A Channel Model to Deal with Distributed Noises and Nonlinear Effects in a Fiber System with Distributed Raman Amplifiers

Nowadays, the distributed fiber Raman amplifier (FRA) has become more and more popular in long-haul fiber systems, owing to its lower noise figures and weaker nonlinear effects in the link. The critical issue in distributed FRAs is the presence of various kinds of noises and their interactions with the signal. However, the existing Raman channel models and their numerical solving methods can only partially describe how the randomly distributed noises interact with the signal. This causes the difficulties in analyzing the distributed FRA precisely and the inconveniences for the applications and the maintenance of FRA systems. In this paper, we propose a modified Raman channel model to describe more comprehensively the interactions between the distributed noises and the signal under the influence of loss, distributed gain, dispersion, and nonlinear effects in the distributed FRA systems. With the comparisons of the error–vector magnitude (EVM) curves, our model can get lower errors in the experimental results regarding bidirectional pumped FRA single-span fiber systems and multi-span systems with backward-pumped FRAs.

Influence of Gain Dispersion on the Propagation Characteristics of Ultrashort Optical Pulses in Distributed Fiber Amplifiers

The physical model for the optical pulse propagating in the amplifier has been established. Within the normal dispersion region of the fiber, the evolution of the optical pulse in the amplifier has been simulated numerically, and the influence of gain dispersion on the propagation characteristics of the pulse has been discussed emphatically. The results show that, during the propagation of the ultrashort optical pulse in a distributed fiber amplifier, group velocity dispersion and Kerr nonlinear effect will affect the pulse profile and spectrum. For the ultrashort optical pulse with a wideband spectrum, the influence of gain spectrum on the propagation characteristics of the optical pulse, in essence, is that of gain dispersion, which is equivalent to a loss mechanism and will generate gain narrowing effect, so it should be considered in the design of an amplifier system.