Finding the fundamental mode in a curved wave guide

mode

#1

Hi,

I’m very new to Lumerical but I think I have gotten somewhat of a hang of the very basics. I’m not sure that this is the correct category for my problem, so please let me know if I should post my problem somewhere else.

I’m using Lumerical MODE, in which I have set up a fiber with a core/cladding radius of 10/200 micrometers and refractive indices of 1.4509/1.449 and have set it to have a bent radius of 12 cm and I’m studying the modes for a wavelength of 1.0482 micrometers. At first when I only had 20 trial modes, I could not find the mode that was confined to the core. I then tried to define a range of values of the refractive index that the solver could sweep over. I just tried 1.448-1.4509 and eventually I got the message that the maximum number of modes was reached. I then looked in the list of the solutions and found that the mode that was most confined to the core was mode number 81 and 82. My question is if there is a way to restrict the solver to only look for modes in the vicinity of the core or some other way of achieving this? As mode 80 and 83 are not confined to the core at all, but don’t differ much in effective refractive index (they have 1.449930 and 1.449907 respectively) from mode 81 and 82 (they have 1.449922 and 1.449919 respectively), I hope that there is a better way than to set a great span to sweep across.

Also, I wonder if it’s possible to export data of the mode fields (i.e. the amplitude of the mode of interest), effective refractive index and overlap? My last question is if it is possible to make a script or something similar to sweep over different values for the core/cladding refractive index as well as the wavelength and then export the results?

Thanks in advance
Robert


#2

Hi Robert,

These are some good questions. Could you share with us your simulation file so we can check why the confined modes are listed so far down the list? This will also allow us to see if the problem is related to simulation settings or the simulation methodology.


#3

Hi,

Sorry for the late reply, I have been swamped with deadlines lately. I have created a similar file to the one that I wrote about (it has the same parameters and is set up in the same way) as I didn’t save the old one. However, I have not been able to run it as the program crashes when I try to run it. I remember that I had this problem last time as well, but after restarting it a couple of times it stopped, is this normal or is it something wrong with how the file is set up?

I’m attaching the new file that I did save.

Thanks in advance
Robert[quote=“rolindbe, post:1, topic:1889, full:true”]
Hi,

I’m very new to Lumerical but I think I have gotten somewhat of a hang of the very basics. I’m not sure that this is the correct category for my problem, so please let me know if I should post my problem somewhere else.

I’m using Lumerical MODE, in which I have set up a fiber with a core/cladding radius of 10/200 micrometers and refractive indices of 1.4509/1.449 and have set it to have a bent radius of 12 cm and I’m studying the modes for a wavelength of 1.0482 micrometers. At first when I only had 20 trial modes, I could not find the mode that was confined to the core. I then tried to define a range of values of the refractive index that the solver could sweep over. I just tried 1.448-1.4509 and eventually I got the message that the maximum number of modes was reached. I then looked in the list of the solutions and found that the mode that was most confined to the core was mode number 81 and 82. My question is if there is a way to restrict the solver to only look for modes in the vicinity of the core or some other way of achieving this? As mode 80 and 83 are not confined to the core at all, but don’t differ much in effective refractive index (they have 1.449930 and 1.449907 respectively) from mode 81 and 82 (they have 1.449922 and 1.449919 respectively), I hope that there is a better way than to set a great span to sweep across.

Also, I wonder if it’s possible to export data of the mode fields (i.e. the amplitude of the mode of interest), effective refractive index and overlap? My last question is if it is possible to make a script or something similar to sweep over different values for the core/cladding refractive index as well as the wavelength and then export the results?

Thanks in advance
Robert
fibertry.lms (778.5 KB)


#4

Hi,

I had a look at your file and I had to use coarser mesh as my 30GB of RAM was not enough to mesh this structure with the original settings. It was also probably unnecessarily fine for this fairly simple simulation and I think that it might be possibly related to your computer crashing if you did not have sufficient RAM.

As for finding the modes confined to the core - If the mode is truly confined to the core, it should be enough to reduce the size of the simulation region to be around the core without the need to include the full cladding structure and instead use the cladding as background index. I think that the problem is that FDE cannot find any physical modes confined to the core in this case. I tested this by using this simulation setup:

and I no physical modes were found. Note that I had to use PML as the metal boundaries would create an artificial confinement since the structure does not seem to have any bound modes.

Do you have any analytical or experimental results that suggest existence of modes confined in the core of this fibre? If yes, is it possible to share it here?


#5

Hi,

When you say you can’t find any confined modes, are you referring to the straight or bent fiber? The straight fiber should have confined modes, the fundamental mode can be found by solving the eigenvalue equation for the propagation constant in a circularly symmetric fiber derived from maxwell’s equations, see for instance:
Nonlinear Fiber optics by Govind Agrawal

I can see the confined modes for the straight fiber using COMSOL, however, if I bend the fiber COMSOL does not give me any confined modes…which was why I looked for some other simulation tool, but I realize that it might have something to do with boundary conditions that I need to change or something similar…I don’t have much experience with FEM or FDM and was looking for a software that already “had the right settings” to save me some time that I would have to spend on reading up on that.

The refractive indices that I used to set up the fiber are somewhat similar to a fiber we have down in the lab (20/400 core/inner clad diameter), and we’ve coiled it onto a cylinder that has a diameter of about 12 cm…and it still guides light but the modefield is a bit altered due to the stress from the coil, and this was what I wanted to study…there are some papers on this topic but I have not found any “simple formulas” to get the altered modefields from the ones the straight modefields.


#6

Hi,

It seems that the main problem was that both of the cladding and core had the same mesh order=2. The index was not correctly resolved as a result and no physical mode was found. I have modified the core mesh order to 1 and now the bound modes are on the top of the list even with coarse mesh and the bend:

Note that since there will be likely some radiation loss in the bend, you should use PML to absorb the radiated power and get correct loss information. Additionally, you should conduct convergence testing of the PML distance from the core to make sure that there is no artificial absorption.

The modified file:
fibertry FIXED.lms (236.2 KB)