Length Of Transition Region Research Articles

A shower of mid-infrared Raman solitons at designed wavelength of ∼3 μm from a tapered fluorotellurite fiber

Optical temporal soliton generation in optical fibers and resonators has achieved great breakthroughs and found significant applications in many fields. Despite recent advances on optical soliton generation in optical fibers, it remains challenging to realize a shower of Raman solitons at a certain wavelength in optical fibers, i.e. put all the generated Raman solitons at a same wavelength on demand. Here we propose and experimentally demonstrate a shower of mid-infrared Raman solitons at designed wavelength of 3 μm from a tapered fluorotellurite fiber pumped by a 1960 nm picosecond fiber laser. Tapered fluorotellurite fibers have an untapered region length of ∼49 cm and a tapered transition region length of ∼1 cm. The fiber segment at the end of the tapered region has a 2nd zero-dispersion wavelength of ∼3 μm. Firstly, multiple Raman solitons with different wavelengths are generated in the untapered region pumped by the 1960 nm picosecond laser. Then, as Raman solitons propagate into the tapered region, the redshift of all the generated Raman solitons with different wavelengths are accelerated due to the dramatic increase of the nonlinear coefficient. Finally, as Raman solitons move out from the tapered region, all the accelerated Raman solitons are stopped around 3 μm owing to the occurrence of soliton self-frequency shift cancellation, causing the generation of a shower of mid-infrared Raman solitons around 3 μm. This phenomenon may find applications in super-efficient nonlinear light generation and signal processing.

Effect of the Geometries of Ge-Sb-Se Chalcogenide Glass Tapered Fiber on the Sensitivity of Evanescent Wave Sensors

A geometrical optimization study of Ge-Sb-Se chalcogenide glass tapered fiber sensors for concentration sensing of methanol is presented. The effects of the geometrical parameters such as taper waist diameter and transition region lengths on the evanescent wave penetration depth are discussed based on the ray-tracing theory. Numerical analysis results demonstrate that the performance of the tapered fiber sensor can be enhanced by decreasing the transition length of the down-taper combined with increasing the transition length of the up-taper. A homemade tapering platform enables controlling the taper waist diameter and transition region length of the tapered fiber accurately. An experimental study using tapered fiber sensors with different transition region lengths is carried out to detect the concentration of methanol aqueous solution. The highest sensitivity of 0.0120 a.u./% is obtained from the tapered fiber with a down-taper transition length of 5.4 mm and an up-taper transition length of 7.9 mm, which agrees with the trend presented by the simulation results. The effects of symmetric and asymmetric tapered fiber structure on the sensing characteristics are also investigated.

Optical fiber strain sensor with double S-tapers

An optical fiber Mach–Zehnder interferometer based on double S-tapers is proposed for strain sensing. The double S-taper strain sensor is formed by two steps. The fiber is pulled during arc discharging, and a second arc discharge is performed while the fixtures are misaligned to form a S-taper. The transmission spectra were collected with different structural parameters, including the axial offset, taper region diameter, and transition region length. The transmission spectra of each group of parameters are compared and analyzed. This work is meaningful for further research on double S-taper strain sensors. When the axial offset is 60 μm, the S2-taper region diameter is 54 μm, the transition region length is 0.7 cm, the strain experiment is tested from 0 με to 800 με, and the strain sensitivity is 6.63 pm/με. In addition, the stability, reversibility, and temperature sensitivity were experimentally characterized.

Broadband supercontinuum generation from 600 to 5400 nm in a tapered fluorotellurite fiber pumped by a 2010 nm femtosecond fiber laser

We demonstrate broadband supercontinuum (SC) generation from 600 to 5400 nm in a tapered fluorotellurite fiber pumped by a 2010 nm femtosecond fiber laser. All-solid fluorotellurite fibers with a core diameter of ∼6 μm are fabricated by using a rod-in-tube method. Tapered fluorotellurite fibers with an untapered region length of ∼2 cm and a tapered transition region length of ∼1.05 cm are prepared by employing an elongation machine. By using the tapered fiber as the nonlinear medium and a 2010 nm femtosecond fiber laser as the pump source, SC generation from 600 to 5400 nm is obtained, the 30 dB bandwidth of the generated SC light is about 3600 nm, and the corresponding output power is about 0.85 W for a launched average pump power of ∼1.57 W. The spectral broadening in the tapered fiber is caused by higher order soliton compression, Raman soliton, and blue-shifted and red-shifted dispersive wave generation. Our results show that fluorotellurite fibers are promising nonlinear media for generating broadband SC light expanding from visible to 5400 nm.

High-Reynolds number transitional flow simulation via parabolized stability equations with an adaptive RANS solver

The accurate prediction of transition is relevant for aerodynamic analysis and design applications. Extending the laminar flow region over airframes is a potential way to reduce the skin friction drag, which in turn reduces fuel burn and greenhouse gas emissions. This paper introduces a numerical framework that includes the modeling of transition effects for high Reynolds number flows in a high-fidelity, Reynolds–averaged Navier–Stokes (RANS) aerodynamic design framework. The CFD solver uses a discontinuous Galerkin (DG) finite element approach and includes goal-oriented adaptation. The Spalart–Allmaras (SA) turbulence model is used for the closure of the governing equations. In the flow stability analysis, the nonlocal, nonparallel effects that characterize boundary layers are accounted for by using the parabolized stability equations (PSE). Transition onset is obtained through an eN method based on the PSE computations, while a smooth intermittency function includes the transition region length. Numerical results for the NLF(1)-0416 airfoil present good agreement with experimental data, improving the computations when compared to fully-turbulent ones.

Enhanced arc-induced core expansion for mode-field adaptation using a H2-loaded fiber

An efficient method to improve the efficiency of mode-field adaptation between two mismatched fibers with arcs and hydrogen-loading is proposed and demonstrated for the first time. By hydrogen-loading the fiber of a relatively smaller core, the dopant diffusion rate and the mode transition region length were significantly increased. These enhancements contributed to an abrupt diffusion rate difference at the intersection of the fibers and an adiabatic mode transition. For the mode-field adaptation of the two fibers that have a mode-field area ratio of 7.25, the transmission loss was reduced from −3.71 to −0.24 dB by an arc duration of approximately 10 seconds.

1.4-7.2 μm broadband supercontinuum generation in an As-S chalcogenide tapered fiber pumped in the normal dispersion regime.

We report a broadband supercontinuum (SC) generation in chalcogenide (ChG) step-index tapered fibers pumped in the normal dispersion regime. The fibers consisting of As2S3 core and As38S62 cladding glasses were fabricated using the isolated stacked extrusion method. A homemade tapering platform allows us to accurately control the core diameters and transition region lengths of the tapered fibers. An SC generation spanning from 1.4 to 7.2μm was achieved by pumping a 12-cm-long tapered fiber with femtosecond laser pulses at 3.25μm. To the best of our knowledge, this is the broadest SC generation obtained experimentally in tapered fibers when pumped in the normal dispersion regime so far. The effects of waist diameter and transition region length of the tapered fiber on the SC spectral behavior were also investigated.

Optimal Length of Conformational Transition Region in Protein Search for Targets on DNA.

The starting point of many fundamental biological processes is associated with protein molecules finding and recognizing specific sites on DNA. However, despite a large number of experimental and theoretical studies on protein search for targets on DNA, many molecular aspects of underlying mechanisms are still not well understood. Experiments show that proteins bound to DNA can switch between slow recognition and fast search conformations. However, from a theoretical point of view, such conformational transitions should slow down the protein search for specific sites on DNA, in contrast to available experimental observations. In addition, experiments indicate that the nucleotide composition near the target site is more symmetrically homogeneous, leading to stronger effective interactions between proteins and DNA at these locations. However, as has been shown theoretically, this should also make the search less efficient, which is not observed. We propose a possible resolution of these problems by suggesting that conformational transitions occur only within a segment around the target where stronger interactions between proteins and DNA are observed. Two theoretical methods, based on continuum and discrete-state stochastic calculations, are developed, allowing us to obtain a comprehensive dynamic description for the protein search process in this system. The existence of an optimal length of the conformational transition zone with the shortest mean search time is predicted.

Tapered fluorotellurite microstructured fibers for broadband supercontinuum generation.

Fluorotellurite microstructured fibers (MFs) based on TeO2-BaF2-Y2O3 glasses are fabricated by using a rod-in-tube method. Tapered fluorotellurite MFs with varied transition region lengths are prepared by employing an elongation machine. By using a tapered fluorotellurite MF with a transition region length of ∼3.3 cm as the nonlinear medium and a 1560 nm femtosecond fiber laser as the pump source, broadband supercontinuum generation covering from 470 to 2770 nm is obtained. The effects of the transition region length of the tapered fluorotellurite MF on supercontinuum generation are also investigated. Our results show that tapered fluorotellurite MFs are promising nonlinear media for generating broadband supercontinuum light expanding from visible to mid-infrared spectral region.

Coherence property of mid-infrared supercontinuum generation in tapered chalcogenide fibers with different structures

We have numerically investigated the coherence property of mid-infrared supercontinuum generation in tapered step-index chalcogenide fibers with different structures. The pump source is a 4 μm laser with pulse width of 500 fs and peak power of 1 kW. The length ratio is the ratio of transition region length near the laser input to the other transition region length near the output. We calculate the bandwidth and the spectrally averaged coherence of the supercontinuum spectra generated in fibers with different length ratios under the same pumping condition. Numerical results show that as the length ratio increases, the bandwidth decreases from 4.84 μm to 4.11 μm while the spectrally averaged coherence increases from 0.53 to 0.9 and then jitters near the maximum. The length ratio within 1–1.5 is preferable to keep a balance between bandwidth and coherence.

Deflagration-to-detonation transition in mixtures of finely divided ammonium perchlorate with submicron aluminum powder

Deflagration-to-detonation transition in binary mixtures of fine ammonium perchlorate (20-μm grains) with submicron ALEX-L aluminum powder (0.2-μm particles) is studied using high-speed photography and pressure recording with quartz crystal sensors. The test mixtures were loaded in thin-walled quartz tubes of inner diameter 10 mm. The charges had a porosity of ~50%. It has been shown that, even under very mild conditions (low-strength shell and a weak source of initiation), the deflagration mode of mixture combustion easily transforms into the detonation mode. The shortest length of the region of transition from deflagration to normal detonation (not more than 30 mm) was observed for a lean mixture, with an aluminum content of ~5%. The mechanism of transition to detonation involves the stage of convective combustion, resulting in the formation of a brightly luminescent crescent-shaped area behind the primary flame front, which, in turn, generates a forward (in the direction of propagation) and a backward wave. The forward wave gives rise to low-speed detonation, which later transforms into normal detonation. The pressure profile within the region of low speed detonation is measured. A comparison with similar experiments in which ALEX-L alu- minum powder was replaced by ASD-4 aluminum (4 μm particles) shows that ALEX-L sensitizes the mixture, resulting in a dramatic reduction of the length of the transition region, making it possible to produce normal detonation in low-strength shells.

Tapered-EDF-Based Mach–Zehnder Interferometer for Dual-Wavelength Fiber Laser

An all-fiber comb filter using a tapered-erbium-doped fiber in a Mach-Zehnder interferometer structure is presented. The free spectral range, extinction ratio, bandwidth, and interference pattern of the comb filter can be shaped by controlling the taper waist length and the length of up and down taper transition regions. By varying the taper waist length from 5 to 25 mm, the free spectral range changes from 14.7 to 1.0 nm, and the linewidth varies from 3.3 to 0.3 nm, respectively. We demonstrate a tunable dual-wavelength laser by using the tapered-erbium-doped fiber as a gain medium as well as a wavelength-selective element. The laser can be tuned at a resolution of 0.2 nm with a side-mode suppression ratio of up to 46.88 dB and a linewidth of 0.09 nm.

Flow Analysis of the Die and the Torsional Multiplying Element in Mould Based on ANSYS

Polymer melt flowing in the multiplying element and matched die was simulated for the flowing uniformity and pressure distribuiton based on ANSYS. The results of simulation revealed that the height difference between outlet and inlet sections of the channels in multiplying element, the length of transition region and forming region of molding die all affected pressure distribution and velocity uniformity at the outlet.

Joint Residual Stress/Refractive Index Characterization of Large-Mode-Area Erbium-Doped Fibers

The residual stress (RS) and refractive index (RI) distributions of large-mode-area (LMA) erbium-doped fibers (EDFs) are measured for the first time using a high-resolution three-dimensional index-stress distribution measurement method. The effects of fiber manufacturing, cleaving, and arc fusion splicing of a commercially available LMA EDF are concurrently characterized at state-of-the-art stress, index, and spatial resolutions. RS induced via fiber manufacturing results in RI changes as large as <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$1.2 \times 10^{-4}$</tex></formula> RI units through the photo-elastic effect. The relaxation of RS within <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\sim} 30$</tex></formula> <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu$</tex> </formula> m of a cleaved end-face reverses the stress-induced RI changes formed during fiber manufacturing. After fusion splicing, an even larger stress-induced RI change of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$3.5 \times 10^{-4}$</tex></formula> RI units results over an axial distance on the order of millimeters. The diffusion of core dopants reduces the maximum core RI by as much as 21.7% along a transition region length of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim} 400$</tex></formula> <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\mu$</tex></formula> m. These measurements represent the first of many required to develop future ultra LMA EDFs where RS effects and dopant diffusion are absolutely critical for fiber design and performance.

A Conservation-Law Approach to Predicting the Length of the Boundary Layer Transition Region

The length of the boundary layer transition region is predicted by the conservation of momentum, based on the prediction method of the boundary layer transition by the mass conservation. The predicted length decreased in line with wall cooling and an increase of Mach number. Some calculated lengths agreed with experimental results, and others did not. To examine the insufficient agreement, the effect of the wall temperature on the length is discussed. Under the heated-upstream wall condition, which may appear in hypersonic wind tunnel experiments, the length of the transition region and the transition Reynolds number increased in line with the wall cooling in the prediction calculation. Most of the experimental data are in the region between the calculated results with this heated-upstream wall condition and those with the uniform wall-temperature condition. The similarity of the results predicted by the energy conservation to those predicted by the momentum conservation is also discussed.

Transverse Multimode Evolution in Non-Adiabatic Optical Micro/Nanofiber Tapers

The multimode evolution, optical losses and wavelength response of non-adiabatic micro/nano-fiber (MNF) tapers are numerically simulated using a three-dimensional finite-difference beam propagation method. For a non-adiabatic MNF taper, it is illustrated that optical losses vary with the transition region length and the optical wavelength. We explain how the complicated multimode evolutions result in the complicated optical loss and wavelength response properties, especially when the waist diameters are large enough to allow much higher-order modes. These results may offer valuable references for trapping and guiding cold atoms in atom optics and practical application of micro/nano-devices.

Analysis of mixed grade transition in continuous thin slab casting with EMBR

A concentration change during grade transition operation in thin slab casting is investigated through computer simulation and the results are compared with experimental measurements. Fluid flow and mixing patterns in various tundish levels and flow rates were analyzed by a three-dimensional mathematical model. Based on the contained results, a simple, efficient and accurate computational model is suggested to predict the concentration profile at the outlet of the tundish. Based on the model, mixing in and below the mold was analyzed considering electromagnetic braking force. The predicted concentration profiles show good agreements with the measured values. It is found that the lower vortices in the mold are suppressed by the electromagnetic field and a plug-like flow region develops, which decreases the intermixing of two different grades of steel and shortens the length of transition region.

Three-dimensional ribbing instability in symmetric forward-roll film-coating processes

We propose a framework for interpreting the formation, evolution, and spatial persistence of ribbing in coating processes, and present companion ‘quantifying’ parallel spectral element simulations of fully nonlinear unsteady three-dimensional free-surface symmetric forward-roll film-coating fluid flows. The framework couples, by means of a transition region, two well-understood phenomena: the ‘viscous fingering’ instability of a splitting meniscus; and the levelling of viscous films under the effect of surface tension. The transition-region length, Lt, is on the order of the coating film thickness, while the downstream extent of the levelling region – the distance over which ribs persist – Lt, depends on the fluid properties, the flow conditions, and the wavenumber content of the nonlinear meniscus rib profile. Numerical results are presented for the evolution of the coating flow from perturbed unstable two-dimensional steady states to three-dimensional saturated ribbed states for several representative supercritical capillary numbers, Ca, and spanwise periodicity lengths, b. Nonlinear state selection is briefly discussed.

Transition From Compression Waves to a Shock Wave in High-Speed Train Tunnels.

A guantitative investigation is carried out on the generation of a compression wave in highspeed train tunnels and on its transition to a shock wave. Discussions are based on results of experiments which were conducted in a scaled train tunnel simulator. The overpressure level of the compression wave is formulated using a modified Hara-Maeda formula, in which an effective blockage factor is taken into account. The length of the shock wave transition region is estimated from the pressure gradient of the compression wave at the tunnel entrance. The installation of a hood at the entrance is found to disperse the pressure rise, thereby the length of the shock wave transition region is increased. However, the length decreases significantly with an increase in the train speed. Conditions under which the entrance hood works effectively are formulated. Consequently, it is concluded that a critical train speed exists beyond which the entrance hood becomes no longer effective for retarding the transition.

Contamination build-up on ABS and its effects on read/write performance of near-contact recording systems

This paper reports results of experimental investigations of contamination build-up on slider surfaces and its effects on the performance of tri-pad slider-based near-contact recording systems. Results indicate that the head-disk clearance changes, caused by contamination build-up, can be comparable to the designed head-disk clearance. As a result, the drive's performance and reliability can be strongly affected. Results obtained show that the amount of increase in the value of transition region length parameter is greater than the corresponding increase in head-disk clearance. Consequently, the writing performance suffers more severe degradation than the read performance. This is shown by error-rate measurements at the drive level as well as PW50 and NLTS measurements at the component level.