Research into terahertz technology is now receiving increasing attention around the world, and devices exploiting this waveband are set to become increasingly important in a very diverse range of applications. Here, an overview of the status of the technology, its uses and its future prospects are presented.

Cutting-edge terahertz technology

This Review covers the state-of-the-art technologies on photonics-based terahertz communications, which are compared with competing technologies based on electronics and free-space optical communications. Future prospects and challenges are also discussed. Almost 15 years have passed since the initial demonstrations of terahertz (THz) wireless communications were made using both pulsed and continuous waves. THz technologies are attracting great interest and are expected to meet the ever-increasing demand for high-capacity wireless communications. Here, we review the latest trends in THz communications research, focusing on how photonics technologies have played a key role in the development of first-age THz communication systems. We also provide a comparison with other competitive technologies, such as THz transceivers enabled by electronic devices as well as free-space lightwave communications.

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Advances in terahertz communications accelerated by photonics

Recent changes in how people consume multimedia services are causing an explosive increase in mobile traffic. With more and more people using wireless networks, the demand for the ultra-fast wireless communications systems is increasing. To date, this demand has been accommodated with advanced modulation schemes and signal-processing technologies at microwave frequencies. However, without increasing the carrier frequencies for more spectral resources, it may be quite difficult to keep up with the needs of users. Although there are several alternative bands, recent advances in terahertz-wave (THz-wave) technologies have attracted attention due to the huge bandwidth of THz waves and its potential for use in wireless communications. The frequency band of 275 ~ 3000 GHz , which has not been allocated for specific uses yet, is especially of interest for future wireless systems with data rates of 10 Gb/s or higher. Although THz communications is still in a very early stage of development, there have been lots of reports that show its potential. In this review, we will examine the current progress of THz-wave technologies related to communications applications and discuss some issues that need to be considered for the future of THz communications.

Present and Future of Terahertz Communications

A wireless communication system with a maximum data rate of 100 Gbit s−1 over 20 m is demonstrated using a carrier frequency of 237.5 GHz. The photonic schemes used to generate the signal carrier and local oscillator are described, as is the fast photodetector used as a mixer for data extraction.

Wireless sub-THz communication system with high data rate

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

Results of a propagation study focused on rain attenuation on 120-GHz band wireless link are presented. The 120-GHz band wireless link with V polarization was operated on a 400-m long path from December 2007 to September 2008 in Atsugi, Japan. The rain rate distribution for Atsugi is in good agreement with the conditional M distributions. As for rain attenuation, the effect of water on the antenna radome is large, and the experimental results coincide with the ITU and L-P model. Cumulative distributions of attenuation and BER deterioration due to rain were obtained.

Rain attenuation statistics for a 120-GHz-band wireless link

A tunable-filter for a 120-GHz-band receiver MMIC, which is used for a 10-Gbit/s wireless link, was designed and fabricated. The filter can change both the center frequency and bandwidth simultaneously. The filter is a Bessel-type filter and composed of three dual coplanar open stubs and variable-reactance circuits. The filter was monotonically fabricated with InP HEMTs on a single-chip, which occupies only 400 mum times 500 mum. We measured the tuning range of the filter. The center frequency can be adjusted from 118 to 122 GHz with the bandwidth of 15 GHz, and the 3-dB bandwidth can be adjusted from 11 to 16.5 GHz at the center frequency of 120 GHz.

Tunable coplanar filter for F-band wireless receivers

We present a 20-Gbit/s ASK wireless system in the 300-GHz band for kiosk data downloading. The front-ends (transmitter and receiver) are manufactured by using InP-based monolithic microwave integrated circuits with high electron mobility transistors. We successfully demonstrate 20-Gbit/s data transmission using these front-ends at link distances from 50 cm to 1 m. The measured bit error rate is less than 10−6 up to the distance of 1 m.

20-Gbit/s ASK wireless system in 300-GHz-band and front-ends with InP MMICs

We have developed two types of 10-Gbit/s wireless link systems using the 120-GHz band. One employs photonic technologies for the generation, modulation, and amplification of 125-GHz signals. This configuration enables us to separate the photonic millimeter-wave generator and transmitter core, so that we can use the wireless link as a kind of radio-over-fiber system. The other is an all-electronic system that has advantages of compactness and low cost, especially when the transceiver functions are implemented with monolithic microwave integrated circuits (MMICs). Using both of these systems, we succeeded in error-free transmission of 10-Gb Ethernet signals.

10-Gbit/s wireless link systems using the 120-GHz band

The deposition process of tungsten from WF6 during ion beam bombardment has been studied using in situ XPS measurements. The deposition was done using low energy beam of Ar+ and H+2, which is important to reduce damage in the substrate. The adsorption of WF6 on the Si3N4 surface shows a tendency to saturate at high exposure to WF6 gas without the ion bombardment, while metallic tungsten is basically formed on Si surfaces, indicating that WF6 is dissociatively chemisorbed on the Si surface. No fluorine desorbed spontaneously from the Si3 N4 surface. After irradiation with 500 eV Ar+ (0.1 uA/cm2) on the adlayer for 700 s, W(4f) peaks show a chemical shift by about 6 eV toward low binding energy and there is a decrease of the F(1s) peak intensity, indicating that metallic tungsten is formed on the surface by ion beam induced dissociation of WF6. The composition of the deposited layer is W: 93%, F: 5%, and O: 2%. The resistivity of a 50 nm thick deposited layer is about 1.5×10−5 Ω cm.

Toshihiko Kosugi

Papers

Rain attenuation statistics for a 120-GHz-band wireless link

Tunable coplanar filter for F-band wireless receivers

20-Gbit/s ASK wireless system in 300-GHz-band and front-ends with InP MMICs

10-Gbit/s wireless link systems using the 120-GHz band

An x‐ray photoelectron spectroscopy study on ion beam induced deposition of tungsten using WF6