660 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 5, MAY 2002 Experimental Demonstration of All-Optical Regeneration Using an MMI-SOA Jan De Merlier, Student Member, IEEE, Geert Morthier, Senior Member, IEEE, Steven Verstuyft, Tom Van Caenegem, Student Member, IEEE, Ingrid Moerman, Member, IEEE, Peter Van Daele, Member, IEEE, and Roel Baets, Senior Member, IEEE Abstract—A novel all-optical 2R regenerator based on a multi- mode interference coupler (MMI) semiconductor optical amplifier is presented. Static measurements of the transfer function reveal a digital transfer characteristic and a high increase in extinction ratio. The experiments are in good agreement with simulations, which have been done using a modified beam propagation method program. The device has a high tolerance to the MMI length. It has been fabricated in an all-active layout, avoiding the need for active/passive integration and is very compact. Index Terms—All optical, MMI, regeneration, semiconductor optical amplifier. I. INTRODUCTION D UE TO THE accumulation of noise introduced by the am- plifiers, jitter and dispersion, the data signals are seriously degraded while propagating in an optical network. To avoid the electronic bottleneck, the integration of all-optical regenerators will be required in future high-speed wavelength-division-mul- tiplexing (WDM) systems to assure a sufficient network node cascadability. Most of the regenerators, proposed so far, perform regeneration with simultaneous wavelength conversion which makes them complicated devices needing a continuous-wave (CW) laser and a filter at the output [1], [2]. 2R-regeneration making only use of the input signal has been demonstrated using a Michelson interferometer. This type of regenerator has been operated at 40 Gb/s [3]. However, the Mach–Zehnder interfer- ometer (MZI), as well as the Michelson interferometer have the disadvantage that they lead to rather large chip sizes. In addition, to get a good regeneration characteristic, a cascade of several MZIs is needed [4]. Some time ago, we have proposed another type of 2R regenerator based on a multimode interference cou- pler (MMI)-semiconductor optical amplifier (SOA) with the po- tential of a digital static regeneration characteristic and a large increase of extinction ratio [5]. In this letter, the simulation of this device is described and first experimental results are shown. II. CONCEPT The MMI consists of a broad multimodal waveguide with in- and output waveguides to couple the light in and out. When a step-index profile is assumed the following relationship is ob- Manuscript received August 03, 2001; revised January 10, 2002. The work was supported by the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT). The authors are with the Department of Information Technology, Ghent Uni- versity, Ghent, Belgium (e-mail: jan.demerlier@intec.rug.ac.be). Publisher Item Identifier S 1041-1135(02)03251-2. Fig. 1. Schematic of MMI-SOA based regenerator. tained between the different propagation constants of the modes in the broad waveguide [6]: with (1) with the propagation constant of the order mode. The length corresponds to the beat length of the two lowest order modes with the effective index of the ridge and the ef- fective width of the waveguide including the penetration of the modes in the neighbor material. is the wavelength in vacuum. From expression (1) it is possible to derive some self-imaging properties of the input field at certain distances in the wave- guide. In case of a 2 2 MMI as shown in Fig. 1, at a distance of the excited modes interfere such that the input field is coupled to the output waveguide for the cross state. When this component is fabricated in an SOA-layer structure, nonlinear ef- fects influence the behavior of the device. At low input powers, the device works in the linear regime and all the light is cou- pled to the output waveguide for the cross state. When the input power is increased saturation sets in and causes the step-index profile to be disturbed due to the influence of the lateral power distribution on the electron density, which in turn has its effect on the real and imaginary part of the refractive index (2) with the effective refractive index of the ridge, the confine- ment factor, the antiguiding factor, the electron density, the differential gain, the wave vector, and the trans- parency electron density. This implies that the propagation con- stants of the different modes in the broad waveguide are changed in a mode-dependent way and eventually will interfere more constructively at the output waveguide for the bar state. Based on this effect regeneration can be obtained. III. SIMULATION RESULTS The simulations have been obtained using a two-dimensional static beam propagation method, linked to an external program 1041-1135/02$17.00 © 2002 IEEE