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Long-wavelength single-mode optical fiber transmission technology has recently made rapid progress and is now moving into the commercial operation stage. This paper describes the present state of this technology focusing on the research and development work in the Electrical Communication Laboratories. Based on the results of experiments on optical devices and on preliminary system feasibility analysis, it is shown that, at the present time, an optical wavelength in the 1.3 μm band is desirable for large capacity transmission from the viewpoint of attainable repeater spacings. A field trial plan of a 400 Mbit/s optical transmission system for a long-haul trunk which can compete economically with existing digital transmission systems is outlined.