CN110727092A - Off-axis reflection type two-mirror beam expanding system based on free-form surface - Google Patents

Abstract

The invention discloses an off-axis reflection type two-mirror beam expander system based on a free-form surface, comprising a primary mirror and a secondary mirror, the primary mirror is a concave mirror, and the secondary mirror is a convex mirror. The optical structure of the system is relatively compact, there is no central blocking and energy loss, and the system has no real focus, so it is more suitable for the beam expander system based on air medium. The system is designed with free-form surfaces and has three structural forms. The first is that the primary mirror is a free-form surface and the secondary mirror is an off-axis aspheric surface. The second is that the primary mirror is an off-axis aspheric surface, and the secondary mirror is a free-form surface. The three types are that both the primary mirror and the secondary mirror are free-form surfaces, which can be selected according to specific application requirements. The use of free-form surfaces can well correct asymmetric aberrations, improve the beam quality of the beam expander system, increase the field of view, and be used in optical communication systems to improve the probability of communication capture.

Description

An off-axis reflective two-mirror beam expander system based on free-form surfaces

technical field

The invention relates to the field of optical communication and the technical field of optical system design, in particular to an off-axis reflection type two-mirror beam expander system based on a free-form surface.

Background technique

The beam expander system is an important part of the optical communication system. Its main function is to compress the spatial divergence angle of the beam, improve the beam collimation, and expand the emission range. The beam expander system is generally divided into three structural forms: refraction, reflection and reentry. Compared with the refractive beam expander system, the reflective beam expander system is more suitable for the application of large-diameter systems, and there is no chromatic aberration in the system, which can meet the requirements of compact structure and light weight. Compared with the on-axis beam expander system, the off-axis beam expander system has the advantages of no central blocking and high utilization of light energy. Therefore, when the emission aperture is required to be large, and the volume and quality are limited, the reflective beam expander system is mostly used. However, the existing off-axis reflective beam expander systems still have many deficiencies, such as a small effective field of view, insufficient simplification of the structure, and difficulty in correcting asymmetric aberrations. On the premise of not reducing the relative aperture of the system and increasing the number of optical components, it is increasingly difficult to meet the design requirements using traditional spherical and rotationally symmetric aspherical surfaces. In recent years, with the continuous improvement of processing and detection technology, the processing accuracy of emerging freeform surfaces has become higher and higher, making it applicable to practical optical systems. Free-form surfaces have also begun to be used in the design of off-axis imaging systems due to their non-rotational symmetry, abundant degrees of freedom, and strong ability to correct asymmetric aberrations. Due to the many shortcomings of the existing off-axis beam expander systems, such as a small effective field of view, asymmetric aberrations such as coma and astigmatism are difficult to correct, and the beam quality is not high, the use of traditional spherical and rotationally symmetrical aspherical surfaces is becoming more and more important. Difficult to meet design requirements.

SUMMARY OF THE INVENTION

Aiming at the shortcomings of the existing beam expanding systems, the present invention provides an off-axis reflection type two-mirror beam expanding system based on free-form surfaces, the purpose of which is to improve the beam quality of the beam expanding system, increase the working field of view, and further improve the beam quality of the beam expanding system. The energy utilization and acquisition probability of the communication system greatly simplify the system structure.

The technical scheme adopted by the present invention is: an off-axis reflection type two-mirror beam expander system based on a free-form surface, the system includes a primary mirror and a secondary mirror:

The primary mirror is a concave mirror, the secondary mirror is a convex mirror, and the beam expansion system adopts an off-axis, no real focus design, no central blocking and energy loss; the beam parallel to the optical axis is incident on the convex secondary mirror On the upper side, the secondary mirror reflects on the concave primary mirror, and the primary mirror reflects and outputs the expanded beam parallel to the incident beam; the beam expansion system is designed with free-form surfaces, and the design includes three structural forms, the first The primary mirror is a free-form surface and the secondary mirror is an off-axis aspheric surface; the second structural form is that the primary mirror is an off-axis aspheric surface, and the secondary mirror is a free-form surface; the third structural form is that both the primary mirror and the secondary mirror are free-form surfaces . It can be selected according to specific usage requirements.

The beam expansion magnification M of the beam expander system is the ratio of the focal length of the primary mirror and the secondary mirror.

Further, the beam expansion magnification is fixed, and the beam expansion magnification can be set according to specific usage requirements.

Further, the beam expander system is applicable to a wavelength range of 0.4 μm to 12 μm.

Further, the surfaces of the two mirrors in the beam expander system need to be coated with a highly reflective film.

Further, the transformation matrix of the Gaussian beam passing through the telescopic system is:

Wherein, f ₁ and f ₂ represent the focal lengths of the two mirrors respectively, the distance between the two mirrors d=f ₁ +f ₂ +Δ, Δ represents the offset, and M _t =−f ₂ /f ₁ is the magnification of the telephoto system.

further,

Suppose the beam waist of the incident beam is ω ₀ , the wavelength is λ, and the focal parameter is The object distance is s, and after passing through the telephoto system, it becomes a Gaussian beam with a beam waist of ω′ ₀ and an image distance of s′;

Set Δ = 0, there are:

ω′ ₀ = |M _t |ω ₀

Let the initial divergence angle be θ ₀₁ , the divergence angle after passing through the system is θ ₀₂ , and there is an inverse relationship between the far-field divergence angle θ ₀ and the beam waist ω ₀ , namely:

该望远系统的扩束比：The far-field divergence angle is compressed by |M _t | times, and has nothing to do with the object distance and the image distance. When s=f ₁ , s′=f ₂ , that is, the image beam waist is located at the back focal plane of the second lens L ₂ on; when s>>f ₁ +f ₂ ,

The beam expansion ratio of this telescopic system:

further,

The magnification M _t of the beam expander system is the ratio of the focal length of the primary mirror to the secondary mirror. After selecting the relative aperture of the primary mirror, the focal length of the primary mirror and the secondary mirror can be obtained; set the offset amount Δ=0, the distance d between the two mirrors is the difference between the focal length of the primary mirror and the focal length of the secondary mirror, the radius of curvature of the apex of the mirror is twice the focal length, and the radii of curvature of the apex of the primary mirror and the secondary mirror are obtained.

Free-form surface description methods are generally divided into two categories, polynomial description methods and parametric description methods. The polynomial description method uses a series of polynomial combinations to represent the surface shape of optical free-form surfaces, including deformed aspheric surfaces, XY polynomial surfaces, Zemike polynomial surfaces, radial basis function surfaces, and Q-type polynomial surfaces. The parametric description method is based on discrete point fitting to obtain the surface shape of optical free-form surfaces, including B-spline surfaces, non-uniform rational B-spline (NURBS) surfaces, etc.

The surfaces of the two mirrors of the beam expander system need to be coated with a high reflective film, and its applicable wavelength band is wide.

The beam expansion magnification of the beam expansion system is fixed, and the beam expansion magnification can be set according to specific application requirements.

beneficial effect

Due to the many shortcomings of the existing off-axis beam expander systems, such as a small effective field of view, asymmetric aberrations such as coma and astigmatism are difficult to correct, and the beam quality is not high, the use of traditional spherical and rotationally symmetrical aspherical surfaces is becoming more and more important. Difficult to meet design requirements.

The invention innovatively applies the free-form surface to the off-axis reflection type beam expander system, and proposes an off-axis reflection type two-mirror beam expander system based on the free-form surface. The system has the advantages of simple structure and good beam quality. Compared with the traditional quadratic surface, the free-form surface can effectively correct the off-axis belt of the system without changing the relative aperture of the system and without increasing the number of optical elements. The resulting asymmetric aberration improves the beam quality, effectively increases the field of view, and can improve the non-unloading working range of the optical communication system. In addition, the use of an afocal system eliminates the need to use collimating lens elements, and can directly accelerate the subsequent optical paths such as fast reflectors and beam splitters, which greatly simplifies the system structure and enables the sharing of optical components in the coarse and fine tracking systems. Compared with the existing off-axis beam expander system, its advantages are very obvious, and it has strong practical value in optical communication systems.

Description of drawings

1 is a schematic structural diagram of an off-axis two-counter beam expanding system provided by the present invention;

2 is a schematic structural diagram of a free-form surface as the primary mirror of an off-axis two-reverse beam expander system provided by the present invention;

3 is a schematic structural diagram of another off-axis two-reverse beam expander system secondary mirror provided by the present invention as a free-form surface;

4 is a schematic structural diagram of another off-axis two-reverse beam expander system provided by the present invention in which both the primary and secondary mirrors are free-form surfaces;

Figure 5 is a schematic diagram of the Gaussian beam passing through the telescopic system;

6 is a structural diagram of an off-axis two-inverse beam expander system with a free-form secondary mirror provided by an embodiment of the present invention;

Detailed ways

The invention provides an off-axis reflection type two-mirror beam expander system based on a free-form surface, comprising a primary mirror and a secondary mirror, the primary mirror is a concave mirror, and the secondary mirror is a convex mirror. The light beam parallel to the optical axis is incident on the convex secondary mirror, which is reflected on the concave primary mirror by the secondary mirror, and the expanded beam parallel to the incident light beam is output after being reflected by the primary mirror. See Figure 1. In the figure, 1 is the primary mirror and 2 is the secondary mirror.

According to the overall design requirements of the instrument, the requirements for the clear aperture and magnification of the optical system are proposed. Then select the relative aperture of the primary mirror. The choice of the relative diameter of the primary mirror is related to many factors. From the perspective of shortening the length of the lens barrel, of course, the larger the relative diameter of the primary mirror, the better. The processing difficulty is proportional to the cube of the relative diameter, so this value is a combination of several factors. To be determined, generally take 1:3 or 1:4, more and more large telescopes use 1:2 or even larger relative aperture of the primary mirror.

The transformation matrix of the Gaussian beam passing through the telescopic system is:

Wherein, f ₁ and f ₂ represent the focal lengths of the two mirrors respectively, the distance between the two mirrors d=f ₁ +f ₂ +Δ, Δ represents the offset, and M _t =−f ₂ /f ₁ is the magnification of the telephoto system.

物距为s，经望远系统后变为束腰为ω′₀、像距为s′的高斯光束。Suppose the beam waist of the incident beam is ω ₀ , the wavelength is λ, and the focal parameter is

The object distance is s, and after passing through the telephoto system, it becomes a Gaussian beam with a beam waist of ω' ₀ and an image distance of s'.

Set Δ = 0, there are:

ω′ ₀ = |M _t |ω ₀

Let the initial divergence angle be θ ₀₁ , the divergence angle after passing through the system is θ ₀₂ , and there is an inverse relationship between the far-field divergence angle θ ₀ and the beam waist ω ₀ , namely:

该望远系统的扩束比：The far-field divergence angle is compressed by |M _t | times, and has nothing to do with the object distance and the image distance. When s=f ₁ , s′=f ₂ , that is, the image beam waist is located at the back focal plane of the second lens L ₂ on; when s>>f ₁ +f ₂ ,

The beam expansion ratio of this telescopic system:

Please see Figure 5.

The magnification M _t (beam expander M) of the beam expander system is the ratio of the focal lengths of the primary mirror and the secondary mirror. After selecting the relative aperture of the primary mirror, the focal length of the primary mirror and the secondary mirror can be obtained. Generally, the offset amount Δ=0 is set, and the distance d between the two mirrors is the difference between the focal length of the primary mirror and the focal length of the secondary mirror. According to Gaussian optics, the radius of curvature of the vertex of the mirror is twice the focal length, and the radius of curvature of the vertex of the primary mirror and secondary mirror can be obtained.

The beam expansion magnification of the beam expander system is fixed, and the applicable wavelength is wide, and the two mirrors need to be coated with a high reflective film.

There are two off-axis ways of the beam expander system: (1) the aperture is off-axis or the field of view is offset (2) the curved surface is inclined.

The beam expander system is designed with free-form surface, which can correct the asymmetric aberration of the system, increase the field of view, and improve the beam quality. Free-form surface description methods are generally divided into two categories, polynomial description methods and parametric description methods. There are three positions of the free-form surface in the system, the primary mirror is a free-form surface, the secondary mirror is a free-form surface, and both the primary and secondary mirrors are free-form surfaces, see Figure 2, Figure 3 and Figure 4 respectively. In Fig. 2, 1 is a free-form surface primary mirror, and 2 is an off-aspherical secondary mirror; in Fig. 3, 1 is an off-axis aspherical primary mirror, and 2 is a free-form surface secondary mirror; in Fig. 4, 1 is a free-form surface primary mirror, and 2 is a free-form surface primary mirror. Freeform secondary mirror;

In the optical design software ZEMAX or CODEV, set the surface shape of the primary mirror or secondary mirror to a free-form surface such as deformed aspheric surface, XY polynomial surface, Zernike polynomial surface, radial basis function surface, Q-type polynomial surface, or also Custom freeform surfaces can be written using the macro language.

Afocal system refers to an optical system that does not diverge or focus the beam, that is, parallel light enters, parallel light exits, and the equivalent focal length of the optical system is infinite.

For example, based on the above process, we designed a beam expander system with the following parameters:

主镜焦距f₁＝400mm，扩束倍率M_t＝6，次镜焦距f₂＝66.666mm，次镜口径D₂＝16.666mm，两镜间距d＝333.333mm，主镜顶点曲率半径R₁＝800mm，次镜顶点曲率半径R₂＝133.333mm，主、次镜的二次曲面系数均为-1，采用第一种离轴方式，将主镜像+Y方向离轴83.333mm。主镜为凹抛物面的一部分，次镜为采用10项zernike多项式表示的自由曲面。Zernike多项式如下：The wavelength of the incident light is 1550nm, the field of view is 1°, the diameter of the main mirror is D ₁ =100mm, the main mirror (that is, the relative aperture of the primary mirror is 1:4), so

The primary mirror focal length f ₁ =400mm, the beam expansion magnification M _t =6, the secondary mirror focal length f ₂ =66.666mm, the secondary mirror aperture D ₂ =16.666mm, the distance between the two mirrors d=333.333mm, the primary mirror vertex curvature radius R ₁ = 800mm, the secondary mirror vertex curvature radius R ₂ =133.333mm, the quadric surface coefficients of the primary mirror and the secondary mirror are both -1, the first off-axis method is adopted, and the primary mirror +Y direction is off-axis by 83.333mm. The primary mirror is part of a concave paraboloid, and the secondary mirror is a free-form surface represented by a 10-term zernike polynomial. The Zernike polynomial is as follows:

为光线角度坐标。次镜的zernike多项式系数如下表所示：Here, c is the base curvature, k is the aspheric coefficient, N is the number of Zernike coefficients in the series, A _i is the coefficient of the i-th Zernike polynomial, r is the radial ray coordinate, the unit is the lens unit, ρ To normalize the radial ray coordinates,

is the ray angle coordinate. The zernike polynomial coefficients of the secondary mirror are shown in the following table:

The structure of the beam expander system is shown in FIG. 6 . In FIG. 6 , 1 is the primary mirror, 2 is the secondary mirror, 3 is the diaphragm surface, and 4 is the image surface.

The image quality of the beam expander system cannot be evaluated by general MTF or spot diagram, but analyzed by wave aberration and Stollier ratio. When the wavelength of incident light is 1550nm, the maximum value of wave aberration in the whole field of view is 0.0604λ, which meets the requirement that the wave aberration is less than λ/14. The ratio SR is greater than the requirement of 0.8, and the field of view reaches 1°. If the traditional quadratic surface is used for design, the maximum value of the wave aberration in the full field of view is 0.0708λ, the minimum SR of the Stollier ratio in the full field of view is 0.837, and the field of view can only reach 0.8°. Therefore, the free-form surface of the system can effectively increase the field of view. Compared with the traditional quadratic surface, the optical field of view is increased by about 25%.

Claims (8)

1. an off-axis reflection type two-mirror beam expander system based on a free-form surface, the system comprises a primary mirror and a secondary mirror, it is characterized in that: The primary mirror is a concave mirror, the secondary mirror is a convex mirror, and the beam expansion system adopts an off-axis, no real focus design, no central blocking and energy loss; the beam parallel to the optical axis is incident on the convex secondary mirror On the upper side, the secondary mirror reflects on the concave primary mirror, and the primary mirror reflects and outputs the expanded beam parallel to the incident beam; the beam expansion system is designed with free-form surfaces, and the design includes three structural forms, the first The primary mirror is a free-form surface and the secondary mirror is an off-axis aspheric surface; the second structural form is that the primary mirror is an off-axis aspheric surface, and the secondary mirror is a free-form surface; the third structural form is that both the primary mirror and the secondary mirror are free-form surfaces . 2. a kind of off-axis reflection type two-mirror beam expander system based on free-form surface according to claim 1, is characterized in that: The beam expansion magnification M of the beam expander system is the ratio of the focal length of the primary mirror and the secondary mirror. 3 . The off-axis reflection type two-mirror beam expander system based on a free-form surface according to claim 1 , wherein the beam expansion magnification is fixed, and the beam expansion magnification can be set according to specific application requirements. 4 . 4 . The off-axis reflection type two-mirror beam expander system based on a free-form surface according to claim 1 , wherein the applicable wavelength range of the beam expander system is 0.4 μm to 12 μm. 5 . 5 . The off-axis reflection type two-mirror beam expander system based on a free-form surface according to claim 1 , wherein the surfaces of the two mirrors in the beam expander system need to be coated with a highly reflective film. 6 . 6. a kind of off-axis reflection type two-mirror beam expander system based on free-form surface according to claim 1, is characterized in that: The transformation matrix of the Gaussian beam passing through the telescopic system is:

Wherein, f ₁ and f ₂ represent the focal lengths of the two mirrors respectively, the distance between the two mirrors d=f ₁ +f ₂ +Δ, Δ represents the offset, and M _t =−f ₂ /f ₁ is the magnification of the telephoto system. 7. A kind of off-axis reflection type two-mirror beam expander system based on free-form surface according to claim 1, is characterized in that: Suppose the beam waist of the incident beam is ω ₀ , the wavelength is λ, and the focal parameter is

The object distance is s, and after passing through the telephoto system, it becomes a Gaussian beam with a beam waist of ω′ ₀ and an image distance of s′; Set Δ = 0, there are:

ω′ ₀ = |M _t |ω ₀ Let the initial divergence angle be θ ₀₁ , the divergence angle after passing through the system is θ ₀₂ , and there is an inverse relationship between the far-field divergence angle θ ₀ and the beam waist ω ₀ , namely: The far-field divergence angle is compressed by |M _t | times, and has nothing to do with the object distance and the image distance. When s=f ₁ , s′=f ₂ , that is, the image beam waist is located at the back focal plane of the second lens L ₂ on; when s>>f ₁ +f ₂ ,

The beam expansion ratio of this telescopic system:

8. a kind of off-axis reflection type two-mirror beam expander system based on free-form surface according to claim 2, is characterized in that: The magnification M _t of the beam expander system is the ratio of the focal length of the primary mirror to the secondary mirror. After selecting the relative aperture of the primary mirror, the focal length of the primary mirror and the secondary mirror can be obtained; set the offset amount Δ=0, the distance d between the two mirrors is the difference between the focal length of the primary mirror and the focal length of the secondary mirror, the radius of curvature of the apex of the mirror is twice the focal length, and the radii of curvature of the apex of the primary mirror and the secondary mirror are obtained.

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