Embedded micro-mirrors for compact routing of multimode polymer waveguides

Abstract

Performance scaling of computing devices leads to higher bandwidth requirements for the processor package. The limited scalability of high-speed electrical interconnects [79] drives research on optical interconnects and optical printed circuit board (PCB) technologies [2, 14, 22, 52, 80]. The first part of this thesis is concerned with simplifying polymer waveguide routing and compact integration schemes for optical printed circuit boards. The discussed subjects are: Embedded micro-mirrors for in-plane and out-of-plane redirection of the light path (a), and mechanical alignment fiducials for the assembly of optical components (b). Both are then eventually integrated in a compact electrooptical flex board (c). The developed embedded micro-mirrors (a) are an integral part of the waveguide layer. The micro-structures that make up their bodies are fabricated directly onto the lower cladding by UV-laser patterning of a photosensitive resin. Vertical and 45°tilted micro-structures are eventually used as in-plane and out-of-plane micromirrors, respectively. A wet-chemical deposition process is developed to apply the reflective metal layer selectively on the micro-structures. In-plane micro-mirrors with a surface roughness of R a = 20 nm and reflectivity of R = 0.5 are realized. The fabrication processes are compatible to polymer waveguide and PCB manufacturing equipment. The presented mechanical fiducial marker approach (b) enables a precise and adjustment-free mounting of external components to an optical printed circuit board. The achieved positioning accuracy of inserted mechanical adapters is σ axis < 5 µm with respect to the waveguide axis. This yields an acceptable missalignment induced coupling loss of less than 0.5 dB. An electro-optical flex board (c), which integrates the mechanical fiducial markers (b) and the out-of-plane micro-mirrors (a), is fabricated as basis for a compact electrooptical module. The optoelectronic element, which can be a VCSEL-(vertical cavity surface emitting laser) or PD-(photodiode) array, will be vertically coupled to the underlying array of twelve polymer waveguides by the embedded out-of-plane micro-mirrors. The second part of this work is concerned with the analysis of light propagation in waveguide links. Herein proposed is an experimental approach to characterize the modal power coupling in simple waveguide elements. The used modal power coupling matrix relates the input and output modal power distribution of the waveguide elements. A generic optical waveguide link can then be represented as a concatenation of pre-characterized simple waveguide elements. To measure the modal power coupling matrix, the launch of specific modes at the input facet is required. Therefore, an intensity-and a phase-controlling spatial light modulator (SLM), are used to generate the specific modal field profiles. Therewith, lower order modes are successfully launched in a circular step-index fiber. In order to analyze the modal power distribution at the endfacet of the waveguide, an approach based on o...