​@Asuku 

I’m not an expert in fiber optics, but the Mode Field Diameter (MFD) is generally greater than the core diameter. See corresponding section of this article:
https://www.ofsoptics.com/how-to-speak-fiber-geek-article-4-single-mode-fiber-geometries/

I copied the relevant Figure and excerpt here for your convenience. Could it be the reason?

Though the drop you see is quite drastic as you say, I’m not sure its the only explanation.

Take care,

David

Hello ​@David.Nguyen Yes, this is the reason. The MFD > CoreDia. However, how would one model the maximum allowable aperture on Image plane. Especially when deal with multiple beams and you want to isolate each beam to a certain location on the Image plane?

Asuku