Mikhail V. Yashkov

Bismuth-doped-glass optical fibers—a new active medium for lasers and amplifiers

Optical fibers with bismuth-doped silicate and germanate glass cores were fabricated by the modified chemical vapor deposition technique (solution and vapor-phase Bi incorporation). The fibers revealed an efficient luminescence with a maximum in the 1050-1200 nm spectral range, FWHM up to 200 nm, and a lifetime of the order of 1 ms.

High-average-power second-harmonic generation from periodically poled silica fibers

The generation of 236 mW of second-harmonic power in a 32-cm-long periodically poled silica fiber, corresponding to an average conversion efficiency of 15.2+/-0.5%, is reported. This represents the highest normalized second-harmonic conversion and the highest average second-harmonic power ever reported for a periodically poled silica fiber, to our knowledge. The enhancement is attributed to an improved design of the specialty twin-hole fiber and the extension of the nonlinear interaction length.

Effect of the AlPO4 join on the pump-to-signal conversion efficiency in heavily Er-doped fibers.

Heavily Er-doped fibers (EDFs) based on P(2)O(5)-Al(2)O(3)-SiO(2) (PAS) ternary glass have been studied. A unique feature of this glass is the formation of a AlPO(4) join having a structure similar to that of SiO(2) glass and a refractive index below it. It is found that the Er(3+) absorption and emission spectra in the PAS EDFs are defined by the dopant (Al(2)O(3) or P(2)O(5)) present in excess and are close to those of the corresponding binary glass (Al(2)O(3)-SiO(2) or P(2)O(5)-SiO(2)). The presence of the AlPO(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 P(2)O(5)-SiO(2) and Al(2)O(3)-SiO(2) (with 1.5 mol. %Al(2)O(3) and less) binary glasses. The PAS host glass is advantageous in the case of large-mode-area active fibers.

Single-mode all-silica photonic bandgap fiber with 20-μm mode-field diameter

An all-silica photonic bandgap fiber with a cladding index difference of approximately 2 % and diameter-to-pitch ratio (d/Λ) of 0.12 was fabricated and studied. To our knowledge, this is the first report on the properties of photonic bandgap fiber with such a small d/Λ. The fiber is single-mode in the fundamental bandgap. The mode field diameter in the 1000-1200 nm wavelength range is 19-20 μm. The minimum loss in the same range is 20 dB/km for a 30-cm bending diameter. In our opinion, all-silica photonic bandgap fiber can serve as a potential candidate for achieving single-mode propagation with a large mode area.

Management of the high-order mode content in large (40 μm) core photonic bandgap Bragg fiber laser

Very large-mode-area Yb(3+)-doped single-mode photonic bandgap (PBG) Bragg fiber oscillators are considered. The transverse hole-burning effect is numerically modeled, which helps properly design the PBG cladding and the Yb(3+)-doped region for the high-order mode content to be carefully controlled. A ratio of the Yb(3+)-doped region diameter to the overall core diameter of 40% allows for single-mode emission, even for small spool diameters of 15 cm. Such a fiber was manufactured and subsequently used as the core element of a cw oscillator. Very good beam quality parameter M(2)=1.12 and slope efficiency of 80% were measured. Insensitivity to bending, exemplified by the absence of temporal drift of the beam, was demonstrated for curvature diameter as small as 15 cm.

Radiation-Resistant Erbium-Doped Fiber for Spacecraft Applications

Radiation-induced absorption and lasing efficiency of a hermetically coated erbium-doped fiber saturated with molecular hydrogen are studied. It is shown that H2-loading of hermetically coated erbium-doped fibers prolongs their service time in space more than in 5 times, making such fibers promising for space applications.

Effects of Yb3+ and Er3+ concentrations and doping procedure on excitation transfer efficiency in Er-Yb doped phosphosilicate fibers

We have studied the effects of preform fabrication procedure and ytterbium and erbium concentrations on the efficiency of excitation transfer from Yb3+ to Er3+ and lasing on erbium transitions in the range 1.53–1.61 μm in phosphosilicate glass fibers. The results indicate that the fabrication of fiber preforms by different MCVD-based processes has no effect on the excitation transfer efficiency. In the ranges 0.07–0.4wt % Er3+ and ∼ 0.4–8 wt % Yb3+ (Yb/Er ratio from ∼ 5 to 40), the excitation transfer efficiency is determined by the ytterbium concentration and is essentially independent of erbium concentration.

Fabrication and optical characterization of silica fibers with a chromium- and alumina-doped core

Silica fibers having a chromium-doped alumina-rich core have been produced by the MCVD process, and their optical properties have been studied. The results demonstrate that increasing the alumina concentration from 6 to 29 mol % increases the percentage of Cr3+ ions in the fiber core and reduces that of Cr4+ ions. Prolonged heat treatment has been shown to increase the room-temperature near-IR luminescence intensity in the fibers. Raman spectroscopy and X-ray diffraction data indicate that high-temperature heat treatment of the fibers leads to the formation of a crystalline mullite phase in the aluminosilicate glass.

Radiation Resistance of Er-Doped Silica Fibers: Effect of Host Glass Composition

With the aim to find the best-suited host glass for radiation-resistant erbium-doped fibers (EDF), radiation-induced absorption (RIA) is measured and analyzed after γ -irradiation to 3.0-4.5 kGy in 18 silica optical fibers with various concentrations of dopants: Al₂O₃ , P₂O₅ , GeO₂ and Er₂O₃ . The RIA dependence on the content of Al₂O₃, P₂O₅ and Er₂O₃ in singly doped fibers is investigated. RIA of fibers codoped simultaneously with Al₂O₃ and P₂O₅ are found to be significantly lower than those of singly Al₂O₃ -doped fibers. This is explained as the result of the formation of AlPO₄-joins in the silica network. GeO₂ codoping of AlPO₄ -doped silica is shown to further reduce RIA. Nevertheless, the application of H₂ -loading and photobleaching of RIA by 980-nm laser radiation shows that the AlPO₄ - and GeO₂ -codoped silica is outperformed by P-free Al₂O₃- and GeO₂-codoped silica, which is, therefore, concluded to be the best host glass composition for radiation resistant EDFs.

Tm3+-doped CW fiber laser based on a highly GeO2-doped dispersion-shifted fiber

A novel all-fiber laser based on a highly GeO2-doped dispersion-shifted Tm-codoped fiber, pumped at 1.56 µm wavelength and lasing at 1.862 µm wavelength with a slope efficiency up to 37% was demonstrated. The single-mode Tm-doped fiber with the 55GeO2-45SiO2 core was fabricated for the first time by MCVD technique. The laser produces spectral side bands, resulting from the four-wave mixing owing to the shift of the zero-dispersion-wavelength of the fiber to the laser wavelength, thus, making it potentially particularly attractive for dispersion management and ultrashort pulse generation.