Alignment tolerancing of free-space MCM-to-MCM optical interconnects

In order for optical interconnects to become a mature technology they must be amenable to electronic packaging technology. Two main obstacles to including free-space optical interconnects are alignment and heat dissipation issues. This paper presents work that studies the issues of alignment tolerancing over long distance (>10 cm) board-level interconnects. In this work we will demonstrate a computer-aided analysis procedure that permits one to determine the alignment tolerances needed to achieve some system level specification, such as yield or cost. The procedure that we employ relies upon developing a detailed design of the system to be studied in a standard optical design program such as CODE V. Using information from this model, we can determine the integrated power falling on the detector by performing Gaussian propagation and/or general Fresnel propagation (if significant vignetting occurs). With this computer-aided analysis technique, a sensitivity analysis of all the misalignments under study is made on a realistic system to find each misalignment's relative effects (with other misalignments being set to zero) on the power falling on the detector. This information is used to set initial tolerances for subsequent tolerancing analysis and design runs. An alignment tolerancing analysis using Monte Carlo techniques is applied to determine if the yield/cost (yield being defined as the percentage of systems that have acceptable system performance) is acceptable. By utilizing a technique called parametric sampling, a subsequent tolerancing design run can be applied to optimize this yield/cost with very little increase in computation. In this paper, we will study a realistic design example and show that all tolerances can be achieved with current technology.