This issue features photonic technologies that support the Beyond-5G / 6G, post-5G, and IOWN (Innovative Optical and Wireless Network). Ultralow-latency, high-capacity, and large connection numbers are demanded to realize the Beyond-5G. The following seven review papers show how photonics and laser technologies can contribute to meeting those demands.

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High-speed optical interconnects have been attracting much attention in data centers and edge computing networks for saving power consumption by reducing electrical link lengths. A VCSEL is a key component in hyperscale data centers and supercomputer networks because of fascinating properties that provide high-speed operations, low-power consumption, a small footprint, wafer-scale testing, low-cost packaging, and ease of fabrication into arrays. In particular, a VCSEL solution gives us high-speed and low-power consumption, which satisfy requirements. We started the NICT project toward high-speed and low-power consumption, co-packaged optics transceivers based on VCSEL arrays and multi-core fibers (MCFs) for delivering the advantages of power consumption and capacity density per module. In this paper, we present our recent activity on high-speed transverse-coupled-cavity (TCC) VCSEL arrays for co-packaged optics. We developed a single-mode, 1060-nm metal-aperture VCSELs array, which offers a high density I/O platform with a multi-core fiber.

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We first discuss opportunities and challenges facing the introduction of spatial channel networks toward the forthcoming space division multiplexing (SDM) era. Then, we present spatial-channel cross-connect architectures and show that hierarchical spatial bypassing and spectral grooming are beneficial in the SDM era in terms of total node-cost reduction and optical reach extension. Finally, we show an ultra- wideband, compact, low insertion loss and low-polarization-dependent-loss five-core 1 × 8 core selective switch prototype employing a two-dimensionally arranged MEMS array.

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An antenna-coupled electrode electro-optic (EO) modulator exhibits potential for use in applications involving a wireless signal receiver and a millimeter-wave sensor for 5G/beyond-5G mobile communication systems. The suitability is primarily due to the excellent characteristics of passive and low-signal distortion operations. With the design of the antenna-coupled electrode based on the three-dimensional (3D) field analysis of an EO-crystal-film and a low-k glass plate stacked structure, the electric field enhancement factor of over 8000 times of the irradiated wireless field can be obtained along a standing- wave electrode for optical modulation, which facilitates a compact wireless-optical signal converter without external power supply. The designed EO modulator for the 5G-band operation was fabricated using a 50-m-thick LiNbO3 crystal film and a 250-m-thick SiO2 glass substrate and its signal conversion characteristics were demonstrated successfully.

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We briefly show the radio over fiber technology for the realization of converged fiber-wireless access network in millimeter- and terahertz-wave bands. Classification of the radio over fiber systems is discussed with their prospects and issues on implementation. High-precision and low-noise oscillator technology based on an optoelectronics oscillator is also discussed for future multi-level modulation scheme in the terahertz-wave bands.

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This paper presents an efficient use of telecom-based photonics technologies in terahertz communications systems, which include high-speed wireless communications in 300-GHz and 600-GHz bands. System concepts, key devices and system demonstrations are described.

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This paper reviews the recent advancement of microresonator frequency comb (i.e., microcombs) technologies for applications related to the future beyond 5G (B5G) communication that is expected to be launched in the 2030s. The advancement of microcomb technologies is the key to realizing low-noise terahertz wave generation and low-latency wavelength division multiplexing (WDM) data transmission, both of which are essential to support the B5G. We also report on the experimental demonstration of ultra- dense WDM data transmission using an intensity modulation and direct detection (IM-DD) scheme. We discuss that IM-DD communication is relevant to B5G thanks to its simplicity and low latency.

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Due to the exponential increase in the wiring resistance of miniaturized electronic components, the increase in both latency and energy consumption is becoming a serious problem. To overcome the electronic bottleneck, we are developing photo-electronic processors to achieve low-latency and energy savings in-network data processing infrastructure. As fundamental building blocks, we first introduce an ultrasmall silicon-wire “Ψ” gate for low-loss and high-speed optical logic operation. A 3 μm-long 5-input Ψ gate and a multi-bit AND circuit configuration were also investigated and designed for large number input operation. By nesting the interferometer, electric-optical digital-to-analog converters can be formed by only using symmetric Y gates. Such gates and circuits are expected to be used in novel photonics- electronics convergence processors for ultralow latency pattern recognition and vector operations for photonic neural network applications.

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This paper aims to understand the current situation of the training of optical engineers necessary for the development of advanced optical research and industry, and to propose solutions. Firstly, the current situation of efforts in the United States was investigated based on presentations at ETOP 2019. We also investigated the current situation of the supply-demand balance of optical engineers based on a survey of efforts to train optical engineers in Japan and the results of a questionnaire at the “Science Photonics Fair 2019”. The questionnaire survey said the same shortage feeling of domestic optical engineers as the United States, and it was understood that the technical level for the shortage feeling of domestic optical engineers was well consistent with the technology transfer stage indicated by AIST’s technology readiness levels. Based on these current situations, the education system, securing human resources, and maintaining employment were discussed.

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When microfabricating thin metal foils using lasers, the short Rayleigh length of the focusing lens means that defocus caused by the deflection of the metal foil due to the thermal deformation can easily lead to reduced processing quality and cutting ability. In addition, when using pulsed lasers, it is necessary to remove the molten metal from the kerf and prevent debris from reattaching by blowing assist gas during cutting. However, it is impractical to blow high-pressure assist gas because of the risk of deformation and damage to the metal foil. In this study, we devised a novel method for reducing the thermal deformation of the metal foil that occurs during laser processing. Furthermore, when this method was applied to the cutting process, we found that the molten metal was removed from the kerf and the cutting performance was improved, even though no assist gas was used.

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