Study of the gain saturation effect on the propagation of dark soliton in Er +3 -doped, Ga 5 Ge 20 Sb 10 S 65 chalcogenide fiber amplifier Z. Tahmasebi, M. Hatami ⁎ Atomic and Molecular Group, Faculty of Physics, Yazd University, Yazd, Iran Photonic Research Group, Engineering Research Center, Yazd University, Yazd, Iran abstract article info Article history: Received 6 January 2010 Received in revised form 18 June 2010 Accepted 17 September 2010 Keywords: Nonlinear Gain saturation Gain dispersion Dark soliton Bright soliton We study the effect of gain saturation on the propagation of fundamental dark soliton in a nonlinear, dispersive and amplifying medium. The Er +3 -doped, Ga 5 Ge 20 Sb 10 S 65 chalcogenide glass is used for dark and erbium doped silicon glass for bright solitons. The numerical simulations show that dark soliton doesn't split to subpulses unlike bright soliton and also the dark soliton is more stable in the presence of gain saturation and gain dispersion effects. So the chalcogenide glasses are suitable for designing all optical devices. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The propagation of short optical pulses in nonlinear fibers is governed by nonlinear Schrödinger equation (NLS) which contains the dispersion and nonlinear effects of medium. Solitons are special solutions of this equation that propagate unchanged over long distances in the absence of loss. Recently, the propagation of bright solitons has been studied in nonlinear and dispersive fiber amplifiers [1–4]. Fiber amplifiers are usually produced by doping the fiber with rare-earth ions such as erbium, neodymium and etc. The most famil- iar of them is erbium doped fiber amplifiers that are widely used in communication systems at a wavelength of 1.55 μm. This paper simulates the propagation of pulse in erbium doped fiber amplifiers for the case of their gains have been modeled by two level atomic systems [3]. In recent years chalcogenide glasses have been proposed as potential host materials for RE-doped lasers and amplifiers [5]. These glasses are characterized by high refractive index, normal dispersion and high nonlinearity [6,7]. Another chalcogenide glass property is useful to fabricate efficient EDFAs and the capability, for several vitreous compositions, to host high dopant concentration, without ion clustering and concentration quenching effects. All of these properties added to the relative easiness that makes fabrication of chalcogenide glasses very attractive materials to be doped with erbium [8]. Dark solitons are more stable in the presence of noise and spread more slowly in the presence of loss as compared with bright solitons in optical communication systems. These properties provide the means for potential applications of dark solitons in optical communication sys- tems [9]. Chalcogenide glasses with positive group velocity dispersion (GVD) are a good material for propagation of dark solitons as previously one of us designed an all optical ultrafast dark soliton switch [10]. The paper is organized as follows. Section 2 involves basic theory and equations of saturable medium. In Section 3, we will simulate the dark and bright soliton propagations without taking into account the gain saturation effect. In Section 4, we will solve the equation numerically with gain saturation for bright and dark solitons in Er +3 - doped silica and chalcogenide respectively and study its effects on amplification and shape of pulse. 2. Equations In the erbium doped fiber amplifiers, ions may be treated as a two level system both in Er +3 -doped silicon or chalcogenide medium. The response of this two level atomic system is governed by Bloch equations [3–10] so that the susceptibility of the two level systems is obtained as follows: χ a = g p c ω ω−ω a ð ÞT 2 −i 1+ ω−ω a ð Þ 2 T 2 2 ð1Þ where T 2 is the dipole relaxation time about 50–100 fs, ω is the optical frequency, ω a is the atomic resonance frequency and g p is the gain peak [3]. For most dopants, T 2 is smaller than pulse width T 0 (T 0 = 1 ps) and this equation is valid for T 2 ≪T 0 . Furthermore, Eq. (1) shows the homogeneity of the gain profile. Optics Communications 284 (2011) 656–659 ⁎ Corresponding author. Tel.: +98 9131518991. E-mail address: mhatami@yazduni.ac.ir (M. Hatami). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.09.044 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom