Optimization of rib, single mode SiOxNy optical waveguide structure for athermal AWG using PMMA cover layer

This paper presents an optimization and dimensioning study of a rib, single-mode SiOxNy optical waveguide structure for athermal arrayed waveguide gratings using a polymethylmethacrylate cladding layer. The SiOxNy core waveguide layer is fabricated using plasma-enhanced chemical vapor deposition, where the refractive index can be precisely tuned through deposition parameters. This tunability enables control over the refractive index contrast between the SiOxNy core and the polymethylmethacrylate cladding, thereby influencing the temperature dependence of the waveguide’s effective refractive index. The complex effective refractive index of the rib waveguide structure, including thermo-optic effects, was calculated using the Finite element method. Additionally, bending losses as a function of curvature radius were simulated by solving Maxwell’s equations in cylindrical coordinates. In the proposed single-mode rib waveguide design, the temperature dependence of the effective refractive index can be modulated by adjusting the evanescent field penetration into the polymethylmethacrylate layer, which has a negative thermo-optic coefficient. This is achieved by optimizing the waveguide-to-slab height ratio, waveguide width and the refractive index contrast between the SiOxNy and polymethylmethacrylate layers. The proposed rib waveguide structure significantly mitigates temperature-induced central wavelength shifts of arrayed waveguide gratings, achieving a shift of only 0.718 pm/°C, lower than previously reported values 1.4 pm/°C, over an extended temperature range from –40°C to 80°C at 1550nm.