  Core-clad fibers on the basis of high-purity 1300 ppmw Pr(3+)-doped
Ga-Ge-As-Se and 2000 ppmw Pr(3+) -doped In-Ge-As-Se-I glasses have been
prepared. The minimum optical losses of the fibers are at the level of 1 dB/m
in the wavelength range, where there is no influence of the praseodymium
absorption. At present, it is the best result obtained for core-clad rare-
earth-doped chalcogenide fibers with high dopant concentration. The fibers
exhibit the broadband luminescence in the spectral range of 3.5-5.5 micron. The
values of luminescence lifetime at the wavelength of 4.7 micron are within
6.5-8.2 ms closed to those for the bulk glasses obtained previously. Simulation
of laser properties for these core-clad fibers in the three-level cascade
scheme was carried out. The low level of optical losses experimentally achieved
in these fibers is one of main criteria for development of mid-IR fiber
systems.
