fftk - Script command – Ansys Optics

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Returns the spatial wavevector kx associated with the Fourier transform of a function of x.

$$ k=\text{fftk}(x)=\frac{2\pi}{dx.M}[0,...,(M-1)] $$

where M=length(x).

fftk and all related functions have an option (option 1 below) that controls the format used to store the frequency domain data. When working with spectral data it is not possible to switch between formats; there are no functions to convert between formats. This implies that if you use option1=n to produce a spectrum with fft, then you must also use option1=n if you want to pass that same spectral data to invfft. Similarly, if you use option1=n for fft, then you also need to use option1=n with fftw to get the proper frequency vector corresponding to your spectrum. invfft and fftk work in the same way.

Syntax

Description

out = fftk(x);

Returns the spatial wavevector kx associated with a fourier transform of a function of x..

fftk(x,option1,option2);

option1

1 : the standard FFT (zero frequency is at the first element of the matrix). This is the default option.

2 : zero frequency is the first element, but frequencies above the Nyquist frequency are removed.

3 : the FFT is shifted so zero frequency is the central element of the spectrum (more precisely, this means the zero frequency point is at element floor(N/2 + 1), where N is the number of samples). Both positive and negative frequencies are seen

option2

0: no zero padding.
1: zero padding up to the next power of 2 longer than the length of Ex (default).
N: zero pad up to length N if N > length(t). If N <= length(t), it will zero pad up to the next power of 2 longer than the length of t. For the fastest results, N should be a power of 2 and can be entered, for example, as 2^12.

Example

If Ex is a 2D matrix of spatial field values where Ex contains the field values along the axis vectors x and y, then the following code will image the field and the fourier transform of Ex.

image(x,y,Ex);
Ix = abs( fft(Ex) )^2; 
kx = fftk(x); 
ky = fftk(y); 
image(kx,ky,Ix);

See Also

List of commands , fft , fftw , invfft

fftk - Script command

Associated files

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