On the Capacity of Nonlinear Fiber Channels

A fiber-optic transmission line is a nonlinear channel due to the material nonlinear effects. Fiber nonlinearity has become one of the major limiting factors in modern optical transmission systems [1, 2]. Distributed Raman amplification and various return-to-zero (RZ) modulation formats may be employed to reduce the nonlinear impairments, but merely to a limited extent. It is known that the nonlinearity of one fiber line may be compensated by that of another with the help of optical phase conjugation (OPC). However, all previous proposals and demonstrations [3, 4, 5, 6, 7] work partially in fighting the fiber nonlinearity. They either are specialized to only one aspect of the nonlinear effects, or fail to work in the presence of dispersion slope or higher order dispersion effects. Still open to date is an important question: does fiber nonlinearity really impose a fundamental limit to the channel capacity? This paper introduces the notion of scaled nonlinearity, which together with the application of OPC, provides a negative answer to the above question.