Abstract

Saturable absorbers (SA) are extensively used for passive mode locking in ultrashort pulse lasers. Their particular features can be likewise exploited for increasing the transmission capacity in long haul fiber networks where gain is provided by semiconductor optical amplifiers (SOA) [1]. An integrated single chip SOA-SA module (SOASA) is particularly useful and can be manufactured by ion-implantation through one facet of a bulk laser diode chip with antireflection coatings on both sides. In order to maximize the transmission capacity an analytical model is required that describes the SOASA single pulse response as a function of relevant system and signal parameters. To achieve this goal we used the model established in [2]. This SOASA module was likewise used in our experiments. The crucial system parameters of the model are recovery time, saturation energy, small signal gain and Henry factor of both the SOA and the SA section. A typical gain function is shown in Fig.1. Based on the analytical response function expressions for temporal walk off, energy gain and spectral shift can be obtained. The analytical results can be extended toward regular pulse trains and the periodic behavior of the module as a function of the bit rate and the saturation energy can be explained. Based on these findings the bit rate dependence of the energy levels that correspond to stable propagation can be easily derived by numerical means. (Fig.2). The dynamics of the amplified spontaneous emission (ASE) can be included into the description by a simple model. All results are in good agreement with full numerical simulations. Additionally, in-mark noise suppression and walk off compensation could be achieved and explained by using the particular spectral features of the SOASA in combination with bandpass filtering. Optimizing the transmission system on the basis of our analytical model an error free transmission of a 5 Gbit/s signal over 30 000 km could be experimentally proven.

© 2000 IEEE