Mathematical modeling of nonlinear propagation of the signal in the multimode fiber-optic communication lines
Currently more than 99% of the global flow of information are provided by technologies of fiber-optic communication. The annual traffic growth already exceeds the growth of the transmission capacity, and in the nearest years, we may face the problem of the traffic volume exceeding the capabilities of data transmission technologies, if no new technology, providing a significant increase in the transmission capacity of communication lines, will be offered.
Using various digital signal processing methods and modulation formats the data transmission rate of several hundred Tbit/s over standart single-mode fiber was achieved. However, further increasing the capacity of the SSMF is difficult due to limitations of the operating range of fiber amplifiers, high requirements of the signal/noise ratio, restrictions on the power of the signal injected into the fiber.
The development of communication systems based on multi-mode fibers is considered as a promising way for solving the above problem. Multi-mode fibers allow an increase in the transmission capacity of optical networks at the expense of simultaneous transmission of signals through multiple modes of the fiber. The research of the data systems based on multimode fibers started recently, and most of the published works are devoted to the linear regime of the signal propagation which significantly reduces the area of applications of of such fibers. However, it is known that the main limitations of fiber links are the nonlinear effects in the optical fiber. Therefore, the objective of this work was to study the nonlinear propagation of signals in data transmission systems based on multimode fiber over long distances. This work was aimed at obtaining practically realizable multimode fiber communication systems based on numerical experiment.
The main method of research is the mathematical modeling, which currently is a powerful, and sometimes the only possible tool for the study of new generations of fiber-optic communication lines and optimization of existing fiber-optic links to the range of parameters in which physical experiment is not possible due to financial, time or other constraints.
In the course of the study, new models for mathematical modeling of the nonlinear propagation in a multimode fiber in conditions of strong and weak coupling were developed. Also it was developed a numerical algorithm based on finite-difference compact scheme of high order for the solution of the basic equations of mathematical models, which in some cases may be more effective than the split-step Fourier method, which currently is the main numerical methods used in nonlinear fiber optics . For numerical algorithm software complex for high-performance computing was developed.
In the present work we compared weak- and strong-coupling regimes. It was shown that with the growth of the number of modes the strong coupling regime provides a lower level of BER than the weak coupling one. We also investigated the dependence of BER on differential group delay (DGD) between the modes. It was shown that performance increases with increasing DGD.
One of the main challenges of long-distance propagation in multi-mode fibers in weak-coupling regime is the complexity of MIMO receivers that used to equalize for mode coupling. We compared two types of multi-mode fibers to meet this challenge: MMF with low DGD and with compensated DGD (combining fiber sections with DGD of opposite sign). MMF with low DGD demonstrate better performance than fibers with DGD management for long-distance transmission.
Also, research and development of methods of coding and signal modulation formats applicable to multimode fiber-optic lines, which can significantly increase the data transfer rate and reduce transmission errors, were carried out in the work.
The selected line of research, certainly, is an important application and commercial problem. The study results allow us to determine the boundaries of the area of applicability of multimode fiber in modern communication systems and can become the theoretical basis for the development of recommendations and proposals for the design of future fiber-optic communication lines. The results will help to reduce the number of expensive components in the high-speed fiber-optic links and significantly increase the capacity of communication lines.