For the first time, 3-dimensional luminescence spectra (luminescence intensity as a function of the excitation and emission wavelengths) have been obtained for bismuth-doped optical fibers of various compositions in a wide spectral range (450-1700 nm). The bismuth-doped fibers investigated have the following core compositions: SiO₂, GeO₂, Al-doped SiO₂, and P-doped SiO₂. The measurements are performed at room and liquid nitrogen temperatures. Based on the experimental results, the positions of the low-lying energy-levels of the IR bismuth active centers in SiO₂- and GeO₂-core fibers have been determined. Similarity of the energy-level schemes for the two core compositions has been revealed.

Continuous-wave lasing of Bi-doped fiber lasers in a wavelength range of $1150–1215\mathrm{nm}$ with a power of up to $15\mathrm{W}$ has been obtained for the first time. The unsaturable optical losses in Bi-doped fibers have been revealed and their influence on the Bi-laser operation is investigated. Frequency doubling of the Bi-fiber laser radiation is demonstrated that can be used as a yellow light source (wavelength of $580\mathrm{nm}$).

Bismuth-doped fiber lasers operating in the range $1300–1470\mathrm{nm}$ have been demonstrated for the first time, to our knowledge. It has been shown that Bi-doped alumina-free phosphogermanosilicate fibers reveal optical gain in a wavelength range of $1240–1485\mathrm{nm}$ with pumping at 1205, 1230, or $808\mathrm{nm}$.

Revolver optical fibers (RF) are special type of hollow-core optical fibers with negative curvature of the core-cladding boundary and with cladding that is formed by a one ring layer of capillaries. The physical mechanisms contributing to the waveguiding parameters of RFs are discussed. The optical properties and possible applications of RFs are reviewed. Special attention is paid to the mid-IR hydrogen Raman lasers that are based on RFs and generating in the wavelength region from 2.9 to 4.4 μm.

We report the demonstration of a 4.42-μm silicafiber- based Raman laser delivering a record average output power of 1.4 W and operating in a single transversal mode. The Raman laser is based on a hydrogen-filled hollow-core silica fiber, which is pumped by a nanosecond erbium-doped fiber laser at 1.56 μm. In spite of a large quantum defect of a 1.56 to 4.42 μm conversion, the Raman laser demonstrates no heatinitiated problems. Due to the implementation of a linearly polarized pump and virtually all-polarization-maintaining configuration of the Raman laser, the quantum efficiency has reached 53 %, which is a theoretical limit imposed by optical losses (1.1 dB/m) at 4.42 μm in a state-of-the-art revolver hollowcore silica fiber. The developed laser demonstrates the prospects of gas fiber Raman lasers to serve as pump sources for highpower supercontinuum generation in the mid-infrared spectral range. Such sources can find numerous scientific and biomedical applications.