Optical Frequency Combs
40 comments
·January 30, 2025mikewarot
CamperBob2
I can't tell from the spec sheet at https://www.thorlabs.com/_sd.cfm?fileName=MENLO_SMART-COMB-S... whether you have to configure the unit at build time to measure lasers near a given wavelength, or whether it really will measure any arbitrary laser you feed to it.
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.
high_priest
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.
CamperBob2
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.
tzs
Does calibration of an optical frequency comb require that the speed of light be known, either directly or indirectly? By indirectly I mean the case where something you need to use to calibrate your comb depends, directly or indirectly, on knowing the speed of light.
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.
fsh
Frequency combs are optical synthesizers. This means that the frequency of the nth comb mode is exactly n times a radio frequency (+ another radio frequency), where n is a (very large) integer. The speed of light does not matter at all. This is very important since the speed of light (and hence the wavelength) depends on environmental factors such as air pressure and humidity. One way to determine the speed of light (in air) would indeed be to measure the wavelength of a laser whose frequency is calibrated using a frequency comb.
The speed of light in vacuum cannot be measured in SI units since it is the fundamental constants that defines the unit "meter".
packetlost
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!
devmor
What did you do with them? Anything interesting you can share?
packetlost
Worked on atomic clocks, the frequency comb was used to downsample optical frequency ranges to a range that electronics can reasonably handle. A patent for the system can be found here: https://patents.google.com/patent/US20220390902A1/en
tartoran
Aside from frying a device? That one would be interesting to know how it happened.
packetlost
Not sure if it was me or someone else. There are a lot of things that can go wrong, but the thing that would've most likely been my fault is running the device without the seed laser on (or turning off the seed laser while operating).
lutherqueen
> 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
AdamH12113
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.
immibis
And in between you have the terahertz gap, where we have no effective technology to emit or receive these frequencies.
mitthrowaway2
Surely we can at least emit using blackbody radiation?
tliltocatl
Comparing CPU frequency (tens of thousands individual lanes that have to be skew-matched) to single-path stuff is quite apples to oranges. Processor frequency is mostly limited by interconnect and cooling (and silicon, but there are other materials available, it's just not worth switching to those unless you solve cooling). Even thunderbolt transceivers go 40GT/lane=20GHz and that's consumer tech. With InP you can go slightly above 100GHz afaik.
kazinator
But one giga to one tera is just 1000.
From 50 GHz to 500 THz we have 10,000.
ziofill
There is also another type of comb that is extremely useful: the GKP qubit (introduced in https://arxiv.org/abs/quant-ph/0008040). Its wavefunction is a comb.
simojo
I had the privilege to attend a talk by Jun Ye, one of Hall's previous advisees, a few months back about frequency combs. I really felt honored meeting the person who is so tapped into the work being done at JILA. Lots of amazing, mind bending work.
londons_explore
What is the theoretical efficiency of these devices?
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?
dr_dshiv
Amazing! Check out the image of the methane leaks in a gas field.
dtgriscom
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?".
kazinator
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.
CamperBob2
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.)
kazinator
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.
adrian_b
A frequency comb is nothing else but an oscillator that is not producing a continuous sinusoidal wave, but it is producing periodic pulses of sinusoidal waves, and which also satisfies the additional requirement that there must be a precise ratio between the frequency of the sinusoidal signal and the repetition frequency of the pulses.
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.
mercurywells
Red light does go through non-vacuum faster than blue light. They're equal in a vacuum.
GJim
> they show multi-spectral light coming in from the left, they show low-frequency light moving faster than high-frequency light ("Survey says: EEEEEHHHHHNNNNNNK!")
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.
This technology enables frequency counters for laser light.[1] Input a red laser, and you can directly measure its frequency in Hertz with 14 digits of precision.
[1] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=11...