mainly tests (empty) and some theroy plots

power_fading_gif_lambda_variation.m

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+%% ============================================================
+%  IM/DD Power Fading Evolution vs Wavelength (L = 10 km)
+% ============================================================
+
+clear; close all; clc;
+
+%% Fixed fiber parameters
+lambda0 = 1310e-9;       % zero-dispersion wavelength [m]
+S0      = 0.09;          % dispersion slope [ps/(nm^2·km)]
+L       = 10e3;          % fiber length FIXED [m]
+c       = physconst('lightspeed');
+
+%% Frequency grid
+f_max = 150e9;
+f = linspace(0, f_max, 4000);  % [Hz]
+
+%% Figure setup (stable axes for clean GIF)
+fig = figure('Color','w');
+ax = axes(fig);
+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
+g = GifWriter('Name','power_fading_vs_wavelength', ...
+              'DelayTime',0.12, ...
+              'Parallel',false);
+
+%% Wavelength sweep (around ZDW)
+lambda_vec = linspace(1260e-9, 1360e-9, 25);  % 1260–1360 nm
+
+for k = 1:length(lambda_vec)
+
+    lambda = lambda_vec(k);
+
+    %% Dispersion for current wavelength
+    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
+
+    %% Power fading transfer function
+    phi = 2*pi^2 * b2 * f.^2 * L;
+    H   = abs(cos(phi));
+    HdB = 10*log10(max(H, 1e-12));
+
+    cla(ax)
+    plot(ax, f/1e9, HdB, 'LineWidth',1.8,'Color','black');
+
+    %% First-null frequency
+    if abs(D_si) > 0
+        f_null = sqrt(c*(0.5)/(abs(D_si)*lambda^2*L));
+        xline(ax, f_null/1e9, 'r--', 'LineWidth',1.2, ...
+            'Label', sprintf('f_{null}=%.1f GHz', f_null/1e9), ...
+            'LabelOrientation','horizontal', ...
+            'LabelVerticalAlignment','bottom');
+    end
+
+    title(ax, sprintf('Power Fading for: 10 km @ %.0f nm', lambda*1e9));
+
+    drawnow;
+    g.addFrame(fig);
+end
+
+%% Compile GIF
+g.compile(fig.Number);
+
+disp(fullfile(g.OutputDir, sprintf('%s_fig_%d.gif', g.Name, fig.Number)));