function [eta, deltaBeta] = calcFwmEfficiency(f_i, f_j, f_k, f_0, Ds, alphaDbPerKm, Lkm)
% FWM efficiency including phase mismatch and attenuation.
% alphaDbPerKm is the power attenuation in dB/km, Lkm is the fiber length in km.

deltaBeta = calcPhaseMatching(f_i, f_j, f_k, f_0, Ds);

alphaNpPerM = alphaDbPerKm .* log(10) ./ 10 ./ 1e3;
Lm = Lkm .* 1e3;

denominator = alphaNpPerM.^2 + deltaBeta.^2;
term1 = alphaNpPerM.^2 ./ denominator;

loss_term = 1 - exp(-alphaNpPerM .* Lm);

if abs(alphaNpPerM) < eps
    term2 = 4 .* sin(deltaBeta .* Lm ./ 2).^2 ./ max((alphaNpPerM .* Lm).^2, eps);
else
    term2 = 1 + 4 .* exp(-alphaNpPerM .* Lm) .* sin(deltaBeta .* Lm ./ 2).^2 ./ (loss_term.^2);
end

eta = term1 .* term2;

end