Boundary selection for ports

Hi

İ am simulating the model files of waveguide: :www.comsol.com/model/download/....dielectric_slab_waveguide.pdf
and i have following question,

İn this model they have used the entire left boundary as excitation port and right boundary as exit port, i was wondering what difference would it make if i connect ports only at the edges of core? İ am attaching images of results, plot1 is when entry and exit is at core ends, plot 2 when excitation is at core end (left side) but exit is the entire right boundary wall and plot3 is when the entire boundaries are used for entry and exit.

What basically i am looking for is i have this 1 D dielectric slab waveguide and i have placed a dielectric microdisk on top of it. İ am getting very leaky modes in waveguide. So i was wondering aside from the wavelength and geometry of the structures what parameters i need to optimize to control the leakage(image attached). İn the example i have ports at the core ends,

thanks and would really appreciate the guidance