Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

Generation of Laguerre-Gaussian modes in Nd:YAG laser using diffractive optical pumping

Abstract: A beam of pulsed laser radiation at the wavelength λ = 0.532 μm was diffracted at a circular diaphragm to produce a hollow-shape intensity distribution in the near field. This light distribution was used to pump an Nd:YAG laser, resulting in the formation of an inversion profile with a minimum at the laser resonator axis and enabling the suppression of the fundamental mode. Under this condition, the oscillation at λ = 1.064 μm of Laguerre-Gaussian modes (LG0m) with low and high values of the azimuthal index m was produced in the gain-switching regime. By changing the geometry of the resonator, the size of its waist parameter was changed, enabling the selection of the mode with an index m that best overlaps with the inversion profile. Oscillation of LG0m modes with indices ranging from m=1 to more than m=200 was obtained. LG0m modes with m ≤ 50 were produced using the dependence of the waist parameter on the length of the resonator near the boundary of its stability region. A LG0m mode of the highest order, m ≈ 240 was obtained by building a miniature laser resonator and using a pair of diaphragms in order to form a sharper ring-shape pumping distribution. Applications of diffractive optical pumping and ``hollow'' LG0m laser beams are discussed.

Probe correction of spherical near-field measurements

Abstract: Probe correction has been implemented in a computer program which calculates antenna far fields from spherical nearfield measurements. The computer program has been applied to near-field measurements on a satellite model, and the accuracy of the computed far field is significantly improved, compared with the results obtained without probe correction.

Near fields of the constant current thin circular loop antenna of arbitrary radius

Abstract: Assuming a known (constant) current distribution on the thin circular loop antenna of arbitrary radius in free space, an exact integration of the vector potential is performed without recourse to approximations. The only restrictions on the solution variables are that the observation point distance must be greater than the loop radius and that the polar angle must run between 0 and /spl pi/. The resulting vector potential infinite series solution possesses a real part composed of linear combinations of complete elliptic integrals of the first and second kind and an imaginary part composed of elementary functions. Thus, it is possible to obtain an exact solution which is valid everywhere that r>a and 0/spl les//spl theta//spl les//spl pi/. The electromagnetic field components of the constant current circular loop antenna are then determined by direct series differentiation. These solutions are valid in the near and induction fields, converging rapidly there, and are also valid in the far field, although many terms of the series are needed for convergence.

Photocurrent mapping of near-field optical antenna resonances

Abstract: A silicon-on-insulator photodetector substrate can be used to produce absolute and quantitative maps of nanoscale optical antenna resonances in the near field.

Microwave holography of large reflector antennas--Simulation algorithms

Abstract: The performance of large reflector antennas can be improved by identifying the location and amount of their surface distortions and then by correcting them. Microwave holography techniques are finding considerable applications as viable tools for performing this task. In these techniques, the complex (amplitude and phase) far-field pattern of the antenna is measured, using a reference antenna. Then, the Fourier transform relationship, which exists between the far field and a function related to the induced current, is invoked to result in the identification of the surface distortions. To critically examine the accuracy of the constructed surface profiles, simulation studies are required to incorporate both the effects of systematic and random distortions, particularly the effects of the displaced surface panels. In this paper, different simulation models are investigated with emphasis given to a model based on the vector diffraction analysis of a curved reflector with displaced panels. The simulated far-field patterns are then used to reconstruct the location and amount of displacement of the surface panels by employing a fast Fourier transform (FFT)/iterative procedure. The sensitivity of the microwave holography technique based on the number of far-field sampled points, level of distortions, polarizations, illumination tapers, etc., is also examined. In addition, the relationships between Az-El and u-v spaces are addressed in the Appendix. Most of the data are tailored to the dimensions of the NASA/JPL Deep Space Network (DSN) 64-m reflector antennas for which the result of a recent measurement is also presented.