Imaging of dipole sources and transfer of modulated signals using phase conjugating lens techniques

The authors discuss the imaging properties and transfer of amplitude and phase-modulated signals through a phase conjugating lens (PCL). The authors outline the mechanisms of the near-field and far-field subwavelength imaging of Hertzian dipole sources using PCL, particularly the authors show that one-dimensional subwavelength resolution of multiple sources is possible in the far-field using a PCL augmented with specially designed scatterers located in both the adjacent vicinity of the sources and in the mirror symmetric positions in the image plane. These scatterers enable evanescent-to-propagating spectrum and its dual, propagating-to-evanescent, field conversion. Thus, the subwavelength information encoded into propagating waves on the source side can be extracted on the image side. Next, for the first time the transfer of amplitude and phase modulated signals through a PCL augmented with evanescent-to-propagating spectrum conversion is discussed and it has been demonstrated that multiple amplitude or phase modulated dipole sources can be distinguished in the far-field with subwavelength resolution without the necessity for numerical post-processing of the received data. From the study conducted here, it is concluded that a system of transmitters/receivers augmented with a PCL and appropriate scatterers operates without the need for any numerical processing of the receive data in order to separate channel information from very close proximity stations.

Active Phase Conjugating Lens With Sub-Wavelength Resolution Capability

Experimental results are presented for the focusing capability of an active phase conjugating lens for a single and a dipole source pair and these are compared with predictions. In addition for a single source we illustrate the ability of the lens to project a null at the lens focus instead of a peak. A scheme is also presented such that when a source or pair of sources is imaged through an identical pair of passive scatterers located symmetrically about the lens that imaging with sub-wavelength resolution is possible. The rationale for the operation of the lens and aberrations observed due to its finite array size is discussed and is supported throughout by means of numerical simulation.

Evaluation of effective groundwater recharge of freshwater lens in small islands by the combined modeling of geoelectrical data and water heads

In small islands, a freshwater lens can develop due to the recharge induced by rain. Magnitude and spatial distribution of this recharge control the elevation of freshwater and the depth of its interface with salt water. Therefore, the study of lens morphology gives useful information on both the recharge and water uptake due to evapotranspiration by vegetation. Electrical resistivity tomography was applied on a small coral reef island, giving relevant information on the lens structure. Variable density groundwater flow models were then applied to simulate freshwater behavior. Cross validation of the geoelectrical model and the groundwater model showed that recharge exceeds water uptake in dunes with little vegetation, allowing the lens to develop. Conversely, in the low-lying and densely vegetated sectors, where water uptake exceeds recharge, the lens cannot develop and seawater intrusion occurs. This combined modeling method constitutes an original approach to evaluate effective groundwater recharge in such environments.
[Comte, J.-C., O. Banton, J.-L. Join, and G. Cabioch (2010), Evaluation of effective groundwater recharge of freshwater lens in small islands by the combined modeling of geoelectrical data and water heads, Water Resour. Res., 46, W06601, doi:10.1029/2009WR008058.]

Far field subwavelength source resolution using Phase Conjugating Lens Assisted With Evanescent-to-Propagating Spectrum Conversion

The imaging properties of a phase conjugating lens operating in the far field zone of the imaged source and augmented with scatterers positioned in the source near field region are theoretically studied in this paper. The phase conjugating lens consists of a double sided 2D assembly of straight wire elements, individually interconnected through phase conjugation operators. The scattering elements are straight wire segments which are loaded with lumped impedance loads at their centers. We analytically and numerically analyze all stages of the imaging process; i) evanescent-to-propagating spectrum conversion; ii) focusing properties of infinite or finite sized phase conjugating lens; iii) source reconstruction upon propagating-to-evanescent spectrum conversion. We show that the resolution that can be achieved depends critically on the separation distance between the imaged source and scattering arrangement, as well as on the topology of the scatterers used. Imaged focal widths of up to one-seventh wavelength are demonstrated. The results obtained indicate the possibility of such an arrangement as a potential practical means for realising using conventional materials devices for fine feature extraction by electromagnetic lensing at distances remotely located from the source objects under investigation

Frequency selective surface using nested split ring slot elements as a lens with mechanically reconfigurable beam steering capability

The design is described of a double layer frequency selective surface which can produce a differential phase shift of 180 ° as the wave propagates through it at normal incidence. The hand of an applied circularly polarized signal is reversed due to the 180° phase shift, and it is demonstrated that the exit circularly polarized output signal can be phase advanced or phase retarded by 180 ° upon rotation of the elements comprising the structure. This feature allows the surface to act as a spatial phase shifter. In this paper the beam steering capabilities of a 10 × 10 array of such elements are demonstrated. Here the individual elements comprising the array are rotated relative to each other in order to generate a progressive phase shift. At normal incidence the 3 dB Axial Ratio Bandwidth for LHCP to RHCP conversion is 5.3% and the insertion loss was found to be -2.3 dB, with minimum axial ratio of 0.05 dB. This array is shown to be able to steer a beam from -40 ° to +40 ° while holding axial ratio at the pointing angle to below 4 dB. The measured radiation patterns match the theoretical calculation and full-wave simulation results. © 2010 IEEE.

Retrodirective Rotman Lens Constraining Factors

The mechanism by which a retrodirective Rotman lens operates is examined theoretically and prediction is compared with measurement. By deriving the reflection matrix based on the phase delay relationship between the beam ports and the array ports we show that if the phase delay difference between neighbouring ports is constrained in a particular way that the reflection matrix becomes an inverse diagonal matrix and the Rotman lens functions as a Van Atta Array hence can perform retrodirective reflection. Further, the primary factors governing the bandwidth and beam pointing error of the lens are elaborated.

Rotman lens based-retrodirective array

A new type of broadband retrodirective array, which has been constructed using a microstrip Rotman lens, is presented. Automatic tracking of targets is obtained by exploiting the conjugate phase response of the beamforming network which is exhibited when the input ports are terminated with either open or short circuits. In addition, the true time-delay property of the Rotman lens gives broadband operation of the self-tracking array when used in conjunction with Vivaldi antennas. The simulated and measured bistatic and monostatic radar cross-section (RCS) patterns of a structure consisting of 13 beamports and 12 array ports are presented at frequencies in the range 8-12 GHz. Significantly enhanced RCS within the scan coverage ±40° is demonstrated by comparing the retrodirective behavior of a 12-element Vivaldi array terminated with and without the Rotman lens. © 2006 IEEE.