Dynamic control of the operation regimes of a mode-locked fiber laser based on intracavity polarizing fibers: experimental and theoretical validation

An intracavity polarizing fiber is proposed to control the emission regime of a passively mode-locked fiber laser. Stable operation in self-starting high and low dispersion soliton mode-locking and 100 GHz multiwavelength regimes is demonstrated through numerical simulations and experimental validation. Mode-locking stability is ensured by a saturable absorber in the ring cavity. The effective selection of operation regime is dynamically carried out by controlling the intracavity polarization state.     

Extensions and Pullbacks in QB-Rings

We prove a new extension result for QB-rings that allows us to examine extensions of rings where the ideal is purely infinite and simple. We then use this result to explore various constructions that provide new examples of QB-rings. More concretely, we show that a surjective pullback of two QB-rings is usually again a QB-ring. Specializing to the case of an extension of a semi-prime ideal I of a unital ring R, the pullback setting leads naturally to the study of rings whose multiplier rings are QB-rings. For a wide class of regular rings, we give necessary and sufficient conditions for their multiplier rings to be QB-rings. Our analysis is based on the study of extensions and the use of nonstable K-theoretical techniques. Presented by S. MontgomeryMathematics Subject Classification (2000) 16D30.Gert K. Pedersen: Deceased 15 March 2004.Partially supported by the DGI and European Regional Development Fund, jointly, through Project BFM 2002-01390, the Comissionat per Universitats i Recerca de la Generalitat de Catalunya and the Danish Research Council.     

Subvarieties of BL-algebras generated by single-component chains

 In this paper we study and equationally characterize the subvarieties of BL, the variety of BL-algebras, which are generated by families of single-component BL-chains, i.e. MV-chains, Product-chain or G?del-chains. Moreover, it is proved that they form a segment of the lattice of subvarieties of BL which is bounded by the Boolean variety and the variety generated by all single-component chains, called ŁΠG. Received: 8 January 2001 / Published online: 12 July 2002     

Highly efficient aldol additions of DHA and DHAP to N-Cbz-amino aldehydes catalyzed by L-rhamnulose-1-phosphate and L-fuculose-1-phosphate aldolases in aqueous borate buffer

Aldol addition reactions of dihydroxyacetone (DHA) to N-Cbz-amino aldehydes catalyzed by L-rhamnulose-1-phosphate aldolase (RhuA) in the presence of borate buffer are reported. High yields of aldol adduct (e.g. 70-90%) were achieved with excellent (>98?:?2 syn/anti) stereoselectivity for most S or R configured acceptors, which compares favorably to the reactions performed with DHAP. The stereochemical outcome was different and depended on the N-Cbz-amino aldehyde enantiomer: the S acceptors gave the syn (3R,4S) aldol adduct whereas the R ones gave the anti (3R,4R) diastereomer. Moreover, the tactical use of Cbz protecting group allows simple and efficient elimination of borate and excess of DHA by reverse phase column chromatography or even by simple extraction. This, in addition to the use of unphosphorylated donor nucleophile, makes a useful and expedient methodology for the synthesis of structurally diverse iminocyclitols. The performance of aldol additions of dihydroxyacetone phosphate (DHAP) to N-Cbz-amino aldehydes using RhuA and L-fuculose-1-phosphate aldolase (FucA) catalyst in borate buffer was also evaluated. For FucA catalysts, including FucA F131A, the initial velocity of the aldol addition reactions using DHAP were between 2 and 10 times faster and the yields between 1.5 and 4 times higher than those in triethanolamine buffer. In this case, the retroaldol velocities measured for some aldol adducts were lower than those without borate buffer indicating some trapping effect that could explain the improvement of yields.     

Relationship between Intrinsic Viscosity and Molecular Weight for Fractionated Cellulose

A relationship between intrinsic viscosity and molecular weight for fractionated cellulose is established. The exponent a of the Mark-Houwink equation is 0.80. The unperturbed dimensions of cellulose are discussed on the basis of the Stockmayer-Fixman and Kurata-Stockmayer viscometric theories, and it is determined that cellulose is not a very rigid macromolecule in nature. This conclusion disagrees with the results obtained by statistical mechanics procedures in which the restriction of β-glucose residues to the C1 conformation was kept. The steric hindrance factor, σ is about 2, and it seems to be independent of the substituents in cellulose if it is evaluated in both cellulose and cellulose derivatives from viscometric data. From statistical mechanics data reported in the literature, σ is estimated as 4.4 for cellulose and cellulose derivatives, such as cellulose nitrate, and this means that the steric hindrances of the substituents have little influence on the rigidity of cellulose derivatives. This view disagrees with the results obtained from the non-Newtonian behavior of cellulose and cellulose nitrate in dilute solutions. By applying the Noda-Hearst theory, the conclusion is reached that cellulose nitrate is a more extended macromolecule than cellulose.     

Local aromaticity of the lowest-lying singlet States of [N]acenes (N = 6-9)

The local aromaticities of the six-membered rings in the two lowest-lying singlet states of [n]acenes (n = 6-9) have been assessed by means of three probes of local aromaticity based on structural, magnetic, and electron delocalization properties. Important differences between the local aromaticities of the closed-shell and diradical singlet electronic states are found. Thus, while the inner rings have the largest aromatic character in the closed-shell singlet states, the outer rings become the most aromatic for the diradical singlet states.     

Alkali treatment of birch kraft pulp to enhance its TEMPO catalyzed oxidation with hypochlorite

Alkali treatment was used to increase the reactivity of birch kraft pulp prior to its 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) catalyzed oxidation with hypochlorite, which is a process commonly applied to prepare nanofibrillated cellulose. On contrary to the traditional use of NaBr as a cocatalyst, TEMPO was activated with HOCl prior to the oxidation. Commonly, the lack of bromide increases the oxidation time and impairs the formation of carboxylic groups. However, the reaction time of the bromide-free TEMPO catalyzed oxidation could be shortened from 2.5 to 0.5 h when the pulp was treated with 1 M NaOH prior to the oxidation (2.4 mmol NaOCl/g pulp). The beneficial effect was obtained even if the alkali treatment was executed at room temperature and only for few minutes. Moreover, the alkali pretreatment enabled selective production of a pulp with carboxylate content as high as 1.6 mmol/g with NaOCl dosage of 4.4 mmol/g. The changes in the cellulosic raw material during the alkali treatment were assessed by water retention value and carbohydrate analysis.     

Unexpected nonadditivity effects in anion-π complexes

Several complexes of fluorine-substituted ethyne, ethene, butadiene, benzene, and [n]radialenes (n = 3-5) with two anions have been optimized at the RI-MP2/aug-cc-pVTZ level of theory. The additivity of the anion-π interaction was studied depending on the number of double bonds and fluorine atoms. Interesting nonadditivity effects were observed in the aromatic and antiaromatic complexes, which were analyzed by partitioning the total interaction energy into individual components, using Bader's theory of "atoms in molecules" and changes in the aromatic character of the ring upon complexation.