Laser Fundamentals II | MIT Understanding Lasers and Fiberoptics

the following content is provided under a Creative Commons license your support will help MIT OpenCourseWare continue to offer high quality educational resources for free to make a donation or view additional materials from hundreds of MIT courses visit MIT opencourseware at ocw.mit.edu just before the break we were talking about the need for a light amplifier an optical amplifier to make a lazy impossible alright so here we where we left off we had the cavity or the resonator that gave us these modes and the spacing between them is C over 2l as we mentioned and now we put the amplifier within this cavity to overcome the losses and I say in general the losses come from from the mirrors and if there's enough gain here let's say this is the location of the gain in frequency and if there is enough gain at this cavity resonance then we will get lazy and we'll get the light output by making this mirror not 100% reflective but a little bit has a little transmission this mirror here so we can leak some the light out so the then the spectral width of the laser then narrows down from initially this to to this essentially this Delta function is Delta F going close to zero and then in the time domain we get as a result of this knowing we get a wave train that's a pure sinusoid constant in frequency and constant in amplitude and that's the sort of the output of a of a typical what shall we say an ideal an ideal laser so now if if somebody then asks you where does where why does the laser have a very narrow spectral width then one can then say that it comes from the fact that we have an optical amplifier in within a within a resonator with enough gain to overcome the losses to give us this extremely narrow line now I'm going to then tell you about how the next property the one that that has to do with diffraction-limited collimation and focusing and so on where does this property come from again in a simple-minded way at this stage and later on we'll go into it in a little bit more more depth well the this the spatial properties really come from the fact of course that we have an amplifier within within the resonator and we have these two mirrors let's take two plane mirrors for example and we'll see later that that indeed the the mode that can be supported within that within that cavity is is essentially a parallel beam that gets reflected backwards and forwards between these two mirrors and the amplifier then overcomes the losses in the cavity and this mirror has little transmission so the output here you would expect to be a collimated beam a plane a plane wave with diffraction limited collimation and if you put a lens will be diffraction limited focusing now not all laser cavities have plane mirrors some of them have curved mirrors like in this case two concave mirrors again the mode shape as we'll see later will be will be different from this will be something like that and then if we let some of the light leak out through one of the curved mirrors you'll see it it will be diverging just because it's diverging it doesn't mean that it's a bad beam it just means that with lenses we can convert it to any sized beam we want and focus it and and so on so it's really it's the cavity plus the amplifier that that gives us this very uniform wave that we we talked about before in fact if we can shape the the output of the laser beam is these two outputs excuse me with with lenses and then we can create here's some small focus spot here maybe slightly bigger focus we can then create a narrower call made it beam from what we started with or larger collimating and so on it's all done with with the use of good optics outside outside the laser cavity so this all these properties to do with the with the spatial effect of the laser beam then come from from the cavity plus the amplifier all right now we want to talk about where where does high power come from excuse me well the high power comes from obviously from the from the amplifier because it's the amplifier that that gives us gain to overcome the losses and if we have more gain left then that can be converted to to power out so this so the the bigger the amplifier then the more power we'll get as simple as that so sooner this later we will find out about how an amplifier works so if we can come up with a bigger amplifier and how to make a big amplifier then we'll really know the secret of making making high-power lasers so it's in the hands of the of the amplifying the the tuning the tuning range where does the tuning range come from well let me give you again a very simple simple explanation let's go back to to the laser that we had two mirrors separated by distance L in a cavity and then we had the cavity mode over here and then this is the the amplified bandwidth this is where the amplifier likes to provide gain and then we said that the output of the laser would be would be this delta function at the cavity resonance now these modes here are not fixed because they depend on on L so if I change L the length of the cavity I can move these modes around so if I move this cavity mode around if I moved it just a little bit here then this frequency here will be dragged the laser frequency will be dragged with it and the tuning range is really the width of the bat of the bandwidth of the of the amplifier because if I tune the cavity outside that I get no output output at all now here is a an amplifier that has a bandwidth that's bigger than what I showed you before which means now that the tuning range can be can be wider now depending again is say on the width on the bandwidth of the of the amplifier and if I in this particular case if this mode leaves the amplifier this end there's the other one that will come in so I will always get some output from the laser but the intensity can can vary depending on where I am with respect to ganker while over here in this previous one if the mode is outside the amplified bandwidth I get no light output at all so since so then the in summary then the tuning of a laser then depends on the bandwidth of the amplifier the wider the bandwidth the the wider the tuning range okay so now the the remaining property that we talked about before is the is the very short pulse width generation so the question is with us where does this come from again in a simple simple-minded way again here to explain this I need to to give you a little bit of a background and then start with with what we've done before if you had a delta function in in frequency space the in the time domain you have essentially an oscillation a sinusoidal oscillation with a constant amplitude and a constant frequency this is what you know we've been talking about before now if I looked at the intensity remember this is now at optical frequency maybe 10 to the 15 Hertz or so so it's very high frequency I cannot observe it I don't have any way of observing it but if I looked at the intensity associated with this using a square law detector what I get is essentially a constant output constant intensity output associated with this way so if I have then a delta function in frequency I have then intensity would be would be constant so this is completely opposite to generation of short short pulse this is essentially you know CW CW laser but now let's let's see what happens to this constant output when I add when I add another frequency if I have now two frequencies let's say that's that are oscillating one at F one f2 here they are different in wavelength and difference in the frequency separated let's say by F sub s now in the when I look at that when I combine them and look at them in the time domain I get something like this which is like a beat frequency or is a beat frequency the the the period between these Peaks t big t is just simply 1 over F sub s the separation in frequency but the actly if you look at the intensity associated with this wave looks something like this which is the envelope of this beat it's no longer constant just by simply adding two ways of different frequencies and we're all familiar with beating effects of when two singers sing with two different two different frequencies then you'll hear the beat so basically now I've transformed a constant intensity to a to assign a soil intensity it's not quite a short pulse yet but it's it's sort of getting there alright so now let's look at let's look at what happens when we add another yet another frequency alright here it is now we have three F 1 F 2 F 3 they are in the time domain I add them up and then give me some waveform like this when I Square this I get the intensity to look like this as you can see now this width is beginning to look like a pulse but again not quite again the the repetition between these two peaks the period between here and here the time between the two peaks or the two sort of pulses at this stage is again 1 over FS where FS is the spacing between these modes and we're assuming they are constant constant spacing between these frequencies now you can think you can add more and more frequencies and let's see and let's see what what happens now I've simplified the picture and I just put the intensity output that we've had before remember when we had only 1 was constant intensity when we had to we we have this sinusoidal behavior 3 is like this 4 if you add 4 begins like this 8 if you eight then now it's beginning to look like a pulse and then when you go to 16 frequencies that are instantly space then indeed begins to really look like a like a pulse the period okay the separation between these pulse strains is again 1 over F as the frequency spacing between the modes and the pulse width which is the key thing here the pulse width pulse width is given by 1 times n the number of of modes times F sub s so n times F sub s is really the the total separation in frequency between the first and the last and the last frequencies or modes alright so then then you can see here that the shorter the pulse alright if you want a very short pulse then you want to make n time some FS very large which means either n is very large or F sub s is is very large or it's certainly the product has to be very large so when that's the bandwidth of the essentially the of this the spectrum of these of these modes so the key thing then is to have let's say in this case lots of modes lots of modes extending over a wide frequency range and and then that would give us this this short short pulse alright big question now is where can we find all these frequencies from in a lazy alright so so let's let's go back and remind ourselves what we just mentioned about lasers we said that if we have a wide gain bandwidth gain medium amplifier and we have then lots of modes lots of cavity modes under the this this amplifier then and if there's enough gain to overcome the losses in all these modes and I would get essentially for four frequencies well before we only talked about one but now I can generate four frequencies out of this amplifier if the amplifier allows me to oscillate at all these frequency that's all very nice so we can see that this is the beginning of of having many modes so if you can imagine an amplifier that has this has much bigger bandwidth in this then I can have more and more more of these modes oscillating alright and then so of course if I have just remind you if I have you know sixteen of them then essentially and in a certain spacing between them but basically I can get a narrow pulse coming out of the laser here without essentially doing anything now the I didn't mention a certain key key issue here and that is when and then we can go back and show you when we when I combine the sine waves I mean let me go back to this this viewgraph here when I combine these sine waves here you know the the these waves had to be in phase somewhere they had to start start together and if they don't then you wouldn't get this kind of behavior because if the relative phases are way off and different than then you're in real trouble so so the let me see if I can find my view graph here and this shows that what happens when when all the sixteen modes in this case are all in phase or start together then indeed this is what I'll get I get this pulse width as given by 1 over NFS so this is a special condition I have to satisfy in order to generate this narrow pulse width and here's an example what happens if only ten of the sixteen start in phase and not all sixteen of them then I get a messy looking output like this and it's not like this at all so in order to to generate very short pulses out of out of lasers then then I need to get all these modes under the gain curve to start together and that is the the topic of of phase locking or mode locking and we will talk about that later so so lasers can generate very very short pulses only if you can get all the modes all these modes to go and and they have to start together they have to be phase locked or mode locked and so on and with that that's the output of the laser becomes the series of short pulses and if the bandwidth of the amplifier is very very broad then you can get these short pulses like picoseconds and and shorter alright so now we've talked about so far we've talked about the unique properties of lasers we talked about in a simple way how these properties come about now we're ready to to put all that stuff together and see how how a typical simple laser really works right so now let's let's start talking about the the simple laser so here the title of this session as I had before is the operation of a simple laser and I have even a demonstration for you at this stage the the lasers that that we have simple ones go helium neon laser and because the amplifier is a helium neon amplifying we'll talk about how that works in a little while now here the they have two kinds of lasers one where the the amplifier tube is terminated by the two mirrors and and the called internal mirror laser and the other one and this is quite typical for a lot of lasers where the the the amplifier is terminated separately not at the mirrors and the mirrors are are again separated from from the from the amplifier and usually these are windows called there's a funny angle brewster window so that to minimize reflections again we'll mention that later but basically here what I would like to show you in in this demonstration is simple ample lasers like these now this one here for example it can do very much because can't adjust anything it's all fixed but this one here if this mirror is is an adjustable mount then I can adjust them move the mirror around misaligned and so on you can see how how easily it is to to stop the laser from lazing and also what happens as a function of mirror adjustment so let's let's now go to to the demonstration number one which is essentially on the on a simple-minded laser and show you how how it works from close so now let's go look at this demonstration now we're ready to take a close look at a laser and see how it ticks and why it ticks we've picked on a helium-neon laser because a helium-neon laser is a very simple one and also it's one of the first lasers in fact the first laser was the Ruby laser and helium neon was was right after it so it was the second laser action that that was observed also the red light from a helium neon laser is familiar to almost every one of you are especially when you go to the supermarket and see it at the the checkout counter usually a helium neon laser a red helium neon laser is used so let me start by showing you what a helium neon laser looks like here is a helium neon laser at least a helium neon laser they all different ah now what I would like you to see here is that it's made up of a discharge tube which is this which is the the amplifier that is necessary for laser action and then the discharge tube is terminated by by two mirrors here's a mirror on on this side you can see it is one mirror and and here is here is the other mirror okay so it's a it's a small this is a small laser and and the mirrors are sealed right onto onto the discharge tube now let's see this laser in action and we have it already set up for you over here and then all I have to do is to turn the power supply on and count to five and the laser will come on and here it is lasers on and see here is the the glow in the in the discharge and you see a a pink streak and that's where the light goes backwards and forwards and gets amplified and and here is the output mirror the other mirror is sealed and the output from the mirror here then goes on to onto onto the screen and that's that's it as long as we power the discharge and have enough gain to overcome the losses and we choose the mirror transmission appropriately the you'll get laser action now with this kind of a laser it's very difficult to to adjust anything and to play with it so what I'm going to do just so that you can see a little bit more of how a laser works I'm going to go on and and and have a laser that that the mirrors in which the mirrors are external to the two the two now in order to to have to separate the mirrors out from the from the amplifier from the discharge tube in this case we have to seal the we have to seal the the discharge tube now here is you can see here if we take a close-up at this here is a a sealed discharge tube doesn't have mirrors on it but has windows let's let me show to you again here is is the is the amplifier section and here is the cathode and the anode is is over is over here so this charge then runs along this line over here okay this capillary tube over here now let's focus at the ends we don't have as you can see we don't have square mirrors you can see we don't have windows I should say not mirrors because we're going to have the mirrors external to to the discharge tube but you can see that the windows are sealed but they're not square to the to the two and the angle in fact is called the Brewster angle cause at that angle there is no reflection from the glass surfaces for a certain polarization so that's why one uses Windows at the Brewster angle so that so that there is no no reflection the windows and you can see both ends now if we take a look look at the other end over here close look at the other end you see that again both windows are are at Brewster's angle so here is then the the amplified tube and now we have another one a similar one that is placed and I'll position it in the same way that is placed over here that held held in place over here in this setup and and there's wires running to to run the discharge so here is then the the the amplified tube inside inside this laser and let me again point to the mirrors now here is here is one mirror and here is the other mirror and each mirror is held in a hefty mirror mounts and the adjustments are over here we have two adjustments over here and then we have similar adjustments over here all right so here is then the tube the amplify too the windows are terminated with windows at Brewster's angle and then we have the two mirrors so now I'm going to turn the discharge on and here it is and let's see if the laser is lazy alright so if I put a card here see that indeed laser is lazing so what i'm doing here is reflecting it by this mirror here then this mirror here here's the laser beam and again on to onto the screen so on the screen now we have essentially two spots so one is coming from the laser with the fixed mirrors that's this one and then the other one the other spot is coming from the laser with the adjustable mirrors here so what I'm going to do is first block the laser with the fixed mirrors so that we don't have any confusion and the only spot van is from the one from the laser with the adjustable mirrors so now here we are is the laser then all opened up and now I'm going to show you how touchy the alignment is so now if you watch the intensity on the screen as I advise slightly misaligned you can see the light is out already just by very small very small misalignment so here we are it's a peak value and then I go the other way and it's gone so the alignment has to be has to be very very stable all right now I can also adjust horizontal alignment again show you how how how touch everything is so here we are here is then in a nutshell is the is the amplifier section the longer it is of course the more gain we have and and the mirrors are placed this case about 50 centimeters apart one this one is a flat mirror and this one is a spherical mirror and and the the alignment has to be has to be very stable because a small misalignment would would create a lot of loss and then we just don't have enough gain in the amplifier to overcome these losses and the laser quits I hope you've enjoyed watching this first demonstration on what goes on inside inside the laser the of course the key thing is the amplifier and now I think we need to talk about how does a light amplifier work so this is what I have now for the next topic is it how does really an optical amplifier really work the so what what we mean by an optical amplifier we need light going in to this magic box here and we need the light enhanced or increased or amplified at the at the other end so the question is what is this magic box here that takes light from one end and actually amplifies it and sends it out at the other end all right that's that's the key question the so in order to answer this we have to we have to see what we understand about the interaction of radiation or light with atoms or molecules so let's let's start with that now we're very familiar for example with light interacting with the material and getting absorbed that we see that all the time that sometimes completely absorbed and sometimes it's not completely absorbed like a piece of glass for example if you send light through a piece of glass its uncoated normally incidents and so on you lose about 8% or so for both both sunny so other materials will lose more and then we see that all the time all right so first let's understand what that is and once we understand what absorption is then then let's see how then we can understand amplification so we'll start first by looking at absorption all right so here is the in order to understand absorption we need a model for for the app all right and as I say in this course we keep everything very simple so that we can get a basic intuitive kind of understanding of what's going on rather than complicate things with a lot of math now a model for for an atom all right is where let's say an atom has several energy levels or orbits or what have you and the atom is the ball here's the atom is is in its ground state or lowest energy state at at the beginning and all atoms essentially are at the lowest energy States at the beginning now when light comes in from over here let's say these are two photons coming in they will interact with the electrons at say in this atom and will excite the electrons to to some higher state or even higher States in this case I'm only taking a very simple model just to level system now if the if the light wavelength light frequency is not correct the the atom will not be all the electrons here will not be excited at all but if the light frequency or wavelength is a certain value of a certain value then the the electrons here can be excited and I'm now on I'm going to call it the atoms can be can be excited from one level to another level and when the atom is is excited from one level to another level then essentially it takes it takes energy from the light or it takes photons from the light to to get excited because now the the the atom itself has now higher energy than he started with and the of course the light gave up its energy to to the atom so it has less energy when when it leaves the app and this is generally called absorption and then let's say a few more words about that here again this picture that I had in terms of just straight waves here is a big wave coming in and a little wave coming out and that essentially is is absorption I've labeled here e1 and e2 for the energy levels and it turns out that the frequency where you get maximum absorption or the wavelength or frequency you get maximum absorption is is given by the difference in energy e2 minus e1 divided by H which is Planck's constant and that gives you the frequency that of at resonance and if you for example put put you put a detector out here and you can see that as you tune the frequency of the of this light source you have no absorption at the beginning that all the intensity comes through and then when you're close to resonance what you will see is you dip like this and back to no absorption so over here there is some absorption and this absorption occurs at this resonance frequency determined by the energy level difference divided by Planck's constant okay the wavelength of course is proportional to that as you know frequency times wavelength equals the speed of light so that either you talk about sequences or wavelength in this course I'd like to talk about frequency because it's easier to talk about frequencies and then wavelength the now if again the the amount of absorption will depend on on the number of atoms or molecules that you have and the width for the absorption again depends on the particular transition in that particular atom now if you don't believe me you can do an experiment and here's how you might do the the experiment you take this variable wavelength light source and you send it into selected vapor a vapor cell and you put a detector at the other end again you have now lots of atoms let's say or molecules you send the light in and some of these going to be excited you're going to go into this excited state and the light is reduced and that's what what we call a stimulated absorption and then we generate the same curve as we had before so if you look at the app of the detector as a function of frequency or wavelength then you would see you would see this dip occurring at this resonant frequency and this way you check out where this absorption occurs and and the width the width of it now the interesting thing here is that you note you'll notice that if this is a glass tube you'll notice the radiation coming out the other side now that radiation is when you see before the the excitation all the atoms were in the ground state and that radiates there was no radiation coming out from the sign but when when you excite these atoms to an excited state then another process comes in that's called spontaneous emission because these atoms that are in the excited state in lots of cases they don't like to stay up there they like to to come down and when they decay so called decay when they decay they emit a photon in a random direction and the wavelength or frequency of this photon is is equal to two the one that brought it brought it up so essentially the light that was absorbed from from the original beam is now sent into into all these directions okay so now we've learned about what stimulated absorption is and what spontaneous spontaneous emission ease question is what about game what we're after is amplification all right so let's see how amplification then comes about now for amplification to understand amplification of light we reverse the picture instead of starting with the atom in the ground state which way it likes to be normally we're going to start with the atom in the excited state so when when a photon comes in now there is no absorption that can take place but what can happen if the again if the frequency or wavelength of this photon is just right at the resonance frequency of this transition then what it what can happen is that this light now will force this atom to come down because it's let's say there's no other energy level to go up it can even push it up if there was an energy level here then it could kick it up but in this particular case we say we only have two levels so the only way for this atom to go is down and then of course when when an atom comes down remember Adam had energy up here when it comes down it will release a photon okay so so this photon let's say is the original photon here and this photon over here is due to the release of the photon from from this excited atom as it gets pushed by by this source here so now what we're seeing we're seeing that we're getting two photons okay when coming out when we had only one going in and as a result of of the excited atom has been pushed down to the ground state and this is the the essential or the basics of stimulated stimulated emission or gain because now here you see that light light can be amplified alright so let's talk a little bit more more about this as I said before just like in absorption the resonance frequency is again determined by the energy level difference divided by Planck's constant all right now in this particular case if what you see when you watch at the output you see that indeed the light gets increases now instead of decreases we had before when you hit when you hit resonance again the amount of increase will depend on again the of atoms that you have and the width will also depend on the on the transition of that for that particular atom or molecule but it's the width is exactly the same as that in absorption that we had before and and in fact the the size of the of the gain is also as I said depends on the number of atoms and in this case number of atoms in the excited state and would be exactly the same as if the process were reversed and all the atoms were in the ground state and the size of the absorption would be exactly the same as the size of the amplification and certainly the widths are the same and again if you don't if you don't believe me you do a little experiment alright and you do this little experiment here you take a light source and tunable light source and now you send it into into this amplifier we're not saying what kind of amplify it is right now but basically it's a it's a way of amplifying light it's an amplifier that can amplify light so again you take this light source tunable you send it in and you measure the output on a detective basically this is the this is the amplifier has all the atoms that say in the this excited state you send the light coming in and if you hit the resonance is that we hit the resonance and you start driving some of these atoms down and the output is essentially going to be bigger than the than the input just like we said before at a certain frequency you're going to get you're going to get the amplification of the slide now what about the while we're at this what about the spontaneous emission because before we said when on all the atoms are in the excited state there will be a spontaneous emission well that's true because if you wait long enough see if you had a condition like this where there's no light coming in all the atoms are excited if you wait long enough then all these atoms will decay spontaneously by themselves emitting light in all direction that's bad news because there's no more no more amplification but when when when the light comes in yes it will interact with atoms and bring them down and and then the light will be amplified but some of the other atoms here will also spontaneously decay and will emit radiation in all direction this is no good to you because these this light is lost and not available for for amplification so the whole secret then of amplification is to make sure that the these atoms here are used up to generate amp gain to come down driven by this field here and not by an out drop down by themselves and emit spontaneous emission that's a tricky thing and you have to make sure you can handle that now to summarize amplification is the following we need four for whether I just look at loss first when when we have a loss loss lossy device it means that we have before we only talked about one atom let's say in a state here I can put lots of atoms or molecules in let's say in the ground state but fewer in the in the excited state so if there is more in the lower than the upper level then the net effect is absorption so if N 1 which is the number in the lower level is bigger than n 2 then the net effect is absorption gain is when there is more in the upper than low because if n 2 is bigger than n 1 and we have net gain and from now on I'll be referring to it as a population inversion because normally atoms like to be in the ground state to start with and not in the in the excited state so I have to as we'll see later I have to do something to put the atoms up here and and the it is the net difference that will give me the will give me the game ok so now it's only a matter of how to put more atoms in the upper state than in the lower state that will give me that will give me the game all right question is how does the laser start so I have gained so what so I'm going to sit around waiting for the laser how does a laser stuck all right so that's my my next question is that how does a laser sorry no I wasn't looking at my the question is now I should hold off a little bit but how to make an optical amplifier and now let me just I got carried away not looking at my my visuals let me go back to fundamentals again I have all the atoms let's say to start with here in a typical atomic system molecular system but I need in order to get amplification I have to have more in the upper than then the lower so I need a so-called pump this pump could be as we'll see later could be optical field could be a discharge lamp could be a laser could be collision could be all kinds of things we'll talk about later but basically for now let's call it a pump and that's pump the job of this pump is take some of these atoms and put them upstairs here otherwise you know right now it's like an absorbent this material acts like an absorber over here it still looks like an absorber because you have more in the lower than in the upper and as I keep transferring atoms from lower level to the upper level I get to a stage where let's say in this case I have 4 & 4 and it's not is neither gay nor absorption if actually the material becomes becomes transparent even if I increase the light doesn't matter because I the net effect is I have equal number in the upper and lower and we call that saturation and there is no amplification or gain so we started as an absorber and with this kind of a pump we we created less absorption but no no gain so in order to to get gain I have to do a little trick all right so this this trick is is the following I cannot use a two-level system I have to use one more all right so we can accomplish eight this by adding three levels to this particular atom or molecule in this case to start with all the atoms are here in the lower level to start with as usual then you find put this pump and this pump again could be optical pump or or whatever this pump if it's if it resonates with the transition now from the first to the third level it can take some of these atoms and put them and put them up here now if we put them up here as soon as you put some atoms up here there is a population difference between level 3 and level 2 so now you have amplification for for transitions for light at this at this frequency but but if look at level 1 and level 3 that's still absorption or what have you but certainly no amplification so the amplification occurs as soon as you put one atom in level 3 because level 2 is is empty so you can create n 3 began n 2 which is the condition for amplification as soon as you pump one atom into this into this level so the amount of gain that you get will depend of course on n 3 minus n 2 on the population difference as we talked before and there'll be a constant and multiplies it that depends on again the particular transition the particular atom and so on but the key thing is that it's a population difference if n 3 is not bigger than n 2 and forget it there is no gain so the frequency for the amplification for the amplified light will be at the at this transition frequency which is e 3 minus e 2 divided by by H which is Planck's constant again if you want to do a experiment then then you do the then this is the following experiment you take again this amplifier you take this tunable source and the amplifier now has a pump the pump then takes atoms from here and puts them up here and creates a population inversion and then you tune this you tune this light source here and then you'll see that when you look at the output you see that again there will be amplification at the specific frequency that coincides this is with this transition alright so I'm sure now that after hearing me talk about all this I'm sure you'd like to see that indeed an optical amplification takes place or the amplification of light takes place and and and I we have a demonstration for you again using a helium neon laser as an amplifier at least the helium neon part as the amplifier and and and we do we will measure will actually measure amply amplification of light we're all familiar with with absorption and attenuation of light but in this particular demonstration will be amplification of light so I think now we're ready to you enjoy this demonstration so let's go and look at it as we have already learned the most important component in a laser or the heart of the laser is the gain medium or the optical amplifier this is no ordinary amplifier this is an amplifier for light and in this demonstration we're going to show you and hopefully convince you that indeed light can be amplifying the amount of amplification is not so huge but I'm sure will make it convincing enough so that you get the feel that indeed that light can be can be amplified the setup we're going to use is is here you're going to have a laser which is going to be a light source here's the here's the output of the of the laser we're going to reflect it by this mirror here and then this mirror over here now laser beam then enters this this optical amplifier now this optical amplifier is is essentially this so it's a discharge tube helium neon gas mixture that will give gain amplification or gain at 6328 angstroms in fact the light enters this window here and then comes out at the other end and that's what we have essentially are mounted here right below okay so the then the output through the through this amplifier and the amplifier right now is turned off then goes on to onto a detector over here and then the output of the detector then goes on to a scope onto oscilloscope over here and and also we look at the output on a on a digital on a digital digital meter alright so we have two ways of looking at the same aim at the same output alright now we're ready to to set to set zero and so first what I'm going to do is then block block the beam of light over here and let's look at the zero on the scope and the zero on the on the meter now the meter says double zero eight is not quite zero and the reason for that is because we have room light hitting the detector so what I'm going to do to get rid of that I'm going to put this little hood over the detector now we see that the output of the of the meter now is indeed is indeed zero and also hopefully then the output of the scope here this is will be our zero on the on the oscilloscope now if I take this card away let the light go through you can see that now the output on the scope has changed and the meter reads around three six four or six three or vert there about which is the output the laser now all I have to do is block block the laser beam and we go back to zero on the meter as well as zero on the scope here we are laser beams back again and then we get that same value again now we're all familiar with absorption of light and I just want to just demonstrate it just for reference I'm going to put a piece of glass in front of the of the laser beam and we know that glass normal incidence has reflectivity of about four percent per surface so I should get an attenuation of about eight percent or so when I put this piece of glass in the in the beam of light now you can see the meter has dropped now to three thirty three hundred and well if I can keep it still three hundred twenty something and if I take the piece of glass away you know we go up to the previous number of three sixty something so you can see we have about an attenuation of about eight or so percent now what I'm going to do now is put this piece of glass before the detector also and indeed I'm going to hold it against the the tube here so that I don't shake too much and again you can see that the attenuation is is also about about eight percent or so here let me put it in the front and and also then I'll put it in the back again and hopefully I'll put it in the same position you can see this OOP hold it still and then take it away and then roughly we get this eight percent attenuation so now we've demonstrated and indeed light can be can be easily easily attenuated now we come to this crucial demonstration of gain so now I want you to then watch the both the scope and and the meter as I turn on the the amplifier so here I'm going to now turn on the amplifier and remember this number is around three sixty three or so three sixty four now you can see on the scope we jumped a little bit and and the meter has gone up to three eighty something which is an increase of about five percent and we turn it off again you can see on the scope went down in fact just watch the scope for a little while I'm going to turn it on and off very fast so that you get a feel that here goes up a little bit and download it since it's only a few percent it's difficult to see it on the scope the way I have it set it up have it set up but it's much easier to see it on the meter so you can see that with amplification we have 380 something with no amplification then we're back to 360 something again roughly it's about 5% amplification now you may wonder that maybe what we're getting is when I turn on the amplifier that we're getting light from the amplifier that hits histor detect it's not really amplification so to prove this what I'm going to do is block the beam of light going into the amplifier so that is no light going onto the deck that we back to our zero on the meter and now what I'm going to do I'm going to turn on the the amplifier just by itself just to see if there's any light from the amplifies falling on the detector so here we go watch the meter and the scope and here is the amplifier on again you can see that there's no change on the meter or the scope is off and do it again on and and off so indeed we've shown that light from the amplifier or spontaneous emission from the amplifier is not hitting the detector and increasing the output so again let me just redo it again for you here is here is the light falling on the detector without amplification again 360 something and here comes the amplification again so yes close to 380 a little bit less than we had before maybe the amplifier is getting a little old here we are without amplification we have this value now I hope you've become a believer in that you've seen for yourself that light indeed can be can be amplifying the the last topic before we leave this session is on how does the laser start you know just because we have inverted population we have between let's say level 3 and level 2 and but how does it laser really start we have gain yes but how does it get started because at the beginning there was no light at this at this frequency of wavelength all right there's plenty of pump light but there's no light to start the stimulated emission going but it turns out that the light that the starts the laser going is spontaneous emission we talked before when Adams are left alone in the upper State they decay to a lower level and they can emit photons all right in all three directions now the spontaneous emission that the photon that is emitted along the axis of the cavity that's the one that gets the the stimulated emission going and I'll show it here here is the spontaneous emission along the axis it hits another atom here that's in the excited state so it gets amplified and then hit some more gets even more amplified and so on as he goes backwards and forward then this builds up and and you get enough gain to overcome the losses and the laser will oscillate and if one of the mirrors has some transmission then you get you get the output so indeed that's how an amplifier works and that's how a laser gets started and then when we come back for our next session we'll talk about the laser beam properties and other properties and so on

Channel: MIT OpenCourseWare

Views: 152,397

Rating: 4.9109254 out of 5

Keywords: lasers, fiberoptics, fundamentals, power, intensity, energy, wavelength, spot size, collimation, tuning range, spectral width, efficiency, size and weight

Id: urbZ8CTceu0

Length: 54min 49sec (3289 seconds)

Published: Wed Mar 21 2012