The high-frequency terahertz (THz) waveband (100 GHz to 10 THz), which is sandwiched between the visible-light and millimeter wavebands, combines the characteristics of both. Accordingly, the terahertz waveband is expected to be applied in fields like ultra-high-speed wireless transmission (i.e., transmission rate of over 100 gigabits per second)—utilizing its light-like broadband performance—and security—utilizing the penetrating power of millimeter waves for “imaging” in safety (i.e., no ionization effects). However, in regard to the terahertz waveband, technologies for easily generating terahertz waves and for detecting them with high sensitivity are unavailable, so terahertz application have so far been limited. Applying ultrahigh-speed electronic device technology developed in conjunction with the enhancement of capacity of NTT’s communication network, we are continuing research aimed at creating integrated circuits (ICs) and functional modules operating in the terahertz waveband and promoting application of the terahertz waveband to provide new services.
An example of applying the broadband performance of the terahertz waveband to ultrahigh-speed short-distance wireless communication is described as follows. By utilizing the 300-GHz band (which is part of the terahertz waveband in which attenuation during propagation of signals through the atmosphere is low) and applying IC technology, we are aiming to achieve wireless communication at high data rate (i.e., above 10 Gbit/s). In the case that a terahertz-band signal is processed by an IC, challenges such as propagation loss in the IC’s internal wiring (due to high signal frequency) and waveform distortion (due to high data rate) must be overcome. Given such challenges, it is essential that ICs and modules operating in the terahertz waveband have characteristics such as low loss, high output power, and flat group delay.
Combined to form an ultrahigh-speed short-distance wireless communication system utilizing the 300-GHz, a wireless-use IC was designed, and packaged in a compact waveguide-module. A photo of the prototyped IC is shown in Figure 2. In the IC, a modulator and a power amplifier (which are required components of a transmission unit for wireless transmission) are monolithically integrated. High output power and low-loss wiring were achieved by multi-parallelization of the amplifier, and high data rate was achieved by a travelling-wave modulator. Moreover, by using low-loss and wideband waveguide-to-IC transition we designed, the deterioration of the characteristics due to packaging was negligible. By applying this module high-speed operation (i.e., 20 Gbit/s) was confirmed (Fig. 3).
As for applications of a high-speed wireless communication utilizing the terahertz waveband, short-distance communication, by which large volumes of data are instantaneously extracted by installed terminals from mobile devices like smart phones (Fig. 4), and standardization, of converting optical fibers in datacenters and wired connections within equipment to wireless communication, are being discussed . Moreover, as for applications of imaging utilizing the penetrating power of terahertz waves, technologies for assuring safety and security—such as imaging at airport security checkpoints and in smoky, dusty, and snowy environments—are being anticipated. As a key device for implementing these applications, research and development aiming at creating ICs and compact modules is progressing.
This work was supported in part by the research and development program “Multi-ten Gigabit Wireless Communications Technology at Sub-terahertz Frequencies" of the Ministry of Internal Affairs and Communications, Japan.
 IEEE 802.15.3d “Applications Requirements Document” (Doc Number 14/0304r16) https://mentor.ieee.org/802.15/documents