Meaning of the frequencies vector using fft
You forget that what you did was describing the passband signals (i.e. some waveform on a carrier frequency == on a wavelength) in baseband.
So, of course, after calculating the baseband spectrum, you need to shift it back where it came from - to the carrier frequency. It's as easy as that.
If this isn't clear to you, you might want to revisit what (complex equivalent) baseband and passband are.