Add some older Theory projects from the past years

Functions/Theory/calcFWM/calcFwmEfficiency.m

@@ -0,0 +1,23 @@
+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