How do I plot the mode group index versus wavelength?

Date	Category
2023-12-18 21:52:10	Mode Solver

To plot the group index of the optical modes calculated using the Tidy3D mode solver, you can follow the example below. We have considered a 500 x 220 nm silicon-on-insulator (SOI) waveguide operating between 1.5 and 1.6 \(\mu\)m.

import numpy as np
from tidy3d.plugins.mode import ModeSolver
from tidy3d.plugins.mode.web import run as run_mode_solver

# Define the waveguide.
waveguide = tidy3d.Structure(
    geometry=tidy3d.Box(size=(tidy3d.inf, 0.5, 0.22)),
    medium=tidy3d.Medium(permittivity=3.47**2),
)

# Build a simulation object including the waveguide.
sim = tidy3d.Simulation(
    size=(10, 2.5, 1.5),
    grid_spec=tidy3d.GridSpec.auto(min_steps_per_wvl=20, wavelength=1.55),
    structures=[waveguide],
    run_time=1e-12,
    boundary_spec=tidy3d.BoundarySpec.all_sides(boundary=tidy3d.PML()),
)

# Plane where we want to solve the modes.
plane = tidy3d.Box(center=(0, 0, 0), size=(0, 2.5, 1.5))

# Mode specification.
mode_spec = tidy3d.ModeSpec(
  num_modes=4,
  target_neff=3.47,
  group_index_step=True,
)

# Build the mode solver.
wvl = np.linspace(1.5, 1.6, 51)
freqs = tidy3d.C_0 / wvl
mode_solver = ModeSolver(
  simulation=sim,
  plane=plane,
  mode_spec=mode_spec,
  freqs=freqs,
)

# Run the server-side mode solver.
mode_data = run_mode_solver(mode_solver)

# Plot the mode group index.
n_group = mode_data.n_group.values.squeeze()
fig, ax = plt.subplots(1, 1, figsize=(6, 4), tight_layout=True)

for mid in range(mode_spec.num_modes):
    ax.plot(wvl, n_group[:, mid], label=(f"mode {mid}"))

For more details on how to set up, run, and visualize the solver results, please refer to this notebook.