%% ============================================================
%  IM/DD Fading Notch – λ_null vs. Bandwidth (Fixed 10 km)
% ============================================================

clear; clc;

%% Fiber and dispersion parameters
lambda0 = 1310e-9;   % Zero-dispersion wavelength [m]
S0      = 0.09;       % Dispersion slope at ZDW [ps/(nm²·km)]
L       = 10e3;      % Fiber length [m]
c       = physconst('lightspeed');

%% Frequency sweep (defines the desired first-fading notch)
f_targets = linspace(40e9, 150e9, 200);   % [Hz]
f_GHz = f_targets / 1e9;

%% Compute wavelength λ_null for each target f_null
[lambda_vec, Dacc_vec] = lambda_for_first_null_full(f_targets, L, lambda0, S0);
lambda_nm = lambda_vec * 1e9;   % Convert to nm
Dacc = Dacc_vec;                % [ps/nm]

%% ------------------------------------------------------------
% Plot λ_null vs. f_null for 10 km fiber
% ------------------------------------------------------------
cols = cbrewer2('Paired',10);
figure('Color','w'); hold on;

hLine = plot(lambda_nm, f_GHz, ...
    'LineWidth', 2, ...
    'DisplayName', sprintf('L = %.1f km', L/1000), ...
    'Color', cols(2,:));

xlabel('Wavelength  λ [nm]');
ylabel('First fading notch  f_{null} [GHz]');
title('IM/DD Fading Notch Position vs. Wavelength');
grid on; box on;

lim = (lambda0.*1e9) - [8, 40];
xlim([lim(2) lim(1)]);
yticks([56,75,90,112]);

%% ------------------------------------------------------------
% Custom DataTip Template
% ------------------------------------------------------------
% Add accumulated dispersion value to the DataTip
hLine.DataTipTemplate.DataTipRows(1).Label = 'λ [nm]';
hLine.DataTipTemplate.DataTipRows(2).Label = 'f_{null} [GHz]';

% Create a new row for Dacc
dRow = dataTipTextRow('D_{acc} [ps/nm]', Dacc);
hLine.DataTipTemplate.DataTipRows(end+1) = dRow;

%% ------------------------------------------------------------
% Helper function: lambda_for_first_null_full
% ------------------------------------------------------------
function [lambda_vec, Dacc_vec] = lambda_for_first_null_full(f_target, L, lambda0, S0)
    c = physconst('lightspeed');
    S0_si = S0 * 1e3;   % ps/(nm²·km) -> s/(m³)

    lambda_min = 1260e-9;
    lambda_max = 1360e-9;

    f_target = f_target(:);
    N = numel(f_target);

    lambda_vec = zeros(N,1);
    Dacc_vec   = zeros(N,1);

    for k = 1:N
        RHS = c * 0.5 / (f_target(k)^2 * L);

        fun = @(lambda) -(S0_si/4).*(lambda - (lambda0^4)./(lambda.^3)).*lambda.^2 - RHS;

        try
            lambda_sol = fzero(fun, [lambda_min, lambda0 * 0.999]);
        catch
            lambda_sol = lambda_min;
        end

        lambda_sol = min(max(lambda_sol, lambda_min), lambda_max);
        lambda_vec(k) = lambda_sol;

        D_lambda = (S0_si/4) * (lambda_sol - (lambda0^4)/(lambda_sol^3)) / 1e-6; % ps/(nm·km)
        Dacc_val = D_lambda * (L/1000);                                          % ps/nm
        Dacc_val = min(max(Dacc_val, -100), 100);

        Dacc_vec(k) = Dacc_val;
    end
end