Discussion Forum

Hi,

I am currently doing a 'Mode analysis' study of a anisotropic dielectric rectangular waveguide (the cladding is air, the core is GaSe and boudary condition: PEC). I have two dispersion curves for the permitivity(one for the ordinary and an other for the extraordinary permitivity). In a certain range of wavelength, the ordinary permitivity(e_ord) is negative and the extraordinary permitivity(e_ex) is positif. In that range, i have normally an infinite number of guided modes. Lets say that i want to search the TM1,2 mode(for example). I know two method for doing that. 1) I ask COMSOL to search a lot of modes in a region where i think the effective index of the TM1,2 mode will be. Then i visually select the TM1,2 mode. 2) I implement the dispersion curve of a approximative model( slab waveguide in my case because the height of the waveguide is very small comparatively to the width). So i implement the TM2 theoritical curve as a initial guess for the effective index and ask COMSOL to find the 5 modes closest to this curve by doing a parametric sweep of the wavelength.

The problem is here. In the region where e_ord is negative and e_ex is positive, there are so many modes possible that the 5 closest modes find are the TM2,30 TM4,20 TM4,23 TM3,12 TM2,12 for example. So i do not alway found the TM1,2 mode desired and if i increase the number of modes to search, the method 2 look like the method 1 (that i want to avoid). I want to get, for example, the TM1,2 dispersion curve whitout doing it manualy.

On other words, my question is: How can i ask COMSOL(directly or indirectly) to found a specific mode (TM1,2 for exemple)? The purpose is to draw the disperson curve of this specific mode.

Thanks, Hassan Khalil