Hi, I have a question about the way Meep treats Gaussian pulses. Suppose you specify the parameter fwidth to be df. Then, the bounds of the pulse in frequency space will be -df/2 and +df/2. In the time domain, the width of the pulse is characterized by 1/fwidth. And this width is not the actual FWHM of the pulse in time, but is proportional to it.
The thing that confuses me is the parameter cutoff. As described in Meep Reference, cutoff determines the time t0 at which peak of the pulse occurs: t0 = tstart + cutoff * width. Therefore, increasing cutoff increases the duration of the pulse in time. Shouldn't this affect fwidth? Why is fwidth independent of cutoff? My attempt at answering this: Cutoff only controls how sharply the pulse in time is turned on/off at its tails, and does not affect its width. The smaller the cutoff, the sharper the pulse's edges are in time. And I am guessing that describing these sharp edges in frequency space requires higher frequency Fourier components. Meep Reference states that increasing cutoff decreases the amount of high frequencies introduced by the start/stop of the source. "Introduced" is still unclear to me as to how it concretely affects a simulation. Maybe these high frequency components are responsible for the wiggles you would usually see in a power spectrum near the edges of the bandwidth (the df)? Thank you, -Simon
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