Thanks vignes,
The “Scan” section is split into two panes. If I select the region of the cladding in the upper pane, the parameters I can change in the lower pane are only the wavelength range and the steps, as shown in Fig. 1. This means that the cladding width is a constant. In addition, if the profile width is selected the profile width can be scanned within the specific limits as shown in Fig. 2. However, I note from the OptiFiber documents that this profile width, I think, is the total width of the whole fiber profile, as it said that the profile as a whole within user-specified limits, for example between 0.8 and 1.2 of the original width. This means that all spatial dimensions of the original design are proportionally shrunk/expanded at regular steps from 0.8L to 1.2L, where L is the initial width.
So is there still no way to calculate the mode index depending on the cladding width? The .fcd file is attached here.