How are the output modes sorted?

Date	Category
2023-12-18 22:11:57	Mode Solver

After running the Tidy3D mode solver, the results are returned within a ModeSolverData object. The solver always computes the num_modes modes closest to the given target_neff. If filter_pol==None, they are simply sorted in order of decreasing effective index. If a polarization filter is selected, the modes are rearranged such that the first n_pol modes in the list are the ones with the selected polarization fraction larger than or equal to 0.5, while the next num_modes - n_pol modes are the ones where it is smaller than 0.5 (i.e. the opposite polarization fraction is larger than 0.5). Within each polarization subset, the modes are still ordered by decreasing effective index. In the example below, the TE modes are returned first in the mode list.

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,
  filter_pol='te',
)

# Build the mode solver.
freq0 = tidy3d.C_0 / 1.55
mode_solver = ModeSolver(
  simulation=sim,
  plane=plane,
  mode_spec=mode_spec,
  freqs=[freq0],
)

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

# Get the mode polarization fraction.
print("TE polarization fraction:")
print(np.asarray(mode_data.pol_fraction['te']).squeeze())