Bo Vest Pedersen

The design of erbium-doped fiber amplifiers

An accurate model for the erbium-doped fiber amplifier is presented. The model is used to design the index profile of the doped fiber, optimizing with regard to efficiency for inline- and preamplifiers as well as for power booster amplifiers. The predicted pump efficiencies (maximum gain to pump power ratios) are in agreement with experimental results presented in the literature. The choice of codopant is shown to be very significant for the pump efficiency when pumping in the 0.98 mu m. The pump efficiency in the 0.98- mu m pump band is shown to be twice the pump efficiency in the 1.48- mu m pump band. >

Varying degrees of Apis mellifera ligustica introgression in protected populations of the black honeybee, Apis mellifera mellifera, in northwest Europe.

The natural distribution of honeybee subspecies in Europe has been significantly affected by human activities during the last century. Non‐native subspecies of honeybees have been introduced and propagated, so that native black honeybee (Apis mellifera mellifera) populations lost their identity by gene‐flow or went extinct. After previous studies investigated the remaining gene‐pools of native honeybees in France and Spain, we here assess the genetic composition of eight northwest European populations of the black honeybee, using both mitochondrial (restriction fragment length polymorphisms of the intergenic transfer RNAleu‐COII region) and nuclear (11 microsatellite loci) markers. Both data sets show that A. m. mellifera populations still exist in Norway, Sweden, Denmark, England, Scotland and Ireland, but that they are threatened by gene flow from commercial honeybees. Both Bayesian admixture analysis of the microsatellite data and DraI‐RFLP (restriction fragment length polymorphism) analysis of the intergenic region indicated that gene‐flow had hardly occurred in some populations, whereas almost 10% introgression was observed in other populations. The most introgressed population was found on the Danish Island of Læsø, which is the last remaining native Danish population of A. m. mellifera and the only one of the eight investigated populations that is protected by law. We discuss how individual admixture analysis can be used to monitor the restoration of honeybee populations that suffer from unwanted hybridization with non‐native subspecies.

Diversity of Plasmodium falciparum populations and complexity of infections in relation to transmission intensity and host age: a study from the Usambara Mountains, Tanzania

Diversity and complexity of infections with Plasmodium falciparum were described from cross-sectional surveys in November-December 1996 in 6 villages in the Usambara Mountains, Tanzania, where transmission ranged markedly from 0.03 to 91 infective bites per individual per year. Forty-eight samples, stratified for age and parasite densities, were examined from each village (n = 288). Genotyping was performed by a nested PCR method using primers specific for allele families of genes for the merozoite surface protein 1 (msp-1) and merozoite surface protein 2 (msp-2). A high degree of genetic diversity was found within each village but there were no differences found among the 6 villages. Poisson regressions showed significant effects of host age, village and interaction between host age and village on the complexity of infection. There was a positive, non-linear relationship between complexity of infection and transmission intensity with a maximal number of genotypes found per individual even at high transmission intensities. Furthermore there was a significantly lower complexity found in adults (> 15 years) as compared to children (< 15 years) in the lowland village. This difference was not found as transmission intensity decreased. By comparing data from the same geographical area, using the same methods, and taking into account confounding factors, the present study provides evidence for an effect of both age and transmission intensity on complexity of infection with P. falciparum.

Detailed theoretical and experimental investigation of high-gain erbium-doped fiber amplifier

A full-scale numerical model for the erbium-doped fiber amplifier has been developed that incorporates realistic index and erbium-concentration profiles as well as the spectral distribution of amplified spontaneous emission (ASE). The high accuracy of the model is demonstrated by comparison with a comprehensive set of data, including gain, ASE, and pump power, obtained for a well-characterized Er-Al-doped fiber. An absorption-to-emission cross-section ratio of 1.0 was measured at the gain peak. Pumping at 654 nm, the excited state absorption was observed to be insignificant. A high gain of 39.6 dB was achieved in the experiment.<<ETX>>

Reproduction and hybrid load in all-hybrid populations of Rana esculenta water frogs in denmark

Abstract All‐hybrid populations of the water frog, Rana esculenta, are exceptional in consisting of independently and to some extent sexually reproducing interspecific hybrids. In most of its range R. esculenta reproduces hemiclonally with one of the parental species, R. lessonae or R. ridibunda, but viable populations of diploid and triploid hybrids, in which no individuals of the parental species have been found, exist in the northern part of the range. We test the hypothesis that nonhybrids arise every year in these all‐hybrid populations, but die during larval development. Microsatellite markers were used to determine the genotypes of adults and abnormal and healthy offspring in three allhybrid populations of R. esculenta in Denmark. Of all eggs and larvae, 63% developed abnormally or died, with some being nonhybrid (genomes matching one of the parental species), many being aneuploid (with noninteger chromosome sets), a few being tetraploid, and many eggs possibly being unfertilized. The 37% surviving and apparently healthy froglets were all diploid or triploid hybrids. In all three populations, gametogenesis matched the pattern previously described for all‐hybrid R. esculenta populations in which most triploid adults have two R. lessonae genomes. This pattern was surprising for the one population in which triploid adults had two R. ridibunda genomes, because here it leads to a deficiency of gametes with an R. lessonae genome and should compromise the stability of this population. We conclude that faulty gametogenesis and mating between frogs with incompatible gametes induce a significant hybrid load in all‐hybrid populations of R. esculenta, and we discuss compensating advantages and potential evolutionary trajectories to reduce this hybrid load.

Differential susceptibility across honey bee colonies in larval chalkbrood resistance

Chalkbrood susceptibility of in vitro reared honey bee larvae was investigated. Larvae were grafted from 3–4 colonies headed by pure mated queens of Apis mellifera carnica, A. m. ligustica and A. m. mellifera, respectively. Three day old larvae were fed with different dosages of Ascophaera apis spores and a clear dose-response relationship was shown. Over the whole experiment LD50 estimates ranged from 55 to 905 spores. The response differed significantly (up to a factor ten) between colonies of the same subspecies. The mean time to death decreased with increased dose, with more larvae dying faster after eating more fungal spores. The A. m. ligustica larvae used in this study were less susceptible to A. apis than A. m. mellifera and A. m. carnica larvae. However due to the limited number of colonies included and the high variation shown we cannot predict that any A. m. ligustica colony is better adapted to cope with A. apis than colonies of A. m. carnica and A. m. mellifera.ZusammenfassungLarven aus verschiedenen Honigbienenvölkern wurden mit Kalkbrut-Sporen (Ascosphaera apis) infiziert. Die Larven stammten von Königinnen, die sich an isolierten Paarungsplätzen (z.B. Inseln) natürlich gepaart hatten. Wir verwendeten Königinnen von Apis mellifera carnica, A. m. ligustica und A. m. mellifera. Dadurch konnten wir sowohl zwischen den drei Gruppen als auch innerhalb der Gruppen die Unterschiede in der Anfälligkeit gegen eine Kalkbrutinfektion testen. Dreitägige Larven wurden dazu mit unterschiedlichen Dosen von A. apis-Sporen gefüttert, während die Kontrollgruppe nicht kontaminiertes Futter erhielt. Alle Larven wurden täglich unter dem Mikroskop beobachtet und nach den Kriterien „lebend“, „tot“ oder „durch Pilzbefall getötet“ (mit Pilzhyphen am Körper) eingeteilt. Es gab einen klaren Zusammenhang zwischen der gefütterten Sporen-menge und der Anzahl infizierter Larven. Es waren zwischen minimal 55 und maximal 905 Sporen notwendig, um 50 % der behandelten Larven zu töten (Tab. I). Die Unterschiede in der Anfälligkeit betragen also mehr als den Faktor 10 und erwartungsgemäß nahm die durchschnittliche Überlebensdauer mit zunehmender Sporendosis ab (Abb. 1). Bei der höchsten verabreichten Sporendosis von 10000 Sporen gab es keine Unterschiede in der Überlebensdauer zwischen bzw. innerhalb der Gruppen. Wurden die Larven aber nur mit 1.000 Sporen gefüttert, traten zwischen den Gruppen signifikante Unterschiede auf. Insgesamt weisen die LD50—Werte und die durchschnittliche Überlebensdauer darauf hin, dass die in dieser Untersuchung verwendeten A. m. ligustica-Larven weniger anfällig gegen A. apis-Infektionen waren als Larven von A. m. mellifera und A. m. carnica. Allerdings werden aufgrund der begrenzten Anzahl an Testvölkern und der hohen Variation innerhalb der Unterarten weitere Daten benötigt, um zuverlässige Vorhersagen zur spezifischen Anfälligkeit bzw. Toleranz verschiedener Apis-Unterarten gegenüber einer Kalkbrutinfektion zu machen. Die großen Unterschiede in der Kalkbrut-Anfälligkeit zwischen den verschiedenen Völkern zeigen aber, dass es durchaus ein Potential für die Zucht auf Kalkbrut-Resistenz gibt.

Power requirements for erbium-doped fiber amplifiers pumped in the 800, 980, and 1480 nm bands

The authors examine relative merits of exciting Er/sup 3+/ amplifiers at the three wavelengths for which high-power laser diodes are available at 800, 980, and 1480 nm. Model calculations are confirmed by a detailed experimental comparison of the power requirements for pumping in the 800-nm band and at 980 nm. To obtain comparable performance with respect to gain and noise figure, 7-8 dB more power is required when pumping in the 800-nm-band.<<ETX>>

Optimization of Pr/sup 3+/:ZBLAN fiber amplifiers

Experimental parameters have been measured and used in a quantitative model of Pr/sup 3+/-doped fluorozirconate fiber amplifiers. The optimum cutoff wavelength was determined to be 800 nm and the gain for 400 mW of pump was found to increase from 12 to 34 dB if the NA was increased from 0.15 to 0.25. Lengthening the metastable state lifetime from 110 to 300 mu s would significantly improve amplifier performance while concentration quenching can appreciably degrade it.<<ETX>>

Detailed design analysis of erbium-doped fiber amplifiers

When pumping the erbium-doped fiber amplifier at 0.98 and 1.48 mu m, the optimum cutoff wavelength for step profiles with arbitrary numerical aperture is shown to be 0.80 and 0.90 mu m, respectively. The use of a confined erbium profile can improve the gain coefficient up to 45%. The index raising co-dopant is shown to be very significant for the gain coefficient when pumping at 0.98 mu m.<<ETX>>

Detailed comparison of two approximate methods for the solution of the scalar wave equation for a rectangular optical waveguide

Two approximate methods for the determination of the fundamental mode of an optical waveguide with rectangular core cross section and step refractive-index profiles are presented and analyzed thoroughly. Both methods are based on Galerkins method. The first method uses Hermite-Gauss basis functions and the second uses the guided and nonguided slab waveguide solutions as basis functions. The results are compared with results from an accurate circular harmonic analysis. Both methods provide values of the normalized propagation constant with errors less than 0.1% for practical rectangular single-mode waveguides. The slab waveguide method is the fastest, and even when only one slab waveguide mode is used the propagation constant for the fundamental mode can be calculated with an error of less than 1%. The slab waveguide method also gives very accurate results for the propagation constant for higher order modes. >