arXiv subjects

Kathirvel Nallappan

Publications and source records attributed to Kathirvel Nallappan.

9 recordsLinked to original sources

arXiv2021

Add drop multiplexers for terahertz communications using two-wire waveguide based plasmonic circuits

Terahertz (THz) band is considered as the next frontier in wireless communications. The emerging THz multiplexing techniques are expected to dramatically increase the information capacity of THz communications far beyond a single channel limit. In this work, we explore the THz frequency-division multiplexing modality enabled by novel add-drop multiplexer (ADM) design. Based on modular two-wire plasmonic waveguides fabricated using additive manufacturing and metallization techniques, we demonstrate four-port THz ADMs containing grating-loaded side couplers for operation at ~140 GHz carrier frequency. Particular attention is payed to the design of plasmonic waveguide Bragg gratings and directional couplers capable of splitting broadband THz light in spectral and spatial domains, respectively. Finally, we demonstrate multiplexing and demultiplexing of THz signals with bit rates up to 6 Gbps using the developed ADMs. We believe that proposed plasmonic circuits hold strong potential to provide robust integrated solutions for analogue signal processing in the upcoming THz communications.

physics.app-ph↗

arXiv2021

Microstructured fiber links for THz communications and their fabrication using infinity printing

In this work, a novel infinity 3D printing technique is explored to fabricate continuous multi-meter-long low-loss near-zero dispersion suspended-core polypropylene fibers for application in terahertz (THz) communications. Particular attention is paid to process parameter optimization for 3D printing with low-loss polypropylene plastic. Three microstructured THz fibers were 3D printed using the standard and infinity 3D printers, and an in-depth theoretical and experimental comparison between the fibers were carried out. Transmission losses (by power) of 4.79 dB/m, 17.34 dB/m and 11.13 dB/m are experimentally demonstrated for the three fibers operating at 128 GHz. Signal transmission with BER far below the forward error correction limit (10-3) for the corresponding three fiber types of lengths of 2 m, 0.75 m and 1.6 m are observed, and an error-free transmission is realized at the bit rates up to 5.2 Gbps. THz imaging of the fiber near-field is used to visualize modal distributions and study optimal fiber excitation conditions. The ability of shielding the fundamental mode from the environment, mechanical robustness and ease of handling of thus developed effectively single-mode high optical performance fibers make them excellent candidates for upcoming fiber-assisted THz communications. Additionally, novel fused deposition modeling (FDM)-based infinity printing technique allows continuous fabrication of unlimited in length fibers of complex transverse geometries using advanced thermoplastic composites, which, in our opinion, is poised to become a key fabrication technique for advanced terahertz fiber manufacturing.

physics.optics↗

arXiv2020

Dispersion Limited versus Power Limited Terahertz Transmission Links Using Solid Core Subwavelength Dielectric Fibers

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 main challenges and tradeoffs 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, and data bitrates 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 meters 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.

physics.app-ph↗

arXiv2019

Super-resolution Orthogonal Deterministic Imaging Technique for Terahertz Subwavelength Microscopy

Terahertz subwavelength imaging aims at developing THz microscopes able to resolve deeply subwavelength features. To improve the spatial resolution beyond the diffraction limit, a current trend is to use various subwavelength probes to convert the near-field to the far-field. These techniques, while offering significant gains in spatial resolution, inherently lack the ability to rapidly obtain THz images due to the necessity of slow pixel-by-pixel raster scan and often suffer from low light throughput. In parallel, in the visible spectral range, several super-resolution imaging techniques were developed that enhance the image resolution by recording and statistically correlating multiple images of an object backlit with light from stochastically blinking fluorophores. Inspired by this methodology, we develop a Super-resolution Orthogonal Deterministic Imaging (SODI) technique and apply it in the THz range. Since there are no natural THz fluorophores, we use optimally designed mask sets brought in proximity with the object as artificial fluorophores. Because we directly control the form of the masks, rather than relying on statistical averages, we aim at employing the smallest possible number of frames. After developing the theoretical basis of SODI, we demonstrate the second-order resolution improvement experimentally using phase masks and binary amplitude masks with only 8 frames. We then numerically show how to extend the SODI technique to higher orders to further improve the resolution. As our formulation is based on the equation of linear imaging and it uses spatial modulation of either the phase or the amplitude of the THz wave, our methodology can be readily adapted for the use with existing phase-sensitive single pixel imaging systems or any amplitude sensitive THz imaging arrays.

physics.optics↗

arXiv2019

3D-printed Terahertz Photonic Bandgap Waveguide-based Fluidic Sensor

A Bragg waveguide-based resonant fluidic sensor operating in THz band is studied. A fused deposition modeling 3D printing technique is employed to fabricate the sensor where the liquid analyte is flowing in the microfluidic channel integrated into the waveguide cladding. The analyte refractive index-dependent resonant defect state supported by the fluidic channel is probed by tracking the resulting absorption dip and phase change of the core-guided mode on waveguide transmission spectra. The proposed fluidic sensor can open new opportunities in applied chemical and biological sensing as it offers a non-contact measurement technique for monitoring refractive index changes in flowing liquids.

physics.optics↗

arXiv2017

Live Streaming of the Uncompressed HD and 4K Videos Using Terahertz Wireless Links

Taming the Terahertz waves (100 GHz-10 THz) is considered the next frontier in wireless communications. While components for the ultra-high bandwidth Terahertz wireless communications were in rapid development over the past several years, however, their commercial availability is still lacking. Nevertheless, as we demonstrate in this paper, due to recent advances in the microwave and infrared photonics hardware, it is now possible to assemble high performance hybrid THz communication systems for real-life applications. As an example, in this work, we present design and performance evaluation of the photonics-based Terahertz wireless communication system for the transmission of uncompressed 4K video feed that is built using all commercially available system components. In particular, two independent tunable lasers operating in the infrared C-band are used as a source for generating the Terahertz carrier wave using frequency difference generation in a photomixer. One of the IR laser beams carries the data which is intensity modulated using the LiNbO3 electro-optic modulator. A zero bias Schottky detector is used as the detector and demodulator of the data stream followed by the high-gain and low-noise pre-amplifier. The Terahertz carrier frequency is fixed at 138 GHz and the system is characterized by measuring the bit error rate for the pseudo random bit sequences at 5.5 Gbps. By optimizing the link geometry and decision parameters, an error-free (BER<10-10) transmission at a link distance of 1m is achieved. Finally, we detail integration of a professional 4K camera into the THz communication link, and demonstrate live streaming of the uncompressed HD and 4K video followed by analysis of the link quality.

physics.app-ph↗

arXiv2017

Channel Multiplexing in Wireless Terahertz Communications Using Orbital Angular Momentum States

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.

physics.optics↗

arXiv2017

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

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

physics.optics↗

arXiv2017

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

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.

physics.optics↗