%% ------------------------------------------------------------
% Plot: Maximum usable IM/DD bandwidth vs wavelength
% ------------------------------------------------------------

% Fiber and dispersion parameters
lambda0 = 1310e-9;   % [m]
S0      = 0.08;      % [ps/(nm²·km)]
L       = 10000;      % [m]
c       = physconst('lightspeed');

% Wavelength range around ZDW
lambda_vec = linspace(1250e-9, 1350e-9, 200);  % [m]

% Compute D(lambda) using full model
lambda_nm  = lambda_vec * 1e9;
lambda0_nm = lambda0 * 1e9;
D_lambda   = (S0/4) .* (lambda_nm - (lambda0_nm.^4) ./ (lambda_nm.^3));  % [ps/(nm·km)]

% Convert D to [s/m²]
D_si = D_lambda * 1e-6;  

% Compute first null frequency (f₀) for each wavelength
f_null = sqrt(c*(0.5) ./ (abs(D_si).*lambda_vec.^2*L));  % [Hz]

% Plot
figure('Color','w');
plot(lambda_vec*1e9, f_null/1e9, 'LineWidth', 1.6);
grid on; box on;
xlabel('Wavelength [nm]');
ylabel('First Fading Null Frequency [GHz]');
title(sprintf('IM/DD Bandwidth Limit vs. Wavelength  (L = %.1f km)', L/1000));

% Highlight useful bandwidth thresholds
yline(25, '--', '25 GHz','Color',[0.4 0.4 0.4],'LabelHorizontalAlignment','left');
yline(50, '--', '50 GHz','Color',[0.2 0.6 0.2],'LabelHorizontalAlignment','left');
yline(100,'--', '100 GHz','Color',[0.6 0.2 0.2],'LabelHorizontalAlignment','left');

legend('First fading notch (f_{null})','Location','best');