Performance Of OFDM System Over Multimode Fiber Link

Multimode fiber (MMF) is widely used in short-distance optical communications for its advantages as follows:high efficiency on coupling with laser source, convenience for fiber connecting operation, and low system cost. MMF is also the transmission media defined by the Ethernet standard. The transmission distance is demanded to reach 300m for lOGb/s signals by Ethernet standard IEEE802.3aq, however, the mode dispersion exist in MMF will cause the inter-symbol interference (ISI), which restricts the bandwidth and transmission capacity. We have to introduce other technologies to satisfy the high requirement, such as subcarrier multiplexing (SCM), Electric field dispersion compensation (EDC), and so on. All of them can enhance the performance of system. Recently, one of the research hotspots is introducing the Orthogonal Frequency Division Multiplexing (OFDM) to fiber communication. Many works have been done, such as 300m MMF system using directly modulation and long-haul optical transmission system using SMF. Related experimental results demonstrate that OFDM can efficiently improve the system transmission performance. It also verifies the advantage of OFDM technology on high-speed communication and resistance to mode dispersion.This research is mainly about performance analysis of short-distance MMF communication system based on OFDM. We put forward to analyze the mode dispersion of MMF by using optical angular spectrum. By giving the space distribution form of the output light from semiconductor laser, we can obtain its angular spectrum by space Fourier transforming. Then we get the transmission modes and the normalized ratio of power distribution, which are useful to calculate the mode transmission delay time in MMF. Finally we obtain the MMF channel's pulse response to analyze the effects of the mode dispersion to signal pulse.Based on the theory described below, we build the model of MMF transmission system with directly modulation and indirectly modulation to realize the signal transmitting and receiving. In order to compare the quality of the two transmission systems, numerical estimation of the mainly system performance parameters has been made, such as rates, distance, BER, SNR, Q factor, and so on.The results show that the angular spectrum method is effective to solve the normalized ratio of power distribution and transmission delay time, as well as analyzing the effects of mode dispersion to light pulse transmission. Transmission performance can be obviously promoted by employing OFDM technology. The transmission distance can reach 300m for signals with the frequency of 40-160 Gb/s, which satisfies the demand of the next generation Ethernet. The transmission performance of CO-OFDM system is superior to the counterpart using direct modulation, but more complicated and expensive.