# i copied it from the book i know the error is in the omega definition(line 12) but i dont know how to solve it please help

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abed on 3 Dec 2013
Commented: Afroja Akter on 12 May 2017
clear all;
distance=15;
beta2=1;
N=1;
mshape=input('m=0 for seech, m>0 for super-gaussian= ');
chirp0=0;
nt=1024;tmax=32;
stepnum=round(20*distance*N.^2);
deltaz=distance/stepnum;
dtau=(2*tmax)/nt;
tau=(-nt/2:nt/2-1)*dtau;
omega=(pi/tmax)*(0:nt/2-1) (-nt/2:-1);
if mshape==0
uu=sech(tau).*exp(-0.5i*chirp0*tau.^2);
else
uu=exp(-0.5*(1+1i*chirp0).*tau.^(2*mshape));
end
%%%%%
temp=fftshift(ifft(uu)).*(nt*dtau)/sqrt(2*pi);
figure;
subplot(2,1,1);
plot(tau,abs(uu).^2,'--k');hold on;
axis([-5 5 0 inf]);
xlabel('time');
ylabel('power');
title('input and output pulse shape and spectrum');
subplot(2,1,2);
plot(fftshift(omega)/(2*pi),abs(temp).^2,'--k');hold on;
axis([-.5 .5 0 inf]);
xlabel('frequency');
ylabel('power');
%%%%%
dispersion=exp(0.5*1i*beta2*omega.^2*delta2);
hhz=1i*N^2*deltaz;
%%%%%
temp=uu.*exp(abs(uu).^2.*hhz/2);
for n=1:stepnum
ftemp=ifft(temp).*dispersion;
uu=fft(f_temp);
temp=uu.*exp(abs(uu).^2.*hhz);
end
uu=temp.*exp(-abs(uu).^2.*hhz);
temp=fftshift(ifft(uu)).*(nt*dtau)/sqrt(2*pi);
subplot(2,1,1)
plot(tau,abs(uu).^2,'-k')
subplot(2,1,2)
plot(fftshift(omega)/(2*pi),abs(temp).^2,'-k')
Walter Roberson on 12 Jun 2016
anu chauhan,
You have put at least one |(| in the wrong place. You have somehow made fftshift into a reference to a variable instead of making it a call to the MATLAB fftshift function. Perhaps
plot(fftshift(omega/(2*pi)), abs(temp).^2, 'k')
Afroja Akter on 12 May 2017
Hi ,
I solve this problem in Matlab, but my problem is when i try to solve in Fortran95. Could anybody help me in fft loop portion solution in fortran.
Thanking Afroja

Wayne King on 7 Dec 2013
Edited: Wayne King on 7 Dec 2013
I'm assuming that you are missing brackets around the vector in omega
distance=15;
beta2 = 1;
delta2=1;
N=1;
mshape=input('m=0 for seech, m>0 for super-gaussian= ');
chirp0=0;
nt=1024;tmax=32;
stepnum=round(20*distance*N.^2);
deltaz=distance/stepnum;
dtau=(2*tmax)/nt;
tau=(-nt/2:nt/2-1)*dtau;
omega=(pi/tmax)*[(0:nt/2-1) (-nt/2:-1)];
if mshape==0
uu=sech(tau).*exp(-0.5i*chirp0*tau.^2);
else
uu=exp(-0.5*(1+1i*chirp0).*tau.^(2*mshape));
end
%%%%%
temp=fftshift(ifft(uu)).*(nt*dtau)/sqrt(2*pi);
figure;
subplot(2,1,1);
plot(tau,abs(uu).^2,'--k');hold on;
axis([-5 5 0 inf]);
xlabel('time');
ylabel('power');
title('input and output pulse shape and spectrum');
subplot(2,1,2);
plot(fftshift(omega)/(2*pi),abs(temp).^2,'--k');hold on;
axis([-.5 .5 0 inf]);
xlabel('frequency');
ylabel('power');
%%%%%
dispersion=exp(0.5*1i*beta2*omega.^2*delta2);
hhz=1i*N^2*deltaz;
%%%%%
temp=uu.*exp(abs(uu).^2.*hhz/2);
for n=1:stepnum
ftemp=ifft(temp).*dispersion;
uu=fft(ftemp);
temp=uu.*exp(abs(uu).^2.*hhz);
end
uu=temp.*exp(-abs(uu).^2.*hhz);
temp=fftshift(ifft(uu)).*(nt*dtau)/sqrt(2*pi);
subplot(2,1,1)
plot(tau,abs(uu).^2,'-k')
subplot(2,1,2)
plot(fftshift(omega)/(2*pi),abs(temp).^2,'-k')
However there are other problems, nowhere do you define/compute what delta2 is. I have just set it equal to 1, but I have no idea, you'll have to figure that part out.
Also, there is mistake where f_temp is confused with ftemp.