Abstract: Similar to ytterbium doped laser materials laser operation with thulium doped media is possible within a quasi-three level scheme, which especially for pulse pumped lasers is a drawback for efficient laser operation, as a significant amount of energy is required to bleach out the laser medium. Since this energy cannot be extracted, it is lost for the amplification process. Hence, operation of such lasers at cryogenic temperatures seems to be an appropriate solution. For further modeling and derivation of design rules for future laser systems based on such a scheme reliable spectral data is needed. We will present absorption and emission measurements on Tm:YAG as a function of temperature in the range from 80 K to 300 K covering both the absorption bands around 800 nm and the emission bands up to 2.1 μm. The spectral measurements were carried out on two samples of Tm:YAG with doping levels of 2 at.% and 8 at.%. Precautions for reabsorption effects were taken to allow for accurate results over the whole measurement range. From these measurements we have derived absorption and emission cross sections and radiative lifetimes. By comparing the latter values to values obtained by highly accurate measurements of the lifetime using the pinhole method we could also estimate the quantum efficiency.
Abstract: Fiber lasers enjoy an excellent reputation as power-scalable diode-pumped solid-state laser concept. Their immunity against thermo-optical issues is combined with high efficiency and performance. The properties, challenges and perspectives of fiber lasers will be discussed.
Abstract: A new one-dimensional analytical model of a thin double layer foil interaction with a laser pulse is presented. It is based on one-dimensional electrodynamics. This model can be used for the study of high intensity laser pulse interactions with overdense plasmas, leading to frequency upshifting, high order harmonic generation, and ion acceleration in different regimes.
Abstract: A new Cryogenic Current Comparator with eXtended Dimensions (CCC-XD), compared to earlier versions built for GSI, is currently under development for a non-destructive, highly-sensitive monitoring of nA-intensities of beams for larger beamline diameters planned for the new FAIR accelerator facility at GSI. The CCC consists of a:
1) flux concentrator,
2) superconducting shield against external magnetic field and a
3) superconducting toroidal coil of niobium which is read out by a
4) Superconducting Quantum Interference Device (SQUID).
The new flux concentrator (1) comprises a specially designed highly-permeable core made of nano-crystalline material, in order to assure low-noise operation with high system bandwidth of up to 200 kHz. The superconducting shielding of niobium (2) is extended in its geometric dimensions compared to the predecessor CCC and thus will suppress (better -200 dB) disturbing magnetic fields of the beamline environment more effectively. For the CCD-XD readout, new SQUID sensors (4) with sub-μm Josephson junctions are used which enable the lowest possible noiselimited current resolution in combination with a good suppression of external disturbances. The CCC-XD system, together with a new dedicated cryostat, will be ready for testing in the CRYRING at GSI in spring 2017. For the application of a CCC in the antiproton storage ring at CERN a pulse shape correction has been developed and tested in parallel. Results from electrical measurements of two components (1 and 4) of the new CCC-XD setup will be presented in this work.
Abstract: State-of-the-art ultrafast fiber lasers currently are limited in peak power by excessive nonlinearity and in average power by modal instabilities. Coherent beam combination in space and time is a successful strategy to continue power scaling by circumventing these limitations. Following this approach, we demonstrate an ultrafast fiber-laser system featuring spatial beam combination of 8 amplifier channels and temporal combination of a burst comprising 4 pulses. Active phase stabilization of this 10-armed interferometer is achieved using LOCSET and Hänsch-Couillaud techniques. The system delivers 1 kW average power at 1 mJ pulse energy, being limited by pump power, and delivers 12 mJ pulse energy at 700 W average power, being limited by optically induced damage. The system efficiency is 91% and 78%, respectively, which is due to inequalities of nonlinearity between the amplifier channels and to inequality of power and nonlinearity between the pulses within the burst. In all cases, the pulse duration is ~260 fs and the M2-value is better than 1.2. Further power scaling is possible using more amplifier channels and longer pulse bursts.
Abstract: The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Thus, it is of high interest to develop efficient mitigation strategies to further enhance the performance of fiber laser systems. By actively modulating the pump power of an Yb-doped rod-type fiber amplifier, it was possible to weaken the thermally-induced refractive index grating along the fiber and, thus, to mitigate TMI to a large extent. A significant advantage of this approach is that it can be easily integrated in any existing fiber-laser system since no further optical components are needed. A function generator connected to the pump diode driver was used to achieve the modulation. With this setup we were able to extract a fully stabilized beam at ~ 1.5 times above the TMI threshold. Furthermore, a stabilization of the beam was still feasible at an average output power of 628 W, which is more than three times higher than the free-running TMI threshold of that particular fiber under identical conditions (e.g. seed power). This is the highest average output power reported from a single-channel rod-type fiber amplifier with a high-quality stabilized beam, to the best of our knowledge.
Abstract: n this contribution, we present a spatio-temporal coherent beam combining setup in a proof-of-principle experiment with an entirely fiber-coupled front-end. Unlike in previous experiments, where the temporal pulse division was achieved using free-space optical delay lines, the pulses are taken directly from the pulse train of the oscillator. Thereby, the free-space paths and the alignment requirement are cut in half. The combination inevitably remains in free-space considering application in high-power lasers. For the combination of 4 temporally separated pulses, a combining efficiency larger than 95% is demonstrated. The efficiency is largely independent of the combined pulse energy and temporal contrasts close to the theoretically estimated maximum are reached. Potentially, this approach allows for self-optimization of the combination due to the many degrees of freedom accessible with the electro-optic modulators.
Abstract: The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Even though a significant amount of knowledge on TMI in single-pass fiber amplifiers has been generated in the last years, relatively little is known about this effect in multi-pass amplifiers and oscillators. In this contribution TMI is experimentally investigated in a double-pass fiber amplifier, for the first time to the best of our knowledge. The TMI threshold was found to be significantly lower in the double-pass configuration than in the single-pass arrangement. Furthermore, the investigations unveiled a complex dynamic behavior of the instabilities in the double-pass fiber amplifier.
Abstract: Thulium-doped fiber lasers are an attractive concept for the generation of mid-infrared (mid-IR) ultrashort pulses around 2 μm wavelength with an unprecedented average power. To date, these systems deliver >150 W of average power and GW-class pulse peak powers with output pulse durations of a few hundreds of fs. As some applications can greatly benefit from even shorter pulse durations, the spectral broadening and subsequent temporal pulse compression can be a key enabling technology for high average power few-cycle laser sources around 2 μm wavelength. In this contribution we demonstrate the nonlinear compression of ultrashort pulses from a high repetition rate Tm-doped fiber laser using a nitrogen gas-filled hollow capillary. Pulses with 4 GW peak power, 46 fs FWHM duration at an average power of 15.4 W have been achieved. This is, to the best of our knowledge, the first 2 μm laser delivering intense, GW-pulses with sub 50-fs pulse duration and an average power of >10 W. Based on this result, we discuss the next steps towards a 100 W-level, GW-class few-cycle mid-IR laser.
Abstract: High power OPCPAs above 10 W at short-wave IR wavelengths (SWIR: 1.4 - 3 μm) may be limited because of thermal heat dissipation in the nonlinear crystals. In this work we provide up-to-date measurements of the absorption coefficients of the nonlinear crystals used at these wavelengths and simulations of the thermal effects on critical parameters. In particular, power scaling limits will be discussed.
Abstract: The noise characteristics of high-power fiber lasers, unlike those of other solid-state lasers such as thin-disks, have not been systematically studied up to now. However, novel applications for high-power fiber laser systems, such as attosecond pulse generation, put stringent limits to the maximum noise level of these sources. Therefore, in order to address these applications, a detailed knowledge and understanding of the characteristics of noise and its behavior in a fiber laser system is required. In this work we have carried out a systematic study of the propagation of the relative intensity noise (RIN) along the amplification chain of a state-of-the-art high-power fiber laser system. The most striking feature of these measurements is that the RIN level is progressively attenuated after each amplification stage. In order to understand this unexpected behavior, we have simulated the transfer function of the RIN in a fiber amplification stage (~80μm core) as a function of the seed power and the frequency. Our simulation model shows that this damping of the amplitude noise is related to saturation. Additionally, we show, for the first time to the best of our knowledge, that the fiber design (e.g. core size, glass composition, doping geometry) can be modified to optimize the noise characteristics of high-power fiber laser systems.
Abstract: Ultrashort-pulse laser systems are an enabling technology for numerous applications. The stability of such systems is especially crucial for frequency metrology and high precision spectroscopy. Thulium-based fiber lasers are an ideal starting point as a reliable and yet powerful source for the nonlinear conversion towards the mid-IR region. Recently, we have demonstrated that nonlinear self-compression in a fused silica solid-core fiber allows for few-cycle pulse duration with up to 24 MW peak power using a high-repetition rate thulium-based fiber laser system operating at around 2 μm wavelength . This experiment operates near the self-focusing limit of about 24 MW for circular polarization, which increases the requirements for the system stability due to the risk of a fiber damage. Here, we present a self-protecting nonlinear compression regime allowing for long-term operation and high output-pulse stability with very similar output performance.
Abstract: Transverse mode instabilities (TMI) have become a very serious problem for the further scaling of the average power of fiber laser systems. Recently the strong impact that photodarkening (PD) has on the TMI threshold of Yb-doped fiber laser systems has been revealed. This is a remarkable finding since it opens the door to a significant increase of the average power of fiber laser systems in the near future. The key to achieve this is to reduce the amount of PD losses in the fiber, which can be done with an optimization of the glass composition in the fiber. In this work we perform a theoretical study on the impact that co-dopants such as Al and P have on PD and on the TMI threshold. This analysis tries to find the optimum glass composition from the point of view of TMI. It is shown that in a short rod type fiber, changing the glass composition only leads to a modest increase of the TMI threshold due to the degradation of the cross-sections. This demonstrates that the optimization of the glass cannot be done attending only to the PD losses at the cost of the laser cross-sections. In spite of this, changing the glass composition can bring benefits in pulsed operation in terms of the stored energy. Additionally, other fiber geometries different from the rod-type can benefit in a greater degree by introducing co-dopants in the glass.
Abstract: Experimental demonstrations of Tm-doped fiber amplifiers (typically in CW- or narrow-band pulsed operation) span a wavelength range going from about 1700 nm to well beyond 2000 nm. Thus, it should be possible to obtain a bandwidth of more than 100 nm, which would enable sub-100 fs pulse duration in an efficient, linear amplification scheme. In fact, this would allow the emission of pulses with less than 20 optical cycles directly from a Tm-doped fiber system, something that seems to be extremely challenging for other dopants in a fused silica fiber. In this contribution, we summarize the current development of our Thulium-doped fiber CPA system, demonstrate preliminary experiments for further scaling and discuss important design factors for the next steps. The current single-channel laser system presented herein delivers a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. Special care has been taken to reduce the detrimental impact of water vapor absorption by placing the whole system in a dry atmosphere housing (<0.1% rel. humidity) and by using a sufficiently long wavelength (1920-1980 nm). The utilization of a low-pressure chamber in the future will allow for the extension of the amplification bandwidth. Preliminary experiments demonstrating a broader amplification bandwidth that supports almost 100 fs pulse duration and average power scaling to < 100W have already been performed. Based on these results, a Tm-doped fiber CPA with sub-100 fs pulse duration, multi-GW pulse peak power and >100 W average power can be expected in the near future.
Abstract: The focus of these lectures is on the quantum vacuum subjected to classical electromagnetic fields. To this end we explicitly derive the renowned Heisenberg-Euler effective action in constant electromagnetic fields in a rather pedagogical and easy to conceive way. As an application, we use it to study vacuum birefringence constituting one of the most promising optical signatures of quantum vacuum nonlinearity.
Abstract: The assistance of an intense optical laser pulse on electron-positron pair production by the Breit-Wheeler and Schwinger processes in XFEL fields is analyzed. The impact of a laser beam on high-energy photon collisions with XFEL photons consists in a phase space redistribution of the pairs emerging in the Breit-Wheeler sub-process. We provide numerical examples of the differential cross section for parameters related to the European XFEL. Analogously, the Schwinger type pair production in pulsed fields with oscillating components referring to a superposition of optical laser and XFEL frequencies is evaluated. The residual phase space distribution of created pairs is sensitive to the pulse shape and may differ significantly from transiently achieved mode occupations.
Abstract: In modern laser-based ion acceleration systems, the field distribution of the focused laser beam at the position of the target strongly influences the overall characteristics of the resulting ion beam. To obtain an unidirectional and quasi mono-energetic ion beam, a flat-top field distribution of the focused laser beam is optimal. This can only be achieved, by using a beam-profiling system that reshapes the incident laser beam into an Airy-shaped field distribution in the far field. Here, we present an extensive design study of such a beam-profiling system based on two free-form mirrors. In order to realize the rings of zero intensity, corresponding to the roots of the Airy-function, strong curvature peaks on the first mirror are necessary. Additionally, the alternating phase in between these rings can only be achieved with grooves on the second mirror. These aspects actually raise the question, if the used purely geometric optical modeling approach is still valid. Therefore, our design study is entirely accompanied with wave-optical simulations to identify influences of diffraction within the beam profiling system. We find that especially the grooves on the second mirror are mandatory, not only to ensure the alternating phase, but also to realize the roots of zero intensity of the Airy-function. On the other hand, these grooves cause diffraction effects in the beam-profiling system that slightly degrade the at-top focal field. These influences are in the range of a few percent and cannot be further avoided.
Abstract: In this chapter, the microscopic characteristics of a bright, short-pulsed source of Ti Kα radiation are studied. This x-ray emission is generated from fast electrons that are generated when a relativistically intense laser pulse interacts with a solid metal surface. The electrons have average energies significantly exceeding the ionization threshold of the K-shell (5 keV) and give rise to K-radiation when the K-shell recombines with a lifetime of a few femtoseconds only. Hence the duration of the Kα emission is dominantly determined by the time these fast electrons are present. But at the same time, the electrons also generate a solid-density plasma state at several tens of electronvolts temperature (e.g., several 100 000 K). This alters the emission probabilities of the Kα source, potentially effecting the brightness of the x-ray source. These mechanisms and possible optimizations are subject of this chapter.
Abstract: The photoelectric effect, the emission of electrons from a metal surface after absorbing light, was explained by Einstein's model, where light particles (photons) must have a minimum energy (frequency) to ionize atoms. The number of excited atoms is proportional to the intensity (the number of photons delivered). However, when the light is supplied by very intense, very fast pulses from lasers, the number of ionized atoms will depend on the electric field strength - the amplitude of the light seen as an electromagnetic wave. This change occurs because ionization occurs via quantum tunneling through the relevant energy barrier during a short time window near the maxima of the electric field. Isolated attosecond pulses recently enabled studies of the dynamics of tunneling ionization of atoms in gases. On page 1348 of this issue, Schultze et al. experimentally show that atoms in a solid are also excited via the tunneling process.
Abstract: Hochleistungslaser ermöglichen es inzwischen, relativistische Elektronenpulse mit bemerkenswerten Eigenschaften zu erzeugen. Neben der äußerst kurzen Beschleunigungslänge sind vor allem die kleine Quellgröße und auch die kurze Pulsdauer interessant. Mit diesen Elektronenpulsen lässt sich zudem elektromagnetische Sekundärstrahlung im Kiloelektronenvolt-Bereich erzeugen. Mit dieser Röntgenstrahlung würden auch Universitäten relativ kompakte Röntgenquellen zur Verfügung stehen, mit denen sich Effekte beobachten lassen, die auf äußerst kurzen räumlichen und zeitlichen Skalen ablaufen. Momentan ist diese Art von Forschung nur an großen, konventionellen Synchrotron-Beschleunigern möglich.
Abstract: In cancer treatment it is highly desirable to identify and /or classify individual cancer cells in real time. Nowadays, the standard method is PCR which is costly and time-consuming. Here we present a different approach to rapidly classify cell types: we measure the pattern of coherently diffracted extreme ultraviolet radiation (XUV radiation at 38nm wavelength), allowing to distinguish different single breast cancer cell types. The output of our laser driven XUV light source is focused onto a single unstained and unlabeled cancer cell, and the resulting diffraction pattern is measured in reflection geometry. As we will further show, the outer shape of the object can be retrieved from the diffraction pattern with sub-micron resolution. For classification it is often not necessary to retrieve the image, it is only necessary to compare the diffraction patterns which can be regarded as a spatial fingerprint of the specimen. For a proof-of-principle experiment MCF7 and SKBR3 breast cancer cells were pipetted on gold-coated silica slides. From illuminating each single cell and measuring a diffraction pattern we could distinguish between them. Owing to the short bursts of coherent soft x-ray light, one could also image temporal changes of the specimen, i.e. studying changes upon drug application once the desired specimen is found by the classification method. Using a more powerful laser, even classifying circulating tumor cells (CTC) at a high throughput seems possible. This lab-sized equipment will allow fast classification of any kind of cells, bacteria or even viruses in the near future.
Abstract: Different pumping schemes for soft X-ray lasers have been investigated at the PHELIX laser facility, including a double-target seeding approach at 18.9 nm. A technical feasibility study of using a Mo XRL beam of several μJ as an excitation source for heavy-ion spectroscopy in a storage ring has been carried out. XRL photon numbers and the beam transport under ultra-high vacuum conditions over almost 30 m are the major challenges.
Abstract: We present a novel approach for the construction of a high energy, high power burst mode laser system, based on diode pumped cryogenically cooled Yb:CaF2. The system consists of a frontend producing pulses of 300 fs duration with 1 MHz. Bursts of 1000 subsequent pulses are cut from the continuous train by an electro optical modulator. Afterwards the duration of the individual pulses is stretched to 50 ps.
The amplifier system consists of two amplifiers. Both amplifiers utilize mirror based relay imaging schemes to allow for a sufficient number of extraction passes for achieving efficient energy extraction. The goal parameters of the system are to achieve a total energy of 5 J per burst with a repetition rate of 10Hz.
Amplification results for the first of two amplifiers are demonstrated. A total output energy of 480 mJ was achieved corresponding to an optical to optical efficiency from absorbed pump energy to extracted energy of more than 17%. Single pulse energies of up to 7.5mJ are generated when changing to less pulses per burst.
To achieve a constant energy from pulse to pulse during the burst we present a technique based on the modulation of the laser diode current during one pulse. With this technique the gain variation during the burst was than 5% peak to peak.
Abstract: Advanced high intensity laser matter interaction experiments always call for optimized laser performance. In order to further enhance the POLARIS laser system, operational at the University of Jena and the Helmholtz-Institute Jena, in particular its energy, bandwidth and focusability, new amplifier technologies have been developed and are reported here. Additionally, existing sections were considerably improved. A new multi-pass amplification stage, which is able to replace two currently used ones, was developed in close collaboration with the MPQ (Garching). The new basic elements of this amplifier are well homogenized pump modules and the application of a successive imaging principle. By operating the amplifier under vacuum conditions a top hat beam profile with an output energy of up to 1.5 J per pulse is foreseen. The already implemented POLARIS amplifier A4 was further improved by adapting an advanced method for the homogenization of the multi-spot composed pump profile. The new method comprises a computer-based evolutionary algorithm which optimizes the position of the different spots regarding its individual size, shape and intensity. The latter allowed a better homogenization of the POLARIS near field profile.
Abstract: A Cryogenic Current Comparator is a highly sensitive tool for the non-destructive online monitoring of continuous as well as bunched beams of very low intensities. The noise-limited current resolution of such a device depends on the ferromagnetic material embedded in the pickup coil of the CCC. Therefore, the main focus of research was on the low temperature properties of ferromagnetic core materials. In this contribution we present first results of the completed Cryogenic Current Comparator for FAIR working in a laboratory environment, regarding the improvements in resolution due to the use of suitable ferromagnetic core materials.
Abstract: An absolute and exact measurement of the intensity of charged particle beams - extracted from an accelerator or circulating in a Storage Ring - is one of the major problems of beam diagnostics. Also the measurement of socalled dark currents, generated by superconductive RF accelerator cavities at high voltage gradients to characterize the quality of these components becomes more and more important for the commissioning of new accelerators (XFEL at DESY). The Cryogenic Current Comparator (CCC) based on high precision LTS SQUIDs is an excellent tool to solve these problems.
Abstract: Online monitoring of low intensity (below 1 μA) charged particle beams without disturbing the beam and its environment is crucial for any accelerator facility. For the upcoming FAIR project a beam monitor based on the Cryogenic Current Comparator principle with an enhanced resolution was developed. The main focus of research was on the low temperature properties of the ferromagnetic core material of the superconducting pickup coil. The pick-up coil transforms the magnetic field of the beam into a current that is detected by a high performance low temperature dc Superconducting QUantum Interference Device (LTS-DC-SQUID). The penetration of the pick-up coil by interfering magnetic fields is highly attenuated by a meander shaped superconducting shielding. The Cryogenic Current Comparator is able to measure DC beam currents, e.g. as required for slow extraction from a synchrotron, as well as bunched beams. In this contribution we present first results of the improved Cryogenic Current Comparator working up to now in a laboratory environment.
Abstract: The present status and properties of charge-changing processes—electron capture and electron loss—are considered for heavy many-electron ions colliding with neutral atoms over a wide energy range E = 10 keV/u–100 GeV/u. The role of single- and multiple-electron charge-changing processes is discussed, and a brief description of available computer codes for calculation of the corresponding cross sections is presented. Experimental data for electron-loss and capture cross sections for germanium, xenon, lead, and uranium ions colliding with H, N, Ne, Ar, and Xe targets are given in comparison with numerical calculations applying different theoretical models as well as semiempirical formulae.
Abstract: We introduce a method to suppress prepulses of pulse picking systems due to the limited extinction ratio of polarization gating systems. By matching the round trip times of the oscillator and the subsequent regenerative amplifiers, leaking pulses are hidden below the temporal intensity pedestal of the main pulse. With this method, prepulses at the temporal position equal to the time difference of the round trip times of the cavities could be suppressed completely.
Abstract: Using state-of-the-art high-power laser systems, we are able to routinely generate extreme energy densities and focused light intensities in a controlled laboratory environment. During the interaction of these laser pulses with matter a plasma is generated that can both sustain and support huge electric fields. These can be used as a novel type of accelerator structure for electrons and ions having properties favorable for a large number of future applications.