Research On Microwave Photonics Frequency Multiplication System Based On Intensity Modulators

Broadband linear frequency modulated continuous-wave(LFMCW)signal and swept signals are widely used in radar,communication and other fields.Especially in radar,the wider the bandwidth,the better accuracy and resolution can be achieved in speed measurement,rang measurement and imaging radar.At present,the Broadband LFMCW signal generated by the electrical method is mainly through DDS(Direct Digital Frequency Synthesis)technology.Due to the limitation of the device and the sampling frequency,the bandwidth of the output signal of this method has certain limitations.At present,the microwave photonics technology has the advantages of large bandwidth,high stability,anti-electromagnetic interference,etc.Especially,the external modulation method has the advantages of simple system,small size,and generating quality microwave signals.This thesis introduces the research background and development status of optical external modulation technology firstly,and briefly introduces the common modulator principles,and then experiments and analyzes several external modulation systems,and proposed the schemes of bias point controlling of DPMZM.And proposed a new scheme based on the frequency multiplication system.The main content and results are as follows:1.Based on the dual-parallel Mach-Zehnder modulator(DPMZM),a frequency quadrupling experiments,a frequency octupling experiments and a 16-tupling frequency experiments were carried out,and the influencing factors in the experiments are analyzed separately.First of all,the quadrupling frequency is achieved through single modulation system.The analysis shows that the radio frequency spurious suppression ratio(RFSSR)can reach 35 dB,if the effective modulation depth is 0.6488.In the experiment,we obtained a frequency sweep signal with RFSSR of at least 20 dB.The phase noise accorded with the theoretical value of 12 dB.After 1 hour of observation on the power of the quadruple frequency signal,it is found that the power of the output signal changes by 0.2 dB,and the broadband LFMCW signal of 1 GHz was generated by the system.Due to the difficulty of controlling the Bias point of the DPMZM by single modulation system,a dual modulation quadruple-frequency experiment was performed and measured the 90 ° bridge and the half-wave voltage of the DPMZM,and the question of the doublefrequency suppression during the frequency sweep was analyzed.In order to verify the feasibility of the cascaded system,we made experiments to verify a 4 × 2 system,which generates 8.96 GHz signal with the low-frequency signal of 1.12 GHz,and the RFSSR of the output signal is 25.42 dB.Finally,a 12.64 GHz signal is generated by a 4 × 4 system based on two frenquency quadrupling systems cascaded.However,due to the presence of spurs and overlapping frequency bands after the frequency multiplication system,it is difficult for this scheme to generate a 4-12 GHz frequency-swept signal through 250-750 MHz.At the same time,a method for controlling the Bias point of DPMZM is proposed based on a frequency quadrupling system,and specific theoretical derivation is carried out.2.Considering the shortcomings of the frequency quadrupling system based on DPMZM,a new system scheme is proposed.This scheme is based on the modulation of two intensity modulators to achieve to achieve a wider LFMCW signal.Optical filter in this system avoids overlapping of LFMCW signal during frequency multiplication systems.Through the derivation of the theoretical formula and the simulation verification of the swept signals of the system by optisystem simulation software,the pure 3GHz signal was obtained through experiments.