Frequency Conversion Technique Based On Microwave Photonics

The technology of frequency conversion based on microwave photonic is widely used in various fields of modern society,such as broadband wireless access networks,satellite communication systems,intelligent transportation systems,electronic warfare systems and so on.Compared with the traditional electronic frequency conversion,the photonic microwave frequency conversation has many advantages.It can effectively improve the signal transmission rate,reduce the transmission loss and avoid electromagnetic interference.At present,the technology of all-optical frequency conversion has shown a strong competitiveness in the field of communications and become a research focus both at home and abroad.Firstly,in this paper we introduce some common methods of microwave photonic frequency conversion,and the principle of the structure based on cascade MZM(Mach-Zehnder modulator)is analyzed in detail.Next,a new design of microwave frequency conversion is proposed.The design is based on Dual Output MZM as a first-stage modulator and the input microwave signal is modulated at the minimum bias point.A Mach-Zehnder modulator working as a second-stage modulator is connected to a output path with larger optical power in order to modulate the local oscillator signal.Finally,the two signals are mixed in a power combiner and the photo-detector will produce the converted signal.And the theoretical derivation and simulation of frequency conversion scheme are carried out.In this experiment,Bragg grating filter is added to eliminate the time imbalance of the signals from the two paths.According to the different positions of the fiber filter,the experiment is divided into three structures: down-conversion based on filtering a sideband after combination,down-conversion based on filtering a sideband before combination and up-conversion based on filtering a sideband before combination.Note that improving the system gain and spurious-free dynamic range is the key to a microwave frequency conversion system.It is found that the structure of microwave photonics frequency up-conversion based on filtering a sideband before combination shows the best results.The system gain(excluding the gain of the photo-detector)is about-45.5dB,which is much higher than that of the cascade modulator,and the spurious-free dynamic range is up to 99.6 dB·Hz2/3.For a double-sideband signal,the microwave power received by photo-detector will reduce periodically due to dispersion after long distance optical fiber transmission.In this experiment,the microwave signal becomes a single sideband after filtering another sideband,so the microwave signal after conversion is able to transmit over long distance and the value of error vector magnitude can meet the basic requirements of transmission.