Large ring resonator visible wavelength



Im using VarFDTD in MODE solutions to simulate relative large (R=15-30 microns at 500nm ) ring resonators in polymer n=1.57. Im doing some variation in the radius varying it from 15 microns to 25 and studying the transmittance (only one port) and im also doing it for different gaps.

Imagine that I star running my simulations with a fixed gap 400 for 3 different radius R=15 R=20 and R=25. For the first one the peak doesn’t drop to zero and we are overcoupled. If we increase the radius R=20 I obtain even less transmitance (more coupling length-stronger coupling) so we are even more over coupled. This trend is normal when i increase the radius (R=16,17,18,19 ) each radius, but when i arrive around R=24-25 breaks this trend and thetransmission peak drops aproximating to zero. For my point of view I would still expect a more overcoupled resonator (since we are increasing the couplign region increasing the radius). I sincerely do not know what it is happening. I have already try to improve simulation time but it did not get what i expect (i do not get anoying ripples or something).

I do not know if it is better to do it in IC i do not have a lot of time to train myself in new softwares.

LEt me know if someone need printscreen or something.



Hi, @miguel.diez-garcia

Could you please share your simulation file?
P.S.: One of the reason that pop up in my mind is that the coupling length so large that the field first enters the ring segment in then couples back, as in a directional coupler.


Hi, thanks for your answer.

I have check the mode in videos and with monitors and it did not seem the reason. form my understanding the power has to go Inside the resonator guide and then comes back but it is not the case i think

Here is my file. I’m working with the TE mode.


Lossyringresonator.lms (912 KB)


Hi @miguel.diez-garcia
I saw your simulation file and i noticed that the source is very close to the coupling region. Also, some essential monitors are missing. Please, read carefully the Lumerical’s Knowledge base about ring resonators


What you are saying can be explained as follows:

Gap= 400 nm and R= 15 um is undercoupled i.e. losses > coupling

As the Radius increases the losses decrease due to less bending. At the same time the coupling increases.

To double check this hypothesis, I encourage you to re-run the R=15 um simulation with Gaps = 200 nm, 250 nm, 300 nm. I have done this before and I know that for those dimensions you are using and TE mode: 400 nm is undercoupled.

Update me with what you find. I am happy to follow up with you on this topic.


Thank you, do you think that the source proximity to the coupling region can create a problem? I placed there to reduce the simulation time.

What kind of important monitor do you mean? Time monitor? I use to adapt the simulation time to avoid peak ripples.


----- Mail d’origine -----



I have repeated what you told me and i find the same behaviour, when i increase the gap the peak drops progresively which from my understanding means that i’am overcoupled. If we where undercoupled increasing the gap will mean that the transmited power at rresonance will increase.
Maybe im doing some errors during the mode choice?

What i do to select the TE mode first i put the gap I aim, then i go to the effective index in varFDTD and i select 100 Ey polarization. Then I go to the source and selec the second mode 100 TE Ey polarization.

By this way I obtain the critical coupling around 420nm for R=15
Here the monitor time

Thank you very much