A catadioptric dual-band afocal optical system

Abstract

The present invention discloses a catadioptric dual-band afocal optical system, which comprises a primary mirror, a secondary mirror and a dual-band lens group which are coaxially arranged in sequence according to the light propagation direction, wherein: the primary mirror is used to receive incident light and reflect it to the secondary mirror; the secondary mirror is used to reflect the light reflected by the primary mirror to the dual-band lens group; the dual-band lens group is used to refract the light reflected by the secondary mirror and then emit it. The catadioptric dual-band afocal optical system of the present invention has the advantages of large field of view, compact size, and coverage of medium and long-wave infrared dual bands, and is particularly suitable for airborne environments with large apertures, long focal lengths, but strict requirements on volume and weight.

Description

Technical Field

[0001] The invention relates to the technical field of optical design, and in particular to a refractive-reflective dual-band afocal optical system. Background Art

[0002] In recent years, large-aperture optical systems have been used more and more widely. When detecting distant targets, they have the advantages of high energy concentration and high resolution. And as the characteristics of the target become more and more complex, the infrared radiation detection of the target is not limited to a single band. It is more necessary to integrate the medium and long wave bands to improve the radiation characteristics of the target, so as to better realize the detection and identification of the target under complex backgrounds. Large-aperture optics all use reflective optical systems. Compared with transmissive optical systems, reflective optical systems have many advantages such as long focal length, large aperture, and high quality of chromatic aberration-free imaging. The widely used coaxial reflective optics are based on the Cassegrain system, but they generally have defects such as small field of view, poor balance between obstruction ratio and length contradiction, and limited to one band.

[0003] The existing dual-band catadioptric optical system and the dual-band common aperture are Cassegrain systems composed of primary and secondary mirrors, and the light splitting is performed after the focus of the primary and secondary mirror systems. The system only uses the common aperture optics composed of the primary and secondary mirror systems, which is actually reflective optics. Therefore, it is impossible to achieve a large field of view. Moreover, the light splitting after the focus makes the system very long and large in size. In addition, the dual band is mainly in the short-wave infrared (1μm~3μm) and medium-wave infrared, but the band cannot be extended to the long-wave infrared. Summary of the invention

[0004] The embodiment of the present invention provides a catadioptric dual-band afocal optical system for achieving large aperture, large field of view, and mid- and long-wave infrared dual-band performance.

[0005] The embodiment of the present invention provides a catadioptric dual-band afocal optical system, which includes a primary mirror, a secondary mirror and a dual-band lens group coaxially arranged in sequence according to the light propagation direction, wherein:

[0006] The primary mirror is used to receive incident light and reflect it to the secondary mirror;

[0007] The secondary mirror is used to reflect the light reflected by the primary mirror to the dual-band lens group;

[0008] The dual-band lens group is used to refract the light reflected by the secondary mirror and then emit it.

[0009] Optionally, the primary mirror is a concave mirror, and the secondary mirror is a convex mirror;

[0010] Among them, the surface shape of the primary mirror is a preset value, and the surface shape of the secondary mirror is an axisymmetric high-order aspheric surface.

[0011] Optionally, the product of the combined focal length of the primary mirror and the secondary mirror and the system magnification of the catadioptric dual-band afocal optical system is within a preset range.

[0012] Optionally, the dual-band lens group is a positive refractive power lens group, and the dual-band lens group includes three continuously arranged first lenses, second lenses and third lenses.

[0013] Optionally, the first lens is a negative lens with a concave surface facing the secondary lens;

[0014] The second lens is a biconvex positive lens;

[0015] The third lens is a positive lens with its concave surface facing the secondary lens;

[0016] The focus of the primary image plane of the catadioptric dual-band afocal optical system is between the secondary mirror and the first lens.

[0017] Optionally, the first lens, the second lens and the third lens are made of different materials respectively.

[0018] Optionally, the total length of the catadioptric dual-band afocal optical system is less than or equal to 150 mm, the system obstruction ratio is 0.2≤α≤0.5, the system magnification satisfies 1 / 6≤r≤1, the system F number satisfies 1.4≤F≤4, and the field of view angle of the system is 3°≤θ≤4°;

[0019] The primary mirror aperture D satisfies 200mm≤D≤500mm.

[0020] Optionally, the surfaces of the first lens, the second lens and the third lens are all high-order aspherical surfaces.

[0021] Optionally, the system bands of the catadioptric dual-band afocal optical system meet the requirements of medium wave 3 μm to 5 μm and long wave 7.7 μm to 10.3 μm.

[0022] This embodiment proposes a refractive-reflective dual-band afocal optical system, which has the advantages of a large field of view, compact size, and coverage of mid-wave and long-wave infrared dual bands. It is particularly suitable for airborne environments with large apertures, long focal lengths, but strict requirements on volume and weight.

[0023] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented according to the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0025] Figure 1 A schematic diagram of the optical path of a catadioptric dual-band afocal optical system according to an embodiment of the present invention;

[0026] Figure 2 This is a medium-wave imaging MTF diagram of the catadioptric dual-band afocal optical system according to an embodiment of the present invention;

[0027] Figure 3 This is a long-wave imaging MTF diagram of the catadioptric dual-band afocal optical system according to an embodiment of the present invention;

[0028] Figure 4 A diagram showing medium wave distortion and field curvature of a catadioptric dual-band afocal optical system according to an embodiment of the present invention;

[0029] Figure 5 Graphs showing long-wave distortion and field curvature of the catadioptric dual-band afocal optical system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0030] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0031] The embodiment of the present invention provides a catadioptric dual-band afocal optical system, such as Figure 1 As shown, according to the light propagation direction, it includes a primary mirror 1, a secondary mirror 2 and a dual-band lens group which are coaxially arranged in sequence, wherein:

[0032] The primary mirror 1 is used to receive incident light and reflect it to the secondary mirror 2;

[0033] The secondary mirror 2 is used to reflect the light reflected by the primary mirror 1 to the dual-band lens group;

[0034] The dual-band lens group is used to refract the light reflected by the secondary mirror and then emit it.

[0035] The present embodiment is a catadioptric dual-band afocal optical system based on a Cassegrain system. The present invention adds a dual-band lens group on the basis of the reflective type of the Cassegrain system, thereby increasing the field of view of the system. The catadioptric dual-band afocal optical system performs dual-band spectroscopy after the output end, and has the advantages of a large field of view, compact size, and coverage of mid- and long-wave infrared dual bands. It is particularly suitable for airborne environments with large apertures and long focal lengths but strict requirements on volume and weight.

[0036] Optionally, the primary mirror is a concave mirror, and the secondary mirror is a convex mirror;

[0037] Among them, the surface shape of the primary mirror is a preset value, and the surface shape of the secondary mirror is an axisymmetric high-order aspheric surface.

[0038] Specifically, in this embodiment, if Figure 1 As shown, the primary mirror 1 is a concave mirror, and the secondary mirror 2 is a convex mirror. In one embodiment of the present invention, the surface shape of the secondary mirror is an axisymmetric high-order aspheric surface, and the surface shape of the primary mirror can be K=-1.

[0039] In this embodiment, the secondary mirror uses a high-order aspheric surface to further eliminate coma. The specific analysis process is as follows:

[0040] The aberration coefficient of the cassette system satisfies:

[0041]

[0042]

[0043]

[0044]

[0045]

[0046] Where β is the ratio of the system focal length f′ to the primary mirror focal length f′1, b is the back intercept of the primary mirror. e1 and e2 are the eccentricities of the primary and secondary mirrors respectively. Ⅰ ~S Ⅴ They are the spherical aberration, coma, astigmatism, field curvature and distortion of the system respectively.

[0047] Considering the processing and testing technology, the primary mirror 1 in this embodiment uses a traditional card-type parabolic mirror. The eccentricity of the parabolic mirror is In order to ensure the compactness of the system structure, the choice of b is determined according to the system space size. The focal length of the primary mirror is determined according to the F number of the primary mirror. Considering the actual processing capacity of large-aperture parabolic mirrors, the F number in this embodiment is not less than 1.5. Let S Ⅰ =0, S Ⅱ = 0, that is, the primary and secondary mirrors eliminate the spherical aberration and coma of the system, and the solution is The combined focal length of the primary and secondary lenses.

[0048] The residual aberrations of the subsequent lens group are astigmatism, field curvature and distortion. The aberrations of the coaxial spherical system are:

[0049] S I '=∑luni(i'-u)(i-i')

[0050] S Ⅱ '=∑luni(i'-u)(i-i')(i p / i)

[0051] S Ⅲ '=∑luni(i'-u)(i-i')(i p / i) 2

[0052] S Ⅳ '=∑j 2 (n'-n) / nn'r)

[0053] S Ⅴ '=∑(S Ⅲ +S Ⅳ )(i p / i)

[0054] Among them, S' Ⅰ and S' Ⅱ Corrected to 0. The remaining astigmatism, field curvature and distortion should ensure that the aberration of the entire system is 0. That is, S Ⅲ +S' Ⅲ =0, S Ⅳ +S' Ⅳ =0, S Ⅴ +S' Ⅴ =0.

[0055] Using the above primary aberration formula, the initial structure of the system can be calculated. In a typical card telescope system, both the primary and secondary mirrors use parabolic mirrors, which can only correct spherical aberration. In this embodiment, the secondary mirror uses a high-order aspheric surface, which can further eliminate the coma caused by the increase in field of view. The subsequent lens group uses a high-order aspheric surface based on the initial structural parameters, which can further optimize the imaging quality of the system.

[0056] Optionally, the product of the combined focal length of the primary mirror and the secondary mirror and the system magnification of the catadioptric dual-band afocal optical system is within a preset range.

[0057] Specifically, in this embodiment, the system magnification r of the catadioptric dual-band afocal optical system and the combined focal length f0 of the primary and secondary mirrors satisfy: 146≤f0×r≤876.

[0058] Optionally, the dual-band lens group is a positive refractive power lens group, and the dual-band lens group includes three continuously arranged first lenses 31, second lenses 32 and third lenses 33.

[0059] Optionally, the first lens 31 is a negative lens with a concave surface facing the secondary lens;

[0060] The second lens 32 is a biconvex positive lens;

[0061] The third lens 33 is a positive lens with the concave surface facing the secondary lens;

[0062] The focus of the primary image plane of the catadioptric dual-band afocal optical system is between the secondary mirror and the first lens 31 .

[0063] Specifically, in this embodiment, if Figure 1 As shown, the dual-band lens group has positive refractive power, and the dual-band lens group includes three consecutively arranged first lenses 31, second lenses 32 and third lenses 33. The dual-band lens group is used to diverge the focus of the intermediate image plane into parallel light. The first lens 31 is a negative lens with a concave surface facing the secondary mirror; the second lens 32 is a double-convex positive lens; the third lens 33 is a positive lens with a concave surface facing the secondary mirror. The focus of the primary image plane of the catadioptric dual-band afocal optical system is between the secondary mirror and the first lens 31, satisfying 100% cold aperture efficiency, and the distance between the rear surface of the third lens 33 and the primary mirror 1 satisfies: L2≥0.

[0064] In this embodiment, a dual-band lens group is used to achieve medium and long-wave dual-band imaging; the dual-band lens group is composed of three lenses, which corrects the chromatic aberration of the dual band and achieves an imaging effect close to the diffraction limit.

[0065] Optionally, the first lens, the second lens and the third lens are made of different materials respectively.

[0066] Optionally, the surfaces of the first lens, the second lens and the third lens are all high-order aspherical surfaces.

[0067] Specifically, in long focal length, large aperture, dual-band optical systems, in addition to the above primary chromatic aberration, there are still large chromatic aberrations, which seriously affect the imaging quality of the optical system. Infrared dual-band chromatic aberration correction has always been very difficult. Traditional chromatic aberration correction methods are to eliminate chromatic aberration by matching different dispersion materials, but infrared materials are scarce, and dual-band chromatic aberration compensation often leaves a large "secondary spectrum".

[0068] In this embodiment, the reflective primary and secondary mirror system does not produce chromatic aberration, while the chromatic aberration of the transmission component is mainly caused by the dispersion of materials in different bands. In order to balance the chromatic aberration of the system, this embodiment also analyzes the dispersion characteristics of different materials in the material selection stage to select materials with small dispersion. For example, in this embodiment, the first lens, the second lens and the third lens are made of ZnSe, Ge and ZnS materials respectively. The dispersion coefficients of different infrared materials in the medium and long wave bands are shown in Table 1.

[0069] Table 1 Dispersion coefficients of different infrared materials in the mid- and long-wave bands

[0070] Material <![CDATA[Refractive index N aver > <![CDATA[ν1(MWIR)]]> <![CDATA[ν2(MWIR)]]> <![CDATA[ν1 / ν2]]> Ge 4.00 103.58 958 0.107 ZnSe 2.42 192.42 58.04 3.315 ZnS 2.22 113.02 22.94 4.926

[0071] According to Table 1, the dispersion of commonly used germanium materials in the medium wave band is large, while the dispersion of ZnS and ZnSe in the medium and long wave band is relatively small. In this embodiment, large chromatic aberration can be avoided by reasonable optical focal power matching. At the same time, in order to cover the dual-band requirements of the system and to correct the dual-band chromatic aberration of the system well, this embodiment adopts a harmonic diffraction optical element in the lens group. Specifically, the harmonic diffraction optical element is to process a binary step surface on the surface of the optical part to form a harmonic diffraction effect. In this embodiment, the harmonic diffraction optical element needs to meet the following conditions to eliminate chromatic aberration:

[0072] The optical system with harmonic diffraction surface should meet the following requirements in the mid-infrared band (band 1) and the long-wave infrared band (band 2):

[0073] The total optical power should meet the following requirements:

[0074] The axial chromatic aberration of band 1 should be equal to zero:

[0075] in are the optical powers of the three refractive lenses in band 1, is the optical power of the harmonic diffraction surface in band 1. are the dispersion factors of the material in band 1, is the dispersion factor of the harmonic diffraction surface in band 1.

[0076] Similarly, the axial chromatic aberration of band 2 should be equal to zero:

[0077] Therefore, by selecting appropriate parameters of the harmonic diffraction optical component in this embodiment, the imaging and aberration correction requirements of the system can be met in two bands, while making the system design compact, with a small number of pieces and a high transmittance.

[0078] In this embodiment, the diffractive optical element has the property of a flat image field. By using the diffractive optical component, the image quality after the field of view is enlarged can be further improved.

[0079] In summary, in order to increase the field of view, this embodiment adopts:

[0080] (1) The secondary mirror uses a high-order aspherical surface to eliminate coma;

[0081] (2) Use a lens group behind the intermediate image plane to eliminate distortion and astigmatism;

[0082] (3) Use harmonic diffractive optical elements to eliminate field curvature and chromatic aberration and increase the field of view.

[0083] In order to achieve excellent dual-band imaging, the following are adopted:

[0084] (1) Use infrared materials with low dispersion characteristics in medium and long waves;

[0085] (2) Harmonic diffraction optics is used to correct dual-band residual chromatic aberration.

[0086] Table 2 Surface parameters

[0087]

[0088] Table 2 lists an implementation of the surface parameters of the refractive-reflective dual-band afocal optical system of the present invention. In Table 2, the radius of curvature refers to the radius of curvature of each surface, and the spacing refers to the distance between two adjacent surfaces. For example, the spacing of surface S1 is the distance between surface S1 and surface S2. The glass material and remarks columns are the materials used to make the lens and the lens number. In addition, in Table 2, surfaces S1 and S2 are respectively the optical reflection surfaces of the primary mirror 1 and the secondary mirror 2, surfaces S3 and S4 are respectively the surfaces of the first lens 31 away from and close to the second lens 32; surfaces S5 and S6 are respectively the surfaces of the second lens 32 away from and close to the third lens 33; surfaces S7 and S8 are respectively the surfaces of the third lens 33 away from and close to the aperture stop4.

[0089] Table 3 Aspheric coefficients

[0090] Surface serial number K A B C S1 -1 0 0 0 S2 0 1.3042e-007 -5.36796e-012 1.1249e-016 S3 0 6.8266e-007 2.4079e-010 5.58163e-014 S4 0 -1.2309e-006 9.2731e-010 -7.6353e-013 S5 0 -2.9011e-006 2.938961e-009 0 S6 0 -1.0489e-006 -4.8248e-014 6.8475e-011 S7 0 -1.2060e-006 1.59722e-010 -2.34568e-014 S8 0 -5.3855e-007 -2.8269e-011 0

[0091] Table 3 lists an implementation method of an aspheric coefficient in an embodiment of the present invention. In this embodiment, the displacement of the aspheric surface in the optical axis direction based on the vertex of the surface is defined as follows:

[0092] Z=(1 / R)*Y^2 / [1+[1-(1+K)*(Y / R)^2]^0.5]+A*(Y^4)+B*(Y^6)+C*(Y^8)

[0093] Among them, Z represents the displacement in the direction of the optical axis, Y represents the height of the optical axis, R represents the paraxial curvature radius, K represents the cone coefficient, and A, B, and C represent the aspheric coefficients respectively.

[0094] Optionally, the total length of the catadioptric dual-band afocal optical system is less than or equal to 150 mm, the system obstruction ratio is 0.2≤α≤0.5, the system magnification satisfies 1 / 6≤r≤1, the system F number satisfies 1.4≤F≤4, and the field of view angle of the system is 3°≤θ≤4°;

[0095] The primary mirror aperture D satisfies 200mm≤D≤500mm.

[0096] Optionally, the system bands of the catadioptric dual-band afocal optical system meet the requirements of medium wave 3 μm to 5 μm and long wave 7.7 μm to 10.3 μm.

[0097] Specifically, Table 4 lists the main parameters in one embodiment of the present invention.

[0098] Table 4 System parameters

[0099]

[0100] Table 4 includes the obstruction ratio, F number, field angle, magnification, aperture, etc. Since this embodiment uses only three lenses to achieve a large magnification and field of view, as well as a high-definition image quality effect of dual bands, and the system structure is compact, and the volume and weight are light, it has the effect of improving performance and simplifying the lens structure compared to traditional technologies.

[0101] Figure 2 and Figure 3 : is a graph of the optical transfer function (MTF) of the medium-wave and long-wave imaging of the catadioptric dual-band afocal optical system of the present invention, wherein the horizontal axis is the number of line pairs per millimeter (line pair per millimeter), and the vertical axis is the contrast value, Figure 4 and Figure 5 The field curvature and distortion diagrams for medium and long waves are shown in Figure 2. Figures 2 to 5 It can be seen from the graphical curve that the MTF, field curvature, distortion and RMS diameter of the mid-wave band are all within the standard range.

[0102] It can be seen that the infrared continuous zoom lens in this embodiment has good imaging quality.

[0103] In summary, the catadioptric dual-band afocal optical system of the present invention is based on the reflective type of the Cassegrain system, and a lens group is added to achieve an increase in the field of view of the system.

[0104] The embodiment of the present invention adopts a dual-band lens group to achieve medium and long-wave dual-band imaging; the dual-band lens group is composed of three lenses, which corrects the chromatic aberration of the dual band and achieves an imaging effect close to the diffraction limit.

[0105] The dual-band lens groups in the embodiments of the present invention all adopt high-order aspheric surfaces, making the structure of the entire lens group more compact, realizing an optical system in which the length of the lens group is limited to the inside of the primary and secondary mirrors, greatly reducing the length of the optical system and greatly reducing the volume and weight of the system.

[0106] The refractive-reflective dual-band afocal optical system proposed in the embodiment of the present invention has the advantages of large field of view, compact size, and coverage of infrared dual bands. It can solve the urgent needs of airborne long-range detection equipment and adapt to the development of systems with large aperture, long focal length, and strict requirements on volume and weight.

[0107] It should be noted that, in this article, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or device including the element.

[0108] The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0109] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation modes, which are merely illustrative rather than restrictive. Under the guidance of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the present invention and the claims, all of which are within the protection of the present invention.

Claims

1. A catadioptric dual-band afocal optical system, characterized in that: According to the direction of light propagation, it includes a primary mirror, a secondary mirror and a dual-band lens group which are coaxially arranged in sequence, wherein: The primary mirror is used to receive incident light and reflect it to the secondary mirror; The secondary mirror is used to reflect the light reflected by the primary mirror to the dual-band lens group; The dual-band lens group is used to refract the light reflected by the secondary mirror and then emit it; The dual-band lens group is a positive refractive power lens group, and the dual-band lens group includes three consecutively arranged first lenses, second lenses and third lenses; The first lens is a negative lens with a concave surface facing the secondary mirror; The second lens is a biconvex positive lens; The third lens is a positive lens with its concave surface facing the secondary lens; The focus of the primary image plane of the catadioptric dual-band afocal optical system is between the secondary mirror and the first lens; The system magnification r of the catadioptric dual-band afocal optical system and the combined focal length f0 of the primary and secondary mirrors satisfy: 146≤f0×r≤876.

2. The catadioptric dual-band afocal optical system according to claim 1, characterized in that: The primary mirror is a concave mirror, and the secondary mirror is a convex mirror; Among them, the surface shape of the primary mirror is a preset value, and the surface shape of the secondary mirror is an axisymmetric high-order aspheric surface.

3. The catadioptric dual-band afocal optical system according to claim 1, wherein: The first lens, the second lens and the third lens are made of different materials respectively.

4. The catadioptric dual-band afocal optical system according to claim 1, characterized in that: The total length of the catadioptric dual-band afocal optical system is less than or equal to 150 mm, the system obstruction ratio is 0.2≤α≤0.5, the system magnification satisfies 1 / 6≤r≤1, the system F number satisfies 1.4≤F≤4, and the system field of view angle is 3°≤θ≤4°; The primary mirror aperture D satisfies 200mm≤D≤500mm.

5. The catadioptric dual-band afocal optical system according to claim 1, wherein: The surfaces of the first lens, the second lens and the third lens are all high-order aspherical surfaces.

6. The catadioptric dual-band afocal optical system according to claim 1, wherein: The system bands of the catadioptric dual-band afocal optical system meet the requirements of medium wave 3 μm to 5 μm and long wave 7.7 μm to 10.3 μm.

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