Femtosecond Soliton Collapse by Frequency Modulation Instability in Erbium-Doped Fiber Amplifiers

Abstract

Recent experiments [1-3] have reported the amplification of femtosecond solitons in erbium doped optical fibers. Whenever the spectral width of the soliton is as wide as the linewidth of the amplifier, the gain that is seen by the pulse is reduced with respect to that of a weak continuous wave signal. However, it has been observed that the pulse width remains virtually unchanged, at least over some initial stage of the amplification process. Physically, this is due to the fact that the pulse shape is determined by two competing effects. From one side, the amplified pulse tends to remain a soliton and therefore to compress its time width. On the other hand, the finite spectral width of the gain leads to preferential amplification of the central part of the pulse spectrum and this would lead to pulse broadening. If these two effects balance each other, the output pulse width may remain almost undistorted. This compensation is however an unstable phenomenon.