Research On Microwave Photonic Signal Generation Technology Based On Cascaded Modulation

Compared with traditional microwave technology,generating triangular and square waveforms based on microwave photonic can overcome the bottleneck of electronics and has the advantages of high frequency,large bandwidth,low cost,and anti-electromagnetic interference.It has great value for modern radar system and wireless communication system.A photonic method to generate frequency-doubled microwave waveforms based on cascaded Mach-Zehnder modulators(MZM)were proposed and experimentally demonstrated.Firstly,the±1^st-order optical sideband signals are generated by a Mach-Zehnder modulator.And then the±1^st-order sideband signals are divided into two mutually perpendicular polarized beams by a polarization beam splitter.The signals in X-polarization are modulated by a MZM to generate the±3^rd-order optical sideband signals.Then,the±1^st-order optical sideband signals in Y-polarization and the±3^rd-order optical sideband signals in X-polarization are polarization multiplexed and photodetected to obtain the 2^nd-order and 6^th-order electrical harmonics.Finally,by properly adjusting the amplitude ratio between the 2^nd-order and 6^th-order harmonic signals,frequency-doubled triangle and square microwave waveforms can be realized.Based on the above scheme and principle,the model of microwave photonic frequency-doubled waveform generation based on cascaded MZM was constructed,and 60GHz triangular and square microwave waveforms are obtained in simulations.The root mean square errors(RMSEs)of the two waveforms are 0.0332and 0.3201,respectively.The effects of extinction ratio and modulation index of the modulator on the RMSEs were investigated.On this basis,the experimental system was built,and 2GHz frequency-doubled triangle and square microwave waveforms are realized,with RMSEs of 0.0989and 0.3906 respectively.The results show that the system has the advantages of simple structure,easy implementation,low cost and flexible frequency adjustment.On the other hand,considering the development of optical integration,a method to generate frequency-doubled microwave waveforms based on MZM and micro-ring resonator(MRR)was proposed.Firstly,±1^st-order optical sidebands and±3^rd-order optical sidebands are generated by the MZM and separated by the MRR.Then,they are photodetected to generate the2^nd-order and 6^th-order electrical harmonics respectively.By properly adjusting the amplitude ratio between the 2^nd-order and 6^th-order harmonic signals,frequency-doubled triangle and square microwave waveforms can be realized.Based on the above principle,the model of microwave photonic frequency-doubled microwave waveform generation based on the MZM and the Add-Drop micro ring resonator was constructed.However,due to the small rectangular coefficient of transmission spectrum of the Add-Drop micro ring resonator,waveforms with low RMSE can not be realized.In order to improve the edge steepness of filter's spectrum,a simulation model of frequency-doubled microwave waveforms generation based on MZM and cascaded micro ring resonator(CMRR)was constructed.By using the transmission spectrum of CMRR with flat top,the triangle and square microwave waveforms are realized.In addition,by adjusting the amplitude coupling coefficient of CMRR and the frequency of the input microwave signal,the microwave waveforms with frequency tuning range of 5?16GHz can be achieved.The RMSEs of triangle and square microwave waveforms at 16 GHz are 0.0371 and0.3311,respectively.On-chip integration can be realized with this method,which greatly reduces system volume and cost.