Learning Forum

[Victoria Zhang]

I tried to simulate the effective index and propagation loss of silicon nitride waveguide. I adopted PML boundary conditions and chose the "correct backward propagation modes" in the Mode Advanced Options. The option has avoided the negative propagation loss of TM mode, but they show a negative refractive index of TM modes.    

 

[Amrita Pati]

Hi Victoria,

My first thought is that the simulation span (especially in the y-max direction) has to be properly set to avoid this. But my first question is, when you uncheck the "correct backward propagating modes", are you seeing gain?

Regards,
Amrita

[Victoria Zhang]

Yes. When I uncheck that correct backward propagating mode, I will get a gain.

[Amrita Pati]

Hi Victoria,

I will recommend unchecking that option for now. I believe the the gain is most likely because of the PML. When you are simulating a bent waveguide, the radiation from the waveguide (a lot of which is opposite to the direction of the bend) can interact with the PML boundaries to create artificial gain/loss. So, it is recommended to limit these as much as possible by increasing the simulation span. I will ask you to run the simulation for different simulation spans, this is also called convergence testing. Start at the current value and then increase the FDE simulation span, at some point you would see the losses converge to a single value even even when you increase the simulation span. This would be the actual loss you are looking for.

Regards,
Amrita