tailieunhanh - Microwave & millimeter wave technologies from photonic bandgap devices to antenna and applications phần 16

Công nghệ cơ khí hay kỹ thuật cơ khí là ngành ứng dụng các nguyên lý vật lý để tạo ra các loại máy móc và thiết bị hoặc các vật dụng hữu ích. Cơ khí áp dụng các nguyên lý nhiệt động lực học, định luật bảo toàn khối lượng và năng lượng để phân tích các hệ vật lý tĩnh và động, phục vụ cho công tác thiết kế trong các lĩnh vực như ô tô, máy bay và các phương tiện giao thông khác, các hệ thống gia nhiệt và làm lạnh, đồ dùng gia đình, máy. | Modelling and Design of Photonic Bandgap Devices a Microwave Accelerating Cavity for Cancer Hadrontherapy 441 several and important differences between optical and microwave resonators some parameters as quality factor Q are very useful for both kind of cavities. The quality factor Q is defined as Q rooU W 1 where 00 is the resonance frequency U the electromagnetic energy stored in the cavity and W the lost power. Losses in the dielectric material and radiations from small apertures can cause the lowering of Q. The Q factor allows to evaluate also the filter bandwidth defined as Aro roo 1 Q 2 where Aa is the range between two frequencies at which the signal power is 3 dB lower than the maximum value. It can be shown that at a given frequency the Q-factor increases with an increasing order of mode. As seen in the previous paragraph a resonant cavity can be obtained by introducing a defect in a photonic crystal in order to modify its physical properties. In the case of a defectless structure electromagnetic waves can not propagate when the operative frequency is inside the bandgap in which a narrow band of allowed frequencies can be achieved breaking the crystal periodicity through a suitable defect. Light localization is used in the PBG based microcavity design to optimize the Q-factor which depends on the geometrical and physical properties of the defect. Lattice defects are constituted by dielectric regions of different shapes sizes or refractive index values. By changing one of these parameters in the defective region we can modify the mode number of the resonance frequency inside the cavity. Moreover the spectral width of the defect mode is demonstrated to decrease rapidly with an increasing number of repetitions of the periodic structure around the cavity region so improving the selectivity of the resonance frequency inside the bandgap. The excellent performances of PBG structures have been used to develop resonators characterized by high values of Q-factor .