Microwave Signal Generation And Processing Based On Electro-optic Phase Modulator

Microwave photonics is an interdisciplinary area that studies the interaction between microwave and optical signals. With the frequency of wireless communication entering millimeter-wave region and the bandwidth increasing in optical communication network, microwave photonics will play a more important role in the next generation communication network. To realize the interaction between microwave and optical signals, an external electro-optic modulator is indispensable in most microwave photonic systems. As the most simple electro-optic modulation structure, electro-optic phase modulators can be made up of any crystals or organic polymers with electro-optic effects. In comparison to common Mach-Zehnder intensity modulators, phase modulators offer several advantages, such as simple structure, low loss, and no biasing circuit required. The microwave signal generation and processing based on electro-optic phase modulators is deeply studied in this thesis. The main content of the thesis is divided into five parts as follows:Firstly, the theory and practice in phase modulation (PM) to intensity modulation (IM) conversion is reviewed. PM signals can be converted into IM signals by using dispersive optical fiber, optical filters, and stimulated Brillouin scattering. We propose and experimentally demonstrate that a fiber Bragg grating (FBG) can be used to simultaneously realizing PM-IM conversion and efficiency improvement in fiber-optic links.Secondly, microwave photonic filters based on phase modulators are extensively investigated. A bandpass microwave photonic filter with a phase modulator is simulated and discussed. By utilizing the properties of phase-modulated optical signals, microwave photonic filters with negative coefficients can be conveniently constructed. We experimentally demonstrate a single-bandpass filter based on PM and stimulated Brillouin scattering.The third part is focused on the application of phase modulators in Ultra-wideband (UWB) systems. The optical methods to generate UWB signals utilizing phase modulators are discussed and analyzed. A tunable UWB filter based on a phase modulator and a narrow-band FBG is proposed and experimentally demonstrated. The 3-dB passband of the photonic UWB filter is about 3.3-10.6 GHz, and the phase of the filter is quite linear.All-optical microwave generation based on phase modulators is investigated in part four. We theoretically prove that only even-order electrical harmonics exist when a carrier-suppressed phase-modulated signal is detected by a photodetector. With the help of Brillouin-assisted carrier-filtering and PM, a high quality microwave signal is experimentally obtained.The last section is about photonic microwave frequency conversion with phase modulators. After concluding the different photonic microwave frequency conversion methods, we propose an approach to upconverting a microwave signal by using a photonic crystal fiber Brillouin laser and a phase modulator. The 3 GHz and 5 GHz input signals are respectively upconverted to 12.76 GHz and 14.76 GHz. Utilizing PM instead of IM, the input intermediate frequency signal will not appear at the output electrical spectrum of the photodetector.