Strengthen WDM and IM/DD optical tests

Functions/Theory/Dispersion/power_fading_gif.m

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+%% ============================================================
+%  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)));