Hi Andreas,
If your source is very narrow band, does the shift reduce to zero?
I was having a similar problem:
http://www.mail-archive.com/meep-discuss@ab-initio.mit.edu/msg02073.html
Steven explained in a different thread why this is the case in meep:
http://www.mail-archive.com/meep-discuss@ab-initio.mit.edu/msg00456.html
His argument is that broadband simulations are only used for transfer
responses where you normalize results, so it doesn't matter if the pulse
has a shift (or isn't even really gaussian).
In your case you don't have normalization. You just set off a pulse in
your cavity and measure the resonances. If I understand correctly, the
only way around the problem would be to run several narrowband
simulations (this defeats the benefit of a time domain simulation).
I would much rather prefer it if the gaussian sources of meep behaved
the way one would expect them to, but this seems to cause some stability
issues.
Kind Regards,
Matt
On Sat, 27 Sep 2008, Andreas Francke wrote:
> Dear Steven and list,
>
> I'm simulating with Meep resonant cavities like add-drop ring and disk
> resonators and obtain the resonant spectrum with the flux function or
> harminv. Everything seems fine except that I noticed how the spectrum
> is always red or blue shifted relative to the source spectrum (i.e.,
> using a Gaussian input pulse, the Gaussian profile of the output
> resonant spectrum is clearly shifted). It looks like that the the
> bandwidth of the source determines the output resonant peak positions
> (while, except for fwidth df, all the code remains unchanged): larger
> source bandwidth returns the same (or negligibly altered) FSR but with
> the spectrum profile largely red-shift, even of dozens of nm on a 1500
> nm center source wavelength. The dimension of the cell seems to have a
> similar effect: in order to accomodate two cavities, doubling the cell
> size, leads to a double redshift. Moreover, I noticed that at some
> point when the refractive index is set beyond a threshold (say between
> 3.2 and 3.3 in my case) suddenly the spectrum is split, i.e. one sees
> two Gaussian spectra, one blue- the other red-shifted abruptly in an
> even more unpredictable manner. Working with higher resolution or
> simulation times won't change much the situation.
>
> I'm confused. I'm certainly missing something important here, but I
> can't give myself an explanation of this. Is it a numerical effect or
> is there some obvious physical explanation? Can anybody help?
>
> Andreas.
>
>
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