What happens when you reflect a Laser beam back on itself?
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- Опубликовано: 11 июн 2024
- Episode 63
#laser
#electronicscreators
What happens when you reflect a Laser beam back on itself?
This unusual Laser system from a Particle Counter does exactly that!
Why?
how does it work?
Let's find out!
Sams Laser FAQ The PMS/REO External Resonator Particle Counter HeNe Laser
www.repairfaq.org/sam/laserhe...
External Passive Cavity Laser patent:
patents.google.com/patent/US4...
Previous teardown of the Laser Particle counter:
• LASER Particle Counter...
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0:00 Intro
0:12 Helium Neon Lasers!
0:53 Brewster Window Laser
1:46 Unusual Particle Counter Laser
2:44 Sam's Laser FAQ
4:02 Patent External Stabilized Passive Cavity
8:25 Laser Teardown
9:05 Optical Bench Setup
9:45 Laser Demo
12:43 Credits 
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This was fascinating! I recall playing with a HeNe laser in 8th grade science in about 1970. I "killed it" by reflecting the beam back into the output and was VERY relieved that it came back on after being power cycled. Now I'm going to have to go read more about HeNe lasers to learn exactly what I did! Thank you.
If sold to a school presumably the manufacturer ensured the failsafe because it's guaranteed some curious kid is going to try to send the beam back from where it came.
Haha! That sounds like some shit that I would have done. A real, "Oh crap..." moment.
I am guessing (and it is a guess) that the laser sends a startup voltage pulse for power on. Then maybe u shorted the steady-state by clearing the population inversion (so then drawing too much current) . Then I suppose an internal circuit -breaker triggered
@@soundsparklol
Wow. In the 90s we got to put baking soda in a paper volcano
This is a classic strategy we use to ensure our lasers are properly back-reflecting so that we can use it for alignment.
laserpointerforums has more answers..
I did something like this during a laser course at a local college back in 83-84. It was so much easier and quicker to achieve feedback than random chance of wiggling a mirror, hoping for a beam, and tightening those tiny screws back down hoping not to lose the beam. My system just spun a strip of white paper between the two lasers and aligning two dots on the paper as it interrupted the beam. Easy peasy. Its output was almost double its rated output and I was quite proud of myself. My instructor was more concerned about me frying it and had me knock the mirrors out of alignment just enough to bring it back down to its rating.
Doomsday device: Reflecting the power back into the tube amplifies the power with each pass as it passes through the external optical cavity. The energy rapidly climbs until it begins to cause the air to laze as an optically pumped UV nitrogen laser. Power continues to climb as both lasers bounce back and forth in a positive feedback loop until temperatures climb enough to begin to first cause chemical reactions in the air (ie N2+O2 -> NOx, etc), then as the power continues to increase, the air undertakes fusion, which if sustained further, causes fusion ignition across the entire atmosphere in a rippling wave of destruction as all the atmosphere fuses, destroying all life on earth. Then we have to wait for another event for life to begin again. Don't try this at home.
That was wicked!
For the nerdy, the air isnt dense enough to sustain. Same question at Las Alamos nuclear tests.
@@baloog8 you are right, that is in the Oppenheimer movie, but it is only barely unsustainable on its own, but if you have an exponentially increasing power source continually feeding it ..
Is there a way I could donate towards the building of this device?
Until your reflector/ equipment melts anyway
got to love those "bare" HeNe tubes glowing...
Have you ever had a fluorescent tube that didn't want to light so you'd slide your finger down it and it would light? Our lab's longest HeNe tube would do that too.
I worked with helium neon lasers at Intec back in the 1970s. We use them for automated inspection systems along with a scanning multifaceted mirror system and a photo multiplier tube. The whole thing was timed like a television scan line system and we could see a trace of the light levels across the width of the product web on an oscilloscope.
"Product web"?
Sounds nice.
- spider
Don't ever reflect a lazer beam unto itself, as it may open up an interdimensional portal allowing dogmen to enter into our reality. 🤔
Better dogmen than trumpists
But that also means there's a possibility that it may open an interdimensional portal allowing catgirls to enter our reality.
worth the risk
Now I want to...
Just for the novelty if nothing else.
and reveal your stupidity..
You never know how many furries have a laser...
Everyone when I walk in front of the projector dressed head to toe in retro reflective paint:
joe momma
Since it says that it forms a resonant cavity, doesn't that mean the distance needs to be calibrated to some small fraction of a wavelength, so that a standing wave pattern can be formed by the back-reflected beam? But that's like maybe 600 nm! Is this really made that precise?! WOW if it is.
theoretically, yes.
practically, no, because every minor fluctuation in optical length (from temperature, density of the air in between, ground shock, etc...) is much bigger than any misalignment of the mirror itself could cause. These fluctuations don't usually ever matter, because even a laser doesn't emit on exactly 1 frequency - it still has a bandwidth - and if the wave coming back into the laser medium doesn't resonate anymore, a close neighboring frequency will be amplified instead and a different mode will take over.
Ive been an electronics geek over 40 years. I have always had a love for lasers. Had my first HeNe tube and power supply kit from MPJA and All Electronics co when i was 8. I have owned dozens of lasers over the years from HeNe to Diode to Co2 to Nd:Yag never heard of or seen a speaker used like this. Very interesting.
Now you have new experiment material for a few more years! ;o)
A laser beam is LITERALLY FORMED by reflecting the beam back onto itself repeatedly.
But one is a half-glazed mirror, so the most-excited photons can escape.
@@throughthoroughthought8064A photon just is or is not. So, what do you mean with most-excited?
@@gbormann71 That was my assumption; guess someone more knowledgeable than us will have to answer that.
Don't cross the streams! -Ghostbusters
fascinating thank you. having stared through a Brewster window I have an appreciation of this cool “vintage” technology.
I use it for Ramen analysis too.
I use mine for ramen noodles, but waving it back and forth across the pot fast enough, long enough, to cook the noodles tears up my wrist!
Awesome. I have never heard of that before. So cool. Thank you!
always wondered about using a third mirror, now I know. Especially the need to vibrate mirror, etc.
thanks
3am and i have to sleep but this video peeks my curiousitu
Very cool. Super interesting use of a multi mode laser. You don't see that often.
For the diy test with the other HeNe, try adding a lens and put the flat mirror at the focal point to make a retro reflector that will match the divergence. An AR coated lens might be needed.
I will try this in the lab if I ever get some free time.
Of interest, a very similar principle is used in HeNe ring laser gyros to modulate them around their instability point.
I found your channel while researching how to de-bayer raspberry pi cameras, and I appreciate the hell out of your work. Catching up on your library now, but may I ask what you think is the most current way for a maker to record video in the UV spectrum? Does your video from 2 years ago have any potential updates? Or, hopefully, is there an easier/cheaper solution than burning the layer off myself?
However it turns out, thank you for getting me started!
Not sure I understand the purpose of the oscillator here - Does the cavity light come out modulated at the same frequency?
Maybe the OC on this tube has different reflectivity to a standard HeNe.
Wonder if this would work with yellow or green HeNe tubes.
The oscillator drive a piezo element with a mirror coating on it do decouple the external cavity. I have not measured the light output in the time domain, but I suspect there would be measurable ripple, but I also expect the laser linewidth would be broadened to some degree. There is a lot to investigate here, so example, what if the frequency is varied.
The OC will be difficult to measure in-situ, one thing of note, the radius of curvature on the OC is high enough to be noticeable by eye.
It will very possibly work with other tubes. I have a German made Green He-Ne and if you back reflect the beam into itself, it will also generate Red at 633.8nm at the same time. It would be interesting to see what happens to it.
Wow, this was fascinating! Thank you.
Thanks!
I learned about how lasers work in the 67 using rubies but never have understood doide lasers.
I used to have quite a collection of those old tubes. They were so fun to work with back in those days. Good times.
They are beautiful right?
Thats pretty neat, I have some fancy optics on my laser cutter to make sure light does not get bounced back into the laser cavity.
Yep, ordinarily reflecting laser beams back in on themselves is bad, but if done in a controlled fashion you can get interesting behaviour!
I remember when I was about 11, doing a phd on fire. I was sitting by the window on a cool autumn day with a reasonable magnifying glass burning paper with the suns rays. I then had the brilliant idea to try using mirrors. I reached a point where I decided to reflect the tightly focussed light back at the sun and the stupid child I was , I was overcome by fear that I might blow up the sun so I did it anyway. That day I truly understood reflected light and the energy of absorbed light.
6:33 *Could this be used to produced pulsed laser, somehow?*
The speaker has to be still at it's max & min extension (when it switches direction), so at that point I assume the laser could become aligned (light is VERY fast) & can pass through a mirror that is filtered to let through a specific frequency?
Or something like that?
Most interesting. Almost always something new that I've never seen in detail on here!
Thanks! There is allsorts of cool, but obscure and forgotten tech hidden in old journals and patents!
Kids today play with some stuff that was seriously expensive in my youth. It's a good thing though. Could you imagine ten year old me with a laser? Kids didn't get much supervision back then, and we were only partially domesticated. . .
I got my first He-Ne Surplus from a Laserdisc player, been hooked ever since. LOL partially domesticated :-D
Well considering I know that the eye doesn't dilate when a laser is pointed right into it, and that I know this without doing any online research, shows that it's probably good I didn't have access to more powerful lasers back then.
Military flashlights, lazers & fuelless plasma torches are quality tools.
LED's have a tricky history even.
Why does the DOD patients office not like civil engineers so much? The reasons technology advancement information is getting blacked out is not likely from any of our own.
The military,
They are so anti-tool advancement in the scientific community!
It's not the users that are lead,
It's the tools that are dangerous.
Is the answer NOT to patent any advancement not intended for commercial purpose? I know there might be still some risk of IP seizure or restraint, but isn't the main way tech is bottled up via patent applications?
There are more rare technologies that would require a paradigm shift.
Interesting... Now I have to take a few different solid-state lasers and see what they can do to themselves and each other. What can go wrong?
My days at M23A on the NRC campus in Ottawa during the early 90s was the peak of R&D in Canada. Laser laboratories to the max!
I was lucky enough to use a 30-watt Cu-vapour pumping a dye laser used to pump an OPO to produce an IR beam for semiconductor spectroscopy.
❤❤
You need a powerful laser for ramen analysis?! I just use my sense of taste 😜
Thanks Les. While studying I made some research on HeNe lasers in semiclassical theory, especially in population inversion in some situations. This partly falls in that range. I can not immediately sort out what is exactly happening hre without studying it in more detail. Hoever, this is interesting.
Thanks!!!
Good useful info!
Congrats on Ieee publishing your paper
Thanks and thanks! Hopefully I will get more published :-)
Very cool! I do wonder if the required oscillator frequency related to the linewidth of the comb of modes in the cavity.
If you'd use would the AOM you have as a frequency shifter with a stationary mirror, would also work?
I wonder if the power is consistent through the entirety of the beam path. I do wonder if anything like standing waves would be a thing here?
Fascinating! Makes me wa t to play with old HeNe laser again.
He-Ne's are beautiful!
Figured it was a Brewster window tube but this is way cooler!
He's crossing the streams.
Did someone say lasers?
"Lasers"
Welp, time to start another year long project
That particular speaker in the audio world would be called a "servo-driven speaker" as it appears in the patent to have a feedback coil in the speaker.
I see a lot of power in the passive chamber that is killed by everything done inside it while operating. How can useful power be extracted from this device, to do work after generation, especially continuous work? Have you tried splitting out half the power, for example, and does that shut down the boost, too?
If you wad the mirror on some rails and you could move it back and forward on the micron level could you move it so you get phase cancelation of the laser?
what would happen if there's no oscillator and the two regions are coupled?
Light never ceases to amaze me. It does so many strange things.
Watching my laser engraver work is pretty amazing. Like I can cut into stone with it to engrave images onto stone using nothing more then photons
Is DIY Raman spectroscopy in any way achievable, do you think?
That is really interesting thank you.
Thanks!
Enjoyed this. Thanks. I look forward to scoping your channel. Subscribed. Cheers
Thanks! Plenty of new stuff in the pipeline as well!
I noticed when i point a green laser at some orange colored things the spot changes color, but the reflection from the spot near by is still green. Why?
Hi Les: Very interesting. The modulator here is a bit like an "Active Q-Switch" used in high Q lasers
Yeah I dont fully understand what is going on here.
When he first described it at the beginning, my first thought was, "oh, that's a Q-switch". The YAG units I worked with were much more efficient and IIRC they consisted of a solid crystal driven by an RF signal. It was easy to get a peak power of a couple hundred kilowatts from a 100 watt resonator, although of course the average power couldn't be more than what it would do in CW.
I don't think it's like a Q switch. Perhaps in effect, but not in principle. I've never played with a HeNe laser, but know that Nd:YAG lasers have an envelope of frequencies they'll accept ( individual nanometers wide ). I think this works by increasing the level of excited photons by driving the gain medium with more than one wavelength via doppler shifting.
huh... thinking about it, what does the oscillator do really? and why do the frequency and distance to the output coupler matter?
If we vary these, can we get a cavity with peak power beyond the 1 watt range?
This is probably designed to produce the Maximum Q by the manufacturer, but there is a fair amount of experimentation that can be done here, including varying the frequency and perhaps even trying other mirrors as well.
The oscillator and piezo/speaker are constantly changing the external cavity length, the goal being to avoid feeding back too much energy into the HENE tube with a phase that would kill the lasing action. The frequency and amplitude fed to the piezo must be sufficient to give an adequate rate of change of cavity length to minimise the time spent with the external cavity feeding back energy into the laser tube with destructive phase.
Fourier Transformed IR and Dichroic mirrors give that a go and see what happens
In the 80s my dad worked for Coeherent General and we had these types of lasers laying around the house and I got to play with them all the time. I was the cool kid with a laser. At the time the company was using lasers for precision cutting ceramic tile layers for the space shuttle's heat shield. I wish I still had some of those failed samples but they were thin and broke very easily. I had no idea what I had at the time.
That sounds amazing! Yeah, same, there is lods of cool stuff that has passed through my hands, and now it's gone! As a consequence, these days I'm a bit of a hoarder!
Yay, a new video! 🤩
NGL, I still dont fully understand how this works to ramp up the power. I get the doppler shifting and all that but how is this thing working exactly?
completely noob with lasers here, is the output coupler a partial mirror?
Yes. Normally >90% reflectivity.
Are there any tools to measure (not lab quality) the laser output that isn't $1000+? I've only started messing with lasers for fun but curious to know the true power output of what I've been playing with, I only know how to measure the electrical current but that doesn't really mean much as far as energy output.
I would mount the cube dump on a Pizo Electric disk so it will be off-axis enough to block the cavity as you did with the card and sweep it through the audio spectrum so it will allow to Chirp the beam to pass, kinda like a poor-man AOM and see if it will kick out some huge power burst if you have a mind to investigate it, and maybe just use a cube beam splitter in there and dump part of it out. I would! but have not one of those laser tubes. :)
Not quite on topic but I have an IOT device that incorporates the mandatory LED power indicator. The LED is a typical red 0602 size surface mount type. What makes it peculiar is that when it its on it is surrounded by a mist of red dots (interference pattern, I guess) that extends to at least 5mm from the source in all directions. I've never seen this before and it is only very recently that I have come across another example of this effect. I'm wondering if anyone can explain what's going on.
I've always wondered about that in particular. Guess you have the answer. My original hint of having the question was knowing the output couplers on them are a ridiculously high value (99% reflectivity for example) so at say 5mW that should translate to something like 500mW or half a watt of optical power intracavity. (I did some bad math and forgot to convert from percent to decimal when doing the ratio division to give the scale factor. This has been corrected though.)
I've shone a cat toy LED laser into a mirror and all it did is made it dim down but it returns to normal brightness when removed from the mirror.
This is a whole field of research as far as sensing - optical feedback interferometry or self-mixing interferometry. Though not really done with gas lasers nowadays.
you need the Deflektor chiptune soundtrack fo this video
or in better lay-terms... the vibrating mirror creates a situation in which 99.9%+ of the power of the laser is sharing cavity with the active cavity but not at the same time... it is out of phase for all but a nearly infinitely small amount of time... creating a near infinite number of 5mw lasers that are out-of-phase with the main one that are regulated by angular loss.
Remember kids... light is neither a particle NOR a wave NOR DOES IT ACT LIKE EITHER... the thing that detects it is both vibrating and solid and thus those characteristics are imbued in its reaction.
I would love to see a video on building a Gamma Spectrometer. Using a photo diode to see individual photons of light flashes from a crystal that’s hit with gamma radiation is really cool.
What happens if you fire a laser beam into the inside of a super highly reflective mirrored sphere and then close flush the entry point? Would the beam be perpetual or eventually be lost as heat and if so how long could a beam theoretically last before doing so?
In the early 80's and I'm pretty sure it still applies, the collated beam would bounce from a 100% reflective mirror at one end of the laser, be it a CO2 gas tube or crystal rod like Nd YAG, to the partially reflective mirror at the other end as the intensity of the beam amplified before exiting the laser. Redirecting that beam back into the laser? Might result in additional amplification. Don't know. Never tried.
Basically they glued a mirror onto a speaker like I have done as a kid.
Whilst everyone should love all things laser and how they work, :-D, can we also see some more videos about applications? I haven't yet seen anyone build a spectrophotometric device on a budget. Hydroponic nutrient analysis would be a worthwhile place to start....
Was it my imagination, or were there, what looked like, interference patterns flicking through the beam? It wasn't random bits of dust, but more organized.
I remember being in 10th grade and some dude burned me with a laser it felt weird it wasnt like a fire burn lasers are awesome
Massive thanks to Sam Goldwasser and Don Klippstein for their contributions to hobby electronics. I remember using library computers & dialup internet in the early 2000s to save (to floppy disk), or print pertinent pages from their sites. I owe a lot of my understanding of lasers, optics, xenon strobes etc. to them!
Absolutely, I feel exactly the same. Giants of our time!
Couldn't you get the same beam amplification effect by shining a laser into a high finesse fabry-perot cavity? The coherence length would need to be pretty high, though
This is very interesting how this is setup.
I have a Small Yellow HeNe laser, and by reflecting the output back in on itself using another HeNe Mirror, I was able to cause the output to flip to Red 632.8nm..
It's still not clear to me how this works. Is the ~1W gain due to the arrangement “storing“ light in the passive space? i.e. the decoupled beam bounces back and forth, and builds up to 1W equilibrium after a short period of time?
Was wondering the same. In the primary cavity, the beam builds up optical power by repeatedly passing through the population inverted laser medium.
The beam can not pick up energy outside the primary cavity, because there is no laser medium.
But if the trick is that this secundary beam picks up energy if re-enters the primary cavity, it will still have to mode-compete with the primary beam. (partially deplete the population inversion)
The best part is to drop particles of diamonds into the laser and watch them float giving the tractor beam affect
Converting electrons into photons or emerging them out of the ether that's already there?
So does the light in the second cavity enter back into the first and cause a second lasing to happen at the same frequency which then "adds" onto the initial power? Like what happens at the atomic scale with the helium atoms? I understand that the electrical energy causes the electrons to jump up in energy level and then fall back to release a photon at that specific wavelength right? But how does the back reflected photons which are out of phase with the original beam cause more photon release inside the gas? Or is it just wave interference causing more brightness? How can constructive interference increase actual power in the second cavity? It's not free energy, so the second amplification must happen in the first cavity and have something to do with the already paid xcited gas molecules I'm guessing.
Can you figure out a way to transfer energy from one battery to another battery just by using a lazer beam?
You destabilize the cavity, resulting in mode hopping and other funky stuff.
One other thing I missed. I don't think Doppler shifting has as much to do with the external cavity gain as previously supposed though it might play a role. I think it might be a form of forced mode hoping so to speak. So you rapidly change the effective length of of the resonator the end result is a series of beat waves that are nearly full power with out it staying in that state for too long. It's like moving the slide on a whistle that is frequency stabilized at a stupidly high harmonic. So any gain will be alternating between full gain coupling and none. Hopefully someone will get around to testing that.
Interesting. Yeah, I plan to mount this on a rig and perform some measurements. I suspect line broadening must happen as well. I wonder if there are modern applications...
They are reflected back in on themselves, inside the device, that's how the laser amplification works 👌
(For those who didn't yet watch it)
I wonder what kind of lasers were used by the RAF during the late 70's and early 80's? I seem to remember Laser Rangefinders being standard fitment on the nose of many aircraft especially noticeable on the Harrier GR3 and Jaguar GR1. It was probably classified information in those days.
They were 10 Megawatt Pulsed YAG Lasers. The tech is declassified now, and they can be had occasionally from MOD surplus stores. I have one under the bench that will find its way into a video at some point!
Try other wavelenth mirrors with it, maybe a yellow one if you got it. Adjust so the focus coincides. Also try adding a AOM and see if you can cavity dumo the light. Also they used the same trick here with argon lasers for medical gas analyzers in the 1990s. ❤
It would be worth a shot. I'm interested to try other piezo elements as well.
One of my Philips laserdisc tubes has an external mirror glued in place on the output ! Looks like a repair of some kind, maybe to "boost" output ?
That's weird. Is it a corrective of focussing optic?
@LesLaboratory maybe ? I have four in total and its just the one that has it ! It's bonded on at a 45 degree angle at the output end !
Would not it affect the breaking of the cavity amplification Process if you interact with the beam inside! then how it's even possible to use that beam?
As happened here 11:36
This is what is special about this, this is an extracavity design, so a regular He-Ne laser with a third mirror. In a Brewster windowed tube, Lasing would cease entirely.
i'm surprised the "1 watt" is available outside the oscillating cavity. These lasers are extremely difficult to get to lase. I would expect any scheme to divert energy would make it stop lasing.
It's pretty weird. The catch is, you can't get the 1 Watt out of the cavity, it's just trapped in there. the moment you try to couple light out, you are back down to fractions of a percent of what is available in the cavity.
Wouldn't this be pumped phase conjugation
I thought your were going to say that reflecting a LASER beam back on to itself could be used as a columnator for a telescope, I have such a laser for that purpose. BTW: I have two old, previously functioning HeNe LASER tubes, I know one is from Spectra Physics and it was bright!! It had a rating of 6.9 mW on it, but I suspect it was much more than that. I have no use for them, plus a solid-state HV power supply that powers them. When the tube sputtered and went out, I'm told that someone with the right equipment can revive them. If you're interested in them we can work out how to get them to you, only the cost of shipping. I'm in the SF Bay Area. I am cleaning up my electronics lab and really hate to junk things that potentially someone else could use.
The whole point of a laser is to bounce light back and forth between two mirrors until it's coherent.
It is the decoupled cavity that is of interest here.
Lasers are cool 😎
I wish AS make video on same topic.
what "smoke" do you use?
Heated Glycerine, the same stuff that's in smoke machines. I used to use magican, but it makes an oily mess.
@@LesLaboratory I see, Im looking for an easy smoke or vapor source that can be used as a crude beamvisualizer at will and easy to store without to much degradation.
I saw some small almost carkey size devices that can make smoke that goes for around 50 bucks, but it does seem like its intended for magician, and if it makes an oily mess, that is far from ideal.
So im curious what other people in practise are using and the pro / cons of it..
Thx for another great vid Les.
10:26 Isn't a square wave just about the worst signal to use in this application, considering it spends a lot of time at a constant voltage (i.e. no movement of the mirror hence no doppler shift). It might be interesting to see if using a triangular wave makes a difference.
Likely the voltage swaps before the transformer is saturated. I believe switchmode power supplies feed the transformers with square waves too.
With a blue laser it burns them out. And or melts the lens then that contains the heat and burns it out.
thats the beginning of constructing a real lightsaber.. like a reverse power coupling!!!
So the frequency doesn't need to be 'tuned' at all as there's no adjustment if you did have a pot on there to tweak could it improved any or is it not overly important please ?.... Sensational colour even on my monitor must be awesome in the lab.........cheers !!
The only requirement seem to be that it's fast enough. There is no pot, but I could try it on a signal generator, and sweep it from 1Hz to 500kHz and see what it does 😉 I wonder if there is a modern use for this effect. He-Ne beams are always beautiful, fire nicer than any Laser Diode. Cheers!
@@LesLaboratory Yep:)
I used to troll thrift stores in the 80s looking for Philips LaserDisc players.....
In the 90's and early 2000's, those Lasers turned up at surplus stores real cheap, but you can hardly get them now, and when they show up they are $$$
Same. Edmund Scientific & Spectra Physics were my "dealers" for gas lasers. Good times.
First gen laser copier optics decks with HeNe tube and supply, scanning mirror, and AO modulator were cheap and plentiful on the surplus market in the early '90s.
From experience I can report: You'll have a high chance of converting a laser diode into an incandescent diode.
Fascinating
I am confused - where is the mirror? And what happens?