When you have a normalization run and a scatter run, the pulse is
shifted in both simulations. The peak is shifted in both. It doesn't
matter where the peak is because at each frequency you're only looking
at the power/fields relative to the normalization run. This comparison
will be correct no matter where the peak is.
For a single run simulation however, the location of the peak is
important.
I think what you could do is run an empty simulation with no resonator
in order to determine how large the shift is. You could then correct
your results by this shift. It's a nonideal solution because it wastes
a simulation. Be careful though - I'm not sure if the pulse gets
shifted in frequency or if it is actually _stretched_ in frequency.
Maybe someone who knows more could weigh in.
Best,
Matt
On Sun, 28 Sep 2008, Andreas Francke wrote:
Hi Matt,
thanks for your answer. Let me answer personally since apparently I'm
the only one who does not get the point here.
You are right, indeed I didn't use the normalization suggested in the
manual (divide by the incident power at each frequency to get the
transmission spectrum, once with only the incident wave and no
scattering structure, and once with the scattering structure, where
the first calculation is used for normalization), but compared the
flux between an input- and an output-port in the same structure during
a single run. I understand that this is possibly a less precise
determination of the power transmission, but it works to a certain
degree. Anyway, I don't see why the normalization suggested in the
manual should also correct for the wavelength shift? As I understand
it corrects only over the power (of the resonance peaks, in my case).
Whereas I need to simulate as realistically as possible not only the
transmitted power but also the precise resonance positions (over a
bandwidth of 0.1 centered on a normalized frequency 0.65, i.e. about
100 nm centered on 1550 nm). Therefore, true is that the shape of the
spectrum doesn't matter for transmitted power, but the resonances will
be shifted nevertheless. Or not?
Best, Andreas.
--- Sab 27/9/08, matt [EMAIL PROTECTED] ha scritto:
Da: matt [EMAIL PROTECTED]
Oggetto: Re: [Meep-discuss] Cause of wavelength shifts in Meep?
A: Andreas Francke [EMAIL PROTECTED]
Cc: meep-discuss@ab-initio.mit.edu
Data: Sabato 27 settembre 2008, 17:47
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
