Research Of The Transmitter Of Optical Orthogonal Frequency Division Multiplexing System

With the increasing capacity demand for network transmission, the requirements for optical communication network are also growing, while the development of digital signal processing technology and integrated circuits makes the implementation of OFDM concept to the optical communications becomes a hotspot in recent years.Optical OFDM combines the advantages of OFDM and optical communication technology, which can construct high spectral efficiency、 high capacity、low-cost optical transmission network. Typically, the optical OFDM system is divided into the direct detection of optical orthogonal frequency division multiplexing system(DD-OFDM) and the coherent detection optical orthogonal frequency division multiplexing system(CO-OFDM), while OFDM can also combine with WDM technology which can improve the capacity. The superior performance of optical OFDM technology leads to a great potential in the long-distance communications and optical access network applications, however the structure of the modulator characteristics and transmitter structure directly affects the performance of the system.This paper is to address this problem, the impact of the transmitter modulation characteristics and structure of the modulator on system performance are analyzed, and the structure is optimized based on various scenarios of applications.The main contents are as follows:1. The basic principles of OFDM technology and implementation are analyzed and compared with the features of common optical OFDM systems.2. Analyzes the influence of transmitter on the DD-OFDM, which including the nonlinear influence of optical MZM and the modulation structure is optimized baseband single sideband modulation using I/Q modulator. First introduces a typical system and built single-sideband DD-OFDM simulation platform, followed by study the effects of MZM nonlinear, including the modulation voltage and the bias point, the number of different sub-carriers and modulation formats, finally use of optical I / Q modulator and adjust the input signal to achieve a single-sideband modulation to replace the dual drive modulator modulator.3. Analyzes the influence of transmitter on the CO-OFDM system. First the typical modulation methods of CO-OFDM system are introduced, followed with a new modulation method of structural optimization based on the existing system, which use the predistortion circuit and dual drive modulator to replace the traditional optical I / Q modulator. The mathematical derivation and the simulation analysis comparison show that it has better performance and availability. Finally, comparative analysis the pulse shaping filter commonly used to get the root raised cosine shaping filter has a better system performance.4. The performances of OFDM-WDM system under the speeds of 400Gbit/s and 1Tbit/s are studied respectively. It is shown that the implementation of WDM theory to the OFDM system is entirely feasible.