Photodarkening mitigation in Yb-doped fiber lasers by 405 nm irradiation

We investigate the impact of 405 nm radiation on photodarkening evolution in Yb-doped fiber. Simultaneous photodarkening and photobleaching effects induced by 976 nm and 405 nm radiations respectively were investigated in a 1070 nm laser

Radiation hardening techniques for rare-earth based optical fibers and amplifiers

Er/Yb doped fibers and amplifiers have been shown to be very radiation sensitive, limiting their integration in space. We present an approach including successive hardening techniques to enhance their radiation tolerance. The efficiency of our approach is demonstrated by comparing the radiation responses of optical amplifiers made with same lengths of different rare-earth doped fibers and exposed to gamma-rays. Previous studies indicated that such amplifiers suffered significant degradation for doses exceeding 10 krad. Applying our techniques significantly enhances the amplifier radiation resistance, resulting in a very limited degradation up to 50 krad. Our optimization techniques concern the fiber composition, some possible pre-treatments and the interest of simulation tools used to harden by design the amplifiers. We showed that adding cerium inside the fiber phosphosilicate-based core strongly decreases the fiber radiation sensitivity compared to the standard fiber. For both fibers, a pre-treatment with hydrogen permits to enhance again the fiber resistance. Furthermore, simulations tools can also be used to improve the tolerance of the fiber amplifier by helping identifying the best amplifier configuration for operation in the radiative environment

Ставлення істориків українських земель другої половини XIX — початку XX ст. до релігії та церкви

У статті показані головні підходи істориків того періоду до важливих суспільних явищ, підкреслені відмінності підходи позитивістів та прихильників інших наукових парадигм.The article shows the main approaches of historians of that period to these important social phenomena, stresses the differences between the approaches of the positivists and supporters of other scientific paradigms

Subseasonal coastal-trapped wave propagations in the Southeastern Pacific and Atlantic Oceans : 2. Wave characteristics and connection with the equatorial variability

The objective of this study is to compare the characteristics of the oceanic teleconnection with the linear equatorial dynamics of two upwelling systems along the southwestern South American and African continents at subseasonal time scales (<120 days). Altimetric data analysis shows that the coastal variability remains coherent with the equatorial signal until 27 degrees S in the southeastern Pacific (SEP), while in the southeastern Atlantic (SEA) it fades out south of 12 degrees S. To explain this striking difference, our methodology is based on the experimentation with twin regional model configurations of the SEP and SEA Oceans. The estimation of free Coastal-Trapped Waves (CTWs) modal structures and associated contribution to coastal variability allows inferring and comparing the characteristics of each CTW mode in the two systems; namely, their forcings, amplitude, dissipation rate, and scattering. Results show that the Pacific subseasonal equatorial forcing is only 20% larger than in the Atlantic, but important differences in the relative contribution of each baroclinic mode are reported. The first baroclinic mode dominates the eastern equatorial Pacific variability, while in the eastern equatorial Atlantic, the second mode is the most energetic. This leads to a drastic increase in the dissipation and scattering of the remotely forced CTW in the SEA sector, compared to the coastal SEP. Concomitantly, south of 15 degrees S, the subseasonal coastal wind stress forcing is substantially more energetic in the SEA and participates in breaking the link between the equatorial forcing and the coastal variability. Our results are consistent with the solutions of a simple multimode CTW model. Plain Language Summary The Humboldt and the Benguela upwelling systems are connected to the equatorial variability. Part of the incoming eastward equatorial wave energy is transmitted southward along the South American and African coasts as Coastal-Trapped Waves, where they imprint on the ecosystem variability. At subseasonal time scales (<120 days), altimetry reveals that the coastal variability remains coherent with the equatorial signal until 27 degrees S in the southeastern Pacific, while in the Atlantic counterpart it fades out south of 12 degrees S. To explain this striking difference, we compare the characteristics of coastal waves between the two systems: their forcing at the equator, their dissipation and scattering along their propagation, and the energization by the coastal wind stress. We use a variety of ocean models of different complexity ranging from regional general circulation models to simple linear coastal models. Results show that the difference between the two systems regarding the connection with the equatorial variability can be attributed to the distinct characteristics of their equatorial forcing. The latter favors fast and weakly dissipative coastal wave in the Humboldt. Off southwestern Africa, the equatorially-forced coastal-trapped waves dissipate at approximate to 13 degrees S and the subseasonal coastal wind stress forcing which is energetic south of 15 degrees S, participates in breaking the link between the equatorial and coastal variabilities

Tidal cycle control of biogeochemical and ecological properties of a macrotidal ecosystem

In some regions, tidal energy can be a key factor in the generation of variability in physical and biogeochemical properties throughout the water column. We use a numerical model resolving tidal cycles and simulating diversity in phytoplankton to assess the impact of tidal mixing on vertical stability and phytoplankton community (total biomass and diversity) in a macrotidal sea (Iroise Sea, France). Two different time scales have been considered: semidiurnal and spring/neap tidal cycles. Our results show that the latter is the one primarily influencing the phytoplankton growth conditions by modifying the vertical stratification. During spring tide, the growth is rather light limited, whereas neap tide conditions lead to vertical stabilization and better light conditions in the shallow surface layer. The transition from high to low tidal mixing conditions is thus associated with a total phytoplankton biomass increase (caused by the rapid development of fast-growing diatoms) and reduced phytoplankton diversity

Longitudinal zonation of macroinvertebrates in an Ecuadorian glacier-fed stream : do tropical glacial systems fit the temperate model ?

1. The ecology of glacier-fed streams at temperate latitudes has been intensely studied in recent years, leading to the development of a well-validated conceptual model on the longitudinal distribution of macroinvertebrate communities downstream of the glacier margin (Freshwater Biology, 2001a; 46, 1833). However, to our knowledge, the ecology of tropical glacier-fed streams has not yet been studied. 2. We sampled benthic macroinvertebrates and measured environmental variables at nine sites between 4730 and 4225 m altitude along a 4.3 km stretch of a glacier-fed stream 40 km south of the equator in the Ecuadorian Andes. Our goal was to study the longitudinal distribution of the fauna in relation to environmental factors and to compare this with the conceptual model based on temperate-arctic glacier-fed streams. 3. Total density of invertebrates differed considerably at the two highest altitude sites; 4600 m-2 at a pro-glacial lake outlet and only 4 m-2 at a site originating directly from the glacier snout. Otherwise, there was a downstream decrease in density to about 825 m-2 at the three lowest sites. Taxon richness increased with distance from the glacier, very similar to the pattern predicted. A total of 28 taxa were collected; two at the glacier snout, seven at the nearby pro-glacial lake outlet, 13 at site 2 (< 400 m from the glacier) and 20 at the lowest sites. 4. The numerical percentage of Chironomidae (Diptera) decreased downstream from 100 to 44%. The subfamily Podonominae was numerous at the highest sites but became much less important further downstream. The Orthocladiinae were important both in numbers and species at all sites, while Diamesinae were numerous only in the middle of the reach studied and were completely absent from the upper three sites. The limited importance of the Diamesinae, and its replacement by Podonominae, is different from the pattern typically observed in north-temperate glacier-fed streams. This could be because of the fact that the genus Diamesa is missing from the Neotropics. 5. Stream temperature and channel stability explained most of the variability in faunal composition and richness, supporting the model. Stability increased systematically downstream while temperature did not. Surprisingly, no classical kryal zone (T-max < 4 degrees C) was found, as even the site closest to the glacier snout (50 m) had a T-max of 15 degrees C and no site had T-max < 8 degrees C. We propose that this might be a general feature of equatorial glacial streams

Influence of Ce3+ Codoping on the Photoluminescence Excitation Channels of Phosphosilicate Yb/Er Doped Glasses

International audienceThe luminescence properties of the Yb/Er-doped phosphosilicate preforms used for the design of active optical fibers were investigated under a tunable laser excitation from ultraviolet to infrared domain. We demonstrated that codoping the glass matrix with Ce3+ ions strongly influences the infrared emission associated with Er3+ ions, it enhances the energy transfer from Yb3+ to Er3+ ions, and it provides an additional ultraviolet excitation channel for the emission of both Yb3+ and Er3+ ions. The excitation/emission pathways are discussed on the basis of models proposed in literature for other systems