Research On High Multiple Frequency Technologies Of Optical Microwave Signal Generation-based Intensity Modulators

At present,high-frequency and high-quality millimeter-wave signals have played an important role in radio over fiber（Ro F）systems,wavelength division multiplexing（WDM）systems,radar systems,satellite communication,and next-generation wireless communications.The traditional method of microwave signal generation in the electrical domain is limited by the electronic bottleneck and it is difficult to generate a high-frequency millimeter-wave signal with high tunability and a high-frequency multiplier factor.Photonic generation techniques have been widely used due to their advantages of low loss,large bandwidth,wide tuning range,and immunity to electromagnetic interference.There are four main photonic generation methods of millimeter-wave,including the simple direct modulation method,external modulation method,optical phase lock loop method,and optical heterodyne method.Among these techniques,external modulation based on LiNbO₃ Mach-Zehnder modulator（MZM）for optical frequency multiplication has significant advantages in generating microwave signals with high quality spectrum and high multiply frequency due to the high tunability,high stability,compact structure,lower cost,and high spectral purity.In order to generate microwave signals with high frequency multiplication factor,the existing schemes mostly adopt cascaded or parallel modulator structures,cooperate with electrical or optical filters,and increase the modulation coefficient to filter and suppress redundant sidebands,resulting in low utilization rate of optical sidebands、decreased stability and feasibility of the system.In this thesis,based on two sets of cascaded MZMs,we proposed a scheme for generating the 16-tupling microwave signal.Then,based on a set of parallel and a set of cascaded dual-drive Mach-zender modulators（DDMZMs）,a scheme of generating32-tupling microwave signal is proposed.Without using any filters and with a small modulation depth,the two schemes both generate microwave signals with high radio Frequency spurious rejection ratio（RFSSR）and optical sideband rejection ratio（OSSR）.Firstly,a scheme to generate 16-tupling microwave signal based on two groups of cascading MZMs is proposed.The optical signal input by the continuous laser（CW）to the two cascaded MZM of the first group are modulated by the RF modulation signal,and converted into electrical signals by the square rate detection of photodetector（PD）.Then the obtained electrical signal is loaded on the second group of cascaded MZM as a remodulation signal to generate a 16-tupling microwave signal.In the simulation process,the modulation depth m of these two groups MZMs is 0.8 and 2,respectively,the extinction ratio is 40 dB and all are located at the minimum transmission point.The factors affecting the stability of the system and the quality of the microwave signal are analyzed by varying the MZM’s extinction ratio and modulation depth,the optical power,and the initial phase of the RF signal.Simulation results show that under optimized operating parameters,the proposed scheme generates microwave signals with OSSR and RFSSR of 24 dB and 64 dB,respectively.Secondly,Secondly,a scheme based on a set of parallel DDMZMs and a set of cascaded DDMZMs is proposed to generate a 32-tupling frequency microwave signal.Among them,the two DDMZMs in parallel are located at the maximum transmission point with a modulation depth m of 2.408,and the two DDMZMs in cascade are located at the minimum transmission point with a modulation depth m of 2.1.The extinction ratio of the modulators is 40 dB.The simulation also analyzes the influence of these system parameters and the risk point of the modulator on the generation of microwave signal by changing the extinction ratio of DDMZM,the optical power input to the modulator,the initial phase of RF signals and the modulation depth of the cascade DDMZM.When the modulation depth of all modulators is appropriate and the system parameters are optimized,a 32-tupling microwave signal with OSSR of 27 dB and RFSSR of 34 dB is obtained.