Recently, the total amount of traffic has increased exponentially due to the expansion of wireless backbone networks, the widespread use of smartphones, and commercialization of video services and cloud computing. Accordingly, there was an urgent need to further enlarge communication capacity. For this reason, the rapid introduction of large-capacity transmission systems using an “optical digital coherent method” has been strongly endorsed. In this method, phase-modulated signal light is demodulated to intensity modulation by interfering with the local light source on the receiving side. To procced from this point, a tunable light source with a narrow linewidth is required.

Various methods have been proposed to realize a wideband wavelength tunable light source. The award winners researched and developed a tunable light source based on a phase-shift DFB laser, which has superior single mode stability and field-proven reliability. The light source is a monolithic integrated laser in which functional elements consisting of 12 DFB laser arrays, optical couplers, and semiconductor optical amplifiers are formed on the same substrate. The monolithic integration was realized by crystal growth and dry etching technologies.

Furthermore, in order to realize narrower linewidth and higher optical output simultaneously, they have devised an integrated laser and integrated a Distributed Reflector (DR) laser array and an Arrayed Waveguide Grating (AWG) coupler to create a new tunable light source (Fig. 1). This laser has achieved unparalleled performance with an optical output power of 19 dBm and a linewidth of less than 70 kHz. Moreover, it was designed to facilitate the stable mass production of a relatively complicated monolithic integrated laser.

They also developed an industry standard Integrable Tunable Laser Assembly (ITLA) equipped with electronic circuits. To support digital coherent communication, it was essential to reduce the noise of the control circuit. Fig. 2 shows the ITLA product lineup that has been commercialized and put into practical use. As a result of strengthening the compensation function that suppresses deterioration of wavelength stability and linewidth characteristics due to the influence of the external environment and operating conditions, the size has been reduced to 1/3 of the initial size. Low power consumption was achieved even when driving at high output under elevated temperatures in the environment.

The award winners have developed a comprehensive technology for a narrow linewidth wavelength tunable light source for digital coherent communication, a compound semiconductor technology that monolithically integrates multiple optical functional elements, a small package technology that uses resin bonding, and a low noise control circuit technology. Globally, they were the first to develop and commercialize a world-leading a wavelength tunable light source characterized by high output power of 19 dBm, high wavelength stability, and with a narrow linewidth of less than 70 kHz.

The commercialized light source contributes to solving the urgent global problem of the need to increase communication capacity triggered by large-scale data centers, electronic commerce, and web conferencing, to name but a few. The award winners have clearly demonstrated their global preeminence by being the first to develop and put into practical use a narrow linewidth tunable light source for digital coherent communication, which is an essential technology for increasing communication capacity. A global market share of 40% or more for this product was achieved.

As described above, the light sources developed by the award winners and now widely used throughout the world are products that are indispensable for the full-scale utilization of 5G, and their achievements have contributed significantly to the field of electronic information and communication worldwide making them worthy recipients of the Society's Achievement Award. Moreover, these achievements have already received high recognition as evidenced by the conferring of the Kenjiro Sakurai Memorial Award and the Okochi Memorial Production Award.

References

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