"OFC 2026," the world's largest international conference on optical communications, was held from March 15 to 19 in Los Angeles, USA. At the conference, NTT announced two major research achievements that received high acclaim as "top-scored" papers: the realization of a next-generation photodetector and the development of the world's highest-capacity optical submarine cable system using multi-core fiber.
The photodetector has, for the first time in the world, achieved a 200-GHz-class operating speed of the for optical signals at the 1,310 nm optical wavelength used in data centers, along with the highest responsivity (responsivity-bandwidth product of 115 GHz·A/W) at that same speed. Indium phosphide compounds are used as the device material. It possesses long-term reliability at a practical level as an ultrahigh-speed, highly responsive photodetector. As a result of conducting high-temperature conduction tests at 200 degrees Celsius for 200 hours, a durable life of over 50 years can be expected at an operating temperature of 85 degrees Celsius, which is required for data centers.
In short-distance connections such as between computer racks in data centers, the introduction of 800 Gigabit Ethernet using transmitters and receivers with a current signal speed of 100 Gbaud is progressing. For 3.2 Terabit-class Ethernet, whose introduction is expected in the future, signal speeds in the 200-Gbaud to 400-Gbaud-class will be required. The developed photodetector can be an achievement that has made great strides toward the practical application of such 3.2-Terabit-class ultrahigh-speed optical communication. The key technology is the realization of a high-speed, high-responsivity device by confining light in a thin absorbing layer through interference of light.
By employing an interference-type perpendicularly incident structure and confining the optical operating region inside, the density of dark current flowing through the sides of the device, which leads to failure, was suppressed, with the aim of improving device reliability.
Furthermore, in order to precisely align the photoelectric conversion region within the photodetector with the position where the signal light is irradiated, a semiconductor convex lens was built into the back surface of the device, expanding the light-receiving area and improving the misalignment error by more than two times. This is also expected to reduce costs when commercializing the product.
The second achievement, the optical submarine cable system, is a 192-core system with the world's highest capacity. To construct it, a 4-core optical fiber was realized by creating and multiplexing four light pathways (cores) within glass of the same diameter as currently used optical fibers, and a maximum of 48 of these were implemented into a submarine cable with a diameter of approximately 20 mm.
In addition, they developed joint boxes to connect the optical submarine cable and terrestrial optical cables, joint boxes to connect these optical submarine cables to each other on the seabed, and MCF cable termination frames to connect multi-core optical fibers and existing optical fibers. These products have all achieved sufficient performance for commercial use.
This developed 192-core optical submarine cable system can expand communication capacity to four times that of conventional systems while maintaining a cable structure equivalent to existing optical fibers. It is an achievement that brings a bright outlook for the economic development and practical deployment of future submarine networks, such as through the reduction of laying costs.
NTT stated that it aims for practical deployment of domestic submarine cables around 2029.
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.