%% ============================================================
%  IM/DD Power Fading Evolution GIF (1 km -> 20 km)
%  Uses the provided GifWriter (serial mode)
% ============================================================

clear; close all; clc;

%% Fiber and system parameters
lambda0 = 1310e-9;       % zero-dispersion wavelength [m]
lambda  = 1275e-9;       % operating wavelength [m]
S0      = 0.09;          % dispersion slope [ps/(nm^2·km)]
c       = physconst('lightspeed');

%% Derived quantities (length-independent)
D_lambda = (S0/4) * (lambda*1e9 - (lambda0*1e9)^4/(lambda*1e9)^3); % ps/(nm·km)
D_si     = D_lambda * 1e-6;                                        % s/m^2
b2       = -D_si * lambda^2 / (2*pi*c);                            % s^2/m

%% Frequency grid
f_max = 150e9;
f = linspace(0, f_max, 4000);  % [Hz]

%% Figure setup (keep it stable for nicer GIFs)
fig = figure('Color','w');
ax = axes(fig); %#ok<LAXES>
hold(ax,'on'); grid(ax,'on'); box(ax,'on');
xlabel(ax,'Frequency [GHz]');
ylabel(ax,'Magnitude [dB]');
ylim(ax,[-30 0]);
xlim(ax,[0 f_max/1e9]);

%% GIF writer (serial mode; simplest)
g = GifWriter('Name','power_fading_evolution', 'DelayTime',0.12, 'Parallel',false);

%% Loop: 1 km to 20 km
L = [1:20,19:-1:1];
for L_km = L
    L_meter = L_km * 1e3; % [m]

    % IM/DD transfer function (power fading)
    phi = 2*pi^2 * b2 * f.^2 * L_meter;
    H   = abs(cos(phi));
    HdB = 10*log10(max(H, 1e-12)); % avoid -Inf for deep notches

    % Clear and redraw (stable axes)
    cla(ax);

    plot(ax, f/1e9, HdB, 'LineWidth', 1.8, 'Color','black');

    % Analytic first-null frequency marker
    f_null = sqrt(c*(0.5)/(abs(D_si)*lambda^2*L_meter));
    xline(ax, f_null/1e9, 'r--', 'LineWidth', 1.2, ...
        'Label', sprintf('f_{null}=%.1f GHz', f_null/1e9), ...
        'LabelOrientation','horizontal', ...
        'LabelVerticalAlignment','bottom');

    title(ax, sprintf('Power Fading for: %.0f km @ 1275 nm', L_km));

    drawnow;

    % Add frame to GIF
    g.addFrame(fig);
end

%% Done
g.compile(fig.Number);

disp(fullfile(g.OutputDir, sprintf('%s_fig_%d.gif', g.Name, fig.Number)));