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.

The causes of SNR fluctuation and nonlinear distortion, uncovered during field trials of PFM optical video transmission systems, are investigated theoretically and experimentally. Degradations are found to be significant in long distance transmission at a wavelength around 0.85 μm. This is because PFM pulse jitter closely relating to the degradation is caused by laser mode competition and its interaction with fiber material dispersion. However, at a wavelength around 1.3 μm, where fiber material dispersion is negligible, only small nonlinear distortion due to feedback light from connectors is observed. Thus, good picture quality is maintained over long distance transmission.