Study On High-speed Multispan Nonlinear Optical Fiber Communication Systems Based On Advanced Perturbation Techniques

In high-speed multispan nonlinear optical fiber systems, nonconstructive effects of fiber nonlinearity and dispersion can significantly degrade signal quality, which limits the transmission capacity. Therfore, mitigating or compensating these impairments to increase information capacity becomes a crucial research component for optical communication. In this dissertation, a weighted perturbation technique is proposed for digital backward propagation (DBP) to compensate fiber nonlinearity and dispersion jointly in both wavelength-division multiplexing (WDM) and polarization-division multiplexed WDM (PDM-WDM) systems, which can acquire a more accurate compensation result and reduce computational complexity effectively. A piecewise perturbation technique is developed to examine the information capacity of the multispan optical fiber communication system with multiplicative noise as a nonlinear channel. The relationship between the nonlinear channel capacity and input signal power with various Kerr nonlinear coefficients and bit rates is analysed rigorously. With the purpose of reducing computational load, a simplified perturbation technique is presented when step size is short. Furthermore, a modified low-order perturbation technique is derived in multispan optical fiber transmission systems. Based on it, a more accurate semi-analytical solution of nonlinear Schrodinger equation (NLSE) can be obtained.A weighted perturbation technique is proposed and basing on this technique, an improved digital backward propagation (IDBP) is investigated to compensate Kerr effects and dispersion jointly in WDM systems. In this weighted perturbation technique, a non-iterative weighted concept is presented to replace the iterative in the analytical recursion expression, which can dramatically simplify the complexity and improve accuracy compared to the traditional perturbation technique (TPT). Furthermore, an analytical recursion expression of the output after backward propagation is obtained initially, which the inter-channel walk-off effect and the combined nonlinear distortion cased by Kerr nonlinearity and dispersion can be consisted in. The research indicates that about6Nspan more multiplications per sample per channel will be required for conventional DBP (CDBP) than IDBP. When launch power is higher than-2dBm, IDBP allows a lower oversampling rate and is more accurate than CDBP for nonlinearity compensation, especially about2.4dB benefits than CDBP at3dBm with one step size per span.With the application of weighted perturbation technique in PDM-WDM systems, a semi-analytical solution of coupled Manakov equations can be obtained. Moreover, an analytical recursion expression of the output after backward propagation is derived. Based on this, the improved digital backward propagation (IDBP) is extended into PDM-WDM systems. A rigorous analysis of the computational cost is carried out and numerical simulations are performed in the corresponding transmission system with various parameters. The results indicate that about17Nspan more multiplications per sample per channel will be required for CDBP than IDBP in PDM-WDM systems. Comparison with CDBP, there will be46percent improvement of Q-factor for IDBP at2dBm with high Kerr nonlinear coefficient and3.5dB benefits at-1dBm with24channels.A piecewise perturbation technique is developed to examine the information capacity of the multispan optical fiber communication system with multiplicative noise as a nonlinear channel and a semianalytic expression of the conditional PDF is obtained initially. What is more, the information capacity is deduced analytically when chromatic dispersion, Kerr nonlinearity, fiber losses, and ASE noise play the importance role simultaneously. The researches indicate that there will be no help to improve the channel capacity by improving input signal power. For a higher speed transmission system, the channel capacity appears to be a larger peak value, then follows it with weakening quickly with the input power increasing, which means a larger value of the channel capacity can be obtained for a high bit rate (>40Gbit/s) transmission channel at the input power levels such that Pin,<9mW.A simplified perturbation technique is put forward to reduce computational load when step size is shorter than30km, which can replace TPT for numerical simulation. Additionally, in order to compensate the deviation from ignoring the high-order perturbation solution, a modified low-order perturbation technique is derived in multispan optical fiber transmission systems. The results of numerical simulations carried out show that this technique provides about2.1dB benefits than TPT at8mW with120km transmission distance and performs even better with larger transmission distance and launch power.The overall results of this dissertation illustrate that the four proposed perturbation techniques can provide more accurate results to improve the performance of high-speed multispan nonlinear optical fiber systems and calculate it information capacity analytically. This related study has significant theorical and practical value for increasing the system capacity and transmission rate.