%% 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
f_null_2 = sqrt( c * 0.5 ./ (S0_si .* abs(Delta_lambda) .* lambda0.^2 .* L) );
L  = 2e3;       % m
f_null_10 = sqrt( c * 0.5 ./ (S0_si .* abs(Delta_lambda) .* lambda0.^2 .* L) );

cols = cbrewer2('Paired',10);
figure('Color','w');hold on
cnt = 2;
for L = 10%[2,5,10]
    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+2;
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-[5,35];
xlim([lim(2) lim(1)]);
ylim([40,130])