| Since the21th century, the network bandwidth has gradually become the essential resource of our daily life because more and more people depend on Internet to handle the business or personal affairs. According to the report, the telecommunication traffic data and network bandwidth of China’s transport network showed a trend of substantial increase in recent years, whose annual growth rates have exceeded200percent. If supposing the node capacity of transport network increase by40percent annually, the node capacities of up to1Pbit/s will appear in2024and link layer rates could reach Tera-bit/s at the same time. The rapid development of bandwidth demand creates enormous stress for the construction of network infrastructure. The high speed and large capacity optical communication system will be the vital infrastructure of the next generation networks. For optical transmission technology beyond100G, orthogonal frequency division multiplexing (OFDM) is a very competitive scheme. Except for the excellent robustness against chromatic dispersion (CD) and polarization mode dispersion (PMD), the compact arrangement of subcarrier’s spectrums offers a high spectral efficiency in the optical orthogonal frequency division multiplexing system. The higher order modulation and electronic signal processing can also be implemented more flexible and easier.However, one major drawback of a optical OFDM system is its sensitivity to fiber nonlinearity. As many subcarriers are superimposed and combined in time, the OFDM signal has a noise like amplitude with a very large dynamic range and exhibits a very high peak to average power ratio (PAPR). Additionally, the frequency interval is so narrow that the walk-off among subcarriers is weak and the four wave mixing (FWM) can be induced more remarkably. The accumulation of these factors intensifies the nonlinear inter-carrier interference, which can significantly degrade the performance of OFDM system and result in unacceptable quality of transmission. Most recently reported nonlinear mitigation methods proposed for optical OFDM system rely on the offline digital signal processing. Although they can achieve a good compensation effect, the algorithms utilized by them is highly complex and make great demands on calculating time. Their abilities for the capacity expansion of systems are also restricted. The real-time experimental demonstrations of nonlinear mitigation are few in number and can’t obtain the satisfactory results up to this time. Therefore, in order to deeply investigate the generation and processing mechanism of nonlinear impairment for multicarrier systems, this paper has made the intensive theoretical analysis and simulation modeling of above-mentioned problems supported by national science and technology projects and a number of innovative research achievements have been gained. The main academic contributions are listed as follows.Firstly, in order to solve the problems created by the nonlinear inter-carrier interference for optical OFDM system, this paper introduced the idea about the alternate permutation of sub-carrier energies into the transmitter-side electronic signal processing. Based on it, a novel nonlinear mitigation scheme employing pulse shaping has been proposed. Through adopting the waveform with small pulse width, this method can remove the overlapping between the adjacent subcarriers, reduce the occurrence probability of FWM effectively and achieve good nonlinear interference mitigation even if the high-dispersion fiber links are used. Numerical results showed that our approach can improve the Q-factor by3dB for a system with more than30000ps/nm of chromatic dispersion generated by2000km of standard fiber.Secondly, based on the mathematical model for approximate analysis of FWM, this paper developed the idea about the cancellation of unwanted signals in a phased array and designed an approach to suppress the nonlinear physical impairment for optical OFDM system. The proposed method can modify the power configuration of the fiber links with the aid of the tunable devices inserted deliberately, which cause the phasor contributions from the individual fiber spans to cancel each other. Now, the multi-span nonlinear effective length reaches it’s minimum value and the nonlinear interference has been reduced to the manageable proportions. Because the generating and receiving process of optical signals aren’t disturbed and no more electronic signal processings are needed, this method doesn’t occupy the extra bandwidth resource and it’s hardware requirement is also very low.Thirdly, in order to eliminate the impact of nonlinear inter-carrier interference, the non-optimized dispersion maps are commonly used for optical OFDM system. Moreover, the real-time scheduling and routing in dynamic transparent optical networks, which are widely regarded as the critical infrastructure for the core network of the future, requires fast and accurate evaluation of transmission penalty caused by nonlinear kerr effects with non-optimized dispersion maps. However, the conventional method using nonlinear phase shift can only be applied to assess the nonlinear penalty with optimized dispersion maps. For this issue, this paper addressed the problem of how to build an efficient evaluation approach for non-optimized dispersion maps and proposed a novel method based on pulse broadening factor to accurately evaluate nonlinear penalty. Numerical simulation has been used to investigate the feasibility of our novel method in Return-to-Zero Differential Quadrature Phase-Shift Keying (RZ-DQPSK) systems and results show that our approach can achieve good evaluation performance even with non-optimized dispersion maps.Finally, performing signal processing and mitigation of physical impairments in the optical domain might promote the rapid development of all-optical orthogonal frequency division multiplexing system. But the execution processes of these information-processing functions depend on the all-optical signal processing devices with enhanced performance properties, for example, the delay elements based on slow light. Therefore, in the view of the propagation laws for optical signal in periodically structured electromagnetic media, this paper have studied the slow-light based optical signal processing in photonic crystal and relevant techniques. A novel optical delay element based on slow light has been designed using the polyatomic photonic crystal and a new optimization method has been proposed. The propagation process of optical pulses in the proposed structures are also investigated using numerical simulation. Our works provide important theoretical basis for the potential application offered by the polyatomic photonic crystal in future all-optical signal processing technology. |
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