I was also hopeful the video would have actual info on how they work, but no such luck. Just a lot of "Are they cool, or what?".
Well, I did get an idea about what the thing actually is: basically a signal consisting of a mixture of frequencies, precisely spaced. Techniques to generate some of the bands include nonlinear mixing. Turns out, light can undergo distortion, so you can get intermodulation distortion to generate colors not present in the inputs.
The unclear part is the details of how the frequency comb is hooked together with the radio frequency domain in a feedback loop to control the comb. I.e. where in the RF domain we have the precise frequency reference we'd like to convey to the optical domain.
As I understand it, two effects are involved. One is the laser's pulse repetition rate that determines the frequency spacing between adjacent comb lines. This is on the order of hundreds of MHz, so it can be measured with a photodiode detector and phase-locked like any other RF signal.
The other effect is the carrier (light) phase shift that occurs from one pulse to the next. Assuming the pulse rate has been stabilized, nulling out this carrier phase shift is equivalent to stabilizing the laser's frequency. The photodiode can't see the carrier cycles, of course, but if the comb spans at least one octave in frequency, there will be a detectable beatnote between the second harmonic of the fundamental F (which like you say is always present to some extent given various nonlinearities in any real-world system) and the comb line at the beginning of the next octave. Driving this difference frequency to zero stabilizes the actual lightwave carrier.
As far as stabilizing the signal from the photodiode is concerned, that's just a matter of mixing it with a signal from the desired frequency standard to get the difference frequency that you steer to zero by tuning the laser. Some systems care about locking at a specific phase, others are OK with just getting the frequency right.
Disclaimer: treat the above with healthy skepticism, as IANAPhysicist and have never actually had my hands on this sort of hardware. Corrections actively solicited.
(Edit: Actually I like o1-pro's explanation better than mine: https://i.imgur.com/L3b7S8v.png -- although the same disclaimer obviously applies.)
It is simple to make a pulsed oscillator, but making one where the pulse frequency and the sinusoidal frequency maintain a fixed ratio is not at all simple, especially when the frequency ratio is very large, like what is needed when the pulse frequency must be low enough to drive a digital counter and the sinusoidal frequency must be in the optical range, up to ultraviolet light.
If you have such an oscillator, by tuning the low frequency you can obtain light with an accurately known frequency. Alternatively, by tuning the high frequency to match light with a known frequency, e.g. produced by an optical atomic clock, you can obtain a pulse train with a known frequency, which may be used as a reference frequency, e.g. for a digital clock.
Has Sir never seen a rainbow?
May I kindly refer Sir, to Isaac Newton's prism experiment, as lovingly depicted on the cover of Pink Floyd's 'Dark Side of the Moon' album.
Somehow four orders of magnitude sound too less for the transition from radio to light, but it makes sense. A i9 processor works at ~6 GHz, and light is at the THz range
From 50 GHz to 500 THz we have 10,000.
[1] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=11...
Under "Measurement Wavelength" it says "Choose one in the 630-2000 nm range," and often that type of specification means you have to specify the nominal frequency up front when you order it. Either way, it seems to be a big step forward in commercializing this stuff. The comb hardware I've seen takes up a good chunk of an entire room.
Apparently, it is a big step towards purely optical network switching.
I'm curious because of something the professor did early on in the introduction to optics class I took in college. He picked up a metal ruler and said we were going to measure the speed of light. Everyone laughed (which was fine because he intended it as a joke).
He then set the ruler on a table, directed a laser to reflect at a shallow angle off the ruler onto the blackboard.
The ruler's lines were raised which made it act like a diffraction grating and there was a visible interference pattern on the blackboard.
He then traced the pattern on the blackboard with chalk, turned off the laser, and used the ruler to (1) measure the distance from where it had been to the blackboard, and (2) the spacing of the lines in the diffraction pattern.
From this and the known frequency of the laser and the known spacing of the lines on the ruler the speed of light is an easy calculation.
This was meant as a joke because usually the frequency of light is calculated using methods that depend on knowing the speed of light, so all that was really happening was the he used a rule to very that the frequency calculation of the laser had been done correctly.
But if you could accurately get the frequency without that depending on knowing the speed of light then you could actually measure the speed of light with a ruler.
The speed of light in vacuum cannot be measured in SI units since it is the fundamental constants that defines the unit "meter".
Could it be used for example to combine multiple different frequencies of light into one higher frequency to excite a solar cell at exactly the bandgap energy so no energy is wasted?
