Analysis And Design Of Terahertz Detection Scheme For Millimeter Wave Applications

With development of terahertz technology for imaging, communication and spectroscopy need of new sources and detectors has emerged as only solution to overcome this terahertz (THz) gap. Role of System compatible antennas and interconnect, which may also play a pivotal role in the development of reliable terahertz systems used in field of imaging, communication and spectroscopy, is very important for future developments. For these devices traditionally used wideband antennas, for instance log spiral and log periodic antennas, with large dimensions have come forward as bottle neck to expand this field for high resolution imaging tool. While conventionally used compact antennas, for instance double slot antenna, are narrowband antennas and are not very appealing to fulfill the requirements of wide bandwidth and high gain. Waveguide based designs suffer from fabrication issues at high frequency. Moreover large dimensions and high cost restrict its use for terahertz (THz) applications. A lot of work has been done on dipole antennas for several decades to improve its bandwidth as well as radiation performance with compact electrical dimensions. Many dipole antennas, like bow tie and twin dipole, have been used in terahertz range but poor coupling of such antennas to quasi optical receiver and low directivity in some designs are reported. Furthermore all these antennas are linearly polarized while circular polarization is basic property required in antennas to be used for communication systems.This dissertation addresses designing and characterization of two novel wide band circular polarized (CP) terahertz (THz) antennas to be used for terahertz imaging and communication. A wideband circular polarized cross bow tie antenna having stable radiation patterns working at terahertz range is proposed and optimized to improve its radiation and polarization characteristics. Designed antenna shows constant impedance for reflection coefficient (S11) lower than-10dB,3dB axial ratio (AR) band width of50%and symmetrical radiation patterns. Geometrical modification of proposed antenna is proposed to achieve circular polarization and to control impedance of cross bow tie antenna to connect it to other active device. The other proposed novel antenna is compact circular polarized clover leaf antenna with symmetric patterns and good gain. W band performance of planar clover leaf antenna obtained by FEM based HFSS shows wide band behavior. Parametric study of both antennas has been carried out using finite element method (FEM) based software HFSS to study how dimensions influence antennas performance. This parametric study will be very helpful to get better performance and use these novel designs for other range of frequencies. A conventionally used circular polarized log spiral antenna has also been designed in this work to make a comparative analysis of conventional and proposed designs. In order to verify designed antenna’s performance cross bow tie antenna is fabricated and tested at X band. Cross bow tie antenna showed satisfactory performance at X band.Quasi optical approach is adopted for tackling substrate mode loss issues. A hybrid method is proposed which is based on combination of finite element method (FEM) used to get far field radiation of planar antenna. This far field radiation is used as source to lens which is further processed using ray tracing inside lens. Further Fresenel coefficients are used to obtain electric and magnetic field and then diffraction theory is used to obtain radiation pattern of dielectric lens coupled antenna structure. This proposed hybrid method have been used in this dissertation to evaluate far field characteristics of both designs with lens structure and find most appropriate dimensions of lens to get best performance of designs with lens. Results obtained by hybrid method showed reduction of substrate mode loss and high broadside gain as well as directivity. In order to validate results of hybrid method a comparative analysis of hybrid method and other software results has been done. Comparison of these methods has shown similarity of both method’s results emphasizing hybrid method used here is good enough for evaluation of large apertures.Array performance of designed antennas at back of quartz lens is analyzed. Proposed focal plane array (FPA) consist of novel cross bow tie and clover leaf antenna on quartz substrate behind the quartz hyper hemispherical lens. FPA performance is studied at low and high frequencies to determine the pixel pitch and number of effective pixels. Off axis performance of FPA is studied carefully to form a densely packed FPA with uniform patterns to high resolution imaging system. Some geometric variations are also made to improve off axis pixel patterns and overcome the reflection losses from lens. Compact dimensions of proposed planar antennas are resulted in densely packed array as compared to conventional self complementary antennas. Uniformity of far field patterns is achieved by proper placement of adjacent pixels.Most of the previously reported work has been focused on basic properties of linearly polarized slot antenna as feed on silicon or quartz substrate lens but such narrow band structures have different resonant behavior at such high frequency. Therefore numeric study of substrate lens mounted circular polarized antenna has discussed and clarified some issues related to substrate lens mounted circular polarized antennas. As well as array performance of circular polarized antenna in focal plane array format with quartz lens is also analyzed. These study deal with polarization diversity with substrate lens, effect of off axis displacement on circular polarized behavior of lens antenna which determine number of pixels for any application, far field patterns variation due to lens’s internal reflections, directivity variation due to internal reflections and losses.Finally future work with possible achievements is presented. Implementation of proposed design for multichannel detector and for making focal plane arrays for large imaging systems is expected to achieve great success because of compact dimensions and good radiation performance. For wireless communication use of reflector or cavity with these planar antennas will proved to be very useful and work on it will be of great importance.