Modal index

[saima buzdarParticipant]

In OptiFDTD after modes calculation we get data like this:

[ Modes ] [ Modal Index ] [ Polarization ]
Mode 1 ( 1.35890436, 0.00000283 ) FVect TE
Mode 2 ( 1.35909016, -0.00000458 ) FVect TM
what does modal index show?

[Abdallah IsmailParticipant]

Hi Saima,
My understanding to the modal index is that each mode has different beta,
in general , Beta= n * 2*pi/lamda
so for different modes, there is different values for Beta and hence different values for n.
So these different values of n corresponding to different modes is called modal index or effective refractive index.

[Damian MarekParticipant]

Abdallah gave a good description. I’m just going to add that you can extract some important parameters from the propagation constant (modal index). For example, the imaginary component of a complex propagation constant will give you information about the loss of the mode. The loss can change from mode to mode. The different modal indices also results in intermodal dispersion, which is one of the main reasons most systems use single mode fibers. It is also important information for when you are studying coupling of modes, in a butt coupled situation it will tell you how much power is reflected.

[saima buzdarParticipant]

Thanks Abdallah and Damian Marek
Actually i want to calculate confinement loss and dispersion, for this i need img{neff} and Re{neff} parameters.
From Abdullah’s answer i assume Modal index is neff
But from Damian merek i assume modal index is showing propagation constant which is as:
propagation constant=alpha+iBeta
please you people can remove this confussion?

Thanks.

[Abdallah IsmailParticipant]

Damian is describing propagation constant which is related to the refractive index by the relation mentioned above.
I think that the modal index is the effective refractive index and the imaginary part describes gain or loss.
Best Regards,