Reply To: FDTD:: simulation of directional coupler

[Zhongdi Peng]

Yes, I put different override mesh sizes in different regions. Like the mesh around WG which has width=2um allows WG separated into 25 grids; mesh in a coupling gap of 1um sets it to have 20 grids. As you see in the image below, the override mesh does not cover the region near the Y/Z's boundary, which is defined by nonuniform mesh with accuracy=1. But the mesh affects the boundary in X min/max, which has mesh size = (0.08,0.08,0.04) um.

Summarize my setting:

a). dt=0.99 by default

b). Boundary: Metal in Y/Z and Standard PML in X with layer=32

c). Mesh: The coupling region has override mesh (0.08,0.05~0.08,0.02~0.04)um, Mesh close to Y/Z boundary is set by nonuniform mesh with accuracy=1 (but will be affected by other override mesh), other override mesh = (0.08,0.08,0.04) um

Result: it can converge to shutoff=0.0015, but later diverge (shown below). Which is similar to the result I got in the Method A (result shown in the aboved posts)

To answer your questions,

1. The mode at the end facet of output is single-mode:

2. input and output intensity (Tnet) versus frequency, 3dB is roughly at 1600nm:

Light propogation at 1573nm:

Even through it converge to 0.0015, it diverged after that. Can I trust the result?

Personally I do not think there is further optimization choice for me except increasing the layer of PML in X direction:

1. dt can not change. I previously used 0.9 instead but it diverge faster.
2. Changing Metal boundary at Y/Z to PML does not help. The result in aboved Mehod A used standard PML in all boundaries but diverge at a similar shutoff = 0.002.

The PML layer is already 32, how many layers you suggest to set?

 

Zhongdi