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Lens Antenna Arrays for Coherent THz Cameras

Periodic Reporting for period 3 - LAA-THz-CC (Lens Antenna Arrays for Coherent THz Cameras )

Reporting period: 2018-10-01 to 2020-03-31

The THz region was limited to applications in radio astronomy and space science. In recent years, THz systems have expanded into many more areas of science, defence, security, and non-destructive industrial applications. Microwave based THz cameras have demonstrated the highest sensitivity at large distances. However, their current state of the art is comparable to the first analog photographic cameras characterized by long exposition times. Two fundamental problems have to be addressed to change this situation: technologically, there is the lack of integrated coherent arrays with high power and sensitivity; and theoretically, a field representation to characterize analytically these systems is missing.

I propose to tackle the technological problem by exploiting the coherency between small lens antenna arrays coupled to actuated lenses to overcome the sensitivity problem and achieve instantaneous refocusing (i.e. zooming). The proposed antenna technology is based on a recent breakthrough that I pioneered: micro-lenses excited by leaky waves with seamless integration in silicon technology. This antenna enables the fabrication of large fly’s eye cameras in just two wafers, and promises one order of magnitude better scanning performances than previous solutions. An analytical model to investigate the electromagnetic response of coherent THz arrays is the enabling tool for optimizing the camera performances. I will develop this tool by combining advance spectral antenna techniques with coherent
Fourier Optics. This model will not only be used in new beamforming techniques, but also for the characterization of future THz telecommunication links.

This project will make the first significant strides in developing the next generation of coherent THz imaging cameras. The outcome of this project will be instrumental in pushing today's costly THz niche applications into the main stream, and possibly pole vault THz systems into the 21st century communication society.
Modelling of Quasi-Optical Systems:
• Development of a Coherent Fourier Optics (CFO) theory for deriving the Plane Wave Spectrum in Quasi-Optical Systems.
• Application of the CFO for the optimization of Quasi-Optical Systems in Reception
• Development of a software tool based on this theory with a set of key canonical problems that will be made freely available to other researchers working in the area.
• Extension of the Coherent Fourier Optics theory for the analysis of multi-cascade optical components, in particular, for Fly’s eye Focal Plane arrays: a transactions papers is under review
• Analysis methodology for designing electrically small lenses in the near-field of the feeding antennas.
• Analysis of periodic and non-periodic grating structures, embedded in lens antennas, for the optimization of the beam-forming performances.
• Application of the PWS theory to a more general problem on the observable fields by antennas in complex scattering environments. .

New Coherent Array Antenna Architectures:
• Fly’s Eye Antenna System for Future Tbps Wireless Communications:
The requirements for wireless data streaming are exponentially increasing today with nearly everyone possessing a smartphone or similar device packed with every sort of app to stay in contact and share one’s life with the world. Nowadays, these demands cannot be met in densely populated environments such a sport stadium. We have envisioned a solution that can provide 200 times more data rates by using a base station made of fly’s eye lenses at THz that can make 80000 user connections simultaneously.
• Scanning Lens Phased Arrays:
The use of a small number of lens antennas as active phased array elements allows for electrically large array periodicity. This strategy facilitates the array integration of submillimeter-wave active technology, while exploiting the lens steering capabilities to achieve the desired beamforming, and avoiding grating lobes. An electronic phase shifting between the high frequency signals enables the steering of the array factor. The multiple grating lobes present in such array pattern can be mitigated by the intrinsic true-time delay steering properties of the lenses. By introducing a micro-metric lateral translation of the lens array layer via piezo-electric motor, the element lens pattern can be steered freely in a wide angular range leading to the dynamic steering of a very high directly beams. This new lens array approach is capable to achieve a considerable low-profile and low-cost solution, since the required lenses have a much lower profile than a single lens with an equivalent directivity.
• Pulsed THz Radars:
A mm range resolution Imaging Rardar, based on coherent focal plane arrays embedded in dense dielectrics, has been envisioned exploiting LT-GaAS technology. A white paper explaining the main antenna architecture and expected SNR has been written.
• Wide Field of View Focal Plane Arrays:
Exploiting the new CFO technique developed in this ERC, we can now properly shaped the lenses in a Fly's eye leaky wave array to increase the field of views in near-field security imagers from 2500 pixels to 10000 pixels with out compromising the image quality.

Proof of concept demonstrators:
• Demonstration of the integration of piezo-motors with a new leaky-wave antenna using artificially generated dielectrics in silicon for enabling wide beam steering in phased arrays.
• Design, fabrication and characterization of 40% leaky wave feeds, including circular polarization, for the Fly’s eye lens scenario.
• Demonstration of high power levels (>1mW) of antenna array based pulsed sources on LT-GaAs in synergy with another ERC grant, AATTSI.
The use of a Fourier Optics approach for co-analyzing quasi-optical systems together with the detectors has been applied, for the first time, to THz imaging cameras. The proposed approach leads to one order faster computation time that standard techniques. Moreover since it is based on having the fields expanded in terms of plane waves gives the designer a clear physical picture of the problem. The proposed theory is finalized, a free accessible tool will be provided to other researchers working in the field.

The target of this ERC is to develop coherent antenna architectures with multiple directive beams exploiting the integrated lens arrays that can open the path for future applications in the area of imaging and communications.
Four new architectures have been already presented in international conferences. The coming period will be dedicated to demonstrate the potential of these architectures with several proof of concept prototypes.