MATLAB：将信号FFT转换为频率，然后将IFFT转换回时域，不完全是第一个信号

Question

I am trying to reproduce the changes on a ~30 femtosecond laser pulse. I create and display this pulse in time just fine. Also, the fft of this pulse apperas fine on matlab (central frequency and width are very fine). But when I backtransform the transformed one using the ifft() function, the pulse is moved in time (posssibly due to phase change?? I don't know) and also the peak maxima are different.. What could be the cause of this? I am using ifft the wrong way?

The code I am using is this :

    atto=1e-18;
c = 299792458;
femto=1e-15;
lamda0=800e-9;
f_0=c/lamda0;
omega0=2*pi*c/lamda0;
T=2*pi/omega0;
a=2*log(2)/((36.32*femto)^2);
Fs=3/atto; %samplying rate
t=-200*femto:1/Fs:200*femto;
Efield=exp(-a.*(t-T).^2).*exp(1j.*omega0.*(t-T));

nfft=2^nextpow2(length(Efield));
Efieldfft=fft(Efield,nfft);
f=(0:nfft-1)*Fs/nfft;
omega=2*pi*f;
figure(1)
plot(t,Efield)
xlabel('s [fs]')
ylabel('Amplitude')

figure(2)
plot(omega,abs(Efieldfft))
xlim([2e15 2.8e15])
xlabel('omega [rad]')
ylabel('Amplitude')

figure(3)
plot(f,abs(Efieldfft))
xlim([3.3e14 4.1e14])
xlabel('frequency [Hz]')
ylabel('Power')

test=ifft(Efieldfft,length(t));

figure(4)
plot(t,test)
xlabel('s[fs]')
ylabel('amplitude')

Answer 1

That's because you are modifying the length of the FFT compared with the length of your time axis. To see this, replace

nfft=2^nextpow2(length(Efield));

by

nfft=length(Efield);

You'll find that figures 1 and 4 are now equal, up to numerical precision errors.