# Bragg grating sections#

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Bragg gratings are often used in waveguides, such as optical fibres, which can reflect light of certain frequencies while transmitting others. This is typically achieved by periodically changing the refractive index or dielectric constant in a section of the waveguide, and the reflective and transmitting frequency bands are controlled by appropriately designing the periodicity and material or geometry parameters of the grating.

In this example, sections of two Bragg gratings will be simulated. The first one involves a waveguide with a perfectly-aligned corrugation on either side, which causes it to act as a reflector. The second one is similar, but with the corrugation on one side misaligned with the corrugation on the other side, so that the structure primarily transmits power.

Reference: Xu Wang, Yun Wang, Jonas Flueckiger, Richard Bojko, Amy Liu, Adam Reid, James Pond, Nicolas A. F. Jaeger, and Lukas Chrostowski, “Precise control of the coupling coefficient through destructive interference in silicon waveguide Bragg gratings,” Opt. Lett. 39, 5519-5522 (2014)

:

# basic imports
import numpy as np
import matplotlib.pylab as plt

# tidy3d imports
import tidy3d as td


[21:02:21] WARNING  This version of Tidy3D was pip installed from the 'tidy3d-beta' repository on   __init__.py:102
PyPI. Future releases will be uploaded to the 'tidy3d' repository. From now on,

           INFO     Using client version: 1.8.2                                                     __init__.py:120


## Structure Setup#

First, the geometry of the structure is defined. Both waveguides are set up in the same simulation side-by-side.

:

# materials
Air = td.Medium(permittivity=1.0)
Si = td.Medium(permittivity=3.47**2)
SiO2 = td.Medium(permittivity=1.44**2)
# SiO2 = td.material_library['SiO2']['Horiba']

# set basic geometric parameters
wg_height = 0.22
wg_feed_length = 0.75
wg_feed_width = 0.5
corrug_width = 0.05
num_periods = 50
period = 0.324

shift = period / 2
corrug_length = period / 2
wg_length = num_periods * period
wg_width = wg_feed_width - corrug_width

wavelength0 = 1.532
freq0 = td.C_0 / wavelength0
fwidth = freq0 / 40.0
run_time = 1250e-15
wavelength_min = td.C_0 / (freq0 + fwidth)

# place the two waveguides so that their centres are half a free-space wavelength apart
wg1_y = wavelength0 / 2
wg2_y = -wavelength0 / 2
wg_separation = wavelength0 / 2

# small buffer added to structures so they extend into the PML a bit
pml_buffer = period

# waveguide 1
wg1_size = [wg_length + pml_buffer, wg_width, wg_height]
wg1_center = [0, wg1_y, wg_height / 2]
wg1_medium = Si

# waveguide 2
wg2_size = [wg_length + pml_buffer, wg_width, wg_height]
wg2_center = [0, wg2_y, wg_height / 2]
wg2_medium = Si

# corrugation setup for waveguide 1
cg1_size = [corrug_length, corrug_width, wg_height]
cg1_center_plus = [
-wg_length / 2 + corrug_length / 2,
wg_width / 2 + corrug_width / 2 + wg1_y,
wg_height / 2,
]
cg1_center_minus = [
-wg_length / 2 + corrug_length / 2,
-wg_width / 2 - corrug_width / 2 + wg1_y,
wg_height / 2,
]
cg1_medium = Si

# corrugation setup for waveguide 2
cg2_size = [corrug_length, corrug_width, wg_height]
cg2_center_plus = [
-wg_length / 2 + corrug_length / 2,
wg_width / 2 + corrug_width / 2 + wg2_y,
wg_height / 2,
]
cg2_center_minus = [
-wg_length / 2 + corrug_length / 2 + shift,
-wg_width / 2 - corrug_width / 2 + wg2_y,
wg_height / 2,
]
cg2_medium = Si

# substrate
sub_size = [td.inf, td.inf, 2]
sub_center = [0, 0, -1.0]
sub_medium = SiO2

# create the substrate
substrate = td.Structure(
geometry=td.Box(center=sub_center, size=sub_size),
medium=sub_medium,
name="substrate",
)

# create the first waveguide
waveguide_1 = td.Structure(
geometry=td.Box(center=wg1_center, size=wg1_size),
medium=wg1_medium,
name="waveguide_1",
)

# create the second waveguide
waveguide_2 = td.Structure(
geometry=td.Box(center=wg2_center, size=wg2_size),
medium=wg2_medium,
name="waveguide_2",
)

# create the corrugation for the first waveguide
corrug1_plus = []
corrug1_minus = []
for i in range(num_periods):
# corrugation on the +y side
center = cg1_center_plus
if i > 0:
center += period
plus = td.Structure(
geometry=td.Box(center=center, size=cg1_size),
medium=cg1_medium,
name=f"corrug1_plus_{i}",
)

# corrugation on the -y side
center = cg1_center_minus
if i > 0:
center += period
minus = td.Structure(
geometry=td.Box(center=center, size=cg1_size),
medium=cg1_medium,
name=f"corrug1_minus_{i}",
)

corrug1_plus.append(plus)
corrug1_minus.append(minus)

# create the corrugation for the second waveguide
corrug2_plus = []
corrug2_minus = []
for i in range(num_periods):
# corrugation on the +y side
center = cg2_center_plus
if i > 0:
center += period
plus = td.Structure(
geometry=td.Box(center=center, size=cg2_size),
medium=cg2_medium,
name=f"corrug2_plus_{i}",
)

# corrugation on the -y side
center = cg2_center_minus
if i > 0:
center += period
minus = td.Structure(
geometry=td.Box(center=center, size=cg2_size),
medium=cg2_medium,
name=f"corrug2_minus_{i}",
)

corrug2_plus.append(plus)
corrug2_minus.append(minus)

# full simulation domain
sim_size = [
wg_length + wavelength0 * 1.5,
2 * wavelength0 + wg_width + 2 * corrug_width,
3.7,
]
sim_center = [0, 0, 0.0]

# boundary conditions - Bloch boundaries are used to emulate an infinitely long grating
boundary_spec = td.BoundarySpec(
# x=td.Boundary.bloch(bloch_vec=num_periods/2),
x=td.Boundary.pml(),
y=td.Boundary.pml(),
z=td.Boundary.pml(),
)

# grid specification
grid_spec = td.GridSpec.auto(min_steps_per_wvl=20)



## Source Setup#

A mode source is defined for each waveguide.

:

# mode source for waveguide 1
source1_time = td.GaussianPulse(freq0=freq0, fwidth=fwidth, amplitude=1)
mode_src1 = td.ModeSource(
center=[-wg_length / 2 + period, wg1_y, wg_height / 2],
size=[0, waveguide_1.geometry.size * 2, waveguide_1.geometry.size * 2],
mode_index=0,
direction="+",
source_time=source1_time,
mode_spec=td.ModeSpec(),
)

# mode source for waveguide 2
source2_time = td.GaussianPulse(freq0=freq0, fwidth=fwidth, amplitude=1)
mode_src2 = td.ModeSource(
center=[-wg_length / 2 + period, wg2_y, wg_height / 2],
size=[0, waveguide_2.geometry.size * 2, waveguide_2.geometry.size * 2],
mode_index=0,
direction="+",
source_time=source2_time,
mode_spec=td.ModeSpec(),
)



## Monitor Setup#

To visualize the field distribution in the waveguides, a monitor is placed in the xy plane cutting through both waveguides. A pair of flux monitors is also placed on the far side the demonstrate the transmission and reflection characteristics.

:

# create monitors
monitor_xy = td.FieldMonitor(
center=[0, 0, wg_height / 2],
size=[wg_length, 2 * wavelength0 + wg_width + 2 * corrug_width, 0],
freqs=[freq0],
name="fields_xy",
)

freqs = np.linspace(freq0 - 2 * fwidth, freq0 + 2 * fwidth, 200)
monitor_flux_aligned = td.FluxMonitor(
center=[wg_length / 2 - period, wg1_y, wg_height / 2],
size=[0, waveguide_1.geometry.size * 2, waveguide_1.geometry.size * 2],
freqs=freqs,
name="flux_aligned",
)

monitor_flux_misaligned = td.FluxMonitor(
center=[wg_length / 2 - period, wg2_y, wg_height / 2],
size=[0, waveguide_2.geometry.size * 2, waveguide_2.geometry.size * 2],
freqs=freqs,
name="flux_misaligned",
)



## Create Simulation#

All the structures, sources, and monitors are consolidated, and the simulation is created and visualized.

:

# list of all structures
structures = (
[substrate, waveguide_1, waveguide_2]
+ corrug1_plus
+ corrug1_minus
+ corrug2_plus
+ corrug2_minus
)

# list of all sources
sources = [mode_src1, mode_src2]

# list of all monitors
monitors = [monitor_xy, monitor_flux_aligned, monitor_flux_misaligned]

# create the simulation
sim = td.Simulation(
center=sim_center,
size=sim_size,
grid_spec=grid_spec,
structures=structures,
sources=sources,
monitors=monitors,
run_time=run_time,
boundary_spec=boundary_spec,
)

# plot the simulation domain
f, (ax1, ax3) = plt.subplots(1, 2, tight_layout=True, figsize=(10, 6))
sim.plot(x=0, ax=ax1)
sim.plot(z=wg_height / 2, ax=ax3)


           INFO     Auto meshing using wavelength 1.5320 defined from sources.                     grid_spec.py:510

:

<AxesSubplot: title={'center': 'cross section at z=0.11'}, xlabel='x', ylabel='y'> ## Run Simulation#

:

# run simulation
import tidy3d.web as web


           INFO     Using Tidy3D credentials from stored file.                                           auth.py:70

[21:02:23] INFO     Authentication successful.                                                           auth.py:30

[21:02:24] INFO     Created task 'bragg' with task_id 'a70f4ce5-cc7d-4cee-9a2c-46170488fe86'.         webapi.py:120

[21:02:27] INFO     Maximum FlexUnit cost: 0.115                                                      webapi.py:252

           INFO     status = queued                                                                   webapi.py:261

[21:02:30] INFO     status = preprocess                                                               webapi.py:273

[21:02:35] INFO     starting up solver                                                                webapi.py:277

[21:02:46] INFO     running solver                                                                    webapi.py:283

[21:05:58] INFO     status = postprocess                                                              webapi.py:306

[21:06:03] INFO     status = success                                                                  webapi.py:306

[21:06:04] INFO     Billed FlexUnit cost: 0.115                                                       webapi.py:310

           INFO     downloading file "output/monitor_data.hdf5" to "data/bragg.hdf5"                  webapi.py:592

[21:06:06] INFO     loading SimulationData from data/bragg.hdf5                                       webapi.py:414

           WARNING  Simulation final field decay value of 0.00406 is greater than the simulation      webapi.py:420
shutoff threshold of 1e-05. Consider simulation again with large run_time
duration for more accurate results.


## Field Plot#

The frequency-domain fields are plotted in the xy plane cutting through the waveguides. We notice that the grating with aligned corrugation effectively reflects power at the design frequency, while the misalignment in the second grating causes it to be mostly transmissive.

:

plt.rcParams.update({"font.size": 14})
# plot fields on the monitor
fig, ax = plt.subplots(tight_layout=True, figsize=(13, 4))
sim_data.plot_field(
field_monitor_name="fields_xy", field_name="Ey", val="real", f=freq0, ax=ax
)


           INFO     Auto meshing using wavelength 1.5320 defined from sources.                     grid_spec.py:510

:

<AxesSubplot: title={'center': 'cross section at z=0.11'}, xlabel='x', ylabel='y'> ## Transmission and Reflection#

The observations made in the field plot above can be confirmed by plotting the flux recorded by the flux monitors as a function of frequency. In the region of the design frequency, indicated by the dashed black line, the drop in flux for the aligned-corrugation grating confirms its reflective property.

:

plt.rcParams.update({"font.size": 14})
# plot transmitted flux for each waveguide

fig, ax = plt.subplots(figsize=(9, 4))
ax.plot(
td.C_0 / freqs * 1e9,
sim_data["flux_aligned"].flux,
"-b",
label="Aligned corrugation",
)
ax.plot(
td.C_0 / freqs * 1e9,
sim_data["flux_misaligned"].flux,
"-r",
label="Misaligned corrugation",
)
ax.set(xlabel="wavelength (nm)", ylabel="flux", yscale="linear", xscale="linear")
ax.grid(visible=True, which="both", axis="both", linewidth=0.4)
lims = ax.get_ylim()
ax.plot([td.C_0 / freq0 * 1e9, td.C_0 / freq0 * 1e9], [lims, lims], "--k")
plt.legend(loc="best", prop={"size": 14})
plt.tight_layout() [ ]: