I August 1997 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG OPTICS COMMUNICATIONS ELSEVIER Optics Communications 140 (1997) 289-295 Pulse propagation through a nonlinear optical amplifier P.L. Chu, Boris A. Malomed ‘, G.D. Peng zyxwvutsrqponmlkjihgfedcbaZYXWVU Optical Communications Group. School of Electrical Engineering. University of New South Wales, Sydney 2052, Australia Received 10 September 1996; revised 15 January 1997; accepted 3 April 1997 zyxwvutsrqponmlkjihgfedcbaZYX Abstract We report an investigation of pulse propagation through a recently proposed nonlinear amplifier composed of asymmetrical twin-core fibre. While the pulse is amplified and the associated noise is suppressed, we find that the pulse is strongly chirped. While the chirp in the centre of the pulse can be easily eliminated by inserting a short piece of dispersive fibre right after the amplifier, the chirp in the wings may remain and lead to the eventual destruction of the pulse. This residual chirp can be removed through a damped and radiative oscillation by judicious adjustment of the parameters of the amplifier and the dispersive fibre. Finally, we consider the effect of noise in the pulse. We discovered a threshold of the noise amplitude below which the noise is suppressed (which must be a characteristic asset of the nonlinear amplifier). Above the threshold, the noise tends to shift the centre of the pulse. 0 1997 Elsevier Science B.V. 1. Introduction The idea to use nonlinear optical amplifiers (NOAS) for reshaping of pulses Csolitons) in long optical communica- tion lines has attracted attention recently [l-3]. The main advantage provided by these amplifiers is its amplification of the useful signal (soliton) without simultaneously ampli- fying the noise, thus helping to reduce the soliton jitter, which is one of the most fundamental problems in the soliton-based optical communications [5]. The NOA simu- lated numerically in Ref. [l] was based on the nonlinear- optical loop mirrors [6]. In Ref. [2], a semi-analytical approach was developed for a simplified model of NOA, assuming a combination of instantaneous linear attenuation and cubic amplification. Transformation of a soliton in such a model could be easily calculated analytically, and then the soliton content of the resultant pulse was found by means of numerical solutions of the corresponding Za- kharov-Shabat equations. As a result, it was demonstrated that, in certain cases, such an “abstract” nonlinear ampli- ’ Permanent address: Department of Interdisciplinary Studies. Faculty of Engineering. Tel Aviv University. Tel Aviv 69978. Israel. E-mail: malomed@eng.tau.ac.il. fier can provide amplification of the soliton with a very high quality, generating fairly small amounts of radiation. In another recent work [3], we have put forward another type of NOA, based on a nonlinear optical device very different from that in Ref. [l], viz., a nonlinear optical coupler (dual-core fiber), in which the straight core (i.e., the one into which the signal is launched) is assumed to be Er-doped, and therefore has gain and nonlinearity, while the cross core is lossy. The nonlinearity arises from the resonant transition of the erbium ion from ground state to the excited state. For a moderate concentration of erbium in the fibre, the nonlinearity can be lo5 times larger than that of silica fibre [4]. Usually, a v phase shift can be achieved with milliwatts of optical power in metres of erbium-doped fibre. The analysis in Ref. [3] was limited to the cw input signal, for which the output-versus-input (OVI) characteristic, i.e., the power of the output signal versus the input power, was found numerically by solving a simple system of ordinary differential equations (ODES) governing the transfer of energy of the cw signal between the active and passive cores. In some (physically reason- able) cases, the characteristic provided exactly the proper- ties sought for, i.e., attenuation of a weak input whose power was below a certain threshold, and amplification above the threshold. 0030-4018/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO30-4018(97)00184-3