How lasers work - a thorough explanation

hi i'm paul from physics high lasers now these are quite ubiquitous that is you can find them fairly cheaply these days and you can use them to do all matter of things such as pointed things such as at my wedding ring lasers are quite common in industry and medical devices they're used for example in eye surgery they're used in industrial applications and of course every time you go to the supermarket and get your groceries scanned they use lasers but what makes a laser a laser let's turn the light off and examine the laser by using this deodorant spray so what you notice is three things first of all it's monochromatic that is it only produces one color secondly we have a beam that is coherent that is the waves that are coming out are all in phase with one another and thirdly we have what we call a collimated beam that is a very nice focused beam but why is it monochromatic why is it coherent and why is it collimated and what does laser actually stand for well today i'm going to discuss the working principles behind a laser so stay tuned [Music] now laser stands for light amplification by the stimulated emission of radiation now we're going to explore that but that means but we also want to address why it is monochromatic coherent and of course collimated so to start off with we need to have a look at the structure of the atom which is really about the substance that actually generates the photons that we want to have for the laser now we have here the bohr model the atom and i have a video that you can have a look at where i explore the ball model now it is a simplistic model and needless to say that the atom is far more complex where the liquid lips and probability clouds around the nucleus but in this case this model will suffice to help us understand what's going on because the electrons do exist in what we call discrete orbits they don't radiate energy in these orbits and so they have various energy levels and so my electron can exist in one energy level it can exist in the other energy level and there could be more further up each case it is a step up in terms of its energy but they are discrete energy levels or quantum energy levels that means that the energy between one energy level and the other one is a very discrete amount and this is where we're going to have an excitation going on here so i will have a photon coming in and if and only if that photons energy is exactly equal to the difference between these two energy levels that photon is absorbed by my electron and as a result my electron jumps at energy level and so has a higher energy state and then what happens almost immediately is that an electron jumps back to the lower energy level what we refer to as the ground state and so it drops down and therefore releases energy but guess what it releases the energy with exactly the same photon of energy being released so that is under the principle of what we refer to as e is equal to h f where f is the frequency of my photon h is planck's constant and e of course is the energy now that explains to us why it is monochromatic as we'll see as we go on all the photons we're going to be talking about are exactly the amount of energy that we are dealing here with their energy levels and so therefore we will only produce photons of a very specific amount of energy and hence it's monochromatic let's move on and let's look now a bit closer because we've talked about emission and in this case we've got spontaneous emission photon coming in electron getting excited dropping back down and then spontaneously emitting the same photon energy but we want a stimulated and so what we could have is a situation like so we have my electron here and it of course is stimulated by my photon comes in because my electron that is absorbing that energy jumps an energy level now what if now for example while this electron is in this stimulated stage what happens if it encounters another photon and in this case it absorbs that energy but before it drops down or whilst it drops down into its energy level it now releases two photons and so what we get here is an increase of photons and so what we have here is an amplification i started with one photon and end up with two photons so in that sense that explains the implication process though there's more to it as you'll see but the thing is is that the second photon is exactly the same so in other words it is the same wavelength the same phase same everything so we say it's coherent because they are in phase with each other now why they are actually going to be exactly the same is actually a quantum phenomena that i'm not going to delve into now and that could be something that you can look into further but needless to say we have now a duplication of our photon and so that explains why it's coherent because they are always going to be in phase every photon we're going to be producing will have exactly the same frequency and phase as a result so i can have a photon going in and a single photon going out which is spontaneous emission i can have a photon going in and if the electron is already in the stimulated stage i can have two come out and so now i've got stimulated emission so what happens if now if those two photons encounter other stimulated electrons well we start off with two of course then we got four we have eight then we have 16 and of course that continues on and so what we get this is cascade effect of all these photons being generated as long as they encounter stimulated electrons but here is the problem the time it takes for the photon to start in its excited state to back to ground state is very very quick so what is the probability that we have multiple atoms in the excited state well really really small so small in fact that really this is not going to produce our cascading effect and there's a problem right there at least in the simplistic model so we need to find a way of increasing the population of our excited state and so what we get now is what we call population inversion let me explain what that means so in this case my population is in its ground state now i've got here six representative electrons in their ground state and so the number of electrons in the excited state is going to be definitely different to the number of electrons in the ground state so clearly our n2 is less than n1 if we want to have x a continuing cascading effect we need that to be reversed we need more in the n2 state than the in one state and so what we want is what we refer to as a population inversion because the population is inverted we've got more in the excited state and less in the ground state so how do we get them up there remember as i said to you when they're actually pushed up there they very quickly jump back so even if we actually have a few stimulator before we have anyone encountering another electron very quickly you'll find they will be jumping back down to their grain state and as a result the number n ends up being mostly in the ground state so how do we solve that problem well the solve the problem is by introducing a material that you can actually have a third level or a level three material and so what we want to do is stimulate our electron beyond to the level that we want and how do we do that well we have here our second we're a second and now a third level so imagine i fire a white light photon now you're gonna say hold on a white light photon doesn't exist you are correct actually what we have is let's say light that has basically white so it has multiple photon wavelengths in there and hopefully one of those photons of course will excite it up to this level right here and so now what we have is our super excited electron but the materials chosen here that these two energy levels or this energy levels is very very unstable and so what happens is as i pump the white light in we have our electrons jumping up into this third level here but very very quickly it jumps down into this second level and so what we now get is an electron that is in the second level that will stay there a little longer now this state here is referred to as the metastate and the beauty about the metastate is that the time that the electron exists in the metastate is actually a little longer up to a thousand times longer than let's say in the normal situation so what that means is you're going to now get a situation where you're going to have a lot more electrons sitting in this metastate for a certain period of time which means if i now have my photon coming in and it's experiencing an electron in that meta state very quickly what we're going to get is that electron jumping down and we there will produce two photons it's more probabilistic for us to produce more photons that way so in essence what we get is this so here is multiple atoms and what we start to see in these electrons and see in these atoms we start to see pairs of photons coming off as they come out and then of course they're interacting with other atoms as well and then what you're going to get of course is an increasing or a cascading number of photons being generated in your material but you can see a problem is that they're all going in different directions we're not going to get let's say a strong focused beam how do we do that well the first thing we need to do is with our atom as i said to you is we need to apply some sort of energy source now it can be a light source but it can also be an electrical source as well so this is what the stimulation aspect so in this case we're using light and so there's our light aspect we've talked already about the fact that it's amplified and we've already talked about that it's by stimulated emission so we've actually covered most of the terms already of the term laser but what we want to do is increase the effect so how do we do that now the first thing we do is we add mirrors what does that do well we have of course photons going all different directions but any photon that is going in let's say that direction is going to reflect back and go back in that direction and then of course when it gets to the other side it's going to reflect back in that direction and so forth it's going to go backwards and forwards and every single time you start to see a stimulated emission you're getting more and more photons as it goes backwards and forwards and backwards and forwards in essence if we then look at the light in terms of its wave phenomena what we end up setting up is a standing wave it's actually what we call a resonance and so what we get here is a huge amplification as we get to generate a standing wave of photons basically going backwards and forwards every single time increasing exponentially as they encounter electrons now at this stage we've got it in a tube with two mirrors but the beauty here of course is is that along this path it's very narrow anything that goes in the other direction will be bouncing off the mirror and goes out to the side but we're going to definitely get an increasing effect along the line here perpendicular to our mirrors now being a standing wave is that that standing wave is determined by the wave formula which is f is equal to nv over 2 l where n is basically equal to the different harmonics in this case i've got a harmonic of two the reality is is that the harmonic we generate here is in the thousands the frequency of course is the frequency of the photon and v of course is the speed of light and the length is the length of the tube so in other words if you set the length of the tube right you'll create the right resonance for the wavelength that you're interested in and in this case for example a red wavelength let's say 632.8 nanometers which is the wavelength for a helium neon laser so now that we have set up a standing wave we need to now somehow let the light out well i need to change the transparency of my mirror now by changing the transparency of a mirror usually only about about one percent so in other words it's now 99 reflective i'm going to get my some of my light going out and i have my laser beam because the light now is only what is going perpendicular to my mirror along that line that central line it now explains why we get a really tight beam and it is collimated and in some lasers what they might also do is put a small lens to really adjust for any imperfections that may exist so in summary let's quickly review what do we use to pump the energy into our tube we used light what did we end up getting we ended up getting more photons so we had amplification how did we do that well we had to have emission but it had to occur with electrons that were already stimulated so we had stimulated emission and as a result we get a nice monochromatic coherent collimated beam which we can call radiation well i hope that has helped you understand how a laser works please like share and subscribe and put a comment down below if this has been helpful for you and consider supporting me via patreon my name is paul from physics high take care and bye for now

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Keywords: laser physics course, laser beam physics, laser physics definition, laser modern physics, laser quantum physics, laser simple physics, stimulated emission, light amplification, Laser physics, stimulated emission in laser, how lasers work, how do lasers work, light amplification in laser, how do lasers work physics, stimulated emission of radiation, population inversion in laser, laser physics lecture, what is laser, how lasers work animation, how lasers work a complete guide

Id: DA7a_v96Jsw

Length: 13min 54sec (834 seconds)

Published: Thu Dec 17 2020