1,675,323 research outputs found
Near-field to far-field transition of photonic crystal fibers: symmetries and interference phenomena
The transition from the near to the far field of the fundamental mode
radiating out of a photonic crystal fiber is investigated experimentally and
theoretically. It is observed that the hexagonal shape of the near field
rotates two times by pi/6 when moving into the far field, and eventually six
satellites form around a nearly gaussian far-field pattern. A semi-empirical
model is proposed, based on describing the near field as a sum of seven
gaussian distributions, which qualitatively explains all the observed phenomena
and quantitatively predicts the relative intensity of the six satellites in the
far field.Comment: 7 pages including 6 figures. Animated version of Fig. 5 is available
at http://www.crystal-fibre.com/technology/movie.gi
Phase Space Engineering in Optical Microcavities I: Preserving near-field uniformity while inducing far-field directionality
Optical microcavities have received much attention over the last decade from
different research fields ranging from fundamental issues of cavity QED to
specific applications such as microlasers and bio-sensors. A major issue in the
latter applications is the difficulty to obtain directional emission of light
in the far-field while keeping high energy densities inside the cavity (i.e.
high quality factor). To improve our understanding of these systems, we have
studied the annular cavity (a dielectric disk with a circular hole), where the
distance cavity-hole centers, d, is used as a parameter to alter the properties
of cavity resonances. We present results showing how one can affect the
directionality of the far-field while preserving the uniformity (hence the
quality factor) of the near-field simply by increasing the value of d.
Interestingly, the transition between a uniform near- and far-field to a
uniform near- and directional far-field is rather abrupt. We can explain this
behavior quite nicely with a simple model, supported by full numerical
calculations, and we predict that the effect will also be found in a large
class of eigenmodes of the cavity.Comment: 12th International Conference on Transparent Optical Network
Towards Bridging the Gap between Near and Far-Field Characterizations of the Wireless Channel
The "near-field" propagation modeling of wireless channels is necessary to
support sixth-generation (6G) technologies, such as intelligent reflecting
surface (IRS), that are enabled by large aperture antennas and higher frequency
carriers. As the conventional far-field model proves inadequate in this
context, there is a pressing need to explore and bridge the gap between near
and far-field propagation models. Although far-field models are simple and
provide computationally efficient solutions for many practical applications,
near-field models provide the most accurate representation of wireless
channels. This paper builds upon the foundations of electromagnetic wave
propagation theory to derive near and far-field models as approximations of the
Green's function (Maxwell's equations). We characterize the near and far-field
models both theoretically and with the help of simulations in a line-of-sight
(LOS)-only scenario. In particular, for two key applications in multiantenna
systems, namely, beamforming and multiple-access, we showcase the advantages of
using the near-field model over the far-field, and present a novel scheduling
scheme for multiple-access in the near-field regime. Our findings offer
insights into the challenge of incorporating near-field models in practical
wireless systems, fostering enhanced performance in future communication
technologies.Comment: Published at IEEE International Conference on Communications (ICC)
2024. IEEE Copyright protecte
A new equivalent dipole-moment source reconstruction method for IC radiated emissions based on near-field scanning
In this paper, a new dipole-moment model based on near-field scanning for representing equivalent IC radiation emissions on a PCB (Print Circuit Board) is presented. One set of dipoles (both electric dipoles and magnetic dipoles) are used to characterize near-field and far-field radiation from the source. In order to build the model, tangential electric field and magnetic field on a Huygens\u27s box enclosing the radiation source are needed. Both the phase and magnitude of these tangential fields are obtained either in simulation or in measurement by near-field scanning method. Two steps are established to reconstruct the dipole moment. Firstly, a set of dipole array is used to predict far-field radiation from the source with the least-square method. Then, another set of dipoles which make contributions to the near-field only are added in for accurate calculation of near-field. The dipoles used for predicting far-fields are further distributed into dipole arrays but the value summation of them is equal to each of the dipole used for the far-field matching. All types of dipoles used have the same number and are distributed on the area of the same size and positions. Finally, these two sets of dipole together are able to characterize both near-field and far-field radiation from the source simultaneously. Two examples are used to demonstrate the approach. A U-shape trace model is built in HFSS and the dipole-moment model is validated by simulation. In another example, IC radiation is modeled by dipole-moment and the method is verified by measurement data based on near-field scanning technique. This new dipole-moment model can predict both the near-field and far-field radiation well which is useful in analyzing radio-frequency interference issues --Abstract, page iii
Shape-dependence of near-field heat transfer between a spheroidal nanoparticle and a flat surface
We study the radiative heat transfer between a spheroidal metallic
nanoparticle and a planar metallic sample for near- and far-field distances. In
particular, we investigate the shape dependence of the heat transfer in the
near-field regime. In comparison with spherical particles, the heat transfer
typically varies by factors between 1/2 and 2 when the particle is deformed
such that its volume is kept constant. These estimates help to quantify the
deviation of the actual heat transfer recorded by a near-field scanning thermal
microscope from the value provided by a dipole model which assumes a perfectly
spherical sensor
On the field configuration in magnetic clouds
Interplanetary magnetic clouds are represented by cylindrically symmetric equilibrium solutions of the MHD equations. The radial magnetic pressure gradient of the force free field is balanced by the curvature stress. The field inside is essentially parallel to the cylinder axis, far outside it is oriented in azimuthal direction. These configurations therefore differ from the nonselfconsistent model where the field lines are tightly wound even near the axis
Computer program documentation for a subcritical wing design code using higher order far-field drag minimization
A subsonic, linearized aerodynamic theory, wing design program for one or two planforms was developed which uses a vortex lattice near field model and a higher order panel method in the far field. The theoretical development of the wake model and its implementation in the vortex lattice design code are summarized and sample results are given. Detailed program usage instructions, sample input and output data, and a program listing are presented in the Appendixes. The far field wake model assumes a wake vortex sheet whose strength varies piecewise linearly in the spanwise direction. From this model analytical expressions for lift coefficient, induced drag coefficient, pitching moment coefficient, and bending moment coefficient were developed. From these relationships a direct optimization scheme is used to determine the optimum wake vorticity distribution for minimum induced drag, subject to constraints on lift, and pitching or bending moment. Integration spanwise yields the bound circulation, which is interpolated in the near field vortex lattice to obtain the design camber surface(s)
An Improved dipole-moment model based on near-field scanning for characterizing near-field coupling and far-field radiation from an IC
Radio Frequency Interference (RFI) problems are critical issues in wireless platform design. The accurate noise model of integrated circuits (ICs) is needed to help designers to diagnose and predict RFI problems. In this dissertation, an improved IC radiated emission model based on near-field measurements is proposed. The regularization technique and the truncated SVD method are employed together with the least square method to calculate the dipole moments from the near-field data. This dipole model has clear physical meaning: the electric and magnetic dipoles represent the voltage and the current in the circuit, respectively. One application of this dipole model is the prediction of heat sink radiation. In order to accurately predict the fields excited by a heat sink, an approach is proposed in this paper to include the exact excitation of the heat sink, which is described by some dipole moments constructed from the near-field scanning of the integrated circuit beneath the heat sink. Another contribution of the work is the proposal of effective dielectric properties of layered media for cavity model applications. With the effective properties. the cavity model can be generalized for either parallel plates or metal enclosures containing multiple dielectric layers. In the fourth paper a unified s-parameter (multimode s-parameter) representation for a multiport passive structure is proposed. Both mixed-mode and single-ended s-parameters arc included in the unified representation, which makes it more convenient to characterize structures --Abstract, page iv
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