I worked closely with some of these devices at a previous employer. Might've even fried one at some point. I'm pretty excited about what other things they can be used for!
Once optical comb sources become economically available off the shelf, they should be game-changers in several fields from spectroscopy to time/frequency work. Right now everything you can buy is still around 6 figures AFAIK.
> seamlessly connected to optical waves that oscillate at 10,000 times higher frequencies
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
Microwave communication goes comfortably into the tens of gigahertz range, and visible light is in the hundreds of terahertz. So it is about a factor of 10,000.
Perhaps I'm being pedantic, but in the video, when they show multi-spectral light coming in from the left, they show low-frequency light moving faster than high-frequency light. ("Survey says: EEEEEHHHHHNNNNNNK!")
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?".
Don't be too hard on the video or article. I just went through the Frequency_comb Wikipedia article and I'm still none the wiser.
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.
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.)
How the pulse rate of the laser determines the frequency between comb lines is unclear. Frequencies are in hundreds of THz; pulsing is way, way slower.
Plus don't you need the fundamental frequency of the comb to follow the radio-frequency references, not just the spacing.
What is clear is that this may indeed be beyond an optical comb appreciation video produced by the NIST for the general public.
I worked closely with some of these devices at a previous employer. Might've even fried one at some point. I'm pretty excited about what other things they can be used for!
Nice to see some more posts about light communications on HN. I have had the pleasure talking with people who developed this tech & see it in action.
Apparently, it is a big step towards purely optical network switching.
Once optical comb sources become economically available off the shelf, they should be game-changers in several fields from spectroscopy to time/frequency work. Right now everything you can buy is still around 6 figures AFAIK.
> seamlessly connected to optical waves that oscillate at 10,000 times higher frequencies
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
Microwave communication goes comfortably into the tens of gigahertz range, and visible light is in the hundreds of terahertz. So it is about a factor of 10,000.
And in between you have the terahertz gap, where we have no effective technology to emit or receive these frequencies.
But one giga to one tera is just 1000.
From 50 GHz to 500 THz we have 10,000.
Perhaps I'm being pedantic, but in the video, when they show multi-spectral light coming in from the left, they show low-frequency light moving faster than high-frequency light. ("Survey says: EEEEEHHHHHNNNNNNK!")
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?".
Don't be too hard on the video or article. I just went through the Frequency_comb Wikipedia article and I'm still none the wiser.
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.
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.)
How the pulse rate of the laser determines the frequency between comb lines is unclear. Frequencies are in hundreds of THz; pulsing is way, way slower.
Plus don't you need the fundamental frequency of the comb to follow the radio-frequency references, not just the spacing.
What is clear is that this may indeed be beyond an optical comb appreciation video produced by the NIST for the general public.
Red light does go through non-vacuum faster than blue light. They're equal in a vacuum.