ISSN: 2229-6948(ONLINE) ICTACT JOURNAL ON COMMUNICATION TECHNOLOGY, DECEMBER 2011, VOLUME: 02, ISSUE: 04 427 IMPACT OF COHERENT AND INCOHERENT CROSSTALKS AND POWER PENALTY ON THE OPTICAL CROSSCONNECTS Suvarna S. Patil 1 , Bharat S. Chaudhari 2 and Baojun Li 3 1 Bharati Vidyapeeth University College of Engineering, Pune, India E-mail: sschorage@gmail.com 2 International Institute of Information Technology, Pune, India E-mail: bharatc@isquareit.edu.in 3 State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-Sen University, Guangzhou 510275, China Abstract Optical cross-connects are one of the most important components in the dense wavelength division multiplexer based optical networks. The crossconnects suffer from crosstalk due to the different wavelength light path channels during the switching process leading to the deterioration in bit error rate (BER) and hence in the system performance. This paper presents the study of impact of coherent and incoherent crosstalk and power penalty on the optical cross-connects in WDM Networks. The effect of accumulation of coherent crosstalk at different stages of crossconnect has been also investigated and analyzed for the blocking probabilities. Results of coherent and incoherent crosstalk are compared to identify their impact on the working of the cross-connect. The results show that the crosstalk increases with increase in either the number of wavelengths per fiber or the number of input fibers. The result also illustrates decrease in the interference penalty by correlating the crosstalk contributions with each other at the appropriate phase angle. We show that an acceptable blocking probability due to crosstalk is achievable for active wavelengths in the WDM network. The present study can be used to model the possible number of routing stages in such networks. Keywords: Coherent Crosstalk, Incoherent Crosstalk, Optical Cross-Connects, Blocking Probabilities, Power Penalty 1. INTRODUCTION Optical networks are considered as a promising solution for the next generation optical networks fulfilling the increasing demand of bandwidth for the applications with high Quality of Service (QoS) requirements. Optical networks process the signals in optical domain enabling the faster and reliable communication through low power and compact optical integrated circuits (OICs) [1]. Switching and routing of the light packets is important process in optical networks and has to ensure correct delivery of packet to the appropriate port without incorporating any error. In optical networks, optical cross- connect (OXC) is responsible for switching and routing of the light packets entirely in the optical domain [2]. The major impairments found in optical networks include ASE (Amplifier Spontaneous Emission) noise from optical amplifiers, crosstalk from OXC nodes as well as attenuation, dispersion and nonlinear effects from the optical fibers. A typical wavelength division multiplexed (WDM) OXC is composed of wavelength-selective and switching elements to route individual wavelength channels from several inputs to several outputs. While traveling through an optical cross-connect node, an optical signal experiences optical crosstalk due to narrow spacing between the light carrying waveguides. The crosstalk is contributed by the adjacent input-output WDM channels and delayed version of the desired signal that travels through different optical paths inside the crossconnect. Crosstalk in OXC can be classified on the basis of Interferometric delay time. If the Interferometric delay time is shorter than the light source coherence time then the crosstalk is treated as coherent crosstalk, while Interferometric delay time is longer than the light source coherence time, the crosstalk is treated as incoherent crosstalk [3]. In earlier studies [4], the crosstalk analysis is done for the static wavelength router structure. The study shows that the interference power penalty depends on the linewidth of the laser source. The parameters like extinction ratio, input power, bit error rate (BER) are not the part of analysis. While considering the coherent crosstalk in optical crossconnect it is necessary to consider the phase relation amongst all the interfering signals. The crosstalk specification requirement will increase drastically as the noise power increases linearly with number of stages of OXC. Here in our study we showed how the signal to interference ratio varies with the number of stages of optical crossconnect. In [5], various topologies of OXC are studied. OXC based on space switch is one of the topology considered there. The scalability of the OXC is studied in function of the number of wavelength channels. It shows that the crosstalk increases with increasing number of channels and optimal performance for a certain throughput is obtained if the number of fibers equals the number of wavelengths. In our work, the performance analysis of OXC is carried out for coherent and incoherent crosstalk by considering phase, the coherence time and the linewidth of the laser source. For computation of crosstalk power penalty, 1-dB power penalty criterion is considered. We have studied the traffic carrying capacity of the OXC node to achieve required BER for these crosstalk. The organization of the paper comprises of four sections. Section 1 gives introduction to the types of crosstalk in OXC. The parameters such as source linewidth, the input power, BER and non-zero extinction ratio contribute to excessive power penalty. The basic OXC structure and the impact of these parameters on crosstalk are studied in the Section 2. Analytical results are discussed in Section 3. Finally the conclusion is discussed in Section 4. 2. ANALYSIS OF CROSSTALK Optical in-band crosstalk occurs when a signal and interferers have close value wavelengths. As a consequence, the signal and interferers are within the pass band of practical optical filter. Generally such a filter is located at the front end of DOI: 10.21917/ijct.2011.0059