Electric field propagation through waveguide window

Hello everyone,

I am studying effect of fused silica window inserted into circular waveguide. I have modeled it using 2D axial symmtery and simulate for TE01 mode at 140GHz frequency. I have attached two images 1. efield propagation without window 2. efield propagation with window.

Can anyone tell me why number of wavelengths are decreased inside window?