This video is taken from the FDTD 100 course on Lumerical University.

## Transcript

Frequency domain field monitors return data in the frequency domain by taking the Fourier

transform of the time domain fields.

By default, CW normalization is applied, which means that the data is also normalized by

the spectrum of the source pulse to give the result as though the source spectrum is uniform.

In other words, by default, the result from a frequency domain monitor at a given frequency

point is the response as though the source is injecting a continuous wave at the specified

frequency with amplitude 1.

You can review the CW normalization method in the FDTD algorithm section of the course.

There are two available frequency domain field monitors: the frequency-domain field profile

monitor and the frequency-domain field and power monitor.

The only difference between the two monitor types is the spatial interpolation method

used.

The profile monitor uses the specified position interpolation and the power monitor uses the

nearest mesh cell interpolation method.

In most cases, the results from these two monitors will be very similar.

We recommend using the Frequency Domain Field and Power monitor unless you are an advanced

user and have a detailed understanding of the monitor interpolation options.

By default, the frequency-domain field monitors return the E and H field components as well

as T - the net power transmission through the monitor.

However, you can also choose to record the Poynting vector components in the data to

record tab.

Transmission results are only returned if the monitor type is linear or 2D if running

a 2D simulation, or for a 2D if running a 3D simulation.

The transmission result is only available if either the Poynting vector or output power

is selected in the Data to record tab.

If all E and H field components are recorded by the monitor, it’s possible to project

the fields from the monitor to get the far field angular distribution.

This result is something that is calculated as a post-processing step after the simulation

has been run, so when you first try to visualize the farfield result of the monitor, if it

hasn’t yet been calculated, a window will pop up asking you to select the settings you

want to use for the far field projection calculation.

Examples of when you might want to get the far field angular distribution are if you

are interested in the radiation pattern of an antenna, or scattering pattern from a particle

or defect.

In the next unit, we will demonstrate the far field projection settings in the graphical

user interface.

There is settings in the “Spectral averaging and apodization” tab for frequency domain

field monitors which allow you to choose to calculate and return a full or partial spectral

average result or return field results from a specified portion of the time signal by

applying apodization.

Apodization is useful if you want to get the field profile of resonant fields without including

fields from the initial source that is injected to excite the resonance, or effects of ending

the simulation early, truncating the time signal.

There is more detail about how and when to use these advanced features in the related

links below.