Hi, Dr. Sun, thank you so much for your answer.

Here are more details about the purpose of simulation:

A linear sin taper wavegide inside the SiO2 cladding and a Si-based substrate added as well.  The background index of FDTD is setted to 1 refers to air.  The dimension of simulation region is 12um wide and 14um tall, upper boundary of FDTD region contains “air layer”, and lower boundary is inside Si-substrate.   

I want to use FDTD solver to check the after 50-micron-long propagation, if there any radiation loss or absorbtion happens, mainly for TM-mode. Another simulation using the same stucture with EME solver has been conducted. However, from EME results, there is no clear answer to indicate that substrate-absorbtion or radiation loss happen. Because after a taper-length sweep, the transmission could always be convergent. From EME solver, for this taper, the transmission is stablized after 100 microns. FDTD simulation length is setted to half of the stucture, it is just to simplify and reduce the time of simulation. 


Follwing the given links, I have double-checked settings in FDTD simulation:


  1. simulation time, it is changed to 4000fs, that should be enough for my simulation. Meanwhile, the autoshutoff is applied to 1e-5 and the mode source is setted to min/max at 1.54/1.55;
  2. source mode, because the simulation mainly focus on wavelength around 1.55um, so the wavelength min/max is 1.54/1.56 um, and from the mode source editor, it only needs few fs to propagate;
  3. power monitor, because of the small range of min/max wavelength, the transmission curve of a mode expansion monitor(same location as power monitor at 50-micron-position) looks smooth


But I didn’t add time-monitor inside simulation. However from the settings and curve above, It looks like the result is correct.  If the EME results are reliable, which after 100 microns lang the transmission in EME is convergent to nearly 1,  in FDTD  simuation, the transmission is only about 95%. I am not clear about the difference of those two results. 

above is the EME results and follwed is FDTD results with mode expansion:

and this is the T directly exported from power monitor without mode expansion:

I have captured the T from power monitor and T-total/T-forward in same image:


Thanks in advance, Dr Sun.