The Field Evaluation Test of The 32Mb/s Optical Fiber Transmission System

    It was expected that the optical fiber communication systems would have a very large impact on the forthcoming communication networks. However, in order to make them practical in the actual network, it was very important to precisely specify and reveal the fundamental characteristics of the optical fiber, thus defining the research and development items to be pursued.

    For the above purpose, a field evaluation test was conducted in 1975 by NTT with many research people involved in this project. In particular, Dr. S.Shimada, Dr. M.Koyama and Dr. T.Miki were the leaders of this project from the early planning to completing this test.

    The test used an 8 -core 1 km step-index optical fiber cable laid in the 400 m conduit of Yokosuka ECL, NTT. The total length of the optical fibers amounted to 64 km. The system was a hybrid system of 32 Mb/s and 8 Mb/s with optical repeaters specially designed for this practical environment.

    This project achieved the following results:
(1) A new optical fiber transmission theory for analyzing multimode fibers. The perturbation method was applied to this theory. The theory clarified the relationship between the loss due to higher modes and the resulting bandwidth.
(2) A design method for optical transmission systems characterized by the limited bandwidth due to the Gaussian lossy channel.
(3) The Swept-frequency method to precisely measure the optical fiber transmission characteristics.
(4)The design method of the optical repeaters for the 32Mb/s and 8 Mb/s hybrid systems.
(5) Optical connectors used in hazardous environments such as cable conduit and ducts and various measurement equipment necessary for practical optical systems.
    This test clarified the characteristics of optical fiber transmission systems with step-index multimode fibers and 0.85 μm semiconductor lasers, thus enabling development of the following high performance optical systems with single mode fibers and very large-capacity optical repeaters.

    In 1978, The Institute of Electronics, Information and Communication Engineers, Japan presented the Achievement Award to Dr. Koyama, Dr. Shimada and Dr. Miki for their contributions to conducting this field evaluation test.

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