Introduction Modern high-speed optical fiber communication systems employ for information transmission only a narrow spectral range, 1530 - 1610 nm, defined by the gain bandwidth of the Er-doped fiber amplifier. At the same time, silica-based optical fibers have low optical losses in a considerably broader range. In particular, the spectral region where the optical losses are under 0.4 dB/km and which might be used for information transmission is 400 nm in width (1300 - 1700 nm). However, efficient fiber amplifiers based on rare earth-doped fiber are missing for the spectral ranges 1300 - 1520 and 1610 - 1700 nm. So there is a great demand for new active materials suitable for the creation of fiber lasers and optical amplifiers operating in these spectral bands. Previous attempts to develop efficient transition metal-doped active materials have been unsuccessful.
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