阅读说明：本技术 一种基于自由曲面的离轴反射式两镜扩束系统 (Off-axis reflection type two-mirror beam expanding system based on free-form surface ) 是由 谷茜茜 崔占刚 亓波 马浩统 于 2019-10-08 设计创作，主要内容包括：本发明公开了一种基于自由曲面的离轴反射式两镜扩束系统,包括主镜和次镜,主镜为凹面反射镜,次镜为凸面反射镜。该系统的光学结构相对紧凑,无中心遮拦和能量损失,且系统无实焦点,更适合基于空气介质的扩束系统。该系统采用自由曲面进行设计,有三种结构形式,第一种是主镜为自由曲面、次镜为离轴非球面,第二种是主镜为离轴非球面、次镜为自由曲面,第三种是主镜和次镜都为自由曲面,可以根据具体的使用要求进行选择。采用自由曲面可以很好地校正非对称像差,提高扩束系统的光束质量,增大视场,应用于光通信系统,提高通信捕获概率。(The invention discloses an off-axis reflection type two-mirror beam expanding system based on a free-form surface. The system has a relatively compact optical structure, no central blocking and no energy loss, has no real focus, and is more suitable for the beam expanding system based on the air medium. The system is designed by adopting a free-form surface, and has three structural forms, wherein the first form is that a primary mirror is the free-form surface and a secondary mirror is an off-axis aspheric surface, the second form is that the primary mirror is the off-axis aspheric surface and the secondary mirror is the free-form surface, and the third form is that the primary mirror and the secondary mirror are both free-form surfaces and can be selected according to specific use requirements. The adoption of the free-form surface can well correct asymmetric aberration, improve the beam quality of a beam expanding system, increase the field of view, be applied to an optical communication system and improve the communication capture probability.)

1. The utility model provides a two mirror beam expanding systems of off-axis reflection formula based on free-form surface, this system includes primary mirror and secondary mirror, its characterized in that:

the primary mirror is a concave mirror, the secondary mirror is a convex mirror, and the beam expanding system adopts an off-axis design without a real focus and without central blocking and energy loss; the light beam parallel to the optical axis is incident on the convex secondary mirror, reflected to the concave primary mirror by the secondary mirror, and reflected by the primary mirror to output a beam-expanded light beam parallel to the incident light beam; the beam expanding system is designed by adopting a free-form surface, and the design comprises three structural forms, wherein the first form is that a primary mirror is the free-form surface and a secondary mirror is an off-axis aspheric surface; the second structure mode 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 the primary mirror and the secondary mirror are both free-form surfaces.

2. The free-form surface based off-axis reflective two-mirror beam expander system of claim 1, wherein:

the beam expanding multiplying power M of the beam expanding system is the ratio of the focal lengths of the primary mirror and the secondary mirror.

3. The free-form surface based off-axis reflective two-mirror beam expander system of claim 1, wherein: the beam expansion multiplying power is fixed, and the beam expansion multiplying power can be set according to specific use requirements.

4. The free-form surface based off-axis reflective two-mirror beam expander system of claim 1, wherein: the applicable wave band range of the beam expanding system is 0.4-12 mu m.

5. The free-form surface based off-axis reflective two-mirror beam expander system of claim 1, wherein: the surfaces of two reflectors in the beam expanding system need to be plated with high reflection films.

6. The free-form surface based off-axis reflective two-mirror beam expander system of claim 1, wherein:

the transformation matrix of the gaussian beam through the telescopic system is:

wherein f is₁，f₂Respectively representing the focal lengths of two mirrors, the distance d between the two mirrors being f₁+f₂+ Δ, Δ represents the amount of detuning, M_t＝-f₂/f₁The magnification of the telescopic system.

7. The free-form surface based off-axis reflective two-mirror beam expander system of claim 1, wherein:

let the beam waist of the incident beam be omega₀Wavelength is λ and focal parameter is

let Δ equal to 0, have:

ω′₀＝|M_t|ω₀

let the initial divergence angle be θ₀₁The divergence angle after passing through the system is theta₀₂Far field divergence angle θ₀And the girdling waist omega₀There is an inverse relationship between, namely:

far field divergence angle is compressed | M_tI times, independent of both object and image distances, when s is f₁When s ═ f₂I.e. the image beam waist is located at the second lens L₂On the back focal plane; when s > f₁+f₂When the temperature of the water is higher than the set temperature,

8. the free-form surface based off-axis reflective two-mirror beam expander system of claim 2, wherein:

the magnification M of the beam expanding system_tThe beam expansion multiplying power M is the ratio of the focal lengths of the primary mirror and the secondary mirror, and after the relative aperture of the primary mirror is selected, the focal lengths of the primary mirror and the secondary mirror can be obtained; and setting the misalignment delta to be 0, setting the distance d between the two mirrors as the difference between the focal length of the primary mirror and the focal length of the secondary mirror, and setting the curvature radius of the vertex of the reflector as a double focal length to obtain the curvature radius of the vertex of the primary mirror and the vertex of the secondary mirror.

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 expanding system based on a free-form surface.

Background

The beam expanding system is an important component of an optical communication system, and mainly has the functions of compressing the spatial divergence angle of a light beam, improving the collimation of the light beam, expanding the emission range and the like. Beam expansion systems generally fall into three structural forms, namely a refractive type, a reflective type and a return type. Compared with a refraction type beam expanding system, the reflection type beam expanding system is more suitable for application of a large-caliber system, chromatic aberration does not exist in the system, and the requirements of compact structure and light weight can be met. Compared with a coaxial beam expanding system, the off-axis beam expanding system has the advantages of no central blocking and high light energy utilization rate. Therefore, when the emission aperture is required to be large and the volume and mass are limited, a reflective beam expanding system is often adopted. However, the existing off-axis reflective beam expanding system has many disadvantages, such as small effective field of view, not simple enough structure, difficult correction of asymmetric aberration, etc. On the premise of not reducing the relative aperture of the system and not increasing the number of optical elements, the traditional spherical surface and the rotationally symmetric aspheric surface are increasingly difficult to meet the design requirements. In recent years, with the continuous improvement of processing and detection technologies, the processing precision of emerging free-form surfaces is higher and higher, so that the free-form surfaces can be applied to practical optical systems. The free-form surface has a series of advantages of non-rotational symmetry, rich degree of freedom, strong capability of correcting asymmetric aberration and the like, and is also used for designing an off-axis imaging system. Due to the defects of the existing off-axis beam expanding system, such as small effective field of view, difficulty in correcting asymmetric aberrations such as coma aberration and astigmatism, low beam quality and the like, the design requirements are more and more difficult to meet by using the traditional spherical surface and the rotationally symmetric aspheric surface.

Disclosure of Invention

Aiming at the defects of the existing beam expanding system, the invention provides an off-axis reflection type two-mirror beam expanding system based on a free-form surface, and aims to improve the beam quality of the beam expanding system, increase the working field of view, further improve the energy utilization rate and capture probability of an optical communication system and greatly simplify the system structure.

The technical scheme adopted by the invention is as follows: an off-axis reflection type two-mirror beam expanding system based on a free-form surface comprises a primary mirror and a secondary mirror:

the primary mirror is a concave mirror, the secondary mirror is a convex mirror, and the beam expanding system adopts an off-axis design without a real focus and without central blocking and energy loss; the light beam parallel to the optical axis is incident on the convex secondary mirror, reflected to the concave primary mirror by the secondary mirror, and reflected by the primary mirror to output a beam-expanded light beam parallel to the incident light beam; the beam expanding system is designed by adopting a free-form surface, and the design comprises three structural forms, wherein the first form is that a primary mirror is the free-form surface and a secondary mirror is an off-axis aspheric surface; the second structure mode 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 the primary mirror and the secondary mirror are both free-form surfaces. And may be selected according to specific use requirements.

The beam expanding multiplying power M of the beam expanding system is the ratio of the focal lengths of the primary mirror and the secondary mirror.

Furthermore, the beam expansion multiplying power is fixed, and the beam expansion multiplying power can be set according to specific use requirements.

Furthermore, the applicable wave band range of the beam expanding system is 0.4-12 μm.

Furthermore, the surfaces of two reflectors in the beam expanding system need to be plated with high-reflection films.

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

wherein f is₁，f₂Respectively representing the focal lengths of two mirrors, the distance d between the two mirrors being f₁+f₂+ Δ, Δ represents the amount of detuning, M_t＝-f₂/f₁The magnification of the telescopic system.

Further, in the above-mentioned case,

let the beam waist of the incident beam be omega₀Wavelength is λ and focal parameter isThe object distance is s, and the object distance is changed into the beam waist of omega after passing through a telescope system'₀A Gaussian beam with an image distance s';

let Δ equal to 0, have:

ω′₀＝|M_t|ω₀

let the initial divergence angle be θ₀₁The divergence angle after passing through the system is theta₀₂Far fieldDivergence angle theta₀And the girdling waist omega₀There is an inverse relationship between, namely:

far field divergence angle is compressed | M_tI times, independent of both object and image distances, when s is f₁When s ═ f₂I.e. the image beam waist is located at the second lens L₂On the back focal plane; when s > f₁+f₂When the temperature of the water is higher than the set temperature,

beam expansion ratio of the telescopic system:

further, in the above-mentioned case,

the magnification M of the beam expanding system_tThe beam expansion multiplying power M is the ratio of the focal lengths of the primary mirror and the secondary mirror, and after the relative aperture of the primary mirror is selected, the focal lengths of the primary mirror and the secondary mirror can be obtained; and setting the misalignment delta to be 0, setting the distance d between the two mirrors as the difference between the focal length of the primary mirror and the focal length of the secondary mirror, and setting the curvature radius of the vertex of the reflector as a double focal length to obtain the curvature radius of the vertex of the primary mirror and the vertex of the secondary mirror.

Free-form surface description methods generally fall into two broad categories, polynomial description methods and parametric description methods. The polynomial description method is a method for representing the surface shape of an optical free-form surface by adopting a series of polynomial combinations, and comprises a deformed aspheric surface, an XY polynomial surface, a Zemike polynomial surface, a radial basis function surface, a Q-type polynomial surface and the like. The parametric description method is a method for obtaining the surface shape of an optical free-form surface based on discrete point fitting, and comprises a B-spline surface, a non-uniform rational B-spline (NURBS) surface and the like.

The surfaces of the two reflectors of the beam expanding system need to be plated with high-reflection films, and the applicable wave band of the beam expanding system is wide.

The beam expanding multiplying power of the beam expanding system is fixed, and the beam expanding multiplying power can be set according to specific use requirements.

Advantageous effects

Due to the defects of the existing off-axis beam expanding system, such as small effective field of view, difficulty in correcting asymmetric aberrations such as coma aberration and astigmatism, low beam quality and the like, the design requirements are more and more difficult to meet by using the traditional spherical surface and the rotationally symmetric aspheric surface.

The invention innovatively applies a free-form surface to an off-axis reflection type beam expanding system and provides the off-axis reflection type two-mirror beam expanding system based on the free-form surface. The system has the advantages of simple structure, good beam quality and the like, adopts the free-form surface, and can effectively correct the asymmetric aberration caused by the off-axis of the system on the premise of not changing the relative aperture of the system and not increasing the number of optical elements by comparing with the traditional quadric surface, thereby improving the beam quality, more effectively increasing the field of view and improving the non-unloading working range of the optical communication system. In addition, the afocal system is adopted, and a collimating lens element is not needed to be used, so that subsequent light paths such as a fast reflecting mirror, a spectroscope and the like can be directly accelerated, the system structure is greatly simplified, and the optical elements of the coarse tracking system and the fine tracking system can be shared. Compared with the existing off-axis beam expanding system, the off-axis beam expanding system has obvious advantages and has strong practical value in an optical communication system.

Drawings

FIG. 1 is a schematic structural diagram of an off-axis two-mirror beam expander system according to the present invention;

FIG. 2 is a schematic structural diagram of a free-form surface of a primary mirror of an off-axis two-mirror beam expanding system according to the present invention;

FIG. 3 is a schematic structural diagram of a free-form surface of another off-axis two-mirror beam expanding system secondary mirror according to the present invention;

FIG. 4 is a schematic structural diagram of a free-form surface of both primary and secondary off-axis two-mirror beam expanding systems provided by the present invention;

FIG. 5 is a schematic diagram of a Gaussian beam passing through a telescopic system;

fig. 6 is a structural diagram of an off-axis two-mirror beam expanding system in which a secondary mirror is a free-form surface according to an embodiment of the present invention;

Detailed Description

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

The requirements of the optical system such as the clear aperture and the magnification are provided by the overall design requirements of the instrument. The relative aperture of the primary mirror is then selected. The relative caliber of the primary mirror is selected according to various factors, from the viewpoint of shortening the length of the lens barrel, the larger the relative caliber of the primary mirror is, the more beneficial the primary mirror is, the processing difficulty is in direct proportion to the cube of the relative caliber, so the numerical value is determined by combining several factors, generally 1: 3 or 1: 4, and the relative caliber of the primary mirror which is 1: 2 or even larger is increasingly adopted by large telescopes.

The transformation matrix of the gaussian beam through the telescopic system is:

wherein f is₁，f₂Respectively representing the focal lengths of two mirrors, the distance d between the two mirrors being f₁+f₂+ Δ, Δ represents the amount of detuning, M_t＝-f₂/f₁The magnification of the telescopic system.

Let the beam waist of the incident beam be omega₀Wavelength is λ and focal parameter is

The object distance is s, and the object distance is changed into the beam waist of omega after passing through a telescope system'₀Gaussian beam with image distance s'.

Let Δ equal to 0, have:

ω′₀＝|M_t|ω₀

let the initial divergence angle be θ₀₁The divergence angle after passing through the system is theta₀₂Far field divergence angle θ₀And the girdling waist omega₀There is an inverse relationship between, namely:

far field divergence angle is compressed | M_tI times, independent of both object and image distances, when s is f₁When s ═ f₂I.e. the image beam waist is located at the second lens L₂On the back focal plane; when s > f₁+f₂When the temperature of the water is higher than the set temperature,

beam expansion ratio of the telescopic system:

please refer to fig. 5.

The magnification M of the beam expanding system_t(beam expansion magnification M) is the ratio of the focal lengths of the primary and secondary mirrors. After the relative aperture of the primary mirror is selected, the focal length of the primary and secondary mirrors can be obtained. Generally, the misalignment amount Δ is set to 0, 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 curvature radius of the vertex of the reflector is twice the focal length, and the curvature radius of the vertex of the primary mirror and the curvature radius of the vertex of the secondary mirror can be obtained.

The beam expanding multiplying power of the beam expanding system is fixed, the applicable wave band is wide, and two reflectors need to be plated with high reflective films.

The off-axis mode of the beam expanding system has two modes: (1) the aperture off-axis or offset field of view (2) tilts the curved surface.

The beam expanding system is designed by adopting a free-form surface, so that the asymmetric aberration of the system can be corrected, the field of view is increased, and the beam quality is improved. Free-form surface description methods generally fall into two broad categories, polynomial description methods and parametric description methods. The positions of the free-form surfaces in the system can be three, the primary mirror is a free-form surface, the secondary mirror is a free-form surface, and the primary mirror and the secondary mirror are free-form surfaces, which are respectively shown in the attached figures 2, 3 and 4. In fig. 2, 1 is a free-form surface primary mirror, and 2 is an off-aspheric secondary mirror; in fig. 3, 1 is an off-axis aspheric 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 secondary mirror;

in the optical design software ZEMAX or CODEV, the surface shape of the primary mirror or the secondary mirror is set to be a free surface shape such as a deformed aspheric surface, an XY polynomial surface, a Zernike polynomial surface, a radial basis function curved surface, a Q-type polynomial surface and the like, or a user-defined free surface shape can be written by using a macro language.

The afocal system is an optical system which does not diverge or focus light beams, namely parallel light enters and parallel light exits, and the equivalent focal length of the optical system is infinite.

For example, according to the above process, we have designed a beam expanding system with the following parameters:

the wavelength of incident light is 1550nm, the visual field is 1 degree, and the aperture of the primary mirror is D₁100mm, main mirror(i.e., the relative aperture of the primary mirror is 1: 4), therefore

Focal length f of primary mirror₁400mm, beam expanding multiplying power M_t6, secondary mirror focal length f₂66.666mm, minor caliber D₂16.666mm, the distance d between two mirrors is 333.333mm, and the curvature radius R of the vertex of the primary mirror₁800mm, secondary mirror vertex radius of curvature R₂133.333mm, the coefficients of the quadric surfaces of the primary mirror and the secondary mirror are both-1, and the primary mirror image is off-axis 83.333mm in the + Y direction by adopting a first off-axis mode. The primary mirror is a portion of a concave paraboloid and the secondary mirror is a free-form surface represented by a 10-term zernike polynomial. The Zernike polynomials are as follows:

where c denotes the base curvature, k denotes the aspherical coefficient, N denotes the number of Zernike coefficients in the series, A_iIs as followsi terms Zernike polynomial coefficients, r is the radial ray coordinate in lens units, ρ is the normalized radial ray coordinate,

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

the structure of the beam expanding system is shown in figure 6, wherein 1 in figure 6 is a primary mirror, 2 is a secondary mirror, 3 is a diaphragm surface, and 4 is an image surface.

The image quality of the beam expanding system can not be evaluated by using a general MTF or a point diagram, but is analyzed by using wave aberration and a Stonell ratio. When the wavelength of incident light is 1550nm, the maximum value of the wave aberration in the full field of view is 0.0604 lambda, the requirement that the wave aberration is less than lambda/14 is met, the minimum value of the Stonell ratio SR in the full field of view is 0.880, the requirement that the Stonell ratio SR is more than 0.8 is met, and the field of view reaches 1 degree. If the traditional quadric surface is adopted for design, the maximum value of wave aberration in the full field of view is 0.0708 lambda, the minimum value of the Stonell ratio SR in the full field of view is 0.837, and the field of view can only reach 0.8 deg. Therefore, the system can effectively increase the visual field by adopting the free curved surface, and the optical visual field is improved by about 25 percent compared with the traditional quadric surface.