The problem of nonlinear self-focusing of Gaussian laser beams is reformulated in terms of a variational principle. By means of approximating Gaussian functions, expressions are obtained for the equilibrium radii and nonlinear frequency shifts of stationary self-trapped laser beams. The nonsteady propagation is given an illuminating form in terms of a potential function description. The analysis confirms the recent results obtained by moment theory as opposed to those based on paraxial ray approximations.
The GRAS instrument on the Metop-A satellite provides more than 600 radio occultation measurement profiles per day. The instrument is characterized by its wide antenna coverage, high signal-to-noise ratio and an ultra-stable clock reference. The conventional dual-frequency tracking of GPS signals is under dynamic atmosphere conditions complemented by open loop tracking with sampling of the signal at a 1 kHz rate, providing an unprecedented view of the signal spectral environment. This paper presents the instrument performance as derived from analysis of in-orbit measurement data. We show that the noise figure is low enough to enable mapping of external radio noise variations over the earth's surface. An error propagation model is presented to relate instrument characteristics to bending angle performance. This model is also used to illustrate the relation between filter bandwidth, resolution and measurement noise. The Doppler model, guiding open loop measurements, is found to be accurate to better than 20 Hz with a possibility for improvement to 10 Hz. The high performance at low altitudes enables the presence of surface reflections at the -20-dB level to be identified in more than 50% of the occultations. The potential performance improvements for next generation receivers are discussed.
Leaky wave enhanced feed arrays for the improvement of the edge of coverage gain in multibeam reflector antennasLlombart, N.; Neto, A.; Gerini, G.; Bonnedal, M.; de Maagt, P.J.I.
Abstract-The impact of mutual coupling between neighboring radiators in an imaging array configuration in the presence of a dielectric super-layer is investigated. The super-layer generally aims at increasing the directivity of each element of the array. However, here it is shown that the directivity of the embedded element patterns are reduced by a high level of mutual coupling. Thus a trade off between directivity enhancement and close packing of the array elements must be found depending on the bandwidth and the pattern requirements.Index Terms-Electromagnetic bandgap (EMB) materials, leaky waves (LWs), leaky wave (LW) antennas, reflector antenna feeds.
The radial intensity profile and nonlinear wavenumber shift of an electromagnetic self-trapped cylindrical beam of radiation propagating in a nonlinear medium is investigated analytically using a variational technique involving trial functions. Good agreement is obtained with previous numerical results.
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