Regarding the spacing of the comb lines that's simply a fundamental property of the time and frequency domains. If you take the Fourier transform of a pulse train you will have comb of lines in the frequency domain where the spacing is the inverse of the pulse period. This depends on the size of the optical cavity. The carrier frequency oscillates underneath (at >100 THz frequency as you correctly point out). You can stabilise the offset frequency i.e. the position of the first line, by f-2f sabilization as described by the other poster. We still need to relate this to an absolute frequency and we do that using accurately known gas absorption lines. That way we have a comb of frequency lines at > 100 THz and more than an octave wide, where we know the absolute frequency with very high accuracy.