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

This paper describes a 120-GHz-band quadrature phase shift keying (QPSK) modulator and demodulator fabricated on microwave monolithic integrated circuits (MMICs) for a 10-Gbit/s wireless link. The MMICs were fabricated using 0.1-µm-gate InP HEMTs and coplanar waveguides. The direct-conversion modulator consists of hybrid couplers, gain-control amplifiers, and combiners. The delay demodulator has a 200-ps delay line, distribution amplifiers, a voltage-controlled phase shifter, and detectors. The modulator and demodulator MMICs were mounted in WR-8 waveguide modules to evaluate the characteristics of 10-Gbit/s transmission. The bit error rate (BER) for 10-Gbit/s pseudorandom binary sequence (PRBS) 27–1 data was smaller than 10−10.

10-Gbit/s QPSK modulator and demodulator for a 120-GHz-band wireless link

A wafer-level backside process for an InP substrate is developed for stable operation of InP-based submillimeter-wave monolithic ICs. The developed backside process consists of thinning a 3-inch InP substrate down to 50 μm, forming dense vias, and patterning backside metal, and dicing the 50-μm-thick InP substrate. Vias can be formed with a narrow edge-to-edge gap of 35 μm and a small deviation of 1.8 % across the whole 50-μmthick substrate. We also successfully demonstrate backside metal patterning with good alignment accuracy of better than 1 μm for the devices formed on the topside and crack-free dicing of the fragile 50-μm-thick InP substrate.

Wafer-level backside process technology for forming high-density vias and backside metal patterning for 50-μm-thick InP substrate

We present a high-directivity photonic emitter with a high-gain antenna and a waveguide-output photodiode module. The photodiode module is integrated with UTC-PDs and HEMT amplifiers. A wireless link with the emitter achieved 10-Gbit/s data transmission.

High-directivity photonic emitter for 10-Gbit/s wireless link

This paper presents a hermetic sealing technique for rectangular waveguides and packages to improve the reliability of systems in the F band (90–140 GHz). The technique features a new sealing structure, which is a laminate composed of two sealing plates and a spacer with a WR-8 waveguide. The sealing plate has the fused glass window and separates inside air from the ambient atmosphere. The laminated structure with the two sealing plates provides larger passband width than that with one sealing plate and is suitable for broadband wireless systems. The insertion loss of the fabricated sealing structure is 0.35 dB at 125 GHz. From 116 to 136 GHz, the return loss and a variation of group delay are better than 15 dB and 10 ps, respectively. We verified that the sealing structure has good simulation fidelity and that packages sealed with this technique meet the hermetic-seal standard in MIL-STD-883.

Hermetic Sealing Technique for F-band Waveguides and Packages

This paper describes 10-Gbit/s wireless transmission systems using the 120-GHz band. System configurations, key devices/modules, transmission characteristics, and experiments on transmitting uncompressed high-definition (HD) video signals are discussed.

Toshihiko Kosugi

Papers

10-Gbit/s QPSK modulator and demodulator for a 120-GHz-band wireless link

Wafer-level backside process technology for forming high-density vias and backside metal patterning for 50-μm-thick InP substrate

High-directivity photonic emitter for 10-Gbit/s wireless link

Hermetic Sealing Technique for F-band Waveguides and Packages

10-Gbit/s Wireless Transmission Systems Using 120-GHz-Band Photodiode and MMIC Technologies