Strengthen WDM and IM/DD optical tests

Functions/Theory/Dispersion/dispersion_first_notch_10km.m

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+%% ------------------------------------------------------------
+% 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');