Amplification of ultrashort optical pulses in fiber amplifiers

Abstract

An erbium-doped fiber amplifier has the potential of amplifying ultrashort optical pulses1,2 to microjoule levels because of its wide gain spectrum and high saturation energy. A modeling of the amplification process in such amplifiers should include not only the gain saturation and gain dispersion associated with the erbium dopants but also groupvelocity dispersion and Kerr nonlinearity associated with the host material. Furthermore, the soliton effects should be expected to play an important role because of anomalous dispersion. In this paper we derive a propagation equation for the pulse amplitude by including the above effects and solve it numerically to study the amplification process. Two cases are considered separately. In the first case, the pulse energy is assumed to remain below the saturation level, so that gain saturation is negligible. The input pulse is compressed as it is amplified without significant changes in the pulse shape. Gain dispersion sets the ultimate limit on the extent of pulse compression.3 In the second case, gain saturation is included, but gain dispersion is neglected. The pulse shape is distorted because its leading edge sees larger gain than the trailing edge sees. We study the temporal and spectral changes occurring during the amplification process through detailed numerical simulations.