Mode Transition Region Research Articles

A highly sensitive temperature sensor based on long period fiber grating coated with polymer-bilayer-membrane

This study introduces a highly sensitive temperature sensor based on long-period fiber grating (LPFG). The sensor is composed of a long period fiber grating covered by a higher RI (RI) layer of polyvinyl alcohol (PVA) and a lower RI layer of polydimethylsiloxane (PDMS). The PVA layer enables the sensor to operate in the mode transition region with high sensitivity, and the PDMS layer further enhances the temperature sensitivity owing to its high thermos-optic coefficient. In addition, the sensor can mitigate the influence of humidity due to the hydrophobic properties of PDMS. Experimental results show that the temperature sensitivity can achieve to 1232.3 pm/℃ within the range of 20℃-100℃ for an LPFG with a period of 363 μm coated with a 265 nm PVA layer and a 20 μm PDMS layer.

Long-Period Fiber Grating Probe: An Improved Design Suitable for Biosensing

Add-layer sensitivity of long-period fiber grating (LPFG) near mode transition (MT) has been studied in reflection configuration to realize a highly sensitive LPFG-based sensor probe suitable for biosensing. The dependency of the sensitivity on the separation between the resonant cladding modes considered during MT has also been investigated. The separation between two consecutive cladding modes was precisely increased or stretched to a desired value by reducing the cladding diameter. The probe was formed by cleaving the mode stretched LPFG (MSLPFG) through the grating region. The MSLPFG was designed to operate near MT by deposition of electrostatic self-assembled (ESA) polymeric overlay layers on the grating surface. We demonstrate that the polymeric overlay not only increases the sensitivity of the sensor but also eliminates undesirable multiple resonant bands that appear due to the introduction of arbitrary phase while cleaving the grating. Add-layer sensitivity of the MSLPFG sensor probe was obtained and found to be ~2.0 nmWL/nmTH with a significantly enhanced peak attenuation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim -30$ </tex-math></inline-formula> dB around the MT region.

Label-free detection of vitamin D by optical biosensing based on long period fiber grating

In this work, we report a fiber optic biosensor for the label-free detection of vitamin D, by targeting the major circulating form of D3 in human body, i.e., 25-hydroxyvitamin D3 (25(OH)D3). The sensing platform relies on a long period grating (LPG) written into an unconventional fiber, i.e., double cladding fiber (DCF) with W-type refractive index profile. The sensitivity to surrounding medium is enhanced by chemical etching of the outer cladding as the working point of the device is tuned to the mode transition region. The DCF structure allows to combine the enhancement of the sensitivity (up to −1400 nm/RIU) with a good visibility of the grating resonance band (>10 dB) and long-term stability due to an all-silica structure. Moreover, the LPG transducer is coated with a nanosized layer of graphene oxide to provide carboxylic functional groups (-COOH) for the grafting of the biological recognition element for vitamin D3, i.e., a 25(OH)D3 specific antibody. To the best of our knowledge, this is the first evidence of a fiber optic biosensor for the detection of vitamin D3, which was performed using concentrations within the clinically relevant range of 1–1000 ng/mL and achieving a low limit of detection below 1.0 ng/mL in buffer solution that is well comparable with the state of the art. Finally, the performance of the biosensor was also evaluated in a complex medium with interfering proteins at physiological concentration obtaining promising results.

Numerical study on edge tone with compressible direct numerical simulation: Sound intensity and jet motion

A two-dimensional model of the edge tone is studied by a highly accurate and reliable method of direct numerical simulation of the compressible Navier-Stokes equations, and used to verify key features observed in previous experimental and numerical studies, and to discover new features related to the jet motion and the edge tone generation mechanism. The first and second modes of the edge tone that are numerically reproduced agree well with Brown’s equation. In the mode transition region, dynamical mode transition is observed at a fixed jet velocity. For both first and second modes, the pressure distributions are antisymmetric with respect to the edge plate, and the sound intensity is proportional to the fifth power of the jet velocity. These results are consistent with the edge tone being radiated from a dipole-like source. Spatial profiles of the velocity and the velocity variance of the oscillating jet are also investigated for each mode over a range of the jet velocity including the mode transition regime. The amplitude of the velocity oscillation becomes constant with increasing jet velocity, while a measure of the amplitude of the velocity variance profile, which is introduced to characterize the strength of the jet fluctuation and named the ’fluctuation strength’, is proportional to the third power of the jet velocity. Some properties of the fluctuation strength correspond to properties of the sound intensity, including the first mode having larger amplitude than the second mode, and the way of deviating from the power law at smaller values of jet velocity and in the mode transition region. It is proposed that the third-power law exhibited by behavior of the fluctuation strength could be related to the increase of the skewness observed in the velocity profile with increase of jet velocity, and a model calculation is used to support this proposal.

Long period grating in double cladding fiber coated with graphene oxide as high-performance optical platform for biosensing

In this work, the development and testing of a novel fiber-optic based label-free biosensor is presented, whose performance were verified through the detection of C-reactive protein (CRP) in serum. The device is based on a long period grating fabricated in a double cladding fiber with a W-shaped refractive index (RI) profile. As a result, the working point of the device was tuned to the mode transition region by chemical etching of the outer fiber cladding, obtaining a significant enhancement of the RI sensitivity and an excellent visibility of the grating resonances due to the mode transition in an all-silica structure. The fiber transducer was coated with a nanometric thin layer of graphene oxide in order to provide functional groups for the covalent immobilization of the biological recognition element. A very low limit of detection of about 0.15 ng/mL was obtained during the detection of CRP in serum, and a large working range (1 ng/mL – 100 μg/mL) of clinical relevance has been also achieved.

Formation of Central Mode Water based on two zonal hydrographic sections in spring 2013 and 2016

Two zonal high-density hydrographic sections along 41° N and 37.5° N east of Japan were occupied in April 2013 and June 2016 to examine the formation of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in relation to fronts and eddies. In the 41° N section traversing the meandering subarctic front, the denser variety of CMW (D-CMW) and TRMW was formed continuously on both sides of the front, except for the part of the section located south of the Kuroshio bifurcation front where the lighter variety of CMW (L-CMW) and D-CMW was formed instead. L-CMW and D-CMW were also formed in the eastern part of the 37.5° N section between the Kuroshio Extension front and the Kuroshio bifurcation front, but were hardly formed in the western part of the section west of the bifurcation point of the two fronts. D-CMW and TRMW pycnostads in the western part of the 41° N section observed in April 2013 tended to exhibit more than one core (vertical minimum of potential vorticity), which might be formed by destruction of deep winter mixed layers. Such multiple-core structure was also observed in L-CMW and D-CMW pycnostads in the eastern part of both the sections south of the Kuroshio bifurcation front in June 2016, being particularly abundant in three anticyclonic eddies. It was likely to be formed by the exchange of low-potential vorticity water among the eddies and the ambient region in association with eddy-to-eddy interaction, suggesting a new mechanism of mode water subduction.

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.

Ultrasensitive long-period gratings sensor works near dispersion turning point and mode transition region by optimally designing a photonic crystal fiber

A novel and ultrasensitive long-period gratings (LPGs) sensor based on photonic crystal fiber (PCF) has been proposed and theoretically investigated. The surrounding refractive index (SRI) sensitivity can be improved enormously by optimizing structure parameters of PCF to ensure LPGs work near dispersion turning point (DTP). Based on this, mode transition (MT) has been also designed and realized in LPGs based on PCF for the first time to further explore the theoretical limit of sensitivity. The simulation results illustrated that the optimized SRI sensitivity is 67,600 nm/RIU in low RI measurement region (1.334–1.344), which is much higher than that of other reported LPGs sensor based on PCF in previous literatures. The competitive advantages make the proposed sensor has a great application prospect in biosensing field, such as biomolecules detection and label-free immune sensing.

New understanding of “mode transition” of coated long period fiber grating

Based on the concept of effective refractive index (ERI) and the power density distribution of cladding mode, we reinvestigate the phenomenon of “mode transition” of coated long-period fiber grating (LPFG), and gives a new physical image for “mode transition”. By modifying the ERI range of cladding mode, we point out that the step-like growth of ERI is the self-property of each cladding mode, it has nothing to do with “mode transition”. Thus, there is no transition process between high and low order modes, and we rename the “mode transition region” to “mode barrier region”. Furthermore, the power density distribution of cladding mode shows some lower-order cladding modes will guided by the overlay in mode barrier region. Then, we present the distribution of sensitivity to SRI in mode barrier region, which provides a correct theoretical guidance for high-sensitivity coated LPFG sensors design. Finally, we propose two new characteristics of mode barrier region. First, a new kind of dual-peak resonance based on mode barrier region can be used for high sensitivity sensing and it expand the application scope of dual-peak resonance, meanwhile, it can avoid cross-sensitivity. Second, the coupling coefficient of even-order cladding mode will increase in mode barrier region, which solves the problem that the too small coupling coefficient of even-order cladding mode. Furthermore, the coupling coefficient of every cladding mode has higher response to overlay thickness, surrounding refractive index (SRI) and some other parameters in mode barrier region, so the sensitivity can be greatly improve for an intensity-modulated LPFG sensor.

Investigation of electromagnetic mode transition and filtering of an asymptotically single-mode hollow THz Bragg fibre

The mechanism for electromagnetic (EM) mode transition and filtering in an asymptotically single-HE11-mode hollow THz Bragg fibre is investigated. We designed, fabricated, and measured Bragg fibres with an asymptotically single-mode pattern, achieving measured signal propagation loss of better than 3 dB m−1 at 0.265 THz and with an operating frequency range from 0.246 to 0.276 THz. Mode transition and filtering effects are both verified by 3D full-wave simulations using measured material properties, with geometrical parameters extracted from the fabricated Bragg fibre prototypes. By optimizing the coupling efficiency between the free-space Gaussian beam and the guided Bessel function mode, the optimum distance of mode transition from the Gaussian-beam excitation into the guided mode is calculated to be ~13.7 free-space wavelengths in our fibre, to ensure fast EM-field convergence to the desired asymptotically single-mode mode pattern in the Bragg fibre. After this mode transition region, the electric field amplitude ratio between the desired HE11 mode and the main competing HE12 mode is approximately 7 times, with the HE12 mode attenuation being more than 10 dB m−1 larger than the fundamental HE11 mode; the results indicate that our fibre is one of the best candidates for low-loss asymptotically single-mode terahertz signal interconnections.

Digital Noninverting-Buck–Boost Converter With Enhanced Duty-Cycle-Overlap Control

The nonideal effects of the comparator and dead time in a synchronous controlled dc-dc converter adversely affect the stability of a four-switch noninverting-buck–boost (NIBB) converter. The pulse-skipping phenomenon occurs in the mode-transition region near the boundary between the step-down and step-up regions, and this phenomenon leads to an unstable output voltage and an unpredictable output voltage ripple. However, these two results may damage the entire power system and application system. This brief proposes an enhanced duty-cycle-overlap control technique for a digitally controlled NIBB converter. The proposed technique offers two duty cycle limitations for various conditions in the mode-transition region and ensures the stability of the digital controller and output voltage. Moreover, this technique involves combining the duty cycles of both step-down and step-up modes for deriving an accurate value of the output voltage. The experimental results derived from a digital controller implemented through a field-programmable-gate-array-based platform revealed that the output voltage of the NIBB converter was stable throughout the transition region. The observed input voltage of the converter, provided by a Li-ion battery, was 2.5–4.5 V, and the output voltage was typically 3.3 V, which is suitable for communication systems, audio systems, and I/O pad power supplies. The switching frequency was 1 MHz, and the maximum load current was 500 mA.

Synoptic observation of Central Mode Water in its formation region in spring 2003

Hydrographic data east of Japan from five research cruises and Argo profiling floats in spring 2003 have been analyzed to examine the relationship of the formation of Central Mode Water (CMW) and Transition Region Mode Water (TRMW) in late winter 2003 to thermohaline fronts and mesoscale eddies. TRMW and the denser variety of CMW (D-CMW) were formed continuously just south of the subarctic frontal zone between 155°E and 165°W with little relation to eddies, suggesting that the absence of the permanent thermocline and halocline in this area is essential for the formation. The lighter variety of CMW (L-CMW) was formed south of the Kuroshio bifurcation front and east of 165°E, partly in an anticyclonic eddy associated with the Kuroshio Extension. Some portion of D-CMW and L-CMW likely had been subducted to the permanent pycnocline by crossing southward the Kuroshio bifurcation front and the Kuroshio Extension front, respectively. In contrast, the formation of these waters in the western regions was inactive and was significantly different from that described previously using multiyear Argo float data. West of 155°E, TRMW and D-CMW were formed only in two anticyclonic eddies that had been detached from the Kuroshio Extension 1–2 years ago. L-CMW was hardly formed west of 165°E, which might be related to the upstream Kuroshio Extension being in its stable state characterized by low regional eddy activity.

Investigation of the Packaging-Induced RF Attenuations and Resonances in a Broadband Optoelectronic Modulator Module

In this paper, the influence of packaging-induced RF signal degradation on an optoelectronic modulator module is investigated. A directly modulated laser (DML) is modeled and packaged in a butterfly-type package. A distributed 3-D electromagnetic model is built based on this laser module. In the packaging assembly procedure, impedance mismatching and ground discontinuity on microwave transmission will cause unwanted signal decays and resonances. We specify the RF degradation in three regions: 1) the RF connector, 2) the RF substrate, and 3) the mode transition region between the optoelectronic subsystem and the package. The RF transmission characteristics in these regions are extracted and analyzed in detail. The results indicate that by optimizing the packaging design, strong resonances and signal decays can be eliminated or compensated over a wide frequency range. The measured scattering parameters show that the proposed packaging assembly has a resonance-free bandwidth of 31.2 GHz, and the DML module exhibits a wide 3 dB bandwidth of 15.1 GHz.

Roles of mode waters in the formation and maintenance of central water in the North Pacific

This study describes the three-dimensional distributions of the Turner angle (Tu) and the potential vorticity (PV) of the main pycnocline water in the subtropical North Pacific (10–50°N, 120°E–120°W) using a large in situ CTD data set taken by the Argo profiling floats during June to October of 2001–2009 to clarify the detailed distribution of the central water and the mode waters as well as the relationship between these water masses. The ventilated part of the main pycnocline water (σθ < 26.7 kg m-3) in the subtropical gyre generally displays a sharp peak in Tu value of 59° in the histogram. The Tu histograms for 10° × 10° geographical boxes mostly show that the mode for the Tu value is 59° too, but they also show some regional differences, suggesting some types of relations with the North Pacific mode waters. To further investigate this relationship, the appearance probability density function of the central water (defined as the main pycnocline water with Tu = 56°–63°) and those of the mode waters with PVs lower than the critical value on each isopycnal surface were analyzed. The distribution area of the central mode water (CMW) corresponds so well with that of the central water that a direct contribution of the CMW to the formation and maintenance of the central water is suggested. On the other hand, the distribution areas of subtropical mode water (STMW), Eastern STMW, and transition region mode water do not correspond to that of the central water. Nevertheless, indirect contributions of these mode waters to the formation and maintenance of the central water through salt finger type convection or diapycnal mixing are suggested.

Optical fibre long period grating based selective vapour sensing of volatile organic compounds

The chemical sensing capabilities of an optical fibre long period grating (LPG) coated with a functional material are explored. The LPG period and coating thickness are chosen such that the mode transition region, characteristic of LPGs coated with nanostructured coatings, corresponds with the phase matching turning point, ensuring optimum sensitivity. The functional material is a calixarene, which is shown to exhibit sensitivity to the aromatic compounds toluene and benzene, while being relatively insensitive to the aliphatic hydrocarbon hexane. A nanoscale cavity in the calixarene molecule entraps the VOC molecule, thereby altering the refractive index of the coating and influencing the transmission spectrum of the LPG. A sensitivity to toluene of 231 ppmv has been achieved, based on a spectrometer resolution of 0.3 nm. This is a weak molecular interaction thereby ensuring the sensor gives a reversible response when the VOC is removed. The recovery time is of the order of 15 s.

Electrical characterization of an inductively coupled gaseous electronics conference reference cell

Plasma parameters and particularly the external electrical operational parameters are examined with respect to mode transition and hysteresis. The external electrical parameters such as antenna current (Ic) and antenna voltage (Vc) amplitudes are measured after the matching network and discussed for an inductively coupled argon discharge. A wide range of discharge conditions by varying applied power (up to 150 W at 13.56 MHz), gas pressure (0.7–7.5 Pa), and electrostatic coupling strength are measured for a gaseous electronics conference radio frequency reference cell. The effect on the power coupling efficiency by varying electrostatic coupling strength is studied via implementing two distinct grounded Faraday shields, in addition to the original nonshielded condition. A brief discussion on the evolution of floating potential, plasma potential, electron density, and electron energy distribution function with power and pressure is also presented mainly in context to mode transitions and hysteresis. Relatively smooth transitions in the plasma parameters and in the external electrical parameters are measured close to H to E mode transition region. Contrary to plasma parameters, however, the reverse transition from E to H mode was found to be abrupt in external parameters. The plasma parameters are measured using a commercial Langmuir probe, whereas the antenna currents and voltages are measured using a homebuilt Rogowski coil and capacitor divider, respectively.

A slow mode transition region adjoining the front boundary of a magnetic cloud as a relic of a convected solar wind feature: Observations and MHD simulation

We identify a planar, pressure‐balanced structure bounded by sharp changes in the dynamic pressure plastered against the front boundary of the magnetic cloud which passed Earth on 20 November 2003. The front boundary of the magnetic cloud (MC) is particularly well‐defined in this case, being located where the He++/H+ number density ratio jumps from 4 to 10% for the first time and the proton plasma beta decreases sharply from ∼1 to ∼0.001. The feature, estimated to have a length scale ∼50 RE in the Sun‐Earth direction, bears close resemblance to a slow mode transition region in that the magnetic pressure decreases, the plasma pressure increases, and their temporal variations are anticorrelated. Using a 2‐D MHD simulation, we hypothesize that a pressure‐balanced structure was encountered by the MC en route to Earth. Our calculations reproduce qualitatively the major features of the observations. Using a simplified geometry suggested by the observations, we find that the lateral deflection speed of the plasma is less than the lateral expansion speed of the MC. We infer that the structure traversed the MC sheath in ∼20 h, consistent with its crossing of the MC's shock at 0.6–0.7 AU. The finding is consistent with the recent paradigm according to which solar wind plasma and magnetic field tend to pile up in front of interplanetary ejecta because the expansion of the ejecta hinders the shocked solar wind plasma from deflecting effectively around the object. Also, the inferred “age” of the layer contiguous to the surface of the MC, the earliest relic of its passage through the inner heliosphere, is in agreement with general estimates.

Electron energy distribution function close to the mode transition region in an inductively coupled gaseous electronics conference reference cell

The electron energy distribution function (EEDF) in the E to H mode transition region of an inductively coupled argon discharge has been studied experimentally. The EEDF, which has a Maxwellian- or Druyvesteyn-like shape (depending on pressure) in both “pure” modes, shows a trend to a bi-Maxwellian shape in the vicinity of both the E to H and the H to E mode transitions. Moreover, the normalized electron energy probability functions closely before the E to H and the reverse H to E mode jumps are almost identical, indicating a similar power coupling at both transition points.

Steady state slow shock inside the Earth's magnetosheath: To be or not to be? 2. Numerical three‐dimensional MHD modeling

Large‐scale structures in the dayside terrestrial magnetosheath are studied in this paper. The question about existence of the steady state slow MHD shock in the inner magnetosheath is one of the principal theoretical and experimental problems which has not been solved yet. In part 1, the original observations have been revisited. In this part, the numerical results of the three‐dimensional MHD model are presented and discussed. It is shown that for the given observations the density enhancement identified previously as a slow mode transition region is explained by temporal variations of the magnetic field and density observed in the supersonic solar wind. There are three factors resulting in the density increase in the slow mode transition region: A density increase in the upstreamflow, a decrease of the IMF modulus, and a new IMF orientation which forms a quasi‐parallel bow shock in the subsolar region. We show that numerical solutions obtained by non‐stationary and stationary MHD models are significantly different for the same solar wind conditions. We discuss how the magnetopause motion may enlarge the observed variations of the magnetosheath density. Also we answer the question why the existed numerical MHD models do not predict the slow shock in the steady state solution and show how the magnetosheath flow may be organized without such shock.

Temporal variations in the magnetosheath: Comparison between MHD calculations and observations for one event on September 17, 1978

Plasma density and magnetic field data from ISEE 3 on September 17, 1978 upstream of the Earth's bow shock are used to calculate a 3D MHD flow in the dayside magnetosheath. The numerical results are compared with the ISEE 2 magnetosheath data along a complete dayside crossing from the magnetopause to the bow shock. Such approach gives an opportunity to estimate the ability of the MHD numerical model to predict temporal variations of the MHD parameters in the magnetosheath. We discuss properties of a particular structure in the inner magnetosheath identified previously by Song et al., as a slow mode transition region (Geophys. Res. Lett. 17 (1990) 2035). Our results show that the quantitative characteristics of the density and magnetic field in this structure may be explained by temporal variations of these parameters in the supersonic solar wind if two different time shifts obtained by Hubert (Geophys. Res. Lett. 28 (2001) 1451) are taken into account. This study stresses importance of the interplanetary magnetic field variations in the formation of MHD flow in the magnetosheath.