One- and two-way 6.3 Mbit/s optical transmission systems employing wavelength-division-multiplexing (WDM) are designed for application to telephone networks. Multiplexing at two wavelengths, 1200 and 1300 nm, as well as that at 810 and 890 nm, is found to be effective after taking the application fields and present fabrication techniques for optical multiplexers into account. Requirements for optical multiplexers are clarified through SNR design in which crosstalk is dealt with effectively. Optical multiplexers composed of newly developed dielectric multilayer thin-film filters are designed to meet the systems' requirements, and good transmission quality is confirmed for the systems by experiments.

An evaluation of the influence of mode partition noise on error rate performance in a high bit rate optical fiber transmission system is presented. First, it is experimentally clarified that the intensity in each longitudinal mode of a laser diode fluctuates, although the intensity for the total mode is constant. It is also established that this fluctuation causes degradation of the error rate performance after transmission through a long optical fiber. The fluctuation is named "mode partition noise." Next a simple model for the fluctuation is proposed. The characteristics of the fluctuation are discussed on the basis of this model. Optical waveform fluctuation is found to be introduced by mode partition noise in the course of transmission through a long despersive transmission medium. This optical waveform fluctuation and its frequency spectrum are calculated. Finally, the error rate performance is evaluated and specifications required for a laser spectrum to attain a given repeater spacing are clarified.

The hybrid digital transmission system, involving 2Rrepeaters between 3R (Reshaping, Retiming and Regenerating)-repeaters, is a suitable system for fiber-optic transmission. The 2R (Reshaping and Regenerating)-repeatered line enables bit rate free transmission, but cannot suppress the accumulated timing jitter in transmitting pulse streams. The trial 1-8 Mbit/s transmission system, using step-index multimode fibers and light emitting diodes, was designed and tested. Multirepeater transmission experiments using optical fiber cable laid in an underground cable tunnel were also performed. The timing jitter accumulations were measured through the multi-repeatered line. Measured timing jitter values were in good agreement with theoretical values. Experimental results show satisfactory feasibility of a fiberoptic hybrid digital transmission system using step-index or gradedindex multimode fibers.