# Single frequency CW source

This section describes the CW normalization used to convert time domain data to frequency (steady-state) data. The CW normalization option should be used whenever you collect frequency domain information.

Solvers: FDTD, VarFDTD

Single frequency simulations in FDTD Solutions
Sources in FDTD Solutions always inject a pulse (i.e. broadband), even when the user is only interested in collecting data at a single frequency. Pulse sources are more efficient for collecting broadband information, and at least as efficient when collecting information at a single frequency. Therefore, there is no reason to use CW sources. The CW response of a structure can easily be obtained from a simulation that used a pulse source.

The following screenshot shows the source Frequency/Wavelength settings tab. In this screenshot, the source wavelength is set to a single frequency of 500nm. Even thought the frequency span is zero, a pulse is still used. The time domain signal of the pulse, and the associated power spectrum, are shown in the figures on the right side of the window.

When the source frequency span is set to zero (i.e. single frequency), then the frequency domain monitors will automatically measure the response of the system at that frequency only.

Note: Changing the normalization state
By default, the CW normalization option is enabled. This is the correct setting
for the majority of simulations. However, if you want to disable the CW
normalization, go to the Settings - Normalization state menu and select No
normalization. You can also use the cwnorm and nonorm script commands.


Creating a CW source
While we strongly recommend against using a CW source, it is possible to create them in FDTD Solutions. For example, it is possible to create a source time signal like the one shown below. For more information on creating custom source time signals, see the custom source time signal page. The source pulse shown in this screenshot is from the simple non-linear laser gain example simulation.

Your data analysis may be more complicated when using a CW source. The frequency domain normalization will not work properly with a CW source. The CW normalization requires that the time domain fields decay to zero by the end of the simulation.

Note: To reproduce the above screenshot, reduce the simulation time in the FDTD region from 20ps to 1ps, then open the source properties and zoom in to the start of the source time signal.

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