Free-form surface-based entrance and exit pupil coaxial space gravitational wave detection telescope and design method thereof

Abstract

The invention provides an entrance and exit pupil coaxial space gravitational wave detection telescope based on a free-form surface and a design method thereof, which are used for solving the problem that an existing entrance and exit pupil coaxial off-axis five-reflection space gravitational wave detection telescope cannot give consideration to low TTL coupling noise and low backward stray light distribution at the same time. Or introduction of more free-form surface entrance and exit pupil coaxial space gravitational wave detection telescopes has the technical problems that the system size and the error source are increased, and the installation and adjustment difficulty is increased. According to the entrance and exit pupil coaxial space gravitational wave detection telescope based on the free-form surface, on the basis of entrance and exit pupil coaxial design, the fourth mirror is set to be the free-form surface reflecting mirror, the incident angles of the third mirror and the fourth mirror are the same, low far-field wavefront distortion is achieved on the basis of reducing TTL coupling noise, and high wavefront quality is guaranteed; meanwhile, in consideration of the influence of a free-form surface processing technology on system performance, incident angles of the third mirror and the fourth mirror are uniformly designed, so that backward stray light can be effectively reduced.

Description

Technical Field

[0001] The present invention relates to a space gravitational wave detection telescope, and in particular to an entrance-exit pupil coaxial space gravitational wave detection telescope based on a free-form surface and a design method thereof. Background Art

[0002] Compared with traditional ground-based gravitational wave detection telescopes, space gravitational wave detection telescopes have longer baselines and lower background noise, and can detect gravitational wave signals in lower frequency bands. At present, mainstream space gravitational wave detection telescopes use ultra-long baseline laser interferometry technology to extract characteristic signals. For example, the LISA plan uses three satellites to form an equilateral triangle with a side length of 2.5 million kilometers, and uses laser beams to travel back and forth between satellites to form an interference light path. When gravitational waves pass by, space-time distortion will slightly change the length of the light path (about picometer level changes), causing the laser interference signal to produce a phase difference. This weak signal is amplified by the ultra-long baseline accumulation effect, and finally the characteristic signal caused by gravitational waves is extracted from the environmental signal.

[0003] As an important component of space gravitational wave detection technology, the space gravitational wave detection telescope has two functions: collecting picowatt-level weak laser signals sent by satellite base stations millions of kilometers away and sending watt-level laser signals from local base stations to satellite base stations millions of kilometers away, thereby realizing the "transmitting and receiving" function of gravitational wave detection laser signals and forming a stable two-way detection link of millions of kilometers between the two base stations. Therefore, the space gravitational wave detection telescope requires an extremely high signal-to-noise ratio, which is specifically manifested in the need for extremely low far-field wavefront distortion, extremely low TTL (optical path jitter) coupling noise, and extremely low backward stray light.

[0004] like Figure 1 As shown, the existing off-axis four-mirror space gravitational wave detection telescope with coaxial entrance and exit pupils includes a system aperture 01, a primary mirror 02, a secondary mirror 03, a triple mirror 04, a quadruple mirror 05 and a system exit pupil 06, wherein the primary mirror 02 is a parabolic reflector, the secondary mirror 03 is a hyperbolic reflector, and the triple mirror 04 and the quadruple mirror 05 are both spherical reflectors. However, since the entrance and exit pupils are not coaxial, this solution introduces additional TTL coupling noise to the entire system. Although the industry currently adopts a solution with coaxial entrance and exit pupils to effectively reduce TTL coupling noise, it uses an off-axis five-mirror structure to achieve this. Figure 2 , Figure 3As shown in FIG. 1 , the off-axis five-trans space gravitational wave detection telescope with coaxial entrance and exit pupils includes two typical structures: an “S” type off-axis five-trans structure with coaxial entrance and exit pupils and a “δ” type off-axis five-trans structure with coaxial entrance and exit pupils, which respectively include a coaxial aperture 001, a primary reflector 002, a secondary reflector 003, a triple reflector 004, a quadruple reflector 005, a quintuple reflector 006 and a coaxial exit pupil 007. The difference between the two is the setting direction of the quadruple reflector 005 and the setting position of the quintuple reflector 006. However, both of the above structures cannot take into account both low TTL coupling noise and low backward stray light distribution, resulting in the inability to further improve the overall detection performance of the system. Summary of the invention

[0005] The purpose of the present invention is to solve the technical problems that the existing off-axis five-transverse space gravitational wave detection telescope with coaxial entrance and exit pupils cannot take into account both low TTL coupling noise and low backward stray light distribution at the same time, resulting in the inability to further improve the overall detection performance of the system, or the introduction of more free-form surfaces in the space gravitational wave detection telescope with coaxial entrance and exit pupils greatly increases the system size and error sources, and the difficulty of installation and adjustment is also greatly increased. A free-form surface-based entrance and exit coaxial space gravitational wave detection telescope and a design method thereof are provided.

[0006] The technical concept of the present invention is as follows: in order to realize a space gravitational wave detection telescope that requires an extremely high signal-to-noise ratio, that is, extremely low far-field wavefront distortion, extremely low TTL coupling noise and extremely low backward stray light, the present invention first adopts the idea of ​​entrance and exit pupil coaxiality to reduce TTL coupling noise; secondly, since the introduction of more free-form surfaces will greatly increase the system size and error sources, and the difficulty of installation and adjustment will also be greatly increased, the present invention only uses Zernike polynomials to optimize the free-form surfaces of four of the mirrors to introduce optimization degrees of freedom, thereby achieving lower far-field wavefront distortion. However, due to the limitations of free-form surface processing technology, the roughness of the surface of the four mirrors is relatively high, and higher roughness will increase the distribution of backward stray light. Therefore, the present invention creatively combines the influence of the incident angle in the reflected light path on the distribution of stray light, and divides the 180° turning angle of the overall light path evenly on the three mirrors and the four mirrors, and each reflecting mirror undertakes a 90° turning of the light path, that is, the incident angles of the three mirrors and the four mirrors are set to 45° respectively, which can effectively reduce backward stray light and meet the signal-to-noise ratio requirements of the space gravitational wave detection telescope.

[0007] In order to achieve the above purpose, the technical solution provided by the present invention is as follows: A free-form surface-based entrance-exit coaxial space gravitational wave detection telescope, comprising an aperture stop, a primary mirror, a secondary mirror, a third mirror, a fourth mirror and an exit pupil; The primary mirror is a parabolic reflector, the secondary mirror is a hyperbolic reflector, the third mirror is an ellipsoidal reflector, and the fourth mirror is a free-form reflector; The aperture stop and the primary mirror are sequentially located on the incident light path of the far-field parallel laser, and the primary mirror and the secondary mirror form a Cassegrain structure; The three mirrors are located on the laser light path reflected by the secondary mirror, the four mirrors are located on the laser light path reflected by the three mirrors, and the exit pupil is located on the laser light path reflected by the four mirrors; the three mirrors, the four mirrors and the exit pupil are all located on the side of the primary mirror away from the aperture stop, and the four mirrors and the exit pupil are located on the central optical axis of the aperture stop; The incident angles of the three mirrors and the four mirrors are the same, both recorded as , Determined by the following formula:

[0008] In the above formula, is the off-axis amount of the primary mirror, is the length of the entire space gravitational wave telescope optical system, is the distance between the four mirrors and the exit pupil, is the distance between the primary mirror and the secondary mirror, is the distance between the secondary mirror and the tertiary mirror; for The upper boundary value of for The lower boundary value of ; The primary mirror, secondary mirror, third mirror and fourth mirror do not interfere with each other.

[0009] Furthermore, the incident angles of the three mirrors and the four mirrors are The angle is 45°, which allows the optical axis to pass through two 90° turns and achieve coaxial design of entrance and exit pupils, greatly reducing backward stray light.

[0010] Furthermore, the four mirrors use Zernike polynomials to perform free-form surface optimization.

[0011] Furthermore, the central optical axes of the aperture stop, primary mirror, secondary mirror, third mirror, fourth mirror and exit pupil are located in the same vertical plane, which can avoid introducing additional off-axis aberrations and meet the requirements of miniaturization of the optical system.

[0012] In addition, the present invention also provides a design method for the above-mentioned free-form surface-based entrance-exit coaxial space gravitational wave detection telescope, comprising the following steps: Step 1: Determine the distance between the primary mirror and the secondary mirror according to the entrance pupil diameter and geometric dimensions of the space gravitational wave detection telescope to be designed, and then build a Cassegrain structure with the primary and secondary mirrors coaxial; propose the position of the primary image plane of the space gravitational wave detection telescope to be designed, and de-axis the aperture according to the position of the primary image plane; optimize the variable parameters of the Cassegrain structure after the aperture is de-axis to ensure that the imaging of the Cassegrain structure is located at the primary image plane, and optimize the imaging quality to approach the diffraction limit; Step 2, placing the three mirrors on the laser light path reflected by the secondary mirror, so that it is located behind the primary image plane, and the reflecting surface is set in the direction of the primary mirror, so that the laser reflected by the three mirrors is emitted to the side of the primary mirror away from the aperture stop; placing the four mirrors on the laser light path reflected by the three mirrors, and at the same time, the four mirrors and the exit pupil are located on the central optical axis of the aperture stop; Step 3, select the four-mirror type as the Zernike Fringe vector height, and optimize the variable parameters of the optical system after adding three mirrors and four mirrors based on the incident wavelength λ, entrance pupil diameter, field angle, focal length and compression ratio of the space gravitational wave detection telescope to be designed, using wave aberration as an evaluation index; Step 4, respectively performing wave aberration analysis, TTL coupling noise analysis, and backward stray light analysis on the optical system optimized in step 3; if the results of each analysis meet the corresponding requirements, the variable parameter optimization of the optical system is completed, and step 5 is executed; if any analysis result does not meet the corresponding requirements, the incident angle of the three mirrors and the off-axis amount of the four mirrors are adjusted, and the variable parameters of the current optical system are re-optimized until the results of each analysis meet the corresponding requirements; Step 5, perform tolerance analysis on the optical system, allocate mirror processing tolerances and installation tolerances for the primary mirror, secondary mirror, third mirror, and fourth mirror; then use the Monte Carlo method to simulate and analyze the mirror processing tolerances and installation tolerances for the primary mirror, secondary mirror, third mirror, and fourth mirror to ensure that the overall optical system meets the pass rate requirements and complete the design of the free-form surface-based entrance-exit coaxial space gravitational wave detection telescope.

[0013] Further, in step 4, the result of the wave aberration analysis must satisfy: the wave aberration is better than λ / 80; The results of the TTL coupling noise analysis must meet the following requirements: TTL coupling noise is less than 25 pm / μrad; The result of the backward stray light analysis must meet the following requirements: the backward stray light energy suppression ratio reaches 10 -10 Magnitude.

[0014] Further, in step 1, the variable parameters of the Cassegrain structure include the radius of curvature of the primary mirror, the radius of curvature of the secondary mirror, and the conic coefficient of the secondary mirror; In step 3, the variable parameters of the optical system after adding three mirrors and four mirrors include the curvature radius of the primary mirror, the curvature radius of the secondary mirror, the cone coefficient of the secondary mirror, the curvature radius of the third mirror, the cone coefficient of the third mirror, the curvature radius of the fourth mirror, the cone coefficient of the fourth mirror and the Zernike polynomial aspheric coefficient of the four mirror.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. The free-form surface-based entrance-and-exit coaxial space gravitational wave detection telescope provided by the present invention, on the basis of the entrance-and-exit coaxial design, sets four mirrors as free-form surface reflectors, and the incident angles of the three-mirror and the four-mirror are the same, thereby achieving lower far-field wavefront distortion (low wave aberration) on the basis of reducing TTL coupling noise, and ensuring high wavefront quality; at the same time, considering the impact of free-form surface processing technology on system performance, the incident angles of the three-mirror and the four-mirror are uniformly designed, which can effectively reduce backward stray light.

[0016] 2. The present invention sets the incident angles of the three-mirror and the four-mirror to 45°, so that the optical axis can achieve two 90° turns and entrance and exit pupil coaxial design, which greatly reduces backward stray light. Under the premise of meeting the signal-to-noise ratio requirements of the space gravitational wave detection telescope, the subsequent supporting mechanical structure has a smaller weight, and the overall structure is compact and has better stability, which effectively improves the detection performance and reliability of the entire system.

[0017] 3. The present invention introduces the design of free-form surfaces on the basis of coaxial entrance and exit pupils, and uses Zernike polynomials to optimize the free-form surfaces, thereby achieving the purpose of correcting aberrations, improving system imaging quality, and making the structure compact. While ensuring high wavefront quality, the TTL coupling noise caused by wave aberrations can be further reduced, so that the optical path stability, backward stray light distribution and system assembly and processing feasibility of the space gravitational wave detection telescope are taken into account. Compared with the existing off-axis five-transverse space gravitational wave detection telescope with coaxial entrance and exit pupils, the present invention has the advantages of lower assembly difficulty and smaller errors introduced in the later processing links.

[0018] 4. The design method of the free-form surface-based pupil-coaxial space gravitational wave detection telescope provided by the present invention is based on the Cassegrain structure with coaxial primary and secondary mirrors, and at the same time, the four-mirror type is selected as the Zernike Fringe (Zernike fringe) vector height. The wave aberration is used as an evaluation index and the geometric constraint of the pupil coaxiality is adopted to optimize the optical system. There are fewer optimization variables, the method is simple and practical, and good wave aberration can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the structure of an existing off-axis four-transmission space gravitational wave detection telescope with different entrance and exit pupil axes; Figure 1 The reference numerals are: 01 - system aperture, 02 - primary mirror, 03 - secondary mirror, 04 - triple mirror, 05 - quadruple mirror, 06 - system exit pupil.

[0020] Figure 2 It is a schematic diagram of the structure of an existing "S"-shaped off-axis five-trans space gravitational wave detection telescope with coaxial entrance and exit pupils; Figure 3It is a schematic diagram of the structure of an existing "δ" type structure off-axis five-trans space gravitational wave detection telescope with coaxial entrance and exit pupils; Figure 2 , Figure 3 The reference numerals are: 001 - coaxial aperture, 002 - primary reflector, 003 - secondary reflector, 004 - triple reflector, 005 - quadruple reflector, 006 - penta reflector, 007 - coaxial exit pupil.

[0021] Figure 4 It is a structural schematic diagram of an embodiment of an entrance-exit coaxial space gravitational wave detection telescope based on a free-form surface according to the present invention; Figure 4 The reference numerals are: 1 - aperture stop; 2 - primary mirror; 3 - secondary mirror; 4 - third mirror; 5 - fourth mirror; 6 - exit pupil. DETAILED DESCRIPTION

[0022] In order to make the advantages and features of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] A free-form surface-based coaxial space gravitational wave detection telescope with pupil and entrance, such as Figure 4 As shown, the space gravitational wave detection telescope is an off-axis four-mirror structure, specifically including an aperture stop 1, a primary mirror 2, a secondary mirror 3, a third mirror 4, a fourth mirror 5 and an exit pupil 6. In order to avoid introducing additional off-axis aberrations and meet the requirements of miniaturization of the optical system, the central optical axes of the overall optical system in this embodiment are all located in the same vertical plane.

[0024] Among them, the primary mirror 2 is a parabolic reflector, the secondary mirror 3 is a hyperbolic reflector, the aperture stop 1 and the primary mirror 2 are sequentially located in the incident light path of the far-field parallel laser, and the secondary mirror 3 is located in the laser light path after being reflected by the primary mirror 2, so that the primary mirror 2 and the secondary mirror 3 form a Cassegrain structure. The third mirror 4, the fourth mirror 5 and the exit pupil 6 are all placed on the side of the primary mirror 2 away from the aperture stop 1, that is, the third mirror 4, the fourth mirror 5 and the exit pupil 6 are all located on the non-reflective surface side of the primary mirror 2.

[0025] The third mirror 4 is an ellipsoidal reflector, which is located on the laser light path after being reflected by the secondary mirror 3. In order to make the overall optical system compact and realize the coaxial structure of the entrance and exit pupils, the reflective surface of the third mirror 4 is set toward the direction of the primary mirror 2, so that the laser reflected by the third mirror 4 is emitted to the side of the primary mirror 2 away from the aperture stop 1. The fourth mirror 5 is a free-form surface reflector, which is located on the laser light path after being reflected by the third mirror 4, and the fourth mirror 5 and the exit pupil 6 are both placed on the central optical axis of the aperture stop, thereby realizing the coaxial design of the entrance and exit pupils based on the free-form surface.

[0026] In order to reduce the system's backward stray light distribution, since the off-axis amount of the three mirrors 4 is 10 -4mm level, it can be approximately considered that the optical axis after reflection by the secondary mirror 3 is parallel to the entrance pupil optical axis, so the optical path turning angles of the third mirror 4 and the fourth mirror 5 are Same (the turning angle of the three mirrors 4 That is, the angle between the optical axis after reflection by the secondary mirror 3 and the optical axis after reflection by the third mirror 4, and the turning angle of the optical path of the fourth mirror 5 That is, the angle between the optical axis after reflection by the third mirror 4 and the optical axis after reflection by the fourth mirror 5). Correspondingly, the incident angles of the two Similarly, the main functions of the three mirrors 4 and the four mirrors 5 are to collimate the light and give the light path a displacement in the direction perpendicular to the entrance pupil optical axis. The incident angles of the three mirrors 4 and the four mirrors 5 are Determined by the following formula: (1) In the above formula, is the off-axis amount of the primary mirror 2, is the length of the entire space gravitational wave telescope optical system, is the distance between the four mirrors 5 and the exit pupil 6, is the distance between the primary mirror 2 and the secondary mirror 3, is the distance between the secondary mirror 3 and the tertiary mirror 4, for The upper boundary value of for The lower boundary value of .

[0027] In the design task of this embodiment, the length of the entire space gravitational wave telescope optical system is The minimum distance between the four mirrors 5 and the exit pupil 6 is 100 mm. The exit pupil 6 directly faces the subsequent optical interference platform interface, that is, the exit pupil 6 is located 100 mm behind the reflected light path of the four mirrors 5. Considering the aperture of the primary mirror 2 and to ensure that the subsequent light path does not have a central occlusion phenomenon, the off-axis distance of the primary mirror 2 in this embodiment is Designed to be 200mm; the distance between primary mirror 2 and secondary mirror 3 Since the entrance-exit ratio is 100×, the focal length ratio between the front group composed of the primary mirror 2 and the secondary mirror 3 and the rear group composed of the third mirror 4 and the fourth mirror 5 is 100×. Based on the known distance between the position of the primary real image and the secondary mirror 3, plus the distance of the rear group reduced in proportion, the distance between the secondary mirror 3 and the third mirror 4 is obtained. , at this time, the position of the three mirrors 4 can be determined; after the position of the three mirrors 4 is determined, the incident angle can be calculated by formula (1) Range 53°> >38°, the position of the fourth mirror 5 is determined according to the intersection of the laser light path after reflection from the third mirror 4 and the entrance pupil optical axis.

[0028] Due to the limitation of free-form surface processing technology, the surface roughness of the four mirrors 5 will be relatively high, which will increase the distribution of backward stray light. While changing the light propagation path, it also affects the distribution of stray light, mainly in the following two aspects: Geometric shielding effect: large light path turning angle It can make the scattered light at the edge of the primary mirror 2 more difficult to be reflected to the focal plane by the secondary mirror 3, thereby reducing stray light.

[0029] Scattering direction deviation: The scattered light on the surface of the four mirrors 5 may originally propagate along the mirror symmetry direction, but the off-axis design makes the scattered light deviate to the turning angle of the optical path side.

[0030] The scattering caused by the roughness of the free-form surface obeys the distribution of the bidirectional reflectance distribution function (BRDF). In the off-axis design, the turning angle of the optical path Changed the incident angle of the quad mirror 5 and scattering angle Relationship: (2) In formula (2), is the actual reflection direction of the four mirrors 5, , is the surface roughness parameter of the four mirrors 5. Larger optical path turning angle (i.e. smaller angle of incidence ) will cause the stray light distribution to shift to a larger angle, away from the sensitive area of ​​the exit pupil 6 plane. But for the system as a whole, a larger optical path turning angle The result is a longer optical axis axial dimension, and the larger the dimension, the greater the positioning error will inevitably be introduced in the later stage of assembly and adjustment. Therefore, the BRDF scattering model of the four mirrors 5 is numerically simulated by the present invention after the spectral power density PSD of the surface morphology of the measured surface roughness test sample is brought in. When the incident angle of the four mirrors 5 is When the angle approaches 45°, the stray light component caused by the surface roughness of the four mirrors 5 approaches a minimum value, which can effectively reduce the processing requirements for the surface roughness of the reflector. Therefore, after comprehensively considering the backward stray light energy suppression effect and the overall size and adjustment error of the system, the incident angle of the four mirrors 5 is Iterative optimization for 45°, i.e. the turning angle of the optical path The incident angles of the three mirrors 4 and the four mirrors 5 are the same, which is also 45°. After being reflected by the three mirrors 4 and the four mirrors 5, the laser light path realizes a 180° turn as a whole, which is equivalent to the 180° turning angle of the whole light path being evenly divided on the three mirrors 4 and the four mirrors 5, and each reflector bears a 90° turn.

[0031] Since the far-field parallel laser is non-parallel after being reflected by the primary mirror 2, the secondary mirror 3 and the tertiary mirror 4, the rear group composed of the tertiary mirror 4 and the quaternary mirror 5 is designed to treat the first real image formed by the front group composed of the primary mirror 2 and the secondary mirror 3 as a real object and perform secondary imaging, and finally the exit pupil 6 presents parallel light with high wavefront quality.

[0032] The receiving optical path of the free-form surface-based entrance-exit coaxial space gravitational wave detection telescope of the present invention is as follows: the far-field parallel laser emitted by the reference at a distance of millions of kilometers is incident on the reflecting surface of the primary mirror 2 through the aperture diaphragm 1, and is reflected by the primary mirror 2 and then by the secondary mirror 3, and then is reflected by the third mirror 4 and the fourth mirror 5 in sequence, and finally emerges at the position of the exit pupil 6 as a parallel light beam, and is connected to the subsequent optical interference platform.

[0033] The specific design parameters of the primary mirror 2, the secondary mirror 3, and the tertiary mirror 4 in this embodiment are shown in Table 1: Table 1 Optical system parameters

[0034] The specific design parameters of the four mirrors 5 are shown in Table 2: Table 2 Zernike polynomial aspheric coefficients

[0035] In this embodiment, the four mirrors 5 use Zernike polynomials to optimize the free surface (aspheric surface optimization). The Zernike polynomial aspheric surface coefficients use eight high-order coefficients, namely G3, G4, G5, G8, G9, G11, G12, and G17, which increases the degree of freedom in the design of the space gravitational wave detection telescope. It can effectively improve its imaging quality while meeting the system size requirements, so that the system wavefront quality meets the actual detection accuracy requirements.

[0036] At the same time, the present invention also proposes a design method for the above-mentioned free-form surface-based entrance-exit coaxial space gravitational wave detection telescope, which specifically includes the following steps: Step 1: Design of large-aperture optical path at the front end of the space gravitational wave detection telescope.

[0037] According to the optical transmission efficiency requirements of the space gravitational wave detection telescope to be designed, the entrance pupil diameter in this embodiment is determined to be 300mm, and the overall geometric size requirements are further determined. After clarifying the requirements, according to the entrance pupil diameter and overall geometric size requirements of the space gravitational wave detection telescope to be designed, the distance from the aperture stop 1 to the primary mirror 2 is set to 600nm, and then the distance from the primary mirror 2 to the secondary mirror 3 is determined, and the primary mirror 2 and the secondary mirror 3 are built into a coaxial Cassegrain structure. According to the system size requirements, the position of the primary image plane of the space gravitational wave detection telescope to be designed is then determined, and the aperture is de-axised according to the position of the primary image plane. After the aperture is off-axis, the curvature radius of the primary mirror 2, the curvature radius of the secondary mirror 3, and the cone coefficient of the secondary mirror 3 are used as optimization items for variable parameter optimization to ensure that the off-axis Cassegrain structure is imaged at the primary image plane, and the imaging quality of the Cassegrain structure is optimized to approach the diffraction limit.

[0038] Step 2: Design of the image transfer optical path at the back end of the space gravitational wave detection telescope.

[0039] The three mirrors 4 are placed on the laser light path reflected by the secondary mirror 3, so that the three mirrors 4 are located behind the primary image plane, and the reflection surface of the three mirrors 4 is set in the direction of the primary mirror 2, so that the laser reflected by the three mirrors 4 is emitted to the side of the primary mirror 2 away from the aperture diaphragm 1. Then the four mirrors 5 are placed on the laser light path reflected by the three mirrors 4, and the four mirrors 5 and the exit pupil 6 are located on the central optical axis of the aperture diaphragm, so as to realize the structural design of the entrance and exit pupils being coaxial. In order to reduce the distribution of backward stray light in the system, the turning angle of the overall optical path of 180° is evenly divided on the three mirrors 4 and the four mirrors 5. The three mirrors 4 and the four mirrors 5 are responsible for the turning of 90° respectively, and when the incident angle is 45°, the processing requirements for the mirror surface roughness can be effectively reduced. The light path is placed at an angle of 45° with the incident light axis after reflection by the secondary mirror 3, so that the light path is emitted in the direction of the central optical axis of the aperture stop, and is placed at an angle of 45° with the incident light axis after reflection by the third mirror 4. The light path turning angle distribution scheme is obtained by comprehensively considering the requirements of backward stray light distribution and compact optical system. The turning angle of the overall light path of 180° is evenly divided between the third mirror 4 and the fourth mirror 5, and each reflector undertakes a 90° turning of the light path. The parallel laser is formed by reflection by the fourth mirror 5, and the formed parallel laser is emitted in the direction of the central optical axis of the aperture stop 1. At this time, the third mirror 4 and the fourth mirror 5 simultaneously realize the laser collimation function and the light path turning function.

[0040] Step 3: Introduce a free-form surface mirror to optimize the system optical path.

[0041] The surface shape of the four-mirror 5 is selected as the Zernike Fringe vector height, and the 17th-order Zernike polynomial aspheric coefficient variable is introduced to increase the optimization freedom of this system. According to the working wavelength, entrance pupil diameter, field angle and magnification requirements of the space gravitational wave detection telescope to be designed, the curvature radius of the primary mirror 2, the curvature radius of the secondary mirror 3, the cone coefficient of the secondary mirror 3, the curvature radius of the third mirror 4, the cone coefficient of the third mirror 4, the curvature radius of the fourth mirror 5, the cone coefficient of the fourth mirror 5 and the Zernike polynomial aspheric coefficient of the fourth mirror 5 are optimized with wave aberration as the main evaluation index. Among them, the evaluation index of wave aberration is required to be better than λ / 80.

[0042] Step 4: Perform wave aberration analysis, TTL coupling noise analysis, and backward stray light analysis.

[0043] The overall optical system optimized in step 3 is subjected to wavefront aberration analysis, TTL coupling noise analysis, and backward stray light analysis to meet the requirements of high wavefront quality, high optical path stability, and high backward stray light energy suppression ratio of the telescope optical system. Specifically, the wavefront aberration is better than λ / 80, the TTL coupling noise is less than 25pm / μrad, and the backward stray light energy suppression ratio must reach 10 -10 Magnitude.

[0044] If the system analysis results cannot meet the three corresponding requirements at the same time, the incident angle of the third mirror 4 and the off-axis amount of the fourth mirror 5 are adjusted, and the curvature radius of the primary mirror 2, the curvature radius of the secondary mirror 3, the cone coefficient of the secondary mirror 3, the curvature radius of the third mirror 4, the cone coefficient of the third mirror 4, the curvature radius of the fourth mirror 5, the cone coefficient of the fourth mirror 5 and the Zernike polynomial aspheric coefficient of the fourth mirror 5 are re-optimized until all analysis results meet all corresponding index requirements at the same time.

[0045] Step 5: Conduct tolerance analysis and provide guidance accuracy for mirror processing and assembly.

[0046] Perform tolerance analysis on the space gravitational wave detection telescope obtained after processing in step 4. According to the actual capabilities of each link of optical processing, reasonably allocate the mirror processing tolerances such as the surface roughness and surface shape accuracy requirements of the primary mirror 2, the secondary mirror 3, the third mirror 4 and the fourth mirror 5, and reasonably allocate the adjustment tolerances of each link of the installation. Then use the Monte Carlo method to analyze the processing tolerances and adjustment tolerances of the primary mirror 2, the secondary mirror 3, the third mirror 4 and the fourth mirror 5 to ensure that the tolerances of the overall optical system are qualified, so that the yield rate of the space gravitational wave detection telescope meets the requirements, and complete the design of the entrance and exit coaxial space gravitational wave detection telescope based on free-form surfaces.

[0047] Taking the far-field parallel laser signal power of 1W as a reference example, the backward stray light energy suppression ratio of the existing off-axis four-transmission space gravitational wave detection telescope with non-coaxial entrance and exit pupils, two different structures of the existing off-axis five-transmission space gravitational wave detection telescope with coaxial entrance and exit pupils, and the space gravitational wave detection telescope with coaxial entrance and exit pupils based on free-form surface of the present invention were analyzed respectively under the same test environment. The analysis results are shown in Table 3.

[0048] Table 3 Analysis of backward stray light energy suppression ratio of four optical structures

[0049] It can be seen that the backward stray light energy suppression ratio of the free-form surface-based entrance-exit coaxial structure of the present invention is significantly better than the two different structures of the existing entrance-exit coaxial off-axis five-transmission space gravitational wave detection telescope. At the same time, the TTL coupling noise in the embodiment of the present invention is 3.07pm / μrad, which meets the detection practicality index requirement of 25pm / μrad, and the wave aberration in the captured field of view is λ / 126, which meets the far-field wavefront quality index requirement of wave aberration less than λ / 80, ensuring the high signal-to-noise ratio of the space gravitational wave detection telescope.

[0050] The above description is only used to illustrate the technical solution of the present invention rather than to limit it. For ordinary professional and technical personnel in the field, the specific technical solution recorded in the above embodiment can be modified, or some of the technical features therein can be replaced by equivalents, and these modifications or replacements do not make the essence of the corresponding technical solution deviate from the scope of the technical solution protected by the present invention.

Claims

1. A free-form surface-based entrance-exit coaxial space gravitational wave detection telescope, characterized by: It includes an aperture stop (1), a primary mirror (2), a secondary mirror (3), a third mirror (4), a fourth mirror (5), and an exit pupil (6); The primary mirror (2) is a parabolic reflector, the secondary mirror (3) is a hyperbolic reflector, the third mirror (4) is an ellipsoidal reflector, and the fourth mirror (5) is a free-form reflector; The aperture stop (1) and the primary mirror (2) are sequentially located on the far-field parallel laser incident light path, and the primary mirror (2) and the secondary mirror (3) form a Cassegrain structure; The three mirrors (4) are located on the laser light path reflected by the secondary mirror (3), the four mirrors (5) are located on the laser light path reflected by the three mirrors (4), and the exit pupil (6) is located on the laser light path reflected by the four mirrors (5); the three mirrors (4), the four mirrors (5) and the exit pupil (6) are all located on a side of the primary mirror (2) away from the aperture stop (1), and the four mirrors (5) and the exit pupil (6) are located on the central optical axis of the aperture stop (1); The incident angles of the three mirrors (4) and the four mirrors (5) are the same, both denoted as , Determined by the following formula: ； In the above formula, is the off-axis value of the primary mirror (2), is the length of the entire space gravitational wave telescope optical system, is the distance between the four mirrors (5) and the exit pupil (6), is the distance between the primary mirror (2) and the secondary mirror (3), is the distance between the secondary mirror (3) and the tertiary mirror (4); for The upper boundary value of for The lower boundary value of ; The primary mirror (2), the secondary mirror (3), the third mirror (4) and the fourth mirror (5) do not interfere with each other.

2. The free-form surface-based entrance-exit coaxial space gravitational wave detection telescope according to claim 1, characterized in that: The incident angles of the three mirrors (4) and the four mirrors (5) are is 45°.

3. The free-form surface-based entrance-exit coaxial space gravitational wave detection telescope according to claim 1 or 2, characterized in that: The four mirrors (5) are optimized for free-form surfaces using Zernike polynomials.

4. The free-form surface-based entrance-exit coaxial space gravitational wave detection telescope according to claim 3, characterized in that: The central optical axes of the aperture stop (1), the primary mirror (2), the secondary mirror (3), the third mirror (4), the fourth mirror (5) and the exit pupil (6) are located in the same vertical plane.

5. A design method for a free-form surface-based entrance-exit coaxial space gravitational wave detection telescope according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1, according to the entrance pupil diameter and geometric dimensions of the space gravitational wave detection telescope to be designed, determine the distance between the primary mirror (2) and the secondary mirror (3), and then build a Cassegrain structure in which the primary and secondary mirrors are coaxial; propose the position of the primary image plane of the space gravitational wave detection telescope to be designed, and de-axis the aperture according to the position of the primary image plane; after the aperture is de-axis, optimize the variable parameters of the Cassegrain structure to ensure that the image of the Cassegrain structure is located at the primary image plane, and optimize the image quality to approach the diffraction limit; Step 2, placing the three mirrors (4) on the laser light path reflected by the secondary mirror (3), so that the three mirrors (4) are located behind the primary image plane, and the reflecting surface is arranged in the direction of the primary mirror (2), so that the laser light reflected by the three mirrors (4) is emitted toward the side of the primary mirror (2) away from the aperture stop (1); placing the four mirrors (5) on the laser light path reflected by the three mirrors (4), and at the same time, the four mirrors (5) and the exit pupil (6) are located on the central optical axis of the aperture stop (1); Step 3, the surface shape of the four mirrors (5) is selected as the Zernike Fringe vector height, and the variable parameters of the optical system after adding the three mirrors (4) and the four mirrors (5) are optimized based on the incident wavelength λ, entrance pupil diameter, field angle, focal length and compression ratio of the space gravitational wave detection telescope to be designed, with wave aberration as the evaluation index; Step 4, performing wave aberration analysis, TTL coupling noise analysis and backward stray light analysis on the optical system optimized in step 3; If all analysis results meet the corresponding requirements, the variable parameter optimization of the optical system is completed and step 5 is executed; if any analysis result does not meet the corresponding requirements, the incident angle of the three mirrors (4) and the off-axis amount of the four mirrors (5) are adjusted, and the variable parameters of the current optical system are optimized again until all analysis results meet the corresponding requirements; Step 5, perform tolerance analysis on the optical system, allocate the mirror processing tolerance and installation tolerance of the primary mirror (2), the secondary mirror (3), the third mirror (4) and the fourth mirror (5); then use the Monte Carlo method to simulate and analyze the mirror processing tolerance and installation tolerance of the primary mirror (2), the secondary mirror (3), the third mirror (4) and the fourth mirror (5), to ensure that the overall optical system meets the qualification rate requirements, and complete the design of the free-form surface-based entrance and exit coaxial space gravitational wave detection telescope.

6. The design method of the free-form surface-based entrance-exit coaxial space gravitational wave detection telescope according to claim 5, characterized in that: In step 4, the result of the wave aberration analysis must satisfy: the wave aberration is better than λ / 80; The results of the TTL coupling noise analysis must meet the following requirements: TTL coupling noise is less than 25 pm / μrad; The result of the backward stray light analysis must meet the following requirements: the backward stray light energy suppression ratio reaches 10 -10 Magnitude.

7. The design method of the free-form surface-based entrance-exit coaxial space gravitational wave detection telescope according to claim 5 or 6, characterized in that: In step 1, the variable parameters of the Cassegrain structure include the radius of curvature of the primary mirror (2), the radius of curvature of the secondary mirror (3), and the conic coefficient of the secondary mirror (3); In step 3, the variable parameters of the optical system after adding the three mirrors (4) and the four mirrors (5) include the radius of curvature of the primary mirror (2), the radius of curvature of the secondary mirror (3), the cone coefficient of the secondary mirror (3), the radius of curvature of the three mirrors (4), the cone coefficient of the three mirrors (4), the radius of curvature of the four mirrors (5), the cone coefficient of the four mirrors (5) and the Zernike polynomial aspheric coefficient of the four mirrors (5).

Images