Research And Expriments On Generation Of High Frequency Microwave Sigals Based On Electro-Optic External Modulation

Recently, microwave signals with higher frequency are ungently required in electric warfare, radar, and next communication system. Conventional microwave signal generation based on the electrical technology is limited by "electrical bottleneck" so that it cannot meet the requirements of high-frequency applications. As a promising solution, photonic approach has advantages over the electrical one in high frequency, good stability and flexible tunability. In this thesis, theoretical and experimental research on photonic microwave signal generation is demonstrated. The main contents are listed as follows.(1) We start with the working principle and the modulation characteristics of electro-optic Mach-Zehnder modulator. In this thesis, the modulation characteristics are analyzed in detail for the small signal modulation and the large signal modulation. Subsequently, cascaded modulators are used to generate4-,6-and8-fold high-frequency microwave signal. The external modulation scheme, which gets high frequency signal using low frequency driving signal, exhibits excellent tunability and stability.(2) A photonic approach is proposed to generate microwave signals with24-fold frequency of the seed signal, which is based on cascaded modulator structure and four-wave-mixing effects. Both theoretical analysis and experiments are performed. Due to the inherent nonlinear modulation of MZM, multiple harmonics can be generated. By controlling the initial phase of the microwave signal and the modulation depth, high order sidebands can be obtained without filtering. Then, utilizing the third-order ultrafast nonlinear effects in highly nonlinear optical fiber(HNLF), higher-order harmonic components can be obtained. Here, the±12th sidebands are selected by an optical filter and sent to a high-speed photodetector. As a result, a microwave signal with24-fold frequency of seed signal is achieved. Both experiment and simulation are performed to verify the feasibility of the approach. When the microwave drive signal is tuned from11GHz to12GHz, the frequency differences between±12th sidebands vary from264GHz to288GHz with a suppression ratio of35dB for other harmonics.(3) Finally, a photonic approach using a single polarization modulator(PolM) and an optical interleaver to achieve frequency-octupling microwave signal is theoretically investigated and experimentally verified. Under large signal modulation, high order optical sidebands are generated thanks to the modulation nonlinearity from the PolM. By carefully controlling the polarization controller(PC) placed before the polarizer, only even-order sidebands are kept. By adjusting the modulation depth, the power of the4th sidebands is significantly higher than that of the6th sidebands and other sidebands simultaneously. Then an interleaver is adopted to further suppress the optical carrier and the6th sidebands, such that the4th sidebands are purified. According to the relationship between harmonic power and modulation depth, an appropriate modulation depth is specified to ensure minimum clutter harmonics. Here, two filtering approaches are designed based on the optical interleaver filter. By applying interleaver with different channel spaces(i.e.,25GHz and50GHz), continuously adjustable microwave signal with frequency octupling is obtained within four frequency bands. The simulation platform and experiment system are established based on the theoretical analysis, with high-quality octave microwave signals obtained in these two filtering schemes.