Researches On Realization Of Time Reversal System And Related Key Components

With the property of synchronous space-time focusing, time reversal technique has drawn extensive attentions since it was introduced into the field of electromagnetism in 2004, and was theoretically applied in ultra-wideband electromagnetic wave communications, radar detection, and imaging systems. The difficulty in obtaining timereversed microwave electromagnetic signal is the main obstacle of blocking time reversal system to practical applications. This dissertation focuses on the research of realization of time reversal systems and related key components, and the main contents are introduced as follows.The first part mainly researches on the realization of time reversal mirror based on the analog signal processing technology. For one thing, using chirped dispersive delay lines, two kinds of time reversal waveform transformation solutions respectively based on Fourier transform theory and advanced time-lens system are conceptly proposed, theoretically analyzed, numerically calculated, and experimentally verified. Compared to conventional implementation scheme system with chirped dispersive delay lines, the newly suggested systems are more stable with higher time-bandwidth products, and are able to process longer time duration and wider bandwidth, which provide theoretical and numerical basis for physically achieving time-reversed systems. In addition, utilizing transient medium and instantaneous coupling technique, two types of time reversal waveform transformation schemes are presented based on transient system. Theoretical analysis and numerical results are also respectively provided to confirm the scheme feasibilities. This dissertation also emphatically analyses the method of using instantaneous coupling technique to achieve time-reversed signals with related experimental settings. Additionally, numerical software MATLAB and ADS are used to affirm the operability of the experiment scheme. The four proposed systems in this dissertation provide extremely reference value for achieving efficient and real-time time reversal mirror.The second part concentrates on the extension research of time reversal waveform transformation systems based on chirped dispersive delay lines. As the core device of the system, we firstly analyze the design approaches of achieving wide band, miniaturization, low loss dispersive delay lines. Based on characteristic impedance modulation, composite right/left handed transmission line and microstrip dispersion characteristics, this dissertation successively introduces three designment and finally obtained several satisfied dispersive delay lines. The designed structures are used in corresponding systems in this dissertation. In addition, on the basis of linear dispersive delay line and waveform transformation systems, this dissertation presents a demonstration of temporal cloaking / uncloaking at microwave frequencies for the first time. Theoretical formula derivation, numerical calculation and practical implementation considerations with designed chirped dispersive delay lines are discussed in relation to the proposed system architecture and its potential suitability in secure communications. Compared to traditional signal processing systems, since the recovered data emerges with a reversed form in time domain before its final decoding, an extra operation named time-reversal is needed to obtain the correct data, which could help dual-protect the significant signals and leads to a more reliable communication. The proposed method and achieved results indicate a predictable practical implementation in secure communications. Last but not the least, on the basis of the mentioned temporal cloaking system, this dissertation proposes a new approach for hidden wireless communication based on time reversal, and experimentally demonstrates that the presented system could use dispersive delay lines to achieve temporal cloaking / un-cloaking, and the related dispersive delay lines are not required to provide linear group delay response. Thus the proposal simplifies the system structure for temporal cloaking, avoids the drawbacks of easy detection, easy decoding and easy analyzed brought by linear modulation system, and then strengthen the safety and reliability in secret communication.In the third part, the principal contents are super-resolution focusing and imaging detection based on time reversal technique. First of all, using a regular arrayed metal wire array structure, which could provide an evanescent-wave channel, and by changing the signal incentives to the time reversal mirror or changing the distribution of metal wire array, we not only achieves the extraction of source signals and satisfied super-resolution at the original place, but also obtains source reconstruction with super-resolution at different locations, and the best refocusing effect reaches λ/14. The proposed imaging structure has important research significance in off-site extracting and reconstructing source signals, far-field super-resolution imaging and nondestructive detections. Besides, through adding dispersive delay lines to each element of the time reversal mirror, this dissertation proposes a new method for enhancing the imaging effect based on time reversal technique. Two situations have been analyzed, which are target detection in low dielectric but cluttered environment and tumor detection in human dispersive lossy medium, respectively. Corresponding theoretical analysis and numerical calculation are proceeded to demonstrate that the proposed method could help reduce the impact of ambient scatters and enhance the focusing effect at the target or tumor. When compared with traditional time reversal detecting systems, the proposal makes it unnecessary to preestimate the position of the target and removes the need for subsequent complex operations as traditionally required. The proposed method could directly simplify the detection steps and strengthen the detection results.