Add some older Theory projects from the past years

Functions/Theory/calcFWM/wavelength2frequency.m

@@ -0,0 +1,18 @@
+function frequency = wavelength2frequency(wavelength, inputUnit)
+% Convert wavelength to optical frequency.
+% Supported units: m, nm.
+
+c = physconst('LightSpeed');
+
+switch lower(inputUnit)
+    case 'm'
+        wavelength_m = wavelength;
+    case 'nm'
+        wavelength_m = wavelength .* 1e-9;
+    otherwise
+        error('Unsupported input unit "%s". Use "m" or "nm".', inputUnit);
+end
+
+frequency = c ./ wavelength_m;
+
+end