Length distributed measurement of temperature effects in Yb-doped fibers during pumping

We demonstrate a distributed measurement technique to observe temperature changes along pumped Yb-doped fibers. This technique is based on an array of fiber Bragg gratings acting as a temperature sensor line. The Bragg gratings are inscribed directly into the Yb-doped fiber core using high-intensity ultrashort laser pulses and an interferometric setup. We studied the temperature evolution in differently co-doped Yb fibers during optical pumping and identified different effects contributing to the observed temperature increase. We found that preloading of fibers with hydrogen supports the formation of Yb2+ during UV irradiation and has a large impact on fiber temperature during pumping. The proposed technique can be applied to investigate the homogeneity of pump absorption in active fibers and to support spatially resolved photodarkening measurements

Interregionalism's impact on regional integration in developing countries: the case of Mercosur

This article examines the impact of interregionalism on deepening regional integration processes in non-European Union (EU) regions, specifically the Southern Common Market (Mercosur). It considers whether ‘capacity-building’ functions of interregionalism are present in EU–Mercosur relations. It argues that although negotiations for an association agreement might have helped Mercosur survive periods of severe crisis in the past, the terms of the agreement under negotiation were not sufficiently attractive to encourage deeper integration in Mercosur. Moreover, interregionalism cannot be expected to compensate for low institutionalization, nor substitute for weak political willingness to deepen integration. Ultimately, Mercosur alone can decide how far it wants to take its regional integration

Measurement of photodarkening in Yb-doped aluminosilicate fibres at elevated temperature

Photodarkening behaviour in Yb-doped aluminosilicate fibres at elevated temperatures is reported. The fibre was core-pumped at 977 nm and the transmitted powers were monitored at 633 (probe) and 977 nm (pump) simultaneously with ~47% of population inversion of Yb3+ ions. A saturated photodarkening loss was found, which is inversely proportional to temperature and, at ~573 K, the loss was negligible at the pump wavelength. From the decay curves at different temperatures, it was found that the photodarkening involves second-order kinetics to form colour centres

Species-specific responses of wood growth to flooding and climate in floodplain forests in Central Germany

With ongoing climate change, episodes of severe flooding are predicted to become more frequent despite a general trend towards increasing summer drought. We investigated how wood growth of adult trees of two species characteristic of floodplain forests in Central Germany (Fraxinus excelsior L., Quercus robur L.) and two less-typical species (Acer pseudoplatanus L., Carpinus betulus L.) responded to both maximum stream water level and climate, with a special focus on the effects of the extraordinary flood of the Saale River in June 2013 and the extreme drought in summer 1976. Tree-ring widths were measured on wood cores, and standardized ARSTAN tree-ring chronologies were produced. Using variance partitioning as well as linear mixed-effects models, we compared the effects of monthly values for maximum water level, temperature and precipitation sum on tree-ring width. Further, we calculated resistance, resilience and recovery of the tree species to the extreme events of flooding in 2013 and drought in 1976. Wood growth of all species studied, and particularly of F. excelsior, responded positively to the extraordinary flooding in June 2013. However, in the best models for the characteristic floodplain forest species (F. excelsior and Q. robur), mainly precipitation (F. excelsior) or a combination of precipitation and wood growth of the previous year (Q. robur) acted as drivers of wood growth of the current year. In contrast, growth of the less habitat-specific species (A. pseudoplatanus) mainly showed a significant response to the combination of temperature and wood growth of the previous year. C. betulus was the only species studied that benefited from the extreme drought in 1976. However, two years afterwards, only the wood growth of A. pseudoplatanus was still reduced, while F. excelsior and Q. robur fully recovered. In comparison to other regions in Central Europe, the moderate flood regime of the Saale River seems to have the potential to mitigate effects of summer drought in this region, which is one of the driest in Germany. Thus, increased flooding frequency might, to some degree, reduce drought effects brought about by climate change as well

High-energy soliton pulse generation with a passively mode-locked Er/Yb-doped multifilament-core large-mode-area fiber laser

The generation of high-energy pulses from rare-earth-doped fiber lasers has been under intense investigation for the past few years. The pulse energy achievable in mode-locked fiber laser systems is limited by nonlinear effects occurring during propagation through the fiber, mainly Kerr-nonlinearity, avoid self-consistent pulse evolution inside a fiber laser resonator and hinder the pursuit of higher pulse energies from mode-locked fiber lasers. Hence, the employment of low-nonlinearity large-mode-area fibers (LMA) opens the possibility of energy scaling. This has been demonstrated recently by a femtosecond ytterbium-doped fiber laser operating in the anomalous net-cavity dispersion regime emitting a pulse energy as high as 16 nJ. More recently, single-mode operation for high energy amplification in the 1.5 mum wavelength region has been demonstrated based on a multifilament core fiber combining large area and high doping concentration

MIKROPHOT - LASERDIRECT. Hochleistungsstabile Laserfasern durch neue modale Ausbreitungskonzepte Abschlussbericht

Available from TIB Hannover: F04B2029 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung und Forschung (BMBF), Bonn (Germany)DEGerman