Strengthen WDM and IM/DD optical tests

Functions/Theory/Dispersion/dispersion_wavelength_notch.m

@@ -0,0 +1,32 @@
+%% Dependency f_null vs Delta_lambda
+lambda0 = 1310e-9;
+S0 = 0.09;       % ps/(nm²·km)
+L  = 10e3;       % m
+c  = physconst('lightspeed');
+
+% Convert slope to SI
+S0_si = S0 * 1e3;   % s/m³
+
+Delta_lambda = linspace(5e-9, 80e-9, 300); % [m] detuning
+
+cols =    [0.3467    0.5360    0.6907;...
+   0.9153    0.2816    0.2878;...
+   0.4416    0.7490    0.4322];
+figure('Color','w');hold on
+cnt = 1;
+for L = [2,10,40]
+    f_null_10 = sqrt( c * 0.5 ./ (S0_si .* abs(Delta_lambda) .* lambda0.^2 .* L*1e3) );
+    plot(1310-Delta_lambda*1e9, f_null_10/1e9, 'LineWidth',2,'DisplayName',sprintf('%d km',L),'Color',cols(cnt,:));
+    cnt = cnt+1;
+end
+% yticks([56,75,90,112])
+% tickse = 1310-[7.5, 12, 17, 31.5];
+% xticks(flip(tickse));
+
+xlabel('$\Delta \lambda$ from ZDW [nm]');
+ylabel('$F_{null}$  [GHz]');
+grid on; box on;
+lim=1310-[5,60];
+xlim([lim(2) lim(1)]);
+ylim([10,130])
+legend