How do I set a ModeSource?

Date	Category
2023-12-08 13:27:54	Sources

The tidy3d.ModeSource injects a current source in the simulation to excite a modal profile in a finite extent plane. It is commonly used to excite specific waveguide modes in photonic integrated circuits. To illustrate how to set up a tidy3d.ModeSource, let’s consider the case of injecting the first-order transverse electric (TE) mode in a silicon-on-insulator (SOI) waveguide operating at 1.55 \(\mu\)m.

# Source bandwidth.
pulse = tidy3d.GaussianPulse(freq0=1.934e14, fwidth=6.245e12)

# Mode specification.
mode_spec = tidy3d.ModeSpec(target_neff=3.47, filter_pol='te', num_modes = 2)

# Source definition
source = tidy3d.ModeSource(
  center=(0, 0, -2),
  size=(0, 2, 1.5),
  source_time=pulse,
  direction='+',
  mode_spec=mode_spec,
  mode_index=1,
  name="mode_source",
)

You should use the center and size parameters to define a source plane surrounding the waveguide, then adjust the source_time dependence using tidy3d.GaussianPulse. The direction='+' parameter specifies propagation in the positive waveguide axis. The tidy3d.ModeSpec object includes all the specifications of a mode solver, which calculates the optical modes given the material distribution within the source plane. The modes calculated by the mode solver are sorted by their effective indices in descending order. So, we have set the initial mode solver guess to the core refractive index (target_neff=3.47) and chosen filter_pol='te' to make sure it will return first in the list of the TE waveguide modes, starting from the fundamental one. Finally, to inject the first-order TE mode in the waveguide, we set`mode_index=1``.

This example illustrates setting up a tidy3d.ModeSource source.