Optik 122 (2011) 1787–1790 Contents lists available at ScienceDirect Optik jou rnal homepage: www.elsevier.de/ijleo MZ-MMI-based all-optical switch using nonlinear coupled waveguides A. Bahrami ∗ , A. Rostami, F. Nazari Photonics and Nanocrystals Research Lab. (PNRL), Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz 51664, Iran a r t i c l e i n f o Article history: Received 19 May 2010 Accepted 10 November 2010 Keywords: All-optical switch Multi-mode interference Nonlinear directional coupler a b s t r a c t We propose an all-optical switch (AOS) based on Mach–Zehnder (MZ) and Multi-mode interference (MMI) using nonlinear closely coupled waveguides. The device operates by switching between two states of coupled waveguides. In first state the refractive index of waveguides are same and light field will completely couple to nonlinear waveguide in half length of coupler and will back in the second half. We will have  phase difference in this procedure and the input field will appear in Bar-state output. In the second state the refractive index of nonlinear waveguide increase with high intensity control field. In this case, we have lower coupling and change in phase. But, we choose the best refractive index change to obtain the phase change of multiple of 2 necessary for Cross-state in output. The beam propagation method is used to simulate the device operation. © 2010 Elsevier GmbH. All rights reserved. 1. Introduction All-optical switches (AOS) are desired devices in optical telecommunication. They can perform a variety of applications, particularly in signal routing and time division signal processing. One of the major advantages of all-optical switches is that they avoid the need for optic–electronic and electronic–optic (o–e–o) conversions. Such conversions not only limit the versatility and transparency of the system, but they introduce errors to signal. In fact, elimination of such o–e–o conversions will result in a major decrease in the overall system cost, since the equipment associ- ated with these conversions represents the main cost associated in today’s networks [1]. In recent years different materials and configurations have been employed for the development of opti- cal switches. They can be separated by the physical switching mechanism, such as acousto-optic [2,3], thermo-optic [4], electro- optic [5], and even electro-mechanical in nature [6], depending on the application requirements. MZI is a key component in optical switching devices. The principle of an MZI is splitting the input field into two equal fields which interfere at the output again and can produce the input field in one of two outputs. According to the relative phase values of two splitted fields MZI structure can have output fields in output 1 or 2. Two methods in optical switching based on MZI are applying ring-resonator or semiconductor optical amplifier (SOA) in coupling with MZI arms [7–9]. The other meth- ods of switching based on MZI are using MMI couplers or splitters in arms of MZI [10,11]. The problem of these devices is the require- ment of long length for switching. Therefore, these switches cannot ∗ Corresponding author. Tel.: +98 411 3393754/3393724; fax: +98 411 3294629. E-mail address: bahramihomepage@yahoo.co.uk (A. Bahrami). be integrated in small chip area. Some researches are done using nonlinear waveguide couplers [12,13]. Switching by a control beam in a nonlinear coupler is explained in [12], but in this switch the control beam has occupied one of the input ports and also one of the output ports. So, the control and input signals will require to be separated. Switching using nonlinear couplers is a well-known method, in which the data signal must experience two different phases in two different power levels to be switched between output ports. In this paper we propose an all-optical MMI switch based on Mach–Zehnder interferometer and nonlinear directional coupler in one arm of Mach–Zehnder. The device operation is based on switch- ing between two states of nonlinear directional coupler. Switching is done based on changing refractive index with high intensity con- trol field. In state one refractive index of waveguides are same. In this state in according to that length of coupler is equal to 2L c that causes that we have full of input field in output 1 (in Fig. 2). In state 2, we change the refractive index of nonlinear waveguide. In this case, we have a weak coupling and two arms of MZI will be same. So, there is output field in cross-state. The theoretical background of elements of the switch is explained in Section 2. In Section 3, the operation of switch and Simulation results and discussions are reported. 2. Theoretical background In this section, MMI and nonlinear directional coupler are pre- sented and discussed using mathematical principles. 0030-4026/$ – see front matter © 2010 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2010.11.005