Dispersion Fluctuations Research Articles

Monitoring FO-PMD and SO-PMD over Time with Respect to Environmental Conditions

Environmental conditions affect the birefringence of an optical fibre which in turn leads to fluctuations in its Polarization mode dispersion (PMD). Fluctuations in PMD make compensation for it in optical fibres very complicated. In this paper, fluctuations of first-order polarization mode dispersion (FO-PMD), FO-PMD coefficients and second-order polarization mode dispersion (SO-PMD) are monitored over time and also with respect to the environmental conditions. Long term measurements of FO-PMD have been done before by Mudau (2008) using the Generalized Interferometry Technique (GINTY) only. However, in this current study the FO-PMD coefficients and SO-PMD fluctuations were also monitored in addition to making a comparison between the measurements obtained using the Single-ended Dispersion Analyzer (FTB-5700) and the GINTY. These measurements were made on a 14.8 km long deployed aerial optical fibre that links St. Albans to Rockland transmission stations in Port Elizabeth, South Africa. The weather information was obtained from the Kestrel 4500 pocket weather tracker that was set up at St. Albans transmission station. So, for good PMD compensation, PMD should be monitored for a very long period of time or use an instrument that can accurately measure it even when the optical fibre is exposed to harsh environmental conditions.

Designing one-pump fiber optical parametric amplifiers for an insensitive amplification to dispersion fluctuations

Small-signal gain spectrum of one-pump fiber optical parametric amplifiers (1-P FOPAs) is investigated in the presence of dispersion fluctuations. By assuming the dispersion fluctuations along the fiber as a stochastic process and using the matrix approach, the full gain spectrum of 1-P FOPAs is simulated. We have examined the gain spectrum of 1-P FOPAs for a wide range of fluctuation parameters, and it is shown for the first time that there are two specific wavelengths at which the dispersion fluctuations play no role. Analytical formulas are also presented to design 1-P FOPAs for amplifying signals with a gain insensitive to dispersion fluctuations.

Preliminary assessment of terrestrial microalgae isolated from lichens as testing species for environmental monitoring: Lichen phycobionts present high sensitivity to environmental micropollutants

Bioassays constitute a tool for pollution analysis providing a holistic approach and high-quality indication of the toxicity. Microbioassays allow evaluating the toxicity of many samples, implying lower costs and enabling routine monitoring and pollution control. But tests conducted so far are limited to the use of a small number of taxa. Lichens are excellent bioindicators of pollution with great ecological significance. Studies show that the phycobiont is more sensitive to pollutants than the mycobiont. Phycobiont have features such as adaptation to anhydrobiosis and relatively rapid growth in vitro, making them suitable for microbioassays. Our aim is to determine the sensitivity of phycobionts to the pharmaceutical micropollutants carbamazepine and diclofenac as a preliminary step for the development of a toxicity microbioassay based on phycobionts. Optical dispersion and chlorophyll autofluorescence were used as endpoints of toxicity on two algal species showing that suspensions present cyclic and taxon specific patterns of aggregation. Trebouxia TR9 suspensions present a very high grade of aggregation while Asterochloris erici cells do not. Both micropollutants alter optical properties of the suspensions of both species. No significant alteration of chlorophyll autofluorescence by carbamazepine is observed. A. erici chlorophyll autofluorescence is extremely sensitive to diclofenac but the effect is not dependent on the drug concentration or on the time of exposure. Differently, TR9 only shows punctual chlorophyll alterations. Fluctuations in optical dispersion may indicate changes in the population structure of the species, including reproductive strategy. A. erici seems more sensitive to micropollutants, is better characterized and is available from commercial collections.

The Impact of Brand Rating Dispersion on Firm Value

This study examines brand dispersion—variance in brand ratings across consumers—and its role in the translation of brand assets into firm value. Dispersion captures the covert heterogeneity in brand evaluations among consumers who like or dislike the brands, which would affect an investor's decision to buy or sell a stock. The higher the dispersion, the more inconsistent and polarized the brands’ cross-consumer ratings. Multiple analyses on 730,818 brand–day observations provide robust evidence that brand dispersion fluctuations affect stock prices. Brand dispersion has Januslike effects: it harms returns but reduces firm risk. Furthermore, downside dispersion has a stronger impact on abnormal returns than upside dispersion, indicating an asymmetry in brand dispersion's effects. Moreover, dispersion tempers the risk-reduction benefits of higher brand rating in both the short run and long run. Without modeling dispersion, brand rating's impact on firm value can be over- or underestimated. Managers should consider dispersion a vital brand-management metric and add it to the brand-performance dashboard.

Phase-preserving parametric wavelength conversion to SWIR band in highly nonlinear dispersion stabilized fiber

The first successful translation of a phase modulated optical signal over 80 THz, from the near infrared to the short-wave infrared (SWIR) band is demonstrated. A signal, phase-modulated at 10 Gbps, was received in an error-free manner in the SWIR(1.7-2.2 μm) band. A new class of highly nonlinear fiber with reduced dispersion fluctuation was utilized as the platform for this phase-preserving distant parametric conversion.

Spatially-Resolved Characterization of Dispersion Flattened Nonlinear Fiber

Spatially resolved dispersion mapping of dispersion flattened highly nonlinear fiber (DF-HNLF) is reported. The characterization of a nearly dispersionless fiber-type is achieved through a generalized counter-colliding pulse method originally developed for conventional, linear-slope HNLF type. A new algorithm is developed in conjunction with the technique and applied to a 160-m-long DF-HNLF segment. The new method is used to identify the fiber part with largest dispersion fluctuations. A combined ability to spatially resolve DF-HNLF type and measure them with high accuracy carries a large practical importance for wideband mixer construction that requires precise phase matching conditions.

20 THz-bandwidth continuous-wave fiber optical parametric amplifier operating at 1 µm using a dispersion-stabilized photonic crystal fiber

We report the experimental demonstration of a continuous-wave all-fiber optical parametric amplifier in the 1 µm band with a record bandwidth of more than 20 THz and a peak gain of almost 40 dB. This is achieved by using a photonic crystal fiber with a high figure of merit and strongly reduced longitudinal dispersion fluctuations. Due to their unique bandwidth and gain characteristics, fiber parametric amplifiers at 1 µm provide an interesting alternative to solid-state or ytterbium-doped fiber amplifiers for ultrafast optical pulse and signal processing.

Dispersion-stabilized highly-nonlinear fiber for wideband parametric mixer synthesis

Conventional highly-nonlinear fiber (HNLF) designs are optimized for high field-confinement but are also inherently susceptible to dispersion fluctuations. The design compromise prevents fiber-optical parametric mixers from possessing high power efficiency and extended operating bandwidth simultaneously. Using a new fiber waveguide design, we have fabricated and tested a new class of HNLF that possesses a significantly lower level of dispersion fluctuations while maintaining a high level of field-confinement comparable to that in conventional HNLFs. The fiber was used to demonstrate an all-fiber parametric oscillator operating in short-wavelength infrared (SWIR) band with a watt-level pump, for the first time.

Temperature independent 80 Gb/s transmission system using spectral phase modulation-based TDC

A temperature independent 80-Gb/s 100-km transmission system is demonstrated with the use of spectral phase modulation-based tunable dispersion compensator (TDC). The principle of dispersion compensation based on spectral phase modulation as well as the relationship between spectral phase modulation function and group velocity dispersion (GVD) are theoretically studied. TDC based on spectral phase modulation is implemented. The performance of 80-Gb/s transmission system is experimentally evaluated. The non-linear relationship between temperature and temperature-induced dispersion fluctuations is demonstrated through the asymmetric temperature-induced power penalty without dispersion compensation. With respect to the low temperature area, the temperature-induced dispersion fluctuations are smaller than those in the high temperature area. By using the proposed TDC, temperature independent 80-Gb/s transmission is successfully demonstrated under a temperature range of –20 – 60 ?C with a power penalty of less than 0.8 dB.

Statistical instability of physical processes

New parameters describing disturbance of statistical stability on a finite observation interval are proposed. Measuring units that allow to quantitatively characterize disturbance of statistical stability for the known and new statistical instability parameters. It is shown that fluctuations of expected value, which lead to changes of mean expected value, and dispersion fluctuations under certain parameters play an essential role in disturbance of statistical stability. Change ranges of sampling statistical instability parameter values weakly depend on the distribution law.

Simulation of Granular Transport of Geldart Type-A, -B, and -D Particles through a 90° Elbow

The flow behavior of three different classes of granular materials - pulverized coal particles (type-A), glass beads (type-B), and polypropylene beads (type-D) - is numerically investigated based on computational fluid dynamics. Three elbow geometries - square and circular (R/D = 1.5 and 3.0) - are employed to investigate turbulent gas-particulate flow in dilute and dilute-to-dense phase modes of pneumatic conveying. The unsteady Eulerian-Discrete phase approach with renormalized group (RNG) k-ε model is adopted, with the effects of particulate phase velocity on the gas flow, turbulent dispersion, lift forces, and particle-wall collisions incorporated in the model. The phenomenon of particle roping is well captured by the computations. Significant phase separation and particle segregation are observed in the vicinity of the lower wall of the pipe. Different classes of particles exhibit very distinct flow dynamics in terms of particle roping, particle segregation, turbulent dispersion, and particle velocity fluctuations. Numerical predictions for pressure drop, particle, and gas velocity profiles are in satisfactory agreement with experimental data from the literature.

Analysis of concentration fluctuations in gas dispersion around high-rise building for different incident wind directions

This article presents experimental results that illustrate the unsteady characteristics of gas dispersion around a complex-shaped high-rise building for different incident wind directions. A series of wind tunnel experiments were conducted using a 1:30 scale model that represented the real structures under study. The objective of this paper is to study the behaviour of concentration fluctuations through transient analysis. Tracer gas was continuously released from a point source located at different positions, and a time series of fluctuating concentrations were recorded at a large number of points using fast flame ionization detectors. The experimental data were analysed to provide a comprehensive data set including variances and associated statistical quantities. Both the unsteady characteristics of the system and their potential practical impact are presented and discussed. Under crowd living conditions, the air pollutant exhausted from one household could probably re-enter into the neighbouring households, traveling with ambient airflow. Such pollutant dispersion process is defined as air cross-contamination in this study. The results indicate that the wind-induced cross-contamination around the studied type of high-rise building should not be overlooked, and the fluctuating concentrations should be paid attention to particularly during the evaluation of a potential contamination risk. This study can help deepen our understanding of the mechanisms of air cross-contamination, and will be useful for implementing optimization strategies to improve the built environments in metropolitan cities such as Hong Kong.

Dispersive pressure and velocity fluctuations in avalanches-Reply to comment by K. Kelfoun and T. Davies on “A random kinetic energy model for rock avalanches: Eight case studies”

Dispersive pressure and velocity fluctuations in avalanches-Reply to comment by K. Kelfoun and T. Davies on “A random kinetic energy model for rock avalanches: Eight case studies”

Ionospheric perturbations in possible association with the 2010 Haiti earthquake, as based on medium-distance subionospheric VLF propagation data

Abstract. Ionospheric perturbations in possible association with the 2010 Haiti earthquake occurred on 12 January 2010 (with a magnitude of 7.0 and depth of 10 km) are investigated on the basis of subionospheric propagation data from the NAA transmitter on the east coast of the USA to a VLF receiving station in Peru. The local nighttime VLF amplitude data are extensively investigated during the period from the beginning of October 2009 to the end of March 2010, in which the trend (nighttime average amplitude), dispersion and nighttime fluctuation are analysed. It is found that a clear precursory ionosphere perturbation is detected just around New Years day of 2010, about 12 days before the main shock, which is characterised by the simultaneous decrease in the trend and the increases in dispersion and nighttime fluctuation. An additional finding might be the presence of the effect of the Earth's tide one and two months before the main shock, which can only be seen for a huge EQ.

High Resolution Measurement of Arbitrary- Dispersion Fibers: Dispersion Map Reconstruction Techniques

New inverse dispersion reconstruction methods are described. Inverse solutions rely exclusively on processing the end-of-fiber observables obtained by a localized four-photon mixing technique. The accuracy of the new methods was verified by solving the inverse problem associated with physical and synthetic dispersion fluctuation maps in high confinement fiber.

Dipolophoresis in large-scale suspensions of ideally polarizable spheres

The nonlinear dynamics of uncharged ideally polarizable spheres freely suspended in a viscous electrolyte in a uniform electric field are analysed using theory and numerical simulations. When a sphere polarizes under the action of the field, it acquires a non-uniform surface charge, which results in an electro-osmotic flow near its surface that scales quadratically with the applied field magnitude. While this so-called induced-charge electrophoresis yields no net motion in the case of a single sphere, it can drive relative motions by symmetry breaking when several particles are present. In addition, Maxwell stresses in the fluid also result in non-zero dielectrophoretic forces, which also cause particle motions. The combination of these two nonlinear electrokinetic effects, termed dipolophoresis, is analysed in detail by using numerical simulations. An efficient simulation method based on our previous analysis of pair interactions is presented and accounts for both far-field and near-field electric and hydrodynamic interactions in the thin-Debye-layer limit, as well as steric interactions using a novel contact algorithm. Simulation results in large-scale suspensions with periodic boundary conditions are presented. While the dynamics under dielectrophoresis alone are shown to be characterized by particle chaining along the field direction, in agreement with previous investigations, chaining is not found to occur under dipolophoresis, which instead causes transient particle pairings and results in a non-uniform microstructure with large number of density fluctuations, as we demonstrate by calculating pair distribution functions and particle occupancy statistics. Dipolophoresis is also found to result in significant hydrodynamic dispersion and velocity fluctuations, and the dependence of these two effects on suspension volume fraction is investigated.

A statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric VLF/LF propagation and earthquakes

The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over 7 yr from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion, and nighttime fluctuation) and earthquakes with magnitude >6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (<40 km). Finally, the mechanism of seismoionospheric perturbations is discussed.

A path integral method for coarse-graining noise in stochastic differential equations with multiple time scales

We present a new path integral method to analyze stochastically perturbed ordinary differential equations with multiple time scales. The objective of this method is to derive from the original system a new stochastic differential equation describing the system’s evolution on slow time scales. For this purpose, we start from the corresponding path integral representation of the stochastic system and apply a multi-scale expansion to the associated path integral kernel of the corresponding Lagrangian. As a concrete example, we apply this expansion to a system that arises in the study of random dispersion fluctuations in dispersion-managed fiber-optic communications. Moreover, we show that, for this particular example, the new path integration method yields the same result at leading order as an asymptotic expansion of the associated Fokker–Planck equation.

La distribución de ingresos y rentas en América Latina durante el siglo XX: un estudio inicial

Latin America has the most unequal income distribution of any region in the world, yet its historical causes are poorly understood. This paper reports the first exploratory attempt to compute income distributions for the five leading Latin American economies for the whole twentieth century. The methodology produces estimates of factor income dispersion for four skill groups over 19002000, which can be used to generate Gini coefficients. Large fluctuations in dispersion over time are found: countering claims of stability since the colonial past in the recent economic institutions literature; but supporting the findings of economic historians and development economists. A set of explanatory variables (reflecting the impact of international trade, labour quality and macroeconomic imbalances) are tested. The resulting estimation model explains the data well, with all three sets of drivers proving significant; although the measured effects are different across the five countries. The paper concludes that the skill composition of the workforce not only underpins long run trends in functional income distribution as conceived by Kuznets; but also conditions fluctuations in response to exogenous shocks.

Optimization of All-Optical 2R Regenerators Operating at 40 Gb/s: Role of Dispersion

We investigate numerically the interplay between dispersion and nonlinearity for optimizing the performance of an all-optical 2R regenerator based on self-phase modulation and spectral filtering at 40 Gb/s. By considering the extent of improvement in the Q factor (related to level of noise reduction), we show that the ratio of accumulated dispersion to the maximum nonlinear phase shift can be used to predict the performance of regenerators making use of fibers with very different lengths, dispersions, and nonlinear parameters. Our results show that fiber dispersion plays an important role and needs to be properly optimized. In general, fibers with larger dispersion perform better but require higher input powers. We also study the impact of fluctuations in dispersion from their nominal value and show that their impact is much less severe when fiber dispersion is relatively small.