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-Reduction degree regulated room-temperature terahertz direct detection based on fully suspended and low-temperature thermally reduced graphene oxides

Yang Cao, Yajing Zhao, Yingxin Wang, Yue Zhang, Jianguo Wen, Ziran Zhao, Lianqing Zhu,

https://www.sciencedirect.com/science/article/pii/S0008622318311540#!

A series of fully-suspended reduced graphene oxide (RGO) room-temperature THz detectors were fabricated based on low-temperature (from 100 to 350 °C) thermally-reduced free-standing graphene oxide (GO) thin films. The suspended configuration results in a four-fold increase in responsivity and at least one order of magnitude increase in response speed compared to the substrate-supported detector. More importantly, the responsivity can be adjusted over a wide range from 10−2–102 mA W−1 and simultaneously the response speed can be adjusted on the order of tens of milliseconds by only tuning the reduction temperature of GO namely the reduction degree of GO. The regulation mechanism was revealed at the molecular level, i. e., the content of C=O functional group and the O/C ratio inside RGO, which are vary with the reduction degree of GO, are closely related to THz absorbance and electrical conductivity of RGO thin films, respectively. The experimental results demonstrated that the as high as possible content of C=O functional group and simultaneously a moderate O/C ratio can achieve optimal synergy between the THz absorption and electrical conductivity of the RGO thin films, thereby achieving an optimal THz detection performance.