Learning Forum

[sagar]

I am trying to get the transmission data only for the fundamental mode. So I selected Mode expanding monitor and chose the fundamental mode by mode selection option and selected other frequency monitors (placed top of the waveguide) to get data corresponding to the fundamental mode only. As suggested here (https://support.lumerical.com/hc/en-us/articles/360034902433-Using-Mode-Expansion-Monitors) Ttotal is the total power radiated by dipole and Tforward is the power transmission into the selected modal field. For a particular diameter value of the dielectric waveguide, only the fundamental mode will propagate. So I was expecting Ttotal and Tforward to be the same. But I am getting different values. How is it possible where only the fundamental mode is propagating and there are no other modes?

Could you please explain the reason?

[Guilin Sun]

I guess this is a theoretical issue: dipole can excite almost all the modes in a waveguide, and in the near fields, before they form stable patterns (the waveguide modes), there are other intermediate "modes" so I believe you need to extract them from the Ttotal. However probably you cannot if you do not use mode expansion. I would suggest that you think it over before you do the simulation.

If you have lab experience in fiber mode launch, it might be easier for you to understand this issue.