Rectangular Dielectric Waveguide Modes#
This notebook solves a dielectric core inside cladding, classifies guided modes, and plots transverse and longitudinal fields.
from palacetoolkit.mode_solver import WaveguideModeSolver
from palacetoolkit.utils import write_and_finalize_gmsh
from palacetoolkit.viz import view_mesh
import gmsh
import numpy as np
import matplotlib.pyplot as plt
def _set_lc_for_dielectric(a_core, b_core, lc_core, lc_clad):
pts = gmsh.model.getEntities(0)
for _, ptag in pts:
coord = gmsh.model.getValue(0, ptag, [])
in_core = (abs(coord[0]) <= a_core / 2 + 1e-10 and abs(coord[1]) <= b_core / 2 + 1e-10)
gmsh.model.mesh.setSize([(0, ptag)], lc_core if in_core else lc_clad)
def make_dielectric_waveguide_mesh(a_core, b_core, a_clad, b_clad, nx_core=16, ny_core=8, nx_clad=8, ny_clad=8, filename=None):
gmsh.initialize()
gmsh.option.setNumber("General.Verbosity", 0)
gmsh.model.add("dielectric_waveguide")
clad = gmsh.model.occ.addRectangle(-a_clad / 2, -b_clad / 2, 0, a_clad, b_clad)
core = gmsh.model.occ.addRectangle(-a_core / 2, -b_core / 2, 0, a_core, b_core)
_, outmap = gmsh.model.occ.fragment([(2, clad)], [(2, core)])
gmsh.model.occ.synchronize()
core_surfs = [tag for _, tag in outmap[1]]
clad_surfs = [tag for _, tag in outmap[0] if tag not in core_surfs]
gmsh.model.addPhysicalGroup(2, core_surfs, tag=1, name="core")
gmsh.model.addPhysicalGroup(2, clad_surfs, tag=2, name="cladding")
all_surfs = [(2, t) for t in core_surfs + clad_surfs]
outer_bnd = gmsh.model.getBoundary(all_surfs, oriented=False, combined=True)
pec_tags = [abs(tag) for _, tag in outer_bnd]
gmsh.model.addPhysicalGroup(1, pec_tags, tag=1, name="PEC")
lc_core = min(a_core / nx_core, b_core / ny_core)
lc_clad = max(lc_core * max(nx_core / nx_clad, ny_core / ny_clad), 1.5 * lc_core)
_set_lc_for_dielectric(a_core, b_core, lc_core, lc_clad)
gmsh.model.mesh.generate(2)
return write_and_finalize_gmsh(filename, prefix="wg_diel_")
a_core, b_core = 1.0, 0.5
a_clad, b_clad = 4.0, 3.0
eps_core, eps_clad = 4.0, 1.0
mu = 1.0
k0_cutoff_approx = np.pi / a_core / np.sqrt(eps_core - eps_clad)
omega = 3.0 * k0_cutoff_approx
kn_min = omega * np.sqrt(mu * eps_clad)
kn_max = omega * np.sqrt(mu * eps_core)
print(f"Guided-mode window: {kn_min:.4f} < kn < {kn_max:.4f}")
Guided-mode window: 5.4414 < kn < 10.8828
mesh_file = make_dielectric_waveguide_mesh(
a_core,
b_core,
a_clad,
b_clad,
nx_core=16,
ny_core=8,
nx_clad=8,
ny_clad=8,
)
view_mesh(mesh_file)
mu_inv_dict = {1: 1.0 / mu, 2: 1.0 / mu}
eps_dict = {1: eps_core, 2: eps_clad}
solver = WaveguideModeSolver(mesh_file, order=2, mu_inv=mu_inv_dict, eps=eps_dict, pec_bdr="all")
results = solver.solve(omega, num_modes=10, mode_idx=1)
print("Computed modes:")
for i, kn in enumerate(results["kn"], start=1):
if kn_min < kn.real < kn_max:
mode_type = "guided"
elif abs(kn.imag) > 0.1 * abs(kn.real):
mode_type = "evanescent"
else:
mode_type = "radiation/substrate"
print(f" Mode {i:2d}: kn={kn.real:+10.6f}{kn.imag:+10.6f}j [{mode_type}]")
Loading mesh file: /tmp/wg_diel_7ttrgedl.msh
Groups to render transparent: ['air_none', 'air_plastic_enclosure']
Mesh loaded successfully with 2 cell blocks
Found 3122 triangles total
Physical group tags in mesh: {1: 'core', 2: 'cladding'}
FE spaces: ND dofs = 15722, H1 dofs = 6357, total = 22079
Essential DOFs: ND = 224, H1 = 224, total = 448
Solving eigenvalue problem (omega = 5.4414, sigma = -130.279, size = 22079)...
Found 10 modes:
Mode 1: kn = +9.51611978e+00 -0.00000000e+00j <-- selected
Mode 2: kn = +8.98886548e+00 -0.00000000e+00j
Mode 3: kn = +8.06679983e+00 -0.00000000e+00j
Mode 4: kn = +7.86873770e+00 -0.00000000e+00j
Mode 5: kn = +6.41320112e+00 -0.00000000e+00j
Mode 6: kn = +6.00582037e+00 -0.00000000e+00j
Mode 7: kn = +5.85640924e+00 -0.00000000e+00j
Mode 8: kn = +5.54115884e+00 -0.00000000e+00j
Mode 9: kn = +5.31151432e+00 -0.00000000e+00j
Mode 10: kn = +5.27932424e+00 -0.00000000e+00j
Computed modes:
Mode 1: kn= +9.516120 -0.000000j [guided]
Mode 2: kn= +8.988865 -0.000000j [guided]
Mode 3: kn= +8.066800 -0.000000j [guided]
Mode 4: kn= +7.868738 -0.000000j [guided]
Mode 5: kn= +6.413201 -0.000000j [guided]
Mode 6: kn= +6.005820 -0.000000j [guided]
Mode 7: kn= +5.856409 -0.000000j [guided]
Mode 8: kn= +5.541159 -0.000000j [guided]
Mode 9: kn= +5.311514 -0.000000j [radiation/substrate]
Mode 10: kn= +5.279324 -0.000000j [radiation/substrate]
guided = [i for i, kn in enumerate(results["kn"]) if kn_min < kn.real < kn_max]
if not guided:
guided = list(range(min(4, len(results["kn"]))))
n_plot = min(4, len(guided))
fig, axes = plt.subplots(2, max(1, n_plot), figsize=(5 * max(1, n_plot), 8))
if n_plot == 1:
axes = axes.reshape(2, 1)
for j, mode_i in enumerate(guided[:n_plot]):
r = solver.solve(omega, num_modes=10, mode_idx=mode_i + 1)
X, Y, Ex, Ey, Ez = solver.get_field_on_grid(r["Et_vec"], r["En_vec"], nx=80, ny=60)
Et_mag = np.sqrt(np.abs(Ex) ** 2 + np.abs(Ey) ** 2)
En_mag = np.abs(Ez)
ax = axes[0, j]
im = ax.pcolormesh(X, Y, Et_mag, cmap="hot", shading="auto")
ax.plot([-a_core/2, a_core/2, a_core/2, -a_core/2, -a_core/2], [-b_core/2, -b_core/2, b_core/2, b_core/2, -b_core/2], "c--", linewidth=1.25)
ax.set_title(f"|Et| mode {mode_i+1}")
ax.set_aspect("equal")
fig.colorbar(im, ax=ax, fraction=0.046)
ax = axes[1, j]
im = ax.pcolormesh(X, Y, En_mag, cmap="hot", shading="auto")
ax.plot([-a_core/2, a_core/2, a_core/2, -a_core/2, -a_core/2], [-b_core/2, -b_core/2, b_core/2, b_core/2, -b_core/2], "c--", linewidth=1.25)
ax.set_title(f"|En| mode {mode_i+1}")
ax.set_aspect("equal")
fig.colorbar(im, ax=ax, fraction=0.046)
plt.tight_layout()
plt.show()
Solving eigenvalue problem (omega = 5.4414, sigma = -130.279, size = 22079)...
Found 10 modes:
Mode 1: kn = +9.51611978e+00 -0.00000000e+00j <-- selected
Mode 2: kn = +8.98886548e+00 -0.00000000e+00j
Mode 3: kn = +8.06679983e+00 -0.00000000e+00j
Mode 4: kn = +7.86873770e+00 -0.00000000e+00j
Mode 5: kn = +6.41320112e+00 -0.00000000e+00j
Mode 6: kn = +6.00582037e+00 -0.00000000e+00j
Mode 7: kn = +5.85640924e+00 -0.00000000e+00j
Mode 8: kn = +5.54115884e+00 -0.00000000e+00j
Mode 9: kn = +5.31151432e+00 -0.00000000e+00j
Mode 10: kn = +5.27932424e+00 -0.00000000e+00j
Solving eigenvalue problem (omega = 5.4414, sigma = -130.279, size = 22079)...
Found 10 modes:
Mode 1: kn = +9.51611978e+00 -0.00000000e+00j
Mode 2: kn = +8.98886548e+00 -0.00000000e+00j <-- selected
Mode 3: kn = +8.06679983e+00 -0.00000000e+00j
Mode 4: kn = +7.86873770e+00 -0.00000000e+00j
Mode 5: kn = +6.41320112e+00 -0.00000000e+00j
Mode 6: kn = +6.00582037e+00 -0.00000000e+00j
Mode 7: kn = +5.85640924e+00 -0.00000000e+00j
Mode 8: kn = +5.54115884e+00 -0.00000000e+00j
Mode 9: kn = +5.31151432e+00 -0.00000000e+00j
Mode 10: kn = +5.27932424e+00 -0.00000000e+00j
Solving eigenvalue problem (omega = 5.4414, sigma = -130.279, size = 22079)...
Found 10 modes:
Mode 1: kn = +9.51611978e+00 -0.00000000e+00j
Mode 2: kn = +8.98886548e+00 -0.00000000e+00j
Mode 3: kn = +8.06679983e+00 -0.00000000e+00j <-- selected
Mode 4: kn = +7.86873770e+00 -0.00000000e+00j
Mode 5: kn = +6.41320112e+00 -0.00000000e+00j
Mode 6: kn = +6.00582037e+00 -0.00000000e+00j
Mode 7: kn = +5.85640924e+00 -0.00000000e+00j
Mode 8: kn = +5.54115884e+00 -0.00000000e+00j
Mode 9: kn = +5.31151432e+00 -0.00000000e+00j
Mode 10: kn = +5.27932424e+00 -0.00000000e+00j
Solving eigenvalue problem (omega = 5.4414, sigma = -130.279, size = 22079)...
Found 10 modes:
Mode 1: kn = +9.51611978e+00 -0.00000000e+00j
Mode 2: kn = +8.98886548e+00 -0.00000000e+00j
Mode 3: kn = +8.06679983e+00 -0.00000000e+00j
Mode 4: kn = +7.86873770e+00 -0.00000000e+00j <-- selected
Mode 5: kn = +6.41320112e+00 -0.00000000e+00j
Mode 6: kn = +6.00582037e+00 -0.00000000e+00j
Mode 7: kn = +5.85640924e+00 -0.00000000e+00j
Mode 8: kn = +5.54115884e+00 -0.00000000e+00j
Mode 9: kn = +5.31151432e+00 -0.00000000e+00j
Mode 10: kn = +5.27932424e+00 -0.00000000e+00j
