J. Opt. A: Pure Appl. Opt. 1 (1999) 255–261. Printed in the UK PII: S1464-4258(99)97943-6 Performances of optical multi-chip-module interconnects: comparing guided-wave and free-space pathways V Baukens†, G Verschaffelt†, P Tuteleers†, P Vynck†, H Ottevaere†, M Kufner†, S Kufner†, I Veretennicoff†, R Bockstaele‡, A Van Hove‡, B Dhoedt‡, R Baets‡ and H Thienpont† † VUB, Department of Applied Physics and Photonics (TW-TONA), Pleinlaan 2, B-1050 Brussel, Belgium ‡ RUG, Department of Information Technology (INTEC), Sint-Pietersnieuwstr 41, B-9000 Gent, Belgium Received 11 September 1998, in final form 24 December 1998 Abstract. We simulate and compare optical transmission efficiencies, throughputs and interconnection lengths of free-space and POF-based guided-wave multi-chip-module optical interconnection demonstrators for different types of microcavity emitters. Keywords: Optical interconnects, plastic optical fibres, optical design 1. Introduction The performance of future-generation data processing systems will be set by intra-multi-chip-module (intra- MCM) interconnect limitations rather than by the processing performance of the CMOS ICs within these modules [1]. Optical input/output (I/O) over the entire chip area is pursued as a solution to these interconnection problems in the European Community funded ESPRIT project ‘Optically Interconnected Integrated Circuits’ (OIIC) [2]. With this approach, two-dimensional (2D) parallel data transfer will be made possible through the use of opto-electronic emitter and receiver arrays, flip-chip mounted on CMOS circuitry, and interconnected by passive optical pathway blocks (OPB). At the source side of the optical interconnection, both vertical cavity surface emitting lasers (VCSELs) [3] and micro- cavity light emitting diodes (MCLEDs) [4] are considered. As receivers, InP detector arrays will be used. For the optical pathways, two approaches are explored. As a principal choice in this project, a 2D array of small diameter plastic optical fibres (POFs) is used as a guiding transmission medium. An alternative is the construction of a free-space OPB. To that aim manufacturable, high- precision monolithic plastic micro-optical pathway blocks are fabricated using proton irradiation to achieve deep- etch lithography [5]. They integrate microlenses and micromirrors to optically interconnect the flip-chip mounted surface-normal transmitters and receivers. Two requirements have to be fulfilled by these free-space OPBs before they can be considered as possible candidates for optical MCM interconnections. First, the transmission efficiency has to be sufficiently high to satisfy the receiver sensitivity and guarantee the very low bit error ratio required for this class of data transmission links (with bit error rate (BER) = 10 −13 – 10 −15 ). Second, we cannot allow cross-talk between the optical data channels. In this paper we simulate and analyse the performances of the guided-wave optical interconnection demonstrators for different characteristics of the POFs and for different types of microcavity emitters. We also study the performances of the free-space interconnection demonstrators for VCSEL sources. In both cases we derive guidelines for the design of optimized practical configurations. Finally, we compare the different approaches by indicating what interconnection density and what aggregate bit-rates one can expect for each case and for what interconnection distances these optical approaches could possibly be used as electrical wire replacement technologies. 2. POF optical pathway block 2.1. The proof-of-principle demonstrator The POF optical pathway block under study consists of a plastic plate with a 2D array of holes into which a 2 × 8 POF ribbon is inserted (see figures 1 and 2). For this application POFs were preferred over glass fibres because of their lower cost, their smaller bending radius and their large NA. The 1464-4258/99/020255+07$19.50 © 1999 IOP Publishing Ltd 255