Simulating racetrack resonator add-drop filter with MODE 2.5D - Getting incorrect FSR
I am attempting to simulate a racetrack resonator in an add-drop filter configuration (so with two coupled waveguides.) Full 3D FDTD is too demanding, so I have been using the 2.5D varFDTD solver in MODE.
The device parameters are the following: SiN (sampled index file) waveguide, 1um wide by 800nm tall, embedded in SiO2 (index = 1.46). The bending radius of the round sections of the couplers and resonator is 20um. The straight sections are all
12um. The coupling gap is 400nm (but this could be anywhere from 300 to 800nm). I have placed four transmission monitors at the input, through, add, and drop ports, and I am exciting the input waveguide with a mode source set to fundamental mode. I would like to see the transmission from approximately 1540nm to 1560nm. The varFDTD slab mode position is set to one of the racetrack straight segments.
The simulation runs fine, but the Free Spectral Range (FSR) is way off. In experiment, I have consistently observed an FSR of approximately 3nm for this device, but the simulation consistently gives me something approximately twice that.
In the past I have simulated similar (albeit smaller) devices with Silicon waveguides and have managed to replicate experimental results. The exact simulation parameters used there (such as meshing, sim time, varFDTD effective index settings etc) are not giving correct results for the SiN system.
I am aware that broadband simulations in 2.5D can be an issue. I have the varFDTD object set to a broadband simulation, but I can't rule out that something is breaking there because the mode source is single wavelength.
Please let me know if there are any other settings that might be an issue, I am running out of ideas to try and replicate these results.
I have attached some screen shots and am happy to upload an .lms file. Thank you very much for any help.
P.S. I have made sure that the sim time is sufficiently long for the fields to decay, and I have tried numerous different mesh settings up to auto non-uniform accuracy 5.
One strange thing I have noticed is that the mode source calculates the effective index of the mode as approx 1.78, but the varFDTD solver calculates it (when choosing 'user select') as 1.86. I am not sure what causes this discrepancy, and I believe the mode source is correct.