Hi Siraj,

You will find the PML definition section in the Definitions menu, right below the Model menu, on the tree to the left. Right click and select the PML.

You will have to create a region around your device, with the same material of the boundary (silica I believe), and select it in the PML settings. There are some parameters that will need setting. The important ones are the scaling factor and the order of the PML. For the EM models I use scaling 1 and parabolic PML functions. A one-wavelength thick PML region should be sufficient.

I recommend reading the documentation for better understanding of these parameters and their effect. Also, test some variations of the parameters, so you may optimize your PML for your case.

Best,

Hi Gustavo

Thanks for the response

I have given a 2 um thick circular PML boundary around the 2d-surface of the micro structure (silica as the material). The scaling factor is taken as 1. But still i am not getting confinement, which i got by applying the scattering boundary condition.

As you told regarding optimization, I would like to know which all parameters have to be optimized for the PML. If you have some references do please share the link.

Thank you

Without access to your design and PML configuration, I can't tell much. I have tried a quick test with a simple cylindrical silicon-silica waveguide, and had no problem adding a PML.

I also simulated a slica-core PCF (found on the internet) and encountered no problem. I used a 2um uniform PML around the honey-comb PCF, with scaling factor equal 1 and curvature equal 2. The only things I changed were the type of the PML from cartesian to cylindrical and the curvature from 1 to 2. I also used the default meshing size. The honey-comb have 4 latices from core to boundary and it is designed to guide 870nm modes. The simulation also found spurious modes, with fields in the PML-PCF interface, but had no problem finding the PCF modes as well.

What exactly is your COMSOL version? Can you tell a little more about your design and configuration, such as meshing, geometry dimensions and PML parameters?

Best

I am working with Vers. 4.3. I tried with a simple geometry (see attached file).A 2 um thick PML was given with all the other parameters as same as u have given. But still no guidance modes only spurious modes. All the light is moving out at the boundary.

Note the geometry

hole diameter= 2um

pitch= 2.55um

Hi Siraj,

Sorry for the delay. There seems to be a problem in this geometry you sent. Just to clarify, what are the little circles made of? And the large one around them (not the PML)? What material did you chose for the PML region?

I will try this geometry here and see if I find any problems, but I need these information before.

Best

The liitle circles and the larger one is made up of air. The PML is chosen to be silica.

So you are saying that everything is air, but the PML? That can't be. For the previous question we discussed, I would guess that you should have the larger circle and the PML made of silica, and the small circles should be air. With that you would have a silica core PCF.

But as I remember you want a hollow (air) core PCF. So you should also have a circle in the middle of the fiber, correct?

What is the expected wavelength of the first guided mode? I will try it out here later, to see what may be wrong.

Best

Hi Gustavo,

Sorry for the delay, I meant to sat that that all the small circular holes within the the large circle is given as air. The material surrounding the air holes is given to be silica. And as I have read the PML material should be same as that of the material to which it shares the boundary. So, here it is sharing the boundary with the silica region which made me to define silica as the material for the PERFECTLY MATCHED LAYER.

I hope it is clear.

Thank you

Hi Siraj! Happy new year!

I checked your system. What I encountered was that there is a divergence in the PML/Fiber boundary, so the scale of the 2D plot is such that you barely see the guided mode. But if you tweak the plot range, reducing the maximum value, you will see the mode you are looking for. See the image below.

The parameters I used in the PML are:

-type: cylindrical

-Typ. Wavelength: Phys. Def.

-PML scaling factor: 1

-PML scaling curvature: 3 (quadratic function)

I am using COMSOL 5.1, if you find any different parameter name. To check if you are using the correct PML function, try to plotting the function "pml1.detInvT" over a line in the PML region (1D line plot).

I hope it works now. I haven't gone far enough to see why the system is diverging in the PML boundary. Nevertheless the imaginary part of the effective index is very low, so the majority of the EM energy is in the guided mode, not in the PML.

Best

Hi Gustavo and Siraj!

I am a beginner in COMSOL and trying my hands on designing PCF and I tried the above geometry and encountered a problem!

the electric field instead of getting concentrated in the core gets leaked in the PML boundary!

Can you please tell me the wavelength at which the model was computed?

Please refer the image!

by adding a layer and give it a thickness in the clad circle.