Tobias Lamprecht

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.

960 Gb/s Optical Backplane Ecosystem Using Embedded Polymer Waveguides and Demonstration in a 12G SAS Storage Array

An optical backplane ecosystem is described and demonstrated that is capable of multi-Tb/s bandwidth and is based on embedded polymer waveguides, passive optical backplane connectors, and midboard optical transceivers with bandwidth up to 28 Gb/s per lane. These systems provide the highest bandwidth-density, lowest power consumption, while maintaining the signal integrity. Ecosystems built around this architecture will provide the bandwidth-density required for next generation fabric interconnect for storage, switching, and routing applications in future high capacity generations of Data Centers and HPC systems. To demonstrate the applicability of this technology, it was used to provide embedded optical connectivity within a functional data storage enclosure.

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

Integrated Optical Backplane Amplifier

A solution for compensating losses in optical interconnects is provided. Large-core Al₂O₃:Nd³⁺ channel waveguide amplifiers are characterized and tested in combination with passive polymer waveguides. Coupling losses between the two waveguides are investigated in order to optimize the channel geometries of the two waveguide types. A tapered Al₂ O₃:Nd³⁺ waveguide is designed to improve the pump intensity in the active region. A maximum 0.21-dB net gain at a signal wavelength of 880 nm is demonstrated in a structure in which an Al₂O₃:Nd³⁺ waveguide is coupled between two polymer waveguides. The gain can be improved by increasing the pump power and adjusting the waveguide properties of the amplifier.

High-Precision, Self-Aligned, Optical Fiber Connectivity Solution for Single-Mode Waveguides Embedded in Optical PCBs

A fully passive, optical fiber connectivity solution for polymer waveguides embedded in electro-optical printed circuit boards (EOCB) is described and its preliminary results for single-mode applications demonstrated. The connectivity solution is based on a pluggable glass-fiber connector interface and a self-alignment packaging technology using high-precision silicon V-grooves. The V-grooves provide precise positioning of the glass-fiber relative to the polymer waveguide through alignment structures on the EOCB, patterned in the same laser direct writing fabrication step as the waveguide core. We realized an EOCB module with an LC-connector pluggable adaptor interface assembled on one side of the EOCB. With this module, we were able to prove for the first time the usability of our connectivity solution for single-mode applications. Coupling losses as low as 1.2 dB between a standard LC-connector and the single-mode polymer waveguide embedded into the EOCB have been reached. This passive packaging solution provides a cost-effective optical connectivity to wave-guides in EOCBs, which will be required in future generations of optical interconnects.

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.

Butt-coupled optoelectronic modules for high-speed optical interconnects

This paper presents the Butt-coupled silicon optoelectronic modules for high-speed optical interconnects. The optoelectronic (OE) modules is populated by flip-chip bonding either with VCSELs or detectors placed in direct contact with the waveguides without additional optical elements. The light is guided through the Si module by integrated waveguides based on holes etched through the silicon, which are then oxidized and filled with a polymer. This integrated OE module concept requires only one alignment step in the board and is compatible with a multi-layer waveguide approach. Data transmission of up to 5 Gbps has been demonstrated.

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.

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.