# Evanescent waveguide couplers_S Parameter

#1

Hi,
I am following the example of Evanescent waveguide couplers in this link:
https://kb.lumerical.com/en/index.html?pic_passive_waveguide_couplers_evanescent.html
I try to extract s parameter in EME port, but I do not understand why S21 is negative as shown.

Thank you very much!
Qi

#2

The S parameters are complex numbers. S21 is defined as the ratio of electric field at the output to the electric field at the input and based on the relative phase they can have negative values:

$S_{21}=E_2/E_1$

https://kb.lumerical.com/en/metamaterials_s_parameters.html

If you are interested in transmission, you need to choose abs^2 for scalar operation in visualizer: