Airborne high-frequency linear array sweep imaging spectrum system based on optical waveguide device

Abstract

The invention belongs to the technical field of imaging spectrometers, and particularly relates to an airborne high-frequency linear array sweep imaging spectrum system based on an optical waveguide device. The system comprises a telescope objective and a spectrometer and further comprises an optical waveguide device coupled between the telescope objective and a slit of the spectrometer, the optical waveguide device comprises an optical switch and a beam splitting optical fiber which are connected in sequence, the optical fiber array end of the optical switch is coupled to the focal plane of the telescope objective, and the optical switch rapidly switches on and off of all optical fibers at the optical fiber array end. And the beam splitting optical fiber enables a scanning result of the optical fiber array end to correspondingly enter a slit of the spectrograph so as to realize sweep imaging. The linear array sweep imaging spectrum system is innovated and optimized, and the scanning efficiency of the linear array sweep imaging spectrum system is greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of imaging spectrometers, and in particular relates to an airborne high-frequency linear array sweeping imaging spectrometer system based on an optical waveguide device. Background Art

[0002] Spectral measurement and analysis technology is a basic technology in studying the composition and structure of materials. Spectral imaging technology combined with imaging function can simultaneously obtain the spatial information and spectral information of the target. It has become an important research direction in the field of optical remote sensing technology. It has been widely and deeply applied and studied in many fields such as resource exploration, agricultural and forestry remote sensing, water body remote sensing, disaster monitoring, national defense security and deep space exploration.

[0003] According to their imaging principles, spectral imaging instruments can be divided into two categories: swing-scan type and push-scan type. Swing-scan type imaging spectrometers use the aircraft's along-track flight to obtain the field of view in the flight direction, obtain the field of view perpendicular to the flight track through mechanical scanning imaging, and use linear or planar array detectors to receive spectral data of each band; push-scan type imaging spectrometers only obtain the field of view in the flight direction through the aircraft's along-track flight, and use large planar array detectors to realize spectral information collection in the cross-track direction.

[0004] Generally speaking, under the same indicators, the main advantages of push-broom imaging spectrometers are high imaging detection efficiency, and the system does not require swinging motion, its structure is relatively simple, and its size is small; at the same time, the characteristics of pixels in the cross-track direction can be acquired simultaneously also reduce the difficulty of image stitching. However, the field of view of the push-broom imaging spectrometer is limited by the size of the array detector, and the cost of large array detectors is very high, so it is not easy to achieve large field of view detection. In addition, the instrument calibration of the push-broom imaging spectrometer is more difficult, and there is also the phenomenon of spectral bending. In contrast, the structure of the swing-sweep imaging spectroscopy system is relatively complex, the detection efficiency is low, and the swing motion control and image stitching processing are relatively complex; but it has the advantages of large field of view and low cost, and has great application potential in the field of airborne imaging spectroscopy remote sensing.

[0005] Depending on the detector used, the swing-scan imaging spectroscopy system can be divided into two categories: linear array swing scanning and planar array swing scanning. The basic principle of linear array swing scanning: the radiation of each ground pixel enters the dispersion system of the instrument in turn. The dispersion system splits the radiation of a single pixel according to a specific spectral interval and allows them to be received by the linear array detector. The number of pixels in a linear array detector is the number of spectral bands of a single pixel. Assuming that the detector pixels are square, the scanning time T for each line is equal to the ratio of the instantaneous field of view on the ground to the flight speed of the aircraft along the track. When the imaging spectrometer scans each line m The ground pixel resolution in the cross-track direction is the total field of view multiplied by the ratio of the flight altitude to the number of scans. The integration time of a single pixel on the detector isT / m Second.

[0006] Due to the unique imaging method of the linear array swing-scanning imaging spectrometer, its advantages mainly lie in: the optical path system design is less difficult; the imaging field of view is not limited by the size of the linear array detector, and it is easy to achieve large field of view detection, with low detector requirements; the instrument calibration is relatively simple; the spectrum consistency is good, etc. Its disadvantages lie in: the design, installation and control of the moving parts (scanning system) are difficult; the subsequent image processing is difficult because the pixels in the through-track space cannot be acquired at the same time; the system spatial and spectral resolution is limited by the integration time of a single observation point, etc.

[0007] The mechanical scanning part and the spectroscopic detection part of the area array swing scanning imaging spectrometer are similar to those of the linear array swing scanning. The imaging principle of the area array swing scanning is similar to that of the linear array, but the difference is that it uses an area array CCD detector, which can simultaneously perform spectral imaging of multiple lines of ground observation targets.

[0008] In the imaging process of the swing-scan imaging spectrometer, the continuous images obtained must ensure that the observed scene is partially overlapped and not missed, so that the subsequent image stitching can be completed smoothly. Assuming the platform flight speed is , the flight altitude is , the instantaneous field of view of the imaging spectrometer along the track is , the sweeping mechanism rotates per second Second, the spatial dimension of the array detector can be obtained To ensure that the scanned area is not missed and overlapped, the speed-to-height ratio and the sweep speed must meet the following requirements: ; It can be seen from the above formula that under the same speed-to-height ratio and instantaneous field of view, the sweep speed s The value can be changed with n It decreases with the increase of. It can be seen that: compared with the linear array swing scanning imaging method, the scanning speed of the swing scanning mechanism of the area array swing scanning method is lower, which reduces the difficulty of designing, assembling and controlling the swing scanning mechanism of the area array swing scanning method, and the single integration time is longer. However, due to the use of area array CCD detectors, the economic cost of the spectrometer is high, and corresponding calibration and uniformity correction are required. Therefore, it is urgent to design a linear array swing scanning imaging spectrometer to overcome the shortcomings of complex system and low detection efficiency. Summary of the invention

[0009] In view of this, the present invention aims to provide an airborne high-frequency linear array swing scanning imaging spectroscopy system based on optical waveguide devices, so as to overcome the shortcomings of the linear array swing scanning imaging spectroscopy system, such as complex system and low detection efficiency. The present invention innovatively optimizes the linear array swing scanning imaging spectroscopy system, thereby greatly improving the scanning efficiency of the linear array swing scanning imaging spectroscopy system.

[0010] To achieve the above object, the technical solution created by the present invention is implemented as follows: An airborne high-frequency linear array swing scanning imaging spectroscopy system based on an optical waveguide device comprises a telescope objective lens and a spectrometer, and also comprises an optical waveguide device coupled between the telescope objective lens and a slit of the spectrometer, wherein the optical waveguide device comprises an optical switch and a beam splitting optical fiber connected in sequence, wherein the optical fiber array end of the optical switch is coupled to the focal plane of the telescope objective lens, the optical switch quickly switches on and off each optical fiber at the optical fiber array end, and the beam splitting optical fiber makes the scanning result at the optical fiber array end correspond to the slit incident on the spectrometer, thereby realizing swing scanning imaging.

[0011] Furthermore, the beam splitting output end of the optical switch is connected to the beam splitting optical fiber.

[0012] Furthermore, the optical fiber array end includes no less than two optical fibers, and no less than two optical fibers are spliced ​​by a staggered splicing method to achieve seamless splicing in the vertical sweeping direction of the telescope objective.

[0013] Furthermore, the telescope objective lens comprises a single off-axis parabolic mirror, and the telescope objective lens images a target in a near-axis central field of view.

[0014] Compared with the prior art, the invention can achieve the following beneficial effects: (1) The present invention creates an airborne high-frequency linear array scanning imaging spectroscopy system based on an optical waveguide device, which couples the optical waveguide device between the telescope objective lens and the slit of the spectrometer. The image of the scene formed by the telescope objective lens can be transmitted to the spectrometer through the flexible optical waveguide device, and the objective lens can be directly swung, and the front scanning reflector can be omitted, thereby simplifying the system and reducing the volume and mass of the system.

[0015] (2) The present invention creates an airborne high-frequency linear array scanning imaging spectroscopy system based on an optical waveguide device, which optimizes the telescope objective lens only for the near-axis central field of view, thereby eliminating the correction process of many off-axis field of view aberrations. Compared with the conventional linear array scanning imaging spectroscopy system, which generally requires multiple lenses or reflectors for the field of view of the telescope objective lens, the present invention can realize a lightweight linear array scanning telescope objective lens system, thereby reducing the complexity and weight of the system.

[0016] (3) The airborne high-frequency linear array scanning imaging spectroscopy system based on optical waveguide devices created by the present invention uses an optical fiber array coupled to the focal plane of a telescope objective lens, and realizes time-sharing detection of multiple scanning fields of view through optical switch devices, thereby greatly improving the detection efficiency.

[0017] (4) The airborne high-frequency linear array scanning imaging spectroscopy system based on optical waveguide devices created by the present invention utilizes the bendable property of optical fiber to directly scan a lightweight telescope objective without having to swing the entire spectrometer or add a scanning plane mirror, thereby reducing the turntable load and further reducing the turntable weight, volume and power consumption requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation on the present invention. In the drawings: Figure 1 A schematic diagram of the structure of an airborne high-frequency linear array scanning imaging spectroscopy system based on an optical waveguide device according to an embodiment of the present invention; Figure 2 A schematic diagram of the structure of using three coupled optical fibers to cover ground pixels in a single sweep according to an embodiment of the present invention; Figure 3 A schematic diagram of the structure of the optical fiber arranged in the edge field of view of the objective lens according to an embodiment of the present invention; Figure 4 The present invention creates a structure that uses two coupled optical fibers to cover ground pixels in a single sweep as described in an embodiment of the present invention.

[0019] Description of reference numerals: 1. Telescope objective lens; 2. Optical switch; 3. Beam-splitting optical fiber; 4. Spectrometer; 21. Fiber array end; 41. Slit; 42. Detector. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the invention more clear, the invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described here are only used to explain the invention and do not constitute a limitation of the invention.

[0021] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0022] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0023] In the description of the invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the invention can be understood according to specific circumstances.

[0024] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments.

[0025] like Figure 1 As shown, the airborne high-frequency linear array swing scanning imaging spectroscopy system based on optical waveguide devices provided by the present invention includes a telescope objective lens 1 and a spectrometer 4, and also includes an optical waveguide device coupled between the telescope objective lens 1 and the slit 41 of the spectrometer 4, the optical waveguide device includes an optical switch 2 and a beam splitting optical fiber 3 connected in sequence, the optical fiber array end 21 of the optical switch 2 is coupled to the focal plane of the telescope objective lens 1, the optical switch 2 quickly switches the on and off of each optical fiber of the optical fiber array end 21, and the beam splitting optical fiber 3 makes the scanning result of the optical fiber array end 21 correspond to the slit 41 of the spectrometer 4, thereby realizing swing scanning imaging.

[0026] First, due to the particularity of the linear array swing scanning imaging spectrum system, in fact, only the central field of view of the telescope objective lens 1 of the present invention has an impact on the imaging spectrum detection quality of the linear array swing scanning imaging spectrum system, and other fields of view do not work; therefore, the telescope objective lens 1 can be optimized only for the paraxial central field of view to save a lot of correction processes of off-axis field of view aberrations. In theory, only the paraxial chromatic aberration and spherical aberration need to be optimized to a certain extent. Generally speaking, for the transmission type telescope objective lens 1, a double-glued or three-separated achromatic objective lens can achieve good imaging quality; for the reflection type telescope objective lens 1, a single off-axis parabolic mirror can achieve better imaging quality. Therefore, according to the characteristics of the linear array swing scanning imaging spectrum system, only the aberration of the paraxial central field of view is optimized, so that a light and small linear array swing scanning telescope objective lens 1 can be realized, reducing the complexity and weight of the linear array swing scanning imaging spectrum system. Secondly, combined with the bendable optical fiber image transmission, the objective lens can be directly swung and scanned, the reflector can be eliminated, and the system can be further simplified. Finally, multiple optical fibers are arranged "seamlessly" and coupled to the focal plane of the telescope objective 1, corresponding to multiple ground fields of view; and the optical switch 2 is used to quickly switch the on and off of the optical fibers of each field of view, so that a time-sharing detection method can be used to use a spectrometer 4 to achieve rapid scanning and detection of multiple fields of view, greatly improving the detection and scanning efficiency on the basis of the original linear array swing scanning imaging spectrum system. The airborne linear array swing scanning imaging spectrum system needs to realize continuous scanning and detection of the ground area through continuous swing scanning of the scanning mechanism and the flight of the airborne platform. The exposure sequence of the spectrometer 4, the movement speed of the swing scanning platform and the flight speed of the carrier aircraft need to be strictly matched. According to the image shift theory, for the swing scanning detector 42, the lens scanning angle within a single exposure time should be less than 1 / 3 of the angular resolution of the detector 42, and only then can the image blur introduced by the image shift be considered acceptable. In other words, in the process of the optical axis scanning the angular resolution, the spectrometer 4 is in working state for no more than 1 / 3 of the time, and is in idle state for the rest of the time. This is the theoretical basis for the swing scanning detection efficiency to be improved by the time-sharing detection of the spectrometer 4.

[0027] The present invention couples the fiber array end 21 to the focal plane of the telescope objective lens 1, and uses the optical switch 2 to control the on and off of the optical fibers in different fields of view, so as to control the rapid time-sharing 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 swing scanning imaging spectrum system is completely dependent on the integration time of the spectrometer 4 and the switching time of the optical switch 2. At the same time, the volume and weight of the optical switch 2 are also small, which has little effect on the volume and weight of the linear array swing scanning imaging spectrum system, and is suitable for application on unmanned aerial vehicle platforms.

[0028] like Figure 2 As shown, the linear array scanning imaging spectroscopy system of the present invention is used to cover ground pixels with a single scanning. If three optical fibers are coupled, the scanning detection efficiency can be increased by about three times.

[0029] To further illustrate the role of the present invention, a brief calculation and analysis is performed below on a linear array swing scanning imaging spectroscopy system applied to a small UAV platform to demonstrate the improvement in detection efficiency that can be obtained by using the linear array swing scanning imaging spectroscopy system of the present invention.

[0030] The flight altitude of the designed UAV platform is 100m, the ground resolution is 1m, the maximum swing speed of the turntable is 90° / s, the scanning overlap rate in both directions is 40%, the measurement spectral range of the swing scanning imaging spectroscopy system is 1-2.5μm, the spectral resolution is 12.5nm, and the F number is 4. At this time, the corresponding spectrometer 4 slit 41 size 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. On a sunny day, the ground albedo is 0.3, and the exposure time is 5ms, the signal-to-noise ratio of the analysis linear array swing scanning imaging spectroscopy system at a wavelength of 2.4μm is about 91, which can meet the use requirements. At this time, the flight speed of the corresponding UAV is about 0.36m / s. After using the 1×3 optical switch 2, the switching speed between the channels of the optical switch 2 is 2ms, and the optical efficiency is about 0.9. At this time, the signal-to-noise ratio of the linear array swing-scanning imaging spectroscopy system is about 87, and the corresponding UAV flight speed is about 1.2m / s.

[0031] Compared with the addition of the optical switch 2, the signal-to-noise ratio of the linear array swing scanning imaging spectroscopy system is slightly reduced, but the flight scanning efficiency is greatly improved. Therefore, from the theoretical calculation comparison, it can be seen that the present invention significantly improves the scanning efficiency of the linear array swing scanning imaging spectroscopy system.

[0032] It should be noted that the 1×3 optical switch 2 represents a 1×N 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 1, and the "1" output optical fiber is coupled to the slit 41 of the spectrometer 4. Furthermore, by adopting a 1×1 optical path on-off optical switch 2, the on-off sequence and time of the beam splitting output ends of different optical switches 2 can be controlled by a given timing, and the fast time-sharing detection function of different fields of view can also be realized.

[0033] In some embodiments, the beam splitting output end of the optical switch 2 is connected to the beam splitting optical fiber 3 .

[0034] In some embodiments, the optical fiber array end 21 includes no less than two optical fibers, and no less than two optical fibers are spliced ​​by staggered splicing to achieve seamless splicing in the vertical scanning direction of the telescope objective lens 1 .

[0035] It should be noted that when arranging optical fibers without gaps in the vertical sweeping direction, since the optical fibers have outer claddings, it is impossible to directly achieve gapless splicing between the optical fibers. Figure 1As shown in , the staggered splicing method is used to achieve seamless fiber array splicing in the vertical sweep direction to avoid splicing gaps in ground pixels. Figure 3 As shown in the figure, the optical fiber is placed at the edge of the field of view of the telescope objective lens 1, which can make full use of the field of view of the telescope objective lens 1 itself, reduce the angle of the swing scanning process, and thus improve the swing scanning detection efficiency. Assuming that the total field of view of the swing scanning needs to be 60°, and the field of view of the objective lens itself is 30°, then the single swing scanning angle is 30°, which can achieve a 60° swing scanning field coverage, that is, the scanning detection efficiency is increased by 1 times. The ground coverage effect of a single swing scanning is as follows Figure 4 shown.

[0036] According to the different specific parameters of the linear array swing scanning imaging spectroscopy system, the number and arrangement of optical fibers can be slightly adjusted on this basis, but the basic principle is to use the field of view of the telescope objective 1 to reduce the scanning angle and improve the scanning efficiency. Obviously, under this principle, the scanning efficiency improvement ratio of the linear array swing scanning imaging spectroscopy system depends on the ratio of the swing scanning field of view to the field of view of the telescope objective 1.

[0037] In addition, since the number of bending times and bending diameter of optical fiber are limited, the length must be considered when designing the linear array scanning imaging spectroscopy system. While ensuring the bending space of the optical fiber, it should not be too long to avoid unnecessary increase in system weight.

[0038] For the selection of optical switch 2, attention should be paid to whether its available spectral band meets the requirements, the switching state or channel conversion speed, life, repeatability and the light suppression ratio of the switching state. The spectral range of the optical switch determines whether it can be used for the detection of the designed spectral band, the switching state or channel conversion speed determines the detection efficiency of the linear array scanning imaging spectroscopy system, the switch channel conversion life affects the cost performance of the linear array scanning imaging spectroscopy system, and the repeatability and switch light suppression ratio determine the impact of the optical switch 2 device on the detection accuracy of the imaging spectrometer 4.

[0039] In some embodiments, the telescope objective lens 1 includes a single off-axis parabolic mirror, and the telescope objective lens 1 only images the target in the near-axis central field of view.

[0040] 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 the disclosure of the present invention can be performed in parallel, sequentially or in different orders, as long as the desired results of the technical solution disclosed in the present invention can be achieved, and this document does not limit this.

[0041] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An airborne high-frequency linear array scanning imaging spectroscopy 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 slit of the spectrometer, the optical waveguide device includes an optical switch and a beam splitting optical fiber connected in sequence, the optical fiber array end of the optical switch is coupled to the focal plane of the telescope objective, the optical switch quickly switches the on and off of each optical fiber at the optical fiber array end, and the beam splitting optical fiber makes the scanning result of the optical fiber array end correspond to the incident slit of the spectrometer, so as to realize swing scanning imaging.

2. The airborne high-frequency linear array scanning imaging spectroscopy system based on optical waveguide devices according to claim 1 is characterized in that: The beam splitting output end of the optical switch is connected to the beam splitting optical fiber.

3. The airborne high-frequency linear array scanning imaging spectroscopy system based on optical waveguide devices according to claim 1 is characterized in that: The optical fiber array end includes no less than two optical fibers, and no less than two optical fibers are spliced ​​by a staggered splicing method to achieve seamless splicing in the vertical sweeping direction of the telescope objective lens.

4. The airborne high-frequency linear array scanning imaging spectroscopy system based on optical waveguide devices according to claim 1 is characterized in that: The telescope objective lens comprises a single off-axis parabolic mirror, and the telescope objective lens images a target in a near-axis central field of view.

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