% 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 = [0.07,0.09]';
% wavelength axis around ZDW
lambda_nm = linspace(lambda0_nm-60, lambda0_nm+60, 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;
cols = cbrewer2('paired',4);
plot(lambda_nm, D_lin(1,:), '-', 'LineWidth',2, 'DisplayName','Linearized model','Color',[cols(1,:)]);
plot(lambda_nm, D_exact(1,:), 'LineWidth',2, 'DisplayName',sprintf('Exact model'),'Color',[cols(2,:)]);

plot(lambda_nm, D_lin(2,:), '-', 'LineWidth',2, 'HandleVisibility','off','Color',[cols(3,:)]);
plot(lambda_nm, D_exact(2,:), 'LineWidth',2, 'HandleVisibility','off','Color',[cols(4,:)]);


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