15 September 2000 Ž . Optics Communications 183 2000 419–423 www.elsevier.comrlocateroptcom Noise enhancement due to the memory effect in fiber transmission lines with semiconductor optical amplifiers Arcady Shipulin a, ) , Boris Malomed b,1 , Falk Lederer c a Physics Department, UniÕersity of Utah, 1400 S, 115 E, Salt Lake City, UT 84112, USA b Department of Interdisciplinary Studies, Faculty of Engineering, Tel AÕiÕ UniÕersity, Tel AÕiÕ 69978, Israel c Institute of Solid-State Physics and Theoretical Optics, Friedrich-Schiller-UniÕersitat Jena, Max-Wien-Platz 1, D-07743 Jena, Germany ¨ Received 14 June 2000; accepted 18 July 2000 Abstract A considerable increase of the noise contamination with the increase of the pulse-stream’s bit rate in an optical fiber link with semiconductor in-line amplifiers is demonstrated experimentally and numerically. We propose a new explanation for the noise enhancement based on the amplifier’s memory effect. In contrast with the pattern effect, known for the modulated train, the memory-induced noise enhancement manifests itself even in a uniform train. The analytical prediction shows a qualitative agreement with the numerical results. The memory effect is expected to be especially detrimental in the WDM transmission mode. q 2000 Elsevier Science B.V. All rights reserved. The use of multiple-quantum-well semiconductor Ž . wx optical amplifiers SOA 1 has attracted a great deal of attention in the development of medium-range standard fiber networks operating near the zero-dis- Ž . persion wavelength f 1.3 mm . Until now, SOAs were the only commercially available all-optical am- plifiers at this wavelength. Field, laboratory, and numerical experiments with SOAs as in-line ampli- fiers have demonstrated the possibility to reach sta- ble transmission of 10 Gbitrs signals over distances wx of up to 1 500 km 2 . Now the next important step is the implementation of the WDM regime in SOA- wx based fiber networks 3 . ) Corresponding author. Fax: q 1-801-581-4801; e-mail: arcady@physics.utah.edu 1 E-mail: malomed@eng.tau.ac.il With regard to this, it is worth reminding that SOAs have two well-known drawbacks, viz., a rela- tively low saturation energy, E ; 2 pJ, and a gain- s recovery time, t ; 200 ps, comparable with the bit r window for the bit rates ) 5 GHz. One of the known consequences of both drawbacks is the pat- tern effect, when the pulse’s energy depends on its particular position in the modulated train: after a ‘0’, the pulse experiences a higher gain then after a ‘1’. This leads to additional eye-diagram-quality degrada- tion, and, consequently, to reduction of the maxi- mum achievable propagation distance. However, it has been found that, even in the case of a uniform train, where the pattern effect is absent, the com- bined action of saturation and incomplete gain recov- Ž . ery which we will call below a memory effect can lead to an increase of the noise with the bit rate. Experimental measurements of the transmission Ž wx. Q-factor definition is given in, for example, 5 for 0030-4018r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0030-4018 00 00905-6