what does dt, dw and w denotes and how it is calculated in split_step​_fourier_m​ethod.m file.

i am using * split_step_fourier_method.m* in my project of optical fiber. i am using code

i=sqrt(-1); Po=.00064; %input pwr in watts alpha=0; % Fiber loss value in dB/km alph=alpha/(4.343); %Ref page#55 eqn 2.5.3 Fiber optic Comm by GP Agrawal gamma=0.003; %fiber non linearity in /W/m to=125e-12; %initial pulse width in second C=-2; %Input chirp parameter for first calculation b2=-20e-27; %2nd order disp. (s2/m) Ld=(to^2)/(abs(b2)); %dispersion length in meter pi=3.1415926535; Ao=sqrt(Po); %Amplitude %---------------------------------------------------------- tau =- 4096e-12:1e-12: 4095e-12;% dt=t/to dt=1e-12; rel_error=1e-5; h=1000;% step size for ii=0.1:0.1:1.5 %the various fiber lengths can be varied and this vector can be changed z=ii*Ld; u=Ao*exp(-((1+i*(-C))/2)*(tau/to).^2);%page#47 G.P.AGrawal figure(1) plot(abs(u),'r'); title('Input Pulse'); xlabel('Time'); ylabel('Amplitude'); grid on; hold on; l=max(size(u)); %%%%%%%%%%%%%%%%%%%%%%% fwhm1=find(abs(u)>abs(max(u)/2)); fwhm1=length(fwhm1); dw=1/l/dt*2*pi; w=(-1*l/2:1:l/2-1)*dw; u=fftshift(u); w=fftshift(w); spectrum=fft(fftshift(u)); %Pulse spectrum for jj=h:h:z spectrum=spectrum.*exp(-alph*(h/2)+i*b2/2*w.^2*(h/2)) ; f=ifft(spectrum); f=f.*exp(i*gamma*((abs(f)).^2)*(h)); spectrum=fft(f); spectrum=spectrum.*exp(-alph*(h/2)+i*b2/2*w.^2*(h/2)) ; end f=ifft(spectrum); op_pulse(ln,:)=abs(f);%saving output pulse at all intervals fwhm=find(abs(f)>abs(max(f)/2)); fwhm=length(fwhm); ratio=fwhm/fwhm1; %PBR at every value pbratio(ln)=ratio;%saving PBR at every step size dd=atand((abs(imag(f)))/(abs(real(f)))); phadisp(ln)=dd;%saving pulse phase ln=ln+1; end toc; cputime=toc; figure(2); mesh(op_pulse(1:1:ln-1,:)); title('Pulse Evolution'); xlabel('Time'); ylabel('distance'); zlabel('amplitude'); figure(3) plot(pbratio(1:1:ln-1),'k'); xlabel('Number of steps'); ylabel('Pulse broadening ratio'); grid on; hold on; figure(5) plot(phadisp(1:1:ln-1),'k'); xlabel('distance travelled'); ylabel('phase change'); grid on; hold on;

i want to ask what does * 'dt'* and * 'dw'* and * 'w'* denotes? how does this is calculated???