 Now, in this module, we are going to learn the basics of Tysaphire laser, what is there inside and how to use it. But before that, this is what we have learnt and this is where we ended the last module. We talked about Carlin's mode locking, Carlin's mode locking means because of the Gaussian distribution of a TemM00 mode, you have greater intensity at the center of the beam and it falls off as it go towards the edge. This brings in an intensity dependent refractive index which sort of act cycle lens, it is called a Kerlin's and since pulse light, so it focuses the intense beams and does not focus beams that are less intense. And since pulse light is intrinsically more intense, what this Kerlin's thing does is that it can select pulse light over CW light. You can do it actively by putting in an aperture in the path of the beam, but that is not even required, the optics usually take care of it, this is what we said and this is how pulses are produced. So, femtosecond pulses are produced by Carlin's mode locking without us having to do much. In subsequent modules, we are going to talk about active mode locking where we have to introduce something and get the mode locking done that is we have to use something called acoustic optic device, but here that is not even required, thermal lensing takes care of it. And of course, why is it that we get femtosecond pulses and not picosecond pulses by thermal lensing? Because in femtosecond pulses, intensity is much more, you are mode locking many more pulse, many more modes are locked, that is why pulse width is so small. So, this is what you can get by Carlin's mode locking. If you want picosecond pulses and nanosecond pulses, you have to do something. We said that this is the advantage of Carlin's mode locking, it produces pulses. Disadvantage of Carlin's mode locking is that it also brings in charge which effectively is going to broaden the pulse. So, before we can think of making a laser, we have to correct for this chart. How do you correct for this chart? The answer is very simple. Once again, you can go back to this analogy of computation, I enter a running race with Tanuja. All of you agreed that Tanuja is going to be enhanced down because she is a faster runner. But I do not want that. I want to reach the end point at the same time as Tanuja, even though I am a slower runner. So, if I am powerful enough, what will I do? I will make her run more and I will run less. I will take a shortcut, I will not let her take a shortcut. So, difference in speed has to be compensated for by altering path length. It is that simple and that is what is done to compensate for charting. What you have to do is whichever path, whichever light is leading has to go through a longer path. The frequencies that are trailing have to go through a shorter path. And this is done quite easily by using a prism pairs. In fact, it is enough to use one prism pair. I am going to show you an arrangement using two prisms. But what is more common and what we have inside the laser we use is two pairs of prism. And the job of this prism is to do compensation for what is called group velocity dispersion. What is the meaning of group velocity dispersion? The separation in time of different frequencies due to different velocities in medium with a finite refractive index that is called group velocity dispersion. We are trying to compensate for it. So, what do you do? Say you have a chart pulse that has gone in. You can see the chart right here the wavelengths are more, here the wavelengths are shorter. Lesser frequency on this side, lesser frequency on this side. So you make it go through a prism. Everybody knows what happens when it goes through a prism it gets dispersed. Then introduce in the path of the light another prism which is facing opposite and the angles have to be same. Then what will happen is look at this path. What is this color on this side? On the top, green, red. What is the color at the bottom? Yellow. So, what we are showing here is that red has travelled less and yellow has travelled more. Then you put this then it will become a collimated beam. And I will show you in a moment what the advantage of using two prism pairs is. And then it goes to another prism pair. Then again they are brought together and it goes out as a beam. So, here the inherent assumption is that the light marked in red is trailing and the light marked in yellow is leading. So, we have given a bigger path length to the yellow beam, yellow I am not saying yellow color the color that I have used to depict the beam here. And I have given a smaller path length to the frequency that I have depicted in red. So, if red was trailing with respect to yellow in this region then that would be compensated for. Have you understood? So, this is how GVD dispersion is done. Are we clear? Now what is the advantage of using two prism pairs instead of one? The advantage is you can have tunability. How do you have tunability? And this is what we have in our tsunami laser when we open it up the next day we are going to show it. In tsunami laser or many other lasers which are tunable what you do is you introduce a slit in this portion that is why you require a collimated beam otherwise it will be very difficult to calibrate. Suppose you have this slit in this region. Then this part of the light is going to go through other part will be blocked so that will be the modal wavelength. If I move the slit I made some animation but I think I have goofed it up but I think you can understand. If I move the slit the way it is drawn here vertically I bring it down then the light that is depicted in yellow will go through to a greater extent. Move it up. That is depicted in red will go through to a greater extent. This is how we get tunability and I am going to show the picture of a laser at least today there we will see how we do it and when you open it up we will see it even better. This is the advantage. If you use only one prism pair then it is difficult to have tunability. Then it is whatever the system gives you the spectral maximum. But advantage of using two prism pairs is in addition to compensating for GVD you can have a tunable output. Now the laser that we use in our laboratory is a titanium sapphire laser. What is the meaning of titanium sapphire laser? Sapphire I think we discussed earlier what is sapphire? What is sapphire to a chemist and what is sapphire to a layman? To a layman a sapphire is a precious stone right. You make a pendant out of it or you wear it on a ring to a chemist what is sapphire? Aluminum oxide with some doping right. So here the doping is titanium. So you can read how it is made it is a titanium oxide, aluminum oxide melted together and so on and so forth with particular level of doping. Aluminum oxide is the matrix. It is a titanium ion that is the actual active ingredient. There can be other depictions of the energy levels but this is one that is there in tsunami manual and this is what describes the system fairly well. Ground state is a doublet T2G, excited state is a doublet EG okay and when you excite you excite use I am going to show you the spectra you excite at say 530 nanometer or so you can excite there. But then post excitation there is an ultrafast relaxation to the zeroth vibrational level and this is where the emission occurs from. And you can see that the equilibrium bond lengths well the minima of the two potential energy surfaces not a good idea to talk about bond lengths right now but minima of the potential energy surfaces are not in the same position right. So when the emission takes place from here this is where you reach. So this energy gap is more or less 800 nanometer equivalent that is why you excite using green light and you get what they have called IR light. See it better in this absorption and emission spectrum right so you can see absorption spectrum is more or less from 400 nanometer to 650 nanometer the maximum is somewhere near 500, 550 that region emission starts at 600 nanometer maximum is at about 800 nanometer and it goes all the way up to 1050 nanometer or so. So this is what you get to play with you can excite anywhere here and you can get emission anywhere here usually one uses 532 nanometer excitation because that is a very robust laser that is already available new medium ELF or new medium YAG laser that is that technology is so robust that it does not make sense to try to pump with anything else in any case it more or less matches the absorption maximum okay. So that is your pump laser pumping is CW in this case you do not we are going to talk later on about cases where pumping is by a pulse laser but not in case of a titanium sapphire oscillator pumping is by CW green laser okay and this is where the emission takes place. Now looking at this diagram the energy level diagram can you tell me what kind of a system is it is it 2 level 3 level or 4 level system 4 level system what kind of output do we expect from 4 level system continuous way right. So intrinsically you expect titanium sapphire laser to give you CW output and it does mode locking takes place because of your car lens effect and so on so forth. Now with that background let me show you one of the earliest successful designs of a titanium sapphire laser this is called the mundane captain design captain and mundane are two professors of I think University of Colorado at Boulder but they have for a long time now launched a company which is quite successful not so much in India but across the world it is called KM labs and claim to fame of KM labs was this kit that they used to sell at one point of time where you have everything that you require to build an oscillator and you are supposed to put it together and make a titanium sapphire laser yourself that kit is still is available once again on demand this is very simple design but it takes bigger space. So this captain modern laser takes up say 2 feet by 2 feet kind of space on the laser table leaving out the pump laser so square is kind of design. So this is how it works pump laser comes in and then you see one thing that we will say without going into much detail here is that for solid state lasers pumping has to be coaxial for die lasers it is okay if the axis of lasing is this and you pump like this not so for solid state lasers. So pumping has to be more or less along the same line same axis along which lasing takes place so that is achieved always and I am going to show you the ray diagram of our own tricepile laser later it is always used by using one mirror that is your dichroic allows green light to go through does not allow red light to go through so this M3 mirror that is shown here it is a dichroic mirror green light goes straight through in fact even M2 and red light is reflected okay so this is your pump from 530 nanometer typically 4 watt, 5 watt, 3 watt whatever it is CW it is focused on the laser rod the titanium sapphire rod which is shown here as a red trapezoid okay and then emission from this laser rod is captured by these 2 mirrors these are concave mirrors I hope you can see and then they are focused back onto the rod so see we talked about card lens mode locking so this is what sustains mode locked operation even without using a using your an aperture because something the mirrors are aligned in such a way that they are exactly focused where this pulse beam is supposed to get focused so anything that is not focusing will not be in the axis of tracing that is how CW is gotten rid of sometimes there is a little bit of CW contamination you have to play around with the optics and get rid of it okay so now let us go one by one let us take M2 to start with okay M2 catches this focuses it here and then it goes this side goes through a prism pair you see here in this design there is only one prism pair what does it mean you do not have to know ability then it goes and hits M4 which is a plane mirror and it is M4 exactly at right angle so then it is sent back it retraces its path it is focused here from M2 it goes to M1 again M1 is a plane mirror and then it goes to an output coupler which is a partially reflecting mirror through which you get the laser out okay quite a simple design not very difficult to make provided you have all the distances right and that the reason why Captain Martin Kidd became such a hit is that everything was specified so just it was like Lego you have to put things together in the right place and you have your laser and to always titanium sapphire laser starts operation in CW mode because it is 4 level laser right so you have to bring in some kind of disturbance in the cavity that is what kick starts pulse operation so the way in which so well I should say that this figure is from PhD thesis of DS English who is a professor in somewhere in the US right now forgot exactly where and I have used this laser as a poster so what we would do is we would tap a mirror it is an open laser unlike what we now use so go and tap a mirror like this or just disturb the mirror a little bit and then mode locking is done. What we have in our lab looks significantly more complicated in fact if you look at the manual it looks even more complicated because other components are included there I have that is what took a lot of time today I had to erase all those things that we do not have let us see if we can understand the optical layout of the spectrophysics tsunami laser that we use in our lab other lasers from coherent and other places have more or less similar layout but every company has its own design so here we will spend a little time on this because we want to understand exactly why what each component does so you have the pump beam coming in from here on the left there is a Brewster window when we talk about non-linear optics we will discuss Brewster effect goes in so this is the entry point of your tsunami laser go straight P1 means P for pump P1 is a green reflecting mirror so that so P1 as you see is a plane mirror goes to P2 which is a focusing mirror because your pump beam also has to be focused at the crystal for optimal operation P2 a curved mirror concave mirror then see it is going through M3 for the same reason that I discussed when we talked about Captain Martin and design M3 again is a dichroic mirror allows green to go through reflects red and it is see it is more or less coaxial so it goes hits tricepher rod and then it goes straight I am talking to the pump beam now pump beam goes through M2 which is again a dichroic mirror and then there is a beam dump that is where it hits and then it has no further role from this tricepher rod emission takes place once again similar to Captain Martin and design you have 2 mirrors M2 and M3 and here you have the high reflector M1 plane mirror the job of plane mirror is to just send it back send the beam back and make it retrace its path then from there it goes to M3 M4 which is a plane mirror M5 which is a plane mirror and then it goes through what we have discussed already 2 pairs of prism what is the job of 2 pairs of prism GVD GVD compensation and this here is the tuning slate in the portion where the beam is a collinear okay. So basically when you change color using a micrometer screw gauge you move this slit up and down this is actually vertical then M8 M9 well this is the prism pair this M8 M9 are actually increasing the length between P3 and P4 well PRC and PR4 then there is something called acoustic modulator hold on we will come back to it and then it goes to your output coupler and then it goes straight there is something called beam splitter here and first photo derived here we will see what they do but before that suppose you have a tsunami laser here of course we have not had much problem over the last 12 years that we have had it but suppose your laser is not lasing it has happened once or twice how do you get it to lace and this is true not only for a ties of a laser any laser whose cavity you have access to how do you make it lace generally you do not have to do much with the pump beam but you must ensure that the pump beam is horizontal in any good alignment all beams have to be horizontal unless for some design purpose they have to go like this otherwise horizontal it is very very important and here you can see why it has to be horizontal if it is anything but horizontal the entire path gets messed up okay so horizontality of pump beam is an issue then okay now tie sapphire emission is there it is not lacing you put in a card there you will see the fluorescence on the card provided your eyes are not as bad as mine you have to be able to see red nicely okay so you can there are two arms is not it one arm towards m2 one arm towards m3 first thing to do is false on m2 hits m1 and comes back right you have to ensure that m1 is aligned in such a way that the beam retraces its path how do you do that take a white card and punch a hole in it right hold the card in this beam and make sure that the beam goes through while coming from m2 to m1 now when it goes back let us say this is the hole in the card this is how the beam goes from m2 to m1 hits m1 comes back now if it is not going to if it is not retracing its path it will hit here or here so you can see it on the card then you have to play around with controls of m1 so that it goes through this right then this m1 m2 segment is aligned you have to do the same thing here because fluorescence that is captured by m2 has to retrace its path while going back to the crystal and of course you understand the moment you touch m2 not only is the tie sapphire rod to m2 alignment changed m1 and 2 align to also get changed so you might have to do it more than once several times do the same thing on this side and then see the beam goes this way and comes back so once again you have to take your card with the hole somewhere here between m3 and m4 play around with m4 control and make sure that your light going in this direction and this direction travel in the same path when that is done take out the card you will see lazy this is something that unfortunately we do not have to do on a daily basis anymore I am saying unfortunately because once you do it you become an expert in alignment if you do not do it it is a black box and if you ever have to do it somewhere it requires a little bit of practice of course now as we are going to show at the end of this module things have become toys you cannot do anything so maybe that skill is not even required anymore this is what the laser looks like and again we are going to go to the lab and see it we have seen it once already see what you have is this is the side through which the pump beam enters no sorry this is the side through which the pump beam enters from outside you just see some controls in our laser we do not even have this birefringent filter wavelength selector this is a generic diagram we have these two knobs what are these two knobs these are vertical and horizontal controls of the high reflector which one is the high reflector this one right so this blue and green knobs that are there green is for earth horizontal blue is for sky vertical so these are horizontal and vertical controls of the high reflector that is what one usually plays around with in the other end we have similar controls for the output coupler generally we do not touch it because the moment you touch both you are sort of shifting the beam completely and you are not careful it can get misaligned but sometimes we do have to touch output coupler because condition of the lab can change from time to time okay we do not have this gti dispersion thing we do have these two micrometer controls here one for prism dispersion compensation one for wavelength selection what are these two micrometer controls one of them changes the distance between the prisms so when you change the distance between the prism you are essentially changing the path difference among the different modes the other one moves the slit up and down okay so by moving the slit up and down you select the wavelength the modal wavelength and by playing around with the distance between prisms that is how you maximize gvd compensation and ensure that you get a good optimal pulse now there are some problems with this tsunami kind of laser one problem is that the cavity is not sealed so if your lab is not absolutely dust free dust can get in and spoil your optics so regular cleaning might be required fortunately that is not the case in our case and for that is not the case for almost all ultrafast labs in India nobody would invest so much in a laser and keep it in a dirty room but there is another problem of the cavity being not sealed the problem is that no matter how much you try you cannot have a room that is absolutely dry a relative humidity we use demeritifiers and all but at most we can go down to 40% it is important that we go down to 40% but then you can try and use in industrial demeritifier and go down to maybe 15% but that is not good for your health nose bleeds have been reported in labs where too much of dry atmosphere was maintained so some moisture will be there okay even if it is 14% and water absorbs in the region of 950 nanometer onwards so if you are going to tune it then you do not want moisture in the cavity so tuning tunable range is severely affected by the presence of moisture so in a tsunami kind of laser if you want to go up to say 1000 nanometer output then there is no way other than purging the entire cavity with dry nitrogen and we have done it a couple of times and I do not want to do it ever again the one nitrogen cylinder gets over in one day you do one day is experiment the cylinder is always on right and it is an open cavity nitrogen is going out so it is first of all it is cumbersome and secondly it is very highly nitrogen intensive I do not want to do it so because of these problems and also the other issue is technology is moving in a direction where everything has to become a black box user should not worry about how things are of course the purpose of a course is exactly opposite but that is how things work you are a biologist working on two photo microscopy you do not really want to know about modes of lasers and how they are coupled and so on and so forth so this is the state of the art and we have shown this already when we went to the TCSPC lab earlier so everything is sealed in a container advantage of this is that you can have full range of tunability 690 nanometer to 1050 nanometer at the click of a mouse disadvantage is anything goes wrong inside this the laser has to go back to the factory because it is hermetically sealed it cannot be opened anywhere other than the factory where humidity temperature everything well air quality everything is maintained very rigorously. The question that one that should come to one's mind and this is actually a laser in which you have the pump laser on one side and you have the tie sapphire laser on the other side now we should have a question when we see this this year is the laser driver why we had said earlier that you need some kind of a cavity length to have pulsed operation round trip time and so on and so forth. So, in such a small thing how is cavity length maintained and that is the diagram I was looking for and unfortunately did not find this morning we find it I will into it in the presentation and upload. So, what you have there is that you have this tie sapphire crystal and you have two large mirrors and the alignment is such that the light bounces off these mirrors many many times. So, even though the separation between the two mirrors is not much the effective path length is the same as what it is in tsunami that is how compact seal lasers like my tie work okay. So, what we have done today is we have started with principle of mode locking recurrence mode locking and we have given you a little bit of idea about construction of lasers we have not talked about certain things yet one is if you remember that diagram we have not told you what the fast photodiode does we have not told you what the meaning of AOM is. So, in the next module in the next module we are going to go to the lab open up tsunami and show you after that we are going to talk about what acoustic modulators are how they can produce pulse speaker how they can produce pulse operation I told you already that you do not really need the acoustic modulator in this cavity why is it still there is still there because there is this technology called regenerative mode lock and we will discuss that after we have talked about acoustic modulators and we will also talk about electro optic modulators like Pockel cell which can give you not short pulses but large pulses but in this context which are useful in switching the laser pulse outside the cavity once we are done with this discussion we will talk about how amplification of laser is done and then we will talk about optical parametric amplification okay so much for today.