Solving the nonlinear Schrödinger equation or similar calculus is essential for designing today’s long-haul optical communication systems. Associated numerical and analytical approaches were extensively studied over the past four decades; simplifications and adaptions for various applications and purposes have been introduced. Optical fibers installed in long-haul systems possess nonideal features such as birefringence, which some of these adaptions partially address to improve the simulation accuracy. But as the fiber birefringence frequently and randomly changes along a link, finding a mathematical solution is a more complex problem and beyond the task of dealing with the nonlinear nature of the corresponding equations. Recently, a novel propagation phenomenon related to the polarization evolution of a signal has been observed. In links with considerable length, i.e., bridging transatlantic or transpacific distances, the polarization state of a light wave is impacted by neighboring signals via the Kerr nonlinearity in glass. Established formalisms for describing polarization effects based on the nonlinear Schrödinger equation cannot fully capture this phenomenon. Here we discuss a first-order calculus for this problem. We start with high-level reviews of experimental observations to introduce the phenomenon and ways to model regular nonlinear propagation. Then we present a first-order calculus to describe the statistics behind the phenomenon by specifically discussing the interplay between fiber birefringence and fiber nonlinearities.
Part of the book: The Nonlinear Schrödinger Equation