A large swath low polarization scanning imaging system

Abstract

A large-width low-polarization scanning imaging system adopts a primary mirror, a secondary mirror, a tertiary mirror and a first turning mirror to realize 360° rotation scanning imaging, and a half-angle mirror follows at half speed to ensure that the light directions of the half-angle mirror at different scanning positions are the same. After being reflected by the primary mirror, the secondary mirror, the tertiary mirror, the first turning mirror and the half-angle mirror, the light passes through a color separation piece and then enters different lens groups, and finally converges on a focal plane detector to realize large-width imaging within a range of ±57.5° of the nadir. The large-width imaging is realized through rotation scanning, which reduces the difficulty of the optical system, and the field of view of the optical system is small, and the light incidence angles of the primary mirror, the secondary mirror, the tertiary mirror and the first turning mirror are small, so that the polarization suppression of the system is better. In addition, an ultraviolet-visible depolarizer is added in front of the ultraviolet-visible lens group, and a visible near-infrared depolarizer is added in front of the visible near-infrared lens group, so that the polarization of the imaging system can be further reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of space optical remote sensing quantitative detection, and relates to a large-width low-polarization scanning imaging system. BACKGROUND

[0002] High detection sensitivity and high measurement accuracy are the keys to successful information acquisition of a camera, and directly determine the application level of quantitative camera. The polarization characteristics of incident light are formed by atmospheric molecules and aerosol scattering, ocean surface reflection, etc., and the polarization sensitivity will directly affect the detection accuracy of the camera. Polarization suppression design is beneficial to improve the radiometric calibration accuracy, reduce the quantitative inversion error, and improve the quantitative application level.

[0003] The typical polarization imagers abroad include the POLDER series cameras of France and the DOAP series cameras of the United States, which extract several specific spectral polarization information of a target. Among them, the POLDER uses a polarizer to obtain the polarization information of the target, and adopts multiple filter groups to select several typical application spectral bands. The DOAP uses a grating to perform spectral splitting, and adopts an amplitude division principle to measure the polarization information of the target. The above-mentioned imagers abroad mainly aim at specific spectral bands, and have the disadvantage that the number of polarization suppression spectral bands is small.

[0004] The existing ocean series satellites at home, such as the ocean No. 1 satellite, have a polarization suppression design, which mainly realizes low polarization sensitivity by reducing the field of view angle and the light incidence angle of the primary mirror, the secondary mirror and the third mirror. However, in order to ensure the large width of the system, it is difficult to further reduce the field of view angle and the incidence angle of the primary mirror, the secondary mirror and the third mirror, resulting in that the imaging width and the polarization sensitivity of the system are difficult to be considered. In addition, the polarization of the lens group and the detector is not specially suppressed in the ocean No. 1 satellite, which also causes the system to have a large polarization sensitivity.

[0005] In view of the above-mentioned disadvantages of the prior art, the primary mirror, the secondary mirror, the third mirror and the first turning mirror of the present method are integrally rotated by 360° for scanning imaging, which can realize large-width imaging under the condition that the field of view angle of the optical system is very small. The small field of view angle of the optical system can significantly reduce the difficulty of optical design, ensure that the light incidence angle of the primary mirror, the secondary mirror, the third mirror and the first turning mirror is small, and then suppress the polarization sensitivity of the system. In addition, the depolarizer technology adopted by the present system specially suppresses the polarization of the lens group and the focal plane detector, significantly reduces the polarization caused by the lens group and the focal plane detector, and can suppress the polarization in all spectral bands within the detection spectral band range, and the number of polarization suppression spectral bands is more. SUMMARY

[0006] The technical problem solved by the present application is that: the existing imaging system generally adopts an optical system with a large field of view to realize large-width imaging on the ground, but large-field imaging will inevitably bring about the disadvantages of poor polarization sensitivity of the imaging system and limited polarization suppression means of the system. In view of the shortcomings that the existing imaging system cannot simultaneously meet the large-width and polarization sensitivity indicators, the present application adopts a 360° rotation scanning imaging scheme of the primary mirror, the secondary mirror, the third mirror and the first folding mirror, which can realize large-angle scanning imaging within a range of ±57.5° of the nadir point and the imaging width on the ground can reach 3000Km. Through optical system design, the field of view angle of the optical system is controlled within 3°, the light incidence angle of the primary mirror, the secondary mirror, the third mirror and the first folding mirror is controlled within 0-10°, the residual polarization degree of the depolarizer of the lens group and the focal plane detector is controlled within 2%, and finally the polarization sensitivity of the system is ≤1.5%, which not only solves the large-width of the imaging system, but also ensures the suppression of the polarization sensitivity of the scanning camera.

[0007] The technical scheme of the present application is: a large-width low-polarization scanning imaging system, comprising a primary mirror, a secondary mirror, a third mirror, a first folding mirror, a half-angle mirror, a first dichroic plate, a second dichroic plate, an ultraviolet-visible depolarizer, a visible near-infrared depolarizer, an ultraviolet-visible lens, a visible near-infrared lens, a short-wave infrared lens, a long-wave infrared lens, an ultraviolet-visible focal plane detector, a visible near-infrared focal plane detector, a short-wave infrared focal plane detector, a long-wave infrared focal plane detector, a second folding mirror, a third folding mirror, and a third dichroic plate.

[0008] The scanning imaging system adopts an off-axis three-mirror optical system form, and the incident light successively passes through the primary mirror, the secondary mirror, the third mirror and the first folding mirror, is reflected by the half-angle mirror and then enters the first dichroic plate; the reflected light after the first dichroic plate enters the second dichroic plate, and the reflected light after the second dichroic plate is reflected to the ultraviolet-visible depolarizer, then the light enters the ultraviolet-visible lens and finally reaches the ultraviolet-visible focal plane detector; the transmitted light after the second dichroic plate is transmitted to the visible near-infrared depolarizer, then enters the visible near-infrared lens and finally reaches the visible near-infrared focal plane detector; the transmitted light after the first dichroic plate enters the second folding mirror, is reflected by the second folding mirror and then enters the third dichroic plate, is reflected by the third dichroic plate and then reaches the short-wave infrared lens, and finally reaches the short-wave infrared focal plane detector; the transmitted light after the third dichroic plate is transmitted to the long-wave infrared lens, is reflected by the third folding mirror and finally reaches the long-wave infrared focal plane detector.

[0009] The primary mirror, the secondary mirror, the third mirror and the folding mirror rotate as a whole, and the half-angle mirror synchronously follows at half the speed, so that the light emitted by the half-angle mirror is imaged at the same angle.

[0010] The ultraviolet-visible depolarizer and the visible near-infrared depolarizer are respectively composed of two quartz wedges.

[0011] The two quartz wedges are one left-handed quartz and the other right-handed quartz.

[0012] The crystal thickness ratio is 2:1, and the angle between the crystal main axes is 45 degrees, so that the light incident on the surfaces of the UV-Vis depolarizer and the visible near-infrared depolarizer is equally distributed in two orthogonal polarization directions.

[0013] The materials of the primary mirror, the secondary mirror, the third mirror, the first turning mirror and the half-angle mirror are all microcrystalline glass, and silver reflective film is plated on the microcrystalline glass.

[0014] Compared with the prior art, the present application has the following advantages:

[0015] (1) The primary mirror, the secondary mirror, the third mirror and the first turning mirror of the whole 360° rotating scanning imaging system are used to realize large-width imaging through the whole 360° rotating scanning imaging of the primary mirror, the secondary mirror, the third mirror and the first turning mirror, the optical system adopts an off-axis three-mirror type, the light incidence angle of each mirror element of the optical system is effectively reduced, the polarization component generated by each mirror element is small, and therefore the polarization sensitivity of the optical system can be effectively reduced under the premise of ensuring the imaging quality of the optical system. The whole 360° rotating scanning type and the off-axis three-mirror type optical system has obvious advantages in the imaging detection application of medium focal length, large width and low polarization.

[0016] (2) The Lyot depolarizer used in the present application is a typical double-wedge optical depolarizer composed of two quartz wedges, and can realize a residual polarization sensitivity of ≤2%. The double-wedge optical depolarizer has obvious effect in system polarization suppression. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 Principle diagram of the half-angle mirror synchronous following scanning imaging;

[0018] Figure 2 Optical path diagram of a large-width low-polarization scanning imaging system;

[0019] Figure 3 Structure diagram of the Lyot depolarizer; DETAILED DESCRIPTION

[0020] The primary mirror, the secondary mirror, the third mirror and the first turning mirror of the present application are whole 360° rotating scanning, the angle of the incident light incident on the half-angle mirror will change, in order to ensure imaging, the direction of the light emitted by the half-angle mirror needs to be unchanged, so the half-angle mirror needs to follow the primary mirror, the secondary mirror, the third mirror and the first turning mirror at half of the speed of the whole 360° rotation.

[0021] The principle of the half-angle mirror following scanning is as follows Figure 1As shown, when the half-angle reflector is in phase with the primary mirror, the secondary mirror, the tertiary mirror and the first turning mirror, the half-angle reflector is at position AB in the figure, and imaging is at point O at this time; when the half-angle reflector has a following error with the primary mirror, the secondary mirror, the tertiary mirror and the first turning mirror, the actual position of the half-angle mirror is A'B', and imaging is at point O'. It can be seen that when the angle position of the half-angle reflector changes by β, the angle difference of the light exiting direction is 2β. In order to ensure the imaging quality, the light exiting direction of the half-angle reflector needs to be unchanged, and the following speed of the half-angle reflector is half of the 360° rotation scanning speed of the primary mirror, the secondary mirror, the tertiary mirror and the first turning mirror as a whole.

[0022] Due to the 360° rotation scanning of the primary mirror, the secondary mirror, the tertiary mirror and the first turning mirror as a whole, the theoretical imaging range is 360°. Considering the actual structure support and the like, it is very easy to realize large-width imaging in a range of star point ±57.5°.

[0023] The primary mirror, the secondary mirror, the tertiary mirror and the first turning mirror as a whole are adopted in the present application, the optical system adopts an off-axis three-mirror type, the number of mirror elements of the optical system is effectively reduced, and the polarization component generated by each mirror element itself is smaller, so that the polarization degree caused by the optical system can be effectively reduced under the premise of ensuring the imaging quality of the optical system. The integral rotation scanning type and the off-axis three-mirror type optical system have obvious advantages in the imaging detection application of medium focal length, large width and low polarization.

[0024] The Lyot depolarizer adopted in the present application is a typical double-wedge optical rotator composed of two quartz wedges, and can realize residual polarization sensitivity ≤2%. The double-wedge optical rotator has very obvious effect in the polarization suppression of the whole machine, and can adapt to polarization suppression of multiple spectral ranges.

[0025] One embodiment of the present application is that the spectral range of the scanning imaging system is 0.35um-12.5um (a total of 18 spectral ranges), a large-angle scanning imaging in a range of star point ±57.5° can be realized, the imaging width on the ground is 3000Km, the entrance pupil diameter of the optical system is 96mm, the field of view angle is 3°, the light incidence angle of the primary mirror, the secondary mirror, the tertiary mirror and the first turning mirror is not more than 10°, the system includes an ultraviolet visible lens group, a visible near-infrared lens group, a short-wave infrared lens group, a medium-long-wave infrared lens group and a focal plane detector, the ultraviolet visible lens group is provided with an ultraviolet visible depolarizer in front, and the visible near-infrared lens group is provided with a visible near-infrared depolarizer in front. The system has the function of suppressing the polarization sensitivity of all spectral ranges (a total of 15 spectral ranges) within 0.35um-1.6um, and the design index of the polarization sensitivity of the system is less than 1.5%.

[0026] The optical system light path of the embodiment is as follows Figure 2As shown, the incident light rays pass through the primary mirror 3, the secondary mirror 4, the tertiary mirror 5, the first turning mirror 6, and then are reflected by the half-angle mirror 7 to enter the first dichroic plate 8. The materials of the primary mirror 3, the secondary mirror 4, the tertiary mirror 5, the first turning mirror 6, and the half-angle mirror 7 are all microcrystalline glass, and silver reflective film is plated on the microcrystalline glass to achieve a reflectivity of 99%, and the maximum light incidence angle is not more than 10°.

[0027] The reflected light rays passing through the first dichroic plate 8 enter the second dichroic plate 9, and the reflected light rays passing through the second dichroic plate 9 are reflected to the ultraviolet-visible depolarizer 10, and then the light rays enter the ultraviolet-visible lens 12 and finally reach the ultraviolet-visible focal plane detector 16; the transmitted light rays passing through the second dichroic plate 9 are transmitted to the visible near-infrared depolarizer 11, and then enter the visible near-infrared lens 13 and finally reach the visible near-infrared focal plane detector 17.

[0028] The transmitted light rays passing through the first dichroic plate 8 enter the second turning mirror 20 and are reflected to the third dichroic plate 22, and the reflected light rays passing through the third dichroic plate 22 are reflected to the short-wave infrared lens 14 and finally reach the short-wave infrared focal plane detector 18; the transmitted light rays passing through the third dichroic plate 22 are transmitted to the long-wave infrared lens 15, are reflected by the third turning mirror 21, and finally reach the long-wave infrared focal plane detector 19.

[0029] Through the optical system configuration, the light incidence angles of the primary mirror, the secondary mirror, the tertiary mirror, and the first turning mirror are all not more than 10°, which can effectively reduce the polarization sensitivity of the optical system reflector, and the polarization sensitivity of the optical system reflector is better than 0.5%.

[0030] The ultraviolet-visible depolarizer and the visible near-infrared depolarizer of the embodiment adopt Lyot depolarizers (such as Figure 3 As shown, the optical axis of the first crystal of the depolarizer is along the Y direction, the optical axis of the second crystal is in the XY plane and forms a 45° angle with the optical axis of the first crystal, and the materials of the two crystals are the same.

[0031] The phase of the first crystal is:

[0032] The phase of the second crystal is:

[0033] k = d2 / d1 = 2 (3)

[0034] Δn = n e -n o (4)

[0035] In the above formula: n e is the refractive index of the crystal e light, and n on0 is the refractive index of the crystal o light, λ is the wavelength of the incident light, d1 is the central thickness of the first crystal, d2 is the central thickness of the second crystal, k is the thickness ratio of the two crystals, β is the wedge angle of the crystal, and a is the half aperture of the depolarizer.

[0036] In the embodiment, the crystal material of the depolarizer is quartz material, d1=5mm, d2=10mm, and the wedge angle β is 1°. The residual polarization degree index of the depolarizer with the parameters can reach 2%.

[0037] As can be known from the above specific embodiment description, the contribution of the technical scheme of the application to the prior art is that the main mirror, the secondary mirror, the third mirror and the first turning mirror are integrally rotated by 360°, the half-angle mirror is synchronously followed at half speed for imaging, the optical system configuration of the imaging system and the Lyot depolarizer polarization suppression technology are adopted, and the star point ±57.5° (3000Km to the earth) wide imaging is realized by using the technologies, and the polarization sensitivity of the whole machine is better than 1.5%.

[0038] Although the application has been disclosed with the above preferred embodiments, it is not intended to limit the application, and any person skilled in the art can make possible changes and modifications to the technical scheme of the application by using the disclosed technical content without departing from the spirit and scope of the application. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the application, which does not depart from the technical scheme of the application, belongs to the protection scope of the technical scheme of the application.

Claims

1. A wide-swath low-polarization scanning imaging system, characterized in that, It includes a primary mirror, secondary mirror, third mirror, first folding mirror, half-angle mirror, first dichroic filter, second dichroic filter, ultraviolet-visible depolarizer, visible-near-infrared depolarizer, ultraviolet-visible lens, visible-near-infrared lens, short-wave infrared lens, long-wave infrared lens, ultraviolet-visible focal plane detector, visible-near-infrared focal plane detector, short-wave infrared focal plane detector, long-wave infrared focal plane detector, second folding mirror, third folding mirror, and third dichroic filter; The scanning imaging system employs an off-axis three-mirror optical system. Incident light sequentially passes through the primary mirror, secondary mirror, third mirror, and first folding mirror, then is reflected by a half-angle mirror before entering the first dichroic filter for beam splitting. The reflected light from the first dichroic filter enters the second dichroic filter, and the reflected light from the second dichroic filter is reflected to the ultraviolet-visible depolarizer. The light then enters the ultraviolet-visible lens and finally reaches the ultraviolet-visible focal plane detector. The transmitted light from the second dichroic filter is transmitted to the visible-near-infrared depolarizer, then enters the visible-near-infrared lens and finally reaches the visible-near-infrared focal plane detector. The transmitted light from the first dichroic filter is reflected by the second folding mirror and enters the third dichroic filter. The light from the third dichroic filter is reflected to the short-wave infrared lens and finally reaches the short-wave infrared focal plane detector. The transmitted light from the third dichroic filter is transmitted to the long-wave infrared lens, and after being reflected by the third folding mirror, it finally reaches the long-wave infrared focal plane detector.

2. The wide-swath low-polarization scanning imaging system according to claim 1, characterized in that, The primary mirror, secondary mirror, tertiary mirror, and folding mirror rotate as a whole, and the half-angle reflector follows synchronously at half the speed, thereby enabling the light from the half-angle reflector to be emitted at the same angle and form an image.

3. The wide-swath low-polarization scanning imaging system according to claim 1, characterized in that, The ultraviolet-visible depolarizer and the visible-near-infrared depolarizer are each composed of two quartz optical wedges.

4. The wide-swath low-polarization scanning imaging system according to claim 3, characterized in that, The two quartz optical wedges consist of two crystals, one of which is left-handed quartz and the other is right-handed quartz.

5. A wide-swath low-polarization scanning imaging system according to claim 4, characterized in that, The crystal thickness ratio is 2:1, and the crystal principal axes are at a 45° angle to ensure that the light incident on the surfaces of the ultraviolet-visible depolarizer and the visible-near-infrared depolarizer is equally distributed in two orthogonal polarization directions.

6. The wide-swath low-polarization scanning imaging system according to claim 1, characterized in that, The primary mirror, secondary mirror, tertiary mirror, first folding mirror, and half-angle mirror are all made of microcrystalline glass, and a silver reflective film is coated on the microcrystalline glass.

Images