R. Beyeler

Polymer-Waveguide-Based Board-Level Optical Interconnect Technology for Datacom Applications

On the basis of a realized 12times10 Gb/s card-to-card optical link demonstrator, the capabilities of a polymer-waveguide-based board-level optical interconnect technology are presented. The conception and realization of the modular building blocks required for this board-level optical interconnect technology are described in detail. In particular, we report on the fabrication and characterization of board-integrated optical low-loss polymer waveguides that are compatible with printed circuit board (PCB) manufacturing processes. We also explain our fully passive alignment technique, superseding time-consuming active positioning of components and connectors. To realize optical transceiver modules comprising vertical cavity surface emitting laser (VCSEL) arrays with laser drivers and photodetector arrays with transimpedance amplifiers (TIAs), a mass-production concept based on wafer-level processing has been elaborated and successfully implemented.

Development of a low-cost low-loss polymer waveguide technology for parallel optical interconnect applications

We report on the material evaluation, design, fabrication, and characterization of low-loss multimode polymer waveguides that are compatible with standard PCB manufacturing processes for use in large-area high-density high speed optical backplane interconnects.

120 Gb/s Optical Card-to-Card Interconnect Link Demonstrator with Embedded Waveguides

We report on a card-to-card optical interconnect demonstrator with passively aligned butt-coupled optoelectronic modules onto waveguides embedded into the printed circuit board (PCB). After describing selected building blocks, we will present experimental results obtained with the demonstrator hardware consisting of a parallel 12-channel at 10 Gb/s (120 Gb/s) optical card-to-card link.

Passive alignment of optical elements in a printed circuit board

A successful implementation of optics into PCBs (printed circuit boards) requires a precise passive alignment of optical elements relative to the optical waveguides in the board. We tackled this challenge with a novel concept that allows the passive alignment onto a PCB of any optical or optoelectronic building block with a precision of a few micrometers. Markers, structured into a copper layer during manufacturing, are used as a position reference for the polymer waveguide fabrication and for the formation of mechanical alignment features. To form the latter, laser drilling, a standard process for via formation in PCBs, is used. We were able to demonstrate repeated insertions of adapter elements into these alignment slots with a standard deviation of 3 mum for in-plane displacements. Afterwards, optical modules were mounted onto the adapters, using a standard MT interface provided by the adapter. We measured a standard deviation of the order of 5 mum for the in-plane and out-of-plane misalignments of the module with respect to the optical waveguides. The passive alignment concept demonstrated enables accurate and simple plug-in of any kind of element, in particular of optical and opto-electronic elements, into a PCB. The concept is based on established PCB manufacturing processes, which is crucial for the development towards a low-cost optical interconnect technology

Characterization of parallel optical-interconnect waveguides integrated on a printed circuit board

The development of optical interconnects in printed circuit boards (PCBs) is driven by the increasing bandwidth requirements in servers, supercomputers and switch routers. At higher data rates, electrical connections exhibit an increase in crosstalk and attenuation; which limits channel density and leads to high power dissipation. Optical interconnects may overcome these drawbacks, although open questions still need to be resolved. We have realized multimode acrylate-polymer-based waveguides on PCBs that have propagation losses below 0.04 dB/cm at a wavelength of 850 nm and 0.12 dB/cm at 980 nm. Transmission measurements at a data rate of 12.5 Gb/s over a 1-m-long waveguide show good eye openings, independent of the incoupling conditions. In the interconnect system, the transmitter and receiver arrays are flip-chip-positioned on the top of the board with turning mirrors to redirect the light. The coupling concept is based on the collimated-beam approach with microlenses in front of the waveguides and the optoelectronic components. As we aim for large two-dimensional waveguide arrays, optical crosstalk is an important parameter to be understood. Accordingly, we have measured optical crosstalk for a linear array of 12 optical channels at a pitch of 250 um. The influence of misalignment at the transmitter and the receiver side on optical crosstalk will be presented as a function of the distance between waveguide and transmitter/receiver.

Optical links for printed circuit boards

We identify applications for optics-enabled printed circuit boards, together with a list of requirements that have to be met for real-world products. We draw conclusions for the technology choices, and present our approaches and experiments.

Polymer waveguide-based multilayer optical connector

For the realization of a polymer waveguide based optical backplane link for computing applications, we developed a method to passively align multiple layers of polymer waveguide flex sheets in a single MT compatible ferrule. The minimal feature forming the backplane is a 192 channel link. This link is equipped with four MT connector at each end, and is performing a shuffling of the channels. We describe the passive alignment used to realize the connectors. The achieved accuracy demonstrated in a 48 channels connector consisting of 4 polymer sheets carrying 12 waveguides each, is shown to be better than ±5μm. The connection losses between a 48 channel MT fiber connector and the realized polymer waveguide connector were found to be about 2dB. Compared to fiber connectors, the presented concept using polymer waveguides has several advantages. The most relevant are that only few assembly steps are needed, it is based on a totally passive alignment scheme and it can easily be executed by standard pick and place tools.

Flexible optical interconnects based on silicon-containing polymers

Formulations containing silicon-based polymers have been used for the formation of planar waveguides on flexible substrates. The substrate of choice is compatible with the flexible waveguide and is made of materials commonly utilized in the printed circuit board industry. When the flexible waveguide material is combined with the chosen substrate using processes compatible with printed circuit board manufacturing techniques, the resultant optical interconnects display sufficient flexibility, low optical loss (<0.05 dB/cm at 850 nm), and high reliability.

Prospects of a polymer-waveguide-based board-level optical interconnect technology

In the long-distance telecom, local-area, and rack-to-rack link classes, optical interconnects have gradually replaced electrical interconnects. We believe that this trend will be continued in the short-distance card-backplane-card datacom link class. Convincing arguments for the predicted transition from electrical to optical interconnects are bandwidth-length advantages, density benefits, crosstalk reduction, and finally cost considerations. Based on this forecast, we currently develop a board-level optical interconnect technology facing several challenges, such as I) the manufacturing of reliable polymer waveguides, II) the elaboration of simple light-coupling concepts, III) the development of high-speed electro-optical modules, and IV) the application of cost-efficient packaging approaches. The successful mastering of all these tasks is a prerequisite for convincing high-speed system designers and porting optical interconnect technology into future product development plans. In this paper, we will present different achievements of our optical interconnect technology, e.g.: - 10 Gb/s per channel over 1 m link length, - optical link propagation loss below 0.05 dB/cm at 850 nm, - linear link densities up to 16 channels/mm, - feasibility of 2D channel arrays (e.g. 4 times 12), - a fully passive, low-cost alignment concept with a position accuracy of les 5 mum, enabling coupling losses < 0.5 dB, and - electro-optical transmitter and receiver modules operating at 10 Gb/s per channel. Finally, we will report on the successful realization of a 12 times 10 Gb/s card-to-card optical link demonstrator.

Waveguide Technology Development based on Temperature- and Humidity-Resistant Low-Loss Silsesquioxane Polymer for Optical Interconnects

We have jointly developed the material and processes to integrate silsesquioxane optical waveguides on printed circuit boards for high-speed datacom applications, using a low-loss, reliable and commercially available polymer designed for 850 nm.