Outer Mode Research Articles

Dispersion‐compensating photonic crystal fibers with special characteristics

AbstractBased on the modified dual core structure, three kinds of special photonic crystal fibers are presented, which are extremely large negative dispersion, super‐broad band, and large area mode field dispersion‐compensating photonic crystal fibers (DCPCF). For extremely large negative dispersion DCPCF, the peak of negative dispersion reaches −5.9 × 104 ps/(nm km). Super‐broad band DCPCF has broadband large negative dispersion and the dispersion value varies linearly from −380 ps/(nm km) to −420 ps/(nm km) in the C band. The designed large area mode field DCPCF has a peak dispersion of −1203 ps/(nm km) with the inner core mode area of 47 μm2 and outer core mode area of 835 μm2. Furthermore, for the large area mode field DCPCF, the experimental result is also obtained. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 1073–1078, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23306

Experimental Study of Mode Field Evolution of Dual-Core Photonic Crystal Fiber

The accurate mode field profile of high negative dispersion dual-core photonic crystal fiber (DCPCF) is measured. The mode field evolution of DCPCF with wavelength is studied experimentally for the first time. The measurement result shows that no individual inner core mode or outer core mode exists, but two modes coexist simultaneously, and either one of them is dominant. The mode field evolution versus wavelength indicates that the wavelength range where the modes coupling takes place between inner core and outer core is broader than that of theoretical design.

Realistic Design of Large-Hollow-Core Photonic Band-Gap Fibers With Suppressed Higher Order Modes and Surface Modes

This paper theoretically describes effective suppression of higher order modes (HOMs) in realistic large-hollow-core photonic band-gap fibers (PBGFs) and utilizes the use of this class of waveguides for low-loss data-transmission applications and high-power beam delivery systems. The proposed design strategy is based on the resonant-coupling mechanism of central air-core modes with defected outer core modes. By incorporating six 7-unit-cell air cores in the cladding of the PBGF with sixfold symmetry, it is possible by resonantly coupling the light corresponding to the HOMs in a central 19-unit-cell core into the outer 7-unit-cell core, thus significantly increasing the leakage losses of the HOMs in comparison to those of fundamental mode. We consider a realistic PBGF structure with hexagonal airholes having rounded corners and derive a surface-mode-free condition of a silica-ring thickness surrounding the hollow core for both 7-unit-cell and 19-unit-cell cores. Verification regarding the propagation properties of the proposed design is ensured with a PBGF analysis based on a finite element modal solver. Numerical results show that the leakage losses of the HOMs can be enhanced in a level of at least three orders of magnitude over 200-nm wavelength range in comparison to those of the fundamental mode, while in addition, we show that the incorporation of a realistic air core with optimized silica-ring thickness can eliminate surface modes and achieve strong confinement into the central core and very low eta-factor for the fundamental mode.

Studying Single-Wall Carbon Nanotubes Through Encapsulation: From Optical Methods Till Magnetic Resonance

Encapsulating fullerenes, magnetic fullerenes, 13C isotope enriched fullerenes, and organic solvents inside SWCNTs enables to yield unprecedented insight into their electronic, optical, and interfacial properties and to study SWCNT growth. In addition to customary methods of their studies such as e.g., optical absorption or Raman spectroscopy, these efforts enables to employ electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. Encapsulated C60 fullerenes are transformed to inner tubes by a high temperature annealing. The diameter distribution of the inner tubes follow that of the outer ones and their unique, low defect concentration makes them an ideal model system for high resolution and energy dependent Raman studies. The observation of Raman modes of individual inner-outer tube pairs allows to measure the inner-outer tube interaction strength that is also well described theoretically. Reversible closing and opening of SWCNT can be studied in a diameter selective manner by encapsulating C60 and transforming it to an inner tube. The growth of inner tubes can be achieved from 13C enriched encapsulated organic solvents, which shows that the geometry of the fullerene does not play a particular role in the inner tube growth as it was originally thought. In addition, it opens new perspectives to explore the in-the-tube chemistry. Growth of inner tubes from 13C enriched fullerenes provides a unique isotope engineered heteronuclear system, where the outer tubes contain natural carbon and the inner walls are controllably 13C isotope enriched. The material enables to identify the vibrational modes of inner tubes which otherwise strongly overlap with the outer tube modes. The 13C NMR signal of the material has an unprecedented specificity for the small diameter SWCNTs. Temperature and field dependent 13C T1 studies show a uniform metallic-like electronic state for all inner tubes rather than distributed metallic and isolating behavior. A low energy, 3 meV gap is observed that is tentatively assigned to a long sought Peierls transition in the small diameter SWCNTs. Encapsulating magnetic fullerenes, such as N@C60 and C59N opens the way for local probe ESR studies of the electronic properties of the SWCNTs.

Ellipsoidal Core–Shell Dielectric-Gold Nanostructure: Theoretical Study of the Tunable Surface Plasmon Resonance

The dependence of surface plasmon resonance (SPR) characters of core-shell structure ellipsoidal nanorods was investigated as functions of the gold shell width and dielectric constant. A dipole-limit calculation indicated that, there are four SPR absorption peaks occur. The two shorter wavelength peaks are attributed to the transverse resonances, whereas the two longer wavelength peaks are attributed to the longitudinal resonances. Both transverse and longitudinal resonances are split into inner surface and outer surface modes, respectively. When the core size is a fixed value, the two transverse surface plasmon peaks red shift as the gold shell width is increased. On the other hand, both the inner and outer surface modes attributed to the longitudinal surface plasmon peaks blue shifts distinctly and nonlinearly. Further more, both the inner and outer surface plasmon modes are also dependent on the embedding medium and core dielectric constant. The longitudinal SPR from inner surface is more sensitive to the core dielectric constant, whereas the longitudinal SPR from outer surface is more sensitive to the surrounding medium.

Self-similar pressure oscillations in neutron star envelopes as probes of neutron star structure

We study eigenmodes of acoustic oscillations of high multipolarity l ~ 100 - 1000 and high frequency (~100 kHz), localized in neutron star envelopes. We show that the oscillation problem is self-similar. Once the oscillation spectrum is calculated for a given equation of state (EOS) in the envelope and given stellar mass M and radius R, it can be rescaled to a star with any M and R (but the same EOS in the envelope). For l>300 the modes can be subdivided into the outer and inner ones. The outer modes are mainly localized in the outer envelope. The inner modes are mostly localized near the neutron drip point, being associated with the softening of the EOS after the neutron drip. We calculate oscillation spectra for the EOSs of cold-catalyzed and accreted matter and show that the spectra of the inner modes are essentially different. A detection and identification of high-frequency pressure modes would allow one to infer M and R and determine also the EOS in the envelope (accreted or ground-state) providing thus a new and simple method to explore the main parameters and internal structure of neutron stars.

Design of photonic band gap fibers with suppressed higher-order modes: Towards the development of effectively single mode large hollow-core fiber platforms

The objective of the present investigation is to propose and theoretically demonstrate the effective suppression of higher-order modes in large-hollow-core photonic band gap fibers (PBGFs), mainly for low-loss data transmission platforms and/or high power delivery systems. The proposed design strategy is based on the index-matching mechanism of central air-core modes with defected outer core modes. By incorporating several air-cores in the cladding of the PBGF with 6-fold symmetry it is possible to resonantly couple the light corresponding to higher-order modes into the outer core, thus significantly increasing the leakage losses of the higher-order modes in comparison to the fundamental mode, thus making our proposed design to operate in an effectively single mode fashion with polarization independent propagation characteristics. The validation of the procedure is ensured with a detailed PBGF analysis based on an accurate finite element modal solver. Extensive numerical results show that the leakage losses of the higher-order modes can be enhanced in a level of at least 2 orders of magnitude in comparison to those of the fundamental mode. Our investigation is expected to remove an essential obstacle in the development of large-core single-mode hollow-core fibers, thus enabling them to surpass the attenuation of conventional fibers.

Inner and outer rounding of Boolean operations on lattice polygonal regions

Robustness problems due to the substitution of the exact computation on real numbers by the rounded floating point arithmetic are often an obstacle to obtain practical implementation of geometric algorithms. If the adoption of the exact computation paradigm [C.K. Yap, T. Dubé, The exact computation paradigm, in: D.-Z. Du, F.K. Hwang (Eds.), Computing in Euclidean Geometry, in: Lecture Notes Series on Computing, vol. 4, second ed., World Scientific, Singapore, 1995, pp. 452–492, http://cs.nyu.edu/cs/faculty/yap/papers/paradigm.ps] gives a satisfactory solution to this kind of problems for purely combinatorial algorithms, this solution does not allow to solve in practice the case of algorithms that cascade the construction of new geometric objects. In this report, we consider the problem of rounding the intersection of two polygonal regions onto the integer lattice with inclusion properties. Namely, given two polygonal regions A and B having their vertices on the integer lattice, the inner and outer rounding modes construct two polygonal regions A ∩ ̲ B and A ∩ ¯ B with integer vertices such that A ∩ ̲ B ⊆ A ∩ B ⊆ A ∩ ¯ B . We also prove interesting results on the Hausdorff distance, the size and the convexity of these polygonal regions.

Second-order Monte Carlo wave-function approach to the relaxation effects on ringing revivals in a molecular system interacting with a strongly squeezed coherent field

We investigate the relaxation effects on the quantum dynamics in a two-state molecular system interacting with a single-mode strongly amplitude-squeezed coherent field using the second-order Monte Carlo wave-function method. The molecular population inversion (collapse-revival behavior of Rabi oscillations) is known to show the echoes after each revival, which are referred to as ringing revivals, in the case of strongly squeezed coherent fields with oscillatory photon-number distributions due to the phase-space interference effect. Two types of relaxation effects, i.e., cavity relaxation (the dissipation of an internal single mode to outer mode) and molecular coherent (phase) relaxation caused by nuclear vibrations on ringing revivals are investigated from the viewpoint of the quantum-phase dynamics using the quasiprobability ($Q$ function) distribution of a single-mode field and the off-diagonal molecular density matrix $\ensuremath{\mid}{\ensuremath{\rho}}_{\mathrm{elec}\phantom{\rule{0.3em}{0ex}}1,2}(t)\ensuremath{\mid}$. It turns out that the molecular phase relaxation attenuates both the entire revival-collapse behavior and the increase in $\ensuremath{\mid}{\ensuremath{\rho}}_{\mathrm{elec}\phantom{\rule{0.3em}{0ex}}1,2}(t)\ensuremath{\mid}$ during the quiescent region, whereas a very slight cavity relaxation particularly suppresses the echoes in ringing revivals more significantly than the first revival but hardly changes a primary variation in envelope of $\ensuremath{\mid}{\ensuremath{\rho}}_{\mathrm{elec}\phantom{\rule{0.3em}{0ex}}1,2}(t)\ensuremath{\mid}$ in the nonrelaxation case.

Schwertmannite precipitated from acid mine drainage: phase transformation, sulphate release and surface properties

Schwertmannite precipitated from acid mine drainage at the Kristineberg Zn–Cu mine in northern Sweden has been characterised regarding elemental composition, phase transformation as a function of pH and time, SO 4 2 - release and speciation of SO 4 2 - associated with the solid. The elemental analysis gave the composition Fe 8O 8(OH) 5.02(SO 4) 1.49 · 0.5H 2O where approximately 1/3 of the SO 4 2 - is adsorbed to the surface. The conversion of schwertmannite to goethite at pH 9 was complete within 187 days; at pH 6, the conversion was still incomplete after 514 days. Lower pH and relatively high SO 4 2 - concentration decreased the conversion even further. Also temperature was shown to be an important parameter for this process and low temperature (+4 °C) effectively stopped the transformation at pH 3. The release of SO 4 2 - was linear with pH and X-ray photoelectron spectroscopy measurements confirming that the surface bound SO 4 2 - was released before bulk SO 4 2 - . Zeta potential measurements indicate a pH IEP of 7.2 for the schwertmannite sample. Prior to conversion into goethite, the SO 4 2 - associated with schwertmannite was indicated by attenuated total reflectance FTIR spectroscopy to be present both as bulk and surface species. Furthermore, the speciation of surface SO 4 2 - was shown to vary with pH and two predominating species were detected. As pH increases, SO 4 2 - is increasingly coordinated in an outer sphere mode whereas a stronger, possibly inner sphere, complex dominates at low pH.

Paul Klee in the Wizard's Kitchen.

Art has its own reality; it does not exist, as we know it, in a mental form prior to its physical manifestation, nor does it bear a direct relationship to either inner or outer realities. Although an individual painting or drawing may appear to represent the outer world of physical reality or the inner world of the imagination, art is more complex than what our immediate experience may lead us to believe. For example, a picture painted or drawn in a manner described as “realism” seems to resemble, to our perception, the outer world, while expressionistic, surreal and abstract modes of art tend to be associated with an artist’s expression of his individual psychic reality. However, within a work of art, inner and outer realities exist in a dialectical relationship as “a coexistence of irresolvably different elements that have entered into a relationship of evolving, mutually enriching negations of one another” (Ogden, 1989a, p. xi). Furthermore, the emergence of a work of art as its final form is unpredictable because the manipulation of physical materials intervenes between the artist’s original idea (which is altered as he paints or draws) and the completed work. A work of art has a life of its own; it is its own reality. Unlike the progression of knowledge in science, progress as such does not exist in art. For this discussion, I refer to those arts (primarily painting, drawing, and printmaking) practiced by Paul Klee (1879–1940). While there is progress in the technical aspects of making art, such as the discovery of new materials or methods, the development of formal ideas in art over time (for

Type-I ELM precursor modes in JET

Clear low frequency (typically 5–25 kHz) coherent type-I ELM precursor modes have been identified in JET. They are detected through various diagnostics, especially in the ECE and Mirnov signals, but also on the SXR cameras and the multichannel O-mode edge reflectometer. The modes propagate in the direction of the ion-diamagnetic drift and are usually accompanied by a slight rise in the divertor Dα-emission. The precursors show no radial phase inversions and are localized to a few centimetres inside the separatrix, in the pedestal region. The time by which the precursor onset precedes the ELM event varies greatly, and ranges from 0.2 ms up to several tens of milliseconds, though the timescale has no effect on the characteristics of the precursors. During their lifetime, the precursor amplitude is often seen to fluctuate, and the frequency and dominant n-number of the precursors can change rapidly, typically within 1 ms. The range of toroidal precursor mode numbers n which have been observed is 1–13. Parametric studies show that n seems to be mainly prescribed by a combination of the electron pedestal pressure and the (normalized) electron pedestal collisionality , with increasing leading to higher n-numbers. Above a certain edge collisionality (roughly ) the precursors seem to be absent, indicating that a sufficiently high edge current is important to destabilize the precursors. The low-n precursors are known to be external kinks (also known on JET as Outer Modes), while experimental findings and their comparison with stability calculations suggest that the precursor modes with higher n are not pure external kinks but coupled ballooning–kink modes. Several hundred discharges have been analysed, and, in discharges with low to moderate edge collisionality, the precursors could be detected prior to most type-I ELMs, while prior to type-III ELMs the modes were not encountered. In spite of their regular occurrence, there is no evidence of the precursor mode growth rate rapidly accelerating before the ELM, indicating that type-I ELMs on JET are not triggered by these modes.

Monte Carlo wave-function approach to the quantum-phase dynamics of a dissipative molecular system interacting with a single-mode amplitude-squeezed field

We investigate the quantum-phase dynamics of a dissipative molecular system interacting with a single-mode amplitude-squeezed field (with a sub-Poissonian photon-number distribution) using the Monte Carlo wave-function method. As quantum dissipation, we consider molecular coherent (phase) and population relaxations, which are caused by nuclear vibration, and cavity relaxation (the dissipation of an internal single mode to outer mode). In this study, we elucidate the effects of these dissipations on the unique quantum dynamical behaviors of this coupled system, e.g., collapse-revival behavior of Rabi oscillations, using the quasiprobability (Q function) distribution of a single-mode field and the off-diagonal molecular density matrix. It is found that although the amplitude-squeezed field exhibits a distinct revival with larger amplitudes of molecular population in the presence of the molecular phase and population relaxations as compared to the coherent field, a very slight cavity relaxation suppresses the revival of molecular population and the increase in the magnitude of off-diagonal molecular density matrices during the quiescent region in the amplitude-squeezed field case more significantly than in the coherent field case. These features are shown to be closely related to the difference in the dynamics of Q function distributions between amplitude-squeezed and coherent fields.

INVESTIGATIONS OF PULSATIONAL PROPERTIES AND EVOLUTION OF PROPAGATION DIAGRAM IN SOME δ SCUTI STAR MODELS

An investigation on the subject of evolution of propagation diagrams and obtaining the theoretical frequency spectrum of some δ Scuti star models with masses around 2 M⊙ has been presented. The variations of some eigenfunctions throughout the star, the variation of ω2 with log Te and the distribution of nodes of eigenfunctions of some modes along with the evolution of the star have been plotted and examined. For the models at the ZAMS phase, pure acoustic modes in the outer stellar layers and pure gravity modes in the interiors were calculated. A mixed character of low degree nonradial modes was found in evolved models. These modes behave like p modes in the envelope and like g modes in the interior.

Mode Coupling within Inner Cladding Fibers

We report the formation of inner cladding modes in the optical fiber having an inner cladding structure. The inner cladding layer located between the core- and the cladding- layers of a conventional fiber might have, so called, inner cladding mode(s). The brief history of the inner cladding fiber and the spectral properties of the inner cladding mode are presented. By utilizing fiber gratings, the spectral properties of the inner cladding mode formed in Dispersion Compensating Fiber (DCF) are discussed. It was observed that one resonant peak of a long-period fiber grating was not sensitive to the variation on the cladding surface. With a fiber Bragg grating, a small group of unusual resonant Peaks was observed between the main Bragg Peak and the series of usual Peaks resulted from the mode coupling to counter-propagating cladding modes. Within the DCF by using fiber gratings, it is noted, at least one mode can be coupled to the inner cladding mode and a few outer cladding modes are severely affected by the inner cladding of the fiber.

Monte Carlo Wave Function (MCWF) approach to dissipative quantum systems interacting with a single‐mode quantized field

AbstractWe investigate the quantum dynamics of a dissipative molecular system interacting with a single‐mode coherent field (with an initial average photon number 8) using the Monte Carlo wave function (MCWF) method. As quantum dissipation, we consider cavity relaxation (the dissipation of an internal single mode to outer mode) and molecular population relaxation and molecular coherent (phase) relaxation caused by nuclear vibration. In this study, we elucidate the effects of these dissipations on the unique quantum dynamic behaviors of this coupled system, e.g., collapse–revival behavior of Rabi oscillations, using the diagonal and off‐diagonal molecular density matrices. It is found that although slight cavity relaxation suppresses the revival of molecular population similarly to other relaxation effects with larger decay rates the cavity relaxation hardly suppresses the increase in the off‐diagonal molecular density matrix (representing the degree of coherency between the ground and excited states) in the quiescent region, the feature of which is related to the generation of the Schrödinger cat state, in contrast to molecular phase and population relaxations. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Nonlinear stability and structure of compressible reacting mixing layers

The parabolized stability equations (PSE) are used to investigate issues of nonlinear flow development and mixing in compressible reacting shear layers, which are modelled with an infinitely fast-chemistry assumption. Particular emphasis is placed on investigating the change in flow structure that occurs when compressibility and heat release are added to the flow. These conditions allow the ‘outer’ instability modes – one associated with each of the fast and slow streams – to dominate over the ‘central’, Kelvin–Helmholtz mode that exists unaccompanied in incompressible non-reacting mixing layers. Analysis of scalar probability density functions in flows with dominant outer modes demonstrates the ineffective, one-sided nature of mixing that accompanies these flow structures. Colayer conditions, where two modes have equal growth rate and the mixing layer is formed by two sets of vortices, offer some opportunity for mixing enhancement. Their extent, however, is found to be limited in the mixing layer's parameter space. Extensive validation of the PSE technique also provides a unique perspective on central-mode vortex pairing, further supporting the view that pairing is primarily governed by instability growth rates; mutual induction appears to be a secondary process. This perspective sheds light on how linear stability theory is able to provide such an accurate prediction of experimentally observed, fully nonlinear flow phenomenon.

Toward the theory of magnetospheric magnetosonic eigenoscillations: simple theoretical models

The structure of magnetosonic eigenmodes was investigated analytically within the framework of simple two‐ and three‐dimensionally inhomogeneous models. Asymptotic (in the limit m ≫ 1, where m is azimuthal wave number) expressions were obtained for the eigenfrequency spectrum of magnetosonic oscillations in the cavity resonator under the plasmapause, as well as for outer magnetospheric cavity modes, the outer boundary for which is represented by the magnetopause. These oscillations were found to be damped ones. Furthermore, the damping of the eigenoscillations in the cavity under the plasmapause is caused by the escape of some of their energy through the potential barrier to the outer magnetosphere. The damping of the outer magnetospheric cavity modes is due to the fact that the magnetopause represents a partially reflecting boundary, through which a part of the oscillation energy escapes to the solar wind. In terms of an axisymmetric model of the magnetosphere, asymptotic (when m ≫ 1) expressions were obtained for damping decrements of the eigenmodes. The spatial structure of magnetosonic eigenmodes was investigated in a simple three‐dimensionally inhomogeneous model of the magnetosphere. The asymptotic expression was obtained for the spectrum of their eigenfrequencies.

An optical fiber sensor for biofilm measurement using intensity modulation and image analysis

The development of an online sensor to determine the fouling properties of aqueous process fluids is described. A plastic optical fiber with its cladding removed over a sensitized length measures the growth of biofilms in a closed loop water process system by evanescent field attenuation and intensity modulation. The sensor detects material build-up at the core-cladding interface by means of refractive index modulation. A theoretical model is developed showing that an increase in cladding refractive index reduces the intensity of light propagating in the fiber and attenuates the high order modes. The modulation mechanism of the sensor is demonstrated using a CCD camera and frame grabber to record the far field modal distribution of the fiber, when the outer modes are excited. The intensity distribution changes spatially in response to the biofilm deposit on the sensor, indicating evanescent field attenuation.

Discharge optimization and the control of edge stability

Discharge optimization for improving MHD stability of both core and edge was essential for the achievement of record fusion power discharges, in the ELM-free hot ion H mode regime, in the recent JET DT operation. The techniques used to increase edge stability are described. In particular the successful technique of current rampdown used to suppress the outer mode is reported. The increased stability of the outer mode by decreasing the edge current density confirms its identification as an n = 1 external kink. Decreasing the plasma current, however, decreases the ELM-free period, which is consistent with stability calculations that show an earlier onset of the ballooning limit. In order to increase external kink stability without causing a deterioration in the ELM-free period, a compromise was achieved by using plasma current rampdown, while working at the highest plasma current values possible. Results from a plasma current scan show that at the time of occurrence of the first giant ELM, the plasma stored energy, as well as the pressure measured at the top of the edge pedestal increase linearly with plasma current, for a given plasma configuration and power. This is consistent with models of the edge transport barrier, where the transport barrier width is proportional to the ion (or fast ion) poloidal Larmor radius. The MHD observations in DT and deuterium only discharges were found to be similar. Thus the experience gained on the control of MHD modes in deuterium plasmas could be fully exploited in the DT campaign.