 This is where we have stopped. We have introduced Pockel cell which is an electro-optic modulator. Remember earlier we have talked about acoustic-optic modulator, AOM as they are called in short. These are called EOM. So, henceforth you should not get scared or confused if people throw these acronyms at you. AOM, EOM you know what they are. Now we know how it works and we know that our ultimate goal is to obtain Q-switching by using a Pockel cell. If you understood these two the rest is not very difficult to understand. We will just introduce one more term that is Brewster angle. Again this discussion is completely from Silvers book. The book that we have been using so far I find that discussion of Pockel cell is a little easier and qualitative in Silvers book. In fact right after this discussion in the same book you do have a detailed mathematical discussion of the theory. If you are interested you are welcome to study. But we can develop a qualitative and understanding at least from what we are saying. So, this here is a cavity. I have forgotten to write which one is output coupler. So, I will tell you the mirror that is blacker we are saying that is the high reflector. This one that is less black then is the output coupler. So, if you ignore this Pockel cell in the between for now you can think it is a very simple laser cavity right. You have a gain medium which they have written amplifier do not get confused with this between this amplifier and the amplifier used in our lab is just gain medium. And you might see that this gain medium is shown in a little strange shape. It is not this cross section is not a square it is a trapezium. That is because in fact all gain media are kept or cut in this case at what is called Brewster angle. Brewster angle is such that if the surface is at Brewster angle then a particular polarization will be sustained everything else will be reflected. And you will see in a minute a few minutes how that becomes useful. Are we clear about the cavity and in the middle of it we have introduced the Pockel cell alright. So, let us see how Q switching is done in this case. For the moment let us think this is a CW laser. At the end of the discussion we will also mention unfortunately I have not drawn the diagram but I think you will be able to understand. We will say what happens if this is a modlocked laser. I can put in a modlocker here can't I yeah or it can be a this amplifier gain medium that can be ties of your sapphire crystal can it be. So, it can be CW can be modlock does not matter to start with let us say this is a CW laser alright. Now the way it works is this you apply the voltage to Pockel cell and you use a Pockel cell and use a voltage in such a way that polarization of light going in is rotated by 45 degrees. This is a very nice trick that is used ubiquitously in especially at a first spectroscopy right. So, the moment a beam of light passes through if it is linearly polarized the plane of polarization will be rotated not by 90 degrees but by 45 degrees okay. So, let us say we have plane polarized light like this vertically polarized light okay and Pockel cell is powered voltage is on. So, now when this light passes through when it emerges what will be the polarization it will be rotated by 45 degrees. So, well it is not so easy to draw 3 dimensional diagram on 2 dimensional surface. So, please imagine that this tilted arrow means 45 degree rotation of polarization okay. Now this goes hits the output coupler comes back and passes through Pockel cell again. I hope you agree with me that when it passes through Pockel cell again once again it will be rotated by 45 degrees and the light that now emerges is at 90 degrees right. You started with vertical polarization after a round trip through the Pockel cell polarization is now horizontal. This meaning of this doughnut is that this arrow one of the arrowheads is pointing towards you. In the later diagrams I have used a different kind of doughnut okay alright. Now do not forget that this amplifier or gain medium surface is cut at Brewster angle and what did we say? It is only going to sustain one kind of polarization in this case vertical polarization. So, what happens when this horizontally polarized light goes further and impinges upon that surface of the amplifier or gain medium it will not be allowed to enter right it will get reflected. So, it is no longer there. So, as long as the voltage is on the cavity is not there right. So, we have been able to satisfy the initial part in the scheme that we have drawn a little earlier right. This is a high loss cavity and you can actually have control loss in this medium maintain it as high low and then start pumping. As you pump the population of the higher energy level keeps increasing you know then what you do is after an appropriate time switch the voltage off. The moment you switch the voltage off Pockel cell is now if I can borrow terminology from our eminent inorganic chemistry colleagues. It is an innocent piece of optic until now it was non-innocent it was doing things to the polarization of the light now it is not. So, since it is now innocent what will happen is vertically polarized light will pass through without change in polarization hit the mirror come back pass again nothing happens goes through the amplifier now the cavity is on right and after a few round trips what will happen is threshold will be exceeded and light comes out like as a pulse this is Q switched operation and at that moment you are so enrosed at looking at the colorful light pulse you did not notice that there was another arrow that came up voltage is switched on which means the cavity is not there again. So, you understand there is a lot of electronics involved here precise timing is required and that is what limits the kind of pulse that you can get by using Q switching as we have discussed when we talked about TCSPC earlier all electronic components have their response time right. So, it will be impossible for you to keep this cavity on only for a few femtosecond it works best for nanosecond actually hundreds of picosecond may be yes anything less than that if you want a really 10 picosecond 10s of picosecond laser you use mode locking not Q switching femtosecond laser forget about it. But you will see how Q switching is important in femtosecond laser technology as well we will come to that but this is how you can produce a giant pulse of light using Pockel cell as a Q switch you understood all right we ask a question well let me remind you the question we asked all this time we were saying that this is a CW laser right suppose this is a mode locked laser then what will happen Pockel cell voltage is off for say 100 nanosecond or something and let us say this is a titanium sapphire laser. So, some pulses will come in that hundreds of nanosecond what is the separation in time between 2 successive pulses of a ties sapphire laser 12.2 nanosecond. So, if this Pockel cell is opened for 100 nanosecond there will be something like 10 pulses there. So, the output will be a giant pulse made up of small pulses right short pulses will it be a square pulse or will it be something like a Gaussian envelope you understand what I am saying right when the light goes out what goes out is whatever number of pulses are produced in that 100 nanosecond right pulses are 100 femtosecond or smaller what will the output look like will it be if it is a CW pulse output as a function of time is Gaussian is just that full width of maximum is nanosecond and not femtosecond. But if you have a mode locked ties sapphire or alexandrite or some such laser which is in any case producing femtosecond pulses at 80 megahertz then within that 100 nanosecond 10 pulses will come what I am asking is will the pulses be all the same in intensity or will it be something like going up and going down it would not be same because what happens is when it is coming even then it is doing round trips right. So, there will be an increase in energy and then eventually loss will take over then intensity will fall this is something that is important in what we are going to discuss next chart pulse amplification and this is something that we have seen in our lab also if you look at that oscilloscope you see some pulses right and then when you switch it off you do not see half of it that is what it is train of pulses and then one can think about applications like pulse shaping if you understand this alright so far so good. Now to our pockel cells the only things that can we use for Q-switching no you can use our old friend acoustic modulators as well not very difficult to understand use a Bragg cell right any emission that comes will be sent off axis by the Bragg cell most of the time and then it will you can use a suitable frequency of sound whenever it is favorable the cavity will be switched on so you can do Q-switching by AOMs as well EOMs are more popular actually since we are talking about EOMs let us not stop only at Q-switching let us talk about another application which might sound very similar after all I mean things are basic principles are the same right it is what kind of linear combination of things you take that determines what your application is so far we have seen how a pockel cell can be used for Q-switching getting a giant pulse out of a laser. Now let us discuss very briefly how a pockel cell and a thin film polarizer combination can be used to switch or steer a beam in direction that we want once again this is going to come useful in the next discussion the discussion we are going to have in the next module chart pulse amplification here remember earlier what worked was if it was only a pockel cell it would not have worked pockel cell could act as Q-switch because along with it you also used a gain medium at Brewster angle so it is a dialyser we talked about digests the other day the digests were never like this if this is the mirror always at Brewster angle Brewster angle is extremely important in all kinds of laser application because you are dealing with polarized light so here what you do is you use a combination of a pockel cell and a thin film polarizer has anybody ever seen a thin film polarizer is anybody into photography at all somebody has a good camera how good a camera I mean camera is as good as it is but how enthusiastic are you do you use external filters and all of with your camera okay so if you are more of an enthusiast what you will do is you will like to get the best possible picture and very often you will see photographs in which the sky is blue sounds foolish but it is blue like it is usually not to on naked eyes some filter has been used right very often you will see camera man using filters which are colorless sometimes they are you filters sometimes they are polarizers whenever we say polarizer especially chemists we think of that Canada balsam and whatever we think of a nice cube which has been cut from cut diagonally and glued together on that right it is not necessary that always you have to have cube polarizers you can have plate polarizers as well so thin plates they are called thin film polarizer right in fact the polarizers that we use in our TCSPC they are basically plates is not it so something like that so what you do is you right now we are not bothered about where exactly the end mirrors are and all here we have a focal cell and here we have a polarizer and voltage is off if you have a polarizer then it will allow either vertically polarized like to go through it or horizontally polarized like to it and like to go through it depending on what kind of orientation of polarizer you have taken right let us say we work with vertically polarized light voltage is off it goes through and then your polarizer is aligned so without any hassle light goes through and very typically it will this will apparatus will be inside the cavity of a laser we will discuss in next modules a little more detail okay so do you understand if the voltage is off then a vertically polarized light will go through this and well one thing I should say is that this thin film polarizer is kept at an angle to the direction of propagation of the laser very typically it is kept at 45 degrees okay so we have already shown you what happens with this combination of focal cell and TFP kept at 45 degrees or whatever degrees for a vertically polarized light it goes through right it depends on our requirement I mean only taking an example you might want the horizontally polarized light to go through then you choose the orientation of polarizer accordingly alright now what happens when without changing this apparatus right which means thin film polarizer is set so that vertically polarized light will go through I apply a voltage and I now this time I apply a voltage in such a way that polarization is rotated not by 45 degrees but by 90 degrees it is in my hand for a given focal cell you can do actually whatever you want now what will happen vertically polarized light comes when it goes through it becomes horizontally polarized hits this and gets reflected right so whenever I want the light to come in this direction I switch the voltage on for whatever duration I want whenever I want it to go straight I switch it off okay so this is a very efficient way in which you can switch an external beam into another laser cavity or switch the output from there alright now let us think a little bit we had said that we get giant pulses out of queues which laser right that is because we keep it on for some time at least some tens of nanosecond or something let us say I have a good focal cell which I can accurately control with the precision of say 5 nanosecond not impossible 3 nanosecond let us say 5 nanosecond which means I can have a fairly square pulse in that time scale and let us say that this horizontally polar vertically polarized light that we are talking about that is from a femtosecond laser okay so femtosecond laser gives a train of pulses all same intensity and if it is an oscillator your separation between 2 pulses is 12.2 nanosecond what I say is I will switch on the voltage only for 5 nanosecond and then switch it off for some time understood what I am saying what kind of output will I get in that direction in this direction yes but okay let us go in steps you will get a pulse will the pulse with change no no but repetition rate will change right in fact if I want I can switch out one single pulse right I keep it on for 5 nanosecond I mean yeah and then switch it off so anything that comes on the so in 5 nanosecond how many pulses can come if you are dealing with a tie sapphire oscillator only one because the next pulse is 12 nanosecond away so it is like a gate that opens for some time but the timing is such that maybe one person walks through so Q in Q switch what we had is it is like a opening the gate of some office building at the beginning of office hour or end of office hour for that matter lot of people rush through right and this one is like you open in the middle of the night one person is in walks out so beam steering and beam switching in and out of cavity that is where Q switches actually find application we might think that why are we even talking about Q switches because we are interested in ultrafast this is not ultrafast it does have application as we will see and the application is in something called chopped pulse amplification know this people Arthur Ashken Gerard Muro Donna Strickland the names ring a bell who are they they got the Nobel Prize right when yeah last year 2018 Nobel Prize in physics it was split into two half of it went to Arthur Ashken what did Ashken get it for he got it for something called laser tweezers micro manipulation and Muro and Strickland got it for chopped pulse amplification a technology that is used in all ultrafast labs worldwide now okay and that is based on what we have discussed so far so what we have done is we have talked about how to produce short pulses from ultra short to not so ultra short we have talked about how to modulate the repetition rate of the output by cavity dumping and then by Q switching and then now we are going back to a domain of ultrafast lasers where we learn how to amplify a pulse using all this and maybe a little more okay that is what we will do in the next module this is how you do it of course we will discuss it in detail in the next one but today let us at least give you a preview what happens is I hope this is something that you know now what this means it is electric field right electric field versus time plot for a pulse and you have a pulse from an oscillator and you want to amplify it you amplify it by switching the pulse in using Q switch into the cavity of another laser again the second laser has no output coupler only two high reflectors and there it gets amplified how it gets amplified we will learn in the next module but the problem is this you people calculated few weeks ago how much of energy there is per pulse if you are talking about is really small pulse so much of light introduced into another laser is going to destroy the optics and then you use a very neat trick and the trick is converting a difficulty into an advantage when we talked about construction of a Ticephile laser we have talked about chirping right that when ultrasonic pulse goes through some medium red light goes ahead and blue light trace behind of the other way down that causes a broadening of pulses and all that happens because remember we are working with a multi mode laser remember longitudinal modes that we talked about lots of modes are actually here what is done is before introduction of the ultra short pulse into the cavity of the second laser which is called an amplifier it is made incident on a couple of prisms or a couple of gratings which disperse the light and we introduce sharp intentionally you make red light go forward blue light trail behind so what you do is you expand the pulse from say 50 femtosecond or 25 femtosecond to whatever it is you make the pulse wide you make it a 20 picosecond or 2 picosecond or something broad pulse so then the energy that would have been incident on the optics in 20 femtosecond gets spread over maybe 10 times or 20 times or 100 times that time so that is why it is not so hard on the optics then it gets amplified comes out the problem is the output is also charged but you do not want it then you use a compressor again another pair of gratings in a different orientation which compensate for the charge that you are introduced right so while stretching the pulse you make different frequencies travel different paths let us say you have given a smaller path length for red larger path length for blue in the compressor after the pulse has been amplified in the compressor you do exactly the opposite you give now a longer path length for red a shorter path length for blue and then that spread that had taken place that is offset and they come together so once again you have an ultra short pulse the difference is the amplitude has increased from what it was by several orders of magnitude in the laser that we use we get from nano joule to milli joule right so that is what happens here so one thing I forgot to say is that in Q switch lasers the energy that you get is the power that you get is something like petawatt sometimes if you want you can get petawatt what is the meaning of petawatt you are very good with femto nano micro giga what is giga giga is 10 to the power what is 6 giga is 9 then after giga there is 10 to the power peta 10 to the power 15 not minus 15 the other way round so you get petawatt output if you use I mean you can get you do not always get petawatt lasers using your Q switch but the thing is giant pulses very wide here you pack all the energy in small time so you actually you have seen what kind of energies per pulse power per pulse you can get so that is what we are going to take up in the next module.