Pia Fürtjes giving a talk about Generation of Ultrashort pulses via OPCPA in the Infrared spectral region during Laser Colloquium at Ruhr University Bochum on 21th of April 2023.
thank you for the introduction um yeah I’m Pia I’m still a PhD student and I hope I will defend in a couple of weeks um and today I my talk today is going to cover large parts of what I did during my PhD um and the title um of my talk is generation of ultra short laser pses via opcpa in the infrared Spectra region um don’t worry too much um if you don’t know what opcpa is because I’m going to try to walk you through um the the behind that uh you can all follow so okay so um these are the two buzzwords um of the today’s talk so on one hand side it’s the F cycle so we’re working with f cycle pulses or Ultra short pulses uh in the infrared spectral region so if you’re for example three micrometer pulse um then you call it yeah fuse cycle if you can count the numbers of cycle in the pulse in this case for example it’s a 40 fcond pulse uh just to give you an impression of um the pulse durations we talking about so why is the infrared region interesting here you go um so there are for example numerous gases that have characteristics absorption lines uh in the spectral region um above three micrometers and we have also the so-called fingerprint region where for example molecules have very characteristic um absorption lines and that’s why it’s very interesting to have a laser system running there so um just as a recap how do we generate Ultra short pulses or what do we need for that so first of all you need a Broadband coherent Spectrum so you have like multiple spectral components um which have a phase relation to each other um and if and this pulse then is said to be Ted so you have like the different spectr components which are distributed over um the pulse so if we have at now some facee control um and we’re able to align all spectrac components any a given um point in time then um we’re able to generate a few cycop POS um which is um yeah which is then ultimately determined by the Spectra width that we have um and this is also called the F limit so um then if you work with ultra short pulses at high energies uh you run into numerous problems for example damage threshold is a problem and there is a technique which is called tur pulse amplification where um you start with a f cycle pause or a ftoc PA and you apply a defined phase to it um in a so-called stretcher so this can be for example bike material or a grating stretch um and from with that you can make from a fcond pulse a PC pulse then you amplify this pulse and after amplification um you reverse the phase so you just apply the negative phase from the beginning in a so-called compressor um and what you end up with is just an amplified ftoc pulse so this technique has been awarded the Nobel Prize in 2018 to Donna Strickland and muu um though the technique has been known from um 1985 already where Don strien developed this technique in her PhD thesis so this is one part of the title opcpa so this is the CH pulse amplification part so um what is still missing is the optical parametric amplification so this is an amplification process that’s taking place in a nonlinear Crystal um and relies on a kai2 pro process um where you have an energy conversion um between uh pump poles and Signal poles which results in a certain Ider energy so in reality we have a pump pulse which is the most energetic one and also the one with the shortest wavelength um so the one with the most of energy and a signal pulse which we send into um this nonlinear Crystal there we have to overlap them temporarily and um um also spatially plus we need a crystal that allows us um to to make this uh process efficient and um if we’re successful to do that um we basically just take the energy which was initially in the pump pulse um and transferred it into a signal pulse and a so-called Isa Parts in my today’s talk I will spend loads of time just describing how we generate these pump and signal pulses because this is typically the challenging part um if you build an opcpa system and if I talk about characterization I most of the time will talk about the ISA pulses in principle you can also use signal pulses for your experiment this is just depending on your application but um in the today’s talk um I will mainly focus on these Isa pulses so and to make the title complete um if you add some pre-stretching so that you uh send in Picos second pulses and you add some post compression to it um to regain your FC pulses then you have um basically what the what was in the title so then you have built an optical parametric CH pulse amplification stage so the main ingredients for such an opcpa um are that you need a front end uh which provides you with all spectral components for or the seat for the pump and the signal um then you because this is eventually determining the idler wavelength so with the pump and the signal you tune the third idler beam that um comes out of the system um you need broad Spectra in order to support FC duration because we’re still working in the ultra short then you need a high pump energy because this is basically then determining your gain uh to some extent in your system and uh if like here we’re going to work at Mill at a 1 kilohertz rate and there um the translation is quite easy so 10 MJ um equals to 10 wats average power at 1 KZ reputation rate so then you need a suitable nonlinear Crystal so the crystal here needs to have a high nonlinearity in order to make this process or to to enable a high gain um then you need low absorption um so for all ideally for all involved frequencies so between for the pump the signal and deiler which is also uh sometimes critical because um we work with fantos second PES so we have broad Spectra and like broad Spectra from really pump across the signal up to The Idler and last but not least we need um suitable face matching conditions with broad bandwidth um in order to reach a ftoc duration and at at the very end um we also need a suitable stretching and compression scheme so um that we stretch the signal for example prior to the process in a way that we can still compress the Yer afterwards and ideally um we can also do some high order phase compensation up to fourth order for example to really reach the Fier limit so now you might ask the question why are we making such such a complicated system and this is um bit of the motivation why we are doing this so um we aim for this spectral region here so above three micrometers and here you see that there are some gas lasers or like Quantum Cascade lasers which cover the spectral region but there are either the pauses are too long or the power is not high enough so in order to reach this area we use um Optical parametric amplification um these are the rare earth ions um from which yeah they’re also quite quite used here for example the utum theum um which are typically used for this uh for front end and to to provide the pump and the signal for the OPA process uh some of you might be familiar with the tianium sapphire laser this is just the working horse of typical opcpa systems um but what I would like to talk about today is um this Chrome sync Sofi laser you see here um in terms of spectral width for example it has like like quite familiar properties to the titanium Sapphire laser um and that’s why um you also say that the chrom sync sulfite laser is the teaser of the mid infrared so we talk a lot about this chromium SN sulf laser as our frontend laser today and I will present two systems so first of all a midwave infrared opcpa system which runs between five and 8 micrometers and a second opcpa which uh aims for wavelengths Beyond 10 micrometers okay so um as I said um they’re going to be three main parts one time the midwave infrared opcpa then the longwave infrared opcpa and eventually I will talk about some application of uh the longwave infrared so I’m going to start with the first um chapter okay so this is the Chrome St sulfide laser um which has the nice a nice Spectrum which spans from 1.9 to 2.6 micrometers and this system provides us with 30 fcond pulses this is just a commercial system we we we purchase from ipd photonics um and we have pulses with 12.5 nanj of energy at centered roughly at 2.4 micrometer and now we use this crumping sulfide oscillator to seed um our pump and the signal um for our opcpa so we do that by just using the dichroic mirror so we split the Spectrum roughly at 2.1 micrometer and we use the lower part of the spectrum to seed the pump and the upper part to seed the signal so the signal um we checked has still a pulsation of 41 FC um which is short enough um for our application so let’s come to the pump um our pump um is a regenerative amplifier um which is a resonator in double xfold um um yeah um architecture or just like double X for cavity so resonator so in principle PES can just circulate around um we have aium IL um Crystal as a gain medium which is S pump by Tulum fiber um fiber laser so and then uh we C in the two nanj seed from our chrom sync sulfide laser um with a pocket cell and a Lambda half plate with which we can trigger the number of round trips so and then the pulse circulates around um and with every pass through the gang medium it accumulates more and more energy and after 19 round trips um we’re coupling this pulse out again um and now we have 13 MJ of of output power and in numbers this is a gain factor of 10 to the 6 uh which is uh which is high enough uh for our application so to put this into um into numbers uh I come back to the picture I’ve already shown you so this is the Spectrum we have for our pump Source um where we use a CH volume Brack rating to elongate the pulse to stretch it to nanocs and select the wavelength um region of interest um then this is Amplified in a pre-amplifier this is a Tulum fiber amplifier and these pulses are then coupled to the regenerative amplifier I was just talking before so and after circulating around um this regenerative amplifier we end up having pulses which are centered at 251 nanometers with a um bandwidth of 2.7 nanometers the energy that we get out of that um is 13 .4 M which is the highest regenerative amplifier at 2 micrometers um so far we have a nice intensity profile and a reasonable low um uh root mean Square deviation so and then our we need to remove the residual uh CH from this pulse so we send it into a grating compressor and with this grating compressor we were able to show that we have a pulse duration of 2.4 P second so as um I didn’t know how familiar you are with um with PS duration measurements in the p and fantc rate I just quickly want to recap on how we measure this puls duration so photo diets are just too slow um so what we do we we build something that’s looking almost like a Michaelson interferometer where we have an input PSE which is split in a beam splitter into two replica and we have one arm that is set onto um a translation stage and then we have a lens that is focusing the two replica into a nonlinear Crystal or a kite2 medium and then we get a signal which is frequency shifted as you see here um which is uh depending on the overlap between the two pulses so with that you can decouple the time domain into like a spatial domain um and this is something you can then record and if the detector is a photo diet then you say that an autocorrelation and if you detect as a spectrometer then you say then you get a frequency resolved Optical gating trace or short of frog trace and I will have yeah in my talk I will a couple of times talk about um yeah a or like autocorrelation traces and frog traces so um to come back so here is the pulsation of the palum so you see here the dday you introduce uh on this arm and the second harmonic signal you get into this from this K2 Coss okay so I now I have talked about the pump um now we come back to the signal so the signal is the remaining part here so it’s Center at 2.4 micrometers with 10 nanj of energy and here again we have an autocorrelation Trace which shows us the 41 F second and then uh we send this uh pulse into a high nonlinear fluide fiber so this is a so-called zeblin fiber we use with a quite large um nonlinear reflective index and we do something that is called Solan Ramen self frequency shifted pulses so um don’t worry if you don’t know what a Solon is I will talk about this on the next slide um the interesting uh feature about this um fiber is that it has negative dispersion uh Beyond two micrometers so we have simulated that from um the properties we got from the fiber and the geometry and yeah the thing you need to remember is that it’s really it has negative dispersion above 2 micrometers So Sol if you send an ultra short pulse into a fiber in general and this fiber has a dispersion um you end up having a lot longer pulst duration because the different Spectra components inside the fiber they travel with different speed um so the pause gets dispersed but there is another feature if you have a short pulse and a nonlinear fiber with negative dispersion you have the effect um that you can create solons and in this case um you have the same temporal shape um because negative dispersion compensate is compensated by surface modulation so you don’t have spect you don’t have the broadening um so we use um yeah this is this fiber I was just talking about is such a nonlinear fiber with negative dispersion um and of on top of that um we add some Ramen frequency shift um along the fiber um and this is um simulated in the following so with some program we just uh simulated how the Spectrum evalu like evolutes in inside the fiber so you have here the fiber distance it’s 2 meters in such a zeblin fiber um and you see that from this um the input Spectrum um you have a Solon that just grows out of it and then shifts towards longer W wavelengths um over the propagation distance so we checked um if this pulse we get here is really a Solon and and I can confirm for this from this autocorrelation measurement that we have a nicely fcond um pulse um at for example the wavelength of 3 micrometers so this is our signal we can now use uh for o our opcpa and another nice feature about that is that the different spectric components um according to the simulations just travel at different speeds into the fiber and if you think back at the 3 picosecond um pump sores you can now nicely select just one pulse and not running into problems because you for example um amplify spectr components you don’t want to have so oops um from this um yeah I can just say that we now use this signal Solon for our opcpa um with a few nanojoules of energy so this is much more than other system have like other systems sometimes start with P PJ pulse energies and the nice feature also is that this soluton is now tunable so we can tune it between 2.8 and 3.2 micrometer just by changing the intensity in inside the fiber so how does it look like so on the top you see the Spectrum from our Crum Sy sulfide um oscillator and um with every line you have different energies that were coupled into the fiber and you now nicely see that we create a solid T that really that really shifts towards longer wavelength um depending on the input energy and with that we have a very simple system where we can basically tweak our signal frequency um for our opcpa okay so let’s come to the setup um so we have here our front end with the chroming sulfite oscillator at 2.4 micrometer we have our dichroic mirror which splits into two parts the 2 micrometer part which is used for the regenerative amplifier and the remaining part which is sent into the zeblin fiber which creates our tunable signal with roughly 75 ftoc and then um the whole setup looks like the following so we have our signal path here which we stretch in some bike materials we have an aop PDF that’s or short desla that’s an acosto optical program as dispersive filter um you can add some polinomial face onto the signal with that uh so this is our higher order phe compensation in our system and then we have two amplification stage so the first um with uh very low um energy from from the region and both stages are um based on the nonlinear Crystal zinc germanium phosphide and then after the second um amplification stage um we have now this tunable signal pulses um with up to 600 micr of energy and we have a tunable idler wavelength between 5.4 and 6.8 micrometers with up to 400 micr and less than 100 fcond pulse duration but this is details I will come to now so we measured the Spectrum of um the signal and the Ider we get out of the system so on the left hand side you see the signal spectrum and on the right hand side you see the is spectrum and you from the color code you nicely see that um the like if you tune the signal um you also tune The Idler so just by changing the pulse energy um yeah we can shift the signal wavelength in our system uh we get a tunable Ider between 5.4 micrometers and 6 8 micrometers and the spectral width um of up to 400 4.3 terz um allows for for limited puls duration of uh 73 ftocs so now you might wonder why we have these lines on top of our ISO spectrum and the answer is fairly easy this is um just absorption lines in air so this is just water absorption which is a huge problem um in this um spectral region so in order to do further experiments and to characterize we really need to purge the whole system uh which we did with nitrogen so the next measurement you see um we implemented all this so um we perched the system and now you see the so-called fru Trace so on the upper uh upper left you see the fru Trace that has like the delay um for every delay point a whole Spectrum just to give you a feeling like this is a measurement that takes roughly 45 minutes because in this um wavelength region you can uh yeah you only have like monochromators and that’s why you need for every single delay step roughly 40 to 50 F seconds of measurement time just uh yeah to record one Spectrum so you need quite of a stable system to do that um yeah and we Could reconstruct this um this fru Trace with an error of less than um 0.7% and you nicely see that the retrieved Spectrum coincide with the measured spectrum and we could also show that the pulse duration is um 99 F to seconds okay so uh to sum it up so this was the midwave infrared opcpa um with our tunable Isa pulses uh compressed down to 99 ftoc and this is a system that runs up to at up to 400 micr of energy at 1 khz repetition rate so now I want to go to the next step and go even further so um we’re now aiming for longer Isa wavelengths now we are looking like now we have explored kind of um the lower region and we’re now aiming for wavelength above 10 micrometers with which is um a very unexplored area so far so we come back um to our front end setup you’ve seen at the beginning so uh the crumbing sulfide laser with the dichroic miror Z seeding the pump and the signal but this time um we directly use the signal at 2.4 micrometers to combine it um in our opcpa and with that we immediately create our longwave infrared opcpa at 12 micrometers so and this is a great system because this is really Compact and there are no nonlinearities involved so in reality it looks like the following so um we also pre stretch the pul the signal PES um which we then pump in three um Optical parametric stages in gallium selenide so we have a first non-colinear stage that amplifies the PES to 15 microj so from the Nano we got to microj here um then we stretch further with bik material then we have a second um second parametric state where we already achieved 140 microj of signal and then this is changing a bit from before we see the last stage with the idler um and in this last stage we able to uh amplify the iser up to 70 micro which is really record-breaking in uh in everything like in wavelength systems uh above 10 micrometers and this is then centered around 11.4 micrometers which we compress with bike material so this is just s salinite we put here um and in the next slides I want to talk about the characterization of this longwave infrared Ida okay so uh let’s start with the signals so on the bottom here you see um the seat Spectrum we have in theory or not like we have in the experiment from um the the chroming sof laser and when we amplify this spectrum uh we had to realize that um we are losing half of the spectrum um just because of phase matching properties so um These Bars here indicate the phase matching in gallium selenide for our 2 micrometer pump and unfortunately um Gallum selenite um just prohibits us to amplify much below 2.4 micrometers so unfortunately we lose a bit of spectrum here but still we have um a spectrum with 79 nanometers which is still enough for our application so and this uh signal then translates into a idler which where you can see the Spectrum here so you see it’s um cented at 11.4 micrometers with 840 nanometers of bandwidth which in principle supports a foer limit of 172 ftocs um we get an average energy of 65 microj out of it um um with a reasonably okay stability um but a nice beam profile and all this corresponds ultimately to a conversion efficiency of 2.4 2.5% um and a Quantum efficiency efficiency of 15% and we pumping the gallium selenide with 50 gwatt per square cm so um at this point uh we just just wanted to find out how long the pulse duration is and make some yeah some adjustments to it and we found that here we have 183 ftto seconds and you might just realize these little bums on the left and the right hand side um these are unfortunately coming um from our uncoated gallium selenide we have in our autocorrelation setup so this is a feature that does not belong to the pulse this is just the fact that you cannot anti-reflection code Gallum celen um just uh as a hint here um and we wanted to to just see some more face information so we did also frog measurement here again same problem um we have for every delay Point um like 40 FC of measurement so it’s like one and a half hours to record this frog Trace which makes like all the higher order face compensation quite tricky um yeah so and here you could see that we could Could reconstruct this um this Trace with an error of 0.4% and again we have um the reconstructed um spectrum is nicely coinciding with the measured ones and happily um we are reconstructing the same pulse duration as we had with the atic correlation Trace before so then we ask okay what can we do with that that’s the big question of developing laser sources in general uh and so we came up with an experiment uh to make uh some nonlinear spectroscopy in water so this is a fairly simple setup so we have like this output iser from our opcpa we uh compress um the pulse duration then we have a polarizer um to to change the energy um and then we just simply use lenses to focus this down uh into a water cell this water cell is made out of two barium fluoride windows I can just say at this point we initially tried with diamond windows but they they they break quite fast like we we kind of put black dots in it so there must be some carbonization going on so barium fluoride was the only window that actually worked for us and then we have a spacer here which allows us to introduce 12 micrometer um of water and this is not flowing so this is just uh yeah put inside and just uh just use how it is and then we just send the the Ido through it and just record on the other side with some additional lenses um lenses the the transmitted um light so and what like if you ask what do we want to see with that so there’s a librational L2 mode so a hindered rotation of water which is roughly at 660 inverse CM so 14 micrometers roughly in weight fls um and you see the green is the input Spectrum from the opcpa um I just want to emphasize that this is normalized so the the the purple one is of course uh very much uh smaller and then the transmitted you see that the Spectrum which is towards the peak of the absorbance um is more likely to be absorbed um yeah so we can now directly excite this um for the first time this this librational mode which no one has done before because there was actually no energy um at that wavelength so and that’s what we measured um so on the x-axis you see the energy um of our longwave infrared pulse and on the left hand side um the transmission and and we could show that between 3 and 27 microj of energy yeah we could see a change of 1.4% um in transmission and now we would like we we try to interpret this um and to make some sense out of it um and that’s why there was oops a simulation I did so um actually we had a look at the libration mode in a kind of quantum picture so with a lower and an excited state and um with the laser pulse we excite um the water molecules from the ground state to the to the excited state and by doing this um this this water molecule become becomes kind of um transparent for the the photons that come after um so in the like before this um water molecule um reaches its lifetime and decays from the upper level to the lower level it’s kind of invisible for for um the other for the other photons in the same pulse so and then um we try to write this in terms of the rate equation which look quite similar to like um the laser rate equations we all know so we have like a population uh density N1 and N2 we have um like an absorption coefficient which um is then changing with um the intensity of the laser and some Decay time or lifetime um with which the the system decays from the upper to the lower um State and then we included some numbers like how many molecules we expect how large the radiated area is and then you can find some absorption coefficient which is then proportional to the population inversion and the absorbance of uh the water molecules and from this we try to to kind of estimate and calculate the transmission um in the water sample um so and that’s what we did so we I I put now two uh more lines here so this is the expected transmission just from this very simple Quantum picture for like a 10 ftoc and a 20 fcond lifetime um which changeed like where you can see that the data is somewhere between um there has been other work where people were indirectly exciting water um at this frequencies and they also find like they both papers basically agree on that um the lifetime is lower than 10050 seconds but we are not experts in water spectroscopy so I’m really curious what what you say about it um yeah okay um and with that I’m coming to the end so I was talking today about the opcpa in Gallum selenide in the longwave infrared spectral region above 10 micrometers with um a front end um which is at the same time seaing the pump and the signal without nonlinear stages it is a very powerful system it has a record energy of 65 micro at 11.5 4 micrometers with 185 ftocs which corresponds to less than five Optical Cycles um and yeah for the first time we could directly excite this librational L2 band and uh we were trying to estimate the lifetime from this which we kind of put below 20 F second which is just an estimation okay and with that um I thank you for your attention um I thank all my colleagues uh for Technical and mental support um yeah and I just put the other like all the relevant papers uh you can also find the data I was talking about today in yeah thank you