How lasers work

Simplified explanation of laser physics principles: atomic energy levels, spontaneous and stimulated emission, gain, three- and four-level schemes. Also explained are various types of laser: ruby, Neodymium, Helium-Neon, Semiconductor/Diode, Capillary Discharge and Free Electron Lasers.
Contents
00:00 - Introduction
02:04 - Atomic processes
04:54 - Laser gain
06:43 - CW and Q-switching
09:14 - Population inversion
13:12 - Ruby, Neodymium
14:24 - HeNe
15:00 - Diode lasers
17:00 - Unconventional
18:27 - Free Electron
19:47 - LWI
20:33 - Summary
References
[1] “Raman spectroscopy has shed light on the structure of difficult to analyse sulphur-rich polymers”, Central Laser Facility
www.clf.stfc.a...
[2] 3D animation of the Rutherford atom (User: Damek)
commons.wikime...
[3] Y. Diouf et al., “Numerical Study of Density Functional Theory of Multi-electronic Atoms: Case of Carbon and Helium”, American Journal of Nanomaterials 9, 12 (2021).
[4] Xenon Flashtube by Nathan Boor (User: NapoleonoftheNow)
en.wikipedia.o...
[5] S. Hooker and C. E. Webb, “Laser Physics”, (OUP 2010). ISBN: 978-0198506928
[6] Black Marble 2016, NASA's Goddard Space Flight Center
svs.gsfc.nasa....
[7] 3D cut of the LHC dipole, CERN, OPEN-PHO-ACCEL-2014-003-8
home.cern/reso...
[8] L. Yuan et al., “Transient lasing without inversion via forbidden and virtual transitions”, Physical Review A 89, 013814 (2014).

[@MyJusticeDiary]

Glad you're back at videos. One the the hidden gem physics channels

[@jeffstaples347]

Acollierastro in there hah

[@ARandomTroll]

You have an uncanny ability to explain very advanced concepts from the ground up without any egregious omissions or simplifications. This is probably the best explanation of lasers short of a university course. I kind of wish you mentioned stellasers tough. Such a powerful and surprisingly straight-forward concept.

[@ImprobableMatter]

I want to look at them more scientifically first, since I study the solar corona. This video is a good basis to build on with regards to stellasers, though.

[@joegonzalez6241]

I think if you kinda already know what he is talking about. It is easy to understand with his explanation

[@jeffstaples347]

Daaang I was just re-watching your fusion series last night, urging more vids. Really enjoy your realism on all these things. Thanks!

[@Profesor449]

Damn, video was over way too soon. Truly riveting content and the fastest 20 minutes I spend this week. People from lab next door use lasers in Laser doppler anemometry and once gave me a tldr of how their lasers are used and it seemed fascinating. This does too, are you planning to shine more light on lasers in the future?

[@ImprobableMatter]

Yes, with this video I wanted to lay the groundwork for any future discussion.

[@jeffstaples347]

The only physics I was good at was optics in my undergrad. Love lasers and what backyard scientists and pyros of the styro nature get up to with them haha

[@GeoffryGifari]

Laser doppler anemometry? in what ways can it be applied?

[@Profesor449]

@@GeoffryGifari they work with sprays - so there it is probably used to measure velocity of droplets. I'm sure it's more complicated than that, but my memory sux xD

[@GeoffryGifari]

@@Profesor449one droplet at a time? pretty cool

[@vipondiu]

It took me a while to reach this basic level of understanding of how lasing works, I would have understood it much better and quicker with this video, and in just 20 minutes! Great job! The salsa background music was also a random phenomena that was totally unexpected

[@robertkoen5506]

Thank you very much. Gaining a better idea of how lasing works has always been a to-do for me, and seeing this video in my feed has given me the perfect chance. Now I want to build my own ruby laser.

[@sharpshooter740]

This video was 20 minutes long?? It felt like five, really concise and easy to follow. I probably need to watch it a couple more times, but I feel like I finally get the gist of how most lasers operate. Thank you

[@flaneurmd]

I’ve looked at 19 videos on explaining lasers, this one is most complete and easiest to understand. I could learn basket weaving from a good teacher and this is a great one. (Awaiting his course on basket weaving). B.L. Md

[@romuloaugustoando8365]

Thank you for posting this awesome video, I work with lasers (Raman spectroscopy) and this is the best material I ever found on the internet

[@philip2213zz]

As a laser enthusiast, this video was pure bliss. Thank you for making this.

[@oscargraveland]

Probably the most detailed video I have come accross. And without glaring mistakes. Interesting choice to mention Q-switching, before population inversion, but to leave out longitudinal modes.

[@ImprobableMatter]

I guess it's because I used to play a game about internet spaceships called EVE Online where they have "pulse" and "beam" lasers, which I presume are Q-switched and CW respectively.

[@oscargraveland]

@@ImprobableMatter Hahaha.. ok Really well done.

[@m.streicher8286]

This makes a good companion piece to Perun's last upload.

[@Profesor449]

Hell yes, I just listened to his lasers when this popped up. And he specifically said he was skipping the science. Truly the greatest accidental collab

[@m.streicher8286]

​@@Profesor449 Now we just need a video concerning the science of microwave emitters

[@markos.5539]

I've been in laser manufacturing course for 6 months and I still learnt so much.

[@NathanaelNewton]

This is one of my favorite science channels. Thank you for making this content for us all to enjoy

[@isaacmoon6739]

more videos and the end of the work week? sounds like heaven

[@jraines002]

“Get ready for some more butt numbers” is gonna be the new 5th slide in my startup pitch deck

[@hanro726]

Your channel is so valuable for learning about physics and technology, thank you so much for all your hard work!

[@sciencegeekgrandpa8]

👍 Admirable primer, as always! Keep them coming!

[@NomenNescio99]

I am very happy I have found this channel, great content, much appreciated.

[@pizzacrusher4632]

Another superb video. Thank you for making it!!!

[@ALLISMIND]

Your channel is fascinating! Please continue making videos

[@qwertyuuytrewq825]

I was in a ground state until this video hit me )

[@AveAtqueVale01]

What an exceptional video. Thank you!

[@ihmejakki2731]

2:30 "... the photons need to have almost exactly the energy..." Why almost? I've always understood it has to be exactly the energy between the states, do you mean almost as in the confines of the uncertainty principle or something else?

[@ImprobableMatter]

There is some wiggle room. One thing I didn't have time to talk about is line broadening. So for example, due to the thermal motion of the atoms, the photons will be Doppler shifted, some in one direction others in another.

[@mikegofton1]

13:21 😊 , nice 5th element reference.

[@Erik-rp1hi]

Out of High school in mid 70's I got a job that made helium gas Lasers. Cool to watch the glass guy do his thing. Their main product was Laser marking machines for engraving and other low watt stuff. They had the flash lamp YAG lasers in it with Q switch. I think the company's name was Control Laser out of Florida. They sent a lot of their product to Sandia Labs in N.M.

[@m.streicher8286]

"The gain medium must also be a plasma, or it would soon become one."

[@Thehighschoolscientistforever]

Please make another video about the same topic but diving in more detail and discussing the divergence and how it's used in lithography and sci-fi technology like space lasers concentrating sun light on to metal infused glass rods to create lasers and other cool things done with lasers

[@EspHack]

you're lucky to not be my neighbor or something, I could ruin your days with questions

[@GeoffryGifari]

Some questions for the community: 1. Would the length of the optical cavity being an integer multiple of laser wavelength produce an advantage? 2. When the laser reaches equilibrium, would the atoms emit photons in a synchronized way? or would it still be random (in other words, can the atoms themselves get correlated due to multiple passing stimulated emission?) 3. How different are the needs of a laser diode compared to a simple LED? Can a LED be turned into a laser diode? 4. For the free electron laser, does it still operate using transition between discrete energy levels? How does energy transition work in this case? 5. Will the straight beam of a laser light stay that way forever?

[@ImprobableMatter]

OK, I'll try: 1. Yes, you can do something called modelocking, which is related. 2. Yes, you can have some atoms more depleted than others where the electric field of the light is higher, if there is a standing wave. 3. Sort of, in that LEDs use spontaneous emission and diode lasers use stimulated. It's a bit like the Sodium light example. 4. There are no longer and energy levels, because the electrons are free and not in orbit around an atom. The laser light is no dependent on the spacing between the magnets, because this is the scale on which the electrons oscillate. 5. No, there is no way to make any beam of light perfectly straight. It will diffract outwards. A laser beam is still pretty good in this regard, but it will still form a cone of light, not a pure cylinder.

[@GeoffryGifari]

@@ImprobableMatter Thank you for the answers, and please don't feel forced. Maybe other commenters who work in the field can give their two cents

[@GeoffryGifari]

@@ImprobableMatter On the FEL case if stimulated emission can work continuously without fixed energy levels, I wonder if EM waves with laser characteristics (coherence, ...) can be produced for radio/microwave range using just wires, waveguides, antennas and such

[@ImprobableMatter]

A bit like a phased array antenna?

[@GeoffryGifari]

@@ImprobableMatter Hmmm I'll look into that

[@esotericgamedev]

Brilliant video. Going to build simulated lasers in a video game

[@waynetrinklein7210]

"Get ready for some more butt numbers" is my new favorite saying :D Awesome video

[@marckhachfe1238]

When i was 9, wevhad to give a small presentation in front if the class on anything we wanted. I chose lasers. It's fascinating to me that no one in my family has ever been into science, but when i first saw a chemistry kit as a kid, i wanted it SO badly Amazing how we are predisposed to like certain things. Anyway, im sure ny presentation was very simple and limited and wrong, but I do remember feeling like I was very smart when I told them what laser stood for. 😂

[@kannon1610]

Yooo new Improbable Matter content! 🎉

[@simo9445tsns]

great video, thanks

[@aaronmast6437]

Cool video! I have some questions about stimulated emission and the 'avalanche' growth of photons in a gain medium. As I understand, photons are not exactly particles which travel in a fixed direction. There is always a wavefront. Can a wavefront stimulate two excited atoms at the same time? And how does the stimulated emission wavefront appear? Does it grow/amplify the incoming photon's wavefront amplitude?

[@ImprobableMatter]

Your question is really more a question about quantum mechanics in general. The answer is the same as if you imagined a beam of electrons being shot out of a particle accelerator and hitting a crystal. Can an electron interact with two different atoms? Yes. A photon is a quantum particle, which like all particles has something akin to a wavefunction - just like with protons and electrons, it is impossible to measure its position and momentum simultaneously. Actually, the laser is yet another proof of this aspect of quantum mechanics - it always puts out an integer number of such photons like 1,2,3..., never 1.5 for instance. Each photon interacts with atoms around it as it is travelling. It can in principle interact with two excited atoms and stimulate emission. There would then be two photons, but they would be highly correlated. Eventually you would have a beam of millions of photons and then it makes sense to talk about it in the classical sense of a beam of light, because all the quantum uncertainties would average out in a statistical sense.

[@aaronmast6437]

@@ImprobableMatter Wow, thanks for the super detailed answer!

[@billr3624]

Hello, off topic but, was wondering if you could do a show about General Fusion's LM26 machine? Looks like they've suddenly gone in a new direction, giving up on the piston compression idea. Now it's some kind of collapsible liner.

[@1stbn8thmar]

holy fuck WE have missed you!

[@WobblycogsUk]

I could have watched a whole course on this, fascinating stuff. One question though, why do the stimulated photons travel in the same direction and not come out heading in a random direction? I assume it's something to do with quantum mechanics that's way above my pay grade.

[@ImprobableMatter]

In very basic terms: the photon has an oscillating electric field; it makes the electron in the atom oscillate in the same direction; this makes a new photon with an oscillating field in the same direction.

[@christosgravias7976]

Beautiful!

[@Cless]

This is the good stuff

[@B30pt87]

Thank you!

[@bdiddy77777]

Excuse me professor, when will you be bringing out the lasers for me to play with? And will we start with the small lasers, or can we just go straight to the big lasers first?

[@tombouie]

Thks & over my head; Question, ?Would a good intuitive description of atomic particles frozen/localized light?

[@DarnokIksnibiks]

Question, does istope content chage lasing properties of the medium? Does the medium need to be "encriched" to have only spacific isotope or the effect, if any, is too small to care?

[@ImprobableMatter]

Not really. It's all to do with the orbital electrons which don't care too much about the isotope, only the number of protons.

[@andrewmelean8259]

Awesome

[@Muonium1]

Watching on my phone during a break, in my uncomfortably hot cleanroom suit, inside the laser bay for the 2 kilojoule, 2 petawatt laser system, next to the 30kJ, 60 terawatt system seen here kzbin.info/www/bejne/f5umYnaGnql_n80 We are hoping to get funding soon for a 25 petawatt OPAL (Optical Parametric Amplifier Lines) system that would use three-wave mixing in noncollinear optical parametric amplifiers made of large slabs of KDP, and would allow ponderomotive acceleration of electrons to highly relativistic (>10GeV) energies in a single pulse over a few cm distance...among other exotic things.

[@ImprobableMatter]

I don't know if you saw, but I posted a link into my community tab, to a talk I did with the Astrophysical Journal. Free-range, applied MHD there.

[@Muonium1]

​@ImprobableMatter most of the time I am actually watching on desktop, and 8 years after introducing them, yt STILL gives no way of seeing these kinds of posts via that platform unless you go to each individual channel you're subscribed to and click on the community section. It's exasperating the way they run this site. So I'll have a look now.

[@sodakjohn]

Since a laser beam is collimated does it obey the inverse square law?

[@Muonium1]

@@sodakjohn yes, in the farfield all light sources do because of the diffraction limit of the output aperture.

[@jedrek1521]

Wait, how do slower moving planets in the outer solar system have more energy? Is it gravitational potential energy combined with their velocity?

[@ImprobableMatter]

Yes, exactly.

[@jedrek1521]

@ImprobableMatter thank you! Never thought of it as I'm skewed by thinking of the delta v requirements of going out vs in as opposed to energy of the orbit.

[@joegonzalez6241]

If i add magnet can i limit how far the photons go.

[@flamingpineconex5140]

dude you are so goated

[@andrewnelson4148]

While I would like to talk more about laser, given how important doping silicon is to our, microprocessors, photovoltaic solar cells, and now I find out that it effects laser too. I think that we need a video just to understand this important method of production. But back on the subject of laser, I would like to know what a frequency doubler is for a laser? Are they even real?

[@ImprobableMatter]

Yes, you can easily frequency double or triple laser light. It involves a type of crystal which will effectively turn two or three (depending on crystal type) photons into a single photon with double/triple the energy.

[@andrewnelson4148]

@@ImprobableMatter Is that way the wavelength of a laser changes? As the energy that a photon has is related to the wavelength of the photon. Also, what is the laser ablation cap? From what I understand it is when out gassing reduce the ability of material to absorb laser light.

[@Krasbin]

I like that you used the Dutch national flag as an example for the color scheme of the semiconductor laser. 🤔

[@joegonzalez6241]

So if i want to make an electric (plasma) sword ⚔️🗡️. I need to make sure my crystal is long enough to produce a blade With enough power to become solid at x distance

[@rykehuss3435]

Planets orbiting closer to the star must be moving faster to have a stable orbit thus they in fact have more kinetic energy (relative to mass), not less.

[@ImprobableMatter]

They have more kinetic energy, but less total energy. As the text in the video suggests, look up "virial theorem" for more information. Namely, KE = -1/2 PE

[@rykehuss3435]

@@ImprobableMatter How can they have less total energy, if the only practical energy a planetary body has is kinetic energy? what is the rest of this "total energy" made of?

[@ImprobableMatter]

Potential energy.

[@jeffstaples347]

Virial... Acollierastro taught me that word and it is still mysterious

[@xXNoamsChannelXx]

How is this channel so small

[@Overonator]

Why not a 5 level laser or higher?

[@Eupolemos]

13:20 - "Like"

[@floriankohn8448]

Dude, this video ist awesome. I seriously owe you a gallon of beer! I mean it. Feel free to contact me so I can return a favor.

[@leonschumann2361]

17:00 EUV Lithography comes to mind

[@clivedamagedgoods]

"So, back to pumping!"

[@ImprobableMatter]

Don't teach a course on lasers in Scotland.

[@999cathou]

they usually don't work

[@Anohaxer]

they usually do actually

[@slarson9483]

The dam fempto never wants to work when it is raining outside, no matter how well I control the humidity

[@999cathou]

:.))@@Anohaxer

[@ValidatingUsername]

So glad I could buy a laser at a fair growing up 😂

[@benvel3392]

I don't get this comment 😅

[@joegonzalez6241]

8:40 a twofer yeah.

[@brendawilliams8062]

What I mean is the science will tell you that electric is not completely understood and although you can have tech to the moon and back. Not everything is fully understood understood

[@Pasha8204]

Need video in 4k, how work modern smartphone laptop...

[@tomelms864]

Stimulating!

[@Pond770]

I have a totally wrong understanding of atomic structure but I’m a construction worker it ok

[@BODYBUILDERS_AGAINST_FEMINISM]

I see....

[@clearlisted]

15:57 gekoloniseerd

[@prbmax]

How do we know the ground state is the ground state? Let me preface this in the fact that I am not a subject matter expert, I just like to read. The proton or nucleus of an atom is small in relative terms but a proton or nucleus with an "orbiting" electron is many times the diameter of just the nucleus, even at so called ground state. If, our current understanding of ground state is simply not true, that brings up many interesting prospects being energy could be extracted if we could control the process of manipulating atoms to a new lower ground state. This may also explain the missing mass of galaxies, the so called dark matter. Just me pondering some thoughts here.

[@hedgehog3180]

It's because the Bohr model of the atom is wrong. Electrons don't actually orbit the nucleus in neat circular orbits since they are governed by wavefunctions so instead they occupy a probablistic cloud around the nucleus that passes through the nucleus as well. The Bohr model of the atom is useful as a teaching aid to illustrate the concept of energy levels but it doesn't accurately reflect what atoms are actually like. Also we know for absolutely certain what the energy levels are because they are governed by physical laws, this is the insight that made Bohr famous.