According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

Review of terahertz and subterahertz wireless communications

This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.

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Tunable, continuous-wave Terahertz photomixer sources and applications

We report on a new class of polymer photonic crystal fibers for low-loss guidance of THz radiation. The use of the cyclic olefin copolymer Topas, in combination with advanced fabrication technology, results in bendable THz fibers with unprecedented low loss and low material dispersion in the THz regime.We demonstrate experimentally how the dispersion may be engineered by fabricating both high- and low-dispersion fibers with zero-dispersion frequency in the regime 0.5-0.6 THz. Near-field, frequencyresolved characterization with high spatial resolution of the amplitude and phase of the modal structure proves that the fiber is single-moded over a wide frequency range, and we see the onset of higher-order modes at high frequencies as well as indication of microporous guiding at low frequencies and high porosity of the fiber. Transmission spectroscopy demonstrates low-loss propagation (< 0.1 dB/cm loss at 0.6 THz) over a wide frequency range.

/pdf/bendable-low-loss-topas-fibers-for-the-terahertz-frequency-427i253lrq.pdf

Bendable, low-loss Topas fibers for the terahertz frequency range

We present a numerical and experimental demonstration of a waveguide regime in a broad band spectral range for the hollow core microstructured optical fibers (HC MOFs) made of silica with a negative curvature of the core boundary. It is shown that HC MOFs with the cladding consisting only of one row of silica capillaries allows to guide light from the near to mid infrared despite of high material losses of silica in this spectral region. Such result can be obtained by a special arrangement of cladding capillaries which leads to a change in the sign of the core boundary curvature. The change in the sign of the core boundary curvature leads to a loss of simplicity of boundary conditions for core modes and to "localization" and limitation of their interaction with the cladding material in space. Such HC MOFs made of different materials can be potential candidates for solving problem of ultra high power transmission including transmission of CO and CO2 laser radiation.

Demonstration of a waveguide regime for a silica hollow--core microstructured optical fiber with a negative curvature of the core boundary in the spectral region > 3.5 μm.

We propose two designs of effectively single mode porous polymer fibers for low-loss guiding of terahertz radiation. First, we present a fiber of several wavelengths in diameter containing an array of sub-wavelength holes separated by sub-wavelength material veins. Second, we detail a large diameter hollow core photonic bandgap Bragg fiber made of solid film layers suspended in air by a network of circular bridges. Numerical simulations of radiation, absorption and bending losses are presented; strategies for the experimental realization of both fibers are suggested. Emphasis is put on the optimization of the fiber geometries to increase the fraction of power guided in the air inside of the fiber, thereby alleviating the effects of material absorption and interaction with the environment. Total fiber loss of less than 10 dB/m, bending radii as tight as 3 cm, and fiber bandwidth of ~1 THz is predicted for the porous fibers with sub-wavelength holes. Performance of this fiber type is also compared to that of the equivalent sub-wavelength rod-in-the-air fiber with a conclusion that suggested porous fibers outperform considerably the rod-in-the-air fiber designs. For the porous Bragg fibers total loss of less than 5 dB/m, bending radii as tight as 12 cm, and fiber bandwidth of ~0.1 THz are predicted. Coupling to the surface states of a multilayer reflector facilitated by the material bridges is determined as primary mechanism responsible for the reduction of the bandwidth of a porous Bragg fiber. In all the simulations, polymer fiber material is assumed to be Teflon with bulk absorption loss of 130 dB/m.

Porous polymer fibers for low-loss Terahertz guiding.

We report a simple subwavelength-diameter plastic wire, similar to an optical fiber, for guiding a terahertz wave with a low attenuation constant. With a large wavelength-to-fiber-core ratio, the fractional power delivered inside the lossy core is reduced, thus lowering the effective fiber attenuation constant. In our experiment we adopt a polyethylene fiber with a 200 µm diameter for guiding terahertz waves in the frequency range near 0.3 THz in which the attenuation constant is reduced to of the order of or less than 0.01 cm−1. Direct free-space coupling efficiency as high as 20% can be achieved by use of an off-axis parabolic mirror. Furthermore, all the plastic wires are readily available, with no need for complex or expensive fabrication.

Low-loss subwavelength plastic fiber for terahertz waveguiding.

A low-loss terahertz air-core microstructure fiber is demonstrated for terahertz waveguiding. Substantially low attenuation constant less than 0.01cm−1 has been achieved and the guiding wavelength is found to be tunable by linear scaling the fiber size. The experimental results well agree with the simulation based on the finite-difference frequency-domain method, which interprets the guiding mechanism as the antiresonant reflecting waveguiding. The simulated modal pattern shows that most terahertz field is concentrated inside the central hollow air core and is guided without outside interference, which has high potential for guiding intense terahertz waves with minimized loss.

https://ir.nctu.edu.tw:443/bitstream/11536/9680/1/000253237900124.pdf

Terahertz air-core microstructure fiber

We propose and demonstrate a simple leaky structure for terahertz (THz) waveguiding. Different from previously reported air-core THz waveguides, in which a high-reflection-coated cladding layer is designed, the proposed structure here is a simple pipe with a large air core and a thin dielectric layer with uniform but low index. Using commercially available Teflon air pipes, we experimentally confirm that THz waves can be successfully guided in the central air core of 3-m-long pipes with excellent mode qualities, high coupling efficiencies, low attenuation constants, and controllable passband width.

Low-index terahertz pipe waveguides.

We demonstrate a fundamentally mode-locked Yb-fiber laser with 3 GHz repetition rate and ∼206  fs pulse duration. The laser incorporates two enabling technologies: a 1 cm heavily Yb-doped phosphate glass fiber as the gain medium and a high-dispersion (−1300  fs2) output coupler to manage cavity dispersion. The oscillator self-starts and generates up to 53 mW average power.

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3 GHz, fundamentally mode-locked, femtosecond Yb-fiber laser.

We demonstrate a room-temperature-operated all-terahertz (THz) fiber-scanning near-field imaging system. The upright-type THz near-field microscope has a compact size, capable of being integrated with an optical microscope. This transmission illumination near-field system could be a promising tool to distinguish breast cancer from the normal tissue without pathologic staining.

Hung-Wen Chen

Papers

Low-loss subwavelength plastic fiber for terahertz waveguiding.

Terahertz air-core microstructure fiber

Low-index terahertz pipe waveguides.

3 GHz, fundamentally mode-locked, femtosecond Yb-fiber laser.

All-terahertz fiber-scanning near-field microscopy