Norbert Meier

Polymer waveguides for electro-optical integration in data centers and high-performance computers.

To satisfy the intra- and inter-system bandwidth requirements of future data centers and high-performance computers, low-cost low-power high-throughput optical interconnects will become a key enabling technology. To tightly integrate optics with the computing hardware, particularly in the context of CMOS-compatible silicon photonics, optical printed circuit boards using polymer waveguides are considered as a formidable platform. IBM Research has already demonstrated the essential silicon photonics and interconnection building blocks. A remaining challenge is electro-optical packaging, i.e., the connection of the silicon photonics chips with the system. In this paper, we present a new single-mode polymer waveguide technology and a scalable method for building the optical interface between silicon photonics chips and single-mode polymer waveguides.

Development of Versatile Polymer Waveguide Flex Technology for Use in Optical Interconnects

We report on the implementation of novel flexible polymer waveguide interconnects. They are based on newly developed mechanically flexible low-loss silicone waveguides. In addition to meeting the generic requirements of rigid waveguide interconnects, several flex-material challenges were mastered: a) mechanical flexibility permitting waveguide flexing down to radii of 1.0 mm without cracking; b) minimization of waveguide curling induced by the CTE mismatch between flex substrates and polymer layers to enable assembly and connectorization; c) greatly improved cladding adhesion on standard PCB flex substrates, such as polyimide; and d) high environmental stability despite the reduced polymer cross-linking required for better mechanical flexibility. The new waveguides exhibit excellent stability in damp heat (2000 h in 85°C/85% rH) and under thermal shock (500 cycles from -40° to +120°C), and lead-free solder reflow up to 260°C. Using the newly engineered “Dow Corning WG-1017 Optical Waveguide Clad Dev Sample” and the established “Dow Corning WG-1010 Optical Waveguide Core”, we were able to develop a manufacturing process suitable for large areas and offering high process control and stability to produce waveguides having optical loss values of less than 0.05 dB/cm at 850 nm VCSEL wavelength and fulfilling requirements (a) to (d) above. We describe this manufacturing process and how we have overcome the material challenges mentioned. Furthermore, we present characterization and manufacturing results, show demonstrators, and outline the potential of flexible waveguides as versatile electro-optic assembly platform.

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

Flexible, stable, and easily processable optical silicones for low loss polymer waveguides

Photopatternable optical silicone materials have been developed that can be fabricated into flexible polymer waveguides using conventional film processing and photolithography techniques. Waveguides designed for multimode applications have demonstrated loss of 0.05 dB/cm at 850 nm. These waveguides have proven stable in 85% relative humidity and 85°C storage conditions for greater than 2000 hours to date with no degradation in optical performance. Further, this polymer waveguide system has shown sufficient adhesion to flexible polyimide substrates without any curling, and passed bending tests to a 1 mm radius without damage.

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.

Parallel optical interconnects in printed circuit boards

Polymer waveguides embedded in a printed circuit board offer a substantial increase in the achievable bandwidth density compared with todays electrical interconnects. We present our results on the polymer waveguide technology and the building blocks that perform the optoelectronic conversion. Specific challenges in integrating optics in a printed circuit board are addressed. Data transfer measurements are presented.