# Frequency Domain Solver

This tutorial demonstrates Meep's frequency-domain solver which is used to compute the fields produced in a geometry in response to a continuous-wave (CW) source. For details regarding this feature, refer to this FAQ. This example involves using the frequency-domain solver to compute the fields of a ring resonator which has been described in a separate tutorial. First, we will verify that the error in the computed fields decreases monotonically with decreasing tolerance of the iterative solver. And then, we will demonstrate qualitative agreement with the frequency-domain fields computed using a different method: Fourier-transforming the time-domain fields in response to a narrowband Gaussian-pulse source.

Usage of the frequency-domain solver involves only two changes to the original simulation: (1) replace the Gaussian-pulse source with a continuous source, and (2) turn on complex fields since, by default, real fields are used. Everything else remains unchanged.

Since the frequency-domain solver uses an iterative method, there are a couple of things we can do to improve its convergence: (1) use a non-zero smoothing width for the CW source (default is 0) to reduce the high-frequency oscillations produced by its abrupt turn on (which have slow group velocities and are absorbed poorly by PML), and (2) increase the $L$ parameter of the BiCGSTAB-L iterative solver (default is 2).

We will compute the fundamental mode at five different tolerance values chosen on a logarithmic scale. We will then plot the L2 norm of the error in the fields as a function of the tolerance. The simulation script is shown below.

import meep as mp
import numpy as np
from numpy import linalg as LA
import matplotlib.pyplot as plt

n = 3.4
w = 1
r = 1
dpml = 2

fcen = 0.118
df = 0.08
src = mp.Source(mp.ContinuousSource(fcen,fwidth=df), mp.Ez, mp.Vector3(r+0.1))

sim = mp.Simulation(cell_size=mp.Vector3(sxy,sxy),
geometry=[c1,c2],
sources=[src],
resolution=10,
force_complex_fields=True,
symmetries=[mp.Mirror(mp.Y)],
boundary_layers=[mp.PML(dpml)])

num_tols = 5
tols = np.power(10, np.arange(-8.0,-8.0-num_tols,-1.0))
ez_dat = np.zeros((122,122,num_tols), dtype=np.complex_)

for i in range(num_tols):
sim.init_fields()
sim.solve_cw(tols[i], 10000, 10)
ez_dat[:,:,i] = sim.get_array(center=mp.Vector3(), size=mp.Vector3(sxy-2*dpml,sxy-2*dpml), component=mp.Ez)

err_dat = np.zeros(num_tols-1)
for i in range(num_tols-1):
err_dat[i] = LA.norm(ez_dat[:,:,i]-ez_dat[:,:,num_tols-1])

plt.figure(dpi=100)
plt.loglog(tols[:num_tols-1], err_dat, 'bo-');
plt.xlabel("frequency-domain solver tolerance");
plt.ylabel("L2 norm of error in fields");
plt.show()

eps_data = sim.get_array(center=mp.Vector3(), size=mp.Vector3(sxy-2*dpml,sxy-2*dpml), component=mp.Dielectric)
ez_data = np.absolute(ez_dat[:,:,num_tols-1])

plt.figure(dpi=100)
plt.imshow(eps_data.transpose(), interpolation='spline36', cmap='binary')
plt.imshow(ez_data.transpose(), interpolation='spline36', cmap='Reds', alpha=0.9)
plt.axis('off')
plt.show()

if np.all(np.diff(err_dat) < 0):
print("PASSED solve_cw test: error in the fields is decreasing with increasing resolution")
else:
print("FAILED solve_cw test: error in the fields is NOT decreasing with increasing resolution")


The results are shown in the figure below. The error in the fields decreases monotonically with decreasing tolerance of the frequency-domain solver.

As a further validation of the frequency-domain solver, we will compare its fields with those computed using time-stepping. This involves taking the Fourier transform of E$_z$ via the add_dft_fields routine. At the end of the time stepping, these frequency-domain fields are then output to an HDF5 file via output_dft. The script is extended as follows.

sim.reset_meep()

src = mp.Source(mp.GaussianSource(fcen,fwidth=df), mp.Ez, mp.Vector3(r+0.1))

sim = mp.Simulation(cell_size=mp.Vector3(sxy,sxy),
geometry=[c1,c2],
sources=[src],
resolution=10,
symmetries=[mp.Mirror(mp.Y)],
boundary_layers=[mp.PML(dpml)])

where = mp.Volume(center=mp.Vector3(), size=mp.Vector3(sxy-2*dpml,sxy-2*dpml))
dfts = sim.add_dft_fields([mp.Ez], fcen, fcen, 1, where=where)
sim.run(until_after_sources=100)
sim.output_dft(dfts, "dft_fields")

import h5py

f = h5py.File("dft_fields.h5", 'r')
ezi = f["ez_0.i"].value
ezr = f["ez_0.r"].value
ez_dat = ezr + 1j * ezi

eps_data = sim.get_array(center=mp.Vector3(), size=mp.Vector3(sxy-2*dpml,sxy-2*dpml), component=mp.Dielectric)
ez_data = np.absolute(ez_dat)

plt.figure(dpi=100)
plt.imshow(eps_data.transpose(), interpolation='spline36', cmap='binary')
plt.imshow(ez_data.transpose(), interpolation='spline36', cmap='Reds', alpha=0.9)
plt.axis('off')
plt.show()


The left inset of the figure above shows the magnitude of the scalar E$_z$ field, computed using the frequency-domain solver with a tolerance of 10-12, superimposed on the ring-resonator geometry. Note the three-fold mirror symmetry of the field pattern (fundamental mode) and faint presence of the point source. The right inset is for the Fourier-transformed fields of the time-domain calculation. The results are qualitatively similar.