Do you mean you want to simulate the result with varying the distance between the input fiber and the edge coupler, where the gap is in air or other material without light confinement? In this case the gap part will need many modes to build up the fields accurately. you may also consider using import sourece for EME, with FDTD simulation of a fiber and air, and use farfieldexact to calculate the fields at any distance. The calcualted EH fields then can be imported into EME. Both methods work but need some time to experiment for a better results.