WEBVTT - Pew Pew Lasers 0:00:04.160 --> 0:00:07.160 Get in touch with technology with tech Stuff from how 0:00:07.240 --> 0:00:13.680 stuff works dot com. Hey there, and welcome to tech Stuff. 0:00:13.720 --> 0:00:17.440 I am your host, Jonathan Strickland. I'm a senior writer 0:00:17.640 --> 0:00:21.160 with how stuff Works dot com, where we attempt to 0:00:21.280 --> 0:00:26.599 demystify the universe for your education and entertainment. My specialty 0:00:27.040 --> 0:00:32.080 happens to be technology despite my degree in medieval and 0:00:32.159 --> 0:00:35.680 Renaissance English literature. So we're gonna talk about some pretty 0:00:35.720 --> 0:00:38.280 high tech stuff today. Actually, I'm gonna look at a 0:00:38.320 --> 0:00:40.840 topic that we first addressed way back in two thousand 0:00:40.880 --> 0:00:44.199 and eleven with the episode how Lasers Work. That's when 0:00:44.240 --> 0:00:47.840 Chris Pallette, my original co host, and I sat down 0:00:48.159 --> 0:00:49.880 and we talked about a little bit of the history 0:00:49.920 --> 0:00:53.320 of lasers and how they actually operate. But I thought 0:00:53.440 --> 0:00:56.160 it would be better to revisit this, explain it again, 0:00:56.320 --> 0:00:59.560 kind of take a different approach to it. Um. So, 0:00:59.800 --> 0:01:04.600 lasers are awesome and they can do tons of different stuff. Right. 0:01:04.640 --> 0:01:07.360 We can do everything from having a little laser pointer 0:01:07.440 --> 0:01:11.680 to amuse ourselves and our pets, to having a laser 0:01:11.720 --> 0:01:15.560 element inside optical drive so that we can read information 0:01:15.600 --> 0:01:19.560 that's been stored on a disc. Two communications satellites, to 0:01:19.720 --> 0:01:23.000 propelling spacecraft to cutting steel. There's all sorts of things 0:01:23.040 --> 0:01:25.959 we can do with lasers. Oh, we can threaten our enemies. 0:01:26.640 --> 0:01:28.399 We can tie them up and put them on a 0:01:28.440 --> 0:01:32.360 slab and then slowly have a laser creep upward and 0:01:32.400 --> 0:01:35.280 then laugh maniacally as we expect Mr Bond to die. 0:01:35.600 --> 0:01:37.400 We can do all that sort of stuff with lasers. 0:01:38.200 --> 0:01:40.120 So we're gonna talk about what they are, what they 0:01:40.120 --> 0:01:42.720 can do, their history, and maybe some cool trivia about 0:01:42.760 --> 0:01:46.960 lasers as well, and laser related stuff. So let's get 0:01:46.959 --> 0:01:50.160 to it now. First of all, what is the technical 0:01:50.320 --> 0:01:54.680 definition of a laser? Well, laser is an acronym that 0:01:54.760 --> 0:01:56.800 means that it's a word that's made up of the 0:01:56.840 --> 0:01:59.920 initials of other words, right, so it stands for light 0:02:00.080 --> 0:02:04.520 amplification by stimulated emission of radiation. But for most of 0:02:04.560 --> 0:02:07.760 us that doesn't really clear things up. That just raises 0:02:07.800 --> 0:02:10.919 other questions like what do they mean by stimulated emission 0:02:10.919 --> 0:02:14.920 of radiation? And how do you amplify light? So I'm 0:02:14.960 --> 0:02:17.400 gonna go in talk about all of that kind of 0:02:17.400 --> 0:02:19.640 stuff because it's really fascinating and involves a lot of 0:02:19.680 --> 0:02:23.040 science and technology, two things I love to talk about. 0:02:23.560 --> 0:02:29.000 The third thing, obviously being Chaucer's Canterbury tails one that 0:02:29.040 --> 0:02:31.000 applicated with the shoulder at SUTA, but that does not 0:02:31.120 --> 0:02:34.959 really fit with lasers. They didn't have the lasers tail, 0:02:35.200 --> 0:02:38.079 so we're gonna skip Canterbury Tails for this episode. Now, 0:02:38.120 --> 0:02:40.640 a laser is a device that produces a very narrow 0:02:40.760 --> 0:02:44.960 beam of light, and these beams are monochromatic. That means 0:02:44.960 --> 0:02:47.840 they are single colors see single wavelength. That's a very 0:02:47.880 --> 0:02:51.280 specific wavelength of light for each laser and us a 0:02:51.360 --> 0:02:56.400 specific color. So we perceive different wavelengths of light as 0:02:56.440 --> 0:02:58.320 different colors of light. So if you think of your 0:02:58.360 --> 0:03:04.200 roy g BIV, that is actually a a spectrum literally 0:03:04.200 --> 0:03:07.120 a spectrum of colors. That's also a spectrum of wavelengths, 0:03:07.160 --> 0:03:10.680 with red being the longest wavelength and violet being the 0:03:10.720 --> 0:03:16.760 shortest wavelength in the visible spectrum. Uh, the wavelength of 0:03:16.840 --> 0:03:21.080 light depends entirely on the amount of energy electrons release 0:03:21.280 --> 0:03:26.280 within the laser itself. So electrons release energy and in 0:03:26.320 --> 0:03:29.680 the form of photons or light particles, and the color 0:03:29.720 --> 0:03:32.560 of laser you get depends upon the amount of energy 0:03:32.600 --> 0:03:35.360 those electrons are releasing, and the amount of energy they 0:03:35.360 --> 0:03:37.960 release is dependent upon the type of atoms that they 0:03:37.960 --> 0:03:40.920 are connected to, because it all has to do with 0:03:41.360 --> 0:03:45.200 orbits of electrons around nuclei. More on that in a second. 0:03:45.480 --> 0:03:48.800 So the light is also coherent. Now, that does not 0:03:48.960 --> 0:03:51.400 mean it is able to hold a conversation and make 0:03:51.640 --> 0:03:55.480 salient points. It's not that kind of coherent. It means 0:03:55.880 --> 0:04:00.320 that the light is made up of organized photons. Organized 0:04:00.360 --> 0:04:03.360 botons in this case means that they're all traveling the 0:04:03.440 --> 0:04:06.440 same pattern of wavelength that are all in the same 0:04:06.600 --> 0:04:09.840 page as it were. If you look at wavelength, if 0:04:09.840 --> 0:04:12.680 you were to draw a series of waves, they would 0:04:12.720 --> 0:04:15.560 all be lined up exactly, so all the crests and 0:04:15.600 --> 0:04:18.320 the troughs would be lined up along the same points. 0:04:18.360 --> 0:04:22.400 At any point along the wavelength, they would match entirely. 0:04:22.839 --> 0:04:26.039 So that that is what we mean by coherent. It's 0:04:26.040 --> 0:04:29.760 what helps keep the light organized and moving in that 0:04:30.040 --> 0:04:33.200 specific direction you wanted to. And the light is also directional. 0:04:33.320 --> 0:04:36.280 That means the beam is tight and concentrated and remains 0:04:36.320 --> 0:04:39.960 so over great distances. You don't get a lot of 0:04:40.480 --> 0:04:45.520 uh light diverging from that that pathway, and some lasers 0:04:45.520 --> 0:04:49.880 are able to project for miles and miles, like hundreds 0:04:49.880 --> 0:04:54.320 of thousands of miles in some cases or uh without 0:04:54.360 --> 0:04:56.840 having any kind of degradation of the beam, which is 0:04:56.920 --> 0:04:59.080 kind of cool. I mean, it's amazingly cool. Now you 0:04:59.120 --> 0:05:02.760 contrast that with something like a flashlight. Flashlights have a 0:05:02.800 --> 0:05:05.919 beam that spreads out as it travels outward from its source, 0:05:05.960 --> 0:05:09.680 it diffuses, so it's different from a laser. It doesn't 0:05:09.839 --> 0:05:12.880 have the coherence that a laser would have. Um this 0:05:12.960 --> 0:05:16.000 is typical of most light sources. You don't find lasers 0:05:16.040 --> 0:05:21.000 in nature. Lasers are something that we have caused to 0:05:21.279 --> 0:05:24.479 happen because of the natural laws. If it weren't for 0:05:24.480 --> 0:05:28.440 the natural laws, lasers wouldn't work. Obviously, we didn't create 0:05:28.480 --> 0:05:33.120 that out of whole cloth. But it doesn't spontaneously happen 0:05:33.240 --> 0:05:36.640 in nature because you have to have very specific parameter 0:05:36.760 --> 0:05:41.039 set up in order to generate a laser beam. Now, 0:05:41.120 --> 0:05:43.839 to understand why it works the way it does, it 0:05:43.839 --> 0:05:46.280 helps to know how light works, and a light behaves 0:05:46.279 --> 0:05:48.839 both as a wave and a particle. But for this 0:05:48.920 --> 0:05:51.760 bit of the explanation, we're mostly concerned with wave physics, 0:05:51.800 --> 0:05:54.760 even though we'll be talking about photons, the basic unit 0:05:54.880 --> 0:05:57.240 of light, the basic particle of light. A lot in 0:05:57.279 --> 0:06:01.479 this episode. So a light source gifts off waves of light, 0:06:01.600 --> 0:06:05.200 and different colors of light have different wavelengths. Like I said, uh, 0:06:05.360 --> 0:06:08.159 you know, those red wavelengths are longer than the orange ones. 0:06:08.200 --> 0:06:11.760 Infrared waves are even longer, Ultra violet are even shorter 0:06:11.839 --> 0:06:15.400 than violet. So you've got that that different spectrum of 0:06:15.400 --> 0:06:19.839 wavelengths there. When you get down to that violet, you're 0:06:19.880 --> 0:06:22.640 really looking at the shortest wavelengths that we can perceive 0:06:22.800 --> 0:06:26.279 before it just becomes invisible to us. So again, ultra violet, 0:06:26.360 --> 0:06:30.080 we can't see that um Certain classes and dungeons and 0:06:30.160 --> 0:06:33.520 dragons different They can see ultraviolet light, not the rest 0:06:33.520 --> 0:06:38.200 of us. So these waves travel typically out of phase 0:06:38.320 --> 0:06:41.080 from each other from normal light sources. So again, if 0:06:41.120 --> 0:06:44.640 you were to chart those wavelengths, the crests and valleys 0:06:44.720 --> 0:06:48.760 would of each individual photon wouldn't match up right, like 0:06:48.880 --> 0:06:52.200 the crest of one might be matched with the valley 0:06:52.240 --> 0:06:55.680 of another or somewhere else along its wavelength. They wouldn't 0:06:55.680 --> 0:06:59.080 be moving in phase, they'd be out of phase. So 0:07:00.440 --> 0:07:03.719 lasers all line up those light waves at the same 0:07:03.920 --> 0:07:06.720 way so that they are in phase. And that's what 0:07:06.760 --> 0:07:09.920 we mean when we say coherent. That the various photons 0:07:09.920 --> 0:07:12.640 are all in phase with one another, and the way 0:07:12.640 --> 0:07:17.040 you generate lasers makes this happen. Uh, it's kind of cool. 0:07:17.080 --> 0:07:20.360 We'll talk about that again a little bit later. But 0:07:20.440 --> 0:07:24.000 all the photons in the beam have unified wave fronts, 0:07:24.120 --> 0:07:27.560 so they're all moving in exactly the same wavelength a 0:07:27.680 --> 0:07:31.800 exactly the same time. Now, to understand how all of 0:07:31.840 --> 0:07:35.640 this works, it takes. It takes a looking at atoms. 0:07:35.640 --> 0:07:38.000 We have to go back to basic science. So let's 0:07:38.000 --> 0:07:39.640 take a look at an atom. Now. Back in the 0:07:39.680 --> 0:07:43.960 day when I was in school, atoms were depicted as 0:07:44.000 --> 0:07:46.320 being kind of like the orbits of planets, where you 0:07:46.360 --> 0:07:48.440 would have a nucleus in the center, kind of like 0:07:48.480 --> 0:07:52.440 the sun, and electrons would orbit a neat little circles 0:07:52.480 --> 0:07:57.160 around at specific distances from the nucleus. As it turns out, 0:07:57.480 --> 0:08:00.680 things aren't quite so neat and simple. Atrons are in 0:08:00.720 --> 0:08:04.080 an electron cloud that are around the nucleus. It is 0:08:04.120 --> 0:08:09.240 impossible to say with complete certainty, Uh, where an electron 0:08:09.520 --> 0:08:11.920 is at any given moment. You know, you can know 0:08:12.000 --> 0:08:14.600 a position of an electron, but not it's direction or 0:08:14.720 --> 0:08:18.120 vice versa with complete certainty. Heisenberg's and certainty principles is 0:08:18.160 --> 0:08:22.600 a fun thing. But you know, when you have a 0:08:22.680 --> 0:08:25.760 basic atom and you haven't added any energy to the atom, 0:08:25.840 --> 0:08:30.080 it's and it's ground state energy level. That's when it's 0:08:30.120 --> 0:08:34.479 just you know, kind of chilling. Atoms are always in motion. Uh. 0:08:35.080 --> 0:08:38.480 You only get atoms in no motion at all at 0:08:38.480 --> 0:08:42.079 absolute zero, when you're at zero kelvin. That is when 0:08:42.080 --> 0:08:46.560 you have zero atomic movement. But otherwise, atoms are always 0:08:46.559 --> 0:08:50.120 in motion, even in solid objects, they're just not moving 0:08:50.320 --> 0:08:53.480 a lot. When you add energy to atoms, they move more. 0:08:53.800 --> 0:08:58.079 They start to get energized. When you energize atoms enough, 0:08:58.160 --> 0:09:01.120 you can boost them to an excited level. Now, typically 0:09:01.120 --> 0:09:04.440 you do this by applying energy like heat, light or 0:09:04.480 --> 0:09:08.040 electricity to the atom. Uh, Whereas if you want to 0:09:08.080 --> 0:09:10.199 excite me, you just say, hey, they might be giants 0:09:10.320 --> 0:09:11.679 is coming to town. You want to go see them? 0:09:11.720 --> 0:09:14.920 And I mean like, yeah, totally. So you've got atom 0:09:14.960 --> 0:09:17.240 which consists of that nucleus, and you've got the electron 0:09:17.280 --> 0:09:20.680 cloud around it. When you apply energy, it causes the 0:09:20.720 --> 0:09:26.600 electrons to move to a higher orbit around that nucleus. Again, 0:09:26.640 --> 0:09:28.600 since we're talking about a cloud and not just a 0:09:28.679 --> 0:09:31.920 simple orbit circle, you can think of it as meaning 0:09:31.920 --> 0:09:35.199 the electrons move a little further away from the nucleus. 0:09:35.240 --> 0:09:39.160 If you add enough energy, you can strip electrons away 0:09:39.160 --> 0:09:43.760 from the atom entirely. This will create a charged UH 0:09:43.840 --> 0:09:48.120 atom because you will now have an ion. It's gonna 0:09:48.160 --> 0:09:50.640 have a pause a net positive charge because you're gonna 0:09:50.640 --> 0:09:53.640 have protons there and you've pulled away some of the electrons, 0:09:53.640 --> 0:09:56.800 so you've taken some of that balance out. UH. If 0:09:56.840 --> 0:09:59.480 you add enough electric enough energy, I was about to 0:09:59.480 --> 0:10:02.440 say electro steve a really energy. Electricity is one form 0:10:02.520 --> 0:10:04.520 of energy you could add to the ADAM in order 0:10:04.520 --> 0:10:06.959 to do this. But if you didn't add that much, 0:10:07.160 --> 0:10:09.720 like if you had it enough to excite the electrons 0:10:09.760 --> 0:10:13.120 but not strip them away from the nucleus. When you 0:10:13.200 --> 0:10:17.400 remove that source of energy, the electrons will move back 0:10:17.440 --> 0:10:20.640 down to their ground state. They do not quote unquote 0:10:20.760 --> 0:10:24.120 want to be at that excited level. They have a 0:10:24.200 --> 0:10:27.360 ground state that they are naturally inclined to be at. 0:10:28.000 --> 0:10:32.720 But they've absorbed energy. So in order to move back 0:10:32.760 --> 0:10:37.040 down to their normal energy level, they have to give 0:10:37.160 --> 0:10:40.160 up some of the energy that they've absorbed, and they 0:10:40.200 --> 0:10:44.880 do this through emitting a photon. That basic unit of light, 0:10:45.880 --> 0:10:48.800 and once they emit that photon, that's what allows them 0:10:48.800 --> 0:10:52.440 to move back to their ground energy state because they 0:10:52.600 --> 0:10:56.959 no longer have that excess energy inside of themselves. So 0:10:57.520 --> 0:10:59.640 you can think of it as almost being like the 0:10:59.679 --> 0:11:02.439 elect run is too full, like it's eaten too much, 0:11:03.000 --> 0:11:07.120 and then it has little belchy belch or something it 0:11:07.240 --> 0:11:11.280 manages to emit some part of that energy. It is absorbed, 0:11:11.440 --> 0:11:13.840 and now it's feeling more like its old self again, 0:11:14.360 --> 0:11:16.120 and then you can just boost it back up again 0:11:16.160 --> 0:11:22.240 if you want to. So photons are admitted this way 0:11:22.480 --> 0:11:25.800 through in lasers, but that's not the only way we 0:11:26.280 --> 0:11:31.160 generate photons like there are very specific ways of doing 0:11:31.200 --> 0:11:35.200 this in all sorts of applications, and many of them 0:11:35.320 --> 0:11:39.320 are pretty basic. Like your incandescent lightbulb uses the same principle. 0:11:39.960 --> 0:11:43.880 You run an electric current through some wire a filament, 0:11:44.080 --> 0:11:49.600 typically in a vacuum sealed tube a bulb, and running 0:11:49.600 --> 0:11:54.120 the electric current causes the filament to heat up because 0:11:54.160 --> 0:11:57.520 it has resistance to electrical current, so some of that 0:11:57.559 --> 0:12:01.480 electricity gets converted over into heat. As this heats up, 0:12:01.520 --> 0:12:05.440 it excites the atoms within that filament, and as that 0:12:05.679 --> 0:12:10.200 energy source moves through it allows those electrons to come 0:12:10.200 --> 0:12:13.840 back down the the atoms begin to emit photons, and 0:12:13.840 --> 0:12:16.720 then you get this glow. In the case of light bulbs, 0:12:16.720 --> 0:12:19.200 the glow creates the light you would have from an 0:12:19.200 --> 0:12:22.600 incandescent bulb. You could also see the same thing with 0:12:22.640 --> 0:12:24.600 heating elements, Like if you were to look inside a 0:12:24.640 --> 0:12:26.800 toaster and you see that orange glow, Well, that orange 0:12:26.800 --> 0:12:30.480 glow is coming from the heating elements that have had 0:12:30.480 --> 0:12:33.360 their atoms excited. The electrons got boosted to a higher 0:12:33.480 --> 0:12:37.880 energy level and then they came down and started releasing photons. 0:12:37.920 --> 0:12:41.199 So it's not just lasers that do this, but lasers 0:12:41.440 --> 0:12:43.559 take advantage of it in a very specific way. That's 0:12:43.559 --> 0:12:48.280 pretty cool. Now, that's just what's going on with the 0:12:48.280 --> 0:12:51.120 physics side of things. I haven't yet explained how this 0:12:51.360 --> 0:12:54.160 really works with a laser. So a laser uses this 0:12:54.240 --> 0:12:57.080 principle to create those narrow beams of light. And here's 0:12:57.320 --> 0:13:00.280 how they do it. First, you need what is called 0:13:00.320 --> 0:13:04.400 a lazing medium, a laser medium. This is the stuff 0:13:04.760 --> 0:13:07.160 that you're going to use to excite. You know, you're 0:13:07.160 --> 0:13:09.400 gonna excite the atoms in this stuff so that it 0:13:09.559 --> 0:13:13.040 generates the wavelength of light that you want, and so 0:13:13.120 --> 0:13:16.000 the type of stuff you use that's going to determine 0:13:16.000 --> 0:13:18.959 the type of atoms that are present, which in turn 0:13:19.120 --> 0:13:22.600 determines the energy levels of the electrons, which in turn 0:13:22.720 --> 0:13:27.160 determines what color light you're gonna get through the lasing medium. 0:13:27.200 --> 0:13:31.959 All of this is dependent upon, ultimately the source of 0:13:32.000 --> 0:13:36.560 the lazing medium, Like what is that material? The lasing 0:13:36.600 --> 0:13:40.400 medium acts like an amplifier, only this is for optics 0:13:40.520 --> 0:13:43.320 rather than for acoustics. So some people call the lasing 0:13:43.400 --> 0:13:47.319 medium the gain medium or the source of optical gain 0:13:47.400 --> 0:13:50.920 because it's like a microphone gain setting, it is amplifying 0:13:50.960 --> 0:13:56.680 a signal. Then this case, it's amplifying light, not amplifying sound. Uh. 0:13:56.720 --> 0:14:00.120 The gain in this case is that stimulated mission the 0:14:00.160 --> 0:14:03.400 photons I was talking about, and the emission is stimulated 0:14:03.640 --> 0:14:09.280 through an interesting series of events. You start by initially 0:14:10.240 --> 0:14:14.200 adding energy to the lasing medium, and then the photons 0:14:14.240 --> 0:14:18.560 that emits end up stimulating other atoms inside the lasing 0:14:18.600 --> 0:14:21.760 medium that have already been excited, and then you get 0:14:21.800 --> 0:14:27.200 a steady stream of photons that create your laser beam. 0:14:27.200 --> 0:14:30.520 But first you have to add energy into the system. 0:14:30.640 --> 0:14:33.200 You do this from what is called a pump source 0:14:33.280 --> 0:14:38.440 because you are pumping energy into the lazing medium. So basically, 0:14:38.480 --> 0:14:42.240 you pump energy into this medium, you excite some atoms, 0:14:42.680 --> 0:14:47.240 the those excited atoms start to emit photons. Those photons 0:14:47.320 --> 0:14:53.480 will start to h hit other stimulated atoms and that's 0:14:53.480 --> 0:14:57.280 where you get this um stimulated emission. So there are 0:14:57.360 --> 0:15:00.440 lots of different types of lasing media. So, for example, 0:15:00.600 --> 0:15:04.080 there are certain crystals that can serve as a medium. Uh, 0:15:04.200 --> 0:15:10.640 the earliest lasers were ruby lasers, so you would get 0:15:10.640 --> 0:15:13.600 a ruby crystal and that would be your lasing medium. 0:15:14.120 --> 0:15:17.840 You would usually in introduce some impurities. It's called doping. 0:15:18.360 --> 0:15:21.400 You add some impurities to the material in order to 0:15:21.960 --> 0:15:24.800 make this a more efficient lasing medium. Usually it's some 0:15:24.920 --> 0:15:29.200 ions of some sort um and that helps when you're 0:15:29.240 --> 0:15:33.560 actually getting to the part of generating a laser. Those 0:15:33.640 --> 0:15:37.160 are specifically solid state lasers, the ones that use crystals. 0:15:37.280 --> 0:15:41.680 You're using a solid lasing medium. But there are other 0:15:41.680 --> 0:15:44.400 ones as well. There's some that use glasses, some that 0:15:44.560 --> 0:15:48.560 use gases, including reactive gases like chlorine and florine. Those 0:15:48.600 --> 0:15:53.200 are specific types of gas lasers that are called xemer lasers. 0:15:53.600 --> 0:15:57.680 You have semiconductor lasers, which produce, in the grand scheme 0:15:57.720 --> 0:16:00.560 of things, fairly weak lasers. But they also are fairly 0:16:00.600 --> 0:16:03.600 inexpensive to produce, and those are the ones that we 0:16:03.760 --> 0:16:07.560 use in things like CD players, DVD players, Blu ray players, 0:16:07.560 --> 0:16:10.240 that kind of stuff. They tend to be semiconductor lasers. 0:16:10.240 --> 0:16:13.360 They're easy to mass produce, they're less expensive, and they 0:16:13.360 --> 0:16:15.880 aren't so powerful as to cause problems. You don't need 0:16:15.960 --> 0:16:19.040 a CD player laser that could burn a hole through 0:16:19.600 --> 0:16:23.160 the surface of the Earth. That would be ridiculous. You 0:16:23.200 --> 0:16:28.520 can also get liquid medium lasers. These are liquids that 0:16:28.560 --> 0:16:33.280 have various organic dyes special organic dyes d y e 0:16:33.680 --> 0:16:39.200 s that will allow for this stimulated emission of light 0:16:39.480 --> 0:16:43.520 amplified light. Now, the pump is some sort of energy 0:16:43.560 --> 0:16:45.520 transfer that you use to excite those atoms in the 0:16:45.560 --> 0:16:49.160 first place, so that they'll emit those initial photons when 0:16:49.280 --> 0:16:54.680 the electrons calm the heck down. Laser pumps are some 0:16:54.760 --> 0:16:59.160 form of external source of energy. Typically they supply energy 0:16:59.160 --> 0:17:02.120 in the form of either electricity or light, but there 0:17:02.160 --> 0:17:06.840 are other means of pumping a lazing medium with energy 0:17:06.920 --> 0:17:11.800 to create lasers. Light and electricity are the two most 0:17:11.960 --> 0:17:14.680 common ones, but they are not the only kinds. There's 0:17:14.720 --> 0:17:18.240 some that use chemical reactions. There's some that even use 0:17:18.480 --> 0:17:21.320 nuclear reactions, which I think is taking in a little 0:17:21.359 --> 0:17:24.800 far if you're asking me, uh, that's me mostly being 0:17:24.800 --> 0:17:28.480 tongue in cheek. But again, most of the lasers that 0:17:28.760 --> 0:17:32.240 we would encounter throughout our day, those are generated either 0:17:32.400 --> 0:17:37.080 through light or through electricity stimulating the lasing medium. So, 0:17:38.960 --> 0:17:41.920 for example, most most early lasers were using some form 0:17:42.000 --> 0:17:47.600 of arc or flash lamp to stimulate that initial reaction 0:17:48.080 --> 0:17:52.800 within the atoms of lazing medium, like a crystal rod. Uh. So, 0:17:52.840 --> 0:17:55.080 you got your crystal rod with a few impurities in 0:17:55.119 --> 0:17:57.000 it that you have specifically placed in there. You have 0:17:57.119 --> 0:18:02.719 doped this crystal rod. You would wrap a light source 0:18:02.840 --> 0:18:08.800 around this thing, usually within some sort of mirrored chamber, 0:18:09.640 --> 0:18:15.119 and you would flash light impulses against the lasing medium, 0:18:15.200 --> 0:18:19.120 and this would actually excite atoms within the medium, which 0:18:19.119 --> 0:18:23.479 would then give off photons. Now, if there were no 0:18:23.680 --> 0:18:28.359 way for you to keep this reaction going, it would 0:18:28.359 --> 0:18:31.560 be such a small emission of photons that you probably 0:18:31.600 --> 0:18:33.800 wouldn't even be able to tell. You wouldn't even it 0:18:33.800 --> 0:18:38.439 wouldn't be visible to you. However, by tricking it you 0:18:38.480 --> 0:18:41.479 can totally make it visible. So you typically would use 0:18:41.520 --> 0:18:45.040 these mirrors to reflect light back into the lasing medium. 0:18:45.080 --> 0:18:50.119 That includes photons that were admitted during that initial flash, 0:18:50.200 --> 0:18:52.640 and that's what allows you to create a cascade effect 0:18:53.240 --> 0:18:57.840 and create a laser. Generally speaking, you would probably use 0:18:57.920 --> 0:19:01.600 mirrors that would allow the election of any wavelengths of 0:19:01.720 --> 0:19:05.160 light that were shorter than the laser's wavelength would be, 0:19:05.640 --> 0:19:10.760 and allow the transference of light that is longer, wavelengths 0:19:10.800 --> 0:19:14.439 longer than the laser's wavelength that you want. The reason 0:19:14.480 --> 0:19:16.760 for that is that if you were to trap all 0:19:16.800 --> 0:19:20.119 the light within the chamber, you could cause things to 0:19:20.160 --> 0:19:23.640 heat up and create what's called thermal lensing. The actual 0:19:23.720 --> 0:19:27.280 change in temperature would create a lens effect that would 0:19:27.880 --> 0:19:31.040 end up affecting the ability of a laser to be 0:19:31.280 --> 0:19:34.240 directional and coherent. And obviously, if that's your intent, you 0:19:34.240 --> 0:19:37.919 don't want that to happen. So, yeah, Thermal lensing occurs 0:19:37.920 --> 0:19:40.080 when a sample absorbs energy from a laser beam, it 0:19:40.160 --> 0:19:43.800 heats up, it creates this refractive lens that causes beam divergence. 0:19:44.560 --> 0:19:46.960 That's not what you want with a laser. Typically, I mean, 0:19:47.000 --> 0:19:50.359 you might want to design a system that creates that 0:19:50.480 --> 0:19:53.680 splits a beam, but that's different from beam divergence. You 0:19:54.040 --> 0:19:57.480 want that beam to be nice and tight. Uh, typically 0:19:57.720 --> 0:20:03.240 or your your average laser applications. So let's let's imagine 0:20:03.240 --> 0:20:06.719 that we're building a laser and we start with a 0:20:06.840 --> 0:20:10.760 rod made out of ruby. I was gonna say that 0:20:10.800 --> 0:20:13.719 you could have a ruby rod, but we all know 0:20:13.840 --> 0:20:16.440 that he is busy with Corbin Dallas trying to save 0:20:16.480 --> 0:20:19.840 the universe. Shout out to any of you guys out 0:20:19.880 --> 0:20:24.360 there who understand what that reference means. So you've got 0:20:24.480 --> 0:20:27.679 ruby rod, and you've got a flash tube that is 0:20:28.400 --> 0:20:31.719 probably wrapped around the ruby rod, but at least as 0:20:32.000 --> 0:20:35.239 shining can shine on the ruby rod, and you can 0:20:35.320 --> 0:20:38.840 use the flash tube out of like a camera. In fact, 0:20:38.920 --> 0:20:44.439 the earliest lasers were using camera flashbulbs as the source 0:20:44.480 --> 0:20:46.960 of light to start this reaction. It's not like it's 0:20:47.000 --> 0:20:51.160 something super high tech. It's actually pretty cool. And you've 0:20:51.200 --> 0:20:55.199 got a mirrored chamber that surrounds the whole thing on 0:20:55.320 --> 0:20:58.199 the on either end of the rod. So think of 0:20:58.240 --> 0:21:01.720 the rod is like a cylinder. You have put a 0:21:01.840 --> 0:21:05.960 silvered mirror on either end. One side is a pure 0:21:06.040 --> 0:21:09.560 silvered mirror, so it just reflects light. The other one 0:21:09.680 --> 0:21:13.119 is a partially silvered mirror, meaning that it can allow 0:21:13.240 --> 0:21:16.480 some light to pass through. Specifically, you want to design 0:21:16.480 --> 0:21:19.600 it so it allows the wavelength of the laser light 0:21:19.640 --> 0:21:22.159 to pass through, but doesn't allow any other light to 0:21:22.160 --> 0:21:27.120 pass through. Uh, you turn on the flash tube. This 0:21:27.359 --> 0:21:30.359 shines bright light onto the rod, which causes some of 0:21:30.359 --> 0:21:33.960 the atoms in the rod to excite. Then as those 0:21:34.000 --> 0:21:37.439 electrons move back down from their excited stage back to 0:21:37.520 --> 0:21:42.000 the ground level stage, they release photons, and with enough 0:21:42.119 --> 0:21:44.840 energy pumped into the medium, you end up with a 0:21:44.960 --> 0:21:48.840 larger population of atoms that are in an excited state 0:21:49.400 --> 0:21:52.320 then there are atoms in the ground state. When you 0:21:52.359 --> 0:21:57.200 reach that point, it is called a population inversion because 0:21:57.240 --> 0:22:01.399 you've inverted the relationship between a sited atoms and ground 0:22:01.480 --> 0:22:04.919 state atoms. Typically, you would have more ground state atoms 0:22:04.920 --> 0:22:07.880 than excited ones. Once you're able to flip that balance, 0:22:07.920 --> 0:22:10.840 you can create this cascading effect that I've been talking about. 0:22:11.400 --> 0:22:14.959 So you've got more excited atoms than you have ground 0:22:15.400 --> 0:22:19.640 energy level atoms inside of this lasing medium. At that point, 0:22:19.720 --> 0:22:24.200 they started giving off photons, and this is pretty cool. 0:22:24.320 --> 0:22:29.560 What happens next is photons from some of those first 0:22:30.920 --> 0:22:34.160 UH atoms that had been excited and they were calming down. 0:22:34.200 --> 0:22:36.920 If you like, they'll go out and they'll hit other 0:22:37.080 --> 0:22:40.760 excited atoms. So these are atoms that I've already had 0:22:40.760 --> 0:22:47.040 their energy levels boosted by that flashbulb. The photon from 0:22:47.359 --> 0:22:51.600 the first atom, the one that excited then calmed down, 0:22:52.520 --> 0:22:55.919 has just the right amount of energy to cause the 0:22:56.040 --> 0:22:59.000 electron in an excited atom to come back down to 0:22:59.080 --> 0:23:03.239 its ground state and release a another photon. So what 0:23:03.240 --> 0:23:06.959 happens is the atom that it it connects with will 0:23:07.000 --> 0:23:10.480 absorb the photon, then it will emit the photon and 0:23:10.560 --> 0:23:14.280 emit a second photon as its own electron comes down 0:23:14.720 --> 0:23:18.520 an energy level. So you get two photons emitted, the 0:23:18.600 --> 0:23:23.200 initial one that you shot the atom with and then 0:23:23.280 --> 0:23:26.399 the one that that atom produced itself. So this is 0:23:26.400 --> 0:23:30.800 what it's called light amplification. Right, you have amplified the light. 0:23:30.960 --> 0:23:34.280 You started with one photon. Now you have two photons 0:23:34.359 --> 0:23:39.080 and they're moving in phase with one another because well 0:23:39.119 --> 0:23:40.879 because of quantum physics, but I don't want to get 0:23:40.920 --> 0:23:45.960 into that too much. So you get this light amplification 0:23:46.200 --> 0:23:51.000 through that process. Now that you have the light amplification. 0:23:51.280 --> 0:23:54.119 You might as well say, like, well, what are we 0:23:54.160 --> 0:23:57.320 calling this? This whole process where a photon can cause 0:23:57.480 --> 0:24:01.000 another atom to admit a photon, that's the stimulated emission. 0:24:01.600 --> 0:24:03.919 You might think stimulated emission was when you turned on 0:24:03.960 --> 0:24:07.600 the flash bulb. That's not technically correct. The stimulated emission 0:24:07.640 --> 0:24:12.360 part technically comes from these initial atoms that release photons, 0:24:12.480 --> 0:24:16.640 and those cause this chain reaction in the lasing medium. 0:24:16.680 --> 0:24:19.200 So this can happen over and over and over again. Right, 0:24:19.240 --> 0:24:22.720 You're not really the atoms aren't losing any matter in this. 0:24:22.800 --> 0:24:25.760 It's just a process of electrons being boosted up to 0:24:25.800 --> 0:24:28.240 an energy level and then coming back down again, so 0:24:28.280 --> 0:24:32.920 they're releasing energy. They're not losing anything in this. It's 0:24:33.000 --> 0:24:37.399 just a transfer of energy and really a transformation of 0:24:37.440 --> 0:24:42.440 it from one form of light to another. So it's 0:24:42.440 --> 0:24:46.360 fascinating to me that this is something that not only works, 0:24:46.400 --> 0:24:49.840