Multimode spot-size converter for optical MEMS applications

Efficient power coupling is an unavoidable requirement in optical MEMS systems, either from input multimode optical fiber to the optical MEMS structure or between different multimode waveguides integrated on the same chip. In this work, non-imaging tapered structures and concentrators are proposed and discussed. The optimal transmission condition is deduced for linear tapered structures and compared to industry standard ray tracing simulations, taking into account the three-dimensional structure of the micromachined structures. The study is carried out for different etching depths and account for the practical MEMS mirror reflectivity. In addition, compound parabolic concentrators are discussed and compared to the linear ones. The results show that the linear taper has the advantage of smaller length while the CPC coupling efficiency can be higher. For instance, the optimal taper length is found to be 160 μm converting an input spot size of 100 μm to an output spot size of 40 μm, while about 90 % of the power is delivered for an optical MEMS structure height of 100 μm.