Photonic Network Communications, 1:4, 313±322 (1999) # 1999 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands. Performance of Optically Preampli®ed Receivers in WDM Systems Disturbed by Interferometric Crosstalk Idelfonso Tafur Monroy, Eduward Tangdiongga, Huug de Waardt* Eindhoven University of Technology, Telecommunication Technology and Electromagnetics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, E-mail: I.Tafur@tue.nl Received March 8, 1999; Revised July 9, 1999 Abstract. Interferometric crosstalk is a performance limiting factor of major concern in all-optical WDM transmission networks. Interferometric crosstalk arising from performance imperfections in optical components may introduce large power penalties and bit-error rate ¯oors. Optical ampli®ers are often used to increase the signal level incident on a detector so that high receiver sensitivity can be obtained. We investigate theoretically and experimentally the performance of optically preampli®ed, direct detection receivers in the presence of interferometric crosstalk. The model includes an accurate description of ®ltered interferometric crosstalk by using a so-called maximum entropy approach. Experimental results, using both directly and externally modulated light sources, are found to be in good agreement with theory. Keywords: optical crosstalk, wavelength division multiplexing networks, error analysis, optical communication, optical ampli®ers 1 Introduction All-optical WDM networks, comprising optical add/ drop modules and/or optical cross-connects will employ optical components that may introduce crosstalk. Interferometric crosstalk arising from performance imperfections in (de)multiplexer and optical switches may result in large power penalties and bit-error rate ¯oors, e.g. [1±4]. In Fig. 1a is shown an example of a cross-connected optical network. Let us considered a channel at a certain wavelength l 1 at one extreme of the network (mark ``in'' in Fig. 1a). Due to performance imperfections of components in the optical nodes at the other extreme (``out'' in Fig. 1a) the channel will experience crosstalk interference from other channels operating at the same wavelength; inband crosstalk. Channels operating at different wavelengths may also fall within the receiver bandwidth producing interband crosstalk. State-of-the-art integrated optical cross-connects show a typical value of  20 dB of inband crosstalk and a value of interband crosstalk less than  40 dB [5]. The power loss of integrated cross-connects can be high depending on the refractive index of the used materials and on the device design. A loss of 13 dB was reported in [5]. When dilated optical switches are used to improve further the crosstalk performance, the loss will be even higher. All-optical networks employing the advanced integrated cross-connects are likely to incorporate optical ampli®ers to compensate for power losses, and also for sensitivity enhancement resulting in a larger power budget. Hence, it is of importance to study the perfor- mance of optically preampli®ed receivers in the presence of interferometric crosstalk. The main contribution of this paper is the accurate theoretical model for the receiver performance and the valida- tion of its results by relevant experiments. The receiver model includes an accurate statistical description of noise. The statistics of ®ltered interferometric noise is determined by using a maximum entropy approach. The non-Gaussian statistics of detected ampli®ed spontaneous emission (ASE) noise is also included. We present experi- mental results for power penalties, due to interferometric crosstalk, for a system using directly and externally modulated light sources. We found that, regardless of the light source modulation type, optical preampli®cation does not improve the *This work was supportedin part by the European Commission ACTS project AC332 APEX.