Optical Transmission Damage And Its Suppression In All Optical OFDM Systems

Orthogonal frequency division multiplexing(OFDM)system due to its overlapping nature among subcarriers in frequency domain,thus enabling high data rate transmission in bandwidth-limited system.OFDM technology applied in various scenarios has been the research focus of wireless communication and optical communication since 2010.Among them,all-optical OFDM system is a superchannel transmission system that can generate very high spectrum utilization and occupy the whole C-band without requiring high-bandwidth digital-to-analog converter,and has become one of the research hotspots in the field of optical communication.At present,the application of all-optical OFDM systems in commercial optical networks is limited by two major problems.On one hand,although the overlapping subcarriers not only give the system a high spectrum utilization rate,but also lead to a low ability to resist optical damage,especially the ability to resist dispersion and nonlinear effects,due to the small frequency domain interval of the subcarriers.On the other hand compensate for dispersion and reduce the influence of nonlinear effect on all-optical OFDM system,many digital signal processing chips are used in all-optical OFDM system,which leads to high system cost.Therefore,how to improve the ability of all-optical OFDM system to resist optical transmission damage and reduce the transmission cost of the system has become a hot issue in the research of all-optical OFDM system at present.Aiming at the two problems of how to improve the anti-optical transmission damage ability of all-optical OFDM system and how to reduce the system cost,this paper focuses on the influence of optical transmission damage such as dispersion,self-phase modulation,cross phase modulation and four wave mixing effect on the transmission quality of all-optical OFDM system.A scheme to improve the anti-dispersion ability of the system and a scheme to improve the anti-four wave mixing effect of the system are proposed.These two schemes can improve the robustness of all-optical OFDM system to dispersion and nonlinear effects,reduce the use of high-speed digital signal processing chips in some application scenarios,and reduce the system cost.The main work and innovation of this paper are as follows.1.A more perfect transmission model of all-optical OFDM system is established,and the influence of walk off,dispersion and nonlinearity on the transmission quality of the system is analyzed for the first time.In this paper,a nonlinear analysis model is proposed for all-optical OFDM system with obvious inter carrier walk off effect.The improved iterative symmetric split step Fourier method is used to solve the nonlinear Schrodinger equation of all-optical OFDM signal in optical fiber.When the subcarrier walk off effect is obvious,the optical transmission damage caused by dispersion,nonlinear effect and other noises is analyzed numerically.The simulation results show that dispersion is the main factor affecting the transmission quality of all-optical OFDM system.Besides dispersion,four wave mixing also greatly limits the transmission performance of the system.In addition,the performances of all-optical OFDM system with optical cyclic prefix(CP)and all-optical OFDM system with optical guard interval(GI)are compared.The best receiving bandwidth and the best sampling point of sinc filter at the receiving end after inserting optical CP or optical GI are found by simulation.The simulation results show that both optical CP and optical GI can improve the transmission quality of the system to a certain extent,and optical CP can greatly improve the transmission quality of the system.2.A method to improve the anti-dispersion ability of all-optical OFDM system is proposed.By modifying the frequency domain function of the filter,which is used to control the generation of subcarriers,the anti-dispersion ability of all-optical OFDM system is improved.When the sinc filter is replaced by Gaussian filter and the subcarriers are sparse in frequency domain,the BER of each subcarrier is significantly reduced under the influence of dispersion.The simulation results show that the bit error rate of all-optical OFDM system with 32 subcarriers and QPSK modulation format is 8.545× 10-2 after 60 km transmission using sinc filter,and the bit error rate is 1.596×10-3 after replacing sinc filter with Gaussian filter and increasing the subcarrier spacing by 1.5 times,which decreases by more than 50 times.3.A method to improve the resistance of all-optical OFDM systems to four-wave mixing effect is proposed.In this method,by inserting a certain size of optical GI,combined with grouping and delaying mechanism for the subcarrier,the intensity of part of the four-wave mixing effect products is reduced and part of the four-wave mixing effect products are applied into the optical GI,so that it has a small impact on the part containing optical signal in each symbol period.Simulation verification indicates that the scheme can reduce the effects of four wave mixing effect on the system,especially when the inserting size is 0.33 of the optical GI,and each subcarrier is delayed differential for a third of all-optical OFDM symbol cycle system,the BER performance is almost comparable to that of the conventional with no consideration of FWM effect.In conclusion,this method can effectively enhance the system robustness against FWM effect,giving full play to the high data rate transmission in bandwidth limited system.