Learning Forum

[Sept]

I am currently working on a vertical taper in EME. After going through all the convergence testing (mesh convergence, cell convergence) I have been stuck on the mode convergence step.

As it seems even with 200 modes being simulated in my tapered region the solution will not converge ( to me it seems that might be overkill). Could I get some guidance on the issue. 

 

[Amrita Pati]

Hello,

Would you be able to tell me how many modes are you using in the two sections having the straight waveguides? Sometimes, when we have a large number of modes in the tapered section, we also need to use a large number of modes for expansion in the straight waveguides so that the mode-matching is more accurate. 

If you are using a lower number, can you try increasing this value, and then run this convergence test?

Regards,
Amrita

[Sept]

Hi Amrita,

 

thank you for the fast reply. As of now I am simulating the same number of modes in the straight waveguide sections as I am in the taper region. I have also noticed that for this geometry in theory the longer the taper length should be the closer it should get to converging S21 and S12 equaling to 1. According to my results after a certain length my S parameters max out at about .94 and then start dropping in value the longer it is.

 

Sincerely,

 

-Lucio 

[Sept]

Hi Amrita,

 

I was wondering if you had any advice regarding the issue.

[Amrita Pati]

Hi Lucio,

Thanks for confirming! So when you increase the number of modes for the convergence testing, are you simultaneously increasing it for all the cell groups? 

Regarding the transmission, I believe you are using lossless waveguides, is that correct?

Regards,
Amrita

[Sept]

Hi Amrita,

that is correct, in the simulation the waveguides are lossless. If it helps at all I will attach my code along with a screenshot of my sweep (also for debuggin purposes I have seen that turning off csvs and energy conservation helped some of my paramaters conerge.

 

Code: 

 

selectall; deleteall; clear;

 

selectall; deleteall; clear;

 

if(materialexists("TFLN"))

 

{

 

deletematerial("TFLN");

 

}

 

 

####################Defining TFLN###########################

 

#Indices of refraction for substrate & waveguide layers

delta_n = 0;

n_TFLN = [2.21; 2.14; 2.21];

temp = addmaterial("Dielectric");

setmaterial(temp,"name","TFLN");

setmaterial("TFLN", "Anisotropy", 1); # enable diagonal anisotropy

setmaterial("TFLN","Refractive Index", n_TFLN);

 

 

########## SETTING UP VARIABLES THAT CAN BE PARAMETRIZED ##################

wg_taper_length=20e-6;

wg_height=0.3e-6;

slab_height=0.3e-6;

s_angle=pi/3;

add_length=wg_height/tan(s_angle);

wg_width=2.5e-6;

l=10.25e-6;

etch=550e-9;

 

##Setting Wg1

addrect;

set("x min",-3e-6);

set("x max",0);

set("y",0);

set("y span",wg_width);

set("z min",0);

set("z max",wg_height+slab_height);

set("name","Waveguide1");

set("material","TFLN");

 

##Setting Up Waveguide Middle

vtx=[0,0;0,wg_height+slab_height;wg_taper_length,wg_height+slab_height-etch;wg_taper_length,0];

addpoly;

set("vertices",vtx);

set("z",0);

set("z span",wg_width);

set("x",0);

set("y",0);

 set("first axis","x");

 set("rotation 1",90);

 set("name","Taper");

 set("material","TFLN");

 

###Setting Wg2

#addrect;

#set("x min",wg_taper_length);

#set("x max",wg_taper_length+3e-6);

#set("y",0);

#set("y span",wg_width);

#set("z min",0);

#set("z max",wg_height+slab_height-etch);

#set("name","Waveguide2");

#set("material","TFLN");

 

 

 

addmesh;

set("based on a structure",1);

set("structure","Taper");

set("buffer",0.1e-6);

set("dy",100e-9);

set("dz",1e-9);

 

#addmesh;

#set("based on a structure",1);

#set("structure","Waveguide2");

#set("buffer",0.1e-6);

#set("dy",50e-9);

#set("dz",1e-9);

 

addmesh;

set("based on a structure",1);

set("structure","Waveguide1");

set("buffer",0.1e-6);

set("dy",100e-9);

set("dz",1e-9);

 

 

 

yspan=6e-6;

zspan=6e-6;

 

addeme;

set("index",1.4440);

set("x min",-1e-6);

set("y min",-yspan);

set("y max",yspan);

set("z max",zspan);

set("z min",-zspan);

set("y min bc","metal");

set("y max bc","metal");

set("z min bc","metal");

set("z max bc","metal");

set("number of cell groups",2);

set("group spans",[1e-6;wg_taper_length]);

set("cells",[1;50]);

set("subcell method",[0; 0]);

set("energy conservation",1);

set("number of modes for all cell groups",60);

#set("allow custom eigensolver settings",1);

#set("modes",[20;60]);

set("wavelength",1.55e-6);

set("display cells",1);

set("mesh cells y",60);

set("mesh cells z",60);

#set("min mesh step",0.5e-9);

 

 

### Setting up Port 1 ###

select("EME::Ports::port_1");

set("use full simulation span",1);

set("mode selection","fundamental TE mode");

 

 

#### Setting up Port 2 ###

select("EME::Ports::port_2");

set("use full simulation span",1);

set("mode selection","fundamental TE mode");

 

 

[Sept]

Hi Amrita,

 

this is the code with the waveguide added at the end:

 

selectall; deleteall; clear;

 

selectall; deleteall; clear;

 

if(materialexists("TFLN"))

 

{

 

deletematerial("TFLN");

 

}

 

 

####################Defining TFLN###########################

 

#Indices of refraction for substrate & waveguide layers

delta_n = 0;

n_TFLN = [2.21; 2.14; 2.21];

temp = addmaterial("Dielectric");

setmaterial(temp,"name","TFLN");

setmaterial("TFLN", "Anisotropy", 1); # enable diagonal anisotropy

setmaterial("TFLN","Refractive Index", n_TFLN);

 

 

########## SETTING UP VARIABLES THAT CAN BE PARAMETRIZED ##################

wg_taper_length=20e-6;

wg_height=0.3e-6;

slab_height=0.3e-6;

s_angle=pi/3;

add_length=wg_height/tan(s_angle);

wg_width=2.5e-6;

l=10.25e-6;

etch=550e-9;

 

##Setting Wg1

addrect;

set("x min",-3e-6);

set("x max",0);

set("y",0);

set("y span",wg_width);

set("z min",0);

set("z max",wg_height+slab_height);

set("name","Waveguide1");

set("material","TFLN");

 

##Setting Up Waveguide Middle

vtx=[0,0;0,wg_height+slab_height;wg_taper_length,wg_height+slab_height-etch;wg_taper_length,0];

addpoly;

set("vertices",vtx);

set("z",0);

set("z span",wg_width);

set("x",0);

set("y",0);

 set("first axis","x");

 set("rotation 1",90);

 set("name","Taper");

 set("material","TFLN");

 

##Setting Wg2

addrect;

set("x min",wg_taper_length);

set("x max",wg_taper_length+3e-6);

set("y",0);

set("y span",wg_width);

set("z min",0);

set("z max",wg_height+slab_height-etch);

set("name","Waveguide2");

set("material","TFLN");

 

 

 

addmesh;

set("based on a structure",1);

set("structure","Taper");

set("buffer",0.1e-6);

set("dy",100e-9);

set("dz",1e-9);

 

addmesh;

set("based on a structure",1);

set("structure","Waveguide2");

set("buffer",0.1e-6);

set("dy",100e-9);

set("dz",1e-9);

 

addmesh;

set("based on a structure",1);

set("structure","Waveguide1");

set("buffer",0.1e-6);

set("dy",100e-9);

set("dz",1e-9);

 

 

 

yspan=6e-6;

zspan=6e-6;

 

addeme;

set("index",1.4440);

set("x min",-1e-6);

set("y min",-yspan);

set("y max",yspan);

set("z max",zspan);

set("z min",-zspan);

set("y min bc","metal");

set("y max bc","metal");

set("z min bc","metal");

set("z max bc","metal");

set("number of cell groups",3);

set("group spans",[1e-6;wg_taper_length;1e-6]);

set("cells",[1;50;1]);

set("subcell method",[0; 1; 0]);

set("energy conservation",1);

set("number of modes for all cell groups",60);

#set("allow custom eigensolver settings",1);

#set("modes",[20;60]);

set("wavelength",1.55e-6);

set("display cells",1);

#set("mesh cells y",60);

#set("mesh cells z",60);

#set("min mesh step",0.5e-9);

 

 

### Setting up Port 1 ###

select("EME::Ports::port_1");

set("use full simulation span",1);

set("mode selection","fundamental TE mode");

 

 

#### Setting up Port 2 ###

select("EME::Ports::port_2");

set("use full simulation span",1);

set("mode selection","fundamental TE mode");