 So, in this hopefully short module we will talk about Alexandrite and Fiber lasers. So this is what we had discussed in the previous module. We are introducing very new discovery LED pumped Alexandrite lasers and of course if you can pump with LEDs instead of diode pumped solid state lasers that is beneficial in many ways including cost and ease of operation. So, what we said is that we have this 450 nanometer emitting LEDs that are used to build luminescent concentrators. So, let us before going further talk about what luminescent concentrators are. Once again this is not a very new idea. This idea has been around for a long time and it is a popular research topics for materials chemists, materials scientists. The idea is this you have a solar cell okay. In order to capture solar radiation what do you want? You want a large surface area is not it? But many times you also want to concentrate the solar cell you have captured. If you think of a more futuristic kind of applications we have a window right. Let us say I want my window I have big windows in a house I want these windows to act as solar cells which means the sunlight that falls on the windows I want to capture it and I want to utilize the energy. How will you do it? In the conventional design of solar cells if you put panels there then your window is going to become opaque right so is there a way of having a transparent window perhaps may be colored but transparent nevertheless but something that captures energy very efficiently and that will be nice in another way as well if the sunlight is completely absorbed and the energy is sent somewhere else then your room will remain cool okay that is the idea that is what has been utilized here. So the idea is this in case of solar concentration sunlight in case of the application we are talking about light from the LEDs comes from this direction. So this is your concentrator you place the LEDs here and there are several designs of LEDs I mean you have also seen right the value is coming during the value people use this long strings of LEDs and all but you also have the square LEDs right square LEDs are convenient because if you take something like a slide let us say okay microscope slide you can place the LEDs one by the other right and completely cover the surface. So now imagine a situation where you have something like a glass light will something like this on top you have LEDs facing downwards giving out light at the bottom you have LEDs facing upward so from both sides the light from the LEDs falls on the substance of which this block is slab is made of right now in this slab what you have is you have emitters emitters which absorb the LED light and in the ideal case scenario with quantum efficiency of one quantum yield of one emits a light as well in whatever their emission ranges okay then what will happen typically this medium in which the emitters are embedded that has a higher refractive index. So if higher refractive index here lower refractive index here then what will happen many times when light is emitted it is going to undergo total internal reflection right so that is what is shown here you see the squares what it means is at the junction you have an emitter or maybe here you have an emitter emitted light hits this wall then here then here so it goes in a zigzag fashion until it can go out from the side understand what I am saying so if total internal reflection is 100% this never 100% the design is very critical here but in this case in any case you are covering the surfaces by LEDs so if that if no emitted light is allowed to go out from this direction and let us say in this side I have polished it so it acts as a mirror then what will happen the only place from which the light can come out is this face okay so you have directional emission directional emission is once again a very important topic of research that has been going on for the last 7-8 years Lakovic among others has done a lot of work there in our own city in BRC Sarmishtha Datta Chaudhary works in that direction okay have you understood what is going on here so this is a very efficient way of concentrating the light the light is being captured on this surface and that surface and it is being emitted from much smaller surface so naturally it will get concentrated right so that is what is used to deliver a high dosage of light to anything that you keep on this side thus this is how a concentrator works are we clear and different people try different things organometallic complexes organic molecules nanoparticles that is these things are different choices for the emitters and what is the substrate going to be that is another question because ideally what you want is you want quantum efficiency of 1 the emitter should be such that it emits with quantum efficiency of 1 and also total reflection internal reflection should be the thing that if only 10% light gets total internally reflected then it is of no use okay this is how illumination concentrator works so the reason why I wanted to digress and talk about this is that this is really an important topic of current research anyway not only for people working in optics but more so for chemists most of the time chemists are working on things like this okay now let us come back so now you know what this is illumination concentrator now where will it emit it will emit in the region where the active component cerium yag emits and that is this big fat emission spectrum as we have said in the last module this emission spectrum has very strong overlap with the absorption of alexandrite so if you place it correctly and we will see how it is placed alexandrite will you can transfer all this energy to alexandrite and then the emission of alexandrite is stimulated emission from alexandrite is what is used to make the laser remember the laser we are going to we are talking about right now is not a pulse laser the CW laser in fact this application that is shown in this diagram there this alexandrite laser is used not as an oscillator but as an amplifier and since it is a an early report the Tysaphyl laser that is used as an oscillator you can use alexandrite as oscillator as well I am sure it is there or it will come very soon so you take the light out of this continuous with Tysaphyl laser and then see this design may be see this one at the bottom so this green thingy is your luminescent concentration concentrator okay and we say little will all the light will come out from one direction that is where the alexandrite rod is placed so it is very it is a beautiful design and the dimensions are such that the alexandrite rod completely covers that end so whatever light comes out of this luminescent concentrator has to be used to pump that alexandrite crystal that is there alexandrite rod that is there so that is a really nice design then it has been shown that it can act as an oscillator of course but here it is acting as an amplifier what kind of amplifier multi pass amplifier so you see this so light goes in it is reflected here comes here goes out and then it hits a mirror gets sent back this way it does many passes until it goes out after amplification okay this system is not there in the market but maybe when you set up your independent labs you are going to use things like this and I hope you understand that this also would lead to a significant amount of miniaturization compression of size okay now that being said let us not go home with the impression that you cannot make ultra first amplifiers with alexandrite in fact you can and as early as 1996 there have been reports once again you see squares name here square has done a lot of work in this direction so there what they did is 200 femtosecond pulses from ticep higher oscillator the amplified to millijoule level okay and then read the last line if you can this system also amplified femtosecond pulses from a frequency doubled rvm doped fiber laser so that is the other kind of laser we want to talk about we will get there this is the design you have ticep higher laser as the source it is not even shown here you have a stretcher you have a compressor here optical isolated is what separates the incoming and outgoing beam and here you see this pump laser is also alexandrite diode pump alexandrite laser so it is DPSS and the alexandrite rod is used as the gain medium in the amplifier and here you can see the auto correlation trace and that is I think if I remember correctly 300 femtosecond also okay now so that is what we wanted to say about alexandrite another class of lasers which has actually been marketed for a significant amount of time now is ultrafast fiber lasers we will not do a very thorough discussion but at least to get an idea about it you can read this review in nature photonics by Furman and Hartle you might have noticed that Furman also featured in the 1996 paper on alexandrite lasers now ultrafast fiber lasers are marketed by I think 40 companies now so it is not as uncommon as alexandrite even a alexandrite is not so uncommon but ultrafast fiber lasers are catching up peak time and one reason why you might want to use ultrafast fiber lasers is that is first of all they are compact you have a say 10 foot long fiber not 10 foot well let us say you have a 30 feet long fiber do you need a table that is 30 feet long to keep it right you can just coil it and keep it in a small box or even leave it on a table provided you do not have people in the lab who are going to go and hit it with a hammer or something right that is why it is very simple and also in the last 20 years I would say it is connecting fibers maintaining polarization fibers slicing fibers all this has become very easy nowadays you can buy a reasonably cheap accessories with which you can make all sorts of combinations of fibers so ultrafast fiber lasers are actually coming up in a big way now in these lasers you usually have two kinds of fibers one is you need an optical medium for that you have a small stretch of fiber usually you can have the entire fiber like that does not matter but usually you have a small stretch of fiber that is doped with something like a lanthanide ion that is your gain medium and then you have polarization maintaining fiber that makes up the cavity okay so I will show you one design there are many and this design is again from this 2012 paper and you will see the moment we go into fiber lasers our line of thinking is a continuation of what we have done so far for free space lasers but new things start coming in so here in this figure itself do you see the active media do you see the gain media yeah you want a different color YB fiber that is again medium dope fiber rest of it is just simple polarization maintaining fiber so this fiber works in two stages and you see where the difference comes first of all this gain medium fiber gain medium is pumped by 980 nanometer light and here you see there is a combination it is like railway tracks right one coming in and joining the other one this coupler that brings in a different wavelength of light is called WDM wavelength division multiplexer it can also be used to divide paths of two fiber of two kinds of light okay that pump light comes in and then pumps this YB fiber and initially you get CW operation they are isolated and all we do not have to talk about that here we see there is a 6040 coupler 6040 coupler means 60% of the light goes straight still CW 40% of the light enters this small loop what is there in the small loop it is sort of like it is amplification and this is not amplification maybe this is where mode locking takes place in the smaller loop once again you have a pump the 980 nanometer comes in and 1030 nanometer is the radiation the frequency of the wavelength of the light that you get out from this laser again you have this YB fiber it is pumped by this and then you get the output of this coming in here and in this loop you place a mode locking element the mode locking element that has been used here is called ANALM nonlinear amplifying loop mirror but I understand better because I do not know so much about fibers I understand better if I replace this by something like what we have discussed saturable absorber mirror remember saturable absorber mirrors first you have Bragg mirror where light goes in and comes out and when that happens mode locking is achieved because only light that is in phase survives everything else is eliminated and then we said that you put a quantum well in the in front you get a passive mode lock so you can put in something like that so mode lock light comes and joins the loop here and here you have an 80 20 coupler 20% goes out 80% goes back in the loop this way it is so you see the philosophy is the same but with some additional components okay and there are other designs as well so this is more or less what fiber lasers how fiber laser works and then just to complete the discussion without going into too much of detail you can in principle make an amplifier out of it and in fact all fiber amplifiers are marketed already by several companies now one thing that is very much there is that generally these optical fibers with which lasers have been made so far they all work very well in IR most of this fiber was made first of all for optical for this optical communication I remember sometimes you do not have internet because somebody has hacked the fiber underground so that is the telecommunication is why optical fibers became important so here also most of the lasers have been built upon similar same fibers so usually you get an IR laser and then it has to have sufficiently high power so that you can get a second harmonic, third harmonic, fourth harmonic to get visible light or if possible UV light in this design that we have shown all this is fiber but then to get a short pulse you have to use a bulk grating compressor so do not think that we have achieved a situation where it can be fiber and nothing else it is there but if you want good performance many times it is a combination of optical fibers and more conventional components like gratings or prism pairs so ultrafast fiber lasers are coming up big time and well companies market them of course would like to see them replaced isifier completely that has not happened so far but if it happens as we have discussed already there are several advantages perhaps you do not even need a very clean lab if optical fibers rule the roost one thing that I should say is this here there is an issue without using optical fibers if you want to make an ultrafast laser can somebody tell me what it is what do we have in the optical fiber is it free space then it is solid right something like glass yes dispersion is a very major problem so that is why you do not see a stretcher anywhere when you use an optical fiber your the pulse is stretched anyway stretching is not a problem so you have to compress it compression is a more important thing that is why see for compression they have used a grating it is not so easy to do it otherwise so it is on the face of it it might seem to be a disadvantage because it gets stretched but as we know if you are going to do chirp pulse amplification we have to stretch the pulse anyway so actually it turns out to be not such a bad thing after all okay so what we have done in this module and the last one is a very very preliminary discussion of ultrafast lasers and in one case CW laser beyond ties a fire this is not even the beginning if you want to really learn about these systems there is no dearth of papers no dearth of manuals no dearth of material on the web be my guest but the purpose of the course will stop the discussion here and then we are going to next talk about what we have in our lab.