Ring Resonator Produces Wrong Transmission

ring
transmission
varfdtd

#1

Hello, I am attempting to use a pre-determined effective index to model a silicon nitride ring resonator for 800 nm light in MODE solutions. I believe I followed the basic ring resonator tutorial available quite closely, but have not yet been able to produce a reasonable transmission spectrum. I set up the source with 750-850 light and have everything aligned, I believe. The one produced by the varFDTD simulation looks like the following:

Can anyone advise me on why this might occur and how to fix it? I have attached the file below for closer inspection if needed.

Thank you so much!!

https://drive.google.com/open?id=0B4PxXEkVPz88cUtHYXR1dnoydlE


#2

Hi,

I checked your simulation file using the FDE solver to find the group index for the cross section of the straight portion of the waveguide and found a group index of 2.01474. Then I used the formula on the following link to estimate the free spectral range of the waveguide:
https://kb.lumerical.com/en/index.html?pic_passive_getting_started_ring_resonator_mode1_discussion.html

FSR = lambda^2/(ngL) = 1.01 nm
lambda = 0.8 um
ng = 2.01474
L = 2
pi*50 um

This expected free spectral range of about 1 nm is close to the spacing of the peaks in the transmission spectrum of the device that you obtained, so the results seem reasonable.

Since the frequency-domain results is sensitive to the simulation time, you could improve the results by increasing the simulation time to ensure that the fields fully decay by the end of the simulation time, like described here:
https://kb.lumerical.com/en/index.html?ref_sim_obj_frequency_monitors_simulation_time.html

You can also increase the number of frequency points recorded by the transmission monitor to increase the resolution of the spectrum plot.