% plot_dispersion_models
% ------------------------------------------------------------
% Visualizes exact and linearized chromatic dispersion D(λ)
% around the zero-dispersion wavelength (ZDW).
%
% Inputs:
%   lambda0_nm - ZDW in nm
%   S0         - dispersion slope at ZDW [ps/(nm^2·km)]
% ------------------------------------------------------------

lambda0_nm = 1310;
S0 = 0.08;
% wavelength axis around ZDW
lambda_nm = linspace(lambda0_nm-40, lambda0_nm+40, 400);

% exact dispersion model
D_exact = (S0/4) .* ( ...
    lambda_nm - (lambda0_nm^4)./(lambda_nm.^3) );

% linearized model around ZDW
D_lin = S0 .* (lambda_nm - lambda0_nm);

% plot
figure('Color','w'); hold on;
plot(lambda_nm, D_exact, 'LineWidth',2, 'DisplayName','Exact model');
plot(lambda_nm, D_lin, '--', 'LineWidth',2, 'DisplayName','Linearized model');

xlabel('Wavelength  \lambda  [nm]');
ylabel('Dispersion  D(\lambda)  [ps/(nm·km)]');
title('Chromatic Dispersion Around the ZDW');
legend('Location','best');
grid on; box on;