Fundamentals and development of PDFAs are described. Spectroscopic data of Pr3+ in a fluoride glass are presented with a view to understanding the performance of PDFA. An amplification mechanism model which explains PDFA performance is established. On the basis of the model pump schemes which efficiently extract the potential in Pr3+-doped fluoride fiber are discussed in order to construct amplifier modules. Gain characteristics of Pr3+-doped fluoride fibers are clarified. Codoping effect on pump wavelength extension is investigated. LD-pumped PDFA construction and performance are described. PDFAs are shown to be attractive device to upgrade the performance of optical systems at 1.3µm. Furthermore future approaches to PDFA research are discussed.

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.

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.