Fiberoptics Fundamentals | 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 well welcome back for the for the last hour in this in this course and we've discussed all kinds of things about lasers certainly not everything I haven't touched on cue switching cavity dumping and techniques for mode locking and so on but I think I hope if you didn't have too much of a background now at least you can see that there's nothing that magical about lasers that they all make sense and and if you were on some desert island with the bag of tricks you could probably create a laser for yourself now as I mentioned earlier I want to change topics now I want to switch to fiber optics because fiber optics is playing a key role in our lives today and I think it's it's good for for all sorts of people to know how how fiber optics how it works you know how it works and and some of the developments have been going on in fiber optics and maybe a few words about some applications of fiber optics so so here is the the very short course on on basics of fiber optics so the first question one asked well what's a fiber all right now fibers have been developed for for many years way before lasers and they basically look look like this if you look from from the end they look like I have a core you see which is one type of glass and of certain index let's say index N 1 and surrounded by another glass of index and two called the cladding and then the cladding is surrounded by a jacket and depends on the type of material that you use but whatever is a jacket and protects the protects declining the index refractive index and one of the core is bigger than n2 sometimes and to the the cladding material is just plain silicon dioxide and the index the material in the core is silicon dioxide plus some impurity with germanium dioxide to raise to raise the index the question is what's going on in here why play with with all these indices so the the key thing here is a total internal reflection all right total if the propagation of light in five is based on total internal reflection so let's let's then review it his array of light hitting the surface the interface here between N 1 and n 2 between the core and the and the cladding if this angle if this incidence angle is bigger than the critical angle then the light will be reflected and bounce around until it comes out the other end and because just like going from glass to air and the critical angle is given as we remember from high school as signed and minus 1 of n2 over n1 where n2 is the in the smaller index and n1 is Li is the bigger one if this angle of incidence is smaller than theta critical then there will be some transmission of light and not all the light will be will be reflected the just to remind you some more of what's going on let me remind you of the of the perfect mirror when I have a perfect mirror and the light comes in in essentially closed 100% of the light gets reflected and the reflectivity is close to 100% as si is 1 as a function of angle of incidence but if you have again the the glass air interface now we're coming from the glass side then as we vary this this angle the angle of incidence then the the reflectivity of this interface then depends on the polarization but right now we're not going to get involved with polarization but basically starts let's say between glass air starts like 4 percent and for one polarization goes through zero which is the Brewster angle here and then rises at the theta critical 200% and for the other polarization then it doesn't quite go to zero but again still Rises and goes to 100% at the theta critical so it doesn't matter which polarization they're both polarization will go to 100% for theta critical and and Beyond and that's what we take advantage of in fibers and let me tell you this is really 100% because you wouldn't get such low loss fibers if this one now the other interesting thing that I want to point out again from high school stuff is that it is what we call evanescent wave here there's a field that is propagates in the in the vacuum or in the air here but essentially just goes in and comes back again it's not a loss it's just that the when you have a glass air interface there is a field that does penetrate but it's still not considered as a loss but and then and then but then comes back here and gives us a hundred percent while in a perfect reflector there is no field present on the other side because the light 100 percent light gets reflected at the interface okay so if you keep that in mind that there isn't an evanescent there's a field penetration into the lower index medium the smaller index medium in this case it's air all right so with this as as background now let's look at the propagation in a in a fiber now it's not the like I showed you here exactly like I showed you is the Ray the Ray approximation but basically when you do the calculations what you get is a is a mode essentially like just like one in lasers is a field distribution here with a peaks at the center of the core and also extends a little bit into the they have an essence feel this is evanescent extends into the cladding so so so you can see this is the side of size of the core but you see the orange this orange if you can see this orange it's basically extends beyond the size of the core and that's what we call single mode propagation okay here's the field distribution here's the side of the mode so it extends a little bit into into the cladding most of it let's say goes into the core part of it goes into Klein and propagates right down the fight I mean this Ray idea is fine for explanation but basically when you when you do the calculation proper calculation that's what you get you get this for the lowest order mode now the the how how does the light look like when it comes out then then basically let's go back to the Ray picture where the Ray picture remember we said that the angle has to here be either equal to the critical or or bigger and then that then determines the the acceptance angle going in because because rays of light coming in outside this this cone here will be then will will not undergo total internal reflection complete total internal reflection and will well be in trouble because they won't propagate for too far and they'll be attenuated now for the for the light that is within this cone here and by the way this cone is referred to as the numerical aperture and a for the fiber and na is basically the n0 the refractive index of air or vacuum outside sine sine theta zero and theta zero is half this angle here which is determined by then the index difference and so on - so get us to the angle here to be bigger than the critical angle all right so so then that then this acceptance angle here the light within this cone when will propagate and in the end of fiber and when it comes out essentially comes out just like this cone here again with an angle to theta 0 so the light coming out from a fiber is expands pretty fast that's using the Ray picture if you look at the the true picture what happens actually you bring in light that you focus into into the core even though it spills over the core a little bit because of the evanescent wave they will propagate down the fiber and then will come out on the other side almost like like it came in so and this is very close close to the diffraction angle which is of the order of lambda over D where D is the size as we discussed way early in this in this course so even though it propagates as a collimated beam in the fiber but as soon as it leaves the fiber it expands spans very very rapidly all right so that's basically then how in a simple way how light propagates now the fact that we have low loss where does that come from and let me show you some data on on low loss propagation if you if you plot the loss in propagation as DB per kilometer and as a function of wavelength you find that it is a certain wavelength like one point five five or one point three the loss is very low it's about let's say 0.2 percent or 0.1 percent or so per kilometer at these wavelengths now 0.1 percent 0.1 DB I'm sorry 0.1 DB per kilometer is like two percent per kilometer loss and that's that's very small that's much smaller than propagating in wires and the reason why it's small its first of all is because the the total internal reflection is really a hundred percent right there is there is no loss because of that and the reason there's any loss at all is because your your squeezed in from one side on this side you have what we call Rayleigh scattering the fact that I don't want to get into it right now but the fact that that you get scattering essentially from from the fluctuations of density fluctuations within the glass that's on one side so as the wavelength gets shorter the scattering gets larger and larger and then you're in trouble that's the same reason why the sky is blue and so on now on the other side on this side here not shown is the is essentially the material absorption or the infrared absorption and so on for the for the let's say for the quartz for the glass so you are you're boxed in on one side you have Rayleigh scattering the other side going up is the material absorption and that's why you have the lowest loss around here and that's about the best you can do if you want to reduce this loss from let's say 0.1 0.2 DBS per kilometer then you have to use other materials and materials whose material absorption is further into the infrared so I'm afraid we're not going to get into the visible and have the low loss because the because of the Rayleigh scattering so going to be if we're going to go any we're going to do better than this we're going to go further into into the into the infrared which means we have to have the right sources and and the components and so on to work in the in the infrared and and the and a deeper in infrared okay so so now we know a little bit about loss in the fiber and basically it's we're very low loss if if the fiber is is constructed very carefully in the sense that it's it's synthesized from from pure materials and not just the any old piece of glass that you find some way okay synthesized it from scratch all right and it's drawn together and then you you have this core that has slightly higher index than the the cladding so you think everything is fine but there's a problem and that's we're going to discuss just like with lasers everybody here wants the the lowest order mode which is the single spot that comes out but what we find that there are other intensity distribution can come out and again these are called the transverse modes that can propagate in a fiber satisfying Maxwell's equations and so on and the boundary conditions in a fiber and for and this is called if a fiber is then then allows these modes to propagate then it's called a multimode fiber single mode fiber is the fiber that only allows this to go all the others have high loss and they and they don't go so now let's let's spend a couple of minutes on on single mode and multimode fibers before I show you some demonstrations the the in this case here I have this is the core and that's the cladding and the index of the core is is n1 which is higher than the index of the cladding which is n2 down here now the on the x-axis here I'm plotting what we call the V number what's the V number right now we're not we don't have time to go into any detail but the V number is essentially a parameter that depends on on one over the wavelength of the light on the D sub C which is the diamond of the core and the square root of N 1 squared that's N one being the core index and n 2 is the index the square root of n 1 squared minus n 2 square now turns out when this V number has a certain value let's say 2.4 or or so on then you only get if it's if it's if it's that value or less then you only get single mode propagation all the other modes have too much loss and they they don't they don't propagate so single mode fiber then is one where the V parameter which means the the size of the core the wavelength and the difference of in the indices are such that gives you the V parameter is smaller than two point for that single mode if you if the let's say you change the wavelength you make it shorter or so on V will go up and then you'll start getting all these multi modes or there's like I showed you I showed you before and and and then you'll be in trouble so for some application not all applications the in this particular case what I show you is the field distribution and the intensity distribution in the in the fiber here's the core is the size of the core and that's the cladding and you can see here depending on on the parameters that I choose some light from in this lowest order mode in the field will will propagate in the in the cladding and so with the essentially the intensity for the next high order mode okay which is similar to the laser cavity mode the field distribution is something like this and again part of it will will propagate into into in the cladding and when you square it up we look at the intensity looks like this that's what this two spots look like that but the field is really one is let's say positive the other one is it's negative and what you see here is that the the size of the of this higher-order mode is bigger than the lowest auto mode and more of it will propagate in the cladding than in the core so in terms of propagation constant or how fast they propagate this one probably gets more in the in the in the cladding than this one and since the index is smaller in the cladding than in the core then this one will go faster than this one and that's a problem in communications in using trans in this in this next the graphic I show how much again as a function of V number but how much of the light propagates in the core and how much propagates in the cladding so again for V number below this 2.4 or so the for single mode operation it turns out that when the V number is very very small most of the light I'm plotting here power in the core transmitting the core over the total power propagated the through the fiber then it turns out when the V number is very small even the single mode propagates more in the in the cladding than the cord okay because the ratio P core to P is very small and as I approach the multimode operation the the single mode travels let's say about the 80% or so in the core but not in the cladding now when the multi when the onset of multi modes comes about then the multi modes will propagate mostly in the in declining first more in the cladding than the core and then as I increase the V number they again will start coming in to the propagating in the core and then their index starts to approach the index of the single node but in all ways it's lower than that of the the single mode okay so they all travel essentially at at different speeds and also the transverse modes occupy they travel more in the in the in the cladding than in the than in the then the core as compared with the with a single mode now to give you to give you an example for single mode fiber let's say for a diameter of core diameter of five microns and a wavelength of 0.8 microns you need the you need N 1 minus n 2 the difference in the index to be 10 to the minus 3 now if you want to make the core diameter larger then you're going to end up with n 1 minus n 2 even smaller so it's very tricky to make single mode fibers otherwise you have multi mode operation all right so now that I have talked about about single mode and and and multimode fibers I have two demonstrations for you one basically shows single-mode fiber and coupling into it and what the output looks like and also we Bend the fiber and show that whatever you do is still single mode it doesn't change on you all happens is that you kick light out and you reduce the intensity but when we came to multimode something else happens so now the first let's go to the single mode fiber demonstration in this demonstration we're going to illustrate the propagation of light in a fiber glass fiber as we know today optical fibers have a lot of low loss very low loss of the order of one or two DBS per kilometer and of course they're being used for communication as well as other applications like sensors so the setup we have is a is a laser helium neon laser over here is the output from from the laser we're going to reflect it by this mirror and this mirror and then pass it through a lens this lens over here focuses the light into the fiber end and if we can take a close-up of what's going on over here what you would see is then a lens this lens then focusing the light and 500 fiber is very close to the lens and then the rest of the fiber is here all right so here is the rest of the fiber now this fiber are in fact what you're seeing over here is the essentially the plastic jacket the fiber core is about four microns in diameter and the cladding is 125 microns and the rest that you see here is the is the plastic jacket that's why it looks so so visible because it's so thick the other end of the fiber then goes into this holder and the the Chuck here it's in a whole fiber holding a chuck and the output of the of the fiber then is over here onto this onto this little screen now if if we can maybe we can take a close look at the the fiber end here what which shows that what you see over here in fact let me point to it what you see over here is the the cladding essentially we stripped the jacket and what you see here is just the cladding and this is 125 microns while over here over here is the is the fiber with the plastic with the plastic jacket so when you remove the plastic jacket then you have essentially what you see is the it's just a hundred twenty-five micron cladding all right so this is then the fiber and there is the output of the of the fiber now what we see if we can then enhance this and bring it bring it in what we see is the is the single mode behavior of a fiber and looks like almost like a Gaussian kind of spot not quite Gaussian but it looks like a Gaussian kind of spot now what I'm doing now is just it just adjusting the the coupling into into the fiber and it's very touchy because it said the core is only about four four microns all right so this is what then a single-mode our fiber the output from a single mode fiber looks like and as I miss a line here doesn't make any difference all you get is just a loss in intensity the shape of the mode stays stays the same so remember the core is four microns cladding is 125 the wavelength of the light is 63 28 angstroms and the core two index difference is about one part in in 10 to the three so this way you can show that indeed you get single mode propagation now I would like to illustrate some interesting phenomena about fibers so if we get the camera to look over here I want to illustrate how touchy our is the propagation of light in a in a single mode fiber now here is is a piece of fiber and you can see that there's no light scattered from the fiber now all I have to do is bend the fiber when finally beginning to see light that gets its transmitted out of the file gets essentially kicked out the fiber and because because of the bend now and and and the reason for that because it's you start going against the rules of propagation of light in a fiber for example if you take the Ray explanation is that what you're doing you are exceeding or you're changing the angle of light with respect to the to the to the to the fiber which means that if you're if you're below the critical angle then you the light is no longer totally internally reflected and therefore gets kicked out all right so here it is it's very dramatic as soon as you put a little bend in this fiber you can you can kick out a lot of light in fact it's the glow right here now if we can bring in the the output of the fiber into the inset over here now you can see that as I increase the bend alright you can see that the intensity yeah I'll do it even more intensity then drops quite a bit which means I've kicked out almost all all the light by simply putting a bend into in the fiber so the the illustration here then shows that if you leave the fiber alone without sharp bends everything is fine if you put in a band then you can kick out a lot of light and then not much will be will be will be transmitted so you have to be careful you don't put it too tight of an otherwise you fibers brittle and you break the fiber so you have to be be careful how you how you do it in this previous demonstration then what we had was a single mode single mode fiber which means that that's a fiber that satisfied this V number being less than 2.4 or so and and because of the again the choice of the core diameter the wavelength and the difference in the in the in the indices now in this next demonstration we're going to violate this we're going to create a V that is bigger than than this value here by in our case we did it by choosing a fiber that has a that's a bigger cord diameter but we could have also done it with by changing the wavelength and if you just shorten the wavelength go more into into the into the blue side of the spectrum then you can you start to see transverse modes like I showed you I showed you before okay and I think now we're ready to have fun with a multimode fiber now we've replaced the previous fiber with another one the only difference between this fiber and the previous one is that the core diameter is now a little bit bigger it's about 6 microns instead of 4 microns in the previous flight setup is exactly the same again we have the laser here getting reflected by this mirror in this mirror into the lens into this fiber this new fiber and then the the output of the fiber goes onto the screen so let's now take a look at the output of this flight uh-huh so what first thing we see that doesn't look like a single mode at all that's single low and if I change the adjustment if we can have a camera looking at my adjustments then you see that I can get a variety of shapes all I'm doing is changing the adjustment I can get a variety of shapes and clearly it's not like what we had before the single mode beef it'll be behavior that we had before and this is then is so-called multi mode what you're seeing are are different transverse modes that are that can propagate in this fiber because the core diameter is bigger but I'm going to leave it to you then to explain why exactly why you get higher higher mode higher transverse modes propagated in this in this fight again the wavelength is the same the only thing the only difference is that the core diameter is is bigger you can see this one here you get nice dark line in the middle and then then if I align it over here you can see it's blobby it's a mixture of moles that's why you don't see a sharp dark line in the middle till you get a mixture most well remember the other one no matter what I did in the alignment it was still single mode coming out just one lobe coming out all right now multimode fibers have many applications also but for today for a lot of sophisticated sensor applications and communications one uses one generally uses single mode fibers so here is a pretty-pretty mode without a dark line down the down the middle now what I would like to do is show how how touchy this fiber is to to bending in fact if we take a look at the fiber here as soon as I just press on it if we get take a closer look at if I just press on it I can kick light kick light out I just simply press on it I can click leave some light out it's very touchy to to to stress and to bends but I would like to do then is show that I can kick out some of the transverse modes by by simply bending or stressing the fiber what I'm going to do is then bend this fiber and if you watch in the in the inset and see that intensity will go down but if I keep increasing the bend I end up with single mode with a single mode propagation but but we and this illustrates that you can strip off the high order mode by simply bending bending the fiber but the penalty is you get much less light getting propagate so here we have single mode propagation and here we have multi mode or mixture multi modes that are propagating in the in the fiber again you can see that the there's a lot of light that gets kicked out has to be kicked out of the fiber you can see the bend absolutely glows in fact this this bending effect on on the transmission of light in a fiber can be used as a as a sensor sensor of pressure sensor of stress bends and and what happen all right so then in in summary we well that's that was all about transverse modes in fibers it's not easy to see these modes in normal fibers but I think this was a nice opportunity for you to see them and how how they get manipulate so what we know now about fibers then is that there's low loss because the material is correct we go to let's say the right wavelength and we have the they say lowest loss if you need the lowest loss for a lot of other application like sensors and so on you know the the fact that the low loss is not the most critical fact but for certainly for communications in long distance propagation that is that is very critical and then we also learned that that we have to be careful with this V number so that we make sure that we have let's say when we need it we have single mode propagation see the problem with these multi modes they travel at different speeds and then as we showed and then and then it prevents you from from using these fibers for wideband communication so for wideband communication you really need the single mode and also what's nice about propagating it around 1.5 microns or so is that the even the refractive index the variations in there is is also small so it's it's a it's a nice region to propagate for for communication applications but just fibers alone is not everything you still have to make connections you know like couplers you have to have other components and so on so let's now spend spend a few minutes on on fiber optic components like couplers because if you want to split the light by 50/50 or 80/20 or so on you need these directional couplers all done with fibers you need polarized because you don't want to come out of the fiber go through a bulk optic polarizer and go back into the fiber again you'd like to create polarisers right on the fibers you like to also control polarization you'd like to maybe create phase modulator x' frequency shift and so on all in fibers now I'm not going to have time to talk about all these things but I'm just going to talk a little bit about couplers maybe a little bit about phase phase modulators so how do you make now how do we make a coupler how do we transfer light let's say we have a fiber in here and we have a core in it and we're propagating down the core and normally the light would come out here but now I'd like to transfer the light to to another fiber so what one one does here you bring another fiber close to to this one now if with with fibers as we said before they have a core and they have a cladding and the cladding is usually much bigger than the core the core is usually a few microns the cladding as as I mentioned in the demo it's about 125 microns if you bring that close to this one then the the two cladding spacing will not let you transfer any light from one fiber to the other so in order to transfer light what you have to do take advantage of this evanescent wave the part of the evanescent wave that's traveling in declining so what you have to do then you have to shave off the cladding so that the that evanescent wave in in this fiber here fiber one okay if if the if you shave enough of the cladding this evanescent wave can start propagating in the in the second five in fiber two so when it does that then you can transfer energy to two fiber two and depending on this interaction length length of the interaction the closeness of the two cores then you can transfer and if 100% and back to zero so in this plot here we show that as a function of either the separation between the cores d-sub course the core separation or as a function of the interaction length for a given course separation you can transfer as much as a hundred percent delight you can make one hundred percent the light come out at port 2 instead of port one and if you keep changing this coupling then you can bring it back to zero again and up again and so on so that so that by by selecting the the right separation between the cores and the right interaction length then you can have you can transfer any amount of light that you want and and this is great because it's it's pretty low loss and is used of course all the time today these couplers can be what we call the Polish type or the fuse type and so on they're different types of couplers but basically they work on that principle of bringing the two cores close enough together so that the evanescent wave that little tail of that field distribution then will propagate in the in the other core and will drive the dipoles over there all right so that's basically the principle of a of a of a coupler all right and now I wanted to just say again a few words about phase modulator now phase modulators well how do you do that how do you modulate the phase of the light propagating in the fibre well again in many ways because if we stretch the fibre if we do something to the fiber we can change the Ascension of the index let's say in the in the fiber or change the physical length of the fiber and then we get a change in phase Delta Phi that's proportional to either change in the the length of the fiber or change the index of the fiber one way of doing this is to take the fiber and wrap it around a piezoelectric crystal if we put a voltage between the inside the outer surfaces of this crystal then we can change the diameter of this of this cylinder and then in this way we can change the physical length of the fiber or by stressing it you know essentially chain could change also the index but whatever yet you get a change in the phase of the light propagating by by applying a voltage either DC voltage of sinusoidal voltage to this cylinder piezo electric cylinder the another type of of phears electric device is a sheet this is a phase electric strip or sheet where the fiber is just attached to bonded to this you don't have the one is not rigidly to this to the sheet and then by applying a voltage between the upper and the lower surface you can again stretch the stretch the fiber and then you can essentially modulate the light that's propagate so so that's a very neat way you don't have to break the fiber it's still still intact and you can get the phase phase modulation now and then let's say said before there all kinds of neat devices that that use fibers say you can make polarizes frequency shifters and and so on but now there is another field that is the gaining momentum a lot and that's called integrated optics because sometimes to make these all fiber the couplers all fiber devices becomes a little tricky and very delicate so be nice to to replace them with something that's more more robust so I'd like to talk a little bit about integrated optic components and and what what are integrated optics first of all let me let me then explain to you a little bit about what's an integrated optic waveguide is so in this case the instead of it's not it's not a fiber but basically let's say you start with with some material like glass and the new in diffuse some impurity that raises the index and if this is controlled then using photo lithographic techniques and so on that you can raise the index over here so if you look from the side you have a material that has a little higher indexed and then the cladding over here and then you can propagate light down this this waveguide becomes a waveguide and you can and then the light is guided through the way of God just like in a fiber because the index here is higher than once on the side and then you can coat this and protect it and so on but the principle is that you can get single mode or multi mode wave guides in a in a substrate and it's rigid doesn't go anywhere and you can do all kinds of things with it so now with this with this idea here you can they can create a directional coupler either fixed or variable again you can create polarizer by preventing certain polarizations there are one polarization from from going by generating it you can create phase modulated frequency shifters intensity module and so on all using integrated optics so let's look at an example here again is is the substrate and here's the waveguide now if this material here is is before even I get to what I want to talk about is that you have to first couple couple to this waveguide remember the core can be very small just a few microns so you have to take the output from the fiber and couple it into into this course so that's a tricky device but again people have found nice techniques to do that so basically you want to fiber to a couple into this and and this to couple into the fiber going out if you can do that and then and these couplers they stay together then you've got something alright so now how do you create a phase modulator here now if this material is for example is slightly electro optic or electro optic then by putting electrodes here and applying a voltage difference between them then you can change the index either in the cladding or in the core and then and then just by simply applying a voltage in this way you can then phase modulate the light propagating in this section you can do it pretty fast and then you can get very fast phase modulator and if you ramp the phase and so on using certain techniques that we hear about today you can also do frequency shifting are quite easily using this device now if you want to use it as a coupler or as intensity modulator and so on you have to combine several waveguides two waveguides together and then you can create a coupler and so on but here I've chosen this example here where I have one guy that looks like this and then the other guy that looks like that now if I put light in in one arm then then again normally it would come out on over here but depending on the closeness of these two cores then I can get some light that will be coupled into this on the side will come out over here so this is a say a fixed coupler if you if you want to have a variable coupler again if this material of the substrate is is electro-optic then again putting electrodes on the material then you can change the essentially the index here between the between the cores and and this way will change the the coupling the coupling ratio then will change depending on the index so I can couple more light or less light by applying a voltage so this is a neat little device and and then there's also other applications you can also use it as a as a for example as a intensity modulator by by again applying a variable voltage here and then you can transfer you can vary the transfer of light from here to from this port to this sport and and so so it has many many applications sensors and so on so this is a very important field in in fiber optics as long as you can as long as you know how to couple back into fibers and in a rigid way without any reflections or big reflections and and so on so it's a very very very exciting development now what I'd like to do is talk a little bit about fibers for four sensors special fibers for sensors so far the fibers i've talked about are good for for communication okay single mode fibers low loss and and so on but for sensors there are all kinds of have been developed and it's a field that is growing a little difficult get started but it is it is gone now the is today we have what we call polarization maintaining fibers these are fibers that will will if you launch the field at one polarization will stay in that polarization there are fibers that that will only propagate one one polarization the other polarization will not propagate is too much loss and so you have essentially like a very long polarizer linear polarized then we get to what we call coated fibers these are fibers that are coated with special materials that if they are placed in a certain environment then that material will will start stretching and will stretch the fiber with it and then you can make a sensor for that particular species that the fiber is dip into you can also get other types of coatings you can coat the material with with with a conducting material like copper or so on you can make it more sensitive to temperature you can coat the fiber with piezoelectric material and you can make it into a phase modulator or magnetostrictive material and so on you can do all kinds of coatings on fibers and and use them for sensors and the jacketed fiber is almost like the the fiber you put you put it with a jacket and then you make the jacket sensitive to just like I was mentioned before to the to the specie that the fiber is dipped into then they are today it's a big field now the doping of fibers and for example if you dope glass fiber in the atom iam you have and you can pump it with with the laser with a semiconductor laser then you can have you can have a essentially an idiom laser a fiber laser and you can make them into a linear lasers or ring lasers and so on today by doping doping to fiber and and this is the the background for the fiber optic laser because you dope the fiber with all kinds of impurities and then you you get then you pump they have to pump them and then usually these are pumped with a semiconductor laser with some other laser and then you can create all sorts of lasers so you've got to watch this this field now it's going to be it's going to be very big then there's another type of fiber where you expose the cladding you shave off the cladding expose the core all right so by exposing the core means that you're exposing the evanescent way that I've talked about before and that can be used that tail of the evanescent wave can be used to to interact with the vapor or gas or liquid or whatever it is it's placed in and you can use that as a as a chemical sensor and then we have twin core fibers where the two cores are closed and then you can transfer light from one coat to the other all the way down the fiber and this can be used for for temperature stress and and so on is say this is a huge field and I think we need a whole one day on course on on fibers so so now I'm getting close to to the end and I hope I've got some ideas across about the basics of fibers without again without using math without scaring you off but obviously if if you have interest in these things you have to consult the literature or take take other courses in the few minutes remaining I would like to talk about just a few words about some future developments both in lasers and and fiber optics are the in terms of lasers suddenly we're going to see the rise and we're already seeing the rise of semiconductor lasers you know for all sorts of applications CDs and so on but we're also we're going to see them replacing the red helium neon laser today they're already available and in fact you see them as pointers and so on and and and so they've very neat little devices and for applications that don't require strict the wavelength stability and what-have-you I mean these these are great lasers we're also seeing and will seem much more semiconductor lasers that are used to pump solid-state laser and fiber lasers waveguide lasers and and so on and this is a very nice application for semiconductor lasers because again you don't have to worry about the stability of wavelength and so on and then they can they can pump very well-behaved solid-state lasers and today these can be also very big they don't have it very small it can be very big and you have the array of semiconductor lasers that will pump them and then we go from the big to the small we have even micro chip lasers very tiny crystals and then they are pumped again with let's say semiconductor lasers or other lasers but a very tiny and they can be single frequency and tunable and so on and the fiber-optic lasers where we already we already mentioned all sorts of materials being looked at and and pumping sources and then the same film wave guys well we talked about wave guides all you have to do is you dope that waveguide pump it with a with a with a optical pump and then you've got yourself a laser in that in that substrate and we're vacuum ultraviolet lasers are very important in medical applications and other applications and we're seeing lots of new developments in here and also x-ray lasers you know the early early stages they classify the development but now I think the literature now if we see all kinds of work going on in x-ray lasers with again completely exciting area of applications for x-ray lasers now this is related applications the you know I mentioned before fiber optic communication that's a huge application and we're going to see I think not that far we're going to see that their homes and all that will be linked with with fiber optic cables and this way we can have huge bandwidth it's going to be transmitted in in these fibers and then we can send all kinds of information all over the place this areas of materials processing again because of the availability of a variety of lasers that is that certainly is increasing and and the varieties also increase and medical procedures is if you keep up with what's going on it's it's it's incredible what the lasers are being used for procedures and for diagnostics and and so on now here's a he's a tough area data processing lasers are being used but but hopefully the the will increase more you can take very fast Fourier transforms optical techniques but again you have the implementation of them is not it's not that easy now sensors have already mentioned some words about sensors but not just fiber optic sensors but all kinds of optical sensors they are there are incredible devices in the sense that that they're not affected by especially the fiber optic sensors they're not affected by electromagnetic interference and and they can be used in all sorts of hostile kind of environments that normal electrical sensors would not would not work at all because they will be affected by the electromagnetic interference the also because fiber optics fibers have low loss you can have all kinds of distributed the sensors and and and so on because the the fiber is a doesn't have any loss so you can place sort of sensors all over the place and collect data from a large area now I mentioned earlier in precision positioning this is based on interferometry laser interferometry using laser interferometry you can monitor the positions of work pieces and and so on and dimensions and sizes and movements of all kinds of things and you can put them in feedback loops and they improve things for you then laser shows I don't know I don't keep up with laser shows but every time I see them they always fascinate me because I'm always curious about what kind of lasers they're using and today they can use them to write all kinds of patterns and messages and so on for people then there's the computer interconnects these are the fiber optics or integrated optics that are being used for for connect computers or even connecting boards and and hopefully sometime in the near future we will see all optical computers now these will be very fast based on these mode lock very short pulses and but a lot of challenges and hopefully some of these challenges will be overcome then in terms of high-density memories well we can already seeing that now and and then but hopefully they will even be even more dense you're taking advantage of three-dimensional structures and so on and finally I'd love to see some color holography becoming becoming popular so that we can have all sorts of color Holograms around around work places around at homes and so on it's a it was so far there's all kinds of kind of a log of available but the true color holography even though it is feasible we haven't we haven't seen them at least that available so that one can purchase them and put them up on the wall I see that that my time is it's close to coming up to an end and I hope that I've been able to to get some ideas across to you again without without scaring you off suddenly I hope that for those of you who didn't have too much background I hope you have motivate motivated you take some interest in lasers in fiber optics and maybe you can find some new applications for them because the field is is very young and it needs it needs some more energetic people that will come in and especially from from different fields it's not always you want you to always talk to people in the optics and laser if you wants you to be talking to to others in other field maybe they'll recognize they recognize some new applications as we can see they're really so many applications of lasers and fiber optics now before I close I I'd like to thank two of my graduate students Steve Smith and farhad's Erin Aichi now I have put them in a circle here because it's difficult to choose between them although farhad's energy has now graduated Steve Smith is is going to be graduating soon and they've helped considerably in the in the creation of the of the demonstrations and also in the creation creation of the of these visuals I hope that you found them readable and and without without a strain so I think I'm almost then on time and I'd like to thank you for for listening and and if you have any questions or so on I think you have our phone numbers here at MIT and you can call and we can continue with this conversation goodbye for now

Channel: MIT OpenCourseWare

Views: 32,232

Rating: 4.957356 out of 5

Keywords: lasers, fiberoptics, fundamentals, propagation, loss, single mode, multimode, single polarization, amplifier

Id: 0DCrIAxEv_Y

Length: 54min 53sec (3293 seconds)

Published: Wed Mar 21 2012