A parameter sweep can be used to sweep the width of a ridge waveguide while tracking the effective index of the modes. The modes are only supported if the effective index of the mode is larger than the effective index of the slab mode of the slab waveguide. By looking at the plot of the effective index over waveguide width, you can determine the range of waveguide widths where only 1 mode is supported by the waveguide.
Here we present an example (files attached to the article).
The parameter sweep collects the effective index of the first 4 modes in the mode list which are collected by the “data” group inside the FDE solver object using the “getdata” script command:
To determine the effective index of the slab mode, you can use a 1D FDE solver region, and some examples which solve for slab waveguide modes can be found in the waveguides section of the Knowledge Base: https://support.lumerical.com/hc/en-us/articles/360043164634