Switching Characteristics of SOA-based Sagnac Interferometer for Subpicosecond Pulses Morteza Jamali Department of Electrical and Computer Engineering Tarbiat Modares University Tehran, Iran m.jamali@modares.ac.ir Vahid Ahmadi Department of Electrical and Computer Engineering Tarbiat Modares University Tehran, Iran Corresponding author: v_ahmadi@modares.ac.ir Mohammad Razaghi Department of Engineering, University of Kurdistan Sanandaj, Iran m.razaghi@uok.ac.ir Abstract— Semiconductor optical amplifier (SOA) is a favorable element like booster or pre-amplifier in most optical communication networks. In this paper SOA is used as nonlinear element to induce phase difference between data pulses. Switching characteristics are investigated by using a numerical method that describes the dynamic gain response of the SOA to ultrashort optical pulses. By reducing the input pulsewidth to subpicosecond regime, higher speeds and capacities for the optical telecommunications can be achieved. Index Terms— all-optical switch; semiconductor optical amplifier; Sagnac interferometer; ultrashort pulses I. INTRODUCTION Increasing demand for more bandwidth and speed in modern telecommunications and therefore high bit rate applications caused to actuate the recent research into analysis of ultrashort optical pulses. One of the main characteristic of all optical networks is that information remains completely in the optical domain along the process path without opto-electrical (O/E) and electro-optical (E/O) conversions. The implementation of ultra-high speed all-optical networks requires the design and development of all-optical switches as a key communication element. These switches will perform a set of critical network processing functions such as shift register with inverter [1], address recognition [2], full adder [3] and multi/demultiplexing [4]. Sagnac interferometer is a favorable structure for the switching purpose in optical telecommunication networks. The main advantage of this interferometer compared with the other interference structures is high phase stability. The Sagnac interferometer is called with different names in publications such as terahertz optical asymmetric demultiplexer (TOAD) [5] and semiconductor laser amplifier in a loop mirror (SLALOM) [6]. SOA have been investigated for applications in all-optical processing since the devices were first realized from mid 1980s [7]. SOA nonlinearities that can be used for all-optical signal processing mechanisms are cross-gain and cross-phase modulation (XGM and XPM), four-wave mixing (FWM), polarization rotation. In nonlinear SOA-assisted Sagnac switch, SOA is placed inside the loop in such a way that the position of the SOA can be changed with respect to the center of the loop. By this tunability desired phase can be induced to optical pulse and hence switching can be occurred. In this paper we examine the switching characteristics of SOA-based Sagnac interferometer for subpicosecond optical input pulses. All nonlinear effects in subpicosecond are analyzed by use of the modified nonlinear Schrödinger equations (MNLSE) [8]. The model used to solve this coupled MNLSE for simulating the SOA in this circuit follows what has been presented in [9]. We investigate the manner that the performance of this switch is optimized and improved by factors such as SOA small signal gain, the switching pulses energy and width, the Sagnac loop asymmetry, and SOA length. This paper is organized as follows: In Section 2, the principle of operation is described. In Section 3, the theory of interferometric equations and SOA modeling scheme based on modified nonlinear Schrödinger equations is introduced. In section 4, results and discussions of switching characteristics of proposed structure with improved and efficient model are presented. Conclusion is given in Section 5. II. PRINCIPLE OF OPERATION The schematic structure of the SOA-assisted Sagnac switch is shown in Fig. 1. The switch consists of an optical loop formed by the joint output ports of a 2×2 3-dB coupler and SOA. The position of the SOA can be displaced asymmetrically with respect to the center of loop by a distance that is equivalent to a temporal offset of T asym /2 that can be achieved by using optical delay line (ODL). A data input light is injected into the loop through the input port and splits symmetrically to two counter-propagating pulses, (the clockwise (CW) and counter-clockwise (CCW)), of equal amplitudes. The 3-dB coupler induces a π/2 phase difference between its outputs [10]. The data (probe) signal power is small enough so that it does not excite the optical nonlinear properties of the SOA. In the absence of control (switching) pulse (at least 10 times higher power than that of the data) [1], incoming data pulse exit the switch reflection port after a time that needs for passing the switch. Without control pulse the This work was supported by Iran Telecommunication Research Center (ITRC). 728 6'th International Symposium on Telecommunications (IST'2012) 978-1-4673-2073-3/12/$31.00 ©2012 IEEE