Hi @tecklee, in the traveling wave modulator example, the FDE eigenmode solver in MODE Solutions is used to calculate the loss and the phase shift in the waveguide. The FDE solver is particularly suitable in this structure since there is no variation along the propagation direction. However, the FDTD solver is capable of doing the same, at least in some cases. In FDTD, you can use the integrated mode solver within either the Mode source or the Mode expansion monitor to get the effective index (neff) and loss for the mode of interest.
Once you have the value for neff, you can use the script in FDTD to analytically calculate the phase shift and loss for the total length of the waveguide. There is no need to do a full 3D simulation. Please note that for a long structure like a traveling wave modulator, a 3D simulation is not realistic as it will be extremely slow and will require huge computational resources.
Please also note that when simulating a different modulator, the best choice for the optical solver will depend on that specific structure. In the case of this traveling wave modulator, the FDE is the best choice since the structure is uniform along the propagation axis.
For example, you can check the 'interleaved junction modulator’ example on our knowledge base which uses FDTD to calculate the loss and phase shift in the waveguide. This example uses FDTD since the modulator has variation along the propagation direction which makes FDE unsuitable for this simulation.