Guilin Sun
Ansys Employee

EME: it depends on which model you used. Luke's and Phillio's models have virtually zero absorption.

VarFDTD: the mode source must be inside the waveguide that supports a mode. in addition, the simulation time must be long enough to let the signal pass through the waveguide in order to get correct frequency-domain result.

 

Suggestion: since mode has effective refractive index n=n_real+j*n_imag, you do not need to simulate to get the propgation loss. FDTD gives the loss in dB/cm from the n_imag: https://optics.ansys.com/hc/en-us/articles/360034917493-Working-with-lossy-modes-and-dB-m-to-kappa-conversion

You will get the propagation loss by multiplying the length to dB/cm with correct length unit, from FDE mode result.