Abstract

Recently there has been considerable interest in the amplification of solitons with the use of Er-doped fiber amplifiers because of their possible application in long distance optical communication systems. Among the many advantages of this type of active fiber are the high gain (> 30 dB) in the 1500 nm region, the wide gain bandwidth (in the order of 30 nm), the polarization independent gain and the high saturation output power. Especially the large bandwidth makes Er-doped fibers attractive candidates for the amplification of ultrashort pulses (with pulse durations in the order of 100 fsec). Whereas some authors have reported on experimental results of short pulse amplification [1,2,3,4], only little theoretical work has been done in this field up to now [5]. Agrawal [5] and earlier Blow et al [6] have pointed out that bandwidth-limited amplification of ultrashort solitons results in a suppression of the soliton self frequency shift (SSFS). This, however, is in contrast to recent experimental results which show that the SSFS is gain enhanced [1]. We will present numerical results for the amplification of ultrashort fundamental solitons in Er-doped fibers based on an extended version of the nonlinear Schrödinger equation which takes into account the influence of group-velocity dispersion (GVD) including higher order dispersion, self phase modulation, the Raman self scattering effect and the frequency dependent gain and phase shift provided by the fiber amplifier. Our calculations clearly demonstrate the deleterious influence of the SSFS on the amplification of 100 fsec solitons: the energy gain that can be extracted from the amplifier is limited to very small values because the SSFS rapidly shifts the pulse spectrum outside the gain bandwidth.

© 1991 Optical Society of America