Learning Forum

[Mohd. Azarulsani Md. Azidin]

Hi, i try to find propagation loss using Mode Expansion monitor in FDTD module. but i gotten 0 loss so far.

i am running a simple straight waveguide with the parameters as below:

design size: 100um x 100um

SU8 - core (width 4um, height 4um)

Air - cladding

SiO2 - buffer - 1um

Si substrate - 5um

 

is there a proper way to place the monitor? i tried putting at the input or in the middle, it gave the same result = ZERO loss.

or i should i should MODE Eigenvalue solver to get the loss reading?

but i wanted to check loss at a certain waveguide area eg. at input vs output

 

[Amrita Pati]

Hello,

If the material you're using as the core (SU-8) is lossless then ideally the propagation losses in a straight waveguide should be 0. So, I think your results are correct.

I would also like to add that if you want to determine the propagation loss of a mode then you don't have to simulate the structure in time domain or use a Mode Expansion monitor. You can edit the mode source, go to mode selection and choose user select to find and select the desirable mode. Once you have selected the mode from the list of calculated modes, you can obtain modal properties including the propagation losses.





You can also visualize the properties of the selected mode.



Hope this helps.

Regards,
Amrita

[Mohd. Azarulsani Md. Azidin]

Hi Amrita,

it seems that i have finally out the main culprit to my problem earlier. I was using METAL BC instead of PML.

So, when i ran PML, i finally gotten the proper loss readings.

But i was wondering, some of the modes showed a NEGATIVE loss. 

Does this means, there is an error in the calculation/design somewhere or perhaps the material does show negative loss on certain modes?

thanks for your help.

[Amrita Pati]

Hi,

Great to know that you're getting the expected results. If your material fitting has no gain then the negative values of losses are most likely due to artifacts of the boundary conditions. You can try placing the PML further away from the structure. These modes are unphysical modes and they mainly do not affect the results of the guided modes. So, if you're getting the expected results you can ignore these modes.

Regards,
Amrita