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What do you mean by taking figure on the paper?

Anyhow, there is no criteria in general to perfectly find out the right mode just by the numerical value of its effective index. Depending upon your circumstances, special rules might be made up though.

Sometimes when I have to compute the dispersion relation of a particular mode, I spot out the effective index of desired mode at two close/consecutive wavelengths and then use a linear prediction scheme to estimate effective index value for the following/preceding wavelength. Using the fact that dispersion curves are continuous, I compute the difference between the estimated effective index with effective indices of computed modes and pick out the one which minimizes the absolute error. However, it might give wrong answers sometime again depending upon the geometry you are using so manual verification might be desirable to ensure you have what you need.

Hi,

Thanks for the answer. Sorry that I didn't make it clear. What I am trying to do is re simulating a structure from one of the paper. I try to do the same thing what they did in the paper and get the same figure. For that I use eigenmode analysis in the hybrid-mode. I've set the ''search effective mode indices around' part, calculated number of neffs. By changing the some parameters in the structure, each time after calculations I have another set of effective indices. My question starts here how can I guess at each calculation which effective index is the right one. For my case (as far as I know) there is no other way such as dispersion curve to compare and estimate the next one.

If I may guess, changing only one parameter at a time by a small margin would produce only small difference in effective index. This is then similar to my dispersion thing, I change one parameter (lambda) by a small difference and based on the previous two results make a guess using linear extrapolation that the newly computed mode would also have its index somewhere close.

If you are, however, changing multiple parameters simulataneously then I am afraid this trick cannot help your cause. May be looking at the exact problem from a theoretical angle would reveal some useful insights on the pattern of change.

All the best!

Thanks very much for the answer Shakeeb. I'll dig for answers from a theoretical point of view as you suggested. Maybe I can find a way for estimation.

By the way, I would be grateful for any other comments and ideas.

Cheers,
Elif