 So we are discussing the optical parametric amplifier we have in our lab Topaz-C what we have said so far is that we put in the beam the pump beam output of the part of the output of the regenerative amplifier we have using an all mirror telescope and the reason why we need so many why do we need so many mirrors let me ask you yes one is path length you might have to give a particular delay was later on you have some other application right. So for that a part of the beam will go somewhere else so you need to give a path length secondly you need at least two mirrors if you want to align a beam. So as we are going to discuss we already started in the last module we will complete in this module as we will discuss alignment is very critical in this things like this. So generally things where life is much simpler if you try and make the alignment in such a way that at least in every stage the beam is horizontal okay. So we do all our experiments on optical table which in any case has this array of holes where you can put in screws and fix optics the holes have a second purpose they tell you which one is the vertical direction which one is horizontal direction. So if you want to make an instrument and you have to actually build something the easiest way of trying to do it is that try and make your beams go along a line of holes and to start with using a ruler make sure that the height is the same that way later on alignment becomes much easier and if something goes wrong you can take easily. So the steering wheel at least two steering mirrors are required to ensure that the input is straight horizontally well like this it should not be like this it should be like this should not be like this it should be like this right both these directions can be fixed easily if you have at least two steering mirrors and this is what we have shown you we said that you have light coming in from the region and using beam splitter and different optics we speed it into six different paths we have already discussed the first path input beam and we have started talking about the pump beam. Now in this module what we will do is we are going to talk about all the paths that are there inside our topas okay this is what we have presented already light comes in from the region that is path one input pump beam 80 to 90% of it is reflected towards M7 and we will see what happens after M7 towards the end of our discussion. The remaining part goes straight and we said the next pieces of optics that this transmitted beam encounters are L1 and L2 two convex lenses they are arranged in such a way that their focal points are coincident and the L1 and L2 are chosen in such a way that the focal length of L1 is longer than the focal length of L2 so that after the beam passes through this you get once again a collimated beam but a collimated beam of a narrower beam diameter smaller beam diameter than the input beam you require this because later on this beam has to go through several light races and all very broad beam would cause loss of energy and inefficient amplification right but let us see what is there before we draw the path or maybe we will draw the path first this is how it goes well it goes further but we stop it here because we want to discuss something right. So first of all what we have said already is that L1 and L2 serve to decrease the beam diameter secondly the other two things that are there right after this and we are going to try to show you some isometric diagrams of these not photographs so if you get to work on a topaz it would be helpful if you first go through this presentation then read the manual or read the manual along with this presentation look at the isometric diagrams and go to topaz along with the manual and try to compare with the diagram to find out which optics is which in order to run it intelligently you must know what is what otherwise it is baffling and if you do it blindly it is impossible to do anything with it right so what is DP1 and DP2 DP1 and DP2 are two well I think quartz plates the material is not mentioned in the manual but two quartz plates that are set approximate Brewster angle approximate Brewster angle because you want to maintain the polarization now I said approximately it is actually there are little different from Brewster angle so that polarization is not compromised but what you can do is if you tilt these plates a little bit anyone or both together then what happens is the light going through these plates depending on how much you tilt will go through more glass or less glass right and that would cause a change in delay so when you run topaz this delay is called delay 2 in this software the trans topaz so if you change delay 2 what it means is that essentially you are tilting these plates why you have to change delay at all hold on we will come back to it okay and remember we have still not answered that question why is it okay to use lenses okay can you make a telescope using mirrors you can right in principle you can you just use concave mirrors of different focal lengths it is just that you have to tilt them in such a way that the light goes out okay otherwise if the output of these 2 mirrors keep on going keep on oscillating between each other it is of no use but why do we use lenses I mean why is it okay to use lenses we will see later or why it is desirable to use lenses we will see later on but do not forget the moment it goes through these plates then also it is possible that you introduce some chart it is not an all reflective optics topaz okay and that is fine after that if you produce this line what do you get what is the next thing a 1 is an aperture okay that is why it is drawn like this you have this black little thing here black little thing here in the middle there is a hole right so it is something like this okay section of that would look like a 1 and then you have L3 what is L3 another lens what is the shape of the beam that goes into L3 collimated beam now when it goes to another lens what will happen it will get focused okay so why do we need to focus we will see okay but then after that we have this BS2 BS2 is a beam straighter that transmits approximately 20% of the beam okay next piece of optics along this think of this horizontal line okay that is a directional propagation of the beam as we are going to see shortly after that we have VF what could VF be can we guess in this context what could F be F could be you can speak in any case you would not be heard so if you give a wrong answer it is okay yeah F is your right actually filter F is filter and V is variable VF is a variable filter because you want to play around with the intensity of the light that goes through why you have to play around we will see later remember when the principle remember is too good is no good you do not want to put in as much light as you can that means there are things we will see later okay and then you have this M1 M2 and so on and so forth okay so let me draw the beam BS2 we already said transmits about 20% of the white light of the WLG beam this is your path number 3 what is path 3 WLG pump beam that means with this beam that goes through so how much of this what would be the energy in this you put in say 1 milli joule okay 80% of that let us say went on along path 2 in this direction horizontal direction 20% 20% of 1 milli joule is how much 200 micro joule and then we are saying that this beam splitter transmits about 20% so 20% of 200 milli joule is how much 200 micro joule sorry 20% of 200 micro joule is how much yeah yeah 40 micro joule and then you have this variable filter so you can actually control you can make it a little less than 40 micro joule as well not a little you can make it 0 if you want okay so you so that is a maximum you do not want too much of light to go in what is this beam going to do this beam is going to generate white light okay with the experience of doing pump pro spectroscopy we know that if we use too much of energy to generate white light or if you focus too hard then the white light quality is not good right it is unstable there is filamentation many things happen that is why so that is the answer that is why you do not want to use too much of pump and I will show you a photograph of what happens when you use too much of pump once we are done discussing this okay but this is your white light generating pump beam WLG pump beam goes through the aperture L1 L3 focuses it now see what is the role of L3 why does it have to focus why does the beam have to be focused because your white light generation is going to take place in this WLG you see this is basically a sapphire plate to generate white light there it is not sufficient to send a collimated beam you have to focus there so L3 the job of L3 is to focus the beam pump beam for white light generation on the sapphire plate okay can you comment on the relative focal length of say L1 L2 we know already right L1 has to have a longer focal length than L2 can you compare can you comment on where L3 focal length will be should it be longer than L1 should it be shorter than L2 or should it be something in between see this is a cartoon but this is roughly to scale the way I do the cartoon is that I started with that photograph and on that I have drawn the optics so lengths are more or less okay so what do you think just looking at what you see here should the focal length of L3 be more than L1 less than L2 or something in between actually it has to be more than L1 right because what is the separation between L1 and L2 this much so depending on how much you want to shorten the beam maximum focal length of L1 is something like this from here to here but look at where L3 is and look at where WLG is it is this is the focal length of L3 so L3 is actually a long focal length lens it is better that way because you are generating white light what happens what is the difference between focusing using a lens of long focal length and a lens of short focal length finally you focus and focal spot can be well your depending on what lambda is it is the best possible focus you can get is lambda by 2 so if you use a long focal length and if you use a short focal length what is the difference let us say they are acromats so one answer is chromatic aberration might depend on the focal length but let us say we are using acromats chromatic aberration does not even arise so for long focal length the situation is something like this yeah short focal length the situation is something like this this is called soft focusing this is called hard focusing because it is not just the point of focus it is the volume of material through which the light is going through if you do hard focusing then you put in more energy for the same length right so for white light generation or even second harmonic generation sometimes soft focusing is better you do not want to focus too hard so the solid angle is important okay that is why you want to use L3 with long focal length right so then what have we seen so far pump beam has been input most of it has been sent along path 2 which is going to be used for the final amplification of signal small amount of it has been transmitted and this well there is another beam free they do not forget BS2 so far we have neglected the reflection of BS2 but if you look at the light that is transmitted through that light is used to generate white light it is a pump for white light generation okay and we have discussed this path 3 thoroughly so far okay M1 and M2 are of course a pair of mirrors in fact mounted on the same platform and then there is an RP1 what is RP1 let us see if you can guess yeah so it is a halfway plate RP means rotate polarization why do you need a halfway plate because you want to restore the polarization the input polarization is horizontal okay the input polarization for topaz is horizontal for the next one next stage you need vertical polarization okay so what this RP1 does is that it rotates the polarization to 90 degrees and each of these steps is important suppose by mistake you take out RP1 or maybe you rotate RP1 so that the polarization is not turned by 90 degrees what will happen you will not get the next stage amplification okay so we need to know every component alright so white light is generated okay and what we have seen is L3 focuses into focuses this pump into the sapphire plate to get your this thing to get your white light as you remember we talked about delay 2 the problem is I forgotten what the animation is so I am saying things before I show it anyway do you remember what is delay 2 delay 2 we said that this DP1 and DP2 the 2 plates you tilt them a little bit and bring in some delay will that change be large or small naturally is small that is why it is called delay 2 so we have talked about delay 2 first not delay 1 what is delay 1 this M1 and M2 are actually mounted on a translation stage and all these are motorized controlled by the computer so you can move this M1 and M2 for order backward so that should remind you of the retro reflector that we use in femtosecond optical gating or pump probe for that matter right so it can be moved for order backward so that is course right so that is delay 1 so if you actually use topos and if you try to control things by yourself then you will see that you like you can play around with delay 1 you can play around with delay 2 delay 1 is this M1 M2 retro reflector moving back and forth delay 2 is the plates tilting at some angle so delay 1 and 2 are your so delay 1 is course delay delay 2 is fine delay it is important to know this because I mean only after you have played around with the course you want to touch fine delay right I ask you to measure the length of this room what will you do you will get a I want to you to measure it to the nearest millimeter let us say or even nearest micron so are you going to start using a vernier caliper from this end to that end of the room no first you measure with a tape right and the last bit you will perhaps measure with vernier caliper so it is important to not forget that delay 1 is the primary delay course delay delay 2 is a secondary delay fine delay so we should not we want to play with delay 2 only when we are close of course close to what that we have not said yet we will say okay but delay 1 is the course delay that first has to be taken care of alright now what have we done so far we have said that we have generated white light where have we generated white light at WLG which is a sapphire crystal right I have not shown the white light path in the next piece of animation but okay we will come back to that later we are right now I am not showing you white light but you can guess where it will go where will white light go you go straight and it will hit L4 what is the need of L4 it has to collimate right finally you want collimated beams with L3 you have focus the pump beam right so white the pump beam is going to diverge and the white light that you generate is also going to diverge it is exactly the same as what you have in your pump probe setup okay white light will also diverge so you have to capture it and make it collimated that is the role of L4 we have certain things later on we will come back to it there is something more interesting later so see once again we are using a lens and we said it is okay to use lenses maybe it is even desirable we will see why it is okay okay next let me show you the other path path 4 what is path 4 PA1 pump beam what is PA1 power amplifier 1 or in other words the preamplifier so of course we have this BS2 we said 20% of the beam is transmitted 80% naturally is reflected right that is the beam that we use for the first stage amplification of signal so where will it go it will go up M3 maybe I will show you M3 L5 M4 DM then goes through let us follow this path maybe I should have done it step by step anyway so see 80% of this beam that is that impinges upon BS2 is reflected to M3 and this is path 4 PA1 pump beam goes straight then goes through a lens L5 what is the role of L5 again collimation do not forget that this is also a diverging beam converging up to a point then diverging because you have used L3 so L3 not only focuses the beam here it is also going to focus the beam somewhere here yeah so it is going to focus both the arms beams in both the arms so it is not enough to use a lens in the white light generation side you also have to use a lens on the other side which is a pump for the preamplifier so that is L5 essentially it collimates the beam next the collimated beam goes and hits M4 from M4 it comes and hits DM1 what is DM1 nothing to do with the initials of our celebrated colleague DM here means dichroic mirror dichroic mirror is wondering who the celebrated colleague is that is not so difficult figure okay dichroic why do you need to write dichroic mirror here because see white light will come from this side okay that white light has to pass through at the same time you have to get this pump for the first stage amplification going in the same direction that is why you use a dichroic mirror that will reflect pump what is pump 800 nanometer okay and what is white light it more or less ends by 800 nanometer so this dichroic mirror is it going to be so dichroic mirror means essentially a filter right can you tell me will it be a long pass filter or a short pass filter it is reflecting 800 nanometer and it is transmitting say 400 to 800 nanometer well I have given you the answer yeah it is a short pass filter so this dichroic mirror is essentially a short pass filter alright okay how far have we gone so this is taking so much of time as far as slide number 5 and we are into the second module and we are about to break again okay how far have we gone here on one side we have generated white light on the other side we have generated pump that is going to selectively amplify one of the colors of course when I say one of the colors I mean modal color femtosecond pulses they are broad one of the colors from the white light in the next stage okay I would have preferred to get the get it done in this stage in this module but I do not want to rush so we will take a break we will come back and finish this discussion in the next module.