Abstract-Dispersion-limited versus power-limited terahertz communication links using solid core subwavelength dielectric fibers

                                                               

Kathirvel Nallappan, Yang Cao, Guofu Xu, Hichem Guerboukha, Chahé Nerguizian, and Maksim Skorobogatiy

Schematic of the THz wireless and fiber communication links for reliable and versatile intra-/inter-vehicle communication applications.

https://www.osapublishing.org/prj/fulltext.cfm?uri=prj-8-11-1757&id=441881

Terahertz (THz) band (0.1–10 THz) is the next frontier for ultra-high-speed communication systems. Currently, most of communications research in this spectral range is focused on wireless systems, while waveguide/fiber-based links have been less explored. Although free space communications have several advantages such as convenience in mobility for the end user, as well as easier multi-device interconnectivity in simple environments, fiber-based communications provide superior performance in certain short-range communication applications such as multi-device connectivity in complex geometrical environments (ex., intra-vehicle connectivity) and secure communications with low probability of eavesdropping, as well as secure signal delivery to hard-to-reach or highly protected environments. In this work, we present an in-depth experimental and numerical study of the short-range THz communications links that use subwavelength dielectric fibers for information transmission and define the main challenges and trade-offs in the link implementation. Particularly, we use air or foam-cladded polypropylene-core subwavelength dielectric THz fibers of various diameters (0.57–1.75 mm) to study link performance as a function of the link length of up to ∼10  m$\sim 10\mathrm{m}$, and data bit rates of up to 6 Gbps at the carrier frequency of 128 GHz (2.34 mm wavelength). We find that depending on the fiber diameter, the quality of the transmitted signal is mostly limited either by the modal propagation loss or by the fiber velocity dispersion (GVD). An error-free transmission over 10 m is achieved for the bit rate of 4 Gbps using the fiber of smaller 0.57 mm diameter. Furthermore, since the fields of subwavelength fibers are weakly confined and extend deep into the air cladding, we study the modal field extent outside of the fiber core, as well as fiber bending loss. Finally, the power budget of the rod-in-air subwavelength THz fiber-based links is compared to that of free space communication links, and we demonstrate that fiber links offer an excellent solution for various short-range applications.

© 2020 Chinese Laser Press

Abstract-Additive manufacturing of highly reconfigurable plasmonic circuits for terahertz communications

Yang Cao, Kathirvel Nallappan, Hichem Guerboukha, Guofu Xu, and Maksim Skorobogatiy

(a) Schematic of the two-wire WBG (top) and a photo from the top with half of the cage removed (bottom). (b) (I) Schematic of a single period of the WBG that comprises two sections, one containing a blank paper sheet, and the other containing metallized paper with the corresponding transverse cross-sections shown in (II) and (III). The electric field distributions (|𝐸|$|E|$) of principal modes propagating in Section 1–Mode 1 (IV), and Section 2–Mode 2 (V), and Mode 3 (VI) at 140 GHz. (c) Simulated modal electric field distribution (|𝐸|$|E|$) of a WBG in the symmetry plane in the mid gap between the two wires at 140 GHz. (d) Simulated power transmission |S21|2$|{\mathrm{S}21|}^2$ and reflection |S11|2$|{\mathrm{S}11|}^2$ coefficients for WBGs with different number of periods. (e) Experimentally measured transmittance of the paper/metal WBGs of different lengths inserted into a 10 cm-long two-wire waveguide. Transmittance is computed by dividing the grating transmission spectra (by field) by that of an empty two-wire waveguide (reference).

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-9-1112

While in most existing terahertz communications systems, the THz carrier wave is transmitted via free-space channels, the THz waveguide-based integrated solutions can be of great utility at both the transmitter and receiver ends, thus simplifying the miniaturization and mass production of cost-effective THz communications systems. Here we present a new type of modular THz integrated circuits based on the two-wire plasmonic waveguide components fabricated using a combination of stereolithography (SLA) 3D printing, wet chemistry metal deposition, and hot stamping techniques. Particular attention is paid to the design of the optical circuits based on the two-wire waveguides suspended inside a protective micro-sized enclosure. Such waveguides feature low transmission and bending losses, as well as low dispersion. Using such waveguides as basic building blocks, we then demonstrate several key optical subcomponents, such as low-loss broadband 2×1THz$2\times 1\mathrm{T}\mathrm{H}\mathrm{z}$ couplers that use two coalescing two-wire waveguide bends, as well as broadband waveguide Bragg gratings that feature a paper sheet with a periodic sequence of metal strips inserted into the air gap of a two-wire waveguide. Finally, using these developed subcomponents, a two-channel add-drop multiplexer is demonstrated to operate at 140 GHz. We believe that the reported micro-encapsulated two-wire waveguide-based modular platform can have a strong impact on the field of THz signal processing and sensing due to the ease of device fabrication and handling, high degree of reconfigurability, and high potential for real-time tunability.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Increasing Reliability of Terahertz Communication Links Using Onboard Fiber Connectivity

Kathirvel Nallappan, Yang Cao, Guofu Xu, Hichem Guerboukha,  Chahe Nerguizian, Maksim Skorobogatiy

https://ieeexplore.ieee.org/document/9031199

Terahertz (THz) band is the next frontier for the ultra-high-speed communication systems. Currently, most of communications research in this spectral range is focused on wireless systems, while waveguide/fiber-based links have been less explored. Although free space communications have several advantages, the fiber-based communications provide superior performance in certain short-range communication applications. In this work, we study the use of subwavelength dielectric THz fibers for information transmission. Particularly, we use polypropylene-based rod-in-air subwavelength dielectric THz fibers of various diameters (0.57-1.75 mm) to study link performance as a function of the link length of up to ∼10 m, and data bitrates of up to 6 Gbps at the carrier frequency of 128 GHz. Furthermore, we compared the power budget of the rod-in-air subwavelength THz fiber-based links to that of free space communication links and we demonstrate that fiber links offer an excellent solution for various short-range applications.

Abstract-Planar Porous Components for Low‐Loss Terahertz Optics

Hichem Guerboukha,   Kathirvel Nallappan,   Yang Cao,   Mohamed Seghilani,  Jose Azaña,   Maksim Skorobogatiy

https://onlinelibrary.wiley.com/doi/10.1002/adom.201900236

There is a strong interest in using the terahertz (THz) frequency band for applications in sensing, imaging, and wireless communications. To enable many of these applications, compact low‐loss components for beamforming are required. Typically, such components are made using solid dielectric elements with spatially variable thickness, for example, planoconvex lenses or spiral phase plates. However, as losses in dielectrics typically greatly increase with THz frequency, so do the losses of the solid components. This work demonstrates that when introducing low‐refractive index, low‐loss subwavelength inclusions (air holes) into a solid material matrix, the loss of porous components can be greatly reduced compared to the loss of solid components with otherwise identical optical properties, thus opening a way to create efficient optical components even with nominally high‐loss materials. Additionally, porous optical components can be created completely flat as spatially dependent optical path difference is achieved by varying the local porosity rather than the component thickness. This offers additional advantages for free‐space alignment and integration of such components into optical systems. As an example, the design, fabrication, and experimental characterization of planar lenses and planar orbital angular momentum phase plates are carried out. It is then demonstrated how these porous components outperform their all‐solid counterparts.

Abstract-Toward real-time terahertz imaging

Hichem Guerboukha, Kathirvel Nallappan, and Maksim Skorobogatiy



https://www.osapublishing.org/aop/abstract.cfm?uri=aop-10-4-843

Terahertz (THz) science and technology have greatly progressed over the past two decades to a point where the THz region of the electromagnetic spectrum is now a mature research area with many fundamental and practical applications. Furthermore, THz imaging is positioned to play a key role in many industrial applications, as THz technology is steadily shifting from university-grade instrumentation to commercial systems. In this context, the objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution imaging. To this end, we first discuss several key imaging modalities developed over the years: THz transmission, reflection, and conductivity imaging; THz pulsed imaging; THz computed tomography; and THz near-field imaging. Then, we discuss several enabling technologies for real-time THz imaging within the time-domain spectroscopy paradigm: fast optical delay lines, photoconductive antenna arrays, and electro-optic sampling with cameras. Next, we discuss the advances in THz cameras, particularly THz thermal cameras and THz field-effect transistor cameras. Finally, we overview the most recent techniques that enable fast THz imaging with single-pixel detectors: mechanical beam-steering, compressive sensing, spectral encoding, and fast Fourier optics. We believe that this critical and comprehensive review of enabling hardware, instrumentation, algorithms, and potential applications in real-time high-resolution THz imaging can serve a diverse community of fundamental and applied scientists.

© 2018 Optical Society of America

Abstract-Exploiting k-space/frequency duality toward real-time terahertz imaging

Hichem Guerboukha, Kathirvel Nallappan, and Maksim Skorobogatiy

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-2-109

Imaging at terahertz frequencies has recently received considerable attention because many materials are semitransparent to THz waves. The principal challenge that impedes a widespread use of THz imaging is the slow acquisition time of a conventional point-by-point raster scan. In this work, we present a theoretical formulation and an experimental demonstration of a novel technique for fast compressionless terahertz imaging based on broadband Fourier optics. The technique exploits k-vector/frequency duality in Fourier optics that allows the use of a single-pixel detector to perform angular scans along a circular path, while the broadband spectrum is used to scan along the radial dimension in Fourier domain. The proposed compressionless image reconstruction technique (hybrid inverse transform) requires only a small number of measurements that scales linearly with an image’s linear size, thus promising real-time acquisition of high-resolution THz images. Additionally, our imaging technique handles equally well and on an equal theoretical footing amplitude contrast and phase contrast images, which makes this technique useful for many practical applications. A detailed analysis of the technique’s advantages and limitations is presented, and its place among other existing THz imaging techniques is clearly identified.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Exploiting k-space - frequency duality in Fourier optics towards real-time compression-less terahertz imaging

Hichem Guerboukha, Kathirvel Nallappan, Maksim Skorobogatiy

(Submitted on 24 Aug 2017)

https://arxiv.org/abs/1708.07528

We present theoretical formulation and experimental demonstration of a novel technique for the fast compression-less terahertz imaging based on the broadband Fourier optics. The technique exploits k-vector/frequency duality in Fourier optics which allows using a single-pixel detector to perform angular scan along a circular path, while the broadband spectrum is used to scan along the radial dimension in Fourier domain. The proposed compression-less image reconstruction technique (hybrid inverse transform) requires only a small number of measurements that scales linearly with the image linear size, thus promising real-time acquisition of high-resolution THz images. Additionally, our imaging technique handles equally well and on the equal theoretical footing the amplitude contrast and the phase contrast images, which makes this technique useful for many practical applications. A detailed analysis of the novel technique advantages and limitations is presented, as well as its place among other existing THz imaging techniques is clearly identified

Abstract-Channel Multiplexing in Wireless Terahertz Communications Using Orbital Angular Momentum States

Kathirvel Nallappan, Hichem Guerboukha, Mohamed Seghilani, José Azana, Chahé Nerguizian, Maksim Skorobogatiy

(Submitted on 26 Aug 2017)

https://arxiv.org/abs/1708.08005

We present design and experimental validation of the system for the generation of the Orbital Angular Momentum (OAM) states using 3D-printed low-loss metamaterial phase plates for application in the terahertz (THz) wireless communications. By azimuthally varying the hole pattern density within the phase plate, the local effective refractive index is varied, thus also changing the local propagation constant in the azimuthal direction. The OAM of any topological charge can be created by simply varying the thickness of the phase plate. The phase plate with topological charge (m=1) is 3D printed and the amplitude and the phase of the terahertz signal after passing the plate is characterized using the THz-time domain imaging system. Finally, we present the experimental setup and theoretical simulation on the multiplexing and de-multiplexing of several different OAM states for applications in wireless terahertz communication.

Abstract-Transmission of Live Uncompressed 4K Video Using Terahertz Wireless Communication System

Kathirvel Nallappan, Hichem Guerboukha, Chahé Nerguizian, Maksim Skorobogatiy

https://arxiv.org/abs/1708.07721

(Submitted on 24 Aug 2017)https://arxiv.org/abs/1708.07721

We present the design and integration of Photonics based Terahertz wireless communication system using all commercially available system components. The performance of the developed system is then characterized by measuring the Bit error rate (BER) at the data rate of 5.5 Gbps as a function of communication link distance, angular rotation of the detector and bias voltage of the photomixer. With optimized threshold an error free transmission of 5.5 Gbps has been achieved over the link distance of 30 cm. Finally, the design for the integration of 4K camera with the THz communication system is discussed and the transmission of live uncompressed 4K video has been demonstrated successfully.

Abstract-A Dynamically Reconfigurable Terahertz Array Antenna for Near-field Imaging Applications

Kathirvel Nallappan, Jingwen Li, Hichem Guerboukha, Andrey Markov, Branko Petrov, Denis Morris, Maksim Skorobogatiy

(Submitted on 30 May 2017)

https://arxiv.org/abs/1705.10624

A proof of concept for high speed near-field imaging with sub-wavelength resolution using SLM is presented. An 8 channel THz detector array antenna with an electrode gap of 100 um and length of 5 mm is fabricated using the commercially available GaAs semiconductor substrate. Each array antenna can be excited simultaneously by spatially reconfiguring the optical probe beam and the THz electric field can be recorded using 8 channel lock-in amplifiers. By scanning the probe beam along the length of the array antenna, a 2D image can be obtained with amplitude, phase and frequency information.

Abstract-3D printed hollow core terahertz Bragg waveguides with defect layers for surface sensing applications

Jingwen Li, Kathirvel Nallappan, Hichem Guerboukha, and Maksim Skorobogatiy

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-4126

We study a 3D-printed hollow core terahertz (THz) Bragg waveguide for resonant surface sensing applications. We demonstrate theoretically and confirm experimentally that by introducing a defect in the first layer of the Bragg reflector, thereby causing anticrossing between the dispersion relations of the core-guided mode and the defect mode, we can create a sharp transmission dip in the waveguide transmission spectrum. By tracking changes in the spectral position of the narrow transmission dip, one can build a sensor, which is highly sensitive to the optical properties of the defect layer. To calibrate our sensor, we use PMMA layers of various thicknesses deposited onto the waveguide core surface. The measured sensitivity to changes in the defect layer thickness is found to be 0.1 GHz/μm. Then, we explore THz resonant surface sensing using α-lactose monohydrate powder as an analyte. We employ a rotating THz Bragg fiber and a semi-automatic powder feeder to explore the limit of the analyte thickness detection using a surface modality. We demonstrate experimentally that powder layer thickness variations as small as 3μm can be reliably detected with our sensor. Finally, we present a comparative study of the time-domain spectroscopy versus continuous wave THz systems supplemented with THz imaging for resonant surface sensing applications.

© 2017 Optical Society of America

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