So I am interested in a particular directional coupler design (220 nm height x 500 nm width) silicon waveguide that sits around 2um of SiO2 cladding - which I am achieving by setting the background index to SiO2 (1.44) and simulating using MODE.
The problem is that I calculate the splitting ratio using the FDE eigen modes, 46 um coupling length and 300 nm separation to be 49%, roughly correct as the device is meant to be 50/50. When I use the varFDTD simulation, I do not observe 50/50 splitting in the waveguide. Usually depending on what I play with (changing region of simulation), the light all ends up transferring and heading out out of the other waveguide to the input. Is there any obvious fault in my simulation that stops me achieving observing this with the 2.5 FDTD?
Secondly, I would like to be able to get the dispersion of the coupler (that is to say, the splitting ratio as a function of wavelength) between a small range - 1545 - 1555 nm. Is there an efficient way I can implement this, for example using a frequency monitor? I was thinking I could use a transmission monitor on both output waveguides which I can verify sum to 1 in the lossless case, but I wouldn’t be able to account for loss using this method. Also, is it straight forward to change the mode source to say mode #2 rather than #1 and repeat this analysis?
Thirdly, how many significant figures can I trust the output of Lumerical to? Can I trust MODE to give me results accurate to around 10^-8 and how can I make MODE output to this precision, or am I better off using FDTD for this purpose?
Thanks very much. I attached the simulation file below.
DC sim.lms (1.7 MB)