Fast Kerr-like nonlinearity effect on the pulse propagation in passive and active optical fibers with different longitudinal and transverse refractive index shapes

The nonlinear optical fibers can be used as laser components for ultra-short pulse compression in passive mode-locked lasers. The ultra-short pulse generation fast nonlinearity causes small changes of the fiber refractive indices, hence the guided mode characteristics are changed slightly. The nonlinear mode is calculated solving the equation for the small perturbation of the linear mode and this equation can be solved numerically. For larger values of the refractive index the nonlinear mode field and the propagation constant can be obtained simulating the total field propagation in the fibers. The resulting modes characteristics depend on the power in the fiber. The spot size transformation is shown for the nonlinear single mode fiber excited by the linear fundamental mode at different values of the power