Self-Switching Using SOA-Assisted Sagnac Interferometer Vahid Ahmadi a , Morteza Jamali a and Mohammad Razaghi b a Dept. of Electrical and Computer Eng., Tarbiat Modares University, Tehran, Iran. Email: v_ahmadi@modares.ac.ir b Dept. of Electrical and Computer Eng., University of Kurdistan, Sanandaj, Iran. Email: m.razaghi@uok.ac.ir Abstract—We propose and investigate self-switching mechanism by Sagnac interferometer based semiconductor optical amplifier (SOA) for subpicosecond pulses. The various switch characteristics such as phase differentiation between propagated pulses, SOA gain and switch extinction ratio all in time domain are shown. I. INTRODUCTION The switching characteristics of SOA as a nonlinear component in all-optical networks have been of large attention in recent researches. Sagnac switch based on SOA is one of favorite structures to achieve this goal. By using modified nonlinear Schrödinger equation (MNSLE) considering main which includes nonlinear effects, we study gain and phase dynamics of SOA. The analysis is based on improved finite difference beam propagation method (IFD-BPM) [1]. In Sagnac loop, cross phase modulation (XPM) technique is used by employing SOA switching nonlinearities [2,3]. The input light pulse enters the first loop through input port of input coupler and splits unequally into two counter-propagating pulses, (u and v), with π/2 phase difference. In our proposed scheme, SOA is offset from the center of the loop using optical delay line (ODL). This will cause SOA to operate in different regimes for two input pulses. This phenomena, if other Sagnac switch parameters are tuned suitably can lead to sufficient phase difference between SOA’s output counter- propagating pulses required for proper switching mechanism. Nonlinear effects of SOA become more important in subpicosecond regime rather than in the picoseconds regime. The main nonlinear effects are: self phase modulation (SPM), two photon absorption (TPA), carrier heating (CH), spectral hole burning (SHB), Kerr effects and gain dispersion. In subpicosecond regime, besides SPM effect which is the dominant phenomenon for picosecond pulses, the effects of SHB and CH phenomena on pulse shape and spectrum are more noticeable. Due to these effects the switch output pulse is broadened. In comparison to previous works [2,3] in our proposed scheme we don’t need additional pump pulse for switching purpose besides using two independent input and output couplers, which leads to more tunability. Furthermore the double Sagnac structure with symmetric output coupler (coupling ratio 0.5) can be used as a pattern effect compensator [4]. Fig.1. Schematic of Sagnac-based switch with unequal power distribution due to 2×2 couplers. OC: optical circulator, ODL: optical delay line. II. THEORY A. Self-switching Operation Perinciple The operation of the self- switching can be described with the help of the schematic diagram shown in Fig. 1. The switch consists of two optical loops formed by the joint input and output ports of two independent 2×2 couplers and a SOA that can be offset from the midpoint of the loops. When an input pulse enters the loop through one of the input ports of input coupler, it splits asymmetrically to two counter-propagating pulses, (u and v). Each coupler induces a π/2 phase difference between its outputs. The low power optical pulse is injected several picoseconds before the high power optical pulse. These delays cause changes in both gain and refractive index of SOA for two counter-propagating pulses and therefore, propagating pulses experiences different dynamic states. As a result, due to SOA nonlinearities the phase difference occurs between these two pulses. B. Theory of proposed self-switched scheme The optical input pulse injected in one of the input ports [e.g., Port 1, see Fig. 1], is distributed unequally to u and v pulses. The power splitting ratios in input and output coupler are X and 1-Y, respectively. The u and v pulse powers after passing through input coupler are p u = XP in and p v = (1-X)P in respectively, where P in is input power. The high power optical pulse arrives 7 ps (the time needed for a pulse propagated through the SOA cavity) after the low power optical pulse. As mentioned before, SOA induces a nonlinear phase shift (∆ ே௅ ) to optical pulses. When ∆ ே௅ ൌ ߨ, maximum extinction ratio between switch outputs (Port 3 and Port 4) can be reached. The basic interferometric equations that describe the output pulses at the output ports (P3 and P4 respectively), can be written as 3 ൌ  ௨ ሺݐሻ ൅ ሺ1 െ ሻ ௩ ሺݐሻ ൅ 2 ൈ ሺඥሺ1 െ ሻ ௨ ሺݐሻ ௩ ሺݐሻ ൈ ݋ݏሾ ௨ ሺݐሻെ ௩ ሺݐሻሿሻ ሺ1ሻ NUSOD 2012 69 978-1-4673-1604-0/12/$31.00 ©2012 IEEE