Multimode interference (MMI) couplers are important integrated optical components for the optical signal processing and routing. The realization of these components by ion exchange on glass substrates is particularly attractive for low cost integration. The design and analysis of MMI devices have generally been based on the self imaging principle in step-index waveguides, whereas waveguides fabricated by ion exchange on glass are practically graded-index due to the nature of the thermal diffusion of exchanged ions. In addition, the ion exchange process results in a guide with depth that depends on the mask opening (the guide width) which causes a high insertion loss at the interface between single mode and multimode sections of the MMI. To overcome these problems 3D simulation of the ion exchanged MMI structures is strongly required. In this work such 3D simulation is achieved on two levels. First the non-linear diffusion equation describing the ion exchange process is solved numerically using a finite-difference method with a modified algorithm to ensure solution stability for an extended range of nonlinearity. The resultant index distribution is used in a wide angle 3D BPM to simulate the optical field propagation in the structure. This allows accurate prediction of the structure performance under different fabrication and excitation conditions. Based on this simulation technique, 3 dB MMI splitter design with tapered access guides is optimized by both geometrical mask design and process parameter variations. The optimization shows that both the tapering and the use of annealing process can significantly improve the performance of the devices.

Date Published: 12 October 2005
PDF: 7 pages
Proc. SPIE 5970, Photonic Applications in Devices and Communication Systems, 59701G (12 October 2005); doi: 10.1117/12.628563

Published in SPIE Proceedings Vol. 5970:
Photonic Applications in Devices and Communication Systems
Peter Mascher; John C. Cartledge; Andrew Peter Knights; David V. Plant, Editor(s)