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Lens Antenna Arrays for Coherent THz Cameras

Periodic Reporting for period 4 - LAA-THz-CC (Lens Antenna Arrays for Coherent THz Cameras )

Reporting period: 2020-04-01 to 2021-09-30

• Project Abstract

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. 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.

• Project Conclusions

We have developed a theoretical framework based on Coherent Fourier Optics for deriving the Plane Wave Spectrum in Quasi-Optical Systems. This technique is extremely powerful for the optimization of these systems using field-match techniques as well for modelling periodic structures inside Quasi-Optical systems. The most relevant contributions that came out thanks to this framework and that would have not been possible otherwise are:
• We derived the fundamental trade-offs for absorber based focal plane arrays and compared to well-known antenna trade-offs. These trade-offs were applied to the design of THz imaging security systems
•We developed state-of-art wide-band integrated lens antennas exploiting the field match between the QO field and the field radiated by a leaky wave antenna, including also the complex field propagation through in-lens periodic gratings
•We designed focal plane arrays based on lens antennas with state of the art scanning performance for imaging applications.

Besides the theoretical framework, we performed the following key technological demonstrations that enable the efficient exploitation of the THz spectrum for future application areas in sensing and communications:
• Dynamic beam steering of ultra-narrow beams at >100GHz via phased arrays composed of leaky wave lens antennas integrated with piezo-motors
• CMOS integrated focal plane array based on leaky wave connected arrays for diffraction limited imaging in the 200-600GHz bandwidth.
• Photoconductive antennas with leaky wave enhanced radiation fabricated on a micro-metric LT-GaAS membranes for power generation and detection in the 100GHz-1THz bandwidth.
Modelling of Quasi-Optical Systems:
• Development of a CFO theory with application to the optimization of Quasi-Optical Systems (IEEE TAP paper).
• Development of a software tool based on the CFO made freely available to other researchers working in the area (IEEE AP Magazine).
• Analysis methodology for designing electrically small lenses in the near-field of the feeding antennas (IEEE TAP paper).
• Design of in-lens grating structures for optimized beam-forming performances (IEEE TAP paper).

New Array Antenna Architectures:
• Fly’s Eye Antenna System that can provide 200 times more data rates than current base stations by using fly’s eye lens arrays at sub-THz.
• Scanning Lens Phased Arrays with electrically large array periodicity of actuated lenses. 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. 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.
• 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.
• CMOS Integrated Imaging Arrays: We proposed a new imaging array architecture based on dual-polarized overlapped leaky-wave connected array that leads to an enhanced trade-off between sensitivity and resolution in an imaging camera integrated in CMOS technology

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 lens phased arrays at 500GHz (IEEE THz paper).
• Development of leaky wave lens antennas with in-lens gratings, including circular polarization and enhanced scanning properties, for the Fly’s eye lens scenario at 180GHz (IEEE TAP paper).
• Demonstration of non-dispersive leaky wave based pulsed sources based on membrane LT-GaAs at 100GHz-1THz (IEEE paper in preparation)
• Demonstration of dynamic steering with lens phased arrays at 100GHz and 500GHz (IEEE paper in preparation)
• Development of a CMOS integrated focal plane array based wat 200GHz-600GHz (IEEE paper in preparation).
The use of a Fourier Optics approach for co-analyzing quasi-optical systems together with the integrated front-ends 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 was made available to other researchers working in the field.

The target of this ERC is to develop lens antenna integrated array architectures with multiple directive beams exploiting the THz bandwidth to open the path for future applications in the area of imaging and communications.
Five new architectures were envisioned for multiple applications and presented in international conferences and scientific publications. To demonstrate the potential of these architectures, we developed multiple state of the art lens arrays prototypes at multiple frequencies ranging from 100GHz up to 1THz (detailed in the answer to the previous question).
CFO description
Lens Phased array description
Imaging Array description
FlyeEye description