Cladding stress induced performance variation of silicon MMI coupler

Abstract

In this paper, we have shown the effect of stress induced by cladding layer on the performance of silicon MMI coupler. The cladding material is assumed to be SiO2. From the numerical analysis using finite element method (FEM), we have demonstrated that the length of the MMI coupler for a given power splitting ratio is a strong function of stress. Moreover, with temperature the power splitting ratio changes as high as 50% for a MMI coupler having cladding induced stress, compared to a MMI coupler without having cladding induced stress. In this study, two cross-sectional geometries, namely rectangular and trapezoidal are considered to investigate the effect of waveguide cross section on the cladding induced stress. Here, the SiO2 cladding thickness is also varied from 400 nm to 800 nm for both types of geometries. It is found that out-of-plane stress increases with cladding thickness for both waveguide cross-sections and the opposite nature of variation is found for in-plane stress. Trapezoidal waveguide introduces higher in-plane as well as out-of-plane stress compared to rectangular waveguide for any given cladding thickness. Further, the temperature sensitivity of MMI coupler without and with stress having two different power splitting ratios and waveguide cross-sections are studied. The stress induced birefringence is also calculated for the fundamental mode (MMI under consideration supports 16 modes in total) as a function of cladding thickness and operating temperature. The results show that like previously studied waveguide structure, in MMI coupler also the birefringence increases with increased cladding thickness, whereas it decreases with the operating temperature. It is also observed that the stress induced birefringence is more for trapezoidal waveguide cross-section compared to rectangular cross-section. Simulation results reveal that the effective thermo-optic coefficient for a given mode supported by the MMI coupler is independent of cladding thickness, waveguide geometry and stress.