Learning Forum

[Greg Baethge]

This topic 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

See also:

Sources

Units and normalization

Frequency domain normalization

Single frequency simulations in FDTD

Sources in FDTD 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 mearure the response of the system at that frequency only.

 

Creating a CW source

While we strongly recommend against using a CW source, it is possible to create them in FDTD. 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 Creating a custom source time signal page.

 

 

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.

 

[Asha Devi]

 

Hi, 

Can you kindly share with me the script for creating the CW source as shown in the second figure if its available?

With kind regards.

 

[Asha Devi]

Hi,

I found the discussion here very helpful : /forum/forums/topic/how-is-a-monochromatic-source-applied-in-time-domain-simulations-in-lumerical/. And, I have found the script in the following link: https://optics.ansys.com/hc/en-us/articles/360042192813. 

 

With kind regards.