91 lines
2.8 KiB
Matlab
91 lines
2.8 KiB
Matlab
%% ============================================================
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% IM/DD Fading Notch – λ_null vs. Bandwidth (Fixed 10 km)
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% ============================================================
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%% Fiber and dispersion parameters
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lambda0 = 1315e-9; % Zero-dispersion wavelength [m]
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S0 = 0.08; % Dispersion slope at ZDW [ps/(nm²·km)]
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L = 10e3; % Fiber length [m]
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c = physconst('lightspeed');
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%% Frequency sweep (defines the desired first-fading notch)
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f_targets = linspace(40e9, 150e9, 200); % [Hz]
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f_GHz = f_targets / 1e9;
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%% Compute wavelength λ_null for each target f_null
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[lambda_vec, Dacc_vec] = lambda_for_first_null_full(f_targets, L, lambda0, S0);
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lambda_nm = lambda_vec * 1e9; % Convert to nm
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%% ------------------------------------------------------------
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% Plot λ_null vs. f_null for 10 km fiber
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% ------------------------------------------------------------
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% figure('Color','w');
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% plot(lambda_nm,f_GHz, 'LineWidth', 2);
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% grid on; box on;
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cols = cbrewer2('Paired',10);
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figure('Color','w');hold on
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plot(lambda_nm, f_GHz, 'LineWidth',2,'DisplayName',sprintf('%d km',L),'Color',cols(2,:));
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yticks([56,75,90,112])
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f_GHz = [56,75,90,112] * 1e9;
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[lambda_vec, Dacc_vec] = lambda_for_first_null_full(f_GHz, L, lambda0, S0);
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lambda_nm = lambda_vec * 1e9; % Convert to nm
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xticks(round(lambda_nm))
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xlabel('$\Delta \lambda$ from ZDW [nm]');
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ylabel('$F_{null}$ [GHz]');
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grid on; box on;
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lim=(lambda0.*1e9)-[8,40];
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xlim([lim(2) lim(1)]);
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% ylim([40,130])
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%% ------------------------------------------------------------
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% Helper function: lambda_for_first_null_full
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% Stable, single-branch, clamped to O-band
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% ------------------------------------------------------------
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function [lambda_vec, Dacc_vec] = lambda_for_first_null_full(f_target, L, lambda0, S0)
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c = physconst('lightspeed');
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S0_si = S0 * 1e3; % ps/(nm²·km) -> s/(m³)
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% Define O-band boundaries [m]
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lambda_min = 1260e-9;
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lambda_max = 1360e-9;
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f_target = f_target(:);
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N = numel(f_target);
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lambda_vec = zeros(N,1);
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Dacc_vec = zeros(N,1);
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for k = 1:N
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RHS = c * 0.5 / (f_target(k)^2 * L);
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% Normal-dispersion branch (λ < λ0)
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fun = @(lambda) -(S0_si/4).*(lambda - (lambda0^4)./(lambda.^3)).*lambda.^2 - RHS;
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% Solve within normal-dispersion range
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try
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lambda_sol = fzero(fun, [lambda_min, lambda0 * 0.999]);
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catch
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lambda_sol = lambda_min;
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end
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% Clamp to O-band
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lambda_sol = min(max(lambda_sol, lambda_min), lambda_max);
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lambda_vec(k) = lambda_sol;
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% Compute D(lambda) and accumulated dispersion
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D_lambda = (S0_si/4) * (lambda_sol - (lambda0^4)/(lambda_sol^3)) / 1e-6; % ps/(nm·km)
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Dacc_val = D_lambda * (L/1000); % ps/nm
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Dacc_val = min(max(Dacc_val, -100), 100);
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Dacc_vec(k) = Dacc_val;
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end
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end
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