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Fiber-based high-power laser systems are advantageous for defense applications due to their intrinsic stability and resilience against challenging environmental conditions. Using single- or few-mode output fibers, an excellent beam quality can be achieved, allowing for tight focusing over long distance. However, thermally induced transverse mode instabilities (TMI) limit the obtainable diffraction-limited output power. We will show how thermally induced mode instabilities can be conveniently detected in-line in an all-fiber system. Towards this aim, we compare different detection locations and methods and show that characteristic signals can be detected in backwards direction. Expanding the system and applying these detection methods, we will investigate Raman-based high-power amplification for spectral shift in a passive fiber. We present the observation and identification of TMI due to Raman amplification and the tracing of their origin, locating it in the passive fiber section. By comparing different fiber and seed laser parameters, we deduce respective limiting parameters.

We report on the observation and experimental characterization of backward power fluctuations with the temporal characteristics of transverse mode instability (TMI). A quasi-monolithic, counter-pumped amplifier system in 20/400 μm geometry was developed to investigate forward and backward propagating core- and cladding power as well as their temporal evolution. By experimentally observing the backward propagating core power on a photodiode, we can correlate the temporal traces to those in forward direction. The degree of correlation is found to be highly increased above the TMI threshold. Simultaneous investigations on the modal content in forward and backward direction were enabled by a free-space optical coupling between the first and second amplification stage and performed utilizing a high-speed camera (HSC). In the case of TMI mode content fluctuations are found to occur only in forward direction. Additionally, the evaluations reveal a varying core power content in both directions. The forward core power fluctuations are shown to be induced by the partial coupling of higher-order mode (HOM) content to the cladding. Meanwhile the backward core power fluctuations appear to be a consequence of the ones in forward direction induced by backreflections. Our measurements demonstrate the detection of TMI at various amplifier positions and could be helpful for scientific as well as industrial applications.

The analysis of TMI has advanced over the last decade, with added observation parameters depending on the complexity of the experimental system. Increasing levels of information have been extracted, from camera images in the beginning over modal decomposition, time trace and frequency analysis, on towards bi directional measurements at multiple system positions and separation of spectral components. We will give an overview of the evolution of TMI analysis for different model systems and discuss the applicability and the additional insight that can be gained from advanced observation methods.

We present the impact of a modulated dual tone seed on transverse mode instability in a co-pumped, monolithic, Yb-doped fiber amplifier due to an oscillating heat load distribution along the fiber.

The threshold of transverse mode instability induced by stimulated Raman scattering is examined. Therefore, key parameters for TMI in a passive fiber high-power core-pumped Raman amplifier are experimentally investigated and discussed.

The effect of transverse mode instability is a limitation for the power scaling of fiber laser systems, that can originate due to heat caused by stimulated Raman scattering. In this contribution, we experimentally investigate the threshold of transverse mode instability caused by stimulated Raman scattering in a passive fiber. Both, the Stokes seed power and the fiber length of a core-pumped Raman fiber amplifier are varied to systematically study this effect. Mode resolved measurements reveal that the threshold occurs at approximately the same Stokes output power for all tested configurations, independent of the total Raman conversion efficiency. These results increase the understanding of this type of mode instability and show which parameters are important for a further power scaling of high-power Raman fiber amplifiers.

Transverse mode instabilities are a major limitation for power scaling of fiber lasers but have so far only been observed in laser-active fibers. In this contribution we present experimental observations of transverse mode instabilities in a passive fiber. In this fiber, stimulated Raman scattering acted as heat source. To demonstrate the effect, a kW-level ytterbium-doped fiber laser was used as pump for a Raman amplifier. Transverse mode instabilities were only observed in the case with high Raman amplification. Frequency resolved stability measurements at various fiber positions as well as spectral and mode resolved measurements pin their origin to the passive fiber. This observation might help to gain further understanding of transverse mode instabilities and shows limitations of high-power Raman amplifiers.

We investigated the limitations in output power generated by a high power narrow-linewidth Raman fiber amplifier. The pump was produced by a kW-level all-fiber Yb-doped amplifier emitting at 1060 nm, whose seed linewidth could be changed. The Raman seed was a narrow-linewidth signal at 1110 nm co-propagating with the laser at 1060 nm. The main Raman conversion occurred in the passive fiber at the amplifier output. We identified cross-phase modulation (XPM) as a main reason for broadening of the Raman light by using different pump sources, which is a first limitation. An improved setup was limited at approximately 600 W of Stokes output power by a threshold-like onset of a transverse mode instability. Since the instability was not observed without a Stokes seed and the temperatures of the active fiber with and without Stokes seed are equal, this constitutes the first direct observation of transverse mode instabilities (TMI) induced by SRS in a passive fiber.

Nonlinear effects and transverse mode instabilities (TMI) limit power scaling of single-mode fiber lasers. To overcome these limitations not only the fiber design but also laser relevant properties of the actively doped material itself need to be optimized. By being able to fabricate Yb-doped fibers for high power applications in-house, we have direct access to laser relevant material parameters.We fabricated fibers using three different co-doping systems, namely Yb:Al:P, Yb:Al:F, and Yb:Al:F:Ce. Afterwards we characterized and compared their laser relevant properties. All three co-doping systems showed nearly identical background losses and absorption cross-sections. In contrast, we found that the PD losses and the factor between PD losses @633nm and the laser wavelength range (1μm) to be significantly different. The retrieved characterization results were implemented into our simulations tool in order to improve the reliability of predictions. Finally, we characterized the fibers in kW-amplifier setups according to their power scaling limits, especially the TMI threshold. This cycle of fiber fabrication, characterization, and simulation enabled us to identify the impact of individual fiber parameters on the TMI threshold. We demonstrated that the impact of PD loss leads to a reductions of the TMI threshold for Yb:Al:F co-doping system of 13% to 23% (depending on the Yb-concentration). The PD loss for the two other systems was proved to be significantly lower and was found to have no impact on the TMI threshold. We experimentally proved that your in-house Yb:Al:P and Yb:Al:F:Ce fibers performed like PD-free fibers.

Supported by both experimental and simulated results, this contribution demonstrates the heat load distribution in a co-pumped, ytterbium (Yb)-doped fiber amplifier seeded with two different wavelengths can be significantly changed depending on the seed power ratio. Longitudinal temperature measurements in a Yb-doped 10.5 m 20/400 μm fiber confirm a significant shift of the heat load maximum by 3.5 m towards the fiber output when decreasing the seed power ratio from P1030nm/P1080nm = 1.7 to 20. In single-tone operation with a seed power of P1080nm = 3.5 W, the amplifier is limited by the onset of transverse mode instabilities at a power-level of 1950 W. However, dual-tone seeding with a seed power ratio up to P1030nm/P1080nm = 10 reduces the TMI-threshold dramatically down to 1050 W. Additionally we show, that the modal instability threshold is very susceptible to 1030 nm seed noise in the frequency regime up to 10 kHz.

We observed SRS-induced transverse mode instabilities in passive fiber. This was achieved by using a high-power Ytterbium-doped fiber laser as pump for a Raman amplifier whose stability was measured at various fiber positions.

In this paper we present numerical and experimental results revealing that the mode instability threshold of highly Yb-doped, Ce/Al co-doped pedestal fibers is affected by the size of the index-increased pedestal structure surrounding the core. An alternative preparation technology for the realization of large mode area fibers with very large Al-doped silica pedestals is introduced. Three different pedestal fiber design iterations characterized by low photodarkening were manufactured and tested in counter-pumped amplifier setups. Up to 1.9 kW continuous-wave output power of near-diffraction-limited beam quality (M2 = 1.26) was achieved with an 18/200/420 µm fiber of very low NA = 0.042, limited only by the occurrence of mode instabilities.

We present and compare the performance of bidirectionally pumped Yb-doped monolithic amplifier and oscillator setups in 20/400 μm geometry tested up to signal powers of 3.5 kW and 5 kW without the occurrence of transverse mode instabilities and maintaining a single mode beam quality of M2 ~ 1.3. The scaling was primarily limited by the nonlinear effect of Stimulated Raman Scattering. This contribution contains detailed analysis of the temporal and spectral behavior of both configurations. The results show the excellent feasibility of monolithic oscillators and FBG for high power operation, even outperforming the amplifier pendant in terms of output power.

With their advantages like good beam quality, easy thermal management, high robustness and compact size, fiber lasers are one of the most promising solid state laser concepts for high power scaling with excellent beam quality. One issue of further power scaling is the reduction of nonlinear effects, especially Raman scattering, which consequently led to increased mode field areas. However, for large mode area fibers, new challenges, namely transversal mode instabilities (TMI) have to be taken into account. Beside our investigations in the power scaling of ytterbium doped fiber amplifiers up to 4.4kW output power, we present our investigations of the TMI threshold in dependence on bend diameter and absorption length of a well-known, commercial fiber. Within this scope, we used a 13m piece of the fiber and gradually reduced the bend diameter from 60cm slightly below 14cm within a pump wavelength of 976nm. Furthermore, we increased the fiber length to 30 m, presuming the bend diameter of 14 cm and all experimental conditions. However, in a next step, we detuned the pump wavelength up to 980 nm in order to increase the pump absorption length As a result, we achieved 2.9kW of single mode output at a bend diameter of 14cm. The 4.4kW result was obtained with a separately manufactured low-NA fiber, allowing for a slope efficiency of 90% with regards to the absorbed pump light and an extremely temporal stability.

We present the fabrication and properties of active fiber laser materials fabricated by a newly developed solution doping technique. The contribution focusses on Aluminum, Phosphorus, Ytterbium as well as Boron doped SiO2 for the use as fiber laser material. More specifically low doping concentration in the vicinity of the molar ratio of Al2O3:P2O5 = 1:1 will be elucidated. The effect of fabrication parameters on optical properties like refractive index, absorption and emission properties will be covered. Currently it is possible to achieve cw output powers greater than 4 kW using Al, P, Yb doped fibers fabricated with this method. Fibers additionally codoped with Boron are as well suitable for kW class applications as well.

We present and compare highly robust Yb-doped monolithic amplifier and -oscillator setups in 20/400 μm geometry achieving signal powers of 3.5 kW and 5 kW in a bidirectional pumping scheme while maintaining single mode beam quality of M2 ~ 1.3.

We present our recent results of scaling low-NA fibers to high average power by overcoming the TMI-limitations. We obtained a record output power of 4.4kW with a higher stability and smaller bending compared to our previous result.

We have evaluated a pooling approach that can reduce the number of polymerase chain reactions in a screen for selective sweeps by more than an order of magnitude. We show that the complex peak pattern that results from pooling of all samples from a given population is a faithful reflection of the composite pattern of the individual alleles, although with an under-representation of the larger alleles. Candidate loci for selective sweeps can be identified by visual inspection of the pool patterns. We have also implemented a software tool, which can find suitable microsatellite loci in the vicinity of annotated genes.

Mixing enthalpies and Gibbs energies of liquid propylene oxide oligomers with hydroxyl and methoxy end groups and of block and statistical cooligomers of ethylene oxide and propylene oxide in tetrachloromethane are obtained by microcalorimetry and vapour pressure measurements. The thermodynamic excess quantities ΔH, ΔGE and ΔSE of mixing are influenced by induced dipol interactions of the ether segments with CCl4 and by the hydrogen bond interactions of the OH end groups. An expanded theoretical treatment using the Huggins approach of quasichemical equilibrium between the contacting segment surfaces is successfully applied on the experimental results.

In a previous paper we reported the enthalpies of mixing of oligomers of ethylene oxide with different end groups in tetrachloromethane. This paper shows the applicability of the modified theoretical treatment, based on a quasichemical equilibrium of the contacting segmental surfaces originated from Huggins, to our experimental results. The experimental data of ethylene oxide oligomers with methoxy end groups can be described with the energy parameter Δeαβ, the equilibrium constant Kαβ and the surface ratio rσ assuming a chemically uniform surface of the oligomer in CCI4. Ethylene oxides with strong polar OH end groups can be treated successfully with an extended version of the theory with two additional parameters K and ΔΔe for systems containing three different kinds of chemical units.

After the sequencing of the human genome, the publication of the genome of our nearest relative, the chimpanzee (Pan troglodytes) provided groundbreaking data improving the understanding of the recent human evolution. There are about forty million changes, most of them single nucleotide substitutions, which teach us about ourselves, both in terms of similarities and differences with chimpanzees. From a medical point of view differences in incidence and severity of diseases are of special importance to pinpoint novel targets and to develop innovative therapies. This analysis focuses on the cognition that chimpanzees rarely suffer from cancer. To elucidate possible reasons for this finding, we compare differences regarding apoptosis and DNA-repair on different levels of chromosome organization, gene structure, post-transcriptional and post-translational modifications to functional changes in protein structures. The result is a complex pattern of subtle variances and a few large-scale changes.

The surprisingly low number of about 25,000 genes in the human genome [1] confirmed a fairly accurate estimate given by King and Jukes in 1969 based on population genetical arguments [2]. On the other hand, the number of different transcripts vastly exceeds gene number. This fact intensified the search for alternatively spliced genes. Recent results [1,3,4-7] suggest that more than 60% of the human genes are alternatively spliced, some of them with a myriad of different splice forms. Alternative splicing is found in all higher eukaryotic species in varying frequency. In this paper we focus on a particular form of alternative splicing, the so-called mutually exclusive exon usage (MEEU). In most known examples mutually exclusive exons are arranged in cassettes of highly similar exons suggesting that they have been derived by exon duplication [8-10]. Since classical gene-finding programs may fail to correctly predict such genes [11-16], we present a method, which is based on local similarity of exons, to detect gene candidates with mutually exclusive exon usage. We have screened the entire genome of D. melanogaster and found five new genes with MEEU in addition to eight previously described cases. Additional 1703 candidate regions of putative mutually exclusive exons were identified.

Sequence comparison by alignment is a fundamental tool of molecular biology. In this paper we show how a number of sequence comparison tasks, including the detection of unique genomic regions, can be accomplished efficiently without an alignment step. Our procedure for nucleotide sequence comparison is based on shortest unique substrings. These are substrings which occur only once within the sequence or set of sequences analysed and which cannot be further reduced in length without losing the property of uniqueness. Such substrings can be detected using generalized suffix trees. We find that the shortest unique substrings in Caenorhabditis elegans, human and mouse are no longer than 11 bp in the autosomes of these organisms. In mouse and human these unique substrings are significantly clustered in upstream regions of known genes. Moreover, the probability of finding such short unique substrings in the genomes of human or mouse by chance is extremely small. We derive an analytical expression for the null distribution of shortest unique substrings, given the GC-content of the query sequences. Furthermore, we apply our method to rapidly detect unique genomic regions in the genome of Staphylococcus aureus strain MSSA476 compared to four other staphylococcal genomes. We combine a method to rapidly search for shortest unique substrings in DNA sequences and a derivation of their null distribution. We show that unique regions in an arbitrary sample of genomes can be efficiently detected with this method. The corresponding programs shustring (SHortest Unique subSTRING) and shulen are written in C and available at http://adenine.biz.fh-weihenstephan.de/shustring/ .

Folinsäurebedürftige Bakterien zeigen nach Adaptation mit 2-Amino-6-oxy-pteridin-aldehyd-8 + p -Aminobenzoyl-l-glutaminsäure bzw. p-Aminobenzoesäure oder mit 2-Amino-6-oxy-8-d-arabo-tetraoxybutyl-pteridin + p-Aminobenzoyl-l-glutaminsäure bzw. p-Aminobenzoesäure so starkes Wachstum wie mit Folinsäure. Verantwortlich für die Adaptation ist nur der Aldehyd bzw. die Tetraoxybutylverbindung. Durch mikrobiologische Versuche wurde nachgewiesen, daß Folinsäure bzw. Pteroinsäure entsteht. Die Synthese folgt dem Massenwirkungsgesetz. Sie wird durch Sulfonamide gehemmt. Das Wachstum der aflaptierten Stämme in folinsäurehaltigem Medium wird im Gegensatz zu dem der Ausgangsstämme durch den Aldehyd oder die Tetraoxybutylverbindung nicht mehr gehemmt.

Die in der ersten Arbeit1 mitgeteilten Befunde über die Einteilung der Bakterien hinsichtlich ihrer Hemmbarkeit durch Sulfonamide und ihres Folinsäurebedürfnisses in die Gruppen A, B und C (Auhagen) wurden durch Untersuchungen weiterer Bakterien vermehrt. Vergleichende Hemmungsversuche mit dem Folinsäure-Antagonisten Aminofolinsäure ergaben, daß die Stämme der Gruppen B und C hemmbar, der Gruppe A meist nicht hemmbar sind. Die von uns nach üblicher Methodik sulfathiazol-resistent gezüchteten Stämme zeigten weder in ihrer PAB-, noch Folinsäuresynthese eine Änderung. Bei einem sulfathiazol-resistenten Coli-Stamm wurde die Folinsäuresynthese erst durch höhere Konzentrationen an Sulfathiazol als beim Originalstamm gehemmt.

Es wurde das Folinsäurebedürfnis von Milchsäurebakterien, Staphylokokken, Streptokokken, Proteus-, Coli-, Pyocyaneus-, Fluorescens-, Dysenterie-, Typhus- und Diphtherie-Stämmen untersucht. Es ergab sich, daß nicht nur ein Teil der Milchsäurebakterien, sondern auch 40% der geprüften Staphylokokkenstämme Folinsäure unbedingt zum Wachstum benötigen. Es konnte ein neuer Test zur Folinsäurebestimmung mit Staph. aureus ausgearbeitet werden, der dem bekannten Test mit Sc. faecalis R in Empfindlichkeit und Spezifität zumindest gleichwertig ist.

Die durch verschiedene Nitroverbindungen verursachte Wachs- tumshemmung wurde an einer Reihe von Bakterien und Pilzen sowie an der Keimung von Kressesamen in vitro untersucht. Einige nitrierte Derivate des Phenyl-thienyl-(2)-sulfids und des Phenyl-furyl-(2)-sulfids wurden dargestellt.

Nachdem in der vorhergehenden Arbeit bei 25 Staphylokokkenstämmen (St. aureus, St. albus, Micrococcus aureus) in einem halbsynthetischen Medium das Folinsäure-Bedürfnis festgestellt worden war, wurde die Wachstumswirkung von Lactoflavin, Pantothensäure, Xanthin und einigen weiteren Stoffen, teilweise quantitativ, bestimmt. Pantothensäure bzw. Lactoflavin waren für den vierten Teil der untersuchten Stämme unbedingt zum Wachstum erforderlich. Bei hoher Citratkonzentration muß auch die Konzentration von Mn¨ und Mg¨ hoch gewählt werden. In verdautem Casein kommen nicht unbedingt notwendige, aber meist stark zuwachsfördernde Substanzen vor, die in säurehydrolysiertem Casein nicht mehr enthalten sind. Darüber hinaus scheinen noch weitere Substanzen für eine wirkliche optimale Ernährung von Staphylokokken notwendig zu sein.

S-Methyl-, S-Athyl- und S-Phenyl-pantethein sind aus Natriumpantothenat und den entsprechenden Thioathanolamin-Derivaten auf dem Weg uber das gemischte Anhydrid der Pantothensaure mit Athylkohlensaure synthetisiert worden. Alle besitzen in kleiner Konzentration geringe Wuchsstoffwirkung bei Lactobacillus helveticus V 80, die bei hoherer Konzentration in eine durch Lactobacillus-bulgaricus-Faktor (LBF, Pantethein) und Pantothensaure aufhebbare Hemmung ubergeht. Wahrend diese bei den Alkyl-Derivaten nach einigen Tagen uberwunden wird, ist die durch das Phenyl-Derivat erzeugte stabil. Es handelt sich dabei um einen echten kompetitiven Vorgang, wobei 30–70 Moll. Hemmstoff je Mol. Wuchsstoff das Bakterienwachstum zu 50% unterbinden. Das auf analogem Weg synthetisierte Pantothensaureamid besitzt keine Hemmwirkung und vermag den Wuchsstoff bei 3 verschiedenen Mikroorganismen, allerdings in hoherer Konzentration, vollstandig zu ersetzen.