Atsushi Kanno

Terahertz (THz) signal processing is crucial for enabling key applications in the THz band. However, conventional electronics-based solutions face several limitations, largely due to the limited roll-off characteristics of electronic devices, which make multiplexing and demultiplexing densely THz signals challenging. Photonic technology offers unique advantages, including wide bandwidth, low loss, steep roll-off, and high flexibility. In this paper, we present a photonic approach for simultaneous THz signal reception, demultiplexing, and downconversion by converting multiplexed THz signals into the optical domain and leveraging advanced photonic signal processing. As a proof-of-concept demonstration, we validate the system for millimeter-wave signals in the W band and THz signals in the 300-GHz band, achieving aggregate capacities of 150 Gb/s and 220 Gb/s, respectively. We also discuss the applicability and outlook of the proposed system for key THz-band applications, including multiple radio access, multi-user access networks, and integrated access and backhaul. The proposed system can pave the way for THz communications in 6G and beyond.

The spatial diversity gain of 2-dimensional photodetector array (2-D PDA) in a mobile FSO system is investigated experimentally. A 2-D PDA with maximum-ratio diversity combining provides a robust optical link against the distortion of beam shape and misalignment, and improves the effective FoV without costly adaptive optics.

An optically reconfigurable metasurface antenna for Future G beam steering applications is demonstrated experimentally. Optical reconfigurability is achieved by using photodiode arrays to provide a photovoltage to varactors, which in turn provide the required phase shift to enable beam-steering. Illumination of the photodiodes is provided over free space by using a galvo scanner to deflect a laser beam which is delivered from a high-power laser via a multicore fiber. In addition to delivering power over fiber, the same multicore fiber is also used to deliver a 64-QAM 5G NR signal to the metasurface antenna unit. Measured EVM values ranging from 3% to 5.7% for a beam-steering range of 58° are obtained. The use of a galvo scanner for free space illumination of the metasurface antenna not only eliminates the requirement for on-antenna DC bias networks (thus reducing the system complexity), but also allows greater control over illumination of the photodiode arrays, resulting in continuous beam-steering over 58°.

We proposed a 0.3-mm large-aperture multi-stacked PIN-PD operated up to 20 Gbps to mitigate optical alignment tolerance in mobile optical wireless communications (OWCs). By reducing PIN junction capacitance using multi-stacked layers via fabrication, we achieved a 38-fold increase in 3 dB bandwidth (7.6 GHz) compared to a conventional 0.3-mm large PIN-PD. Assuming 10–20 km/h mobile communication applications at a short distance of 20 m, we developed a mobile OWC transceiver combined with a multi-stacked PIN-PD. In the mobile OWC demonstration considering a tracking latency of -6 at 10 Gbps and -3 at 20 Gbps on a 2.1 m long optical bench (relative speed = 13.7 km/h at 20 m) for 1 hour.

Wireless communications using highly directive terahertz (THz) waves exhibit lower immunity to obstructions than microwaves, limiting their applications. This study demonstrates an obstacle-tolerant THz wireless link established by a self-healing Bessel beam at 300 GHz. The Bessel beam is generated by sending a Gaussian beam through a dielectric axicon lens. Furthermore, experiments were conducted to investigate the short-range transmission (98 mm) reception characteristics with and without the dielectric cubic obstacle (7.5 λ in size) and the metallic obstacle (22 λ in length and 8 λ in width) in the Gaussian beam and self-healing Bessel beam cross sections. The maximum attenuation of the received power due to obstruction was 8.8 and 2.2 dB for the Gaussian and Bessel beams, respectively, when the dielectric obstacle is placed in the middle of the transmission path (49 mm from a transmission lens). This study further investigated the bit error rate (BER) characteristics (1 Gbps, on–off keying) with the dielectric obstacle crossing the beam cross section. When the obstacle crosses the Gaussian beam, the BER degrades as the obstacle approaches the optical axis, breaking the wireless link. In contrast, when the obstacle crosses the Bessel beam cross section, the BER is maintained at <3.8 × 10−3 (the forward error correction limit), and the wireless link is maintained. A self-healing beam, such as the Bessel beam, makes the THz wireless link more tolerant than the Gaussian beam to obstacle and expand applications for THz wireless communications.

The system performance of a wireless signal receiver, in which the modulated RF signal is up-converted to the optical domain based on the phase modulation then down-converted to the baseband by the optical coherent detection, has been examined.