Damian,
First of all thank you for your answer.
As for the equation’s reference, yes I do: – Bahaa E. A. Saleh, Malvin Carl Teich – Fundamentals of Photonics, Chapter 7, page 244 – ISBN: 0471213748 (Electronic) 0471839655 (Print).
What I would like to find is the relationship between the number of modes and the waveguide’s width and thought that through OptiFDTD should be able to achieve this.
Best regards.