FFT: How to connect two waves?

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Sat, 11/17/2012 - 04:46 am

Hello,

I have a problem with the FFT. Neither existing topics in this forum nor the IGOR manual can help.
I must say that I am not an expert concerning IGOR!

I have to transform an interferogram into an IR-spectrum.

I measured an interferogram an loaded two waves: the optical path difference (already changed the wave scaling) on the bottom-axis and the intensity on the left-axis.
But I can only do a FFT with one wave - do I have to connect the two waves first? How can I do that?

Can you please explain to me, how I get the IR spectrum out of the two waves I loaded?

Thank you!
If I understand your description properly, you should first create a magnitude wave by taking the square root of the intensity wave. Be sure that your phase wave is in radians; if not convert degrees to radians.

Then make a polar (magnitude, phase) complex wave using
Duplicate/O AmplitudeWave, ComplexPolarWave
Redimension/C ComplexPolarWave
ComplexPolarWave = cmplx(AmplitudeWave, PhaseWave)
MatrixOP/C/O ComplexRectWave = p2rect(ComplexPolarWave) // convert to (x,y) complex

Then apply the FFT operation to ComplexRectWave using whatever options are appropriate (see the Help file). You will most likely want the Magsqr (/OUT=4) output option to view the IR spectrum.

November 17, 2012 at 06:48 am - Permalink

Thank you for your answer!

I made a FFT as you described, but it did not work. Probably the input-data is not suitable and has to be modified first. The only things I ever did with IGOR were quite simple, so I am a bit overstrained with IGOR itself.

Now I have an appointment with an expert at my university.

November 17, 2012 at 09:27 am - Permalink

Perhaps I misunderstood your description, and was over-complicating things. If your optical path difference wave has uniformly spaced increments, it may be serving as the axis for the interferogram intensity output. If this is indeed the case, then it may be just a matter of setting the scale of the "intensity" wave to match the OPD points:
Setscale/P x, OPDwave[0], OPDwave[1]-OPDwave[0],"units" IntensityWave

Then take the FFT of IntensityWave.
It will be most convenient if you use "units" for the IntensityWave scaling so that the FFT output will have your desired frequency units in the spectrum. You may also find it useful to first multiply the OPDwave by some scale factor, depending on what units the instrument is providing for the OPDwave.

November 18, 2012 at 06:42 am - Permalink