Study Of Microwave Photonics Frequency Down-Conversion Method Based On Intensity And Phase Modulation

Microwave photonic frequency down-conversion is a new type of frequency reduction technology. It has been widely used in many fields such as signal reception, remote sensing, array radar, and satellite communications. Microwave photonic frequency down-conversion technology is with larger bandwidth, stronger anti electromagnetic interference, smaller transmission attenuation characteristics compared to the traditional microwave frequency down-conversion technology. It makes up for the shortage of the microwave frequency down-conversion technology in the process of dealing with the high frequency and wideband microwave signals at the bandwidth and the processing speed. Meanwhile, microwave photonic frequency down-conversion technology is required for higher device integration degree and smaller thermal effects, which has the advantages in the aspect of equipment cost and volume, and also in line with the current trend in the development of system miniaturization, so that it has a broad application prospect.In this paper, the development of microwave photonic and the research status of microwave photonic frequency down-conversion technology is summarized, and the performance characteristics of some classical microwave photonic frequency down-conversion system are analyzed. Besides, the operating principles of the Mach-Zehnder intensity modulator (MZM), phase modulator (PM) and photoelectric detector (PD) are expounded, and the measurement methods about the DC and RF half-wave voltages of MZM are proposed and experimental verified respectively. According the research result, we purpose the microwave photonic frequency down-conversion system based in MZM-PM cascade structure, and analyze the frequency down-conversion principle theoretical. Meanwhile, the simulation experiments are carried out in Opti System to verify the feasibility and measure the performances for this system. The main contents of the research work are as follows:1. The operating principle of MZM, PM and PD is derived theoretically, and their expression of input and output optical field is obtained. The basic definition of main performance evaluation parameters for frequency down-conversion system, system gain, noise figure and spurious free dynamic range (SFDR) are described in detail, the calculation method of which are deduced briefly.2. Methods of measuring about direct current (DC) and radio frequency (RF) half wave voltages of MZM are put forward and verified, and its trend of changes in different modulation conditions is analyzed. Aiming to DC half-wave voltage, it can be calculated by observing the output intensity change period of the MZM when loading a triangular wave on the DC-bias interface. This method doesn't like the extremal method, which need to measure the max or min point of the MZM output intensity. Hence, it has advantages over the extremal method in accuracy. Besides, a feedback control about DC-bias is designed and demonstrated to overcome the influence of environment and the instability of MZM operating point, which provides an important prerequisite for the construction of inverter experimental system. For RF half-wave voltage, we measured it by using the spectrum analysis method, which obtaining the RF half-wave voltage by measuring the intensity ratio of odd-order modulation sidebands. The RF half-wave voltage is calculated by spectrum information of the MZM output, so the method is stronger to anti-interference and can measure the RF half-wave voltage with different modulation frequency or power. By comparing the experimental results, the increase of the RF half-wave voltage with increscent modulation frequency is stronger the effect of modulation power on the RF half-wave voltage, which shows the modulation frequency has more influence for the value of RF half-wave voltage,3. The microwave photonic frequency down-conversion system based on MZM-PM cascade structure is proposed. The frequency conversion principle of the system is derived theoretically and the simulation experiment is carried out. The performance parameters of the system are simulated and calculated. After that, the experimental system was set up in the laboratory, and realized the frequency conversion of the high frequency microwave signal, which verified the feasibility of the MZM-PM cascade structure. At the same time, the system gain and SFDR performance test are completed on the system, the maximum gain is-34.65 dB and the optimal value of SFDR is SFDR=108.62 dB/Hz2/3, this results are basically consistent with the simulation results. Through the contrast experiment to the MZM-PM cascade structure and the common cascade MZMs structure, the MZM-PM cascade structure is better at system gain and SFDR, which proves that the MZM-PM cascade structure has the ideal down conversion ability, and strong development potential as well.