WEBVTT - TechStuff Classic: Pew Pew Lasers 0:00:04.440 --> 0:00:12.400 Welcome to tech Stuff, a production from iHeartRadio. Hey there, 0:00:12.480 --> 0:00:16.080 and welcome to tech Stuff. I'm your host, Jonathan Strickland. 0:00:16.280 --> 0:00:19.479 I'm an executive producer with iHeart Podcasts and How the 0:00:19.520 --> 0:00:22.279 Tech Are You. It's a Friday. It's time for a 0:00:22.360 --> 0:00:25.759 tech Stuff Classics episode, and that means we go back 0:00:25.800 --> 0:00:28.600 into our archives and pick an episode to play for 0:00:28.680 --> 0:00:32.680 your pleasure. This one originally published way back on June fourteenth, 0:00:32.760 --> 0:00:38.800 twenty seventeen. It has the wonderful title Pew Pew Lasers Enjoy. 0:00:39.880 --> 0:00:41.880 So we're going to talk about some pretty high tech 0:00:41.920 --> 0:00:44.360 stuff today. Actually, I'm going to look at a topic 0:00:44.400 --> 0:00:47.600 that we first addressed way back in twenty eleven with 0:00:47.760 --> 0:00:51.000 the episode how Lasers Work. That's when Chris Palette, my 0:00:51.200 --> 0:00:54.360 original co host, and I sat down and we talked 0:00:54.400 --> 0:00:56.640 about a little bit of the history of lasers and 0:00:56.720 --> 0:00:59.880 how they actually operate. But I thought it'd be better 0:01:00.040 --> 0:01:02.760 to revisit this, explain it again, kind of take a 0:01:02.920 --> 0:01:07.880 different approach to it. So, lasers are awesome and they 0:01:07.880 --> 0:01:10.800 can do tons of different stuff. Right. We can do 0:01:10.880 --> 0:01:14.640 everything from having a little laser pointer to amuse ourselves 0:01:14.640 --> 0:01:19.080 and our pets to having a laser element inside optical 0:01:19.200 --> 0:01:22.080 drive so that we can read information that's been stored 0:01:22.120 --> 0:01:26.840 on a disc. To communications satellites, to propelling spacecraft to 0:01:26.920 --> 0:01:29.080 cutting steel. There's all sorts of things we can do 0:01:29.080 --> 0:01:32.679 with lasers. Oh, we can threaten our enemies. We can 0:01:32.680 --> 0:01:35.080 tie them up and put them on a slab and 0:01:35.120 --> 0:01:38.720 then slowly have a laser creep upward and then laugh 0:01:38.800 --> 0:01:41.640 maniacally as we expect mister Bond to die. We can 0:01:41.680 --> 0:01:44.280 do all that sort of stuff with lasers. So we're 0:01:44.280 --> 0:01:46.200 going to talk about what they are, what they can do, 0:01:46.280 --> 0:01:49.400 their history, and maybe some cool trivia about lasers as well, 0:01:49.840 --> 0:01:53.520 and laser related stuff. So let's get to it now. 0:01:53.560 --> 0:01:58.200 First of all, what is the technical definition of a laser? Well, 0:01:58.240 --> 0:02:01.240 a laser is an acronym that means that it's a 0:02:01.280 --> 0:02:04.240 word that's made up of the initials of other words, right, 0:02:04.520 --> 0:02:08.720 so it stands for light amplification by stimulated emission of radiation. 0:02:09.600 --> 0:02:12.240 But for most of us that doesn't really clear things up. 0:02:12.440 --> 0:02:15.239 That just raises other questions like what do they mean 0:02:15.320 --> 0:02:19.480 by stimulated emission of radiation? And how do you amplify light? 0:02:20.080 --> 0:02:22.800 So I'm going to go in talk about all of 0:02:22.800 --> 0:02:25.040 that kind of stuff because it's really fascinating. It involves 0:02:25.040 --> 0:02:28.079 a lot of science and technology, two things I love 0:02:28.160 --> 0:02:33.960 to talk about. The third thing obviously being Chaucer's Canterbury 0:02:34.080 --> 0:02:36.400 tails one that I plicate with the shoulder asuta, but 0:02:36.440 --> 0:02:39.600 that does not really fit with lasers. They didn't have 0:02:39.639 --> 0:02:42.720 the laser's tail, so we're gonna skip Cannabar retails for 0:02:42.800 --> 0:02:45.480 this episode. Now, a laser is a device that produces 0:02:45.520 --> 0:02:50.280 a very narrow beam of light, and these beams are monochromatic. 0:02:50.360 --> 0:02:53.320 That means they are single color see single wavelength. That's 0:02:53.320 --> 0:02:56.680 a very specific wavelength of light for each laser and 0:02:56.720 --> 0:03:01.600 thus a specific color. So we perceive different wavelengths of 0:03:01.680 --> 0:03:03.799 light as different colors of light. So if you think 0:03:03.840 --> 0:03:09.880 of your roy GVIV, that is actually a spectrum literally 0:03:09.960 --> 0:03:12.840 a spectrum of colors. That's also a spectrum of wavelengths, 0:03:12.880 --> 0:03:16.440 with red being the longest wavelength and violet being the 0:03:16.480 --> 0:03:22.799 shortest wavelength. In the visible spectrum. The wavelength of light 0:03:23.320 --> 0:03:27.359 depends entirely on the amount of energy electrons release within 0:03:27.720 --> 0:03:32.200 the laser itself. So electrons release energy and in the 0:03:32.240 --> 0:03:35.520 form of photons or light particles, and the color of 0:03:35.560 --> 0:03:38.520 laser you get depends upon the amount of energy those 0:03:38.560 --> 0:03:41.440 electrons are releasing, and the amount of energy they release 0:03:41.520 --> 0:03:43.960 is dependent upon the type of atoms that they are 0:03:43.960 --> 0:03:47.600 connected to, because it all has to do with orbits 0:03:47.720 --> 0:03:51.040 of electrons around nuclei. More on that in a second. 0:03:51.200 --> 0:03:54.560 So the light is also coherent. Now, that does not 0:03:54.720 --> 0:03:57.160 mean it is able to hold a conversation and make 0:03:57.360 --> 0:04:00.800 salient points. It's not that kind of co parent. It 0:04:00.880 --> 0:04:04.680 means that the light is made up of organized photons. 0:04:05.640 --> 0:04:08.520 Organized photons in this case means that they're all traveling 0:04:09.000 --> 0:04:11.760 the same pattern of wavelength that are all in the 0:04:11.840 --> 0:04:15.440 same page as it were. If you look at wavelength, 0:04:15.480 --> 0:04:18.200 if you were to draw a series of waves, they 0:04:18.200 --> 0:04:21.200 would all be lined up exactly, so all the crests 0:04:21.240 --> 0:04:24.080 and the troughs would be lined up along the same points. 0:04:24.120 --> 0:04:28.160 At any point along the wavelength, they would match entirely. 0:04:28.600 --> 0:04:31.960 So that is what we mean by coherent. It's what 0:04:32.400 --> 0:04:36.279 helps keep the light organized and moving in that specific 0:04:36.320 --> 0:04:38.960 direction you want it to. And the light is also directional. 0:04:39.040 --> 0:04:42.040 That means the beam is tight and concentrated and remains 0:04:42.040 --> 0:04:45.600 so over great distances. You don't get a lot of 0:04:46.680 --> 0:04:51.600 light diverging from that pathway, and some lasers are able 0:04:51.600 --> 0:04:56.080 to project for miles in miles, like hundreds of thousands 0:04:56.120 --> 0:05:00.680 of miles in some cases, or without having any kind 0:05:00.680 --> 0:05:03.160 of degradation of the beam, which is kind of cool. 0:05:03.240 --> 0:05:05.480 I mean, it's amazingly cool. Now you can trast that 0:05:05.520 --> 0:05:09.240 with something like a flashlight. Flashlights have a beam that 0:05:09.440 --> 0:05:11.680 spreads out as it travels out we're from its source, 0:05:11.880 --> 0:05:15.599 it diffuses, so it's different from a laser. It doesn't 0:05:15.640 --> 0:05:18.760 have the coherence that a laser would have. This is 0:05:18.800 --> 0:05:23.159 typical of most light sources. You don't find lasers in nature. 0:05:23.600 --> 0:05:28.360 Lasers are something that we have caused to happen because 0:05:28.400 --> 0:05:31.039 of the natural laws. If it weren't for the natural laws, 0:05:31.120 --> 0:05:34.839 lasers wouldn't work. Obviously we didn't create that out of 0:05:34.880 --> 0:05:39.839 whole cloth. But it doesn't spontaneously happen in nature because 0:05:39.839 --> 0:05:43.080 you have to have very specific parameters set up in 0:05:43.200 --> 0:05:48.279 order to generate a laser beam. Now, to understand why 0:05:48.440 --> 0:05:50.160 it works the way it does, it helps to know 0:05:50.200 --> 0:05:52.760 how light works. Now, light behaves both as a wave 0:05:52.800 --> 0:05:55.520 and a particle, but for this bit of the explanation, 0:05:55.560 --> 0:05:58.159 we're mostly concerned with wave physics, even though we'll be 0:05:58.160 --> 0:06:01.440 talking about photons, the basic unit of light, the basic 0:06:01.480 --> 0:06:04.680 particle of light a lot in this episode. So a 0:06:04.760 --> 0:06:08.200 light source gives off waves of light, and different colors 0:06:08.200 --> 0:06:11.320 of light have different wavelengths. Like I said, you know, 0:06:11.360 --> 0:06:14.479 those red wavelengths are longer than the orange ones, Infrared 0:06:14.520 --> 0:06:18.160 waves are even longer, Ultraviolet are even shorter than violet, 0:06:18.480 --> 0:06:23.360 So you've got that different spectrum of wavelengths there. When 0:06:23.400 --> 0:06:26.360 you get down to that violet, you're really looking at 0:06:26.360 --> 0:06:29.800 the shortest wavelengths that we can perceive before it just 0:06:29.839 --> 0:06:32.880 becomes invisible to us. So again ultraviolet, we can't see that. 0:06:34.520 --> 0:06:38.080 Certain classes in dungeons and dragons different They can see ultravioletlight, 0:06:38.760 --> 0:06:42.760 not the rest of us. So these waves travel typically 0:06:42.960 --> 0:06:46.040 out of phase from each other from normal light sources. 0:06:46.120 --> 0:06:48.800 So again, if you were to chart those wavelengths, the 0:06:48.960 --> 0:06:53.480 crests and valleys would of each individual photon wouldn't match 0:06:53.600 --> 0:06:57.320 up right, like the crest of one might be matched 0:06:57.320 --> 0:07:00.520 with the valley of another or somewhere else along its waylength. 0:07:00.839 --> 0:07:04.120 They wouldn't be moving in phase, they'd be out of phase. 0:07:04.680 --> 0:07:08.960 So lasers all line up those light waves at the 0:07:09.040 --> 0:07:12.240 same way. So that they are in phase. And that's 0:07:12.320 --> 0:07:15.120 what we mean when we say coherent, that the various 0:07:15.160 --> 0:07:18.120 photons are all in phase with one another. And the 0:07:18.160 --> 0:07:22.800 way you generate lasers makes this happen. It's kind of cool. 0:07:22.840 --> 0:07:26.120 We'll talk about that again a little bit later. But 0:07:26.200 --> 0:07:29.760 all of the photons in the beam have unified wave fronts, 0:07:29.880 --> 0:07:33.440 so they're all moving in exactly the same wavelength at 0:07:33.440 --> 0:07:37.520 exactly the same time. Now, to understand how all of 0:07:37.600 --> 0:07:41.600 this works, it takes a looking at atoms. We have 0:07:41.640 --> 0:07:43.960 to go back to basic science. So let's take a 0:07:43.960 --> 0:07:45.800 look at an atom. Now. Back in the day when 0:07:45.840 --> 0:07:50.320 I was in school, atoms were depicted as being kind 0:07:50.320 --> 0:07:52.400 of like the orbits of planets, where you would have 0:07:52.440 --> 0:07:54.640 a nucleus in the center, kind of like the Sun, 0:07:54.880 --> 0:07:59.400 and electrons would orbit in neat little circles around at 0:07:59.400 --> 0:08:03.480 specific distances from the nucleus. As it turns out, things 0:08:03.520 --> 0:08:06.560 aren't quite so neat, and simple electrons are in an 0:08:06.600 --> 0:08:10.600 electron cloud that are around the nucleus. It is impossible 0:08:10.920 --> 0:08:15.920 to say with complete certainty where an electron is at 0:08:15.960 --> 0:08:18.280 any given moment. You know, you can know a position 0:08:18.320 --> 0:08:21.000 of an electron, but not it's direction. Or vice versa 0:08:21.160 --> 0:08:24.480 with complete certainty. Heisenberg's n certainty principle is a fun thing. 0:08:24.960 --> 0:08:29.400 But you know, when you have a basic atom and 0:08:29.440 --> 0:08:32.240 you haven't added any energy to the atom in its 0:08:33.280 --> 0:08:36.320 ground state energy level, that's when it's just, you know, 0:08:36.480 --> 0:08:41.280 kind of chilling. Atoms are always in motion. You only 0:08:41.280 --> 0:08:45.120 get atoms in no motion at all at absolute zero, 0:08:46.000 --> 0:08:48.080 when you're at zero kelvin. That is when you have 0:08:48.960 --> 0:08:52.640 zero atomic movement. But otherwise, atoms are always in motion. 0:08:52.720 --> 0:08:56.600 Even in solid objects, they're just not moving a lot. 0:08:57.040 --> 0:08:59.640 When you add energy to atoms, they move more. They 0:09:00.080 --> 0:09:03.960 start to get energized. When you energize atoms enough, you 0:09:04.000 --> 0:09:06.960 can boost them to an excited level. Now, typically you 0:09:07.000 --> 0:09:11.000 do this by applying energy like heat, light or electricity 0:09:11.040 --> 0:09:14.320 to the atom, Whereas if you want to excite me, 0:09:14.400 --> 0:09:16.640 you just say, hey, they might be giants's coming to town. 0:09:16.640 --> 0:09:18.760 You want to go see them? And I'm like, yeah, totally. 0:09:19.440 --> 0:09:22.160 So you've got atom which consists of that nucleus, and 0:09:22.160 --> 0:09:25.520 you've got the electron cloud around it. When you apply energy, 0:09:25.640 --> 0:09:29.960 it causes the electrons to move to a higher orbit 0:09:30.160 --> 0:09:33.640 around that nucleus. Again, since we're talking about a cloud, 0:09:33.679 --> 0:09:36.839 and not just a simple orbit circle. You can think 0:09:36.880 --> 0:09:39.320 of it as meaning the electrons move a little further 0:09:39.520 --> 0:09:42.960 away from the nucleus. If you add enough energy, you 0:09:43.000 --> 0:09:46.840 can strip electrons away from the atom entirely. This will 0:09:46.880 --> 0:09:52.400 create a charged atom because you will now have an ion. 0:09:53.400 --> 0:09:56.160 It's going to have a net positive charge because you're 0:09:56.160 --> 0:09:58.600 going to have protons there and you've pulled away some 0:09:58.640 --> 0:10:01.360 of the electrons, so you've taken some of that balance out. 0:10:02.400 --> 0:10:05.200 If you add enough electric enough energy, I was about 0:10:05.200 --> 0:10:08.199 to say electricity, but really energy. Electricity is one form 0:10:08.280 --> 0:10:10.240 of energy you could add to the atom in order 0:10:10.280 --> 0:10:12.720 to do this. But if you didn't add that much, 0:10:12.960 --> 0:10:15.600 like if you added enough to excite the electrons but 0:10:15.679 --> 0:10:19.440 not strip them away from the nucleus. When you remove 0:10:19.640 --> 0:10:23.400 that source of energy, the electrons will move back down 0:10:23.440 --> 0:10:27.040 to their ground state. They do not quote unquote want 0:10:27.280 --> 0:10:30.319 to be at that excited level. They have a ground 0:10:30.320 --> 0:10:33.840 state that they are naturally inclined to be at. But 0:10:34.440 --> 0:10:38.760 they've absorbed energy. So in order to move back down 0:10:38.800 --> 0:10:43.080 to their normal energy level, they have to give up 0:10:43.559 --> 0:10:46.120 some of the energy that they've absorbed, and they do 0:10:46.200 --> 0:10:50.600 this through emitting a photon. That basic unit of light, 0:10:51.600 --> 0:10:54.560 and once they emit that photon, that's what allows them 0:10:54.600 --> 0:10:58.200 to move back to their ground energy state because they 0:10:58.360 --> 0:11:02.520 no longer have that excess energy inside of themselves. So 0:11:03.280 --> 0:11:05.280 you can think of it as almost being like the 0:11:05.440 --> 0:11:08.880 electron is too full, like it's eaten too much, and 0:11:08.920 --> 0:11:12.960 then it has a little belchi belch or something it 0:11:13.040 --> 0:11:17.040 manages to emit some part of that energy. It is absorbed, 0:11:17.160 --> 0:11:19.559 and now it's feeling more like its old self again, 0:11:20.080 --> 0:11:21.880 and then you can just boost it back up again 0:11:21.880 --> 0:11:28.120 if you want to. So photons are emitted this way 0:11:28.200 --> 0:11:31.560 through in lasers, but that's not the only way we 0:11:32.040 --> 0:11:36.920 generate photons, like there are very specific ways of doing 0:11:36.960 --> 0:11:40.920 this in all sorts of applications, and many of them 0:11:41.080 --> 0:11:44.280 are pretty basic. Like your incandescent light bulb uses the 0:11:44.360 --> 0:11:48.680 same principle. You run an electric current through some wire 0:11:49.000 --> 0:11:53.160 a filament, typically in a vacuum sealed tube a bulb, 0:11:54.240 --> 0:11:59.200 and running the electric current causes the filament to heat 0:11:59.280 --> 0:12:03.160 up because as resistance to electrical current, so some of 0:12:03.160 --> 0:12:07.240 that electricity gets converted over into heat. As this heats up, 0:12:07.280 --> 0:12:11.240 it excites the atoms within that filament, and as that 0:12:11.480 --> 0:12:15.959 energy source moves through it allows those electrons to come 0:12:16.000 --> 0:12:19.720 back down, the atoms begin to emit photons, and then 0:12:19.760 --> 0:12:22.440 you get this glow. In the case of light bulbs, 0:12:22.440 --> 0:12:24.920 the glow creates the light you would have from an 0:12:24.960 --> 0:12:28.320 incandescent bulb. You could also see the same thing with 0:12:28.400 --> 0:12:30.360 heating elements, Like if you were to look inside a 0:12:30.360 --> 0:12:32.560 toaster and you see that orange glow, Well, that orange 0:12:32.559 --> 0:12:36.199 glow is coming from the heating elements that have had 0:12:36.240 --> 0:12:39.120 their atoms excited. The electrons got boosted to a higher 0:12:39.240 --> 0:12:43.199 energy level and then they came down and started releasing photons. 0:12:43.679 --> 0:12:46.920 So it's not just lasers that do this, but lasers 0:12:47.200 --> 0:12:49.319 take advantage of it in a very specific way. That's 0:12:49.320 --> 0:12:54.560 pretty cool. I am interrupting this laser focused episode, uh 0:12:54.600 --> 0:12:57.440 huh unintended in order for us to take a quick 0:12:57.480 --> 0:13:09.679 break to think our sponsors. So, a laser uses this 0:13:09.720 --> 0:13:12.480 principle to create those narrow beams of light. And here's 0:13:12.800 --> 0:13:15.720 how they do it. First, you need what is called 0:13:15.720 --> 0:13:19.840 a lasing medium. A laser medium, this is the stuff 0:13:20.200 --> 0:13:22.600 that you're going to use to excite. You know, you're 0:13:22.640 --> 0:13:24.840 gonna excite the atoms in this stuff so that it 0:13:25.000 --> 0:13:28.480 generates the wavelength of light that you want, and so 0:13:28.559 --> 0:13:31.440 the type of stuff you use that's going to determine 0:13:31.480 --> 0:13:34.400 the type of atoms that are present, which in turn 0:13:34.600 --> 0:13:38.079 determines the energy levels of the electrons, which in turn 0:13:38.200 --> 0:13:42.480 determines what color light you're gonna get through the lasing medium. 0:13:42.679 --> 0:13:47.400 All of this is dependent upon, ultimately the source of 0:13:47.440 --> 0:13:52.040 the lasing medium, like what is that material? The lasing 0:13:52.080 --> 0:13:55.880 medium acts like an amplifier, only this is for optics 0:13:55.960 --> 0:13:58.800 rather than for acoustics. So some people call the lasing 0:13:58.840 --> 0:14:02.760 medium the game medium or the source of optical gain 0:14:02.840 --> 0:14:06.360 because it's like a microphone gain setting. It is amplifying 0:14:06.400 --> 0:14:09.080 a signal, but in this case it's amplifying light, not 0:14:09.240 --> 0:14:15.079 amplifying sound. The gain in this case is that stimulated 0:14:15.120 --> 0:14:17.679 emission of photons I was talking about, and the emission 0:14:17.760 --> 0:14:23.000 is stimulated through an interesting series of events. You start 0:14:23.560 --> 0:14:28.560 by initially adding energy to the lasing medium, and then 0:14:28.880 --> 0:14:33.520 the photons it emits end up stimulating other atoms inside 0:14:33.520 --> 0:14:36.520 the lasing medium that have already been excited, and then 0:14:36.600 --> 0:14:41.040 you get a steady stream of photons that create your 0:14:41.120 --> 0:14:45.520 laser beam. But first you have to add energy into 0:14:45.520 --> 0:14:47.640 the system. You do this from what is called a 0:14:47.720 --> 0:14:52.640 pump source because you are pumping energy into the lasing medium. 0:14:53.040 --> 0:14:57.120 So basically, you pump energy into this medium, you excite 0:14:57.120 --> 0:15:02.040 some atoms. Those excited atoms start to emit photons. Those 0:15:02.040 --> 0:15:08.920 photons will start to hit other stimulated atoms and that's 0:15:08.920 --> 0:15:13.000 where you get this stimulated emission. So there are lots 0:15:13.040 --> 0:15:16.160 of different types of lasing media. So, for example, there 0:15:16.160 --> 0:15:20.000 are certain crystals that can serve as a medium. The 0:15:20.040 --> 0:15:26.120 earliest lasers were ruby lasers, so you would get a 0:15:26.160 --> 0:15:29.640 ruby crystal and that would be your lasing medium. You 0:15:29.640 --> 0:15:34.480 would usually introduce some impurities. It's called doping. You add 0:15:34.480 --> 0:15:37.840 some impurities to the material in order to make this 0:15:37.920 --> 0:15:41.000 a more efficient lasing medium. Usually it's some ions of 0:15:41.040 --> 0:15:45.360 some sort and that helps when you are actually getting 0:15:45.400 --> 0:15:50.120 to the part of generating a laser. Those are specifically 0:15:50.160 --> 0:15:53.240 solid state lasers, the ones that use crystals. You're using 0:15:53.360 --> 0:15:57.720 a solid lasing medium. But there are other ones as well. 0:15:57.720 --> 0:16:01.440 There's some that use glasses. Some that you gases, including 0:16:01.480 --> 0:16:05.480 reactive gases like chlorine and fluorine. Those are specific types 0:16:05.520 --> 0:16:09.280 of gas lasers that are called exemer lasers. You have 0:16:09.360 --> 0:16:13.480 semiconductor lasers, which produce in the grand scheme of things. 0:16:13.520 --> 0:16:17.200 Fairly weak lasers, but they also are fairly inexpensive to produce, 0:16:18.080 --> 0:16:19.760 and those are the ones that we use in things 0:16:19.800 --> 0:16:23.280 like CD players, DVD players, blu ray players, that kind 0:16:23.280 --> 0:16:26.560 of stuff. They tend to be semiconductor lasers. They're easy 0:16:26.600 --> 0:16:29.200 to mass produce, they're less expensive, and they aren't so 0:16:29.360 --> 0:16:31.800 powerful as to cause problems. You don't need a CD 0:16:31.920 --> 0:16:35.640 player laser that could burn a hole through the surface 0:16:35.640 --> 0:16:39.080 of the Earth. That would be ridiculous. You can also 0:16:39.720 --> 0:16:44.840 get liquid medium lasers. These are liquids that have various 0:16:45.000 --> 0:16:52.320 organic dyes, special organic dyes, dyes that will allow for 0:16:52.360 --> 0:16:57.720 this stimulated emission of light amplified light. Now, the pump 0:16:57.880 --> 0:17:00.040 is some sort of energy transfer that you use to 0:17:00.080 --> 0:17:02.040 excite those atoms in the first place, so that they'll 0:17:02.040 --> 0:17:07.320 emit those initial photons when the electrons calm the heck down. 0:17:08.040 --> 0:17:13.320 Laser pumps are some form of external source of energy. Typically, 0:17:13.480 --> 0:17:17.080 they supply energy in the form of either electricity or light, 0:17:17.240 --> 0:17:21.680 but there are other means of pumping a lasing medium 0:17:21.680 --> 0:17:26.359 with energy to create lasers. Light and electricity are the 0:17:26.400 --> 0:17:29.879 two most common ones, but they are not the only kinds. 0:17:29.880 --> 0:17:32.679 There are some that use chemical reactions. There are some 0:17:32.800 --> 0:17:36.280 that even use nuclear reactions, which I think is taking 0:17:36.320 --> 0:17:39.520 it a little far if you're asking me, that's me 0:17:39.640 --> 0:17:43.199 mostly being tongue in cheek. But again, most of the 0:17:43.240 --> 0:17:46.120 lasers that we would encounter throughout our day, those are 0:17:46.160 --> 0:17:52.479 generated either through light or through electricity stimulating the lasing medium. So, 0:17:54.400 --> 0:17:57.520 for example, most early lasers were using some form of 0:17:57.840 --> 0:18:02.960 arc or flash lamp to stack emulate that initial reaction 0:18:03.520 --> 0:18:07.160 within the atoms of the lasing medium, like a crystal rod. 0:18:08.160 --> 0:18:10.440 So you got your crystal rod with a few impurities 0:18:10.440 --> 0:18:12.359 in it that you have specifically placed in there. You 0:18:12.400 --> 0:18:17.280 have doped this crystal rod. You would wrap a light 0:18:17.840 --> 0:18:24.240 source around this thing, usually within some sort of mirrored chamber, 0:18:25.080 --> 0:18:29.640 and you would flash light in pulses against the lasing medium, 0:18:30.640 --> 0:18:34.560 and this would actually excite atoms within the medium, which 0:18:34.560 --> 0:18:38.959 would then give off photons. Now, if there were no 0:18:39.119 --> 0:18:43.800 way for you to keep this reaction going, it would 0:18:43.840 --> 0:18:47.040 be such a small emission of photons that you probably 0:18:47.040 --> 0:18:49.640 wouldn't even be able to tell you wouldn't it wouldn't 0:18:49.680 --> 0:18:54.040 be visible to you. However, by tricking it, you can 0:18:54.119 --> 0:18:57.160 totally make it visible. So you typically would use these 0:18:57.160 --> 0:19:00.639 mirrors to reflect light back into the lasing medium. That 0:19:00.720 --> 0:19:05.800 includes photons that were emitted during that initial flash, and 0:19:05.840 --> 0:19:08.840 that's what allows you to create a cascade effect and 0:19:09.400 --> 0:19:13.879 create a laser. Generally speaking, you would probably use mirrors 0:19:13.880 --> 0:19:17.320 that would allow the reflection of any wavelengths of light 0:19:17.400 --> 0:19:21.239 that were shorter than the laser's wavelength would be, and 0:19:21.280 --> 0:19:26.760 allow the transference of light that is longer wavelengths longer 0:19:26.800 --> 0:19:30.040 than the laser's wavelength that you want. The reason for 0:19:30.080 --> 0:19:32.320 that is that if you were to trap all the 0:19:32.400 --> 0:19:35.800 light within the chamber, you could cause things to heat 0:19:35.880 --> 0:19:39.480 up and create what's called thermal lensing. The actual change 0:19:39.480 --> 0:19:43.480 in temperature would create a lens effect that would end 0:19:43.560 --> 0:19:47.280 up affecting the ability of a laser to be directional 0:19:47.320 --> 0:19:49.920 and coherent. And obviously, if that's your intent, you don't 0:19:49.920 --> 0:19:53.520 want that to happen. So, yeah, Thermal lensing occurs when 0:19:53.520 --> 0:19:56.000 a sample absorbs energy from a laser beam, it heats up, 0:19:56.040 --> 0:20:00.240 it creates this refractive lens that causes beam divergence. That's 0:20:00.280 --> 0:20:02.400 not what you want with a laser typically, I mean 0:20:02.440 --> 0:20:05.840 you might want to design a system that creates that 0:20:05.920 --> 0:20:09.399 splits a beam but that's different from beam divergence. You 0:20:09.480 --> 0:20:13.560 want that beam to be nice and tight, typically or 0:20:13.240 --> 0:20:19.400 your average laser applications. So let's imagine that we're building 0:20:19.400 --> 0:20:23.440 a laser and we start with a rod made out 0:20:23.720 --> 0:20:26.639 of ruby. I was gonna say that you could have 0:20:26.640 --> 0:20:29.720 a ruby rod, but we all know that he is 0:20:29.760 --> 0:20:33.679 busy with Corbyn Dallas trying to save the universe. A 0:20:33.720 --> 0:20:35.640 shout out to any of you guys out there who 0:20:35.720 --> 0:20:40.600 understand what that reference means. So you've got ruby rod, 0:20:40.840 --> 0:20:44.760 and you've got a flash tube that is probably wrapped 0:20:44.760 --> 0:20:48.520 around the ruby rod, but at least is shining can 0:20:48.600 --> 0:20:51.040 shine on the ruby rod, and you can use the 0:20:51.040 --> 0:20:54.439 flash tube out of like a camera. In fact, the 0:20:54.520 --> 0:20:59.840 earliest lasers we're using camera flash bulbs as the source 0:20:59.840 --> 0:21:02.400 of light to start this reaction. It's not like it's 0:21:02.440 --> 0:21:06.640 something super high tech. It's actually pretty cool. And you've 0:21:06.640 --> 0:21:10.600 got a mirrored chamber that surrounds the whole thing on 0:21:10.760 --> 0:21:13.639 the on either end of the rod. So think of 0:21:13.680 --> 0:21:17.239 the rod as like a cylinder. You have put a 0:21:17.320 --> 0:21:21.399 silvered mirror on either end. One side is a pure 0:21:21.480 --> 0:21:25.040 silvered mirror, so it just reflects light. The other one 0:21:25.119 --> 0:21:28.600 is a partially silvered mirror, meaning that it can allow 0:21:28.680 --> 0:21:31.879 some light to pass through. Specifically, you want to design 0:21:31.960 --> 0:21:35.040 it so it allows the wavelength of the laser light 0:21:35.119 --> 0:21:37.600 to pass through, but doesn't allow any other light to 0:21:37.600 --> 0:21:43.240 pass through. You turn on the flash tube. This shines 0:21:43.280 --> 0:21:45.879 bright light onto the rod, which causes some of the 0:21:45.880 --> 0:21:50.120 atoms in the rod to excite. Then, as those electrons 0:21:50.200 --> 0:21:53.080 move back down from their excited stage back to the 0:21:53.119 --> 0:21:57.800 ground level stage, they release photons, and with enough energy 0:21:57.840 --> 0:22:00.800 pumped into the medium, you end up with a larger 0:22:00.880 --> 0:22:05.080 population of atoms that are in an excited state than 0:22:05.119 --> 0:22:08.040 there are atoms in the ground state. When you reach 0:22:08.160 --> 0:22:12.840 that point, it is called a population inversion because you've 0:22:12.880 --> 0:22:17.720 inverted the relationship between excited atoms and ground state atoms. 0:22:17.760 --> 0:22:21.280 Typically you would have more ground state atoms than excited ones. 0:22:21.760 --> 0:22:23.920 Once you're able to flip that balance, you can create 0:22:23.960 --> 0:22:28.120 this cascading effect that I've been talking about. So you've 0:22:28.160 --> 0:22:31.560 got more excited atoms than you have ground energy level 0:22:31.560 --> 0:22:35.320 atoms inside of this lazing medium. At that point they 0:22:35.320 --> 0:22:39.840 start giving off photons, and this is pretty cool. What 0:22:39.960 --> 0:22:47.160 happens next is photons from some of those first atoms 0:22:47.160 --> 0:22:49.800 that had been excited and then were calming down. If 0:22:49.840 --> 0:22:54.400 you like, they'll go out and they'll hit other excited atoms. 0:22:54.640 --> 0:22:56.720 So these are atoms that I've already had their energy 0:22:56.800 --> 0:23:03.000 levels boosted by that flash bulb. The photon from the 0:23:03.000 --> 0:23:08.240 first atom, the one that excited and calmed down, has 0:23:08.440 --> 0:23:12.200 just the right amount of energy to cause the electron 0:23:12.400 --> 0:23:14.639 in an excited atom to come back down to its 0:23:14.680 --> 0:23:19.119 ground state and release another photon. So what happens is 0:23:19.160 --> 0:23:23.560 the atom that it it connects with will absorb the photon, 0:23:24.080 --> 0:23:27.000 then it will emit the photon and emit a second 0:23:27.080 --> 0:23:31.040 photon as its own electron comes down an energy level. 0:23:31.280 --> 0:23:35.520 So you get two photons emitted, the initial one that 0:23:35.600 --> 0:23:39.320 you shot the atom with and then the one that 0:23:39.320 --> 0:23:42.400 that atom produced itself. So this is what is called 0:23:42.480 --> 0:23:46.959 light amplification. Right, you have amplified the light. You started 0:23:47.000 --> 0:23:50.080 with one photon. Now you have two photons and they're 0:23:50.119 --> 0:23:54.879 moving in phase with one another because well, because of 0:23:54.960 --> 0:23:56.719 quantum physics, but I don't want to get into that 0:23:56.760 --> 0:24:02.879 too much. So you get this light amplification through that process. 0:24:04.720 --> 0:24:07.159 Now that you have the light amplification, you might as 0:24:07.160 --> 0:24:10.240 well say, like, well, what are we calling this this 0:24:10.280 --> 0:24:14.000 whole process where a photon can cause another atom to 0:24:14.119 --> 0:24:17.560 emit a photon. That's the stimulated emission. You might think 0:24:17.560 --> 0:24:20.200 stimulated emission was when you turned on the flash bulb. 0:24:20.240 --> 0:24:24.400 That's not technically correct. The stimulated emission part technically comes 0:24:24.800 --> 0:24:29.200 from these initial atoms that release photons, and those cause 0:24:29.280 --> 0:24:32.600 this chain reaction in the lasing medium. So this can 0:24:32.640 --> 0:24:35.040 happen over and over and over again. Right, you're not 0:24:35.119 --> 0:24:38.440 really the atoms aren't losing any matter in this. It's 0:24:38.600 --> 0:24:41.359 just a process of electrons being boosted up to an 0:24:41.440 --> 0:24:43.920 energy level and then coming back down again, so they're 0:24:43.960 --> 0:24:48.600 releasing energy. They're not losing anything in this. It's just 0:24:48.640 --> 0:24:53.000 a transfer of energy and really a transformation of it 0:24:53.160 --> 0:24:58.600 from one form of light to another. So it's fascinating 0:24:58.880 --> 0:25:01.800 to me that this is something that not only works, 0:25:01.840 --> 0:25:04.399 but that people were able to figure out would work. 0:25:05.680 --> 0:25:11.600 It's so far into quantum physics and optics and photonics 0:25:11.640 --> 0:25:16.120