Learning Forum

[taynara.deoliveira]

Hi all,

In my model I have a linear taper on top of a polymer waveguide. I am optimizing the taper length for the coupling to the polymer adiabatiactly. 

For that, I would like to know how the neff changes against taper length or taper width. I know this would be possible in FDTD but it would consumes more time I guess ... in the FDE my model is 3D

I think if I simulate in FDTD only the taper, maybe the neff will be different from when I have the whole instructure? In the FDE is quite easy to change the length of the taper, I dont need to create new strucures as I would need for the FDTD (as far as I know). 

In the end, I want to monitor how the effective index changes along the taper until the end of the tip. So, I can evaluate the most critical part. 

Is it possible to do that in the FDE?

Thank you.

Best regards,

Taynara

[taynara.deoliveira]

Sorry, I meant EME, not FDE !!

[Guilin Sun]

So you want to get neff of the taper? 

you can use FDE, sweep the size of the waveguide cross section then you can get neff vs width or height. Make sure you use fixed mesh size and the change of the size is multiple of the mesh size.

EME and FDTD are propagators, not for mode analysis.

 

[taynara.deoliveira]

Hi Giuilin Sun,

Thanks for your reply. I want to get neff of the taper x taper length. So i want to sweep the taper length. I did the cross-section, but that does not give me information I want. 

The sweep also works in the EME? 

I learned something. For each cells in my taper I can see the neff and mode profile ... but of cource I cannot know which length is this because is the optimized length of the taper after the sweep... so phisically I have a taper of 100um but when i optimize and propagate the light with the optimize length the taper should be 660um ... I can create a knew taper with this length but if i need to do that every time will be a bit tiring and takes too long ...

Any other suggestions?

Thank you.

Tay

[Guilin Sun]

There is no such neff for a 3D structure, except you consider it as a cavity. 

When you optimize the taper length, the cross section does not change for each cell. so it does not matter for the neff at each cross section.

Please note both EME and FDE gives neff for the waveguide mode at the waveguide cross section, which is always 2D or 1D, not 3D.

neff is associated with a mode. the mode properies of the taper are different at different cells along the taper. Not sure how do you associate 3D neff to a "mode"? if you can, it is an approximation. Please reconsider its physics. You may try to find some publications to get some information on what you want to get.