Thermal

GDSFactory has an FEM femwell plugin that you can use for thermal simulations. You can simulate directly the component layout and include important effects such as metal dummy fill.

from collections import OrderedDict

import gdsfactory as gf
import matplotlib.pyplot as plt
import numpy as np
from femwell.maxwell.waveguide import compute_modes
from femwell.mesh import mesh_from_OrderedDict
from femwell.thermal import solve_thermal
from gdsfactory.generic_tech import get_generic_pdk
from shapely.geometry import LineString, Polygon
from skfem import Basis, ElementTriP0
from skfem.io import from_meshio
from tqdm import tqdm

/tmp/ipykernel_3432/3426391321.py:9: DeprecationWarning: The 'gdsfactory.generic_tech' module is deprecated and will be removed in a future version. Please update your imports to use 'gdsfactory.gpdk' instead:
  from gdsfactory.gpdk import LAYER, LAYER_STACK, get_generic_pdk
Or for submodules:
  from gdsfactory.gpdk.layer_map import LAYER
  from gdsfactory.gpdk.layer_stack import LAYER_STACK
  from gdsfactory.generic_tech import get_generic_pdk

gf.config.rich_output()
PDK = get_generic_pdk()
PDK.activate()

LAYER_STACK = PDK.layer_stack

LAYER_STACK.layers["heater"].thickness = 0.13
LAYER_STACK.layers["heater"].zmin = 2.2

heater = gf.components.straight_heater_metal(length=50)
heater.plot()

s = heater.to_3d()
s.show()

w_sim = 8 * 2
h_clad = 2.8
h_box = 3
w_core = 0.5
h_core = 0.22
h_heater = 0.14
w_heater = 2
offset_heater = 2 + (h_core + h_heater) / 2
h_silicon = 0.5

polygons = OrderedDict(
    bottom=LineString(
        [
            (-w_sim / 2, -h_core / 2 - h_box - h_silicon),
            (w_sim / 2, -h_core / 2 - h_box - h_silicon),
        ]
    ),
    core=Polygon(
        [
            (-w_core / 2, -h_core / 2),
            (-w_core / 2, h_core / 2),
            (w_core / 2, h_core / 2),
            (w_core / 2, -h_core / 2),
        ]
    ),
    heater=Polygon(
        [
            (-w_heater / 2, -h_heater / 2 + offset_heater),
            (-w_heater / 2, h_heater / 2 + offset_heater),
            (w_heater / 2, h_heater / 2 + offset_heater),
            (w_heater / 2, -h_heater / 2 + offset_heater),
        ]
    ),
    clad=Polygon(
        [
            (-w_sim / 2, -h_core / 2),
            (-w_sim / 2, -h_core / 2 + h_clad),
            (w_sim / 2, -h_core / 2 + h_clad),
            (w_sim / 2, -h_core / 2),
        ]
    ),
    box=Polygon(
        [
            (-w_sim / 2, -h_core / 2),
            (-w_sim / 2, -h_core / 2 - h_box),
            (w_sim / 2, -h_core / 2 - h_box),
            (w_sim / 2, -h_core / 2),
        ]
    ),
    wafer=Polygon(
        [
            (-w_sim / 2, -h_core / 2 - h_box - h_silicon),
            (-w_sim / 2, -h_core / 2 - h_box),
            (w_sim / 2, -h_core / 2 - h_box),
            (w_sim / 2, -h_core / 2 - h_box - h_silicon),
        ]
    ),
)

resolutions = dict(
    core={"resolution": 0.04, "distance": 1},
    clad={"resolution": 0.6, "distance": 1},
    box={"resolution": 0.6, "distance": 1},
    heater={"resolution": 0.1, "distance": 1},
)

mesh = from_meshio(
    mesh_from_OrderedDict(polygons, resolutions, default_resolution_max=0.6)
)
mesh.draw().show()

And then we solve it!

currents = np.linspace(0.0, 7.4e-3, 10)
current_densities = currents / polygons["heater"].area
neffs = []

for current_density in tqdm(current_densities):
    basis0 = Basis(mesh, ElementTriP0(), intorder=4)
    thermal_conductivity_p0 = basis0.zeros()
    for domain, value in {
        "core": 90,
        "box": 1.38,
        "clad": 1.38,
        "heater": 28,
        "wafer": 148,
    }.items():
        thermal_conductivity_p0[basis0.get_dofs(elements=domain)] = value
    thermal_conductivity_p0 *= 1e-12  # 1e-12 -> conversion from 1/m^2 -> 1/um^2

    basis, temperature = solve_thermal(
        basis0,
        thermal_conductivity_p0,
        specific_conductivity={"heater": 2.3e6},
        current_densities={"heater": current_density},
        fixed_boundaries={"bottom": 0},
    )

    if current_density == current_densities[-1]:
        basis.plot(temperature, shading="gouraud", colorbar=True)
        plt.show()

    temperature0 = basis0.project(basis.interpolate(temperature))
    epsilon = basis0.zeros() + (1.444 + 1.00e-5 * temperature0) ** 2
    epsilon[basis0.get_dofs(elements="core")] = (
        3.4777 + 1.86e-4 * temperature0[basis0.get_dofs(elements="core")]
    ) ** 2
    # basis0.plot(epsilon, colorbar=True).show()

    modes = compute_modes(basis0, epsilon, wavelength=1.55, num_modes=1, solver="scipy")

    if current_density == current_densities[-1]:
        modes[0].show(modes[0].E.real)

    neffs.append(np.real(modes[0].n_eff))

length = 320  # um
print(f"Phase shift: {2 * np.pi / 1.55 * (neffs[-1] - neffs[0]) * length}")
plt.xlabel("Current / mA")
plt.ylabel("Effective refractive index $n_{eff}$")
plt.plot(currents * 1e3, neffs)
plt.show()

Phase shift: 4.993708862478653