The fundamental lightwave carrier frequency F is locked by means of the F-2F beatnote. But if I understand it correctly, they normally stabilize the PRF first (which, again, is in the hundreds of MHz). Otherwise, the frequency separation between the second harmonic of F and the comb tooth at 2F will be noisier/driftier.
The business about the PRF determining the comb spacing comes straight out of Fourier. A train of narrow pulses in the time domain is a series of comb lines in the frequency domain, with spacing equal to the PRF. Lots of applications in traditional RF work for this, but femtosecond lasers made it relevant in the optical field as well.
To measure the absolute frequency of the light, as mentioned by cycomanic, one or more well-known quantum transitions is certain to be within range of any octave-bandwidth laser comb. Some of those transitions have line widths in the hundreds of hertz, which gives serious levels of precision if you're working with a THz or PHz carrier. Then you say goodbye to Mr. Fourier and hello to Mssrs Zeeman and Stark.
OK. It seems that the key is the femtosecond capability in the lasers. Even if the pulses are relatively far apart compared to the target frequencies for the comb, if those pulses are super crisp, that will drive energy into that frequency band?
Right. The risetime of the pulses determines how far out the useful comb lines extend (at least that's how it works in the RF world).
Being able to go from IR to UV in one comb spectrum was considered a very worthwhile advance when it happened. Now you can apparently buy a single box that does it, such as the one mentioned in https://news.ycombinator.com/item?id=42890578 . But you have to ask for the price, and usually I find that if I have to ask, I needn't have bothered. :-P
The business about the PRF determining the comb spacing comes straight out of Fourier. A train of narrow pulses in the time domain is a series of comb lines in the frequency domain, with spacing equal to the PRF. Lots of applications in traditional RF work for this, but femtosecond lasers made it relevant in the optical field as well.
To measure the absolute frequency of the light, as mentioned by cycomanic, one or more well-known quantum transitions is certain to be within range of any octave-bandwidth laser comb. Some of those transitions have line widths in the hundreds of hertz, which gives serious levels of precision if you're working with a THz or PHz carrier. Then you say goodbye to Mr. Fourier and hello to Mssrs Zeeman and Stark.