High contrast polarimetric imaging system for direct imaging of exoplanets

Abstract

The application provides a high-contrast polarization imaging system for direct imaging of exoplanets, which comprises a coronagraph system, a relay lens, a polarization modulation assembly based on phase modulation, an imaging lens and a detector. A polarimeter is introduced on the basis of the coronagraph system, a liquid crystal phase retarder is used as a modulation device, different intensity values are obtained by applying different voltages, and finally the Stoeks parameter is inverted. In view of the fact that the planet light has certain polarization characteristics and the star light has no polarization characteristics, the planet light can be distinguished from the star light after passing through the high-contrast polarization imaging system, and high-contrast polarization imaging is realized. Meanwhile, the system introduces a Wollaston prism to form a double light path, so as to reduce the influence of atmospheric turbulence, and meanwhile, additional contrast gain can be obtained, and an imaging contrast gain of about 100 times can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of exoplanet detection, and in particular to a high-contrast polarization imaging system for direct imaging of exoplanets based on a liquid crystal phase retarder. Background Art

[0002] Exoplanet detection primarily involves indirect detection techniques and direct imaging. Indirect detection relies on phenomena generated by the interaction between planets and stars. Direct imaging captures photon information from planets, allowing for spectral observations and analysis of their physical structure and parameters. Due to the significant contrast between stars and planets, planets are often drowned out by the telescope's diffraction photon noise, making direct exoplanet imaging extremely challenging. A common approach is to use a coronagraph to suppress this diffraction photon noise.

[0003] However, imperfect surfaces in telescope and terminal instrument optical systems introduce speckle noise, which drowns out planetary light and prevents direct imaging of mature, cold planets. Stellar radiation is non-polarized and its vibration direction is random. However, light reflected from a planetary atmosphere exhibits specific polarization properties due to its interaction with atmospheric molecules. This difference in polarization between starlight and planetary light allows for the extraction of the planet's polarization signal by introducing a polarization measurement system, thereby eliminating the impact of speckle noise on contrast. Traditional coronagraph systems address speckle noise by introducing multiple deformable mirrors, resulting in high system cost, complex structure, and complex control systems. Summary of the Invention

[0004] The present invention aims to provide a high-contrast polarization imaging system for direct imaging of exoplanets. A polarization measurement system is introduced based on a coronagraph system. A liquid crystal phase retarder (LCVR) is used as a modem. Different intensity values ​​are obtained by applying different voltages. Finally, the Stoeks parameter is inverted to achieve polarization measurement of planetary polarization signals and high-contrast polarization imaging.

[0005] According to a first aspect of the present invention, a high-contrast polarization imaging system for direct imaging of exoplanets is provided, comprising a coronagraph system, a relay mirror, a polarization modulation component based on phase modulation, an imaging mirror, and a detector, wherein:

[0006] The coronagraph system is used to receive starlight from the astronomical telescope and suppress it before outputting it;

[0007] The relay mirror is located behind the focal plane of the coronagraph system and is used to collimate the starlight output by the coronagraph system.

[0008] A phase modulation based polarization modulation component is located at the back focal plane of the relay lens, which is used to modulate the collimated starlight and demodulate the polarization, and then the starlight is imaged on the detector through the imaging lens.

[0009] The phase modulation based polarization modulation component comprises a liquid crystal phase retarder located at the back focal plane of the relay lens and a Wollaston prism located behind the liquid crystal phase retarder; the liquid crystal phase retarder comprises a first liquid crystal phase retarder and a second liquid crystal phase retarder arranged along the optical axis in sequence.

[0010] The starlight collimated by the relay lens is modulated by the first liquid crystal phase retarder and the second liquid crystal phase retarder in sequence, and then the double beams are generated by the Wollaston prism to demodulate the polarization, and finally the light beams are imaged on the detector through the imaging lens.

[0011] As an optional embodiment, the fast axis direction of the first liquid crystal phase retarder is located at a position perpendicular to the horizontal plane, and the fast axis of the second liquid crystal phase retarder is arranged at an angle of 45 degrees with the fast axis of the first liquid crystal phase retarder.

[0012] As an optional embodiment, the first vibration transmission direction of the Wollaston prism is consistent with the fast axis direction of the first liquid crystal phase retarder, and the second vibration transmission direction is orthogonal to the first vibration transmission direction.

[0013] As an optional embodiment, the first liquid crystal phase retarder and the second liquid crystal phase retarder are applied with four different voltages to obtain different phase retardation amounts, so as to modulate the polarization characteristics of the light beams, and then the polarization is demodulated through the Wollaston prism, four intensity images are obtained through the detector in four different modulation modes, four Stokes components are calculated through the four images, and finally the polarization image is obtained.

[0014] Therefore, by arranging the two liquid crystal phase retarders LCVR at an angle of 45 degrees, different voltages are applied to realize different phase retardation amounts. Four different voltages are applied to obtain four images, and finally the polarization image of the full Stokes parameter is obtained, so as to realize high-contrast polarization imaging.

[0015] As an optional embodiment, the coronagraph system adopts a phase introduction type coronagraph system based on liquid crystal spatial light modulation, which comprises a collimator, an aperture stop, a second imaging lens, a polarizer and a liquid crystal spatial light modulator arranged in sequence along the entrance pupil light path; a second relay lens, a Lyot diaphragm and a converging lens arranged in sequence along the exit pupil light path.

[0016] The collimator is used to collimate the light from the telescope, and the aperture stop is used to limit the aperture of the entrance pupil beam.

[0017] The light from the telescope is collimated by the collimating mirror and limited by the aperture diaphragm, and then imaged at a focal plane position via the second imaging mirror and the polarizer, the liquid crystal spatial light modulator is arranged at an angle with the first optical axis of the entrance pupil light path and located at the focal plane position;

[0018] The liquid crystal spatial light modulator is arranged to generate a gray value image according to a preset topological number, to phase modulate the imaging on the liquid crystal spatial light modulator, and generate vortex light;

[0019] The vortex light is collimated by the second relay mirror arranged in the exit pupil light path, then the light in the periphery of the vortex light is shielded by the Lyot diaphragm, and finally projected to the first relay mirror through the converging mirror.

[0020] Compared with the prior art, the significant beneficial effects of the high-contrast polarization imaging system for direct imaging of exoplanets of the present application are that:

[0021] In view of the polarization characteristics of the planet light and the non-polarization characteristics of the star light, the high-contrast polarization imaging system proposed by the present application aims at the speckle noise of the coronagraph system, realizes polarization measurement of the planet polarization signal by introducing a liquid crystal phase retarder LCVR as the core element of the polarimeter, obtains additional gain of imaging contrast, can distinguish the planet light from the star light, effectively suppresses the speckle noise of the system, and can realize two orders of magnitude of contrast gain.

[0022] By adopting the liquid crystal phase retarder as the polarization modulation element, the present application can realize fast modulation, has no mechanical moving parts, avoids light jitter, and can realize high-precision polarization measurement; in order to solve the crosstalk problem caused by atmospheric disturbance, the present application introduces a Wollaston prism to form a double light path, instead of a traditional single beam polarization element, so that polarization separation perpendicular to each other can be measured at the same time, changes caused by atmospheric disturbance are eliminated by polarization subtraction, and the influence of atmospheric turbulence is reduced.

[0023] It should be understood that all combinations of the foregoing concepts and additional concepts described in greater detail below can be seen as being part of the inventive subject matter of the present disclosure unless otherwise stated or otherwise evident in context. Additionally, all combinations of claimed subject matter can be seen as being part of the inventive subject matter of the present disclosure.

[0024] The foregoing and other aspects, embodiments and features of the present teachings can be better understood and appreciated from the following description, taken together with the accompanying drawings. Other additional aspects, embodiments and features of the present teachings will be apparent from the following description. BRIEF DESCRIPTION OF DRAWINGS

[0025] The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component shown in various figures may be represented by the same reference numeral. For the sake of clarity, not every component is labeled in every figure. Embodiments of various aspects of the present invention will now be described by way of example and with reference to the accompanying drawings, in which:

[0026] Figure 1 Schematic diagram of a high-contrast polarization imaging system for direct imaging of exoplanets according to an embodiment of the present invention.

[0027] Figure 2 Schematic diagram of a coronagraph system according to an embodiment of the present invention.

[0028] Figure 3 is an example of a grayscale image applied to a liquid crystal spatial light modulator.

[0029] Figure 4 It is the intensity diagram of the exit pupil position after the starlight is phase modulated. DETAILED DESCRIPTION

[0030] In order to better understand the technical content of the present invention, specific embodiments are given and described below with reference to the accompanying drawings.

[0031] Various aspects of the present invention are described in this disclosure with reference to the accompanying drawings, in which a number of illustrative embodiments are shown. The embodiments of the present disclosure are not necessarily intended to include all aspects of the present invention. It should be understood that the various concepts and embodiments introduced above, as well as those described in more detail below, can be implemented in any of many ways, because the concepts and embodiments disclosed herein are not limited to any embodiment. In addition, some aspects of the present disclosure may be used alone or in any appropriate combination with other aspects disclosed herein.

[0032] This invention is one of the results of the National Natural Science Foundation of China's general project "Research on Key Technologies of Snapshot Linear Stokes Parameter Polarization Spectrometer" (12073056).

[0033] High contrast polarization imaging system

[0034] A high-contrast polarization imaging system for direct exoplanet imaging according to an embodiment of the present invention is used for exoplanet detection through direct imaging. The high-contrast polarization imaging system employs a high-precision polarimeter based on a liquid crystal phase retarder as a technical means of improving the contrast of direct exoplanet imaging. Specifically, a polarization measurement system is introduced after the coronagraph system. The polarization measurement system includes a liquid crystal phase retarder and a Wollaston prism. After modulation by the liquid crystal phase retarder (LCVR) and demodulation by the Wollaston prism, the planetary light is extracted from stellar diffraction photon noise and speckle noise. This system has the advantages of having no mechanical moving parts, a simple structure, low cost, and the ability to achieve additional imaging contrast gain.

[0035] The high-contrast polarization imaging system for direct exoplanet imaging in embodiments of the present invention incorporates high-precision polarization measurement technology into a coronagraph system. Using liquid crystal phase retarders, it avoids the traditional use of rotating wave plates, eliminating mechanical moving parts and complex control systems. Polarization subtraction technology eliminates the effects of atmospheric turbulence, improving imaging contrast and creating a compact system that reduces development cycles and costs. In some embodiments, the aforementioned polarization measurement system can also be applied to other research areas in astronomical optical imaging.

[0036] Combine Figure 1 The high-contrast polarization imaging system for direct imaging of exoplanets in the illustrated embodiment includes a coronagraph system 20 , a first relay mirror 30 , a phase-modulation-based polarization modulation assembly 40 , a first imaging mirror 50 , and a detector 60 .

[0037] The coronagraph system 20 is used to receive starlight from the astronomical telescope and suppress it before outputting it. Figure 1 As shown, reference numeral 10 denotes the incident end face of the skylight telescope.

[0038] The first relay mirror 30 is located behind the focal plane of the coronagraph system 20 and is used to collimate the starlight output by the coronagraph system 20 .

[0039] The phase-modulated polarization modulation component 40 is located at the rear focal plane of the relay mirror 12 and is used to perform polarization modulation and polarization demodulation on the collimated starlight, and then image the starlight onto the detector 60 through the first imaging mirror 50.

[0040] Therefore, the light from the telescope suppresses the star light after passing through the coronagraph system 20 to obtain a certain contrast. The remaining light is collimated by the first relay lens 30, and then polarization modulated and demodulated by the polarization modulation component 40, and the light beam is imaged on the detector through the first imaging mirror 50.

[0041] As a preferred embodiment, the phase-modulation-based polarization modulation assembly 40 includes a liquid crystal phase retarder assembly 41 located at the rear focal plane of the relay lens 12 and a Wollaston prism 42 located behind the liquid crystal phase retarder assembly 41. The liquid crystal phase retarder assembly 41 adopts a dual LCVR (Liquid Crystal Variable Retarder) design, including a first liquid crystal phase retarder 41A and a second liquid crystal phase retarder 41B arranged sequentially along the optical axis.

[0042] The starlight collimated by the first relay lens 30 passes through the first liquid crystal phase retarder 41A and the second liquid crystal phase retarder 41B of the liquid crystal phase delay assembly 41 in sequence for polarization modulation, then passes through the Wollaston prism 42 to generate a dual light beam for polarization demodulation, and finally passes through the first imaging mirror 50 to image the light beam onto the detector 60.

[0043] Therefore, after the starlight passes through the coronagraph system 20, the star diffraction photon noise from the telescope is suppressed, and the imaging contrast can reach 10 -6 ~10 -7 The suppressed starlight is then collimated by a relay mirror, sequentially passed through two liquid crystal phase retarders for polarization modulation, and then passed through a Wollaston prism to generate a dual beam for polarization demodulation. Finally, the starlight is imaged on the detector through an imaging mirror, achieving high-contrast polarization imaging.

[0044] Combine Figure 1 As shown, to address the drawback of speckle noise caused by the system's imperfect optical surface, which suppresses astronomical imaging contrast, embodiments of the present invention aim to provide a high-contrast polarization imaging system based on the polarization characteristics of planetary light, thereby increasing additional contrast gain. Specifically, a polarization modulation component is formed using a liquid crystal phase retarder and a Wollaston prism. By applying different voltages to the liquid crystal phase retarder, different phase delays are obtained, thereby modulating the polarization characteristics of the light beam. The light beam is then demodulated through the Wollaston prism to ultimately obtain the four Stokes polarization parameters (I, Q, U, and V). The liquid crystal phase retarder achieves phase delay of light by controlling the refractive index of the liquid crystal molecules via voltage.

[0045] Therefore, using a liquid crystal phase retarder as a polarization modulator, compared to traditional methods using rotating wave plates, eliminates the need for mechanical moving parts, thus avoiding the impact of beam deviation caused by mechanical motion on measurement, resulting in a simpler system. While achieving contrast gain, astronomical polarization high-contrast imaging can be performed over a full 360° working area, significantly improving detection efficiency.

[0046] To address the issue of atmospheric turbulence affecting ground-based astronomical imaging instruments, an embodiment of the present invention proposes a high-contrast polarization imaging system that includes both polarization state modulation and demodulation via a Wollaston prism. In addition to increasing the instrument's sampling speed in the time domain, the system also utilizes the characteristics of the Wollaston prism to generate dual beams in two mutually perpendicular directions. Polarization subtraction technology effectively suppresses the effects of atmospheric turbulence while also providing additional contrast gain, potentially achieving an imaging contrast gain of ~100 times. Compared to existing solutions that introduce deformable mirrors, the polarization imaging system of the present invention offers lower cost, a simpler system, and no mechanical moving parts, saving R&D costs and time, making it suitable for direct imaging detection of exoplanets.

[0047] In an optional embodiment, the liquid crystal phase retarder may adopt a liquid crystal controlled variable phase retarder, especially a liquid crystal variable phase retarder that can achieve sub-millisecond speed, which is suitable for use in ground-based astronomical observation equipment.

[0048] In an optional embodiment, the aforementioned liquid crystal phase retarder may use, for example, a high-speed liquid crystal variable phase retarder produced by Meadowlark, or a full-wave liquid crystal variable retarder produced by Thorlabs.

[0049] In an optional embodiment, the fast axis of the first liquid crystal retarder 41A is located perpendicular to the horizontal plane, and the fast axis of the second liquid crystal retarder 41B is arranged at 45° to the fast axis of the first liquid crystal retarder 41A.

[0050] The first transmission direction of the Wollaston prism 42 is consistent with the fast axis direction of the first liquid crystal retarder 41A, and the second transmission direction is orthogonal to the first transmission direction.

[0051] In the embodiment of the present invention, the substrate of the Wollaston prism 42 is quartz, the beam separation angle is 1°, and the extinction ratio is greater than 10000: 1. The transmission directions of the Wollaston prism 42 are strictly perpendicular and parallel to the horizontal plane.

[0052] Combine Figure 1 In the illustrated embodiment, the first relay lens 30 and the first imaging lens 50 each include at least one convex lens, and the focal lengths of the convex lenses are the same.

[0053] The detector 60 may be a high-speed sampling CMOS imaging detector or a CCD imaging detector, and is disposed at the rear focal plane of the first imaging mirror 50 .

[0054] In an optional embodiment, the coronagraph system 20 adopts a high-contrast coronagraph system, for example, a coronagraph system based on pupil transmittance modulation, finite-zone modulation, modulation-band transmittance and film thickness control, phase modulation, etc., and after suppressing the stellar diffraction photon noise, the imaging contrast can reach 10 -6 ~10 -7 .

[0055] In a preferred embodiment, the present invention proposes a coronagraph system based on liquid crystal spatial light phase modulation, such as Figure 2 As shown, a liquid crystal spatial light modulator is used to generate a grayscale image at the focal plane position, and phase modulates the imaging at the focal plane. The phase-modulated light is further passed through a Lyot aperture to suppress axial starlight, thereby achieving high-contrast astronomical imaging.

[0056] Coronagraph system

[0057] Combine Figure 2 The coronagraph system 20 of the illustrated embodiment includes: a collimator 21, an aperture stop 22, a second imaging mirror 23, a polarizer 24, and a liquid crystal spatial light modulator 25 arranged in sequence along the entrance pupil optical path; and a second relay mirror 26, a Lyot stop 27, and a converging mirror 28 arranged in sequence along the exit pupil optical path.

[0058] exist Figure 2 In FIG. 1 , O1 represents the first optical axis of the entrance pupil optical path, and the collimator 21 , the aperture stop 22 , the second imaging mirror 23 , and the polarizer 24 are arranged coaxially.

[0059] The second optical axis of the exit pupil optical path is represented by O2, and the second relay lens 26, the Lyot stop 27, and the focusing lens 28 are arranged coaxially.

[0060] exist Figure 2 In the example shown in FIG, the collimator lens 21 is used to collimate the light from the telescope.

[0061] In an optional embodiment, the collimating lens 21 may be formed by at least one convex lens, such as a double convex lens or a plano-convex lens.

[0062] In other embodiments, the collimating lens 21 may also adopt other structures, such as a combination of at least one convex lens and at least one concave lens.

[0063] The aperture stop 22 is used to limit the aperture of the entrance pupil light beam.

[0064] The second imaging mirror 23 is located behind the aperture stop 22 and is set at the rear focal plane of the collimating mirror 21 .

[0065] The polarizer 24 is located behind the second imaging mirror 23 and is used to generate linearly polarized light. As an optional example, the extinction ratio of the polarizer 24 is greater than 10000:1. As a result, the imaging on the liquid crystal spatial light modulator 25 enters a pure phase modulation mode.

[0066] Combine Figure 2 As shown, the light from the telescope is collimated by the collimator 21 and limited by the aperture stop 22, and then imaged at the focal plane position through the second imaging mirror 23 and the polarizer 24. The liquid crystal spatial light modulator 25 is set at a certain angle to the first optical axis O1 of the entrance pupil light path and is located at the focal plane position.

[0067] The liquid crystal spatial light modulator 25 is configured to generate a grayscale value image according to a preset topological number, so as to perform phase modulation on the imaging on the liquid crystal spatial light modulator 25 and generate vortex light; the vortex light is collimated by the second relay lens 26 arranged in the exit pupil light path, and then the light outside the vortex light is blocked by the Lyot aperture 27, and finally converged by the converging lens 28 and projected onto the first relay lens 30.

[0068] The converging mirror 28 may be an imaging lens assembly composed of at least one convex lens, and is used to image the remaining star and planetary light after suppressing the axial star light and stray light.

[0069] As an optional embodiment, the intersection of the first optical axis O1 and the focal plane is used as the main focus of the coronagraph system, wherein a pure phase modulation is generated by applying a grayscale image to the liquid crystal spatial light modulator, and the center of the phase image is aligned with the main focus of the coronagraph system.

[0070] The intersection of the second optical axis O2 of the exit pupil optical path and the first optical axis O1 of the entrance pupil optical path coincides with the main focus of the coronagraph system.

[0071] As an optional embodiment, the angle between the second optical axis O2 of the exit pupil optical path and the first optical axis O1 of the entrance pupil optical path ranges from 0° to 7°.

[0072] As an optional embodiment, the angle between the second optical axis O2 of the exit pupil optical path and the liquid crystal spatial light modulator 20 is in the range of 86.5-90°.

[0073] The angle between the intersection of the first optical axis O1 of the entrance pupil optical path and the liquid crystal spatial light modulator 20 is in the range of 86.5-90°.

[0074] Combine Figure 1 As shown, the liquid crystal spatial light modulator 25 is used as a phase modulator to perform phase modulation on the imaging by generating a grayscale image at the focal plane position, as shown in FIG. Figure 3An example of a grayscale image with a topological number l of 2 is shown. Based on the interference destructiveness of the vortex light itself at the phase singularity, the on-axis light is pushed to the periphery of the pupil, so that most of the energy at the exit pupil position is distributed at the edge position, and the light around the pupil is suppressed by the Lyot aperture 27 set at the exit pupil position to achieve the suppression of on-axis starlight. Finally, the remaining starlight and planetary light are projected to the first relay mirror 30 via the converging mirror 23.

[0075] Combine Figure 1 、 2 In the example shown, starlight (stars, planets) from an astronomical telescope is first collimated into parallel light, then limited by an aperture diaphragm and imaged at the focal plane. At the same time, the grayscale image generated by the liquid crystal spatial light modulator 25 is phase modulated at the focal plane. The modulated starlight is collimated into parallel light again by a relay lens, and then a Lyot diaphragm is introduced at the exit pupil position to suppress on-axis starlight and system stray light. Finally, the starlight is projected onto the first relay lens 30 through an imaging lens.

[0076] As an alternative example, the liquid crystal spatial light modulator 25 can be an XY spatial light modulator manufactured by Meadowlark. This product can achieve a phase modulation of up to 6π, a light energy efficiency of 95%, and can implement pure phase modulation, pure amplitude modulation, and mixed phase-amplitude modulation. This allows for a full 360° working area while maintaining high planetary transmittance, enabling large-area, high-transmittance, high-contrast direct imaging of exoplanets.

[0077] Therefore, the phase-introduction type coronagraph system of the above embodiment of the present invention adopts phase modulation technology. At the focal position of the coronagraph system, the phase is modulated according to the characteristics of the vortex light, and then after passing through the Lyot aperture, the axial starlight is suppressed. This phase modulation method is different from the traditional transmittance modulation, and its advantage is that it can obtain a higher planetary transmittance. By introducing a liquid crystal spatial light modulator at the focal plane position of the system and utilizing the characteristics of vortex light interference decomposition, high-contrast imaging is achieved. By using a commercial liquid crystal spatial light modulator as a key optical element, the R&D cycle and cost can be greatly reduced, and the application of liquid crystal optical devices in the field of astronomical optics can be expanded.

[0078] Conventional transmittance-modulated coronagraph systems, while suppressing starlight, also reduce the transmittance of planetary light, thus requiring longer exposure times for observation, reducing detection efficiency. In an embodiment of the present invention, phase modulation of a liquid crystal spatial light modulator is used to suppress on-axis starlight without reducing the transmittance of planetary light, achieving an imaging contrast of 10 -6 , which can be used for direct imaging observation of exoplanets and can solve the problem of low planetary transmittance in traditional transmittance modulation coronagraphs.

[0079] In the embodiment of the present invention, different contrast values ​​can also be obtained by adjusting the topological number applied by the liquid crystal spatial light modulator 25 to adjust the grayscale images corresponding to different grayscale values.

[0080] In an embodiment of the present invention, the transmittance can also be adjusted by adjusting the aperture size of the Lyot diaphragm 27, wherein the smaller the aperture of the Lyot diaphragm 27, the smaller the transmittance and the higher the contrast obtained.

[0081] In some embodiments, the phase-introduction type coronagraph system of the aforementioned embodiment of the present invention, in addition to introducing the entire liquid crystal spatial light modulator 25 at the focal plane position for phase modulation, also performs modulation at the exit pupil position. According to the requirements of contrast and transmittance, the aperture size of the Lyot diaphragm can be optimized, for example, by adjusting the aperture size of the Lyot diaphragm 27 to adjust the transmittance, wherein the smaller the aperture of the Lyot diaphragm 27, the smaller the transmittance and the higher the contrast obtained.

[0082] Therefore, under specific detection requirements, the ideal energy distribution can be obtained at the exit pupil position by reasonably selecting and designing the aperture size of the Lyot diaphragm, for example Figure 4 The intensity image at the exit pupil position of the example shown shows that most of the energy is concentrated at the pupil edge. This indicates that the Lyot stop in the exit pupil optical path blocks the surrounding starlight and simultaneously suppresses the system's stray light.

[0083] In an optional embodiment, the focal lengths of the collimating lens 21 , the second imaging lens 23 , the second relay lens 26 , and the converging lens 28 are equal, i.e., 400 mm.

[0084] Working principle of the coronagraph system

[0085] Step 1: pre-set the topological number l according to the specific needs of high-contrast imaging. In this embodiment, l=2 is used as an example for description.

[0086] Step 2: Optimize the size of the Lyot aperture to match the topological number l, and set the optimal aperture size and topological number l. For example, the aperture size of the Lyot aperture is 0.8 times the aperture size of the aperture aperture.

[0087] Step 3. Build a coronagraph system according to the design requirements. Starlight enters the system and passes through a collimator (f = 400 mm), an aperture stop, a second imaging mirror (f = 400 mm), and a polarizer (extinction ratio > 10000:1) in sequence before being imaged on a liquid crystal spatial light modulator located at the focal plane. Simultaneously, the liquid crystal spatial light modulator located at the focal plane applies a grayscale image corresponding to the topological number l = 2 to perform phase modulation on the image, generating vortex light after phase modulation.

[0088] Step 4: After being collimated by the second relay lens (f=400mm), the light at the edge of the pupil is blocked by the Lyot aperture 22 arranged at the exit pupil position, and finally the remaining stellar and planetary light is projected to the first relay lens 30 through the converging lens (f=400mm).

[0089] As an optional example, in the aforementioned step 3, the liquid crystal spatial light modulator is controlled by MATLAB software. According to the relationship between the grayscale value and the phase delay amount of the liquid crystal spatial light modulator, MATLAB generates a grayscale value image with a topological number l = 2, and then applies it to the spatial light modulator through MATLAB software.

[0090] The coronagraph system proposed in the present invention uses phase modulation technology to modulate the phase at the focal point of the coronagraph system according to the characteristics of the vortex light. After passing through the Lyot aperture, the axial starlight is suppressed. This phase modulation method is different from traditional transmittance modulation and has the advantage of being able to achieve higher planetary transmittance. By introducing a liquid crystal spatial light modulator at the focal plane of the system and utilizing the destructive interference characteristics of the vortex light, high-contrast imaging is achieved. By using a commercial liquid crystal spatial light modulator as a key optical component, the R&D cycle and cost can be greatly reduced, and the application of liquid crystal optical devices in the field of astronomical optics can be expanded.

[0091] How high-contrast polarization imaging works

[0092] Combine Figure 1 In the example shown, four different sets of voltages are applied to the first liquid crystal phase retarder 41A and the second liquid crystal phase retarder 41B to obtain different phase delay amounts, thereby modulating the polarization characteristics of the light beam, and then performing polarization demodulation through a Wollaston prism. Four intensity images are collected by the detector 60 under four different modulation modes, and four Stokes components are calculated from the four images to finally obtain a polarization image.

[0093] In a preferred embodiment, by calibrating and testing the phase retardation of the liquid crystal retarder, four different phase retardation values ​​corresponding to the first liquid crystal retarder 41A are obtained as follows:

[0094] [p1 p2 p3 p4] = [315 315 225 225]

[0095] The four different phase retardation amounts corresponding to the second liquid crystal phase retarder 41B are as follows:

[0096] [σ1 σ2 σ3 σ4] = [305.264 54.736 125.264 234.736]

[0097] wherein p1, p2, p3, p4 represent the optimal phase retardation amounts of the first liquid crystal phase retarder 41A respectively, so as to make the response time of the first liquid crystal phase retarder 41A shortest.

[0098] σ1, σ2, σ3, σ4 represent the optimal phase retardation amounts of the second liquid crystal phase retarder 41B respectively, so as to make the response time of the second liquid crystal phase retarder 41B shortest.

[0099] Thus, the phase retardation amount corresponding voltage value can be further obtained, for example, the voltage value corresponding to the four different phase retardation amounts of each liquid crystal phase retarder is obtained by an interpolation method.

[0100] In the following, the embodiment of the high-contrast polarization imaging system of the present application is described in combination with specific examples.

[0101] Step 1, according to the contrast measurement and polarization modulation efficiency measurement requirements, the optimal design selection is performed, a polarimeter based on two LCVRs and a Wollaston prism is designed behind the coronagraph system 20, and the fast axis direction and phase retardation amount combination of the LCVR are optimized.

[0102] Different voltages are applied to the liquid crystal phase retarder to obtain different phase retardations, and according to the differences of the Mueller matrices of each element, the relationship between the output Stokes vector and the input Stokes vector can be calculated:

[0103] s out = M p M2M1

[0104] wherein M p is the Mueller matrix of a vibration transmission direction of the Wollaston prism, and M1 and M2 are the Mueller matrices of the first liquid crystal phase retarder 41A and the second liquid crystal phase retarder 41B respectively.

[0105] According to the fast axis azimuth angle of the liquid crystal phase retarder, it is brought into the Mueller matrix, and after simplification, the following is obtained:

[0106]

[0107] wherein p and σ represent the phase retardation amounts of the two LCVRs respectively.

[0108] To obtain the highest system efficiency, in the embodiments of the present application, the phase combination is selected by trial and selection so that the response time of the LCVR is the shortest,

[0109] [ρ1 ρ2 ρ3 ρ4]=[315 315 225 225]

[0110] [σ1 σ2 σ3 σ4]=[305.264 54.736 125.264 234.736]

[0111] According to the above phase combination, the values of the four Stokes components can be obtained:

[0112]

[0113] The above is the solving method in one direction of the Wollaston prism, and the solving method in the other direction is the same, so that the four Stokes polarization parameters can be obtained by controlling the phase retardation of the liquid crystal phase retarder.

[0114] Step 2, calibrate the phase retardation of the liquid crystal phase retarder.

[0115] Since the LCVR is greatly affected by external conditions such as temperature, in actual application, the corresponding relationship (LUT) between the phase retardation and the voltage under the current environment needs to be measured, the fast axis of the LCVR is at an angle of 45° with the transmission direction of the polarizer, and the intensity curves when the two polarizers are perpendicular and parallel are measured respectively,

[0116] By calculating the ratio of the intensities in the two directions, the phase retardation corresponding to the applied voltage is finally determined, and then the voltage values corresponding to the four sets of phase retardation [315 315 225 225], [305.264 54.736 125.264 234.736] are obtained by the method of interpolation.

[0117] Step 3, the starlight passes through the coronagraph subsystem 11, and after being suppressed, a high-contrast image is obtained -6 The starlight passes through the coronagraph system to obtain a high-contrast image, and then collimates through a first relay mirror 30 (focal length f = 400 mm). The collimated light passes through a liquid crystal variable retardation assembly 41 (two LCVRs combined), for example, a full-wave liquid crystal variable retarder (model LCC1223-A) produced by Thorlabs Company. The fast axis direction of the first LCVR is placed perpendicular to the horizontal plane, the fast axis of the second LCVR is at an angle of 45° with the fast axis of the first LCVR, one transmission direction of the Wollaston prism is consistent with the fast axis direction of the first LCVR, the base of the Wollaston prism is quartz, the beam separation angle is 1°, and the extinction ratio is >10000:1.

[0118] The two LCVRs are precisely arranged and installed in sequence, following the determined fast axis orientation. The Wollaston prism is placed on a precision mechanical adjustment mechanism, such as a precision six-axis adjustment mechanism, with both transmission directions of the Wollaston prism strictly perpendicular and parallel to the horizontal plane. The detector 60 is introduced, and the Wollaston prism is rotated so that the two image spots are in the same horizontal direction on the camera target surface. Finally, the image is formed on the detector through the first imaging mirror 50 (focal length f = 400 mm).

[0119] Step 4: An external control system applies four different voltages to the liquid crystal variable delay element 41. Four intensity images are collected under four different modulation modes. The Stokes components are calculated from these four images, ultimately obtaining a polarization image of the point spread function. This allows the polarized planetary light to be clearly visible, distinguishing it from speckle noise and residual telescope diffraction photon noise.

[0120] In summary, the present invention addresses the problem of astronomical polarimetric imaging and proposes a high-contrast polarimetric imaging system. This system has no mechanical moving parts, and the entire system does not require complex auxiliary equipment such as motors. The system is simple, and the LCVR modulation rate is high, which can achieve high-contrast polarimetric imaging over the entire focal plane, effectively solving the problem of static speckle noise in the system affecting direct imaging. Furthermore, the system combines spatial and temporal modulation, replacing traditional linear polarizers with Wollaston prisms, effectively solving the problem of atmospheric seeing affecting measurements.

[0121] While the present invention has been disclosed above with reference to preferred embodiments, this is not intended to limit the present invention. Persons skilled in the art will readily appreciate that various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the claims.

Claims

1. A high-contrast polarization imaging system for direct imaging of exoplanets, characterized in that: The invention comprises a coronagraph system (20), a relay mirror (30), a polarization modulation component (40) based on phase modulation, a first imaging mirror (50), and a detector (60), wherein: A coronagraph system (20) is used to receive starlight from an astronomical telescope and suppress it before outputting it; A relay mirror (30), located behind the focal plane of the coronagraph system (20), for collimating the starlight output by the coronagraph system (20); A phase-modulated polarization modulation component (40) is located at the rear focal plane of the relay mirror (12) and is used to perform polarization modulation and polarization demodulation on the collimated starlight, and then image the starlight onto the detector (60) through the first imaging mirror (50); The phase modulation-based polarization modulation component (40) comprises a liquid crystal phase delay component (41) located at the rear focal plane of the relay lens (12) and a Wollaston prism (42) located behind the liquid crystal phase delay component (41); the liquid crystal phase delay component (41) comprises a first liquid crystal phase retarder (41A) and a second liquid crystal phase retarder (41B) sequentially arranged along the optical axis; The starlight collimated by the relay mirror (30) sequentially passes through the first liquid crystal phase retarder (41A) and the second liquid crystal phase retarder (41B) of the liquid crystal phase delay component (41) for polarization modulation, then passes through the Wollaston prism (42) to generate a double light beam for polarization demodulation, and finally images the light beam onto the detector (60) via the first imaging mirror (50); The fast axis of the first liquid crystal phase retarder (41A) is located at a position perpendicular to the horizontal plane, and the fast axis of the second liquid crystal phase retarder (41B) is arranged at 45 degrees to the fast axis of the first liquid crystal phase retarder (41A); The first vibration transmission direction of the Wollaston prism (42) is consistent with the fast axis direction of the first liquid crystal phase retarder (41A), and the second vibration transmission direction is orthogonal to the first vibration transmission direction; Four different sets of voltages are applied to the first liquid crystal phase retarder (41A) and the second liquid crystal phase retarder (41B) to obtain different phase delay amounts, thereby modulating the polarization characteristics of the light beam, and then performing polarization demodulation through a Wollaston prism. Four intensity images are collected through the detector (60) in four different modulation modes, and four Stokes components are calculated from the four images to finally obtain a polarization image. The four different phase retardation amounts corresponding to the first liquid crystal phase retarder (41A) are as follows: ； The four different phase retardation amounts corresponding to the second liquid crystal phase retarder (41B) are as follows: ； in, , respectively represent the optimal phase delay amount of the first liquid crystal phase retarder (41A), so that the response time of the first liquid crystal phase retarder (41A) is the shortest; in, , respectively represent the optimal phase delay amount of the second liquid crystal phase retarder (41B), so that the response time of the second liquid crystal phase retarder (41B) is the shortest.

2. The high-contrast polarization imaging system for direct imaging of exoplanets according to claim 1, characterized in that: The substrate of the Wollaston prism (42) is quartz, the beam separation angle is 1°, and the extinction ratio is greater than 10000:

1.

3. The high-contrast polarization imaging system for direct imaging of exoplanets according to claim 1, wherein: The vibration transmission directions of the Wollaston prism (42) are strictly perpendicular and parallel to the horizontal plane respectively.

4. The high-contrast polarization imaging system for direct imaging of exoplanets according to claim 1, wherein: The relay mirror (30) and the first imaging mirror (50) each include at least one convex lens, and the focal lengths of the convex lenses are the same.

5. The high-contrast polarization imaging system for direct imaging of exoplanets according to claim 1, wherein: The detector (60) is a CMOS imaging detector or a CCD imaging detector, and is arranged at the rear focal plane position of the first imaging mirror (50).

6. The high-contrast polarization imaging system for direct imaging of exoplanets according to claim 1, wherein: The coronagraph system adopts a phase-introduction coronagraph system based on liquid crystal spatial light modulation, including: a collimator lens, an aperture stop, a second imaging lens, a polarizer, and a liquid crystal spatial light modulator arranged in sequence along the entrance pupil optical path; a second relay lens, a Lyot stop, and a converging lens arranged in sequence along the exit pupil optical path; The collimator is used to collimate the light from the telescope, and the aperture stop is used to limit the aperture of the entrance pupil light beam; Light from the telescope is collimated by the collimator lens and limited by the aperture stop, and then imaged at a focal plane position by the second imaging lens and the polarizer. The liquid crystal spatial light modulator is arranged at a certain angle to the first optical axis of the entrance pupil optical path and is located at the focal plane position. The liquid crystal spatial light modulator is configured to generate a grayscale value image according to a preset topological number, so as to perform phase modulation on the image formed on the liquid crystal spatial light modulator to generate vortex light; The vortex light is collimated by the second relay lens arranged in the exit pupil optical path, and then the light outside the vortex light is blocked by the Lyot aperture, and finally projected to the first relay lens after passing through the converging lens.

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