help me to coding JSA and JSI and deltat

i write this code and i have some problems to about ploting joint spectral amplitude (i dont write plot functions). my alphaa value being 0 and i dont know whats the problem that i comes 0! i attached my figure that i must obtain. thanks for your helps.

clc;

clear;

close all;

%...................parameters...............

lambdaP = 1.55e-6; % pump wavelength

nelithiumP = 2.142; % refractive index lithium niobate for pump lambda

lambdaS = 0.775e-6 ; % signal wavelength

nelithiumS = 2.186; % refractive index lithium niobate for signal lambda

lambdaI = 0.775e-6 ; % idler wavelength

nelithiumI = 2.186; % refractive index lithium niobate for idler lambda

kP = 2*pi*(nelithiumP/lambdaP);

kS = 2*pi*(nelithiumS/lambdaS);

kI = 2*pi*(nelithiumI/lambdaI);

capital_lambda = 17.6e-6;

DeltaK = (kP-kS-kI-(2*pi/capital_lambda)) % phase matching condition

L = 2e-2;

%sigmaP=250e9;

deltat=50e-12;

sigmaP = deltat/(2*sqrt(log(2))); % pulse width

c = 3e8; % speed of light in vacuum

fS = c/lambdaS; fP = c/lambdaP; fI = c/lambdaI;

wS = 2*pi*fS; wP = 2*pi*fP; wI = 2*pi*fI; % angel frequncy

%alpha=exp(-(2*(pi^2)*(sigmaP^2))*((wP-(wS+wI))^2));

%b=wS+wI-wP

alphaa = exp(-((wS+wI-wP)/sigmaP)); % pump envelope function

phi = (sinc(Deltak.*L./2)).*exp(1i.*Deltak.*L./2); % phase matching function

f = alphaa*phi; % joint spectral amplitude

joint_spectral_intensity = abs((f).^2); % joint spectral intensity