 We continue our discussion on chirp pulse amplification as we have said already this is how it is done. Output of oscillator is stretched goes into an amplifier gets amplified but remains chirped and then you compress to remove the chirp to get the desired amplified uncharped output. In the last couple of modules we discussed the stretching and compressing bit. Now we are going to talk about what happens inside the amplifier. The general introduction to this is this. The way you do amplification is first you do seeding. Seeding means introduce a chirped pulse into a pumped gain medium and the gain medium is typically the same as the one that you have used to generate the ultra short pulse in the first place. So if you are using a tie sapphire laser typically you use a tie sapphire gain medium but it is not compulsory. You do have ultrafast fiber lasers which are used to seed tie sapphire amplifiers. For now since we use tie sapphire oscillator and tie sapphire amplifier let us just stick to that okay. So essentially what you do is you introduce a chirped pulse into a pumped gain medium tie sapphire laser then what will happen? The pulse when it goes to the pumped gain medium will find a lot of ions molecules whatever it is in their excited state right. So it is going to cause stimulated emission and that will result in amplification. Now you make this go back into the gain medium again you will further amplification and so on and so forth okay. So you do a number of round trips in order to amplify the light that you have seeded in okay. What is the number of round trips? Is it better to have 5? Is it better to have 500? Is it better if you keep increasing? Maximum is the best? Actually not. As you are going to demonstrate maximum is not the best we will see what that means. And then when you have reached amplification to a sufficient level switch the pulse out of the amplifier and into the compressor. How do you switch the pulse out of the amplifier we will see okay. But this is the general way in which amplification of an ultra short pulse is done after being stretched okay. So we are going to discuss two different kinds of amplifiers one the multi-pass amplifier and second regenerative amplifier. In our lab we have a regenerative amplifier when do you choose which one? You choose a multi-pass amplifier when your pulses are really very short 6 femtosecond 10 femtosecond. But then the output generally does not have as much of power as you can get in region. Region means regenerative amplifier okay. Sometimes if you want an ultra really really ultra short pulse as well as high power you might have to use multi-pass as well as regenerative amplifier. So there are systems in which both combination of both are there okay. So let us see how a multi-pass amplifier works. So I hope it is not very difficult to understand what we have drawn here. We have drawn a gain medium and we have drawn a lot of mirrors M1, M2, M3, M4, M5, M6. Now there is no guarantee that there will be 6 meters. There can be more there can be less and the geometry can be such that M2 can be used as M2 as well as M4 it all depends on how efficiently one can design it okay. So to start with you pump the gain medium and as we have discussed you will find a lot of excited state population in that case. Then the chart output of the stretcher is fed into the gain medium this is called the seed. I hope you are familiar with this can you see the chart here or is it too small okay. So it goes through. So in the first pass itself I hope you will agree that there will be some amplification because it is already pumped right. So if it is a ticep fire crystal that is a gain medium then you will typically pump it with some NDEL laser or something like that okay. We will talk about this pumping laser and all in a little more detail when we talk about regenerative amplification. So now the mirrors are arranged in such a way that after hitting M1 the beam goes to M2 and then M2 sends it through the gain medium once again on to M3. So now this is a second pass through the gain medium so there will be further amplification right. Next step M3 sends the beam to M4 from M4 again it goes to the gain medium to M5 third pass from M5 it goes to M6 and from M6 it makes the fourth pass through the gain medium and goes out and what goes out is the amplified chop pulse alright. Good thing about multipass amplifier is that the only medium through which the beam travels is the gain medium everything else is reflective. That is why it gives you very short pulses. Problem is amplification is not so much because how many mirrors can you put in? It all depends on that if you can put in the 50 mirrors and make that number of passes then perhaps it will be very large but typically this is used so that the pulse does not become too broad and it is useful in applications where you need a really really short pulse and you can afford to compromise on the energy a little bit. Now let us come to the design we have in our lab it is called regenerative amplifier. Here you have a higher output power the difference between a multipass amplifier and regenerative amplifier is that in a region the gain medium is actually inside a cavity. So it is a laser by itself so what you do is okay these green mirrors are the pump mirrors the black mirrors are the two mirrors of the laser the point is both are high reflector mirrors there is no output coupler we have encountered this right when we discuss cavity dumper we have said there is no output coupler by the acoustic modulator black cell you get the beam out. So here it is something like that both are high reflectives you cannot afford to have an output coupler when you are talking about when you are trying to amplify because before the sufficient number of passes is made your beam is going to exceed the threshold and it will go out so you have to use something else we will see what okay. So first of all you pump the gain medium then okay right first I will show you a schematic then I will show you another schematic the second schematic will be a little more detail than the first so you pump it and then the excitation population is built for some time and all and then you put the seed in and typically and we are going to elaborate upon this in the next slide oops put the seed in and typically what you do is in this mirror I forgot one slide anyway I can just tell you about it it hits the gain medium itself and this gain medium of course is the ends are at Brewster angle so it hits the gain medium and then does multiple round trips in the laser cavity itself. Now all these arrows that I have drawn are displaced with respect to each other please do not take that seriously they are all actually the same axis but then if I try to do that here will not be able to see anything except some arrowheads coming up here and there which you might miss alright so this is what happens so it goes around in the cavity and as you would have understood by now in every round trip the beam gains energy or in other words gets amplified and then after the required number of round trips you have some way of switching it out and what you switch out is once again the amplified chap pulse how do you do the switching in how do you do switching out the answer to that comes from what we had learned maybe 3 or 4 modules ago remember we have talked about acoustic modulators electro optic modulators Q switch so the answer here is Q switch and the way it is done is not difficult to understand but you have to do actually a lot of things to make this happen so in the setup that we are going to talk about now this is what is discussed in this interaction laser spectroscopy book and this is more or less the arrangement that we have in our laser our amplifier so we are going to discuss this where you use two pocket cells a quarter wave plate and a thin film polarizer now unfortunately the side I forgot is a simpler well simpler to see design where you do not use two pocket cell use only one simpler to see difficult implement very difficult to implement so if you ever make if you ever have to build your own amplifier please use two pocket cells otherwise this alignment becomes a complete nightmare so there you use something called a Faraday rotator and all so where it is not at all easy maybe next day in later module will start with at least the schematic of that design but this is something that is much more popular now okay now it is placed in such a way that this Brewster angle supports one kind of polarization right either horizontal or vertical the way I have drawn it is that I have said that horizontal polarization from the reaction we are looking goes through the gate medium that is how the Brewster angle is and this thin film polarizer is also such that it transmits that polarization which is sustained in the gain medium that is what you need to remember of course if we cross this polarizer then no beam will ever get through alright so this thin film polarizer allows the same polarization to pass through as the one that is allowed by the gain medium by virtue of its Brewster angle alright so now let us see how this happens this will this is something that we should definitely know okay so first of all we start from a condition where both pocket cells are off remember what pocket cells do if they are powered what do they do if I apply high voltage to pocket cell what does it do this is the pocket cell light passes through this is basically a window of some optical material and it has two electrodes on two sides if you apply high voltage the pocket cell turns the polarization of the light and what we have said is that depending on how much voltage you apply you can make this polarization turn by 45 degrees or 90 degrees or whatever angle you want and you can do it at different extents so if you want you can even generate circularly polarized or elliptically polarized light okay so to start with pocket cells are off that means they are just pieces of glass and see this is why you cannot use this if you want a 6 femtosecond laser because in addition to your gain medium the beam is passing through significant amount of glass 50 femtosecond is fine okay now first both pocket cells are off okay now then you pump it typically you pump not by a CW laser but by a pulsed laser but it is not an ultra short pulse it is by our standards ultra long pulse something like 250 nanoseconds there are advantages of using a pulsed pump first advantage I think we all know by now if you use CW light versus if you use a pulse light in pulse in pulse operation we pack the energy in some small amount of time since we are so used to femtosecond we are being snobbish and we are saying 250 nanoseconds is a long time but 50 nanoseconds we will try counting on counting using some stopwatch you will know how difficult it is so 250 nanoseconds is also a small time right so you pack all the energy in that time that helps and secondly as we are going to discuss later timing is very important in this kind of amplification process if you use pulses it becomes easier to time we will discuss this not only in this module or the next one but also in the module where we actually show you photographs of the amplifier and discuss okay so it is pumped by a pulsed laser so in the gain medium excited state population has grown in that condition introduce the seed and here comes the effect of polarization okay so let us say this vertical polarization is what cannot go through the gain medium and the polarizer then what will happen the seed will be reflected by the gain medium it will come this way are we clear yeah so it will go towards your left where the lambda y4 is it will go in that direction focal cell is switched off nothing will happen but then the moment it reaches the lambda y4 plate it will turn by 45 degrees okay then it goes hits the other mirror then when it comes back and passes through the lambda y4 plate again what will happen will it go back to its original position or will it turn by 45 degrees more thankfully it will turn by 45 degrees more otherwise this would not have worked comes back turns by 45 degrees more and now you have horizontal polarization horizontally polarized light is what can go through the gain medium as well as the thin film polarizer okay so it goes through what do we need to do now we need to make it oscillate in the cavity make it do round trips so that it will get amplified but see now there is a problem the problem is if it goes back in this condition once again at lambda y4 plate it will turn by 45 while coming back it will turn by 45 more it will become vertically polarized so it will not come back that round trip will not take place and then when it comes from the high reflector side and hits the Brewster window it will actually go out in the same direction from which the seat came right it is not as if it will go back and make another round trip it would not you understand the problem so you have to do something that is why the Pockel cell is here now what you do is you switch on the Pockel cell and you switch on the Pockel cell and apply voltage in such a way that it is going to introduce 45 degree rotation of polarization now let us see what happens when this horizontally polarized light comes back after reflection in this mirror comes back goes through gain medium Pockel cell turned by 45 degrees then what will happen when it goes through lambda y4 plate turn by 45 degrees more so it will become vertically polarized lambda y4 plate vertical polarization that will go hit the mirror come back turn by 45 degrees again now when it goes through Pockel cell once again it turns by 45 degrees becomes horizontally polarized all over again now it can do the round trip as long as Pockel cell 1 is powered have you understood so you cannot do this by using passive optics alone you have to do something actively and that is where Pockel cell plays a role okay so it can go here and then it can do round trips okay typically you make it do 10 to 20 round trips why not more we will see shortly okay after that after it has done 10 or 20 round trips what will happen amplification will happen yeah when you have reached the required amplification level then what you do now is now you want to switch it out isn't it so to do that now Pockel cell 2 is switched on and Pockel cell 1 is switched off see it is not necessary to switch off Pockel cell 1 just because you want to take the beam out Pockel cell 1 is switched off because it is no longer has to be switched on and also you have to prepare the amplifier for the next seed pulse so this Pockel cell is switched on in such a way that it turns the polarization by 90 degrees so now what will happen horizontally polarized light comes back from the mirror turns by 90 degrees will it go through the thin film polarizer now thin film polarizer is set so that horizontally polarized light will go through vertically polarized light will be reflected so that is what will happen it becomes vertically polarized here its thin film polarizer and then it goes out and it goes out as once again the amplified shaft pulse right so this is what happens in the laser we use right is any question it is very important that we know the sequence of events here it is also very important that we understand that timing is king that is where all the electronics comes in here all the timing circuits all this has to be precisely timed otherwise it will not happen okay okay let us calculate something what is the round trip time let us say how wide is our laser I think we know that it is kept like the how wide is our laser 2 feet so round trip is 4 feet yeah so 4 feet means what 4 x 30 120 centimeter so how much time what will be the round trip actually it is easier if you keep it in feet 4 nanosecond yeah so every round trip takes 4 nanosecond all right and we have said the pulse that we use to pump is a 250 nanosecond pulse how many round trips can you do while the gain is on say 250 by what is the round trip we said 4 nanosecond right 250 by 4 is how much 240 by 4 60 in principle you can do 60 round trips okay now and when should the next seed come in next seed should come in only after the amplified shaft pulse has left and the next pulse from the pump laser has come this is a periodic process right so how frequently can we get an amplified shaft pulse out of this system who will determine that so what we see is that the game begins with pumping and we are pumping by a pulse laser that would have some repetition rate yeah so each pumping event initiates the process that leads to amplification so for each pump pulse I can get one amplified shaft pulse right so what I am trying to say is that the output of this amplifier is going to be determined the sorry repetition rate of the output of this kind of an amplifier will be determined by the repetition rate of the pulsed pump laser is that right it can never be more than that actually it is exactly equal to it so now tell me what is the output that we have what is the repetition rate of our amplifier is 1 kilo hertz where does that 1 kilo hertz come from the oscillator is 8 mega hertz why is the output 1 kilo hertz because the evolution laser that is used to pump the gain medium of the amplifier operates at 10 kilo hertz what is the meaning of 10 kilo hertz what is the time separation between two pulses 10 kilo hertz is 10 to the power 4 times per second one way that is 10 to the power minus 4 second which means 0.1 millisecond which means 100 micro second right so pulses are separated by 100 micro second and their pulse width full width of max is 250 picosecond 250 nanosecond okay that is what happens so timing is of utmost importance in this kind of pulsed operation that is what we need to know I think we will stop here today even though I do have one more slide it is better that we go back understand this completely come back next day do a quick revision of this thing and then go on to talk about timing and then what is there inside our amplifier okay that is what we will do in the next day.