There are many different types of components used in optical communication systems. Difficulties in interconnecting these elements are responsible for the increased cost and complexity of many systems. One of the main issues is the coupling efficiency between the different components involved. The coupling efficiency is affected by mode mismatches as well as device misalignments. In this paper, we look at the example of a hybrid optical switch to show some of the coupling issues when using otpical interconnects. We first show how the use of tapered polymer waveguides can perform a mode transformation in order to reduce the mode mismatch. We also present results for a Monte-Carlo simulation which shows the effects of the component misalignments in the hybrid device.
The coupling efficiency between laser diode arrays or OEIC components and single-mode fiber ribbons drops rapidly with increasing misalignment tolerance. There are trade-offs between the allowed minimum coupled power and the cost of establishing the required alignment. We demonstrate how tapered polymer waveguides may be used to match the laser mode to the fiber mode, resulting in a module with decreased alignment requirements for a given coupling efficiency. Waveguides with tapered mode profiles have been constructed using photobleaching of a guest/host dye/polymer system. Amoco 4212 polyimide doped with DCM dye was chosen as the waveguide material due to its good thermal stability, and its simple processing which allows multilayer waveguides to be readily fabricated. In particular, waveguides which simultaneously taper the mode both laterally and vertically have been designed. This waveguide system is compatible with assembly of the laser and waveguide components into a module by flip-chip soldering. Our solder self-alignment technique achieves accurate alignment of touching chips through an understanding of the dynamics during bonding.
Surface irregularities on a waveguide are influenced by material composition, the process by which polymer was made, and the technique used to fabricate the end-face. Different end-face preparation techniques (laser ablation, sawing, cleaving, reactive ion etching) produces different kind of surface terrain. Surface features, big compared to the wavelength of light, are represented as phase distortions superimposed on the Gaussian excitation beam, optimized to get maximum coupling efficiency. Similarity between the excitation beam and the fundamental waveguide mode is a measure of coupling loss due to different type of surface errors. To investigate the effect of surface roughness on coupling efficiency, several different surfaces were considered. These included sinusoidal phase gratings, spherical groove, cylindrical groove, wedges and delamination of the waveguide layers. For each tupe of surface error, a critical parameter was identified, and the associated value for that parameter was determined for a given tolerable coupling loss.