Research and Development of a High-speed Digital Single-mode-fiber Transmission System

Configuration of field test transmission line

Fig.1 Configuration of field test transmission line

Mode-partition noise

Fig.2 Mode-partition noise

Configuration of a high-speed digital optical repeater

Fig.3 Configuration of a high-speed digital optical repeater

Increase in loss due to laying of single-mode fiber cable

Fig.4 Increase in loss due to laying of single-mode fiber cable

Researchers have long thought that a superb high-speed digital transmission system could be achieved using single-mode fiber and semiconductor lasers as main elements. Single-mode fiber has low-loss and high-bandwidth characteristics while semiconductor lasers feature high output and high-speed modulation. Before an actual system could be implemented, however, it was essential that the performance limits of both of these elements with regard to transmission distance and bit-rate be determined and that the factors behind those limits be well understood. In this way, a method for configuring an optimal system that does not degrade the outstanding features of single-mode fiber and semiconductor lasers could be developed.

From early on, Nippon Telegraph and Telephone Public Corporation (now NTT) promoted the research and development of singlemode fiber transmission technology for practical use. Beginning in January 1978, NTT performed a series of tests toward a practical high-speed optical transmission system using cable laid out through the premises of its Electrical Communication Laboratories (ECL) in Yokosuka. These R&D efforts culminated in the construction of an optical-fiber transmission line running the length of Japan in February 1983 providing a 400-Mbps transmission system.

The following describes the R&D steps leading to this achievement.
(1) The performance limits of a single-mode fiber transmission system were theoretically clarified and then verified by various transmission experiments, and low-loss single-mode fiber transmission technology, high-speed modulation/demodulation technology, and other key technologies were established (Fig. 1).
(2) It was clarified that increase in noise due to mode partitioning in a semiconductor laser was a major factor behind the performance limits of a single-mode fiber transmission system (Fig. 2).
(3) Transmission characteristics were closely evaluated for various combinations of transmission speeds of 100—800 Mbps and wavelengths of 0.85—1.5 μm.
(4) Methods for configuring an optical transmission circuit, optical receiver circuit, and optical repeater circuit (Fig. 3) having high-speed and high-performance characteristics were clarified, a method for setting practical operating conditions was devised and an optical-repeater design method established, prototype circuits were constructed, and favorable operations were demonstrated.
(5) A method for configuring transmission-line code applicable to high-speed operation was devised and stable operation independent of the signal stream was achieved.
(6) Taking cable-laying tension, temperature variation, and other practical requirements and conditions into account, a unit-type single-mode optical-cable structure was devised that could decrease microbending loss (Fig. 4) and prevent fiber cracks. Experiments revealed that this optical-cable structure had low-loss and wide-bandwidth characteristics and that it could withstand long-distance laying conditions.
(7) An optimal system was designed based on the above research results and a 400 Mbps single-mode fiber transmission system was constructed.
This optical transmission system makes for an economical digital transmission line. It has already been applied to the construction of a trunk transmission line running the length of the Japanese Islands, and it will play an extremely important role in the building of an advanced information-communications system. The above research results are also making a big contribution to the development of high-performance optical transmission systems such as a 1.6-Gbit/s terrestrial system and a 400-Mbit/s submarine system.

This system has played a leading role in the development of single-mode fiber transmission systems in other countries as well. In recognition of their outstanding work in the above R&D process, Takeshi Ito, Kiyoshi Nakagawa, and Yukinori Ishida (Nippon Telegraph and Telephone Public Corporation) received an Achievement Award in 1984 from the Institute of Electronics, Information and Communication Engineers (IEICE).

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