Design and optimization of planar multimode waveguides for high speed board-level optical interconnects

This paper describes the experimental investigation on the influence of geometrical waveguide parameters and material combination (refractive index contrast) onto relevant waveguide characteristics: optical attenuation, mode conversion, assembly tolerances and calculated bandwidth. For achieving waveguide structures with different geometries a test mask with a variation of the line width was developed. The application of a material system with tunable refractive index enables the change of numerical aperture (NA) of the waveguide. The fabricated waveguides will be characterized in their far field in order to define the NA at the waveguide´s output. The realization of these measurements for different waveguide lengths and core cross sections enables the characterization of the numerical aperture change caused by mode conversion and mode dependent attenuation. These phenomena have influence on intermodal dispersion, which limits the bandwidth capacity of the waveguide. The additional near-field analysis on the end face of the waveguide for the aforementioned design parameters variations will illustrate the mode filling and conversion along the waveguide. Additionally, the paper will analyze the influence of waveguide design parameters on the optical propagation loss by using attenuation measurements. On the other hand, the requirement for a robust optical coupling into the board-level optical interconnects call for relaxed assembly tolerances, which are also dependent on geometrical waveguide parameters. Finally the achieved experimental results will be analyzed in order to derive design rules for low-loss, robust optical link basing on multimode waveguides and ultimately enable multi Gbps m performance.