Design Of Direct Modulation Broadband Analog RoF Link

In recent years,the new communication technology represented by 5G mobile telecommunication technique has attracted more and more attention.With the rapid development of communication industry in recent years,the available spectrum resource is increasingly exhausted since it is non-renewable,consequently,wireless access gradually reaches the bottleneck both in bandwidth and speed.Under this background,the new wireless transmission technology represented by radio over fiber(RoF)technology has attracted wide attention.In the direct modulated RoF link,the input RF signal is directly added to the semiconductor laser,and the output spectral intensity changes linearly with the change of the external RF signal.Through the transmission of the long optical fiber link,the output light reaches the photodetector,and the original RF signal with certain attenuation is obtained after the beat frequency.Compared with conventional wireless transmission links,the direct modulated radio frequency link has the advantages of simple structure,sound suitability for fast deployment,large transmission capacity and better security,which may have great application prospects in indoor wireless signal transmission,military communications and 5G signal transmission.In this paper,a direct modulated RoF link with 10 MHz-4 GHz is studied and tested.Compared with indirect modulation method,direct modulated laser has larger spectral chirp.Therefore,in this paper,the generation of chirp and the effect of chirp on the performance of analog RoF link is theoretically analyzed and experimentally demonstrated.The contents are organized as follows:First of all,the advantages and future development of RoF transmission compared with traditional radio link are analyzed,and the research course of the RoF link from home and abroad since 60s of the last century is reviewed.The basic concepts and research status of the radio frequency link system are also introduced.Then,the direct modulated RoF and the externally modulated radio link are modeled respectively.The most basic direct modulated RoF link composed of semiconductor laser,optical fiber and photodetector is analyzed,and the influence of each device performance on the whole link performance is also introduced.Starting from the rate equation,the frequency chirp of the output spectrum of the direct modulated semiconductor laser is studied.The influence of the chirp and the fiber dispersion superposition on the link is deduced after the chirped intensity modulated signal is transmitted through the long optical fiber.Then a direct modulated RoF link is designed,manufactured and tested by using existing devices in our laboratory.Utili(?)ng the high resolution complex spectrometer and radio frequency measuring instruments,the output spectrum chirp of the direct modulated laser and its influence on the link are measured.The actual measurement results are in good agreement with the theoretical derivation,which shows that by controlling the bias current of the direct modulated laser and the temperature in a suitable range,the spectral chirp can be controlled at a low level,thereby enhancing the harmonic characteristics of the whole link.