Int. Zurich Seminar on Communications (IZS), Feb. 18–20, 2004 0-7803-8329-X/04/$20.00 c 2004 IEEE. 196 MIMO Channel for Modal Multiplexing in Highly Overmoded Optical Waveguides Daniel Lenz 1 , Boris Rankov 2 , Daniel Erni 1 , Werner B¤ achtold 1 and Armin Wittneben 2 1 Laboratory for Electromagnetic Fields and Microwave Electronics, 2 Communication Technology Laboratory Swiss Federal Institute of Technology (ETH) Z¤ urich, CH-8092 Z¤ urich, Switzerland Email: {dlenz, erni, baechtold}@ifh.ee.ethz.ch, {rankov, wittneben}@nari.ee.ethz.ch Abstract In order to meet the increasing demand for data rate in modern multi-processor systems optical transmission has become an important technology. Due to cost reasons only highly multimodal waveguides come into question for mass-producible board level interconnects. We present the idea of an optical multiple-input multiple-output (MIMO) channel by having more than one data source at the channel input and a multi-segmented photodetector at the channel output. We develop channel models for the optical MIMO channel and show that orthogonality between modes in a highly multimodal waveguide can be exploited to submit independent data streams over different mode groups (modal multiplexing). Our performance results in terms of bit error rates show that the MIMO scheme outperforms the SISO scheme when the data rate is the same. Hence, optical MIMO schemes allows either to increase the data rate by keeping the amount of intersymbol interference (ISI) constant or to reduce the ISI by keeping the data rate constant. I. I NTRODUCTION Optical transmission has become the future data commu- nication technology on multi-layered Printed Circuit Boards (PCBs) to meet the increasing demand for communication bandwidth of modern multiprocessor systems [1], [2]. For mass-producible board level interconnects only multimode waveguides are applicable due to the inappropriateness of clean room technologies and the relaxed tolerance require- ments compared to singlemode technology. Such waveguides have very large cross sectional dimensions (100 × 100 μm) and are guiding up to several thousand modes. The drawback of multimode waveguides is the mode dispersion caused by the different propagation delays of the modes. Optical communication systems using multimode waveguides are thus rather dispersion limited than noise limited and require the application of equalizers. Argon et al. [3] proposed recently a multi-segmented detector to obtain spatial diversity in a single-input multiple- output (SIMO) multimode ber communication channel. In this work we extend the concept of [3] and present the idea of an optical multiple-input multiple-output (MIMO) channel by having more than one laser source at the channel input and a multi-segmented detector at the output. MIMO communication systems are well known from the wireless communication literature where antenna arrays at the transmit and receive side introduce additional degrees of freedom (spatial dimension) into a wireless communication system [4]. There are two basic space-time processing methods which make use of these degrees of freedom in MIMO systems, namely space-time coding to improve link reliability and spatial multiplexing to increase spectral efciency. With spatial multiplexing it is possible to enhance the data rate without additional cost of bandwidth or power by transmitting simultaneously over spatial subchannels [4]. We introduce here the concept of modal multiplexing where we exploit the orthogonality between modes in a highly multimodal waveguide to submit independent data streams over different mode groups. This may help to overcome the limitation imposed by the intersymbol interference (ISI) caused by signal dispersion since the symbol duration may be relaxed without reducing the data rate. The remainder of the paper is organized as follows: Section II describes the optical communication system and the phys- ical models that are applied in order to obtain the parameters for the channel model that is deduced in Section III. A simple receiver scheme for SISO and MIMO transmission and performance results are demonstrated in Section IV. II. SYSTEM DESCRIPTION AND PHYSICAL MODELS The investigated optical communication system consists of a Vertical-Cavity Surface-Emitting Laser (VCSEL) source which is butt-coupled to a rectangular optical waveguide embedded in a PCB (Figure 1). For the sake of simplicity we assume a straight, perfect waveguide, i.e., no bends and no surface roughness are taken into account. Since the waveguides are embedded in a PCB, the length of the waveguide shall not exceed a few meters. At the receiver side a photodetector array is assumed. The idea is to take the optical communication system as a MIMO system. Therefore multiple inputs are required as well. Among the many possibilities of realizing multiple inputs we propose a segmented VCSEL contact geometry [5] to realize two sources within one VCSEL. Applying the proposed asymmet- ric drive scheme based on individually addressable segmented VCSEL contacts, a preferential excitation of modes with given azimuthal distribution is possible [5]. Hence, in the following sections we assume the VCSEL to excite two dominant independent controllable modes, namely the LP c 11 and the LP s 11 mode [6]. A rigorous treatment of the entire communication system, i.e., laser, far eld transformation, mode coupling and mode propagation through the oversized waveguide, is not possible.