Near-field Radiation Patterns Research Articles

An efficient sound source determination process based on half-space boundary element method

In this study, sound source localization and identification of a refrigerator were accomplished by using the half-space boundary element method (BEM) based on surface velocity measurements. Surface velocities of the refrigerator were measured by a vibration test set-up; and two main dominant excitation frequencies were determined. The velocity data were fed to half-contact BE model of the refrigerator built in our “in-house” BEM computer code. The code calculated surface acoustic pressure distributions. The surface pressures exposed the location of sound sources of the refrigerator. Near-field sound radiation pattern of the localized sources were obtained by the surface pressure-input to the computer code.

Lasing on higher-azimuthal-order modes in vertical cavity surface emitting lasers at room temperature

We report a direct method to observe higher-azimuthal-order whispering-gallery-like modes in vertical cavity surface emitting lasers (VCSELs) at room temperature. Instead of introducing any defect mode, we show that suppression of lower-order cavity modes can be achieved by destroying vertical reflectors with a surface micro-structure. Perfect higher-order modes with an azimuthal number as large as 41 are observed in experiments through collecting near-field radiation patterns, as well as in numerical simulations.

Surface-Structure-Assisted Chaotic Mode Lasing in Vertical Cavity Surface Emitting Lasers

We demonstrate chaotic mode lasing in vertical cavity surface emitting lasers at room temperature, with an open cavity confined laterally by the native oxide layer. Instead of introducing any defect mode, we show that suppression of lower-order cavity modes can be achieved by destroying vertical reflectors with a surface microstructure. Lasing on chaotic modes is observed directly through collecting near-field radiation patterns. Various vertical emission transverse modes are identified by the spectrum in experiments as well as numerical simulations in real and phase spaces.

Evaluating near-field radiation patterns of commercial antennas

A source reconstruction technique from the measured near fields is proposed to obtain a set of equivalent currents that will characterize the forward and backward radiation patterns of an antenna. Once the equivalent sources are determined, the electromagnetic field at any aspect angle and distance from the antenna can be calculated. In this paper, the method is applied to the evaluation of the radiation from commercial antennas at any observation point. The electric field patterns of a DCS base station antenna at 1800 MHz and a horn antenna at 2500 MHz have been calculated and plotted at several distances from the antenna. This method can be used in characterizing the reference volumes or exclusion zones for transmitting antennas dealing with the maximum levels of electromagnetic radiation safe for human exposure, as stated in many national and international regulations

Fast calculation of pulsed photoacoustic fields in fluids usingk-space methods

Two related numerical models that calculate the time-dependent pressure field radiated by an arbitrary photoacoustic source in a fluid, such as that generated by the absorption of a short laser pulse, are presented. Frequency-wavenumber (k-space) implementations have been used to produce fast and accurate predictions. Model I calculates the field everywhere at any instant of time, and is useful for visualizing the three-dimensional evolution of the wave field. Model II calculates pressure time series for points on a straight line or plane and is therefore useful for simulating array measurements. By mapping the vertical wavenumber spectrum directly to frequency, this model can calculate time series up to 50 times faster than current numerical models of photoacoustic propagation. As the propagating and evanescent parts of the field are calculated separately, model II can be used to calculate far- and near-field radiation patterns. Also, it can readily be adapted to calculate the velocity potential and thus particle velocity and acoustic intensity vectors. Both models exploit the efficiency of the fast Fourier transform, and can include the frequency-dependent directional response of an acoustic detector straightforwardly. The models were verified by comparison with a known analytic solution and a slower, but well-understood, numerical model.

Polarization dependence of light intensity distribution near a nanometric aluminum slit

The near-field radiation pattern of a long thin slit (with a width much smaller than the excitation wavelength) in a uniform aluminum surface was measured and modeled by numerical computation. In particular, the interplay between the incident light polarization and the slit width is found to play an essential role in the near-field profile on the back side of the nanoslits. Two-dimensional finite-difference time-domain computer simulations were performed to calculate the near-field intensity profile for different slit widths and metal thicknesses. This method will allow the optimization of three-dimensional near-field radiation patterns for a variety of near-field molecular scanning schemes.

Base station antenna near-field radiation pattern distortion analysis

When estimating the potential radiation hazard of a base station antenna system, one often relies on the antenna far-field radiation pattern, while the estimation often refers to the antenna near-field zone. It is widely accepted that this leads to overestimation of the field amplitudes. Some recent works suggest the use of modified analytical models to avoid the overestimation problem. However, all of these references refer only to a single antenna in the free space. This work shows that potential near-field radiation pattern distortion due to conductive objects in the close proximity of the antenna (like in multiple- antenna configuration) makes many models inapplicable. In that case, in the directions outside the mainlobe of the antenna, sidelobes are shifted and changed in an unpredictable manner. The nulls are either shifted, or filled so they do not really exist. The mainlobe itself can get an additional gain of a few dB in unpredictable directions. This suggests that even the use of the far-field pattern can sometimes lead to underestimation of the field. Considering that many real antennas have some conductive object nearby, especially in multiple antenna configurations, this work suggests that for truly conservative, worst-case electromagnetic field estimation, far-field pattern should be used for the mainlobe, with the uncertainty of a few dB. For all other directions, the protective envelope should be used as the radiation pattern, with the gain in all directions equal to the highest sidelobe gain. No nulls should be considered to exist in the near field radiation pattern.

Near-field radiation of bow-tie antennas and apertures at optical frequencies.

We investigate the ability of the bow-tie slot antenna to generate intense optical spots below the diffraction limit. A commercially available finite-difference time-domain electromagnetic modelling software is used in the numerical simulations. The finite-difference time-domain software is first compared to analytical results at optical frequencies to verify its accuracy. We then present numerical simulations for various geometries involving apertures on thin films and the bow-tie antenna. The transmission efficiency and optical spot size of the bow-tie antenna are compared with those of rectangular and circular apertures on thin metal films. We also investigate the effects of material composition, frequency, and antenna geometry on the near-field radiation pattern using numerical simulations.

Near-field dipole radiation dynamics through FDTD modeling

We use finite-difference time-domain (FDTD) numerical simulations to study horizontal dipole radiation mechanisms and patterns near half-space interfaces. Time snapshots illustrating propagation of wavefronts at an instance in time are included with antenna patterns to provide a visualization tool for understanding antenna radiation properties. Near-field radiation patterns are compared with far-field asymptotic solutions and the effects of electrical properties, antenna height, and observation distance are investigated through numerical simulations. Numerical simulations show excellent agreement with measured data collected over a water-filled tank. Near-field H-plane radiation patterns are broader and contain radiation maxima beyond the critical angle predicted by far-field solutions. A large amplitude E-plane radiation lobe is located directly below the antenna in all simulations, while the two large amplitude sidelobes are less distinct and occur at larger incidence angles than predicted by far-field solutions. Radiation patterns resemble far-field solutions by a distance of 10 wavelengths, except near the critical angle where H-plane radiation maxima and E-plane sidelobes occur at larger incidence angles than predicted by far-field solutions.

Gaussian-beam analysis of a large adaptive reflector antenna (LAR) using a feed-reflector

A novel approach for analysing the quasi-optical large adaptive reflector Cassegrain system is described. In this system a feed-reflector is used to illuminate a hyperboloid sub-reflector with 5-10 m diameter, located 500 m above the ground. In the proposed method of analysis the feed-reflector aperture field distribution is expanded into a set of Gaussian–Laguerre modes. These modes propagate from the feed-reflector aperture in a simple and well defined way. The feed-reflector near-field radiation pattern is calculated at the subreflector location. The subreflector parameters in this system are found by maximising the LAR aperture efficiency, which includes phase and taper efficiencies, and minimising the LAR spillover loss. This process is computationally more efficient than the physical-optics current-distribution method and more accurate than the ray-tracing approach. It also provides a new insight into the operation of the feed-reflector system.

Generation of tsunamis by a slowly spreading uplift of the sea floor

Generation of tsunamis by slow submarine processes (faulting, slumps or slides) is investigated, in search for possible amplification mechanisms resulting from lateral spreading of the sea floor uplift. A linearized solution for constant water depth is derived by transform methods (Laplace in time and Fourier in space), for sea floor uplift represented by a sliding Heaviside step function (i.e. a simplified Haskell source model, with zero rise time). The model is used to study the tsunami amplitude amplification (wave amplitude normalized by the final sea floor uplift) as a function of the model parameters. The results show that, above the source, the amplification is larger for larger uplifted area and for smaller water depth, and is the largest in the direction of uplift spreading, for velocity of spreading comparable to the long period tsunami velocity. Near the source, this amplification could be one order of magnitude. This amplification mechanism seen in the near-field is a form of wave focusing, and is manifested by a high frequency pulse, with amplitude attenuating with distance due to dispersion and geometric spreading. In the far-field, the linear theory predicts maximum amplification equal to one, as predicted by point source models. An analogy between this form of focusing and resonance of a single-degree-of-freedom oscillator, and near-field radiation patterns are discussed. The magnitudes, seismic movements and source durations of selected earthquakes which generated tsunamis are cited in search of conditions which could lead to slow rupture and unusually large near-field amplification.

Electrooptic mapping and finite-element modeling of the near-field pattern of a microstrip patch antenna

A comprehensive electrooptic field-mapping technique is applied to the characterization of near-field radiation patterns above a microstrip patch antenna. The amplitude and phase maps of three orthogonal electric-field components, measured using electrooptic crystals above the patch, also have revealed the transition from the near field to the far field of the radiation pattern. In addition, experimental results have been compared with a finite-element method (FEM) simulation. The measurememts show superior results to the FEM simulation, especially in terms of spatial resolution and data acquisition times. Furthermore, the scattering parameter S/sub 11/ for the patch antenna has been calculated from the electrooptic measurement results of standing waves on the feeding line and compared with results from a conventional network analyzer.

Near-field distributions and radiation patterns of hollow dielectric wall pyramidal horn antennas

Aperture/near-field distributions and radiation patterns of hollow dielectric wall pyramidal horn antennas have been studied theoretically and experimentally. Solution of the Helmholtz's equation is obtained for the TE to x mode, using the separation of variables technique for the horn, assumed to be made of an equivalent homogeneous dielectric material of dielectric constant equal to the effective dielectric constant of the dielectric slabs and inner free space region bounded by these slabs. Transcendental equations for the transverse components of the propagation constants are obtained by applying proper boundary conditions, i.e. the continuity of the electric and magnetic fields at the dielectric–air boundaries. These transcendental equations are solved for the transverse propagation constants for the fundamental mode for the horn of axial length = 10.2cm, and of angles of flare in the H- and E-planes of 8.5° and 6.5°, respectively, at 9.418GHz in the X-band. The expressions for the near-field components of the horn under study are used to obtain the expression for radiation pattern from Kirchoff's diffraction formula. The theoretical aperture field distributions obtained for the Hx 11 mode propagating in the antenna and 11 radiation patterns are compared with corresponding experimental ones.

Electro-optic near-field mapping of planar resonators

A comprehensive experimental and theoretical study for the determination of the electric near-field above planar resonators is presented. The transverse component of the electric field is mapped by external electro-optic (EO) sampling technique with high spatial and temporal resolution. The evolution of the near-field radiation pattern of the investigated 7-GHz planar resonator to the onset of the far-field pattern is traced by measurements at various heights above the sample. Frequency-dependent measurements allow to characterize the field pattern changes when the frequency is swept through the main resonance. Additional undesired resonances are identified by the detected mode pattern. The experimental data are reproduced by simulations based upon an electric field integral equation (EFIE) method.

GaN-based blue/green semiconductor laser

High-power InGaN single-quantum-well (SQW) structure blue/green light-emitting diodes (LEDs) with an output power of 3-5 mW were fabricated. The continuous-wave operation of bluish-purple InGaN multiquantum-well (MQW)-structure laser diodes (LDs) was achieved at room temperature with a lifetime of 35 h. The threshold current and the voltage of the LD were 80 mA and 5.5 V, respectively. Photocurrent spectra of the InGaN SQW LEDs and MQW LDs were measured at room temperature. The Stokes shifts of the energy difference between the absorption and the emission energy of the blue/green InGaN SQW LED's and MQW LDs were 290, 570, and 190 meV. Both spontaneous and stimulated emission of the LDs originated from this deep localized energy state which is equivalent to a quantum dot-like state. When the temperature or the operating current of the LDs was varied, large mode hopping of the emission wavelength was observed. The carrier lifetime and the threshold carrier density were estimated to be 4.7 ns and 1/spl times/10/sup 20//cm/sup 3/, respectively. The optical confinement factor (/spl Gamma/) of the InGaN MQW LDs was estimated to be 0.025 from the measurement of the near-field radiation patterns.

Electro-optic measurement of the electric near-fielddistribution of 7 GHz planar resonator

Mapping of the electric near-field distribution of a free-running planar 7 GHz resonator with a novel femtosecond laser based electro-optic measurement sytem is demonstrated. The measured near-field radiation pattern gives direct evidence for the presence of parasitic patch and microstrip modes predicted by electric field integral equation simulations.

Losses in Y-junction semiconductor laser arrays

Losses in Y-junction semiconductor laser arrays are shown to affect not only the thresholds and differential efficiencies, but also the array near-field mode patterns, threshold discrimination, and radiation patterns.

Observation of sustained self-pulsation in CW operated flared Y-coupled laser arrays

The time evolution of the near-field radiation patterns of CW operated inphase locked flared ‘Y’ coupled diode laser arrays has been observed with a streak camera. The arrays exhibit sustained self-pulsation at frequencies as high as 4 GHz.

Lateral beam collimation of a phased array semiconductor laser

The lateral near-field and far-field radiation patterns of a phase-locked array of gain-guided semiconductor injection lasers are studied. These lasers exhibit a lateral far-field beam divergence full width at half-maximum (FWHM) of 1.5° up to 200 mW of output power per facet. Greater than 60% of the laser output power is concentrated in a low divergence beam at this high optical power. Catastrophic mirror degradation of the laser occurs at ≊250–270 mW cw per facet.

A series approach for the determination of nearfield radiation patterns of axisymmetric transducers under various conditions of baffle

A new technique has been developed for the theoretical determination of the radiated nearfield pressure and particle velocity distributions of axisymmetric transducers. The approach involves expressing the solution to the appropriate wave equation as a series and using a limited number of terms. A method has been developed for applying mixed boundary conditions to such a series, allowing various baffle configurations to be studied. The principle is inherently simple, and permits a wide range of application. Various predictions of the technique have been compared to those of surface integral approaches, over which the technique has several advantages.