Large-depth wide-field-of-view spectrum confocal microscopic endoscopic optical system and imaging method

Abstract

The invention discloses a large-depth wide-field-of-view spectrum confocal microscopic endoscopic optical system and an imaging method. Large-angle incident light is deflected after passing through the curved convex surface of the spherical positive lens of the front lens group, and tends to be gentle relative to the optical axis, so that distortion is corrected; the transition mirror adopts an aspheric surface, further performs residual aberration correction on the incident light beam, and inhibits residual spherical aberration of the front lens group; the rear lens group adopts a concentric structure to correct residual aberration, and adopts a flat-field thick lens to correct field curvature. Materials of optical elements of the system are matched and designed to further enlarge the color focal shift of the system, axial separation is generated at convergence positions of different wavelengths, axial light splitting of light beams of different wavelengths is achieved, the light beams are finally converged on an image plane for imaging, and the color focal shift larger than 1 mm is obtained. By adopting the matching design between the lens group structure and the material, large-depth, wide-view-field and high-resolution microscopic endoscopic imaging is realized, more comprehensive tissue information can be obtained, and the clinical diagnosis requirement is met.

Description

Technical Field

[0001] The present invention relates to the technical field of optical design, and in particular to a spectral confocal microendoscopic optical system with large depth and wide field of view functions and an imaging method. Background Art

[0002] Traditional confocal microscopy technology, which is excited by a single wavelength, can only image a specific tissue depth and one depth plane at a time. Therefore, the use of traditional confocal microscopy technology requires 3D imaging to be completed through mechanical scanning, which has a slow imaging speed and cannot meet the needs of clinical real-time 3D imaging.

[0003] Spectral confocal technology can achieve high-resolution, fast, multi-depth tomographic imaging. Compared with traditional confocal microscopes, spectral confocal microscopes can achieve multi-wavelength, multi-depth imaging without any mechanical axial scanning, but it is difficult to simultaneously meet the requirements of small size, large chromatic focal shift, and small spherical aberration. Before the present invention was made, the document "Fiber-optic large-depth3D chromatic confocal endomicroscopy" (Biomed. Opt. Express 13, 300-313 (2022)) reported a fiber-based large-depth 3D spectral confocal endoscope, which designed a five-piece microscope objective with a large chromatic focal shift to focus light of different wavelengths at different depths. Spherical aberration correction technology was also introduced when designing the objective to solve the technical problem of simultaneously achieving large chromatic focal shift and small spherical aberration. However, the system can only achieve an actual imaging depth of 560μm, which is unable to achieve deep detection and difficult to obtain deep information of the subendothelial tissue of the human gastrointestinal tract. On the other hand, the system has a small field of view, resulting in a small detection range, which cannot meet the actual application needs of endoscopes. The third problem is that the objective lens aperture of the system is far from the national standard for the use of gastroenteroscopes. The national standard for the outer diameter of gastroenteroscopes is less than 3mm, but the outer diameter of the system is 11mm, which is too large, far exceeding the national standard, and is still far from the actual clinical application. Therefore, the system has many problems such as shallow detection depth, small detection field of view and too large aperture, which has limitations in actual use in the medical field. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a spectral confocal microendoscopic optical system and an imaging method thereof which have a simple and compact structure, are easy to process and assemble, have deep imaging depth, high dispersion linearity, a large field of view, and good imaging quality.

[0005] The technical solution adopted by the present invention is to provide a large-depth and wide-field spectral confocal microendoscopic optical system, which includes an object plane, a front mirror group, a transition mirror, a rear mirror group and an image plane; The front lens group is a three-piece nearly concentric symmetrical structure. According to the incident direction of the light, it is a spherical positive lens, a first spherical negative lens, and a second spherical negative lens. The curvature radius of each lens surface is R21, R22, R31, R32, R41, and R42, respectively meeting the conditions of 0.9mm≤R21≤2.3mm, -181mm≤R22≤-180mm, -4mm≤R31≤-3mm, -20.2mm≤R32≤-19.2mm, -3.8mm≤R41≤-3.1mm, and -8.2 mm≤R42≤-7.4mm; the material refractive index n1 of the spherical positive lens ranges from 1.5≤n1≤1.7, and the Abbe number v1 ranges from 59.9≤v1≤62.8; the material refractive index n2 of the first spherical negative lens ranges from 1.8≤n2≤2.0, and the Abbe number v2 ranges from 32.5≤v2≤36.1; the material refractive index n3 of the second spherical negative lens of the front lens group ranges from 1.7≤n3≤1.8, and the Abbe number v3 ranges from 43.7≤v3≤45.4; The transition mirror comprises an aspheric positive lens, the front and rear surfaces of which are both even-order aspheric surfaces; the curvature radii of the front and rear surfaces are R51 and R52, respectively, satisfying the conditions of 1.7mm≤R51≤2.1mm, -5.8mm≤R52≤-5.4mm; the value range of the material refractive index n4 is 1.8≤n4≤2.0, and the value range of the Abbe number v4 is 16.0≤v4≤18.5; The rear lens group includes two concentric spherical negative lenses; the first spherical negative lens and the second spherical negative lens, the rear surface of the second spherical negative lens is a plane; according to the incident direction of light, the curvature radii of the remaining surfaces of each lens are R61, R62 and R71, respectively, satisfying the conditions of 3.6mm≤R61≤4.2mm, 1.0mm≤R62≤1.5mm, -5.2mm≤R71≤-4.2mm; the material refractive index n5 of the first spherical negative lens has a value range of 1.8≤n5≤1.9, and the Abbe number v5 has a value range of 39.4≤v5≤41.3; the material refractive index n6 of the second spherical negative lens has a value range of 1.8≤n6≤2.0, and the Abbe number v6 has a value range of 36.6≤v6≤38.2; The aperture stop of the optical system is arranged at the front surface of the first spherical negative lens of the front lens group.

[0006] The front and rear surfaces of the aspheric positive lens are both aspheric. A Cartesian space rectangular coordinate system is constructed with the intersection of the lens surface and the optical axis as the origin. The incident direction of the light is the positive direction of the Z axis, the positive direction of the Y axis is upward, and the positive direction of the X axis is perpendicular to the paper and inward. The vector height equation Z of the even-order aspheric surface is: , Where c is the curvature; r is the radius; k is the quadratic surface coefficient; a i is the coefficient of the monomial; The front surface of the aspherical positive lens, -2.4≤k≤0.4, the coefficients of each monomial respectively satisfy the conditions: , , , , , ; The rear surface of the aspherical positive lens, -33.7≤k≤-31.7, the coefficients of each monomial respectively satisfy the conditions: , , , , , .

[0007] The imaging method of a large-depth and wide-field spectral confocal microendoscopic optical system described in the present invention comprises the following steps: (1) The spherical positive lens of the front mirror group adopts a curved convex surface with an optical power of 0.26 to 0.65 to accept incident light at a large angle. The light beam is deflected after passing through the convex surface. The deflected light beam tends to be flat relative to the optical axis, thereby correcting the distortion. For the incident light of the near-infrared wide band of 1000nm to 1500nm, the front mirror group adopts matching optical elements of high refractive index, low Abbe number and low refractive index, high Abbe number materials. The optical path difference between light beams of different wavelengths is increased, and the convergence positions of different wavelengths are axially separated, thereby increasing the axial focal shift of the incident light. The convergent light beam is then incident on the transition mirror. (2) The transition mirror is designed by matching the conic coefficient and aspheric coefficient of the front and rear aspheric surfaces to further correct the residual aberration of the incident light beam and suppress the residual spherical aberration of the front mirror group. At the same time, it compensates for the coma and distortion of the rear mirror group to achieve relay imaging. (3) The imaging light beam processed by the transition mirror is then passed through the rear mirror group. The rear mirror group adopts a concentric structure to correct the residual aberration and a flat-field thick lens to correct the field curvature. The materials of the optical elements are matched to further expand the chromatic focal shift of the system. The convergence positions between different wavelengths produce further axial separation, realizing axial splitting between light beams of different wavelengths, and finally converging on the image plane to form an image, resulting in a chromatic focal shift greater than 1 mm.

[0008] Compared with the prior art, the present invention has the following beneficial effects: 1. Aiming at the demand for large detection depth, the present invention adopts a combination of high refractive index, low Abbe number and low refractive index, high Abbe number materials according to the relationship between material properties and optical path difference to ensure that the optical path difference between light of different wavelengths is increased, focusing on different positions on the axis, obtaining a larger axial chromatic focal shift, and ensuring that the system achieves a detection depth of 1mm. At the same time, through the optimization design of the lens group structure, the axial position relationship between different depths of focus of each wavelength is strictly controlled, thereby achieving an approximately linear relationship between the axial chromatic focal shift and the wavelength, ensuring the consistency of sensitivity within the measurement range and the accuracy of actual measurement.

[0009] 2. In order to realize endoscopic microscopic imaging under a wide field of view, the present invention overcomes the influence of field curvature and distortion caused by a wide field of view on endoscopic microscopic imaging. In the structural design of the optical system, an improved Cook three-piece structure is adopted in the front mirror group, and fine aberration correction and optimization design are performed based on aberration theory, so that the intrinsic aberration is small, the influence on the aberration of the subsequent optical group is small, and the influence of distortion is effectively reduced; the rear mirror group adopts a concentric structure, and further corrects the field curvature through a flat-field thick lens. And further, the three mirror groups are jointly optimized and designed, which effectively corrects the problem that aberrations such as field curvature, astigmatism and distortion increase with the increase of field of view, and realizes an ultra-wide field of view of 600μm, which can obtain more comprehensive tissue information. At the same time, compared with the existing spectral confocal endoscopic imaging system, the field of view is significantly improved.

[0010] 3. Aiming at the demand for high-resolution microscopic imaging of endoscopes, the present invention designs the position of the aspheric surface through the first-order aberration theory and aberration characteristics, and designs the aperture stop in front of the aspheric lens, which can accurately suppress spherical aberration without affecting other aberrations. The selection of the aspheric position enhances the system's ability to balance various geometric aberrations, suppresses the residual spherical aberration of the front lens group, and compensates for the coma and distortion of the rear lens group, effectively improving the overall imaging resolution of the system. Compared with the existing near-infrared microendoscopic imaging system, the imaging quality is significantly improved. Aiming at the 12μm pixel size commonly used in near-infrared band detectors of 1000nm to 1500nm, the present invention achieves an optical transfer function of more than 0.6 for the entire field of view of the 1μm to 1.5μm working band at 42lp / mm, with good imaging quality.

[0011] 4. The present invention optimizes the structure of the endoscope under the premise of the system caliber limitation in view of the use requirements of the small-caliber endoscope. The system achieves good imaging quality while meeting the requirements of miniaturization. The micro-microscope objective lens used in the digestive tract provided by the present invention is small in size, with an outer diameter of only 2.8 mm, and can enter the human body through the working channel of a conventional gastroenteroscopy for clinical diagnosis of intestinal diseases, and can meet various optical performance requirements during diagnosis. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 Schematic diagram of the structure of a large-depth and wide-field spectral confocal microendoscopy optical system provided by an embodiment of the present invention; In the figure, 1. object plane; 11. front mirror group; 2. front mirror group spherical positive lens; 3. front mirror group first spherical negative lens; 4. front mirror group second spherical negative lens; 22. transition mirror; 5. fourth aspherical positive lens; 33. rear mirror group; 6. rear mirror group first spherical negative lens; 7. rear mirror group second spherical negative lens; 8. image plane; Figure 2 is a flow chart of a microendoscopic optical system imaging method provided by an embodiment of the present invention; Figure 3 It is a focused light spot array diagram of the full field of view and full working band of the microendoscopic optical system provided by an embodiment of the present invention; Figure 4 is a color focus shift curve diagram of the microendoscopy optical system provided by an embodiment of the present invention; Figure 5 is a transfer function MTF curve diagram of the microendoscopy optical system provided by an embodiment of the present invention; Figure 6 1 is an optical path difference curve diagram of the microendoscopic optical system provided by an embodiment of the present invention at four radial fields of view on the image side. DETAILED DESCRIPTION

[0013] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and embodiments. Example 1

[0014] This embodiment provides a large-depth, wide-field-of-view spectral confocal microendoscopic optical system and imaging method. The optical system consists of six lenses, the object numerical aperture of the optical system is 0.3, the magnification of the micro-microscope objective is 2 times, and the working wavelength is 1000-1500nm. Since the object plane of the microscope objective is connected to the end face of the optical fiber bundle in the entire endoscope probe, the effective field of view that the microscope objective should achieve can be calculated to be 600μm based on the aperture of the optical fiber bundle.

[0015] The performance parameters of the microendoscope provided in this embodiment are shown in Table 1.

[0016] Table 1: .

[0017] See attached Figure 1, which is a schematic diagram of the structure of the microendoscopy optical system described in this embodiment. According to the incident direction of light, the optical elements are: object plane 1, front lens group 11, transition mirror 22, rear lens group 33 and image plane 8 in sequence; wherein the front lens group 11 is a three-piece nearly concentric symmetrical structure, and according to the incident direction of light, it is a spherical positive lens 2, a first spherical negative lens 3, and a second spherical negative lens 4 in sequence, the transition mirror 22 includes an aspherical positive lens 5, and the rear lens group 33 includes a first spherical negative lens 6 and a second spherical negative lens 7.

[0018] The front lens group is a three-piece nearly concentric symmetrical structure. According to the incident direction of the light, it is a spherical positive lens, the first spherical negative lens and the second spherical negative lens. The radius of curvature of each lens surface is R21, R22, R31, R32, R41 and R42, respectively meeting the conditions of 0.9mm≤R21≤2.3mm, -181mm≤R22≤-180mm, -4mm≤R31≤-3mm, -20.2mm≤R32≤-19.2mm, -3.8mm≤R41≤-3.1mm, -8.2mm≤R41≤-3.1mm, m≤R42≤-7.4mm; the material refractive index n1 of the spherical positive lens ranges from 1.5≤n1≤1.7, and the Abbe number v1 ranges from 59.9≤v1≤62.8; the material refractive index n2 of the first spherical negative lens ranges from 1.8≤n2≤2.0, and the Abbe number v2 ranges from 32.5≤v2≤36.1; the material refractive index n3 of the second spherical negative lens of the front lens group ranges from 1.7≤n3≤1.8, and the Abbe number v3 ranges from 43.7≤v3≤45.4; The transition mirror is an aspheric positive lens, and its front and rear surfaces are both even-order aspheric surfaces; the curvature radii of the front and rear surfaces are R51 and R52, respectively, satisfying the conditions of 1.7mm≤R51≤2.1mm and -5.8mm≤R52≤-5.4mm; the value range of its material refractive index n4 is 1.8≤n4≤2.0, and the value range of Abbe number v4 is 16.0≤v4≤18.5; The rear lens group includes two concentric spherical negative lenses; the first spherical negative lens and the second spherical negative lens, the rear surface of the second spherical negative lens is a plane; according to the incident direction of light, the curvature radii of the remaining surfaces of each lens are R61, R62 and R71, respectively, satisfying the conditions of 3.6mm≤R61≤4.2mm, 1.0mm≤R62≤1.5mm, -5.2mm≤R71≤-4.2mm; the material refractive index n5 of the first spherical negative lens has a value range of 1.8≤n5≤1.9, and the Abbe number v5 has a value range of 39.4≤v5≤41.3; the material refractive index n6 of the second spherical negative lens has a value range of 1.8≤n6≤2.0, and the Abbe number v6 has a value range of 36.6≤v6≤38.2; The aperture stop of the optical system is arranged at the front surface of the first spherical negative lens of the front lens group.

[0019] The front and back surfaces of the aspheric positive lens are both aspheric. The Cartesian space rectangular coordinate system is constructed with the intersection of the lens surface and the optical axis as the origin. The incident direction of the light is the positive direction of the Z axis, the positive direction of the Y axis is upward, and the positive direction of the X axis is perpendicular to the paper surface and inward. The vector height equation Z of the even-order aspheric surface is: , Where c is the curvature; r is the radius; k is the quadratic surface coefficient; a i is the coefficient of the monomial; The front surface of the aspherical positive lens, -2.4≤k≤0.4, the coefficients of each monomial satisfy the conditions: , , , , , .

[0020] The rear surface of the aspherical positive lens, -33.7≤k≤-31.7, the coefficients of each monomial respectively satisfy the conditions: , , , , , .

[0021] When the microscope objective is imaging, the object plane is the end face of the optical fiber bundle. The light passes through the front lens group, transition lens and rear lens group in sequence, and finally forms an image on the image plane, which is the cross section of the human tissue at different depths. Different wavelengths are focused on the image plane at different depths, that is, the imaging depth in the gastric tissue. Then, according to the principle of reversibility of light, the light reflected by the tissue at different depths will be reflected to the object plane, transmitted through the optical fiber bundle, and then the images of the cross sections at different depths in the gastric tissue are obtained, thereby constructing an overall three-dimensional image.

[0022] In this embodiment, the front surface of the aspheric positive lens is an even-order aspheric surface. In the vector height equation Z of the even-order aspheric surface, k=-1.468, and the coefficients of each monomial are respectively , , , , , .

[0023] In this embodiment, the rear surface of the aspheric positive lens is an even-order aspheric surface. In the vector height equation Z of the even-order aspheric surface, k=-32.798, and the coefficients of each monomial are respectively , , , , , .

[0024] The parameters of the optical components in this embodiment are shown in Table 2.

[0025] Table 2: .

[0026] Among them, the material refractive index n1=1.59 and the Abbe number v1=61.3 of the spherical positive lens 2 of the front lens group; the material refractive index n2=1.91 and the Abbe number v2=35.3 of the first spherical negative lens 3 of the front lens group; the material refractive index n3=1.74 and the Abbe number v3=44.9 of the second spherical negative lens 4 of the front lens group; the material refractive index n4=1.96 and the Abbe number v4=17.5 of the transition mirror aspherical positive lens 5; the material refractive index n5=1.88 and the Abbe number v5=40.8 of the first spherical negative lens 6 of the rear lens group; the material refractive index n6=1.90 and the Abbe number v6=37.1 of the second spherical negative lens 7 of the rear lens group.

[0027] See attached Figure 2 , which is a flow chart of the imaging method of the microendoscopic optical system provided in this embodiment; when imaging using the microendoscopic optical system provided in this embodiment, the steps are as follows: (1) The spherical positive lens of the front mirror group adopts a curved convex surface with an optical power of 0.314 to accept incident light at a large angle. The light beam is deflected after passing through the convex surface. The deflected light beam tends to be flat relative to the optical axis, and the distortion is corrected. For the incident light of the near-infrared wide band of 1000nm to 1500nm, the front mirror group adopts matching optical elements of high refractive index, low Abbe number and low refractive index, high Abbe number materials. The optical path difference between light beams of different wavelengths is increased, and the convergence positions of different wavelengths are axially separated, thereby increasing the axial focal shift of the incident light. The convergent light beam is then incident on the transition mirror. (2) The transition mirror is designed by matching the conic coefficient and aspheric coefficient of the front and rear aspheric surfaces to further correct the residual aberration of the incident light beam and suppress the residual spherical aberration of the front mirror group. At the same time, it compensates for the coma and distortion of the rear mirror group to achieve relay imaging. (3) The imaging light beam processed by the transition mirror is then passed through the rear mirror group. The rear mirror group adopts a concentric structure to correct the residual aberration and a flat-field thick lens to correct the field curvature. The materials of the optical elements are matched to further expand the chromatic focal shift of the system. The convergence positions between different wavelengths produce further axial separation, realizing axial splitting between light beams of different wavelengths, and finally converging on the image plane to form an image, resulting in a chromatic focal shift greater than 1 mm.

[0028] See attached Figure 3It is a ray tracing point diagram of light passing through the microendoscopy optical system provided in this embodiment. In the figure, the root mean square radius of the point diagram at each wavelength corresponding to the five fields of view of 0 mm (a), 0.1 mm (b), 0.2 mm (c), 0.24 mm (d) and 0.3 mm (e) is less than 4.95 μm, and the geometric radius of the point diagram is less than 15.00 μm, and the imaging quality is good.

[0029] See attached Figure 4 , which is the chromatic focus shift curve of the microendoscopic optical system provided in this embodiment, the axial dispersion and wavelength are linearly fitted using the least squares method, and the result is as follows Figure 4 As shown in the figure, the axial chromatic focal shift of the dispersive objective lens is 1.02mm, indicating that the system has achieved the requirements of large depth detection. The hollow circles in the figure represent actual data points, the solid line is the fitting straight line of the relationship between focal shift and wavelength, and the fitting formula of the relationship between focal shift (axial dispersion δ) and wavelength (λ) is , linear determination coefficient R ２ It is 0.9994, indicating that the designed dispersion objective has an excellent dispersion-wavelength linear relationship, which well guarantees the consistency of sensitivity within the measurement range and the accuracy of actual measurement.

[0030] See attached Figure 5 , which is a transfer function MTF curve diagram on the image plane corresponding to each field of view of the microendoscopic imaging system provided in this embodiment; in the figure, Figures (a), (b) and (c) correspond to the MTF curves of all field of view transfer functions on the image plane corresponding to the wavelengths of 1μm, 1.25μm and 1.5μm of the microendoscopic imaging system provided in this embodiment. Figure 5 It can be obtained that the optical transfer function of the entire field of view from 1μm to 1.5μm working band at 70lp / mm is greater than 0.3, close to the diffraction limit, and the curve is smooth and compact, indicating that the imaging of this system is clear and uniform, and has good imaging quality in the entire band and the entire field of view.

[0031] See attached Figure 6 , which is an optical path difference curve diagram of the microendoscopic imaging system provided by this embodiment at three radial fields of view on the image side; in the figure, Figures (a), (b) and (c) correspond to the optical path difference curve diagrams of the microendoscopic imaging system provided by this embodiment at the fields of view of 0mm (a), 0.2mm (b) and 0.3mm (c) on the image plane, respectively. Figure 6 The ordinates in the figure are all ±0.25λ, among which the curve corresponding to the wavelength of 1.5μm in Figure (c) has the largest optical path difference, which can reach 0.21λ at the edge of the pupil. The remaining curves can satisfy the optical path difference within the pupil range is less than 0.25λ, indicating that the design of the microendoscopy imaging system can be considered to be close to the diffraction limit performance.

Claims

1. A large depth and wide field spectral confocal microendoscopic optical system, characterized in that: It includes an object plane (1), a front lens group (11), a transition lens (22), a rear lens group (33) and an image plane (8); The front lens group (11) is a three-piece nearly concentric symmetrical structure, which, according to the incident direction of light, comprises a spherical positive lens (2), a first spherical negative lens (3) and a second spherical negative lens (4), and the curvature radii of the lens surfaces are R21, R22, R31, R32, R41 and R42, respectively satisfying the conditions of 0.9 mm ≤ R21 ≤ 2.3 mm, -181 mm ≤ R22 ≤ -180 mm, -4 mm ≤ R31 ≤ -3 mm, -20.2 mm ≤ R32 ≤ -19.2 mm, -3.8 mm ≤ R41 ≤ -3.1 mm, -8 .2mm≤R42≤-7.4mm; the material refractive index n1 of the spherical positive lens (2) has a value range of 1.5≤n1≤1.7, and the Abbe number v1 has a value range of 59.9≤v1≤62.8; the material refractive index n2 of the first spherical negative lens (3) has a value range of 1.8≤n2≤2.0, and the Abbe number v2 has a value range of 32.5≤v2≤36.1; the material refractive index n3 of the second spherical negative lens (4) of the front lens group has a value range of 1.7≤n3≤1.8, and the Abbe number v3 has a value range of 43.7≤v3≤45.4; The transition mirror (22) comprises an aspheric positive lens (5), the front and rear surfaces of which are both even-order aspheric surfaces; the radii of curvature of the front and rear surfaces are R51 and R52, respectively, satisfying the conditions of 1.7 mm ≤ R51 ≤ 2.1 mm, -5.8 mm ≤ R52 ≤ -5.4 mm; the value range of the material refractive index n4 is 1.8 ≤ n4 ≤ 2.0, and the value range of the Abbe number v4 is 16.0 ≤ v4 ≤ 18.5; The rear lens group (33) comprises two concentrically structured spherical negative lenses; a first spherical negative lens (6) and a second spherical negative lens (7), the rear surface of the second spherical negative lens (7) being a plane; according to the incident direction of light, the radii of curvature of the remaining surfaces of the lenses are R61, R62 and R71, respectively, satisfying the conditions of 3.6mm≤R61≤4.2mm, 1.0mm≤R62≤1.5mm, and -5.2mm≤R71≤-4.2mm; the material refractive index n5 of the first spherical negative lens (6) has a value range of 1.8≤n5≤1.9, and the Abbe number v5 has a value range of 39.4≤v5≤41.3; the material refractive index n6 of the second spherical negative lens (7) has a value range of 1.8≤n6≤2.0, and the Abbe number v6 has a value range of 36.6≤v6≤38.2; The aperture stop of the optical system is arranged on the front surface of the first spherical negative lens (3) of the front lens group.

2. The large-depth and wide-field spectral confocal microendoscopic optical system according to claim 1, characterized in that: The front and rear surfaces of the aspheric positive lens (5) are both aspheric surfaces. A Cartesian space rectangular coordinate system is constructed with the intersection of the lens surface and the optical axis as the origin. The incident direction of the light is the positive direction of the Z axis, the positive direction of the Y axis is upward, and the positive direction of the X axis is perpendicular to the paper surface and inward. The vector height equation Z of the even-order aspheric surface is: ， Where c is the curvature; r is the radius; k is the quadratic surface coefficient; a i is the coefficient of the monomial; The front surface of the aspherical positive lens (5) has a condition of -2.4≤k≤0.4, and the coefficients of each monomial satisfy the following conditions: , , , , , ; The rear surface of the aspherical positive lens (5) has a value of -33.7≤k≤-31.7, and the coefficients of the monomials satisfy the following conditions: , , , , , .

3. The imaging method of a large depth and wide field spectral confocal microendoscopic optical system according to claim 1, characterized in that The steps include: (1) The spherical positive lens (2) of the front mirror group adopts a curved convex surface with an optical power of 0.26 to 0.65 to receive incident light at a large angle. The light beam is deflected after passing through the convex surface. The deflected light beam tends to be flat relative to the optical axis, thereby correcting the distortion. For incident light of a wide band of near-infrared light of 1000 nm to 1500 nm, the front mirror group adopts optical elements made of matching high refractive index, low Abbe number and low refractive index, high Abbe number materials. The optical path difference between light beams of different wavelengths is increased, and the convergence positions of different wavelengths are axially separated, thereby increasing the axial focal shift of the incident light. The converged light beam is then incident on the transition mirror. (2) The transition mirror is designed by matching the conic coefficient and aspheric coefficient of the front and rear aspheric surfaces to further correct the residual aberration of the incident light beam and suppress the residual spherical aberration of the front mirror group. At the same time, it compensates for the coma and distortion of the rear mirror group to achieve relay imaging. (3) The imaging light beam processed by the transition mirror is then passed through the rear mirror group. The rear mirror group adopts a concentric structure to correct the residual aberration and a flat-field thick lens to correct the field curvature. The materials of the optical elements are matched to further expand the chromatic focal shift of the system. The convergence positions between different wavelengths produce further axial separation, realizing axial splitting between light beams of different wavelengths, and finally converging on the image plane to form an image, resulting in a chromatic focal shift greater than 1 mm.

Images