This paper describes an overview of the special issue for recent research progress on optical fiber technologies for realizing high-speed, high-capacity communication with the use of so-called 3M technology. 3M technology means multi-level modulation, multi-core fiber and multi-mode control. The topics of the present issue cover recent trend of optical fiber technology, high-density multi-core fibers, submarine multi-core fibers, multi-core optical amplifiers, standardization of multi-core fibers, recent progress in high-density conventional optical fiber cables, and optical amplification fiber for satellite optical communication.

View full abstract

This paper presents the latest developments in the 3M technologies for next-generation optical communication, namely Multi-level modulation, Multi-core fiber and Multi-mode control for mode-division multiplexing. We also describe recent innovative technologies including ultra-wideband WDM transmission beyond conventional C and L bands, and novel hollow-core fibers enabling ultra-low loss below conventional fibers.

View full abstract

This paper overviews the evolution of space-division multiplexing (SDM) technology that will contribute to high-capacity transmission in future optical communication systems. It focuses on weakly-coupled multi-core fibers (WC-MCFs), which are expected to be used in the beginning of SDM technologies. The pursuit of the maximum number of cores accommodated in a single optical fiber led to 32-core MCFs within a range that was twice the cladding diameter of standard optical fibers. Attention has recently concentrated on 4-core MCFs with a standard cladding diameter from a practical perspective. This strategy allows for a maximum four-fold increase in transmission capacity while maintaining the outer diameter of optical fiber cables. This paper covers the development of 4-core MCFs with such peripheral technologies as cables and connectivity as well as the results of recent field deployment tests. WC-MCFs offer a promising path towards achieving future high-capacity optical communication systems.

View full abstract

Multi-core fibers are needed for expanding the transmission capacity of submarine optical communication because submarine transmission and cable technologies have advanced, leaving scant room for capacity expansion by enhancing spectral efficiency or increasing the number of fibers per cable. 2-core fibers for submarine cable are becoming the first commercial applications of multi-core fibers because of their optical characteristics that are competitive to those of single core fibers as well as availability of mass-produced fibers.

View full abstract

We review core-pumped and cladding-pumped multicore erbium-doped fiber amplification (EDFA) technology and discuss the configuration of the connections between multicore transmission fibers and multicore EDFAs. The core-pumped one contributes to downsizing per core since the case size is identical as that of the conventional EDFAs. The size of the optical devices using multicore fibers achieved the same size as those using single-core fibers. The cladding-pumped one reduced the power consumption. Lower power consumption for a core than that of the conventional EDFAs was achieved by a C-band cladding- pumped coupled 12-core EDFA. We reviewed techniques that increase cladding pump efficiency. The configuration of the connections depends on the directions of the optical signals on a multicore fiber. Co-directional and bi-directional propagation are respectively supported by low loss splicing of the multicore fibers and low insertion loss of fan-in fan-out devices.

View full abstract

Worldwide research on space division multiplexing (SDM) technology has been conducted intensively since 2010. Recently, the commercialization of a multi-core fiber (MCF) and the deployment of MCF into a new optical submarine system were announced. These initiatives provide a strong push for the future deployment of SDM transmission technology. However, the international standardization of new SDM technology is mandatory before an SDM ecosystem can be established to achieve the smooth deployment of the SDM transmission system, particularly in the terrestrial network. In this paper, we review the status of a standardization activity for an SDM optical fiber in the international telecommunication union, telecommunication standardization sector (ITU-T). We also provide a personal perspective on how to develop a new standard for weakly coupled MCF (WC-MCF) that ensures interoperability. We show that well planned and harmonized standardization activity is required for achieving the SDM ecosystem in a terrestrial network by 2030.

View full abstract

The amount of information that a fiber optic cable can transmit depends on the number of fiber cores installed. However, a simple increase in the number of cores would require a thicker cable diameter, which does not contribute to an efficient network construction. New cable structures, different from conventional loose tube-type or slotted core-type cables, have been developed and have been put into practical use. This paper introduces high-density optical fiber cables that utilize single-core fibers.

View full abstract

High-power fiber-optic amplifiers are essential for optical communications in space. Specifically, erbium and ytterbium co-doped fibers (EYDFs) are key to amplifying a signal power over 10 W. However, cosmic radiation degrades a general EYDF. Doping the EYDF with cerium reportedly mitigates the radiation- induced degradation. This paper shows that germanium-doped EYDF successfully amplifies output power over 10 W with radiation tolerance. These results contribute to progress in spaceborne high-power optical amplifiers and optical communication in space.

View full abstract

In laser peening, a transparent overlay called the plasma confinement layer is typically employed to suppress the expansion of the plasma such that a high-pressure shock wave is generated. Water is usually used as a plasma confinement layer. However, for metallic materials that are sensitive to water, solid media can substitute for water. In this study, laser peening was performed in a dry environment using silicone rubber (polydimethylsiloxane, PDMS) as a plasma confinement layer. Instead of using rigid solid media such as glass, PDMS has been used, which is softer and can effectively adhere to metal surfaces to improve the acoustic impedance mismatch. Because of laser-induced damage, it is difficult to use solid media repeatedly as a plasma confinement layer, which ultimately reduces the efficiency of the laser peening process. Experiments were conducted to explore the appropriate process window for laser irradiation by varying the laser intensity and number of laser shots.

View full abstract