Research On High Dynamic Range Photonic Downconversion System

Due to the applications on wireless communications, satellite communications, radar and military system, the available microwave bandwith is become less and less. In order to solve the problem, hight frequecy millimeter wave signal which is at the frequecy from30GHz to300GHz had been investigated. However, it is hard to proccess the microwave signal at such a high frequecy. Usually, a frequecy downconverter should be used before proccessing. Traditional microwave mixer can achieve this function. But it’s insert loss is very large, especily when the dermination is far away. Photonic downconversion system takes the advantage of low loss in fiber and becomes a recent reseach hotspot. This technology can be used in the many scenario, such as millimeter communications, deep-space exploration, radar array and position system.Nowadays, two reasons limit the development of photonic downconverter. The first one is low convesion efficiency in E/O and O/E device. This will utimatly influence system gain. Secondly, the nonlinear transfer function of modulator will cause distortion, and decrease the dynamic range in the futher. To solve this problems, we reseach the system structure deeply and provide some strategy to overcome it. The main innovation are as follows:1. Full spectrum utilization. Phase modulation is very popular in microwave photonics. As the counteract characteristc of up and down sidebands of phase modulation signal, phase modulation signal can not be detected by PD directly. Some of the sidebands should be filtering by opical filter before detection. That is to say, some optical spectrum is wasted. We propose a full spectrum utilizaiton downcnversion system based on both using the transmitted spectrum and reflected spectrum of FBG. Because all the optical power is used, the link gain improved. Experimental results demonstrate that the conversion efficiency improve6dB, compare to only one spectrm is used. 2. Digital linearization. We reseach the transfer function of the above system and theoretically prove that the optical current can be treated as a simple sine function approximately. By this, we proposed a simple anti-sine function method to suppress the IMD3on DSP module. Experimental results demonstrate that the IMD3can be supressed by17.81dB. And, SFDR is effectively improve5dB.3. Reciprocating optical modulation. By utilizing the velocity mismatch phenomenon is traveling-wave electrooptic modulator, we proposed a simple and high conversion efficiecy photonic downconversion method. In the system, phase modulator is placed betweem two narrow-band FBG. The optical carrier is transmitted in both directions, and modulated by LO and RF signal. Because of the carrier suppression and reciprocating optical modulation, downconversion efficiency is improved. Experimental results demonstrate that the efficiency can be improve by29dB, compare to the traditional method. Balance detector is used, noise floor can be decreased, too.4. linearization in frequency doubling system. frequency doubling system can half off the frequency request at the input port. However, the IMD2is proven to be larger than frequency doubling signal. To solve this problem, we proposed a linearization method based on DPMZM. By careful match of microwave amplitude and phase, the IMD2can be canceled. The experimental results demonstrate that the IMD2can be suppressed by35dB.