The quantum yield (QY) of luminescence of bismuth active centers associated with silica (BAC-Si) in bismuthdoped optical fibers was measured. The luminescence bands peaked at 830 nm and 1400 nm were excited at $800 \mathrm{~nm}, 1240 \mathrm{~nm}$ and 1310 nm. The branching ratio (BR) of radiative transitions at 830 nm and 1400 nm excited at 800 nm was defined. The influence of co-doping with boron on QY and BR is examined.

A new active optical fiber with germano-phosphosilicate core, co-doped with Er and Yb, was developed by means of MCVD technology. This fiber has a high UV photosensitivity to the radiation of the 193 nm excimer laser. This feature allows us to avoid hydrogen loading of this fiber for the inscription of the Bragg gratings forming the cavity of a fiber laser. The parameters of this fiber, such as absorption and gain spectra, as well as photosensi-tivity, have been studied in detail. It was shown that high-performance single-frequency fiber lasers can be developed based on this fiber. A Fabry-Perot laser cavity with two fiber Bragg gratings was created based on a 30 mm long "pristine" (non-hydrogenated) fiber segment by means of the 193-nm radiation of the excimer laser. The slope efficiency of this laser was found to be 17% and the linewidth of about 88 kHz.

Efficient spectral filtering was demonstrated in a single-mode optical fiber by insert high-index rods into the silica cladding. Spectrally selective core mode suppression due to different mechanisms (absorption in high-index rods and enhanced mode leakage near the resonance) was demonstrated.

Ultra-highly-Yb-doped silica-based fibers are of great interest for such application as high-repetition rate mode-lock lasers (for reduction of the cavity length) and single frequency lasers (for increase pump absorption in a short, few cm in length, cavity). Another promising application is high-peak power lasers based on pedestal-supported large-mode-area (LMA) highly-Yb-doped fiber [1] . In the last case utilization of core based on aluminophophosilicate (APS) glass doped with nearly equimolar concentration of Al 2 O 3 and P 2 O 5 is highly desirable. The reason is formation of AlPO 4 join which increase solubility of rare-earth elements and have refractive index close to that of pure silica [2] . Reduction of the core refractive index in this case greatly simplify fabrication of the matched Ge-doped pedestal. At the same time maximum possible Yb 2 O 3 concentration in the core of such fiber is highly desirable to get the shortest possible fiber length.

The single-frequency linearly-polarized laser was implemented entirely in a segment of Er/Yb-codoped phosphosilicate single-mode fiber and Ge-doped polarization-maintain fiber. The slope efficiency was found to be ~9 %. The spectral linewidth of the laser did not exceed 170 kHz.

In this work, a single-frequency continuous-wave fiber laser based on the developed in-house Er-Yb-codoped phosphorosilicate fiber was created as a reference signal source for the 1.55 μm-wavelength band operation. Absorption in this particular fiber at 976 nm was 500 dB/m and 60 dB/m at 1535 nm. Small-signal gain measured at 1550 nm was as high as 30 dB/m. The laser generation spectrum had a single narrow peak centered at the 1551.6 nm-wavelength with line width less than 100 kHz. The total cavity length of the laser was 2.5 cm.

Highly Er/Yb co-doped phosphosilicate fibers with low optical losses were fabricated by the MCVD method, utilizing an all-gas-phase deposition technique developed “in house.” Important characteristics such as photodarkening and photosensitivity of these fibers were thoroughly investigated. An almost negligible impact of photodarkening-induced losses (no more than 1 dB/m) was discovered in the near-infrared spectral region. The photosensitivity of hydrogen-loaded fiber samples was studied using excimer laser irradiation at a wavelength of 193 nm. A photoinduced refractive index value of 1 × 10 − 3 at the total UV irradiation exposure dose of 550 J / c m 2 was achieved. A single-frequency laser with a Fabry–Perot short cavity was designed by inscribing highly reflective ( R > 99.9 % ) and partially reflective ( R = 94.3 % ) fiber Bragg gratings directly in the Er/Yb co-doped phosphosilicate fiber core. The laser showed stable CW output emitting with a narrow peak centered at 1551.5 nm in the absence of any signs of Y b 3 + parasite lasing in a wavelength range from 1000 to 1100 nm.

Single-mode Er-Yb fibers with the core based on a phosphorosilicate glass matrix (up to 6.5 mol.% P2O5) highly doped with fluorine (up to 0.9 wt.%) were fabricated using an all- gas-phase modified chemical vapor deposition method. The core numerical aperture was in the range of 0.07-0.08 relative to the pure silica, which allowed us to increase the single-mode core diameter up to 20 mu m. The slope efficiency in lasers based on the fabricated fibers reached 34% relative to the launched pump power.

— This study determines the limiting concentrations of Er 2 O 3 and Yb 2 O 3 in aluminum-phosphorosilicate (APS) glass containing similar concentrations of aluminum and phosphorus, where the signal amplification in the lightguides is still possible. The structural group of AlPO 4 is evaluated as a codoping additive to quartz glass, which causes an increase in the concentration limit for the embedding (solubility) of the oxides of rare earth elements (REEs). The concentration ratios Er 2 O 3 /AlPO 4 and Yb 2 O 3 /AlPO 4 that provide a high amplification efficiency and a low level of optical loss in lightguides are determined.

Using MCVD, we have produced optically active fibers with a glass-ceramic core containing mullite, 3Al2O3 · 2SiO2, as a major crystalline phase. Thermodynamic prediction of mullite formation conditions has been supplemented by thermal analysis and X-ray diffraction characterization of preform core samples, and fiber heat treatment (annealing) conditions have been established. Luminescence measurements for the fibers annealed under optimal conditions demonstrate that optically active chromium has a crystalline local environment.

We have studied absorption spectra of optical fibres doped with rare-earth ions (Sm3+, Tm3+, Tb3+, Pr3+ and Ho3+) and demonstrated that they are potentially attractive for ensuring wavelength-selective absorption in fibre lasers. Such fibres can be used for pump diode protection against back-reflected light at the operating wavelength of neodymium, erbium, erbium - ytterbium or thulium lasers. A proper choice of a rare-earth element makes it possible to ensure a strong absorption (10-20 dB) at the operating laser wavelength in combination with high transmission (loss under a few tenths of a decibel) at pump wavelengths. To demonstrate the potential of the proposed approach, we have fabricated and investigated Tm3+-doped fibre compatible with the output fibre pigtails of standard semiconductor diodes used for pumping (core diameter, 105 mu m; numerical aperture, 0.22). We have demonstrated the possibility of effectively suppressing light in the 1550-nm range by more than 20 dB even in the case of a high-power (up to 10 W) undesirable signal. Under such conditions, the total pump loss does not exceed 0.5 dB and can be further reduced by optimising the core composition and fibre design.

This study determines the limiting concentrations of Er2O3 and Yb2O3 in aluminum-phosphorosilicate (APS) glass containing similar concentrations of aluminum and phosphorus, where the signal amplification in the lightguides is still possible. The structural group of AlPO4 is evaluated as a codoping additive to quartz glass, which causes an increase in the concentration limit for the embedding (solubility) of the oxides of rare earth elements (REEs). The concentration ratios Er2O3/AlPO4 and Yb2O3/AlPO4 that provide a high amplification efficiency and a low level of optical loss in lightguides are determined.