 So we will continue our discussion of modulocking and then go on to something called cavity dumping. This is where we have got until the last module. We have discussed active modulocking using a modulocker by applying sound frequency using device errors effect within the Raman Nath regime and then we moved on to talk about passive modulocking where what is used is saturable absorber. So and we have finished our discussion saying saturable absorber can not only produce pulses but they can also narrow them down okay. Now what I want to say is saturable absorber works even better if it is not given the work of producing pulses as such but pulses are produced already and the only job of the saturable absorber is to narrow the pulses down. And that is achieved very commonly in dye lasers especially and also in some kinds of solid state lasers by what is called synchronous pumping. In synchronous pumping one pumps the dye laser with not a CW laser but with a pulse laser. So what happens if I pump with the pulse laser what kind of an output do I get from the dye. So we are talking about what I mean 6G dye lasers I have what I mean 6G dye laser I pump it with green light 532 nanometer but the 532 nanometer light itself is say 20 picosecond pulses 80 megahertz or something. What should the output of the dye laser be okay I will give options will the output be CW or will it be pulsed that was very simple question but I am exciting with the pulse laser. So if I excite with pulses should the output also not be passed why the pulses are 20 picosecond lifetime of the dye is say 10 nanosecond say 5 nanosecond and repetition rate is such that that difference between 2 pulses is something like 20 nanosecond should I not get pulses like this every pulse of pump is going to produce a burst of red light and when the pump is off then what will happen there is nothing to produce a burst is not it. So you get an inherently pumped laser but the problem is these pulses that you produce are going to exactly they are not going to be sustained unless the cavity length of the dye laser is exactly the same as the cavity length of the pump laser okay so you have to carefully match the cavity length of the pump laser and the dye laser this is called synchronous pumping. Pumping is by a modulocked India laser or something like that and cavity length is the same as that of the not pumped laser I made a mistake there pump laser that is used for pumping okay. So already the pulse is there and then without going into the math once again there is this 1978 paper which says that in synchronous pumping pulse duration of the dye laser is square root of that of the pump laser. So by synchronous pumping you can actually get shorter pulses than the pump laser itself that is why this was a very common technology for a couple of decades before the advent of titanium sapphire lasers okay. So square root of that of pump laser so the good thing is it is not very difficult to produce a pump laser which has say 20, 30, 50 picosecond full width of max but let us say 25 picosecond pulse width laser that is used for pumping automatically the output of the dye laser is going to be 5 picosecond right square root of that of pump laser alright and I am asking you to believe me on this we will not do the math if you are interested please read this APL paper from 1978 alright. Now I should show you some result this is the result Kafka and Bayer I think where engineers working in spectrophysics where you remember correctly and this is what they produced in the pulsed NDAC pumped synchronous pumped rhodamine 6G laser what is the full width of maximum that you read here 220 femtosecond. So if your pump laser itself has a short enough pulse width then you can actually go down to 220 femtosecond just by synchronous pumping that is how good it is right. But then later on they modified the static the cavity a little bit that is a Kafka and Bayer may not be from spectrophysics I might have made a mistake this optics letters is from spectrophysics so this is an again a laser do you see the cavities let us look at the cavity carefully. Here we can right away see some elements that we have discussed earlier right we see our familiar 4 prisms and that is what will give you tunability anyway. Here this is the output coupler the partially reflecting beam through which the light will go out of course in the other end will be the high reflector and this is what the pump is. Now we have discussed high sapphire already do you see a major difference between pumping geometries of high sapphire laser and die laser remember in titanium sapphire laser the pump green light went through one of the mirrors we said there is a dichroic mirror inside dichroic sapphire why because in case of solid state lasers you get the best result if the pumping is coaxial with the laser lasing axis not so in case of die laser in die laser pumping can be at some angle that angle is also optimized another thing I would like to draw your attention to is what is written here what does it say gain jet what does that mean gain of course is gain medium what I mean 60 die what is the meaning of jet. So as you know when very short pulses go through any medium they get broadened which are thing and all. So in order to get short pulses which are picosecond or lesser you do not want any extra component to come in the path. So gain jet means in these lasers what they do is the die would be stored in a small bucket and it will a lot of die one liter or something lot of die solution rather okay and typically this solution would be in glissarol or some such very highly viscous solvent and there used to be a pump which would circulate this die okay so it would circulate and bring it out through a jet jet means a metal nozzle which would be flattened. So the output of the die through that nozzle would be a flat jet spherical usually if you take a hose pipe what kind of a jet do you get the cross section will be circular right that will not work here you want it to be flat and as thin as possible that is why that jet was made it was very very thin and then it would go into a catcher tube which would be connected to the reservoir it would go back there okay this is this was the state of the art again for our 20 years or so in picosecond and femtosecond lasers one would use jets and in fact even now a lot of people do pump probe experiment where the sample is not a rotating sample or translating sample or anything but a jet because then there is no quarts coming in the path no further broadening of the pulse we really want to go down to small time scales jet is the way out okay but I digress too much right and then here you have this gain jet and here do you see there is a saturable absorber jet what is that do you see I once again circulated in the same manner. So now you see this Sotomayor 6G is already pulsed and you know how good the pulse can be what do you say 220 femtosecond or something like that now the question is by introducing this saturable absorber is there any improvement and this is the answer what is the full width half max now 65 femtosecond no the lower number is correct I do not know why they have written 100 there but I checked with the text 65 femtosecond is what they say from 220 femtosecond it has gone down to 65 femtosecond because now think 220 femtosecond pulse right that means the base would be something like maybe 400 femtosecond from there to say 200 femtosecond or so for that time that is the time required for the light to actually get absorbed even of the intense pulse and to increase to have an increasing amount of bleaching when the threshold is reached then only the pulse propagates that is why the pulse is narrower okay. So synchronous pumping with saturable absorber for a long time used to be the state of the art for producing short pulses of course now it is a thing of the past nobody would do anything other than titanium sapphire laser but it is important that we know the principles at least qualitatively that is why we are discussing this but everything said and done nobody likes to use dye solution and all that anymore because first of all they are messy secondly dyes degrade very fast especially saturable absorber what I mean 60 is stout dye not much happens but do DCI is extremely flaky I have worked with this kind of a setup we would make a do DCI solution today and it would be very intense dark color within a matter of week there would be no color in the jet at all we bleached completely because if you leave do DCI solution on the table under tube light it will get bleached. So that is a very big problem and then when you are done with it where do you throw so much of solvent and there is the 100 other paraphernalia there whenever you have solved the pump can go bad the solvent chamber can leak you have to cool it it is really messy so that is really a thing of past now so right now what everybody wants to use is your solid state solutions and something that is used now very often is a passive mode locker made of well not made of is perhaps not the correct term something what is used is saturable brag brag reflector which act as saturable absorber and therefore passive mode locker but I should explain what a saturable brag reflector is even before going there I should tell you what a brag reflector is does anybody know what is a brag reflector whenever we say brag what do you think diffraction right whenever if you say diffraction what comes to your mind these alternate layers right lot of apertures and all that right. So a brag reflector is something like this you have sort of a mirror and in the example that we discuss here the reflective surface is gallium arsenide and then your alternate layers of high and low reflective index so when light falls on it what will happen light goes in first of all let us not worry about this thing on the top from this direction light goes in gets reflected comes back but then when it comes back since you have different layers what kind of light will come out let me take the question down to an easy level you have a you have a grating okay or you have this crystals and all light falls on it then light goes back everybody knows the rule which tells at which angle it will come out what is the rule braggs law what is it and lambda equal to 2 design theta I am asking an easy question I never ask difficult questions so where does n lambda equal to 2 design theta come from again answer is easy so remember what you studied in childhood you have one reflecting layer another reflecting layer right so you get reflected ray from the first another reflected ray from the second and then what you do is you have to say that you have to have constructive interference so path difference must be integral multiple of lambda by 2 lambda by 2 okay that is how you derived in lambda equal to 2 design theta so in other words so this is a bragg reflector isn't it what I just discussed your crystal or whatever it is a bragg reflector and what is the property of the light that comes out all the rays that constitute that beam are in face yeah right so now if you use a reflector like this light that goes in and then light comes out whatever light is able to come out of this bragg reflector has to have components that are completely in face or in other words mode locked right so when you use a bragg reflector automatically you get mode locking so this is the state of art well we have it is a little later in the slide where these are used as auto in auto start ticephile lasers you might remember that in one of the previous modules I told you that when you work with a homemade ticephile lasers to get the mode locking started we had to tap a meter in the tsunami that we have in our lab there also to get the mode locking started we have to disturb the cavity a little bit however in my tie the new laser that we have the compact laser we do not have to do anything why because my tie uses a saturable bragg reflector and we have not even got to the saturable bragg reflector okay so automatically light that comes out is mode locked okay so that is why one wants to use bragg reflector and if you want it to be a saturable bragg reflector what you do is you introduce what is called a quantum well okay the substance is written here in the what it is used what is the meaning of quantum well your substance with small band gap sandwiched between two layers of another substance which has large band gap so once the exciton gets tapped there is not so easy to come out right what does that mean when will it come out when intense light falls on it so that is the saturable absorbing action okay so bragg reflector with a layer of quantum well is the saturable absorber of choice in present day nobody uses do DCI anymore do DCI sales must have fallen worldwide might of course this is made by layer by layer assembly and this is an example and what I want to draw your attention to today is how old the technology is 1995 1978 I told you is when femtosecond pulses were already there 1995 is when you have the use of saturable bragg reflectors the diagram you see here is for a diode found solid-state laser and I think you can more or less make out all the components what is going on this here is a saturable bragg reflector SBL okay and that is what causes passive mode locking what is the full width half maximum here 100 femtosecond not as good as 65 femtosecond but it is so hassle free that nobody cares and also the comparison may not be fair because there you had a synchronously pumped die laser with saturable absorber here you have a diode found solid-state laser inherently pulse width is going to be more but then now I show you another one from references in the same paper and it is there in a laser focus world article as well this is a self-starting ties a file later once again you see the S there are two different cavity designs but the crux of the matter is that this saturable bragg reflector is used as one of the mirrors now see what the pulse width is 25 femtosecond so good thing is first of all ties a fire itself gets mode locked by KLM into it you are introducing a saturable absorber so the output is as good as it gets so in all not all but in most of the modern-day compact self-starting ties a fire lasers this technology is used now we move on to something that is a little different but it is very easy to get confused between the two so far we have been talking about mode locking right how to produce a short pulse maybe not an ultra short pulse but short pulse nevertheless and why not an ultra short pulse we have showed you that you can go down to 25 femtosecond by mode locking next question is how do you get the pulse out you can get the pulse out by using a an output coupler but especially in the dye laser era the question that people ask is it possible to do better is it possible to use some other mechanism by which there will be further amplification perhaps further narrowing of pulse and we can change the repetition rate so I will give you an example this all those dye lasers rota mean 60 dye lasers that I showed you they are synchronously pumped by green lasers which have a repetition rate of about 70 80 something like that as you understand if you want to do safe fluorescence experiments TCSPC experiments 70 megahertz is too fast you have to cut down the repetition rate how do you cut down repetition rate so as long as dye lasers rule the market cavity dumping was the technology unfortunately titanium sapphire laser cannot be cavity dumped it would have been great if it could be so that is why you have to use something else called pulse picking but let us talk about cavity dumping first if there is time we will talk very briefly about pulse picking and then we will pick it up next day so in cavity dumping you use the same thing you use an acoustic modulator right you take a quartz crystal or something and use a transducer but now you do not work in Raman Nath regime anymore this is the main difference you work in what is called the Bragg regime and Bragg regime condition is exactly opposite of Raman Nath regime here L is much much greater than capital lambda square divided by 2 pi lambda okay that is where cavity dumping is observed best so first of all what do you have to do compared to a modelocker you have to use a higher frequency of acoustic wave right because L has to be much much larger than capital lambda square by 2 pi lambda you have no control over lambda right lambda depends on what kind of laser you are working with you do have control over capital lambda which is the wavelength of light so if L has to be much much larger than its square divided by some constant then that capital lambda should be small or in other words frequency of the acoustic wave should be large also the other thing you have in your hand is a length so you can use a longer interaction length alright you can use thicker crystals of course there is a limit to that because if you use something that is too thick then again pulses will get broaden so you always have to find the optimal length right so what happens when you work in Bragg regime is this remember what happened in Raman Nath regime light passing through was diffracted on the 2 sides and was phase modulated by amount of N omega here that does not happen all the higher order diffraction is eliminated you do not get it why you do not get it we have to read the original papers but we for our purpose you do not even need to know you do not get higher order diffraction at all only 2 rays are sustained the 0 order that is unmodulated omega and the first order omega 0 plus capital omega ok so this is what happens in a Bragg regime ok are we have you understood what happens yeah can I go ahead only 2 beams now and that is important for cavity dumping and this is what a cavity dumper is made up of there is some math after this may be I will skip all of that and I will just show you the final result you can go through the math if you want so here what happens is first of all now here we have an interesting situation all this time we have been saying that in a laser you have a high reflector and we have a an output coupler right in these lasers there is no output coupler all mirrors are high reflector so in absence of cavity dumper what kind of a laser is it you pump it lasing will start right there will be gain but there will be no loss it will never come out right so there is no output cavity dumper is a device that is put into a laser like that to get the pulse out to switch the pulse out how ok this is the same thing it is a part of the laser cavity on this side you can understand there will be the gain medium and all and the other end other high reflector here you can think this is the last mirror m1 in this case is last mirror or first mirror or whatever it is since it is one you can say it is a first mirror last mirror is somewhere else but the crux of the matter is no output coupler here you have m2 both are concave mirrors focusing mirrors and we have this AOM at the focus of both center of the acoustic modulator is at the focus of these 2 mirrors ok maybe I will just take this and stop and start from here next day so think of this this horizontal line horizontal black line is the lasing axis comes here goes through now this acoustic modulator is being operated in Bragg regime so what will happen 0th order 1 will go through and there will be the first order line which is frequency modulated by just little bit ok so now you will have 2 rays then since and it is constructed in such a way that these rays hit well I do not even have to say that this one is at the center right so these rays are made to retrace their path so now what happens when they retrace the path on this side also you will have 2 rays understood one of these will go back to the laser cavity the other one which takes a different path will not go through the cavity is going somewhere else to put in a prism there and it goes out ok so that is how dumping is done what is the advantage we will start discussion from here next day but what is the advantage of this so you see we have used a pulse speaker right what is the difference between cavity damper and pulse speaker in pulse speaker the acoustic modulator is outside the cavity so what are we doing we operate the laser at 80 megahertz and we operate the acoustic modulator at 8 megahertz right so we are taking 10% of the pulses throwing in dumping the remaining 90% of the pulses we will discuss later again what is happening in cavity dumping either the light goes out and you use it or it goes back to the cavity nothing is wasted and one of the photons that go back to the cavity is going to get amplified in the next round trip right so in cavity dumping this is this is the attractive feature of cavity dumping you take out the light at the same time you have further amplification so whereas your power is going to get cut down if you do pulse picking you measure the power before pulse speaker and after pulse speaker if you operate it is a one-tenth ratio it is your power is also one-tenth is not it because you are taking only one-tenth of the pulses but in a die laser first of all if there is no cavity dumping you cannot even measure anything no light will come out but due to cavity dumping what you see is there is an increase in power because while the light is taken out only one part is taken out the remaining part is sent back to the cavity no photon is wasted nothing is dumped this is where we will start from next day