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[zahra mansouri]

 

i am working on waveguide simulation in fdtd and mode solution lumerical. i have a cylinderical waveguide. with mode solution i get the neff and so i have propagation lenght of that. then i design the waveguide in fdtd solution and i use mode source for exciting a special mode that i calculateed propagation lenght for it. but here i cant see the propagation lenght that i calculated in previous step.

my designed propagation lenght is 2 micron. in fdtd i draw a 4micron lenght waveguide and i excite it with only one mode but i cant see attenuation of wave.

can anyone help me?

 

i am working on waveguide simulation in fdtd and mode solution lumerical. i have a cylinderical waveguide. with mode solution i get the neff and so i have propagation lenght of that. then i design the waveguide in fdtd solution and i use mode source for exciting a special mode that i calculateed propagation lenght for it. but here i cant see the propagation lenght that i calculated in previous step.

my designed propagation lenght is 2 micron. in fdtd i draw a 4micron lenght waveguide and i excite it with only one mode but i cant see attenuation of wave.

can anyone help me?

 

[Guilin Sun]

what is the effecgtive index of the mode you used?

Please check if the mode is truncated by PML. If so you will need to increase the simulation span.

In addition, did you let the simulation run long enough and terminate it when the autoshutoff min is reached? you cannot simply use t=neff*L/c, since the source is a pulse.

You will also need to extend the waveguide outside of PML.

Without screnshots of your simulation settings, I just give the above recommendation. Please try.

 

 

[zahra mansouri]

 

dear Guilin Sun

thank you for your reply. i will applied them on my project. i attached my simulation file here.

the effective index of excited mode is 2.82956+0.0494307i.

i would be thankful if you check this file.

 

 

[Guilin Sun]

We are not allowed to download or check customer simulation files in this forum. Since the mode is lossy, it is normal that the field is attenutated. You can calculate it in theory. However, discretization will introduce numerical error.

[zahra mansouri]

i shared a pdf file that contained my simulation shape and settings. in this simulation i used standard pml setting but i also simulated this file for higher pml profile before and the answers hasnt been changed.
the problem is that i cant see calculated propagation lenght along the waveguide. i calculate the propagation lenght 2.4 micron but i cant see wave attenuation in that lenght. i use this below relation for calculating propagation lenght(pl).

pl=1/(2*im(beta))

beta= 2*pi*neff/lambda;

https://drive.google.com/file/d/1o-BtV7o2jj8G5ONvwTqR2BZ90sCi9Y7S/view?usp=share_link

[zahra mansouri]

my project is a cylinderical mim waveguide and i used a magnetooptic material for dielectric layer. when i replace the magnetooptic layer by a isotropic dielectric the lumerical answers seems to be true. i can see the propagation lenght of waveguide when i simulate my project in fdtd 3d simulation.  

can i use the fdtd 3d simulation for observing wave propagation when i used anisotropic material?i defined my material using the code explained on the Lumerical website. the wavelenght is 1550 nano and propagation lenght is about 4 micron. cylinderical radius is about 500 nano

[Guilin Sun]

You can add paramagnetic material in Material BASE and choose anisotropic:

 

If it is for simple propgation in a straight waveguide, it is not necessary to simulate, as the mode will give you the propagation constants.

[zahra mansouri]

i have found the propagation constant and propagation modes with lumerical mode solution but i need to show the wave propagation of a one of the modes along the waveguide.

[Guilin Sun]

Then you can do FDTD simulatioon. Make sure the mode profile is not truncated otherwise it will cause diffraction.

[zahra mansouri]

Dear Guilin Sun

thanks for your advises to my problem. i changed my mesh setting and also extended the simulation region. so i got acceptable response for my waveguide.

now i have another problem. when i simulate my waveguide in mode solution my answers is very sensitive to waveguide radius. my waveguide is a multilayer cylinderical structure and each layer has about 200-300 nanometer radius. when i sweep for 1 nanometer in radius, i get very different and irregular neff that it is not seems to be reasonable. my dielectric layer is a magneto optic material and i havnt teoritical formula for solving neff analytically.

[Guilin Sun]

1nm change is too small, which is smaller than the manufacturing errors. You should use larger incerement, at the integer number of mesh size.

This is for FDE solver I believe. Next time please write a new post whenever you have questions different from the exisiting post.

[zahra mansouri]

Thank you dear Gulin.

As you say if it is because of fde solver? If yes what can I do?

I also tested 10 nanometer step and higher. I wanted to know that how much is  my designed structures error threshold .but I couldn't find any area that my answers become true on it . The mode results changes in every point

Thanks for answering this post. I will write new post for my new questions.

[Guilin Sun]

"You should use larger incerement, at the integer number of mesh size." this is my suggestion. Please get the mesh size. for example, you can use specified override mesh, which you know the mesh size since you set it. or you can use the scrript below:

e1=getresult("mode1","E");

x=e1.x;

nx=length(x);

delta_x=x(1:nx-1)-x(2:nx);

 

e1 is dataset: Introduction to Lumerical datasets 

similarly for y and z if the FDE is in the other cross section. The sweep should use n*delta_x where "n" is an integer number.

 

Please try.

 

[zahra mansouri]

thank you so much dear Guilin Sun

i set my sweep step on  n*delta_x  and i got better answer.

thanks for your guidance