Multimode Polymer Waveguide Components for Complex On-Board Optical Topologies

Multimode polymer waveguides are an attractive transmission medium for board-level optical links as they provide high bandwidth, relaxed alignment tolerances, and can be directly integrated onto conventional printed circuit boards. However, the performance of multimode waveguide components depends on the launch conditions at the component input, complicating their use in topologies that require the concatenation of multiple multimode components. This paper presents key polymer components for a multichannel optical bus and reports their performance under different launch conditions, enabling useful rules that can be used to design complex interconnection topologies to be derived. The components studied are multimode signal splitters and combiners, 90°-crossings, S-bends, and 90°-bends. By varying the width of the splitter arms, a splitting ratio between 1% and 95% is achieved from the 1 × 2 splitters, while low-loss signal combining is demonstrated with the waveguide combiners. It is shown that a 3 dB improvement in the combiner excess loss can be achieved by increasing the bus width by 50 μm. The worst-case insertion loss of 50 × 100 μm waveguide crossings is measured to be 0.1 dB/crossing. An empirical method is proposed and used to estimate the insertion losses of on-board optical paths of a polymeric four-channel optical bus module. Good agreement is achieved between the predicted and measured values. Although the components and empirical method have been tailored for use in a multichannel optical bus architecture, they can be used for any on-board optical interconnection topology.

I. Introduction

Conventional copper-based electrical tracks are fundamentally bandwidth-limited by their aspect ratio [1] and are therefore, considered unsuitable for high-capacity on-board links required in next generation supercomputers and servers [2]. As optics are able to provide higher board-level bandwidth density and power efficiency [3]–[5], considerable research has been conducted to integrate optical links onto printed circuit boards (PCBs). Several different methods for forming hybrid opto-electronic (OE) PCBs have been proposed including free-space optics [6]–[8], conventional optical fiber technologies [9]–[11], and polymer waveguides [12]–[15]. Multimode polymer waveguides in particular are a promising technology for on-board optical links as they can be cost-effectively formed directly onto PCBs, and are able to withstand the high temperature environments associated with the assembly and use of PCBs [16]. To date, various on-board point-to-point multimode polymer-based optical links have been demonstrated, achieving aggregate data rates up to 225 Gb/s (15 × 15 Gb/s) while being integrated on the board with relaxed alignment tolerances [15].