Exact coupled-mode theories for multilayer interference coatingswith arbitrary strong index modulations

The analysis of multilayered interference coatings by coupled-mode theory is considered to be an approximation useful for small index, gain, and/or loss modulations. In this paper, we show that an exact analysis of a multilayer coating with coupled-mode theory is possible by redefining the coupling and detuning coefficients. We derive the correct coefficients for the case of a Bragg mirror consisting of layers with arbitrary high and low refractive indexes. A detailed comparison with coupled-mode theories using the standard coupling and detuning coefficients is presented     

Evolution of Volatile Flavor Compounds During Roasting of Nut Seeds by Thermogravimetry Coupled to Fast-Cycling Optical Heating Gas Chromatography-Mass Spectrometry with Electron and Photoionization

Online multidimensional evolved gas analysis was conducted during the roasting of nuts using fast-cycling optical heating gas chromatography (OHGC) coupled to mass spectrometry with electron ionization and soft single photon ionization (OHGC-EI/SPI-MS). SPI is a semi-selective soft ionization method for organic compounds that produces mainly molecular ions, whereas EI is a hard ionization method that results in fragmentation. Ionization was either done exclusively by one of these methods or both were used alternately. Roasting of the nuts (almonds, Brazil nuts, cashews, peanuts, hazelnuts, pecans, pine nuts, and walnuts) was simulated in a thermal analysis (TA) device at a roasting temperature of 170 °C. The TA device was directly coupled to the OHGC-EI/SPI-MS for quasi-real-time analysis. Multidimensional analysis was possible with a temporal resolution of 1 min. Good chromatographic separation, constant sampling repetition rates, and constant retention times (RTs) were obtained. Peak assignment was performed using the molecular mass information obtained from SPI-MS, the characteristic fragmentation patterns from EI-MS, and the OHGC RTs. The gases that evolved during roasting of each type of nut were monitored online using the TA-OHGC-MS setup. Aldehydes, furans, and pyrazines were detected as flavor compounds. Changes in the compositions of the evolved gases during the roasting process were evaluated. The TA-OHGC-MS method could separate isobaric and isomeric compounds.     

The effect of fruit extracts with polyphenol oxidase (PPO) activity on the in vitro accessibility of iron in high-tannin sorghum

Dephytinized high-tannin sorghum flour was incubated with crude extracts from pear, banana or avocado, respectively, followed by investigation of the effects on the phenolic content and on in vitro accessible iron. All fruits contained polyphenol oxidase (PPO) activity and incubation resulted in significant reduction of phenolic compounds. Incubation with avocado extract resulted in the lowest levels of phenolic compounds, as well as the highest amount of in vitro accessible iron. Peroxidase activity and some organic acids in the fruit extracts might also have contributed to the positive effect on iron accessibility. Nevertheless, incubation of the sorghum flour with the fruit extracts under conditions enabling the PPO to oxidize phenolic compounds, resulted in the highest accessibility of iron. The results from this study suggest that the PPO activity in simple fruit extracts can be utilized to increase the accessibility of iron in dephytinized polyphenol-containing cereal foods.     

Theory of double-chirped mirrors

A theory of double-chirped mirrors (DCMs) for dispersion compensation in ultrashort pulse laser sources is presented. We describe the multilayer interference coating by exact coupled-mode equations. They show that the analysis and synthesis of a coating with a slowly varying chirp in the layer thicknesses can be mapped onto a weakly inhomogeneous transmission line problem. Solutions of the transmission line equations are given using the WKB-method. Analytic expressions for reflectivity and group delay are derived. The solutions show that the main problem in chirped mirror design is the avoidance of spurious reflections, that lead to Gires-Tournois-like interference effects responsible for the oscillations in the group delay. These oscillations are due to an impedance matching problem of the equivalent transmission line. The impedance matching can be achieved by simultaneously chirping the strength of the coupling coefficient and the Bragg wavenumber of the mirror. An adiabatic increase in the coupling coefficient removes the typical oscillations in the group delay and results in broad-band mirrors with a controlled dispersion. Finally, the mirror is matched to air with a broadband antireflection coating. We discuss a complete design of a laser mirror with a reflectivity larger than 99.8% and a controlled dispersion over 300-nm bandwidth. Using such mirrors in a Ti:sapphire laser, we have demonstrated ≈30-fs pulses, tunable over 300 nm, as well as 8-fs pulses from the same setup. A different design resulted in 6.5-fs pulses     

Semiconductor saturable absorber mirrors (SESAM's) for femtosecondto nanosecond pulse generation in solid-state lasers

Intracavity semiconductor saturable absorber mirrors (SESAM's) offer unique and exciting possibilities for passively pulsed solid-state laser systems, extending from Q-switched pulses in the nanosecond and picosecond regime to mode-locked pulses from 10's of picoseconds to sub-10 fs. This paper reviews the design requirements of SESAM's for stable pulse generation in both the mode-locked and Q-switched regime. The combination of device structure and material parameters for SESAM's provide sufficient design freedom to choose key parameters such as recovery time, saturation intensity, and saturation fluence, in a compact structure with low insertion loss. We have been able to demonstrate, for example, passive modelocking (with no Q-switching) using an intracavity saturable absorber in solid-state lasers with long upper state lifetimes (e.g., 1-μm neodymium transitions), Kerr lens modelocking assisted with pulsewidths as short as 6.5 fs from a Ti:sapphire laser-the shortest pulses ever produced directly out of a laser without any external pulse compression, and passive Q-switching with pulses as short as 56 ps-the shortest pulses ever produced directly from a Q-switched solid-state laser. Diode-pumping of such lasers is leading to practical, real-world ultrafast sources, and we will review results on diode-pumped Cr:LiSAF, Nd:glass, Yb:YAG, Nd:YAG, Nd:YLF, Nd:LSB, and Nd:YVO₄     

Properties of Pump-Laser Modules Exposed to Polarization-Dependent and Wavelength-Selective Feedback From Fiber Bragg Gratings

The performance of fiber Bragg grating (FBG) stabilized pump-laser modules is strongly influenced by changes of the state of polarization (SOP) arising from intrinsic and/or induced birefringence of the fiber. Birefringence changes the SOP as the light propagates back and forth between laser and FBG, which are separated by a distance of 1 to 2 m. As a result, the effective wavelength-selective feedback provided by the FBG varies accordingly. Based on the steady-state solution of a set of multimode rate equations we present a new model allowing us to calculate the output characteristics of a grating-stabilized pump-laser module as function of effective feedback. An effective-reflector approach is applied to describe the external cavity between the front mirror and the FBG on the module's front side. The polarization- and wavelength-dependent effective feedback from the grating is taken into account by a feedback parameter in the equations for the effective reflector. We present an extended traveling-wave amplifier model for calculating the TE- and TM-polarized contributions to the power from all output ports of the module. Our model is verified by comparison of simulation results with experimental data for a typical FBG-stabilized pump-laser module. The output characteristics are analyzed as function of effective feedback for both the principal TE-polarization as well as the residual TM-polarization modes.     

400 mW uncooled MiniDIL pump modules

A new generation of wavelength stabilised, uncooled 980 nm pump modules in MiniDIL housings is presented, enabling 400 mW ex-fibre power over a temperature range of 10/spl deg/C to 70/spl deg/C. At 100/spl deg/C 200 mW power is still obtained with a robust fibre coupling scheme.     

Schalenbeulen von Sandwichzylindern mit einem neuen Elastomer als Verbundwerkstoff

Shell buckling of cylindrical sandwich shells with a new elastomer as composite material. Alternatively to cylindrical steel shells a sandwich shell with a new elastomer as composite material is investigated with regard to the stability. The sandwich shell has an inner and an outer steel face, which are bonded adhesively to an elastomer core between them. In cooperation with Krupp Stahlbau Hannover GmbH the institute for steel construction of University of Hannover checked, if the application of this sandwich technology is possible for towers of wind energy converters. Especially the combination with high strength steel is a point of interest. In comparison with numerical buckling analyses the validity of a laminate composites shell theory is proven. Finally the stability of the sandwich construction is investigated against shell buckling due to axial compression and torsion. The results are compared to the stability of cylindrical steel shells.     

Verstärkung der orthotropen Stahlfahrbahn der Schönwasserparkbrücke mittels Stahl‐Elastomer‐Sandwich (SPS)

Rehabilitation of the orthotropic steel bridge deck of the Schöwasserparkbrücke using the Sandwich‐plate‐system (SPS). By the Federal Ministry of Transport a pilot project for strengthening of orthotropic bridge decks was promoted in 2005. The application of a steel‐elastomere‐sandwich‐overlay was selected. This new technology was tested which allows to strenghten orthotropic plates of bridges against hazards from growing traffic actions. The tested method as well as the pilot project at a BAB A 57 highway bridge in Krefeld are described in the following article.