Research On Multimode Fiber Communication System Using Coherent Optical Ofdm

Multimode fiber has high modal dispersion which limits its transmission capacity, how to enhance multimode fiber transmission capacity to become the research focus. The technology has the merits of coherent detection and OFDM. The synergies between coherent optical communications and OFDM are twofold.The coherent system brings OFDM a much needed linearity in RF-to-optical (RTO) upconversion and optical-to-RF (OTR) downconversion. OFDM brings coherent system computation efficiency and ease of channel and phase estimation. CO-OFDM technology can take full advantage of the high frequency band-pass regions of the multimode fiber, and carry high-speed and high capacity communication. It is can carry high-speed and high capacity communication. However, the deep fading in high frequency band pass region of the multimode fiber will affect the system performance. Adaptive modulation (AM) technology can change the modulation scheme according to the channel characterization, and the deep fading in high frequency band pass region of the multimode fiber will not affect the system performance.In this paper, research on CO-OFDM single mode fiber system, mainly including the MMF channel mathematical model, architecture of communications system,2x2 multiple-input multiple-output (MIMO) channels and adaptive modulation. Details are as follows:1) In this paper, the transmission characteristic of mutimode fiber is analyzed, the design of coherent optical OFDM transmission over 200 km multimode fiber is given, which is about two orders of magnitude reach expansion over conventional multimode fiber systems at the channel date rate of 10 Gb/s and beyond. Although this design does not demonstrate all the advantages of multimode fiber, it serves as the first proof-of-concept experiment that multimode fiber may have a role beyond short reach. We anticipate the further research will rapidly improve the performance in multimode fiber system beyond short reach.2) 2x2 MIMO CO-OFDM transmission system of multimode fiber is design, and the effects of polarization-mode dispersion (PMD), polarization-dependent-loss (PDL), and chromatic dispersion (CD) on the transmission system of fiber are theoretical analyzed, and use the channel estimation method to eliminate.The experiment result proved that coherent optical OFDM transmission system of multimode fiber transmission capacity to be redoubled when the system into the 2x2 MIMO technology.21.4Gb/s CO-OFDM signal through 2x2 MIMO CO-OFDM multimode fiber transmission system transmission distance can reach 200km when the OSNR is lOdB, BER is 10"3.3) We propose an adaptive modulation algorithm using threshold allocation of optical signal to noise ratio (OSNR), and introduce the CO-OFDM multimode fiber communication systems based on adaptive modulation (AM).The experimental results show that AM-CO-OFDM reduces 2dB OSNR than that without AM technology in 21.4Gb/s,200km multimode fiber CO-OFDM transmission system.