 So, this is where we had stopped our discussion how do you stretch a pulse and so far we have discussed the simplest possible geometry using a single grating couple of plane mirrors and a concave mirror and we had said that if you want a greater stretch a greater chart then you need multiple passes more often than not as we discussed if you really want a 200 picosecond pulse the path difference would better be something like 6 centimeter. So, either your stretcher will be very big which you do not want or you have to do multiple passes by so that the effective path travel is much more okay so that leads us to little more complicated geometries this geometry is called 3C geometry I am not very sure what the correct pronunciation of 3C is but then let us just call you that so here what you have is you do not have one grating rather you have two parallel gratings and I have not animated this slide but you can see right it comes in similar kind of separation takes place and then it comes here comes in now the path has increased a little bit and again by 3C geometry you can make not only a pulse stretcher but also pulse compressor in fact in the laser that I used as opposed to this was a geometry for the compressor the earlier geometry was the one for a stretcher now more components you have in a system more parameters you have that you can control and get an accurate match but more complicated also becomes okay next let us okay we have already said this but let us say it again so if you put in an uncharved pulse uncharved input in any of these geometries then you get a chart output and you get a stretcher you put in a chart input the output is uncharved then you get a compressor provided the chart that was there in the original pulse is exactly compensated this is perhaps the 8th time I have said it in the last module and the present one but this is something that we should never forget so if you have a negative chart in stretcher generally you want to give a positive chart in compressor and vice versa so different people prefer different things some think it is better to have positive chart in the stretcher negative chart in the stretcher some feel it is the other way round so it is completely dependent on what you want to do alright next move on to another geometry which is known to be quite a good one it is quite popular it is called Martinez geometry in Martinez geometry once again you have two gratings but unlike in tree C geometry the gratings are not parallel if you produce them they would make an acute angle and another thing that is very important is Martinez introduced telescope between the two gratings it is not as if this geometry is not used actually you get a good amount of stretching using this but can you tell me what could be a possible problem in Martinez geometry using lenses right so you are making a short pulse travel through lens so that itself introduces some chart so the question is are you using good lenses or not if you use very good quality lenses in the telescope then actually Martinez geometry works fine but one needs to be careful about that it is not such a big problem because in any case you want to stretch so I am sure you do not want to use it in a compressor but in a stretcher this works and in fact Martinez had shown that the efficiency of this geometry is quite good and one difference between the earlier geometries that we have discussed and this one is that here in Martinez geometry you end up introducing a positive chart okay the gratings are facing are almost an acute angle okay next let us move on to something that is a little more complicated and this complicated arrangement is called an offener triplet okay this is one slide that I should have animated but it would have been too much of work as well so I did not so let us see if you can understand the understand what is going on here what do we have here unlike the other geometries this looks like a more complicated arrangement right because first of all you have a plane mirror then you have a grating after the grating you have a concave mirror followed by a convex mirror followed by this HRR does anybody know what HRR could mean well there is a VRR so H and V would mean horizontal and vertical respectively yeah these are called roof retro reflectors and this is what they look like and usually these are very good because already the angle between the two reflecting planes is defined so you cannot really mess up things there it is so we have something like this in the retro reflectors in our spectrometers in our case we have three sides of a cube this is sort of a hollow cube and the coating can be different it can be aluminum can be gold can be dielectric can be nothing so it depends on what you need so this is a horizontal roof reflector this is a vertical roof reflector so what is happening the input comes hits this mirror and comes to the diffraction grating this number one is the first part on the diffraction grating then where does it go it goes to the concave mirror from concave mirror to convex mirror from convex mirror back to concave mirror and from there it goes back to the diffraction grating once again to form point number two from here the beam goes to a vertical retro reflector and of course if it is a retro vertical roof retro reflector and if it is a retro reflector of course it has to send it back and good thing about retro reflectors is that the incoming and outgoing beams are parallel to each other opposite direction but parallel so it comes back and hits the diffraction grating here at point number three goes straight the concave mirror from concave mirror to convex mirror then back to the concave mirror and back here in point number four from there it goes to the high reflector horizontal retro reflector and then it goes out what is the need for such a complicated arrangement we actually provided the answer already in one of our earlier discussions we have given you the answer what is the need of such a complicated arrangement remember we have said that if you want a good amount of sharp to be introduced then the path length would better be large here path length is large without the stretcher having to be too large okay if you suppose you had only this mirror concave this plane mirror concave mirror and grating you can understand path length would have been maybe one fifth or one tenth of what it is in this arrangement right so this is basically a geometry in which a lot of passes are made so that the effective path lengths is significantly large and you can get a good amount of sharp introduced and the reason why I give you trouble with this kind of a complicated geometry is that in the laser that we use the stretcher is you can say modification of this often a triplet arrangement thankfully it is not as complicated as what it is here but we will see what it is but before going there you must have noticed that we have registered references of papers there if you are interested to study more all you have to do is go to these references and you will get leads to further detail about these things and again before moving on I am sure you can understand how much of effort would go in to design something like this it is not easily done in earlier days everybody worked with homemade lasers typically those lasers would be bulky even now for some applications you have to make your laser typically the laser would be bulky because it is difficult for us to do things that are very complicated and also you do not want to do it because maintenance is an issue but in commercial systems the design is complicated and nowadays at least up to this part generally nobody has to do anything themselves but still it is important that you know as for all you know somewhere sometime you might have to make your own apparatus where this understanding would come handy okay with that we move on to the laser we have we have a one box coherent Libra laser Libra amplifier we are going to talk more about the amplifier in one of the subsequent modules but for now let me just show you the ray diagram for a stretcher and you already see the complication look at the input the trouble starts here the input goes through 2 mirrors that are like this okay and this thing is important if you ever have to align the laser yourself or if you ever have to even see whether the stretcher is working properly or not so let us see what happens this here is the input comes through SM5, SM6 so in Libra user's manual SM means a mirror which is in the stretcher and the number would of course indicate the sequence okay so it goes through SM5 it does not hit any and impinges on the grating okay and it is a simple uncharred beam hitting the grating so on the grating it looks like a spot a circular spot we will come to this 2, 3, 4 relative positions later so this is where it comes goes to SM3 which is a circular big mirror from SM3 it goes to SM4 from SM4 it comes back to SM3 and hits the grating once again and in a correct alignment if your alignment is perfect then this beam number 2 should hit the grating lower than beam number 1 that is point number 1 point number 2 is so far it has done one hit on the grating right so since it has hit the grating once it has been dispersed so beam number 2 which is directly below beam number 1 spot number 2 let us say is no longer a circle you see a stripe a streak has somebody seen when these guys opened up you have seen right so this is actually a streak it is not a circle then okay this is 1 this is 2 goes back here back here then back to SM3 then it hits the grating above spot number 1 and it is still a streak then from here it goes to SM6 sorry think I got it a little wrong 1 2 from 2 it goes to SM5 SM6 back to spot number 3 comes to SM3 goes to SM4 comes back and then when it the last time it hits the grating that dispersion is gone because so many passes again well dispersion is still there but you do not see a streak anymore once again you see a nice circular spot and that is what goes to SM7 and from there it moves out okay so this is the stretcher we have in our laser where we have in our laser we will see in our amplifier and this is the compressor once again the compressor has only one grating earlier I have shown you a design with two gratings right but here since it is a one box laser everything has been made very compact so minimum number of optics has been used so instead of putting another grating what they have done is here it is perhaps not very clear about what it happens here but once again there are several passes and here you can see the relative positions of the beams 1 2 3 and 4 once again the first beam that goes in is circular second and third are streaks fourth one is where the spatial dispersion is taken care of but not spectral dispersion and that is beam number 4 which goes out okay this is what we have so that almost concludes our discussion of stretchers and compressors but then I might have given you the impression over the course of this discussion that if you want to stretch a beam or if you want to compress a beam all that is there at your disposal is grating or prism so you will see in literature designs using not gratings but prisms okay that is understandable but that is not the only thing in fact whatever technology we have is moving towards miniaturization so I think the latest development in this field is something called VHG solid glass volume holographic grating so you remember we talked about the Bragg mirror right Bragg mirror where reflection is from different layers here also what you have is a very small device and so far I do not know anybody else does it there is this company called Ondax which has now been taken over by coherent Ondax market says and the principle was known long ago and in fact when this was demonstrated long ago as well in 1987 optics later as paper but then now this company has developed it and marketed it as a product and it is very simple what you have is you have an optical fiber feeding the pulse into it and as what is it that we have to remember if you are going to use optical fiber in an application like this you better use a polarization maintaining fiber it is very important and in the next module we will see how we use polarization very effectively so typically what you have is you have this fiber going in if it goes in through here then there are since it is a holographic grating there are several reflecting surfaces and different reflecting surfaces reflect different frequencies it is as simple as that so red gets reflected first followed by green followed by blue so if your input was a broad pulse chart pulse provided the chart is such that the trails and blue leads then this kind of an arrangement is going to give you a compressed output otherwise the sequence of reflecting mirrors has to be opposite but the good thing here is that if you take a pair of these VHG stretcher and compressor all you have to do is put in the pulse the initial output of the oscillator uncharped pulse you put it in here let us say then what will happen you get a chart pulse and you get a chart pulse in which red leads and blue follows in the compressor side just turn it around let the beam going from here and not from here then what will happen then it will get compensated so the same piece of optic can be used either as a stretcher or as a compressor depending on which is the input which is the output and I have not seen it but from the pictures on the website and all it seems like something that is about this size so the stretcher that I used 20 years ago was about this size square the stretcher we use now is about this maybe this and the one we are talking about here is this so that is what advent of technology and miniaturization does ok so much for pulse stretching and pulse compression we are going to next discuss the actual amplification bit of chart pulse amplification method.