Uniform bending losses of single-mode fibers are experimentally studied. The effects of a dip in the refractive index at the center of the core, bending radius and fiber parameters on the bending losses were investigated. An experimental equation for the bending loss of a single-mode fiber with a small dip have been obtained as an exponential function of the bending redius and the fiber parameters. It essentially agrees with the theoretical equation.

Low-loss dispersion-free single-mode fibers near 1.5 µm were fabricated by controlling their waveguide dispersion. The deviation of the zero-dispersion wavelength from the calculated one was within 0.02 µm, and the minimum loss of the fiber was 0.46 dB/km at 1.56 µm. The pulse broadening in the 20 km long fiber was measured to be 1.3 ps/km/nm at 1.50 µm. These results confirmed that it is beneficial to use the dispersion-free single-mode fibers for high capacity and long distance transmission media.

The effect of relative refractive index differences of single-mode fibers on zero-dispersion wavelength was investigated. Optimizing the fiber parameters, a single-mode fiber has been realized, which has minimum loss and minimum dispersion at the same wavelength in the 1.5 µm region.

Single-mode fibers are prospective candidates for future high-capacity and long distance transmission media. In particular, low loss single-mode fibers, operating around 1.27 µm wavelength, where the material dispersion of silica glass falls to zero, are very attractive because of their huge bandwidth capability. Until now, however, the attainment of very low loss in the long wavelength region has been confined to only multimode fibers. Single-mode fibers with GeO2 -SiO2 glass system, which have the broadest low loss window ant the long wavelength region, were fabricated. The minimum loss of 0.5 dB/km at 1.3 µm wavelength, where dispersion of the fiber is negligibly small, was obtained by suitably designing waveguide parameters and reducing OH contamination. Also, practical structural features and the OH ion behavior are clarified in relation to reduction of OH absorption, which is the major problem in reducing loss at the wavelength of interest.

Ultimate low loss single-mode fibers have been prepared by reducing excess due to structural imperfections as much as possible. Loss mechanisms based on the fabricated fibers have also been analyzed. Transmission loss has been reduced almost down to the intrinsic material loss. Minimum loss was 0.2 dB/km at 1.55 µm. It was made clear that the dispersion can be reduced to zero at this wavelength region by increasing the waveguide dispersion. It has been confirmed that such single-mode fibers are very useful for high capacity and long distance transmission media.