Abstract
We have systematically investigated soliton amplification and reshaping by a nonlinear optical amplifier consisting of an active second-harmonic-generating element and a piece of passive dechirping fiber with a dispersion different from that in the system fiber. In the active element the fundamental harmonic is damped by enhanced losses, whereas the second harmonic is amplified through the external pumping. By selecting the length of the dechirping fiber we find it possible to achieve practically ideal soliton amplification, viz., low-power input (fundamental) solitons are amplified and then released into the system fiber as virtually unchirped high-power fundamental solitons. We have found that a power gain for the soliton of as much as 20–25 dB can be readily achieved; the length or the dispersion of the dechirping fiber is not critical for the degree of soliton amplification. The dispersion of this fiber can be merely twice that of the standard system fiber, and its length can be <10 km. Moreover, we have found that the dechirping fiber is not always needed, although the effective power gain in these cases is smaller, 10–12 dB. We have further investigated the influence of a random amplitude noise added to the input soliton. We found that the soliton amplification and reshaping scheme proposed is reasonably stable against this noise, especially when the dechirping fiber is not used.