Photonic Processing Techniques Of Microwave Signals And Millimeter-Wave Band ROF System Design Based On Lithium Niobate Modulators

Microwave photonics is an interdisciplinary subject which combines microwave engineering and photonics. It covers microelectronic technology, optoelectronic devices, key enabling techniques, and application issues etc. Its original intention is to introduce photonic technology in microwave systems, which can eliminate the so-called electronic bottleneck and meanwhile lead to many unique advantages, such as low loss, light weight, small size, broadband, immunity to electromagnetic interference (EMI), and flat frequency responses etc. Therefore, several powerful functions which are commonly complex or even impossible in microwave or electrical domain can be implemented with the help of microwave photonic technologies. These unique advantages of microwave photonic technology have resulted in diverse applications, such as signal processing, telecommunications, military or defense systems, aeronautics & astronautics, and medical treatments. As the two major research branches of microwave photonics, photonic processing techniques of microwave signals and radio-over-fiber (ROF) systems have recently attracted great interests throughout the world. However, the corresponding researches are just at the initial stage in China. The domestic level greatly falls behind the overseas state of art in terms of research achievements, strength, and funds. In this dissertation, several key photonic processing techniques of microwave signals and the design issues of ROF systems have been intensively investigated for broadband wireless access applications and military purpose under the sponsorship of national 863 programs and national natural science foundation in China.On the other hand, Lithium Niobate (LiNbO₃) modulators have been widely applied in high-speed digital optical communication systems for implementing electrical-to-optical (E/O) conversion. E/O conversion is also required in a typical microwave photonic system. Due to their unique advantages, LiNbO₃ modulators are also preferred as the E/O conversion devices in microwave photonic systems. With rapid development and commercialization of LiNbO₃ modulators, the cost of an individual device is supposed to be dramatically reduced. Therefore, LiNbO₃ modulators are expected to play more important roles in microwave photonic systems, and to be used for implementation of more functions (not limited to E/O conversion). Furthermore, to our best of knowledge, there are few reported research works about the exploration of LiNbO₃ modulators in microwave photonic systems for achieving more functions. This dissertation is just oriented to extend the application scope of LiNbO₃ modulators. In this dissertation, several novel photonic processing techniques of microwave signals and novel solutions to the ROF system design have been proposed and demonstrated based on the characteristics of LiNbO₃ modulators. In these proposed techniques or schemes, LiNbO₃ modulators are utilized to perform more advanced functions. Moreover, employing LiNbO₃ modulators can bring several benefits, such as simplicity, stability, and high-performance etc. To sum up, the research works in this dissertation will provide important references for future diverse applications of LiNbO₃ modulators in microwave photonic field.The main innovative research efforts are summarized as follows.1. Photonic processing techniques of microwave signals using LiNbO₃ modulators1) Microwave photonic filtersTwo novel structures of microwave photonic filters have been proposed and numerically or experimentally verified using a LiNbO₃ phase modulators and a dual-drive Mach-Zehnder modulator (MZM) respectively. The limitations which typical microwave photonic filters suffer under incoherent operation condition can be eliminated by employing the proposed approaches. Thus bandpass filtering characteristics are realized, and meanwhile the proposed approaches feature further simplicity and flexibility. In one scheme, counter-phase modulation is achieved based on polarization modulation in a phase modulator. Finally, a general frequency response of a microwave photonic filter with negative coefficients has been achieved, which leads to a bandpass filtering characteristic. In the other scheme, equivalent bandpass filtering can be achieved by constructing a compound transfer function based on single sideband modulation. Both the proposed schemes are characterized as tunability and reconfigurability.Considering that the current research on different photonic processing techniques of microwave signals are separate from each other, we for the first time studied the possibility of the convergence of two different processing techniques. The feasibility was demonstrated by theory and simulations. In the second proposed scheme, bandpass filtering can be integrated with all-optical frequency up-conversion in a single ROF system. The intermediate-frequency signals can be simultaneously bandpass-filtered and up-converted to radio-frequency band, which is favorable for the cost reduction of a ROF system.2) Impulse radio-ultra wideband (IR-UWB) pulse generationPhotonic IR-UWB pulse generation techniques can not only overcome several limitations by conventional electrical methods, but also fit into UWB-Over-Fiber systems. In this dissertation, four novel approaches to monocycle or doublet IR-UWB pulse generation have been proposed and experimentally demonstrated. The key devices are MZMs or phase modulators. The experimental results show agreement with the theoretical prediction. The experimental results also indicate that the fractional bandwidth of generated pulses all exceed 100% at the central frequency around 5～6GHz, which is complicated by conventional electrical methods. The four schemes have their own strengths and weaknesses, which lead to different application scenarios. One can choose the suitable schemes according to different requirements on applications and system configuration.Furthermore, although the photonic approaches to IR-UWB pulse generation have been widely reported, there are few reported results on IR-UWB pulse modulation based on microwave photonics. However, all-optical IR-UWB pulse generation and modulation are simultaneously achieved using the proposed four approaches. The corresponding schemes for photonic pulse modulation have been proposed, which can reduce the cost and facilitate the practicability of UWB-Over-Fiber systems.3) Microwave frequency measurementTwo novel methods have been proposed using LiNbO₃ modulators for microwave frequency measurement. Compared with conventional frequency measurement approaches, both the proposed methods feature immunity to EMI, low loss, broad frequency coverage, and fast response etc, which is favorable for electrical warfare applications. In one method, delay and frequency mixing functions have been achieved in the optical domain, which can finally relate microwave power with frequency. The idea originates from the fundamental principle of a conventional instantaneous frequency measurement (IFM) receiver. Single wavelength operation is achieved by using a dual-output MZM, which can avoid additional optical source and wavelength multiplexing devices. The experimental results indicated that the measurement error maintains within 50MHz around the commonly-used 7～12.7GHz. The measurement range can also be tuned. In the other method, the microwave frequency to optical power mapping has been achieved using two cascaded MZMs which are working at carrier-suppressed double sideband modulation mode. The frequency information can be obtained by directly monitoring the optical power, which can exempt photodetectors and ultimately reduce entire system cost. The common ground of both proposed schemes is to perform some functions in the optical domain leaving several simple operations in the electrical domain. Since the transmission time of light in a link is very short, fast response can be achieved.Due to the limitation of our experimental condition, the experimentally demonstrated measurement range was limited. However, the range can be extended to millimeter-wave band using the proposed approaches.2. Millimeter-wave band ROF system design and transmission experimentsThe study on millimeter-wave band ROF system design and transmission experiments has been carried out considering the development trend of millimeter-wave band ROF systems. In the proposed solutions, LiNbO₃ modulators were all used to perform E/O conversion and several other processing functions. At first, a novel scheme has been proposed for high frequency utilization efficiency in full-duplex millimeter-wave band ROF systems. In the proposed scheme, a novel all-optical frequency down-conversion method was achieved by using external modulators. High frequency utilization efficiency was achieved by using an optical interleaver. The proposed scheme can avoid light sources and microwave sources at base station (BS), which can reduce the cost and realize centralized maintenance and management. Secondly, the transmission experiments have been carried out based on polarization multiplexing for hybrid wireless and fixed access. Error-free 25km fiber transmission has been achieved for 2.5Gbit/s fixed service and 2.5Gbit/s wireless service at 20GHz RF band. The power penalty remains within 1dB for each service. Finally, fiber and wireless transmission experiments have been done at 40GHz band. Error-free >2m wireless transmission and 25km fiber transmission have been achieved for 1.25Gbit/s data rate at 40GHz band. The power penalty is not larger than 1dB.To sum up, the entire works in this dissertation are involved in the research field of microwave photonics, and meanwhile cross two subjects (i.e. electronic science and technology & information and communication engineering). The common ground on novelty lies on extending the available application fields of LiNbO₃ modulators by special designs. In terms of contents, the research works cover several key unit processing techniques, ROF system design, transmission experiments, and related application issues. In terms of investigation approach, each work was demonstrated from the following five aspects: research background, theoretical analysis, numerical simulations, experiment verification and result discussions. In terms of application fields, the research works are oriented to broadband wireless access systems based on ROF, electrical warfare systems, radar systems, and several other fields where high-performance signal processing functions are required.