Noise and Crosstalk Analysis of SMZ Switches R. Ngah, and Z. Ghassemlooy Optical Communication Research Lab., School of Engineering & Technology, Northumbria University Tel: +44 191 227 4902, Email: fary.ghassemlooy@unn.ac.uk Abstract: Ultra-high speed OTDM networks require stable and compact all optical switches for demultiplexing and routing. In this paper we investigate the noise and crosstalk characteristics of all optical symmetric Mach-Zehnder (SMZ) switch using a numerical model. Two effects, the relative intensity noise (RIN) and channel crosstalk (CXT) that degrade the performance of the switch are investigated. Numerical simulation results show that RIN and CXT varies with different SOA length, data pulse width, RMS jitter and the bit rate. 1. Introduction All optical switches based on the cross-phase modulation (XPM) in conjunction with interferometric configurations such as Mach- Zehnder interferometers (MZIs), terahertz optical asymmetric demultiplexers (TOADs) and ultrafast nonlinear interferometers (UNIs) are widely studied [1]. Among these the monolithically integrated MZI switches are the most promising due to their compact size, thermal stability, and low–power operation. All optical demultiplexing employing MZI switches at 168 Gb/s has been reported [2]. Considering various MZI configurations, the symmetric Mach-Zehnder (SMZ) structure provides the highest flexibility and shortest switching window [3]. These switches are used in a number of applications such as optical routers. Recently, a 1x2 all optical router employing SMZ switches has been reported [4]. Therefore, understanding their noise and crosstalk characteristics is extremely important in order to reduce power penalty it may incur. This paper presents an analysis and subsequent simulations of the noise and crosstalk characteristics of a SMZ by mean of a computer simulation. The structure of this paper is as follows. The operation principles of the SMZ switch are explained in section 2. In Section 3, the noise and crosstalk characteristics of the switch and the sources introducing them are identified. Numerical simulation results for the noise and crosstalk performance of all optical SMZ switch are presented in Section 4. Finally, in Section 5, the concluding remarks are summarized. 2. Operation Principles Figure 1 shows the block diagram of a typical SMZ switch composed of two semiconductor optical amplifiers (SOAs), one in each arm of the interferometer, and a number of 3-dB couplers. SOAs are positioned in the same relative location within the interferometer. Control and data pulses are fed into switch via 3-dB couplers and co-propagate within the interferometer. With no control pulses, the SMZ is balanced in such a way that all the data signals emerge from the reflected output port (port 2). However, with the control signals present a differential phase shift is introduced between the two arms of the interferometer thus causing the data pulses to be switched to the transmitted port (port 1). Note that the temporal delay between the control pulses determines the nominal width of the switching window, W(t) of SMZ. W(t) at port 1 is given by [5]: { } )) ( cos( . ) ( ) ( 2 ) ( ) ( 25 . 0 ) ( 2 1 2 1 t t G t G t G t G t W ϕ ∆ − + = (1) where G 1 and G 2 are the temporal gain profile of the data pulses, and ∆φ is the phase difference between the data pulses which is related to the gain ratio and the linewidth enhancement factor and is given by [9]: κ ( ) 2 1 / ln 5 . 0 G G κ ϕ − = ∆ (2) Solving (1) requires determination of G 1 and G 2 gains of the data signal at the output of the SOA1 and SOA2 at the temporal point, which are given as, respectively:                 + Γ = ∫ SOA L g dz V z t z g t 0 1 , . exp ) ( G (3)                 + + Γ = ∫ SOA L g delay dz V z T t z g t G 0 2 , . exp ) ( (4) where Γ is the confinement factor, g represents the differential gain of data and control pulses, t the time at which the temporal point of the data pulse enters the amplifier, T delay the temporal delay between the control pulses, z/V g the time increment in the z direction and V g the group velocity of the control pulse 3. Noise and Crosstalk Analysis The noise and crosstalk associated with all optical switches are the relative intensity noise (RIN) and channel crosstalk (CXT). The mathematical models reported in [5-6] are used in these analyses. 3.1 Relative intensity noise