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.

We demonstrate a novel amplification regime in a counter-pumped, relatively long (2 meters), large mode area, highly Yb-doped and polarization-maintaining tapered fiber, which offers a high peak power directly from the amplifier. The main feature of this regime is that the amplifying signal propagates through a thin part of the tapered fiber without amplification and experiences an extremely high gain in the thick part of the tapered fiber, where most of the pump power is absorbed. In this regime, we have demonstrated 8 ps pulse amplification to a peak power of up to 0.76 MW, which is limited by appearance of stimulated Raman scattering. In the same regime, 28 ps chirped pulses are amplified to a peak power of 0.35 MW directly from the amplifier and then compressed with 70% efficiency to 315 ± 10 fs, corresponding to an estimated peak power of 22 MW.

Heavily Er-doped fibers (EDFs) based on ${\mathrm{P}}_2{\mathrm{O}}_5–{\mathrm{Al}}_2{\mathrm{O}}_3–\mathrm{Si}{\mathrm{O}}_2$ (PAS) ternary glass have been studied. A unique feature of this glass is the formation of a $\mathrm{Al}\mathrm{P}{\mathrm{O}}_4$ join having a structure similar to that of $\mathrm{Si}{\mathrm{O}}_2$ glass and a refractive index below it. It is found that the ${\mathrm{Er}}^{3+}$ absorption and emission spectra in the PAS EDFs are defined by the dopant (${\mathrm{Al}}_2{\mathrm{O}}_3$ or ${\mathrm{P}}_2{\mathrm{O}}_5$) present in excess and are close to those of the corresponding binary glass (${\mathrm{Al}}_2{\mathrm{O}}_3–\mathrm{Si}{\mathrm{O}}_2$ or ${\mathrm{P}}_2{\mathrm{O}}_5–\mathrm{Si}{\mathrm{O}}_2$). The presence of the $\mathrm{Al}\mathrm{P}{\mathrm{O}}_4$ join results in the enhancement of the pump-to-signal conversion efficiency in the PAS EDFs as compared with the EDFs based on the ${\mathrm{P}}_2{\mathrm{O}}_5–\mathrm{Si}{\mathrm{O}}_2$ and ${\mathrm{Al}}_2{\mathrm{O}}_3–\mathrm{Si}{\mathrm{O}}_2$ (with $1.5\mathrm{mol.}\%$ ${\mathrm{Al}}_2{\mathrm{O}}_3$ and less) binary glasses. The PAS host glass is advantageous in the case of large-mode-area active fibers.

We propose a novel saddle-shaped Yb-doped fiber design for high power generation in the spectral range near 0.976 μm. The fiber has a central single-mode part with a core diameter of approximately 12 μm and an ultra-thin silica clad of approximately 49 μm. At both ends of the fiber, the core/clad diameters were adiabatically increased up to 20/80 μm to be compatible with ordinary passive fibers. A monolithic 976-nm single-mode continuous wave laser based on the proposed fiber was created. The laser’s power was a record high compared to all-fiber laser schemes, with an output power of 10.6 W. Utilization of a photodarkening-free core glass matrix has allowed us to demonstrate perfect long-term stability (without any power degradation) of the developed laser over 45 hours.

Prospects for average power scaling of sub-MW output peak power picosecond fiber lasers by utilization of a Yb-doped tapered fiber at the final amplification stage were studied. In this paper, it was shown experimentally that a tapered fiber allows the achievement of an average power level of 150 W (limited by the available pump power) with a peak power of 0.74 MW for 22 ps pulses with no signs of transverse mode instability. Measurements of the mode content using the S² technique showed a negligible level of high order modes (less than 0.3%) in the output radiation even for the maximum output power level. Our reliability tests predict no thermal issues during long-term operation (10⁵ hours) of the developed tapered fiber laser up to kilowatt output average power levels.

Silica-based optical fibers with an ultra-high Yb concentration were systematically studied. Three the most commonly used in industry glass matrixes for active fiber core were investigated: aluminosilicate, phosphosilicate and aluminophosphosilicate. For all the glass hosts optical fibers doped with a record high concentration of Yb in a glass core were fabricated utilizing an all-gas-phase deposition based on MCVD technology. The factors limiting increase of Yb content in glasses and fibers were revealed. For the first time it was shown that highly Yb-doped fibers could nearly completely lose their active properties and the most probable reason for that is concentration quenching of luminescence.