I am trying to take a Fourier transform of a femtosecond pulse train and plot the frequency spectrum.
I followed this question and am able to create a pulse train and plot both the time and frequency domain.
My question is, why do the pulses overlap in the time domain graph, since their width is on the order of femtoseconds, and the distance between them (T) is much larger?
Is it an issue with my time vector or am I interpreting the graph incorrectly?
%Using a Gaussian carrier envelope for the e-field, I show a comb of laser %frequencies defining frep, and its representation in freq space. The %sampling frequency is 10 times the freq (f=10*fs, fs=fr so fr=f/10). %Define variables fr=120e6; %rep rate, units: Hz fs=10*fr; %sampling frequency, units: Hz T=1/fs; %period, units: s (~fs) (sampling time) tau=100e-15; %proportional to pulse duration, units: s %tau=(pulse_duration)/sqrt(2*log(2)); wl=2*pi*fr; %carrier frequency, units: Hz ceo=0; %offset freq., can change later %Define electric field L=1500; %sets axis length t=(0:L-1)*T; %time, units: s Es=zeros(length(t)); for n=0:100 %n is the number of pulses (depends on L) for i=1:length(t) Es(n+1,i)=exp(-(t(i)-n*T)^2/tau^2)*exp(-1i*wl*t(i)); end end E=sum(Es,1); %Fourier Transform to frequency F=abs(fft(E)); %Fourier transform the vector Es FT=F(L/2:L-1); %Take out half due to symmetry FTn=FT/max(FT); %Normalized y-axis f=(1/L)*(L/2:L-1)*fs; %Frequency for plotting %Plots versus time and frequency figure(1) plot(t,real(E)) xlim([0 max(t)/2]) xlabel('Time (s)') ylim([-1.2 1.2]) ylabel('Amplitude') figure(2) plot(f,FTn,'k') xlabel('Frequency (Hz)') xlim([min(f) max(f)]) ylabel('Normalized Amplitude')