Airborne high-frequency linear array sweeping imaging spectroscopic system based on optical waveguide devices

Abstract

This invention belongs to the field of imaging spectrometer technology, and particularly relates to an airborne high-frequency linear array oscillating imaging spectrometer system based on an optical waveguide device. The system includes a telescope objective and a spectrometer, and further includes an optical waveguide device coupled between the telescope objective and the spectrometer slit. The optical waveguide device includes an optical switch and a beam splitter fiber connected in sequence. The fiber array end of the optical switch is coupled to the focal plane of the telescope objective. The optical switch rapidly switches the on / off state of each fiber at the fiber array end, and the beam splitter fiber ensures that the scanning result at the fiber array end corresponds to the incident light at the spectrometer slit, achieving oscillating imaging. This invention innovatively optimizes the linear array oscillating imaging spectrometer system, significantly improving its scanning efficiency.

Description

Technical Field

[0001] This invention belongs to the field of imaging spectrometer technology, and particularly relates to an airborne high-frequency linear array swing-scan imaging spectrometer system based on optical waveguide devices. Background Technology

[0002] Spectral measurement and analysis technology is a fundamental technology in the study of the composition and structure of matter. Spectral imaging technology, which combines imaging capabilities, can simultaneously acquire spatial and spectral information of a target and has become an important research direction in the field of optical remote sensing. It has been widely and deeply applied and studied in many fields such as resource exploration, agricultural and forestry remote sensing, water remote sensing, disaster monitoring, national defense and security, and deep space exploration.

[0003] Spectral imaging instruments can be classified into two types based on their imaging principles: oscillating broom and pushbroom. Oscillating broom imaging spectrometers acquire the field of view in the flight direction by observing the aircraft's flight along its orbit, and obtain the field of view perpendicular to the flight track through mechanical scanning imaging. They use linear or area array detectors to receive spectral data for each band. Pushbroom imaging spectrometers, on the other hand, acquire the field of view in the flight direction only by observing the aircraft's flight along its orbit, and use large area array detectors to acquire spectral information in the direction of crossing the track.

[0004] Generally, under the same performance specifications, the main advantages of pushbroom imaging spectrometers lie in their high imaging and detection efficiency, the absence of sweeping motion, relatively simple structure, and small size. Furthermore, the ability to simultaneously acquire pixels along the track-crossing direction reduces the difficulty of image stitching. However, the field of view of pushbroom imaging spectrometers is limited by the size of the area array detector, and large area array detectors are very expensive, making it difficult to achieve a large field of view. Additionally, instrument calibration of pushbroom imaging spectrometers is more difficult, and spectral bending is also present. In contrast, sweeping imaging spectrometer systems have a relatively complex structure, lower detection efficiency, and more complex sweeping motion control and image stitching processing; however, they offer advantages such as a large field of view and low cost, making them highly promising for application in airborne imaging spectroscopic remote sensing.

[0005] Depending on the detector used, oscillating imaging spectroscopic systems can be further divided into linear oscillating and area oscillating types. The basic principle of linear oscillating is as follows: the radiation from each ground pixel sequentially enters the instrument's dispersion system. The dispersion system disperses the radiation from each individual pixel according to specific spectral intervals, allowing them to be received by the linear array detector. The number of pixels in the linear array detector is the number of spectral bands dispersed by each pixel. Assuming the detector pixels are square, the scanning time T per line is equal to the ratio of the instantaneous field of view on the ground to the aircraft's orbital speed. When the imaging spectrometer scans each line... m At this time, the ground pixel resolution in the trajectory crossing direction is the ratio of the total scanned field of view to the flight altitude and the number of scans, and the integration time of a single pixel on the detector is...T / m Second.

[0006] Due to its unique imaging method, the linear array oscillating imaging spectrometer has the following advantages: less difficulty in designing the optical path system; the imaging field of view is not limited by the size of the linear array detector, making it easy to achieve large field of view detection and reducing detector requirements; relatively simple instrument calibration; and good spectral consistency. However, its disadvantages include: high difficulty in designing, assembling, and controlling moving parts (scanning system); difficulty in subsequent image processing due to the inability to acquire cross-track spatial pixels simultaneously; and limitations on system spatial and spectral resolution due to the integration time of a single observation point.

[0007] The mechanical scanning and spectroscopic detection sections of an area-array oscillating imaging spectrometer are similar to those of a linear oscillating spectrometer. The imaging principle of an area-array oscillating spectrometer is similar to that of a linear spectrometer, but the difference lies in the use of an area-array CCD detector, enabling simultaneous spectral imaging of multiple rows of ground objects.

[0008] In the imaging process of a oscillating scanning imaging spectrometer, the acquired continuous images must ensure partial overlap and completeness of the observed scene, thereby enabling successful subsequent image stitching. Assume the platform's flight speed is... Flight altitude is The instantaneous field of view of the imaging spectrometer along the track is The sweeping mechanism rotates every second Next, the spatial dimension of the array detector can be obtained For line data to ensure that the scanned area is not missed and has overlap, the speed-to-height ratio and the oscillating scan speed need to meet the following requirements:

[0009] ;

[0010] As can be seen from the above formula, under the same velocity-to-height ratio and instantaneous field of view, the sweeping speed... s The value can vary with n The value decreases as the value increases. Therefore, compared to linear array scanning imaging, area array scanning has a lower scanning speed, thus reducing the design, assembly, and control complexity of the area array scanning mechanism, and also resulting in a longer single integration time. However, due to the use of an area array CCD detector, the spectrometer's economic cost is high, and corresponding calibration and uniformity correction are required. Therefore, there is an urgent need to design a linear array scanning imaging spectrometer to overcome its drawbacks of system complexity and low detection efficiency. Summary of the Invention

[0011] In view of this, the present invention aims to provide an airborne high-frequency linear array oscillating imaging spectral system based on optical waveguide devices, so as to overcome the disadvantages of linear array oscillating imaging spectral systems such as system complexity and low detection efficiency. The present invention innovatively optimizes the linear array oscillating imaging spectral system, which significantly improves the scanning efficiency of the linear array oscillating imaging spectral system.

[0012] To achieve the above objectives, the technical solution created by this invention is implemented as follows:

[0013] An airborne high-frequency linear array oscillating imaging spectroscopic system based on an optical waveguide device includes a telescope objective and a spectrometer, and also includes an optical waveguide device coupled between the slits of the telescope objective and the spectrometer. The optical waveguide device includes an optical switch and a beam splitter fiber connected in sequence. The fiber array end of the optical switch is coupled to the focal plane of the telescope objective. The optical switch rapidly switches the on / off state of each fiber at the fiber array end. The beam splitter fiber makes the scanning result at the fiber array end correspond to the incident light on the slit of the spectrometer, thereby realizing oscillating imaging.

[0014] Furthermore, the beam-splitting output end of the optical switch is connected to the beam-splitting optical fiber.

[0015] Furthermore, the fiber array end contains no fewer than two optical fibers, and these two optical fibers are spliced ​​in a staggered manner to achieve seamless splicing in the vertical scanning direction of the telescope objective.

[0016] Furthermore, the telescope objective includes a single off-axis parabolic mirror, which images targets in the paraxial central field of view.

[0017] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0018] (1) The airborne high-frequency linear array oscillating imaging spectroscopic system based on optical waveguide devices created by the present invention couples the optical waveguide devices between the telescope objective and the spectrometer slit, so that the image of the scene formed by the telescope objective can be transmitted to the spectrometer through the flexible optical waveguide devices. This can realize the direct oscillating of the objective, eliminate the need for the front oscillating reflector, simplify the system and reduce the system's volume and weight.

[0019] (2) The airborne high-frequency linear array sweeping imaging spectral system based on optical waveguide devices created by the present invention optimizes the telescope objective only for the paraxial center field of view, saving a lot of off-axis field of view aberration correction process. Compared with the conventional linear array sweeping imaging spectral system, which generally requires multiple lenses or mirrors for the telescope objective field of view, the present invention can realize a lightweight linear array sweeping telescope objective system, reducing system complexity and weight.

[0020] (3) The airborne high-frequency linear array oscillating imaging spectral system based on optical waveguide devices described in this invention uses an optical fiber array coupled to the focal plane of the telescope objective lens, and realizes time-division detection of multiple scanning fields through optical switching devices, which greatly improves the detection efficiency.

[0021] (4) The airborne high-frequency linear array oscillating imaging spectroscopic system based on optical waveguide devices created by the present invention utilizes the bendable characteristics of optical fiber to directly oscillate lightweight telescope objectives without having to oscillate the entire spectrometer or add oscillating plane mirrors, thereby reducing the turntable load and thus reducing the turntable weight, volume and power consumption requirements. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0023] Figure 1 A schematic diagram of the structure of the airborne high-frequency linear array swing-broom imaging spectral system based on optical waveguide devices as described in an embodiment of the present invention;

[0024] Figure 2 This is a schematic diagram of the structure of the present invention, which uses three coupled optical fibers to cover ground pixels in a single sweep.

[0025] Figure 3 A schematic diagram of the structure of the optical fiber arrangement at the edge of the objective lens field of view as described in the embodiment of the present invention;

[0026] Figure 4 The structure described in the embodiment of the present invention uses two coupled optical fibers to achieve a single sweep to cover ground pixels.

[0027] Explanation of reference numerals in the attached figures:

[0028] 1. Telescope objective lens; 2. Optical switch; 3. Fiber optic splitter; 4. Spectrometer; 21. Fiber optic array end; 41. Slit; 42. Detector. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0030] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0031] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0032] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0033] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0034] like Figure 1 As shown, the airborne high-frequency linear array oscillating imaging spectroscopic system based on optical waveguide devices provided by the present invention includes a telescope objective 1 and a spectrometer 4, and also includes an optical waveguide device coupled between the telescope objective 1 and the slit 41 of the spectrometer 4. The optical waveguide device includes an optical switch 2 and a beam splitter fiber 3 connected in sequence. The fiber array end 21 of the optical switch 2 is coupled to the focal plane of the telescope objective 1. The optical switch 2 quickly switches the on / off state of each fiber of the fiber array end 21. The beam splitter fiber 3 makes the scanning result of the fiber array end 21 correspond to the incident light on the slit 41 of the spectrometer 4, thereby realizing oscillating imaging.

[0035] First, due to the unique nature of linear array sweeping imaging spectral systems, in reality, only the central field of view of the telescope objective 1 of this invention affects the imaging spectral detection quality of the linear array sweeping imaging spectral system; other fields of view have no effect. Therefore, the telescope objective 1 can be optimized only for the paraxial central field of view, eliminating the need for correction of many off-axis field aberrations. Theoretically, only optimization of paraxial chromatic aberration and spherical aberration is required. Generally, for transmission telescope objective 1, a cemented doublet or triple-separated achromatic objective can achieve good imaging quality; for reflection telescope objective 1, a single off-axis parabolic mirror can achieve good imaging quality. Therefore, considering the characteristics of linear array sweeping imaging spectral systems, optimizing aberrations only for the paraxial central field of view allows for the creation of a lightweight and compact linear array sweeping telescope objective 1, reducing the complexity and weight of the linear array sweeping imaging spectral system. Second, by combining it with flexible fiber optic image transmission, the sweeping objective can be directly used, eliminating the need for a reflector and further simplifying the system. Finally, multiple optical fibers are seamlessly arranged and coupled to the focal plane of the telescope objective 1, corresponding to multiple ground fields of view. By rapidly switching the on / off state of the optical fibers in each field of view using an optical switch 2, a single spectrometer 4 can achieve rapid scanning detection of multiple fields of view using a time-division detection method, significantly improving the detection and scanning efficiency compared to the original linear array oscillating imaging spectrometer system. Airborne linear array oscillating imaging spectrometer systems require continuous oscillating scanning of the scanning mechanism and the flight of the airborne platform to achieve continuous ground area scanning detection. The exposure sequence of the spectrometer 4, the movement speed of the oscillating platform, and the flight speed of the aircraft must be strictly matched. According to image shift theory, for the oscillating detector 42, the angle swept by the lens during a single exposure should be less than 1 / 3 of the angular resolution of the detector 42; only then can the image blur introduced by image shift be considered acceptable. In other words, during the process of the optical axis sweeping across the angular resolution, the spectrometer 4 is only in working condition for no more than 1 / 3 of the time, and idle for the rest of the time. This is the theoretical basis for the improvement of oscillating detection efficiency through time-division detection of the spectrometer 4.

[0036] This invention couples the fiber array end 21 to the focal plane of the telescope objective 1, and uses an optical switch 2 to control the on / off state of the fibers for different fields of view, enabling rapid time-division detection of different fields of view. Under this detection system, since the shortest scanning time is much shorter than the integration time, and the detection efficiency of the linear array sweeping imaging spectrometer system depends entirely on the integration time of the spectrometer 4 and the switching time of the optical switch 2, and considering the relatively small size and weight of the optical switch 2, it has minimal impact on the overall size and weight of the linear array sweeping imaging spectrometer system, making it suitable for UAV-based platform applications.

[0037] like Figure 2 As shown, the linear array oscillating scanning imaging spectral system of the present invention can achieve coverage of ground pixels in a single oscillating scan. If three optical fibers are coupled, the oscillating scanning detection efficiency can be improved by about three times.

[0038] To further illustrate the role of the present invention, a brief calculation and analysis are performed below on a linear array sweeping imaging spectral system applied to a small unmanned aerial vehicle platform to demonstrate the improvement in detection efficiency that can be achieved by using the linear array sweeping imaging spectral system of the present invention.

[0039] The designed UAV platform has a flight altitude of 100m, a ground resolution of 1m, a maximum turntable swing speed of 90° / s, and a 40% overlap rate in both scanning directions. The measurement spectral range of the sweeping imaging spectrometer system is 1-2.5μm, with a spectral resolution of 12.5nm and an F-number of 4. Under these conditions, the slit size 41 of the spectrometer 4 is 0.05×0.25mm, the optical system efficiency of the spectrometer 4 is 0.48, and the average quantum efficiency of the long linear array InGaAs photodetector 42 is 0.85. Under clear weather conditions, with a ground albedo of 0.3 and an exposure time of 5ms, the signal-to-noise ratio of the linear array sweeping imaging spectrometer system at a wavelength of 2.4μm is approximately 91, which meets the requirements. At this time, the corresponding UAV flight speed is approximately 0.36m / s. Using a 1×3 optical switch 2, the switching speed between channels of the optical switch 2 is 2ms, and the optical efficiency is approximately 0.9. At this time, the signal-to-noise ratio of the linear array sweep imaging spectral system is about 87, corresponding to a UAV flight speed of about 1.2 m / s.

[0040] Compared to the case where optical switch 2 was added, the signal-to-noise ratio of the linear array oscillating imaging spectral system decreased only slightly, but the flying scan efficiency was greatly improved. Therefore, theoretical calculations show that the present invention significantly improves the scanning efficiency of the linear array oscillating imaging spectral system.

[0041] It should be noted that the 1×3 optical switch 2 represents a 1×N type optical path switching optical switch 2. The "N" input optical fibers of the optical switch 2 are coupled to the focal plane of the telescope objective lens 1, and the "1" output optical fiber is coupled to the slit 41 of the spectrometer 4. Furthermore, by adopting a 1×1 type optical path switching optical switch 2, the on / off sequence and time of the beam splitting output end of different optical switches 2 can be controlled by giving a timing sequence, which can also realize the function of rapid time-division detection of different fields of view.

[0042] In some embodiments, the beam-splitting output end of the optical switch 2 is connected to the beam-splitting optical fiber 3.

[0043] In some embodiments, the fiber array end 21 includes no fewer than two optical fibers, and the two optical fibers are spliced ​​in a staggered manner to achieve seamless splicing in the vertical swivel direction of the telescope objective lens 1.

[0044] It should be noted that when arranging optical fibers seamlessly in the vertical oscillation direction, because the fibers have an outer cladding, seamless splicing cannot be directly achieved between the fibers. Therefore, it is possible to... Figure 1As shown, a staggered splicing method is used to achieve seamless fiber array splicing in the vertical scanning direction, avoiding splicing gaps in ground pixels. Figure 3 As shown, the optical fiber is placed at the edge of the field of view of telescope objective 1, which can make full use of the field of view of telescope objective 1 itself, reduce the angle of the scanning process, and thus improve the scanning detection efficiency. Assuming that the total scanning field of view needs to be 60°, and the objective lens's own field of view is 30°, then a single scanning angle of 30° can achieve a 60° scanning field of view coverage, which improves the scanning detection efficiency by 1 time. The ground coverage effect of a single scanning is shown in the figure. Figure 4 As shown.

[0045] Depending on the specific parameters of the linear array oscillating imaging spectral system, the number and arrangement of optical fibers can be slightly adjusted, but the basic principle remains the same: utilizing the field of view of telescope objective 1 itself to reduce the scanning angle and improve scanning efficiency. Clearly, under this principle, the magnification increase in scanning efficiency of the linear array oscillating imaging spectral system depends on the ratio of the oscillating field of view to the field of view of telescope objective 1.

[0046] In addition, since there are limitations on the number of bends and the bend diameter of optical fibers, the length of the fiber must be appropriate when designing a linear array oscillating scanning imaging spectral system. While ensuring the bending space of the optical fiber, it should not be too long to avoid unnecessarily increasing the weight of the system.

[0047] When selecting optical switch 2, attention should be paid to whether its available spectral bands meet the requirements, its switching state or path switching speed, lifetime, repeatability, and the light suppression ratio in the switching state. The spectral range of the optical switch determines whether it can be used for detection in the designed spectral bands. The switching state or path switching speed determines the detection efficiency of the linear array oscillating imaging spectrometer system. The switching path switching lifetime affects the cost-effectiveness of the linear array oscillating imaging spectrometer system. Repeatability and the switching light suppression ratio determine the impact of optical switch 2 on the detection accuracy of imaging spectrometer 4.

[0048] In some embodiments, the telescope objective 1 includes a single off-axis parabolic mirror, and the telescope objective 1 only images targets in the paraxial central field of view.

[0049] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0050] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. An airborne high-frequency linear array oscillating imaging spectroscopic system based on optical waveguide devices, comprising a telescope objective and a spectrometer, characterized in that: It also includes an optical waveguide device coupled between the telescope objective and the spectrometer slit. The optical waveguide device includes an optical switch and a splitting fiber connected in sequence. The fiber array end of the optical switch is coupled to the focal plane of the telescope objective. The optical switch quickly switches the on / off state of each fiber at the fiber array end. The splitting fiber makes the scanning result at the fiber array end correspond to the incident light on the slit of the spectrometer, thereby realizing oscillating scanning imaging. The beam-splitting output end of the optical switch is connected to the beam-splitting optical fiber, and the telescope objective is optimized only for the paraxial central field of view.

2. The airborne high-frequency linear array oscillating imaging spectroscopic system based on optical waveguide devices according to claim 1, characterized in that: The fiber array end includes no fewer than two optical fibers, and these two optical fibers are spliced ​​in a staggered manner to achieve seamless splicing in the vertical scanning direction of the telescope objective.

3. The airborne high-frequency linear array oscillating imaging spectroscopic system based on optical waveguide devices according to claim 1, characterized in that: The telescope objective lens includes a single off-axis parabolic mirror, which images targets in the paraxial central field of view.

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