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

Ion beam assisted etching of GaAs has been performed by continuous and pulsed Ga focused ion beam irradiation in Cl2 atmosphere, and etch yield and surface composition have been measured as a function of beam energy and Cl2 flux. It was observed that pulsed irradiation results in a 10 times or more higher etching rate than continuous irradiation. From Auger electron spectroscopy, it was observed that the surface becomes As rich after the etching with the composition depending on the etching condition. The thickness of the surface altered layer extends 7–20 nm depending on the beam energy. These results are discussed based on rate equations.

Characteristics of ion beam assisted etching of GaAs : surface stoichiometry

Advances in Linear Modeling of Microwave Transistors

Optical mixing in InP-based ultra-fast high-electron mobility transistors (HEMTs) using a focused laser beam onto the surface was studied in detail. Position-dependent optical responses in the HEMT and optical signal detection at 10 GHz were demonstrated.

Optical mixing in InP-based high-electron mobility transistors by use of a focused laser beam

A CPW-MMIC (coplanar-waveguide monolithic microwave integrated circuit) with quad-channel receiver IC for 140-GHz passive millimeter-wave imaging system was developed. The IC has four low-noise amplifiers (LNA), Dicke switches, and detectors using Schottokey barrier diode. The Dicke switch has 1dB insertion loss and 8 dB on-off ratio. The LNA has 40 dB gain and 5 dB noise figure. The total power consumption of the IC is 0.39 W. The IC was assembled with a four-antenna array fabricated on a polyimide film. The antenna typically has 9 dB antenna gain and 5 dB loss. This subassembly has a print board connector, therefore we can mount four channel receivers as desired number on the print board easily. The subassembly was finally implemented in 140-GHz imaging system and we confirmed human can be detected at 30 m apart from the imaging system.

A 140-GHz quad-receivers IC and sub-assembly for compact passive imaging sensors

This paper describes Monolithic microwave integrated circuits (MMICs) and MMIC modules for 120 GHz 10-Gbps or more wireless data communications using ASK and QPSK modulation schemes. Record transmission distance of 5.8 km was established using ASK architecture MMICs at a data rate of 10-Gbps and a carrier frequency of 125 GHz. QPSK modulator and demodulator show that they performs full function in back-to-back test at a carrier frequency of 128 GHz at 10-Gbps. Furthermore, the QPSK chips was improved and integrated with low noise amplifier or power amplifier. In result, this QPSK transceiver chipset showed 22.2-Gbps error-free transmission at an output power of 7 dBm and received power -35 dBm in back-to-back measurement.

Toshihiko Kosugi

Papers

Characteristics of ion beam assisted etching of GaAs : surface stoichiometry

Advances in Linear Modeling of Microwave Transistors

Optical mixing in InP-based high-electron mobility transistors by use of a focused laser beam

A 140-GHz quad-receivers IC and sub-assembly for compact passive imaging sensors

Broadband InP MMICs for 120 GHz wireless data communications