flex_rf.tidy3d.ModeMonitor

Type: class │ Base(s): AbstractModeMonitor

Description

Monitor that records amplitudes from modal decomposition of fields on plane.

Notes

The fields recorded by frequency monitors (and hence also mode monitors) are automatically normalized by the power amplitude spectrum of the source. For multiple sources, the user can select which source to use for the normalization too.

We can also use the mode amplitudes recorded in the mode monitor to reveal the decomposition of the radiated power into forward- and backward-propagating modes, respectively.

Practical Advice

For reliable mode decomposition, place mode monitors in straight waveguide sections where the mode profile is well-defined. The monitor should be large enough to capture the full mode profile including evanescent tails — a typical sizing is 3-4x the waveguide width in each transverse dimension.

Extracting mode amplitudes:

amps = sim_data["mode_monitor"].amps

# Forward-propagating power in fundamental mode
T_mode0 = np.abs(amps.sel(direction="+", mode_index=0).values) ** 2

# Backward-propagating (reflection)
R_mode0 = np.abs(amps.sel(direction="-", mode_index=0).values) ** 2

.. TODO give an example of how to extract the data from this mode.

.. TODO add derivation in the notebook.

.. TODO add link to method

.. TODO add links to notebooks correspondingly

Example(s)

mode_spec = ModeSpec(num_modes=3)
monitor = ModeMonitor(
    center=(1,2,3),
    size=(2,2,0),
    freqs=[200e12, 210e12],
    mode_spec=mode_spec,
    name='mode_monitor')

Parameters

size [TracedSize]

Size in x, y, and z directions.

name [str]

Unique name for monitor.

freqs [FreqArray]

Array or list of frequencies stored by the field monitor.

center [TracedCoordinate] = (0.0, 0.0, 0.0)

Center of object in x, y, and z.

interval_space [tuple[Literal[1], Literal[1], Literal[1]]] = (1, 1, 1)

Number of grid step intervals between monitor recordings. If equal to 1, there will be no downsampling. If greater than 1, the step will be applied, but the first and last point of the monitor grid are always included. Not all monitors support values different from 1.

colocate [bool] = True

Toggle whether fields should be colocated to grid cell boundaries (i.e. primal grid nodes).

use_colocated_integration [bool] = True

Only takes effect when colocate=False. If True, dot products and overlap integrals still use fields interpolated to grid cell boundaries (colocated), even though the field data is stored at native Yee grid positions. Experimental feature that can give improved accuracy by avoiding interpolation of fields to Yee cell positions for integration.

apodization [ApodizationSpec] = factory: ApodizationSpec

Sets parameters of (optional) apodization. Apodization applies a windowing function to the Fourier transform of the time-domain fields into frequency-domain ones, and can be used to truncate the beginning and/or end of the time signal, for example to eliminate the source pulse when studying the eigenmodes of a system. Note: apodization affects the normalization of the frequency-domain fields.

store_fields_direction [Direction | None] = None

Propagation direction for the mode field profiles stored from mode solving.

conjugated_dot_product [bool] = True

Use conjugated or non-conjugated dot product for mode decomposition.

mode_spec [ModeSpec] = factory: ModeSpec

Parameters to feed to mode solver which determine modes measured by monitor.

Methods

parallel_adjoint_bases(simulation: Simulation, monitor_index: int)

Return parallel adjoint bases for mode monitor amplitudes.

storage_size(num_cells: int, tmesh: int)

Size of monitor storage given the number of points after discretization.

supports_parallel_adjoint()

Return True for mode monitor amplitude adjoints.