Changing the source bandwidth in FDTD and MODE

This page discusses how to control the bandwidth of the source pulse.

 By default, FDTD and MODE automatically calculate the source pulse shape and bandwith from the Start & Stop wavelength values specified by the user. A pulse is used EVEN when you want steady state, CW results at a single frequency. 

In FDTD, the 'Optimize for short pulse' option is enabled by default, meaning that a very short pulse is typically used. In MODE, this option is disabled by default, leading to a slightly longer pulse (although still quite short).

Optimize for short pulse: enabled

This is the default setting for FDTD, meaning a very short pulse will be used in most simulations.

Optimize for short pulse: disabled

This is the default setting for MODE, meaning a slightly longe pulse will be used in most simulations. Using a slightly longer pulse minimizes the amount of power injected at frequencies outside the range of interest, minimizing numerical stability problems.

For the majority of simulations, the default source pulse settings work well. However, in some special situations, you may need to modify the source pulse settings:

Very wide band pulse (ex. searching for resonant modes)

When studying resonators, the first task is generally to determine the resonant frequencies. A very wide band source can be used to determine all resonant frequencies from a single simulation. Typically the frequency range of interest is 0->fmax. This type of source is easily generated with the Set Frequency frame of the Frequency / Wavelength tab. Set the frequency start value to a small value(zero is not allowed). Set frequency stop to fmax.

 In the following example, the source range is 1-601 THz.  Notice the broad spectrum in the Spectrum vs frequency plot, and the very short source pulse in the Signal vs time plot.

Narrow band source (ex. some non-linear simulations, some divergence problems, etc)

In some some situations, it is necessary to create pulse with a narrow bandwidth, or to minimize the amount of power that is injected outside the specified wavelength range. In such cases, you can disabled the 'Optimize for short pulse' option. If that is not sufficient, you can select the 'set time domain' option and increase the Pulse length and Offset properties as much as needed. If these settings are increases significantly, you may need to increase the overall simulation time.