The stable propagation of ultrashort optical solitons in an ultralong active fiber is investigated. We show that the soliton self-frequency shift is very sensitive to the nonresonance of the carrier frequency and that the linearly frequency dependent gain with positive slope is the effective component of gain spectrum for the compensation of this effect. Furthermore, we demonstrate that the inclusion of nonlinear gain that contains terms proportional tot he second and fourth power of the amplitude can provide stable propagation of ultrashort optical solitons.
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