A diffractive polarization imaging system

Abstract

The present application relates to a kind of diffraction polarization imaging systems, including two major modules of diffraction imaging system, focal plane detector, wherein the diffraction imaging system includes a diffraction primary mirror and a set of Schupmann achromatic structure.Target radiation light passes through front aperture and diffraction primary mirror, then after diffraction primary mirror Schupmann achromatic lens group is used to correct chromatic aberration of light emitted by diffraction primary mirror, and converging lens is used to converge into the focal plane polarization detector.Light can be detected by focal plane detector to 0 °, 45 °, 90 °, 135 ° polarization image, and polarization degree and polarization angle image can be solved by Stokes vector method.The above-mentioned diffraction polarization imaging system uses a diffraction primary mirror, a set of Schupmann achromatic structure and a focal plane detector, four different angle polarization images are obtained simultaneously by one-time exposure, the number of imaging system lenses is greatly reduced, and the cost and complexity of the system are reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical imaging technology, and particularly relates to a diffraction polarization imaging system. BACKGROUND

[0002] Traditional detection means can only record the amplitude information of the target, and it is difficult to record the information of other dimensions of the target, so the polarization detection as a new imaging system has been widely concerned. The polarization detection is mainly divided into five types: time-sharing polarization detection system, amplitude-sharing polarization detection system, aperture-sharing polarization detection system, and focal plane polarization detection system. The above four polarization detection methods use traditional imaging systems. With the development of modern optical technology, people's requirements for the resolution of optical systems are getting higher and higher, which requires the aperture of the optical system to be continuously increased. However, the increase of the aperture brings problems such as high precision required for processing, complex optical system, and heavy weight of the optical system. In this case, a telescope system based on a large-aperture diffractive optical element (DOE) is proposed, and using DOE as the main aperture becomes a new solution.

[0003] The advantages of the diffraction imaging system compared with the traditional imaging system mainly reflect in two aspects. On the one hand, by manufacturing the DOE pattern on a low-weight substrate, the weight of the telescope system can be significantly reduced. On the other hand, the transmission DOE provides a loose surface shape tolerance, which will alleviate the difficulties in surface manufacturing and testing. These advantages make the diffraction imaging technology significantly superior to the traditional imaging technology in large-aperture telescopes. The United States launched the EYEGLASS program in 1999 and the MOIRE program in 2013, and the main mirror structure of both programs is a diffractive optical element.

[0004] The diffraction imaging system at the present stage has a large amount of stray light on the image plane due to the problem of diffraction efficiency, which reduces the contrast of the system imaging. In view of the problems, three kinds of solutions are mainly used at the present stage. One is to use more steps to approximate the blazed structure. The more the number of steps is, the higher the diffraction efficiency of the diffraction element is, but the processing requirement is also greater, so the number of steps cannot be increased unlimitedly. The second is to use a light barrier and the like to reduce the influence of the background light, but this method has limited effect on reducing the influence of stray light and cannot reduce the stray light problem caused by the diffraction efficiency. The third is to use image processing and the like to process the image, but the image processing method will lose part of the imaging information. The above solutions are mainly from the system design and image post-processing, and there are many problems. It is shown that the traditional imaging detection method cannot well solve the problem of the contrast reduction of the imaging system caused by the stray light of the diffraction imaging system. The polarization imaging technology makes up for the defect to a certain extent. The polarization detection can highlight the target hidden in the natural background by using the difference in polarization characteristics between the target and the background. Meanwhile, the polarization detection can eliminate the stray light caused by various environmental factors by using the difference in polarization characteristics between the target light and the background light.

[0005] In summary, the combination of the diffraction system and the polarization imaging technology is not only beneficial to solving the problem of the weight and the system complexity of the imaging optical system of the polarization detection system, but also expected to solve the stray light problem of the diffraction imaging system. Therefore, it is necessary to design a diffraction polarization imaging system. SUMMARY

[0006] In view of the problems of the complex polarization imaging system and the stray light of the diffraction imaging system, the application provides a diffraction polarization imaging system.

[0007] The technical scheme adopted by the application is as follows: a diffraction polarization imaging system, the imaging system comprises a diffraction imaging system and a focal plane detector.

[0008] The lens group of the diffraction imaging system is sequentially provided with a light barrier, a diffraction main mirror and a Schupmann achromatic structure along the light path. The target light is incident on the diffraction main mirror through the light barrier, and the emergent light is converged on the focal plane detector after passing through the Schupmann achromatic structure.

[0009] The emergent light enters the focal plane detector, and four polarization images of different angles are obtained through exposure.

[0010] Preferably, the diffraction main mirror is processed according to the calculated microstructure parameters, and the specific content comprises:

[0011] S1, determining the center wavelength of the work, and determining the optical material used at the center wavelength;

[0012] S2, determine the aperture, F number and focal length of the required diffractive lens;

[0013] S3, solve the microstructure height according to the central wavelength and the refractive index of the required material;

[0014] S4, determine the position of each annular zone according to the aperture, F number and focal length;

[0015] S5, according to the microstructure height and the annular zone position, the position of each step of the diffractive lens is solved;

[0016] S6, according to the obtained micro-lens structure position, the diffractive primary mirror is processed.

[0017] Preferably, the diffractive imaging system adopts Schupmann achromatic structure, and the Schupmann structure is used to eliminate chromatic aberration by means of another piece of diffractive lens with opposite focal power of the primary mirror, and the two traditional lenses in the structure are a relay lens and a converging lens.

[0018] Preferably, the focal plane detector, the light emitted from the diffractive imaging system is converged on the focal plane detector, and a micro-polarizer is added in front of each image element of the detector, and the four adjacent image elements integrate the polarization modulation in different directions, and the four image elements can be equivalent to an image element of a common detector, so that the polarization information in four polarization directions can be obtained through one detection.

[0019] Preferably, the diffractive imaging system is processed by using a structure with four steps or more to ensure the quality of imaging; and the minimum period of the diffractive element in the diffractive imaging system should be kept above 10 times the wavelength to ensure the polarization maintaining performance of the system.

[0020] Preferably, the diffractive imaging system can adjust the aperture and focal length of the diffractive primary mirror, so that the diffractive imaging system has multiple focal lengths and apertures.

[0021] The technical scheme provided by the present application has at least the following beneficial effects:

[0022] 1. The system of the present application is based on a diffractive system, and has simple overall system structure, light overall weight, low processing difficulty and wide system tolerance;

[0023] 2. The detector of the present application is a focal plane detector, and there is no artificial rotating polarizer device, four different polarization images can be obtained at one time, the stability and detection speed of the system are improved, and the system is suitable for polarization imaging of non-static target scene;

[0024] 3. The present application utilizes the difference between background light and polarized light to improve the imaging quality of the diffractive imaging system in complex background and reduce the imaging stray light. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Figure 1 is a schematic diagram of the optical path of the diffraction polarization imaging system of the present application, wherein 1 is an optical stop, 2 is a diffraction primary mirror, 3 is a relay lens, 4 is an achromatic diffraction lens, 5 is a positive lens, and 6 is a split focal plane detector.

[0026] Figure 2 Figure 2 is a schematic diagram of four pixels of the split focal plane detector of the present application.

[0027] Figure 3 Figure 3 is a system transfer function diagram of an embodiment of the present application. DETAILED DESCRIPTION

[0028] The present application aims to provide a diffraction polarization imaging system which can solve the problems of large volume and complexity of the polarization imaging system, and can also solve the problem of imaging contrast reduction caused by stray light of the diffraction system.

[0029] In order to have a further understanding of the purpose, structure, features and functions of the present application, the following will be described in detail in conjunction with embodiments.

[0030] Please refer to Figure 1 , Figure 2 , Figure 1 Figure 1 is a schematic diagram of the optical path structure provided by an embodiment of the present application, Figure 2 Figure 2 is a schematic diagram of four pixels of the split focal plane polarization detector.

[0031] As shown in Figure 1 , the diffraction polarization imaging system provided by an embodiment of the present application includes two modules of a diffraction imaging system and a split focal plane detector. The diffraction imaging system receiving target radiation light is sequentially placed with an optical stop 1 with an aperture of 80 mm, a diffraction primary mirror 2 with an aperture of 80 mm, a Schupmann achromatic lens group, a relay lens 3, an achromatic diffraction lens 4, and a positive lens with a double cemented structure in the light path. The design wavelength of the diffraction primary mirror is 550 mm, the focal length is 800 mm, the surface microstructure is 4 steps, and the minimum period width is 22 um. The achromatic diffraction lens group includes the relay lens 3, the achromatic diffraction lens 4, and the achromatic diffraction lens 4. The surface microstructure of the achromatic diffraction lens is 4 steps, the minimum period is 7.6 um, it is conjugated with the diffraction primary mirror 1 about the relay lens 3, and the two diffraction lenses have opposite dispersion and opposite chromatic aberration. The positive lens with a double cemented structure is placed after the achromatic diffraction lens 4, and the divergent light beam of the achromatic lens group is converged to the split focal plane detection module at a distance of 150 mm.

[0032] The split focal plane detection module includes a polarization split focal plane detector with an aperture of 29 mm. The light emitted from the diffraction imaging system is converged to the split focal plane detector. A micro-polarizer is added in front of each pixel of the detector, and the four adjacent pixels integrate polarization modulation in different directions. The schematic diagram is as shown in Figure 2As shown, four pixels can be equivalent to one pixel of a common detector, so that the polarization information in the polarization directions of 0°, 45°, 90° and 135° can be obtained through one detection, and the Stokes vector of the target radiation light can be derived therefrom, and the expression is as follows:

[0033] S0=I 0° +I 90°

[0034] S1=I 0° -I 90°

[0035] S2=I 45° -I 135°

[0036] wherein I 0° , I 45° , I 90° , I 135° are the intensities in the polarization directions of 0°, 45°, 90° and 135° respectively, S0, S1 and S2 are the Stokes vectors, and the linear polarization degree DoLP and the polarization angle AoP of the target radiation light are further obtained, and the expressions are as follows:

[0037]

[0038]

[0039] The above example provides a diffraction imaging system prototype with a primary mirror of 80mm aperture, the system design focal length is 361.5mm, the design wavelength is 486nm-656nm, the system central wavelength is 550nm, and the diffraction lenses in the system all satisfy 4 steps, the minimum period is greater than 10 times the wavelength, and the polarization maintaining performance is better. Figure 3 As shown in the figure, the abscissa represents the spatial frequency, the unit is line pair / mm, the ordinate represents the MTF value, 0.0000DEG, 0.1400DEG and 0.200DEG represent the fields of view of 0°, 0.14° and 0.2° respectively, T represents the meridian plane, S represents the sagittal plane, DIFF.LIMIT represents the diffraction limit, and the curves one by one correspond to the MTFs of the meridian plane and the sagittal plane in each field of view. The results show that the system imaging quality is better. The above diffraction imaging system prototype is combined with a polarization split focal plane detector with an aperture of 29mm, and the two together constitute a diffraction polarization imaging system prototype, and the imaging quality and the polarization maintaining performance of the system both meet the requirements, and the system as a whole is also relatively simple.

[0040] In different embodiments, the diffraction primary mirror in the diffraction imaging system can adopt different focal lengths and apertures to meet different actual application requirements.

[0041] In summary, the polarized imaging system provided by the application is based on a diffraction imaging system, adopts a split focal plane polarized detector, and simultaneously obtains four polarized images of different angles through one exposure, and the working principle is as follows: target radiation light passes through a front light diaphragm and a diffraction main mirror, then passes through a Schupmann achromatic lens group after the diffraction main mirror to correct chromatic aberration of light emitted by the diffraction lens, and converging lenses are used to converge the light into the split focal plane polarized detector. A micro polarized sheet is added in front of each pixel of the split focal plane polarized detector, and the four adjacent pixels integrate polarized modulation of different directions, and the four pixels can be equivalent to one pixel of a common detector, so that polarized information in four polarization directions can be obtained through one detection, and thus 0°, 45°, 90° and 135° polarized images can be obtained. The front diffraction imaging system can adopt different relative apertures and focal lengths to meet the actual needs of different imaging distances.

[0042] Finally, it should be noted that the above description of the preferred embodiments is relatively detailed, and therefore should not be considered as limiting the protection scope of the application, and those skilled in the art can make some improvements and refinements under the inspiration of the application without departing from the scope protected by the claims of the application, and these improvements and refinements should also be considered as the protection scope of the application, and the protection scope of the application should be subject to the appended claims.

Claims

1. A diffraction polarization imaging system, characterized in that: The imaging system includes a diffraction imaging system and a focal plane detector; Inside the lens group of the diffraction imaging system, an aperture stop, a diffraction primary mirror, and a Schupmann achromatic structure are placed sequentially along the optical path. The target light is incident on the diffraction primary mirror through the aperture stop, and after passing through the Schupmann achromatic structure, the outgoing light converges to the focal plane detector. The emitted light enters the focal plane detector and is exposed to obtain four polarization images at different angles. The focal plane detector is where the light emitted from the diffraction imaging system is focused. A micro polarizer is added in front of each pixel of the detector. The four adjacent pixels integrate polarization modulation in different directions. The four pixels can be equivalent to one pixel of a normal detector. In this way, polarization information in four polarization directions can be obtained in one detection.

2. The diffraction polarization imaging system as described in claim 1, characterized in that: The diffraction primary mirror is fabricated based on calculated microstructure parameters, the specific details of which include: S1, determine the center wavelength of the operation, and determine the optical material to be used at that center wavelength; S2, determine the required aperture, F-number and focal length of the diffraction lens; S3, calculate the microstructure height based on the center wavelength and the refractive index of the required material; S4, determine the position of each ring based on the aperture, F number and focal length; S5. Based on the height of the microstructure and the position of the ring zone, determine the position of each step of the diffraction lens; S6. Fabricate the diffraction primary mirror based on the obtained microlens structure position.

3. The diffraction polarization imaging system as described in claim 1, characterized in that: The diffraction imaging system employs a Schupmann achromatic structure, which eliminates chromatic aberration by using another diffraction lens with the opposite optical power to the primary mirror. The two conventional lenses in the Schupmann structure are a relay lens and a converging lens, respectively.

4. The diffraction polarization imaging system as described in claim 1 or 2, characterized in that: To ensure imaging quality, the diffraction elements in the diffraction imaging system should be fabricated with a structure of 4 steps or more; to ensure the polarization-maintaining performance of the system, the minimum period of the diffraction elements in the diffraction imaging system should be maintained at more than 10 times the wavelength.

5. The diffraction polarization imaging system as described in claim 1 or 2, characterized in that: The diffraction imaging system described above can adjust the aperture and focal length of the diffraction primary mirror, giving it multiple focal lengths and apertures.

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