Research Of Optomechanical Uncooled Infrared Imaging

Infrared (IR) imaging technique plays a critical role in military, medical, meteorologic, agricultural, and industrial applications. Opto-mechanical uncooled infrared imging technique becomes a hotspot of research because of its attractive features like low cost and high theoretical temperature resolution. However, the traditional theoretical models of IR energy conversion efficiency H and thermo-mechanical sensitivity ST are not suitable for substarte-free FPA, and the traditional theory of the optical responsivity (?) does not agree with experiments. To solve these problems, new and absolutely suitable theoretical models are established in this dissertation. Based on the models, substrate-free FPA and optical filtering readout technique are optimized and improved.When substrate-free FPA is radiated by IR radiation, the accurate temperature distribution of substrate-free FPA can be calculated by finite element method (FEM). However, FPA usually contains more than 100,000 pixels, so the calculation will be very large and the calculation time will be very long. Apparently FEM is unsuitable for optimization design of substrate-free FPA. So a theoretical approximate formula is needed. A thermal resistance model of substrate-free FPA is established in this dissertation considering the supporting framework's thermal resisitance. After reasonably simplifying the model, the theoretical expressions of H and ST are derived. Compared to FEM results, the error of the model is less than 10%. The theoretical expression of the thermal response time of substrate-free FPA is also obtained. To validate the expression, a substrate-free FPA contained four imaging areas (the design parameters of the areas are different) is fabricated. The experimental results of the thermal response time well match the theoretical results. The imaging performance of the substate-free FPA situated in atmosphere is deeply analyzed. Analysis shows that the traditional theory can not explain the phenomenon that substrate-free FPA can obtain the thermal image at room temperature even satiated in atmosphere. To explain this phenomenon, a thermal resistance model of the substrate-free FPA situated in atmosphere is established.The error of the traditional optical responsivity theory results from not considering the morror's undesired deformation. In this dissertation, a theoretical optical responsivity model related to deformed mirror is established. Theoretical analysis and experiment both show that the bending deformation will seriously degrade the optical responsivity. To reduce the deformation of mirror, three kinds of structures, namely thin-metal-film structure, enhanced-beams structure and two-layers structure, are designed and fabricated. Experiments demonstrate that:For 180-μm mirror, thin-metal-film structure increases its deformation radius from 5.3mm to 25mm, the optical responsivity correspondingly increases 5.2 times. Enhanced-beams structure makes the deformation radius of the mirror increase 4 times, the optical responsivity increases 15%. The two-layers structure makes the mirror like an I beam, so the deformation of the mirror will be very little. To reduce blindness of production process and avoid unnecessary loses, for the mirror with length L, a critical deformation radius Rc is given. If its deformation radius is bigger than Rc, the deformation of the mirror doesn't need reducing. Theoretical analysis shows that there exists an optimal mirror length which makes the optical responsivity achieve its maximum under a certain mirror deformation. Based on the results, a substrate-free FPA with optimal mirror length is designed and fabricated. The experimental results of the performance of the FPA agree with the theoretical results.The visible light source usually is not an ideal point, and is nonmonochromatic. The influences of the spectral width and the spatial width of the light source on the optical responsivity are discussed. Analysis shows that the influence of the spectral width of LED can be ignored. The optical responsivity rapidly decreases with increasing the spatial width of the light source. However, for the mirror with length L, when the half width of the light source is less than Hc (Hc is defined as the critical half width of the source), the light source can be considered to be a monochromatic point light source.Under the constraint condition that the light intensity must not exceed CCD's full scale, the influence of the filtering location of knife-edge on the optical responsivity is researched. Then a dimensionless theoretical model is established. Analysis shows that:For plane mirror, when knife-edge blocks three quarters of the diffraction spectrum, the optical responsivity reaches its maximum; for deformed mirror, the optimal filtering location of knife-edge relates to the deformation radius. Compared to traditional knife-edge central filtering location, through optimizing the filtering location of knife-edge, for plane mirror, the optical responsivity can increase 80%; for 55-μm deformed mirror (deformation radius 6mm), theoretical analysis and experiment both demonstrate that the optical responsivity can increase 33%.For optical filtering readout technique, a general opitcial responsivity model is established for the first time. The model offers a theoretical basis for optimization design of optical filter. A detailed analysis is carried out for four filters, such as knife-edge, silt, rectangular hole and circular hole. The light utilization efficiency (LUE), optical resolution and dynamic range of the four filters are detailedly dicussed. The analysis results show:slit filtering mechod is always better than rectangular and circular hole; Compare slit filtering mechod with knife-edge filtering mechod, slit filtering mechod has the highest optical responsivity, knife-edge central filtering mechod has the highest LUE, optical resolution and dynamic range, and the performance of knife-edge optimum filtering mechod is between the two filtering methods.Under different filtering methods and different CCD background intensity, the system noise is measured by experiment. It's demonstrated that increaing the background intensity of CCD and improving optical filtering method not only can increase the optical responsivity, but also can decrease NETD efficiently.