The Technology Of The Acousto-Optic Tunable Filter Based On The Hyperspectral Imaging System

In recent years, the researchers have been attracted more and more attentions on the technology of spectral imaging, for its wide applications in the fields of the military detection, fine agriculture and biomedicine. The spectral imaging technology, with the resolving power both on the space and on the spectral, is the perfect combination of the image analysis and the spectral analysis.The study on the prismatic element is one of the key technologies in the area of the spectral imaging. The acousto-optic tunable filter (AOTF) is a newfashioned prismatic element. It has some prominent merits compared with ordinary prismatic elements: AOTF is small and light, its structure is solid without moving parts, both the clear aperture and the incident angular aperture are big, high diffraction efficiency, it can be tuned neatly and fast in the wide field. These merits indicate the large application potential of AOTF in the spectral imaging area. The study on the AOTF being applied on the spectral imaging has become a bright point for the international researchers. But it is incontestable that, the spectral imaging based on the acousto-optic tunable filtering is a new applied science, many basic theories and the practical problems are need to be resolved by the active investigation. In our country, we begin the study of AOTF based spectral imaging comparatively late. At present, our study has not entered the practical phase; we still need to do a great deal of studies. Therefore, the technology of the acousto-optic tunable filter based on the spectral imaging system is selected as the main studied object of this paper. We have done many theoretical studies. It is for the purpose that, the wide applications of the acousto-optic tunable filtering technology on the speatral imaging area would be pushed.Firstly, the development of the spectral imaging technology and AOTF is introduced. The application of AOTF on the spectral imaging and its trend is analyzed. The main studied content and the meaning is made clear.Secondly, the working principle of AOTF is described in detail. AOTF operates on the extraordinary Bragg diffraction. We introduce the acousto-optic interaction physics, the Bragg diffraction, the optical and acoustic characters of the acousto-optic interaction crystal. We describe the working principle of the noncollinear AOTF and the main technical parameters. The parallel tangents momentum-matching condition and the design method of the piezoelectric transducer are also introduced. These theories have made a foundation for the following study on the AOTF based hyperspectral imaging system.Thirdly, we have studied the noncollinear AOTF which is applied on the spectral imaging. We have pointed out that the previous relationship of the acousto-optic interaction existed obvious error. By considering the birefringence together with the rotatory property of TeO2 crystal, the modified relationship of the noncollinear acousto-optic interaction is deduced. This modified relationship is meaningful to improve the design accuracy of the noncollinear AOTF. On the base of the modified acousto-optic interaction relationship and the working principle of AOTF, some important contents in the design of noncollinear AOTF, such as the selection of the optimum incident polar angle and the image resolution have been discussed. An integrated method of designing the noncollinear AOTF has been established.Fourthly, we put forward a theory of designing AOTF with high spectral resolution for the applications of the hyperspectral imaging. AOTF with high spectral resolution is the foundation of good detection performance of hyperspectral imaging system. On the base of the modified acousto-optic interaction relationship, modified formula of the spectral resolution of TeO2 AOTF is deduced; it ensures the accuracy of computing the spectral resolution. In the study, a new method, that is called double-filtering method, is put forward to improve the spectral resolution by the properly assembling two separate AOTFs. We have discussed the basic principle of double-filtering method which can achieve the improved spectral resolution. The two separate AOTFs are designed and the detailed parameters are given by calculation. Through discussing the factors that can influence the spectral bandwidth in double-filtering, it is found that the spectral resolution can be obviously improved when we increase the acoustic frequency interval of the two AOTFs at a certain reference acoustic frequency. We also see a merit of double-filtering method on sidelobe suppression. Since the acousto-optic diffraction efficiency decreases with the increasing of the acoustic frequency interval, we have introduced factor Q which represents the spectral sharpness of the optical signal. By studying the change of Q factor, it is indicated that, an equal point of Q factor exists at a certain reference acoustic frequency when the acoustic frequency interval increases to some extent. At the equal points, the spectral bandwidth is decreased obviously, the acousto-optic diffraction efficiency holds on a higher level. This study confirms that the introduction of the equal points of Q in double-filtering can be used as a functional method of improving the spectral resolution of AOTF with a good signal performance. The method of double-filtering has provided a new way of improving the spectral resolution of AOTF effectively.Lastly, we have designed the VIS/NIR hyperspectral imaging system based on AOTF with high spectral resolution. We have discussed the design idea, basic composition and the key technology about the design of the hyperspectral imaging system. In the design of AOTF imaging modular, the theory of double-filtering is used to realize the high spectral resolution of hyperspectral imaging system. On the base of the principle of electricity induced phase retardation, the polarization control equipment is designed by the liquid crystal variable retarder, and the system is made to have the ability of identifying the polarization property of the object. The designed VIS/NIR hyperspectral imaging system based on AOTF with high spectral resolution is very simple and compact; it can be operated flexibly with a high level of automatization, the need for the application in various fields can be satisfied. Besides, we design an experiment to test the performance of VIS/NIR AOTF based hyperspectral imaging system with high spectral resolution. By describing the main implementation methods and the key technology of these performance testing experiments, we have provided an objective criterion and reference for evaluating the performance of the hyperspectral imaging system.In this paper, we have synthetically developed the corresponding theoretical study on the technology of the acousto-optic tunable filter based on the hyperspectral imaging system. By the investigation of this paper, we have got the modified relationship of the noncollinear acousto-optic interaction, and have acquired the design method of AOTF with high spectral resolution. The VIS/NIR AOTF based hyperspectral imaging system with high spectral resolution which is designed in this paper, would obviously push the development and wide application of the spectral imaging technology.