Dispersion theory scripts

Functions/Theory/power_fading.m

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+%% ============================================================
+%  Minimal IM/DD Power Fading Plot
+% ============================================================
+
+
+%% Fiber and system parameters
+lambda0 = 1310e-9;       % zero-dispersion wavelength [m]
+lambda  = 1275e-9;       % operating wavelength [m]
+S0      = 0.08;          % dispersion slope [ps/(nm²·km)]
+L       = 10e3;          % fiber length [m]
+c       = physconst('lightspeed');
+
+%% Derived quantities
+S0_si = S0 * 1e3;                                    % → s/m³
+D_lambda = (S0/4) * (lambda*1e9 - (lambda0*1e9)^4/(lambda*1e9)^3); % ps/(nm·km)
+D_si = D_lambda * 1e-6;                              % → s/m²
+b2   = -D_si * lambda^2 / (2*pi*c);                  % s²/m
+
+%% Frequency grid
+f_max = 150e9;
+f = linspace(0, f_max, 4000);                        % [Hz]
+
+%% IM/DD transfer function (power fading)
+phi = 2*pi^2 * b2 * f.^2 * L;
+H = abs(cos(phi));
+
+%% Plot
+figure('Color','w');
+plot(f/1e9, 10*log10(H), 'LineWidth', 1.8);
+grid on; box on;
+xlabel('Frequency [GHz]');
+ylabel('Magnitude [dB]');
+title(sprintf('IM/DD Power Fading |H| for λ = %.1f nm, L = %.1f km', lambda*1e9, L/1000));
+ylim([-30 0]);
+
+%% Mark analytic first-null frequency
+f_null = sqrt(c*(0.5)/(abs(D_si)*lambda^2*L));
+xline(f_null/1e9, 'r--', 'LineWidth', 1.2, ...
+       'Label', sprintf('f_{null}=%.1f GHz', f_null/1e9), ...
+       'LabelOrientation', 'horizontal', 'LabelVerticalAlignment', 'bottom');