Study On Generation Of Broadband Microwave Photonic Signals Based On External Electro-Optic Modulation

With the development of modern information technology, current bandwidth resources are insufficient to meet the needs of people. Microwave signals with higher carrier frequencies are required to carry the rapid-growth wireless data traffic. Therefore, exploiting and utilizing bandwidth resource in the high frequency range is imperative. However, conventional technologies for generating, transmitting and processing microwave signals suffer from the limitation of the electrical bottleneck in the high frequency range. Under this scenario, microwave photonics which studies interactions between optical signals and electrical signals has been attracting an ever-increasing interest from both the academic community and industry community. It is an ideal method for solving the problems of the generation, transmission and processing of high-frequency microwave signals due to the inherent advantages of photonic technologies, such as large bandwidth, low loss and electromagnetic immunity. This thesis focuses on the photonic generation of microwave signals and chooses the study on the generation of broadband microwave photonic signals based on external electro-optic modulation as the topic. The aims are to explore new approaches to generate single-tone low-phase-noise microwave signals and arbitrary waveform microwave signals, and to improve performances of the generation systems of microwave photonic signals.In this thesis, we theoretically and experimentally study a technique for generating single-tone low-phase-noise microwave signal based on the optoelectronic oscillator (OEO) and a technique for generating arbitrary waveform microwave signals based on the external modulation, respectively. The corresponding studies include the following aspects:firstly, the amplitude characteristic and the tunability of the modulation index of the OEO are analyzed in detail and a high-speed optical pulse generation scheme based on the OEO is proposed. Moreover, the tuning, side-mode suppression and high-frequency output of the OEO are deeply studied, and two novel OEOs have been proposed and demonstrated experimentally. Secondly, the photonic generation of microwave arbitrary waveforms with a flexible and fine parameter manipulation is studied, and an approach for generating chirped microwave pulses using large-signal phase modulation and an approach for generating triangular microwave pulses based on nonlinear modulation are proposed and experimentally demonstrated. The main achievements are listed below:(1) Iterative equations are derived for describing the oscillating process of the OEO. By numerical simulation, the variations of the oscillating amplitude and the loop gain are studied in detail. The open-loop gain condition for keeping OEO stable oscillation is analyzed and the maximum amplitude of the generated microwave signal by the OEO is present. Moreover, the tunable range of the modulation index is analyzed. Meanwhile, novel applications of the OEO with the specific modulation indices are discussed. A high-speed optical pulse generation scheme based on the OEO is proposed, and return to zero (RZ) and carrier suppressed return to zero (CS-RZ) optical pulse trains are generated in experiments.(2) A multi-loop OEO based on cascaded RDLs is proposed. The system response of the cascaded RDLs is analyzed. Employing the cascaded RDLs as both the energy storage device and the mode selector of the OEO, microwave signal whose frequency equals to the least common multiple of two RDLs'free spectral ranges can be generated. This method can both increase the mode space and enhance the side-mode suppression. Moreover, flexible tuning can be achieved by using the electro-optic and thermo-optic effect.(3) A frequency-doubling OEO based on optical carrier processing using stimulated Brillouin scattering (SBS) is proposed. Under the small-signal modulation, a polarization modulator (PolM) is used to generate the double sideband suppressed carrier (DSB-SC) signal which will be converted to a frequency-doubling microwave signal at a photodetector (PD). Then, Brillouin gain is used to amplify the suppressed carrier and to convert the DSB-SC signal to the DSB one, which leads to the generation of a fundamental frequency signal for keeping a stable oscillation. The proposed OEO can generate microwave signals with high frequencies by using low-frequency electrical devices.(4) A photonic approach for generating chirped microwave pulses based on large-signal electro-optic modulation is proposed. A time domain intensity switch that is formed by an intensity modulator (IM) is used to select different phase modulation ranges of two phase-modulated optical signals. Appling a high-speed PD for beating, the desired chirped microwave pulses are generated. The central frequencies, chirp rates and pulse repetition frequencies of the generated pulses are directly determined by the amplitudes, frequencies and initial phases of the microwave signals applied to modulators, leading to the precise control and flexible tunablity to these signal parameters.(5) A photonic approach for generating triangular microwave pulses with a simple tuning of the pulse repetition frequency is proposed. Under the large-signal modulation, an IM working at minimum transmission point (MITP) and an optical bandpass filter (OBPF) are together utilized to generate positive odd-order sidebands. The amplitude and phase of the suppressed optical carrier are controlled by the optical carrier processing through SBS. In this way, triangular pulses can be generated. In addition, the effects of the SBS gain, nonlinear phase shift of SBS, and carrier to sideband ratio on the pulse shape are analyzed, and an experiment has been performed to verify the effectiveness and simple tunability of the proposed approach.In conclusion, this thesis mainly focuses on the generation of broadband microwave photonic signals using external modulation, including the generations of single-tone low-phase-noise microwave signals, chirped microwave signals and triangular microwave signals, which have important applications in fields of communication system, pulse radar and electronic measurement. Moreover, how to use photonic integrated circuit technology for improving the stability and practicability of the system is worth to be further studied.