Effective and group refractive index extraction and cross-sectional dimension estimation for silicon-on-insulator rib waveguides

The accurate determination of the effective and group refractive indices (n_eff and n_g) of optical waveguides as a function of wavelength is of critical importance to the design of photonic integrated circuits (PICs). This paper demonstrates the extraction of the two parameters of silicon-on-insulator (SOI) rib waveguides using the transmission spectra of two racetrack micro-ring resonators (MRRs) with different perimeters. The extracted n_eff and n_g exhibit an uncertainty of approximately 10^-3. Based on the extracted n_eff(λ), we estimate the cross-sectional dimension of the SOI rib waveguide that constitutes the MRR. This waveguide has a nominal rectangular cross section with a width, height, and slab thickness of 450 nm, 200 nm, and 70 nm, respectively. The estimated cross-sectional dimension is in accordance with the findings of the scanning transmission electron microscopy (STEM) analysis, exhibiting a discrepancy of approximately 1%. The proposed methodology offers a universal approach to n_eff and n_g extraction and a non-invasive method for cross-sectional dimension assessment, which can be applied in different PIC platforms.