How to set the position of the Source and Monitor in devices?

It is recommended to put Source and Monitor in the straight waveguide position, which can reduce the simulation error caused by the grid accuracy.

The Mode Source calculation in FDTD is based on the user-defined 2D cross-section. Consequently, the calculated mode is suited for stable transmission within the same cross-section. If the user positions the Source in a region of the waveguide with structural variations, the discrete Yee cell may introduce significant numerical errors when encountering different waveguide cross-sectional distributions before stable transmission is established. Therefore, it is recommended that the Source be placed in a straight waveguide region extending at least 3 grid cells along the propagation direction. Similarly, the Monitor may encounter the same issue when measuring mode energy; thus, it is advised that the Monitor be surrounded by at least 3 grid cells of straight waveguide.