Thank you very much. Here is a paper (open access) instead: https://opg.optica.org/DirectPDFAccess/4E0B835C-0276-4BF8-91609504AAD67E68_336815/oe-24-5-5026.pdf?da=1&id=336815&seq=0&mobile=no.
This is the exact same structure as in Lumerical's own edge coupler example: https://support.lumerical.com/hc/en-us/articles/360042305354
In this Lumerical example it says "In this example the sub-wavelength grating structure was not included. Instead it was assumed that a graded profile can be used where the effective index of the Si3N4 layer could be continuously varied". (I followed the paper and tried to simulate the full structure with the grating.)
But actually I would like to simulate a different coupler, where instead of silicon nitride layers forming a subwavelength grating, the inverse taper waveguide itself is the grating (Si and SiO2 layers with increasing duty cycle of silicon, so that the light is coupled into a waveguide grating which adiabatically tapers into a solid waveguide after some propagation distance). So there are no silicon nitride layers above the tapered silicon grating waveguide. In the inverse taper, the duty cycle of the silicon/SiO2 layers varies (similar to page 8 of the attached paper) , so the same unit is not repeating. So I cannot use the easy periodic method with EME, right?
I will have to model the entire structure and do either EME or FDTD. To obtain the overlap between the taper output and an SMF-28 fibre mode, should I put a field profile monitor exactly at the edge of the taper for both EME and FDTD? Or can I use FDE only (without EME or FDTD) at the edge of the taper like in the Lumerical example above, even though FDE is only supposed to be used for constant cross sections (which is not the case because the waveguide itself is a tapered grating with alternating Si/SiO2 layers)? Thanks again!