmission optical fiber bundle, which can maximize the coupling efficiency.

[0087] Figure 12 The field curvature and distortion diagram of this embodiment is shown. It can be seen from the figure that the design wavelengths of this embodiment are 0.488 μm, 0.515 μm, and 0.55 μm, and the astigmatism and field curvature within the entire field of view are fully constrained and corrected. The field curvature within the entire field of view is less than 3 μm, so that it can be perfectly matched with the end face of the image transmission optical fiber after grinding and polishing, maximizing the coupling efficiency with the optical fiber bundle. The astigmatism of each wavelength is less than 3 μm, which also maximizes the coupling efficiency of each wavelength within the wide-spectrum fluorescence band. The distortion within the entire field of view is less than 1%, which is less than the distortion that can be detected by the human eye's vision in general, and can maximally ensure that the shape of the imaging target is not distorted.

[0088] Figure 13 The chromatic focal shift curve of this embodiment is shown. It can be seen from the curve that the chromatic focal shift within the design wavelength range is constrained within 2.5 μm, approaching the 2.103 μm of the diffraction limit. This further indicates that the chromatic aberration of the objective lens 10 of this embodiment is fully corrected, maximizing the coupling efficiency with the optical fiber bundle.

[0089] The objective lens 10 provided by the present invention not only corrects conventional aberrations such as spherical aberration, coma, field curvature, and distortion, reaching the diffraction limit state, but also aims at the requirement of "coupling". By matching with the used image transmission optical fiber, it not only has flat-field characteristics and achromatic characteristics, but also specifically enhances the "telecentric" characteristics, greatly improving the coupling efficiency and significantly improving the signal-to-noise ratio and overall performance of various imaging systems such as probe-type confocal microendoscopes and two-photon microscopes.

[0090] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. An objective lens for coupling with an image transmission optical fiber, characterized in that, It includes a first lens, a second lens, a third lens, a fourth lens and a cover glass which are sequentially arranged from the object plane to the image plane. The third lens is a doublet lens. Both surfaces of the first lens bulge towards the image plane. One surface of the second lens close to the object plane bulges towards the object plane and the other surface close to the image plane bulges towards the image plane. One surface of the third lens close to the object plane bulges towards the object plane, the middle cemented surface bulges towards the image plane, and the other surface close to the image plane bulges towards the object plane. Both surfaces of the fourth lens bulge towards the object plane. Both surfaces of the cover glass are flat planes.

2. The objective lens according to claim 1, characterized in that, The focal length of the objective lens is f, and f = 9 mm.

3. The objective lens according to claim 2, characterized in that, The entrance pupil diameter of the objective lens is 9 mm.

4. The objective lens according to claim 2, characterized in that The radius of curvature of the surface of the first lens close to the object surface is r11, and the radius of curvature of the surface close to the image surface is r12. The radius of curvature of the surface of the second lens close to the object surface is r21, and the radius of curvature of the surface close to the image surface is r22. The focal length of the first lens is f1, and the focal length of the second lens is f2, satisfying:

5. The objective lens according to claim 2, characterized in that, The clear aperture of the side of the third lens close to the object surface is R31, the clear aperture of the intermediate cemented surface is R32, and the clear aperture of the side of the third lens close to the image surface is R33. The central thickness of the third lens is t3. The clear aperture of the side of the fourth lens close to the object surface is R41, and the clear aperture of the side of the fourth lens close to the image surface is R42. The central thickness of the fourth lens is t4, and they satisfy:

6. The objective lens according to claim 2, characterized in that, The numerical aperture of the objective lens is equal to that of the image transmission optical fiber.

7. The objective lens according to claim 2, characterized in that, The parfocal distance of the objective lens is 45 mm.

8. The objective lens according to claim 2, characterized in that, The maximum semi-aperture of the objective lens is 8 mm.

9. The objective lens according to any one of claims 3-8, characterized in that The curvature radius of one surface of the first lens close to the object plane is -8.016 mm, the clear aperture semi-aperture is 4.62 mm, the curvature radius of the other surface close to the image plane is -14.077 mm, the clear aperture semi-aperture is 7.61 mm. The material of the first lens is H-ZLAF92, the central thickness is 10.359 mm, and the central thickness between the adjacent surfaces of the first lens and the second lens is 5.000 mm; The curvature radius of one surface of the second lens close to the object plane is 142.037 mm, the clear aperture semi-aperture is 7.63 mm, the curvature radius of the other surface close to the image plane is -44.713 mm, the clear aperture semi-aperture is 7.58 mm. The material of the second lens is H-ZLAF68C, the central thickness is 2.947 mm, and the central thickness between the adjacent surfaces of the second lens and the third lens is 2.000 mm; The curvature radius of one surface of the third lens close to the object plane is 14.792 mm, the clear aperture semi-aperture is 6.98 mm, the curvature radius of the middle cemented surface is -21.763 mm, the clear aperture semi-aperture is 5.17 mm, the curvature radius of the other surface close to the image plane is 20.295 mm, the clear aperture semi-aperture is 4.62 mm. The third lens is composed of two lenses cemented together. The material of the lens close to the object plane is H-ZPK5, the central thickness is 8.852 mm, the material of the lens close to the image plane is H-ZF88, the central thickness is 2.000 mm, and the central thickness between the adjacent surfaces of the third lens and the fourth lens is 5.536 mm; The curvature radius of one surface of the fourth lens close to the object plane is 7.176 mm, the clear aperture semi-aperture is 3.92 mm, the curvature radius of the other surface close to the image plane is 20.972 mm, the clear aperture semi-aperture is 2.88 mm. The material of the fourth lens is H-ZLAF68C, the central thickness is 3.922 mm, and the central thickness between the adjacent surfaces of the fourth lens and the cover glass is 4.216 mm; The material of the cover glass is H-K8, the central thickness is 0.170 mm, the clear aperture semi-aperture of the surface close to the object plane is 0.56 mm, and the clear aperture semi-aperture of the surface close to the image plane is 0.50 mm.

10. The objective lens according to any one of claims 3-8, characterized in that, The radius of curvature of the surface of the first lens adjacent to the object plane is -7.985 mm, the clear aperture radius is 4.62 mm, the radius of curvature of the surface of the first lens adjacent to the image plane is -14.042 mm, the clear aperture radius is 7.60 mm, the material of the first lens is H-ZLAF92, the central thickness is 10.293 mm, and the central thickness between the adjacent surfaces of the first lens and the second lens is 5.000 mm; The radius of curvature of the surface of the second lens adjacent to the object plane is 187.368 mm, the clear aperture radius is 7.64 mm, the radius of curvature of the surface of the second lens adjacent to the image plane is -42.303 mm, the clear aperture radius is 7.60 mm, the material of the second lens is H-ZLAF68C, the central thickness is 2.845 mm, and the central thickness between the adjacent surfaces of the second lens and the third lens is 2.000 mm; The radius of curvature of the surface of the third lens adjacent to the object plane is 14.823 mm, the clear aperture radius is 7.01 mm, the radius of curvature of the cemented surface in the middle is -21.981 mm, the clear aperture radius is 5.21 mm, the radius of curvature of the surface of the third lens adjacent to the image plane is 20.987 mm, the clear aperture radius is 4.67 mm. The third lens is composed of two lenses cemented together. The lens adjacent to the object plane is made of H-ZPK5, with a central thickness of 8.887 mm, and the lens adjacent to the image plane is made of H-ZF88, with a central thickness of 2.000 mm. The central thickness between the adjacent surfaces of the third lens and the fourth lens is 5.644 mm; The radius of curvature of the surface of the fourth lens adjacent to the object plane is 7.179 mm, the clear aperture radius is 3.93 mm, the radius of curvature of the surface of the fourth lens adjacent to the image plane is 20.242 mm, the clear aperture radius is 2.87 mm, the material of the fourth lens is H-ZLAF68C, the central thickness is 3.959 mm, and the central thickness between the adjacent surfaces of the fourth lens and the cover glass is 4.204 mm; The material of the cover glass is H-K8, the central thickness is 0.170 mm, the clear aperture radius of the surface of the cover glass adjacent to the object plane is 0.56 mm, and the clear aperture radius of the surface of the cover glass adjacent to the image plane is 0.50 mm.

Images