Henrik Koblitz Rasmussen

Bendable, low-loss Topas fibers for the terahertz frequency range

We report on a new class of polymer photonic crystal fibers for low-loss guidance of THz radiation. The use of the cyclic olefin copolymer Topas, in combination with advanced fabrication technology, results in bendable THz fibers with unprecedented low loss and low material dispersion in the THz regime.We demonstrate experimentally how the dispersion may be engineered by fabricating both high- and low-dispersion fibers with zero-dispersion frequency in the regime 0.5-0.6 THz. Near-field, frequency-resolved characterization with high spatial resolution of the amplitude and phase of the modal structure proves that the fiber is single-moded over a wide frequency range, and we see the onset of higher-order modes at high frequencies as well as indication of microporous guiding at low frequencies and high porosity of the fiber. Transmission spectroscopy demonstrates low-loss propagation (< 0.1 dB/cm loss at 0.6 THz) over a wide frequency range.

Humidity insensitive TOPAS polymer fiber Bragg grating sensor

We report the first experimental demonstration of a humidity insensitive polymer optical fiber Bragg grating (FBG), as well as the first FBG recorded in a TOPAS polymer optical fiber in the important low loss 850 nm spectral region. For the demonstration we have fabricated FBGs with resonance wavelength around 850 nm and 1550 nm in single-mode microstructured polymer optical fibers made of TOPAS and the conventional poly (methyl methacrylate) (PMMA). Characterization of the FBGs shows that the TOPAS FBG is more than 50 times less sensitive to humidity than the conventional PMMA FBG in both wavelength regimes. This makes the TOPAS FBG very appealing for sensing applications as it appears to solve the humidity sensitivity problem suffered by the PMMA FBG.

Extensional viscosity for polymer melts measured in the filament stretching rheometer

A new filament stretching rheometer has been constructed to measure the elongational viscosity of polymer melts at high temperatures. Two polymer melts, a LDPE and a LLDPE, were investigated with this rheometer. A constant elongational rate has been obtained by an iterative application of the Orr–Sridhar method for specification of the end-plate movement. Agreement has been found with linear viscoelastic measurements performed in shear. Hencky strains up to about 6 have been reached. Steady values of the viscosity have been sustained in some cases for about two Hencky strain units.

High-Tg TOPAS microstructured polymer optical fiber for fiber Bragg grating strain sensing at 110 degrees

We present the fabrication and characterization of fiber Bragg gratings (FBGs) in an endlessly single-mode microstructured polymer optical fiber (mPOF) made of humidity-insensitive high-Tg TOPAS cyclic olefin copolymer. The mPOF is the first made from grade 5013 TOPAS with a glass transition temperature of Tg = 135°C and we experimentally demonstrate high strain operation (2.5%) of the FBG at 98°C and stable operation up to a record high temperature of 110°C. The Bragg wavelengths of the FBGs are around 860 nm, where the propagation loss is 5.1 dB/m, close to the fiber loss minimum of 3.67 dB/m at 787 nm.

Transient filament stretching rheometer II: Numerical simulation

The Lagrangian specification is used to simulate the transient filament stretching rheometer. Simulations are performed for a dilute PIB-solution modeled as a four mode Oldroyd-B fluid and a semidilute PIB-solution modeled as a non-linear single integral equation. The simulations are used to investigate a sequence of filament stretching rheometers. The questions of special and temporal homogeneity of the experiment are investigated. The simulations are compared with data from a specific rheometer by Tirtaadmadja and Sridhar.

Elongational Viscosity of Monodisperse and Bidisperse Polystyrene Melts

The start-up and steady uniaxial elongational viscosity have been measured for two monodisperse polystyrene melts with molecular weights of 52 and 103kg∕mole, and for three bidisperse polystyrene melts. The monodisperse melts show a maximum in the steady elongational viscosity vs. the elongational rate, ϵ, of about two times the limiting value of 3η0 expected for a Newtonian fluid, whereas the bidisperse melts have a maximum of up to a factor of seven times the Trouton limit of 3η0. The Wiest model which incorporates anisotropic drag and finite extensibility may be used to interpret the results in molecular terms.

Fabrication and characterization of porous-core honeycomb bandgap THz fibers

We present a numerical and experimental investigation of a low-loss porous-core honeycomb fiber for terahertz wave guiding. The introduction of a porous core with hole size of the same dimension as the holes in the surrounding honeycomb cladding results in a fiber that can be drawn with much higher precision and reproducibility than a corresponding air-core fiber. The high-precision hole structure provides very clear bandgap guidance and the location of the two measured bandgaps agree well with simulations based on finite-element modeling. Fiber loss measurements reveal the frequency-dependent coupling loss and propagation loss, and we find that the fiber propagation loss is much lower than the bulk material loss within the first band gap between 0.75 and 1.05 THz.

Viscosity overshoot in the start-up of uniaxial elongation of low density polyethylene melts

The transient uniaxial elongational viscosity of BASF Lupolen 1840D and 3020D melts has been measured on a filament stretch rheometer up to Hencky strains of 6–7. The elongational viscosity of both melts was measured at 130°C within a broad range of elongational rates. At high elongation rates, an overshoot or maximum in the transient elongational viscosity followed by a steady viscosity was observed. The steady elongation viscosity was about 40%–50% less than the maximum at high strain rates. The steady elongational viscosity as a function of the elongation rate, ϵ, decreases approximately as ϵ−0.6 in both melts at high strain rates. The transient elongational viscosity, measured at a specific elongation rate at 170°C on the BASF Lupolen 3020D melt, did not follow the time temperature superposition principle based on linear viscoelasticity during the decrease in the transient elongational viscosity towards the steady state.

Porous-core honeycomb bandgap THz fiber

In this Letter we propose a novel (to our knowledge) porous-core honeycomb bandgap design. The holes of the porous core are the same size as the holes in the surrounding cladding, thereby giving the proposed fiber important manufacturing benefits. The fiber is shown to have a 0.35-THz-wide fundamental bandgap centered at 1.05 THz. The calculated minimum loss of the fiber is 0.25 dB/cm.

Simulation of Transient Viscoelastic Flow

Abstract The Lagrangian kinematic description is used to develop a numerical method for simulation of time-dependent flow of viscoelastic fluids described by integral models. The method is shown to converge to first order in the time step and at least second order in the spatial discretization. The method is tested on the established sphere in a cylinder benchmark problem, and an extension of the problem to transient flow is proposed.