7 ms·
Optical Frequency Combs
- packetlost 2y agoI 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 2y agoWhat did you do with them? Anything interesting you can share?
- tartoran 2y agoAside from frying a device? That one would be interesting to know how it happened.
- packetlost 2y agoNot 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).
- CamperBob2 2y agoFirst rule of Lab Club: It's always someone else who broke it. Second rule of Lab Club: see rule #1.
- packetlost 2y agoIt probably wasn't me ;)
- packetlost 2y agoWorked 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 https://patents.google.com/patent/US20220390902A1/en
- high_priest 2y agoNice 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 2y agoOnce 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.
- lutherqueen 2y ago> 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 2y agoMicrowave 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 2y agoAnd in between you have the terahertz gap, where we have no effective technology to emit or receive these frequencies.
- mitthrowaway2 2y agoSurely we can at least emit using blackbody radiation?
- frabert 2y agoI guess we can't effectively modulate it fast enough to make it practical for communication
- lindboe 2y ago"Effective" compared to the state of the art in the bands on either side: in RF/microwave we have very fast arbitrary waveform generators, very nice amplifiers, and well-characterized conductors, and on the optical side we have lasers, lights, fibers, and more. The terahertz gap is so named because it's too high in frequency for our usual RF devices to work well, and too low in frequency for our usual optical devices to work well; terahertz work ends up being a mix of both, taking from either column as needed for a specific application. (You might hear the word "quasi-optical" used in this sense, though I've never heard the dual word "quasi-microwave"!) We do have terahertz devices - they're just very limited compared to devices in adjacent bands, usually stated in terms of power. But there are a lot of hardworking and talented people working on narrowing the THz gap from all sides. It's a very very hot research area at the moment.
- dtgriscom 2y agoPerhaps 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?".
- mercurywells 2y agoRed light does go through non-vacuum faster than blue light. They're equal in a vacuum.
- kazinator 2y agoDon'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 2y agoThe 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 https://i.imgur.com/L3b7S8v.png -- although the same disclaimer obviously applies.)
- ziofill 2y agoThere is also another type of comb that is extremely useful: the GKP qubit (introduced in https://arxiv.org/abs/quant-ph/0008040 https://arxiv.org/abs/quant-ph/0008040). Its wavefunction is a comb.
- mikewarot 2y agoThis 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=11422 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=11...
- CamperBob2 2y agoI can't tell from the spec sheet at https://www.thorlabs.com/_sd.cfm?fileName=MENLO_SMART-COMB-S01.pdf&partNumber=SMART-COMB 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.
- dr_dshiv 2y agoAmazing! Check out the image of the methane leaks in a gas field.
- londons_explore 2y agoWhat 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?
- tzs 2y agoDoes 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 2y agoFrequency 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".
- simojo 2y agoI 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.