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2012

J. Rothhardt, A. M. Heidt, S. Hädrich, S. Demmler, J. Limpert, and A. Tünnermann
High stability soliton frequency-shifting mechanisms for laser synchronization applications
J. Opt. Soc. Am. B, 29 :1257 (May 2012)

Abstract:

We analyze frequency-shifting mechanisms in photonic crystal fibers (PCFs). In contrast to the generally used approach of launching pulses in the negative group velocity dispersion (GVD) region of PCFs, we suggest employing a fiber with a higher zero dispersion wavelength that is pumped in the positive GVD region. Results of a numerical optimization reveal that the amplitude stability of the frequency-shifted pulses can be improved by more than 1 order of magnitude and the timing jitter arising from input fluctuations by 2 orders of magnitude by a proper choice of the fiber dispersion. The presented approach and optimization will improve the performance of timing- and amplitude-sensitive applications, such as nonlinear microscopy and spectroscopy or optical synchronization for optical parametric chirped pulse amplification significantly.

M. Baumgartl, B. Ortac, J. Limpert, and A. Tünnermann
Impact of dispersion on pulse dynamics in chirped-pulse fiber lasers
Appl. Phys. B, 107 :263 (May 2012)

Abstract:

We report on a systematic study of an environmentally stable mode-locked Yb-doped fiber laser operating in the chirped-pulse regime. The linear cavity chirped-pulse fiber laser is constructed with a saturable absorber mirror as nonlinear mode-locking mechanism and a nonlinearity-free transmission-grating-based stretcher/compressor for dispersion management. Mode-locked operation and pulse dynamics from strong normal to strong anomalous total cavity dispersion in the range of +2.5 to -1.6 ps^2 is experimentally studied. Strongly positively chirped pulses from 4.3 ps (0.01 ps^2) to 39 ps (2.5 ps^2) are obtained at normal net-cavity dispersion. In the anomalous dispersion regime, the laser generates average soliton feature negatively chirped pulses with autocorrelation pulse durations from 0.8 ps (−0.07 ps^2) to 3.9 ps (-1.6 ps^2). The lowered peak power due to the pulse stretching allows one to increase the double pulse threshold. Based on the numerical simulation, different regimes of mode locking are obtained by varying the intra-cavity dispersion, and the characteristics of average soliton, stretched-pulse, wave-breaking-free and chirped-pulse regimes are discussed.

Ph. A. Korneev, S. V. Popruzhenko, S. P. Goreslavski, T.-M. Yan, D. Bauer, W. Becker, M. Kübel, M. F. Kling, C. Rödel, M. Wünsche, and G.G. Paulus
Interference Carpets in Above-Threshold Ionization: From the Coulomb-Free to the Coulomb-Dominated Regime
Phys. Rev. Lett., 108 :223601 (May 2012)

Abstract:

The velocity map recorded in above-threshold ionization of xenon at 800 nm exhibits a distinct carpetlike pattern of maxima and minima for emission in the direction approximately perpendicular to the laser polarization. The pattern is well reproduced by a numerical solution of the time-dependent Schrödinger equation. In terms of the simple-man model and the strong-field approximation, it is explained by the constructive and destructive interference of the contribution of the long and the short orbit. Strictly perpendicular emission is caused by ionization at the two peaks of the laser field per cycle, which results in a 2ℏω separation of the above-threshold ionization rings.

G. Weber, R. Märtin, A. Surzhykov, M. Yasuda, V. A. Yerokhin, and T. Stöhlker
PEBSI – A Monte Carlo simulator for bremsstrahlung arising from electrons colliding with thin solid-state targets
Nucl. Instr. Meth. Phys. Res. B, 279 :155 (May 2012)

Abstract:

We present a Monte Carlo code dedicated to the simulation of bremsstrahlung arising in collisions of polarized electrons with thin target foils. The program consists of an electron transport algorithm taking into account elastic electron-nucleus scattering and inelastic collisions with target electrons as well as a treatment of polarized-electron bremsstrahlung emission. Good agreement is found between the predictions of the electron transport code and data stemming from other simulation programs and experiments. In addition, we present first results from the bremsstrahlung simulation which indicate a significant decrease in the degree of linear polarization of bremsstrahlung even for the thinnest gold targets considered.

M. Grieser, Yu. A. Litvinov, R. Raabe, K. Blaum, Y. Blumenfeld, P. A. Butler, F. Wenander, P. J. Woods, M. Aliotta, A. Andreyev, A. Artemyev, D. Atanasov, T. Aumann, D. Balabanski, A. Barzakh, L. Batist, A.-P. Bernardes, D. Bernhardt, J. Billowes, S. Bishop, M. Borge, I. Borzov, F. Bosch, A. J. Boston, C. Brandau, W. Catford, R. Catherall, J. Cederkall, D. Cullen, T. Davinson, I. Dillmann, C. Dimopoulou, G. Dracoulis, Ch. E. Düllmann, P. Egelhof, A. Estrade, D. Fischer, K. Flanagan, L. Fraile, M. A. Fraser, S. J. Freeman, H. Geissel, J. Gerl, P. Greenlees, R. E. Grisenti, D. Habs, R. von Hahn, S. Hagmann, M. Hausmann, J. J. He, M. Heil, M. Huyse, D. Jenkins, A. Jokinen, B. Jonson, D. T. Joss, Y. Kadi, N. Kalantar-Nayestanaki, B. P. Kay, O. Kiselev, H.-J. Kluge, M. Kowalska, C. Kozhuharov, S. Kreim, T. Kröll, J. Kurcewicz, M. Labiche, R. C. Lemmon, M. Lestinsky, G. Lotay, X. W. Ma, M. Marta, J. Meng, D. Mücher, I. Mukha, A. Müller, A. Murphy, G. Neyens, T. Nilsson, C. Nociforo, W. Nörtershäuser, R. D. Page, M. Pasini, N. Petridis, N. Pietralla, M. Pfützner, Z. Podolyak, P. Regan, M. W. Reed, R. Reifarth, P. Reiter, R. Repnow, K. Riisager, B. Rubio, M. S. Sanjari, D. W. Savin, C. Scheidenberger, S. Schippers, D. Schneider, R. Schuch, D. Schwalm, L. Schweikhard, D. Shubina, E. Siesling, H. Simon, J. Simpson, J. Smith, K. Sonnabend, M. Steck, T. Stora, T. Stöhlker, B. Sun, A. Surzhykov, F. Suzaki, O. Tarasov, S. Trotsenko, X. L. Tu, P. Van Duppen, C. Volpe, D. Voulot, P. M. Walker, E. Wildner, N. Winckler, D. F. A. Winters, A. Wolf, H. S. Xu, A. Yakushev, T. Yamaguchi, Y. J. Yuan, Y. H. Zhang, and K. Zuber
Storage ring at HIE-ISOLDE Technical design report
Eur. Phys. J. ST, 207 :1 (May 2012)

Abstract:

We propose to install a storage ring at an ISOL-type radioactive beam facility for the first time. Specifically, we intend to setup the heavy-ion, low-energy ring TSR at the HIE-ISOLDE facility in CERN, Geneva. Such a facility will provide a capability for experiments with stored secondary beams that is unique in the world. The envisaged physics programme is rich and varied, spanning from investigations of nuclear ground-state properties and reaction studies of astrophysical relevance, to investigations with highly-charged ions and pure isomeric beams. The TSR might also be employed for removal of isobaric contaminants from stored ion beams and for systematic studies within the neutrino beam programme. In addition to experiments performed using beams recirculating within the ring, cooled beams can also be extracted and exploited by external spectrometers for high-precision measurements. The existing TSR, which is presently in operation at the Max-Planck Institute for Nuclear Physics in Heidelberg, is well-suited and can be employed for this purpose. The physics cases as well as technical details of the existing ring facility and of the beam and infrastructure requirements at HIE-ISOLDE are discussed in the present technical design report.

J. Habib, O. Guilbaud, B. Zielbauer, D. Zimmer, M. Pittman, S. Kazamias, C. Montet, T. Kühl, and D. Ros
Low energy prepulse for 10 Hz operation of a soft-x-ray laser
Opt. Express, 20 :10128 (April 2012)

Abstract:

The influence on Nickel-like Molybdenum soft-x-ray laser performance and stability of a low energy laser prepulse arriving prior to the main laser pumping pulses is experimentally investigated. A promising regime for 10 Hz operation has been observed. A four times increase in soft-x-ray laser operation time with a same target surface is demonstrated. This soft-x-ray laser operation mode corresponds to an optimum delay between the prepulse and the main pulses and to a prepulse energy greater than 20 mJ. We also show that this regime is not associated with a weaker degradation of the target or any reduced ablation rate. Therefore the role of preplasma density gradient in this effect is discussed.

J. Rothhardt, S. Demmler, S. Hädrich, J. Limpert, and A. Tünnermann
Octave-spanning OPCPA system delivering CEP-stable few-cycle pulses and 22 W of average power at 1 MHz repetition rate
Opt. Express, 20 :10870 (April 2012)

Abstract:

We report on an OPCPA system delivering CEP-stable pulses with a pulse duration of only 1.7 optical cycles at 880 nm wavelength. This pulse duration is achieved by the generation, optical parametric amplification and compression of a full optical octave of bandwidth. The system is pumped by a high average power Yb-fiber laser system, which allows for operation of the OPCPA at up to 1 MHz repetition rate and 22 W of average output power. Further scaling towards single-cycle pulses, higher energy and output power is discussed.

T. Rathje, N. Johnson, M. Möller, F. Süssmann, D. Adolph, M. Kübel, R. Kienberger, M. F. Kling, G.G. Paulus, and A. M. Sayler
Review of attosecond resolved measurement and control via carrier-envelope phase tagging with above-threshold ionization
J. Phys. B, 45 :074003 (April 2012)

Abstract:

A precise, real-time, single-shot carrier–envelope phase (CEP) tagging technique for few-cycle pulses was developed and combined with cold-target recoil-ion momentum spectroscopy and velocity-map imaging to investigate and control CEP-dependent processes with attosecond resolution. The stability and precision of these new techniques have allowed for the study of intense, few-cycle, laser-matter dynamics with unprecedented detail. Moreover, the same stereo above-threshold ionization (ATI) measurement was expanded to multi-cycle pulses and allows for CEP locking and pulse-length determination. Here we review these techniques and their first applications to waveform characterization and control, non-sequential double ionization of argon, ATI of xenon and electron emission from SiO_2 nanospheres.

J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann
Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation
Light Sci. Appl., 1 :e8 (April 2012)

Abstract:

Rare earth-doped fibres are a diode-pumped, solid-state laser architecture that is highly scalable in average power. The performance of pulsed fibre laser systems is restricted due to nonlinear effects. Hence, fibre designs that allow for very large mode areas at high average powers with diffraction-limited beam quality are of enormous interest. Ytterbium-doped, rod-type, large-pitch fibres (LPF) enable extreme fibre dimensions, i.e., effective single-mode fibres with mode sizes exceeding 100 times the wavelength of the guided radiation, by exploiting the novel concept of delocalisation of higher-order transverse modes. The non-resonant nature of the operating principle makes LPF suitable for high power extraction. This design allows for an unparalleled level of performance in pulsed fibre lasers.

F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann
26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality
Opt. Lett., 37 :1073 (March 2012)

Abstract:

We demonstrate a Q-switched fiber laser system emitting sub-60 ns pulses with 26 mJ pulse energy and near-diffraction-limited beam quality (M^2 < 1.3). In combination with a repetition rate of 5 kHz, a corresponding average output power of 130 W is achieved. This record performance is enabled by a large-pitch fiber with a core diameter of 135 µm. This fiber allows for effective single-mode operation with mode field diameters larger than 90 µm even at average output powers exceeding 100 W.

E. Seres, J. Seres, and C. Spielmann
Extreme ultraviolet light source based on intracavity high harmonic generation in a mode locked Ti:sapphire oscillator with 9.4 MHz repetition rate
Opt. Express, 20 :6185 (March 2012)

Abstract:

We report on the realization of an intracavity high harmonic source with a cutoff above 30 eV. The EUV source is based on a high power, hard-aperture, Kerr-lens mode-locked Ti:sapphire oscillator with a repetition rate of 9.4 MHz. The laser is operated in the net negative dispersion regime resulting in intracavity pulses as short as 17 fs with 1 µJ pulse energy. In a second intracavity focus, intensity more than 10^(14) W/cm^2 has been achieved, which is sufficient for high harmonic generation in a Xenon gas jet.

B. Aurand, C. Rödel, H. Zhao, S. Kuschel, M. Wünsche, O. Jäckel, M. Heyer, F. Wunderlich, M.C. Kaluza, G.G. Paulus, and T. Kühl
Note: A large aperture four-mirror reflective wave-plate for high-intensity short-pulse laser experiments
Rev. Sci. Instrum., 83 :036104 (March 2012)

Abstract:

We report on a four-mirror reflective wave-plate system based on a phase-shifting mirror (PSM) for a continuous variation of elliptical polarization without changing the beam position and direction. The system presented and characterized here can replace a conventional retardation plate providing all advantages of a PSM, such as high damage-threshold, large scalability, and low dispersion. This makes reflective wave-plates an ideal tool for ultra-high power laser applications.

S. Fuchs, A. Blinne, C. Rödel, U. Zastrau, V. Hilbert, M. Wünsche, J. Bierbach, E. Frumker, E. Förster, and G.G. Paulus
Optical coherence tomography using broad-bandwidth XUV and soft X-ray radiation
Appl. Phys. B, 106 :789 (March 2012)

Abstract:

We present a novel approach to extend optical coherence tomography (OCT) to the extreme ultraviolet (XUV) and soft X-ray (SXR) spectral range. With a simple setup based on Fourier-domain OCT and adapted for the application of XUV and SXR broadband radiation, cross-sectional images of semiconductors and organic samples becomes feasible with current synchrotron or laser-plasma sources. For this purpose, broadband XUV radiation is focused onto the sample surface, and the reflected spectrum is recorded by an XUV spectrometer. The proposed method has the particular advantage that the axial spatial resolution only depends on the spectral bandwidth. As a consequence, the theoretical resolution limit of XUV coherence tomography (XCT) is in the order of nanometers, e.g., 3 nm for wavelengths in the water window (280 - 530 eV). We proved the concept of XCT by calculating the reflectivity of one-dimensional silicon and boron carbide samples containing buried layers and found the expected properties with respect to resolution and penetration depth confirmed.

I. I. Tupitsyn, Y. S. Kozhedub, V. M. Shabaev, A. I. Bondarev, G. B. Deyneka, I. A. Maltsev, S. Hagmann, G. Plunien, and T. Stöhlker
Relativistic calculations of the K-K charge transfer and K-vacancy production probabilities in low-energy ion-atom collisions
Phys. Rev. A, 85 :032712 (March 2012)

Abstract:

The previously developed technique for evaluation of charge transfer and electron-excitation processes in low-energy heavy-ion collisions [ Tupitsyn et al. Phys. Rev. A 82 042701 (2010)] is extended to collisions of ions with neutral atoms. The method employs the active-electron approximation, in which only the active-electron participates in the charge transfer and excitation processes while the passive electrons provide the screening density-functional theory (DFT) potential. The time-dependent Dirac wave function of the active electron is represented as a linear combination of atomic-like Dirac-Fock-Sturm orbitals, localized at the ions (atoms). The screening DFT potential is calculated using the overlapping densities of each ion (atom), derived from the atomic orbitals of the passive electrons. The atomic orbitals are generated by solving numerically the one-center Dirac-Fock and Dirac-Fock-Sturm equations by means of a finite-difference approach with the potential taken as the sum of the exact reference ion (atom) Dirac-Fock potential and of the Coulomb potential from the other ion within the monopole approximation. The method developed is used to calculate the K-K charge transfer and K-vacancy production probabilties for the Ne(1s^(2) 2s^(2) 2p^(6))-F^(8+)(1s) collisions at the F^(8+)(1s) projectile energies 130 and 230 keV/u. The obtained results are compared with experimental data and other theoretical calculations. The K-K charge transfer and K-vacancy production probabilities are also calculated for the Xe-Xe^(53+)(1s) collision.

R. Lötzsch, I. Uschmann, and E. Förster
Spatially resolved twin domain distribution and lattice parameter variations in the near-surface region of SrTiO_3 single crystals
Appl. Phys. B, 106 :563 (March 2012)

Abstract:

We investigated the twin domain distribution and lattice parameter variations associated with the displacive phase transition in SrTiO_3 by means of X-ray diffraction with high spatial resolution. By using 4.5-keV photons, the probed region is the first micrometer near the surface. We find a very inhomogeneous domain distribution, showing both regions with large monodomains and highly twinned regions, as well as large needle domains. Also, the lattice parameters in these different regions vary substantially.

U. Zastrau, T. Burian, J. Chalupsky, T. Döppner, T. W. J. Dzelzainis, R. R. Fäustlin, C. Fortmann, E. Galtier, S. H. Glenzer, G. Gregori, L. Juha, H. J. Lee, R. W. Lee, C. L. S. Lewis, N. Medvedev, B. Nagler, A. J. Nelson, D. Riley, F. B. Rosmej, S. Toleikis, T. Tschentscher, I. Uschmann, S. M. Vinko, J. S. Wark, T. Whitcher, and E. Förster
XUV spectroscopic characterization of warm dense aluminum plasmas generated by the free-electron-laser FLASH
Laser Part. Beams, 30 :45 (March 2012)

Abstract:

We report on experiments aimed at the generation and characterization of solid density plasmas at the free-electron laser FLASH in Hamburg. Aluminum samples were irradiated with XUV pulses at 13.5 nm wavelength (92 eV photon energy). The pulses with duration of a few tens of femtoseconds and pulse energy up to 100 µJ are focused to intensities ranging between 10^(13) and 10^(17) W/cm^2. We investigate the absorption and temporal evolution of the sample under irradiation by use of XUV and optical spectroscopy. We discuss the origin of saturable absorption, radiative decay, bremsstrahlung and atomic and ionic line emission. Our experimental results are in good agreement with simulations.

M. Baumgartl, M. Chemnitz, C. Jauregui, T. Meyer, B. Dietzek, J. Popp, J. Limpert, and A. Tünnermann
All-fiber laser source for CARS microscopy based on fiber optical parametric frequency conversion
Opt. Express, 20 :4484 (February 2012)

Abstract:

A novel approach for an all-fiber mono-laser source for CARS microscopy is presented. An Yb-fiber laser generates 100 ps pulses, which later undergo narrowband in-fiber frequency conversion based on degenerate four-wave-mixing. The frequency conversion is optimized to access frequency shifts between 900 and 3200 cm^(−1), relevant for vibrational imaging. Inherently synchronized pump and Stokes pulses are available at one fiber end, readily overlapped in space and time. The source is applied to CARS spectroscopy and microscopy experiments in the CH-stretching region around 3000 cm^(−1). Due to its simplicity and maintenance-free operation, the laser scheme holds great potential for bio-medical applications outside laser laboratories.

R. Fraga, A. Kalinin, M. Kühnel, D. C. Hochhaus, A. Schottelius, J. Polz, M.C. Kaluza, P. Neumayer, and R. E. Grisenti
Compact cryogenic source of periodic hydrogen and argon droplet beams for relativistic laser-plasma generation
Rev. Sci. Instrum., 83 :025102 (February 2012)

Abstract:

We present a cryogenic source of periodic streams of micrometer-sized hydrogen and argon droplets as ideal mass-limited target systems for fundamental intense laser-driven plasma applications. The highly compact design combined with a high temporal and spatial droplet stability makes our injector ideally suited for experiments using state-of-the-art high-power lasers in which a precise synchronization between the laser pulses and the droplets is mandatory. We show this by irradiating argon droplets with multi-terawatt pulses.

M. Schnell, A. Sävert, B. Landgraf, M. Reuter, M. Nicolai, O. Jäckel, C. Peth, T. Thiele, O. Jansen, A. Pukhov, O. Willi, M.C. Kaluza, and C. Spielmann
Deducing the Electron-Beam Diameter in a Laser-Plasma Accelerator Using X-Ray Betatron Radiation
Phys. Rev. Lett., 108 :075001 (February 2012)

Abstract:

We investigate the properties of a laser-plasma electron accelerator as a bright source of keV x-ray radiation. During the interaction, the electrons undergo betatron oscillations and from the carefully measured x-ray spectrum the oscillation amplitude of the electrons can be deduced which decreases with increasing electron energies. From the oscillation amplitude and the independently measured x-ray source size of (1.8 ± 0.3) μm we are able to estimate the electron bunch diameter to be (1.6 ± 0.3) μm.

D. R. Atanasov, N. Winckler, D. Balabanski, L. Batist, F. Bosch, D. Boutin, C. Brandau, C. Dimopoulou, H. G. Essel, T. Fästermann, H. Geissel, I. Hachiuma, S. Hess, T. Izumikawa, P. Kienle, R. Knöbel, C. Kozhuharov, J. Kurcewicz, N. Kuzminchuk, S. A. Litvinov, Yu. A. Litvinov, R. S. Mao, R. Märtin, M. Mazzocco, G. Münzenberg, K. Namihira, F. Nolden, T. Ohtsubo, Z. Patyk, R. Reuschl, M. S. Sanjari, C. Scheidenberger, D. Shubina, U. Spillmann, M. Steck, T. Stöhlker, B. Sun, T. Suzuki, M. Trassinelli, I. I. Tupitsyn, H. Weick, M. Winkler, D. F. A. Winters, and T. Yamaguchi
Half-life measurements of stored fully ionized and hydrogen-like I-122 ions
Eur. Phys. J. A, 48 :22 (February 2012)

Abstract:

The half-lives of fully ionized and hydrogen-like (H-like) I-122 ions have been measured in a heavy-ion storage ring. The β^(+)-decay constants for both charge states and the electron capture (EC) decay constant of H-like ions have been determined. The EC-decay constant in H-like I-122 ions λ^(H-like)_(EC) = 7.35(33) · 10^(−4) s^(−1) is, within the uncertainty, the same as the one in neutral atoms. This result is in agreement with the estimates of recent theoretical considerations on the EC-decay of few-electron ions that explicitly take into account the conservation of the total angular momentum of the nucleus plus lepton(s) system and its projections. No firm confirmation could be concluded from our results on the predicted effect that allowed Gamow-Teller transitions become forbidden if the initial and final total angular momenta are not equal.

S. Eyring, C. Kern, M. Zürch, and C. Spielmann
Improving high-order harmonic yield using wavefront-controlled ultrashort laser pulses
Opt. Express, 20 :5601 (February 2012)

Abstract:

In this work we show that it is possible to increase the high-order harmonic yield when using wavefront-shaped laser beams. The investigation of the beam profile near the interaction region shows that the optimized beam is asymmetric and has a larger diameter. Thus, the optimized beam leads to a higher yield even if the peak intensity is lower compared to an unoptimized beam. This indicates that the wavefront of the fundamental laser beam and, accordingly, the focal profile play an important role in the efficient generation of high-order harmonic radiation.

M. Schulz, R. Riedel, A. Willner, S. Duesterer, M. J. Prandolini, J. Feldhaus, B. Faatz, J. Rossbach, M. Drescher, and F. Tavella
Pulsed operation of a high average power Yb:YAG thin-disk multipass amplifier
Opt. Express, 20 :5038 (February 2012)

Abstract:

An Yb:YAG thin-disk multipass laser amplifier system was developed operating in a 10 Hz burst operation mode with 800 µs burst duration and 100 kHz intra-burst repetition rate. Methods for the suppression of parasitic amplified spontaneous emission are presented. The average output pulse energy is up to 44.5 mJ and 820 fs compressed pulse duration. The average power of 4.45 kW during the burst is the highest reported for this type of amplifier.

F. Jansen, F. Stutzki, H.-J. Otto, T. Eidam, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann
Thermally induced waveguide changes in active fibers
Opt. Express, 20 :3997 (February 2012)

Abstract:

Thermally induced waveguide changes become significant for very large mode area fibers. This results in a reduction of the mode-field diameter, but simultaneously in an improvement of the beam quality. In this work the first systematic experimental characterization of the reduction of the mode-field diameter in various fibers during high-power operation is carried out. It is shown that the reduction of the mode-field diameter shows a characteristic behavior that scales with the core size but that is independent of the particular fiber design. Furthermore, the strength of the actual index change is experimentally estimated, and its use to overcome avoided crossings is discussed and experimentally demonstrated.

K. Jansen, F. Karbstein, A. Nagy, and M. Wagner
Lambda_MS from the static potential for QCD with n_f=2 dynamical quark flavors
J. High Energ. Phys., 025 :1 (January 2012)

Abstract:

We determine Lambda((MS)over-bar) for QCD with n_f = 2 dynamical quark flavors by fitting the QQ̅ static potential known analytically in the perturbative regime up to terms of O(α^4_s) and ~ α^4_s ln(α_s) to corresponding results obtained from lattice simulations. This has become possible, due to recent advances in both perturbative calculations, namely the determination and publication of the last missing contribution to the QQ̅ static potential at O(α^4_s), and lattice simulations with n_f = 2 dynamical quark flavors performed at the rather fine lattice spacing of a ≈ 0.042 fm. Imposing conservative error estimates we obtain Lambda((MS)over-bar) = 315(30) MeV.

C. Jauregui, T. Eidam, H.-J. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann
Temperature-induced index gratings and their impact on mode instabilities in high-power fiber laser systems
Opt. Express, 20 :440 (January 2012)

Abstract:

Mode-interference along an active fiber in high-power operation gives rise to a longitudinally oscillating temperature profile which, in turn, is converted into a strong index grating via the thermo-optic effect. In the case of mode beating between the fundamental mode and a radially anti-symmetric mode such a grating exhibits two periodic features: a main one which is radially symmetric and has half the period of the modal beating, and a second one that closely follows the mode interference pattern and has its same period. In the case of modal beating between two radially symmetric modes the thermally induced grating only has radially symmetric features and exhibits the same period of the mode interference. The relevance of such gratings in the context of the recently observed mode instabilities of high-power fiber laser systems is discussed.

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