Suya Feng

Influence of Al3+ and P5+ ion contents on the valence state of Yb3+ ions and the dispersion effect of Al3+ and P5+ ions on Yb3+ ions in silica glass

Three series of Yb3+-doped silica glasses containing different amounts of Al2O3 and P2O5 were prepared successfully by using a sol–gel method. Absorption, excitation and fluorescence spectra of Yb2+ ions in these silica glasses as well as X-ray photoelectron spectra (XPS) of Yb4d were recorded and analysed systematically. It is found out that the addition of Al3+ or P5+ ions has great influence on the redox state of ytterbium ions. With increasing Al3+ ion contents in these silica glasses, more trivalent Yb3+ ions are reduced to divalent Yb2+. In contrast, the increase of P5+ ion contents greatly promotes divalent Yb2+ ions to be oxidized to trivalent Yb3+. The possible redox mechanisms have been explored and discussed in detail. The influence of Al3+ and P5+ ion contents on the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs was also discussed. Both the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs decrease gradually with increasing Al3+ and P5+ ion contents. The decrease of cooperative luminescence of Yb3+ ion pairs indicates a good dispersion effect of Al3+ and P5+ ions on Yb3+ ions in Yb3+-doped silica glass. The results are useful for optimization of fabrication of the high quality Yb3+-doped silica fiber by the composite design of Yb–Al–P co-doped silica glass.

Watt-level Yb-doped silica glass fiber laser with a core made by sol-gel method

A Yb-doped silica glass fiber laser with a core made by sol-gel method is reported. The maximum power of 1.14 W is obtained with a pump power of 5.46 W at a wavelength of 976 nm. The slope efficiency is 34%. The refractive index fluctuation across the core is below 5×10-4 at a doping level of Yb 0.15 mol%, A2O3 4.0 mol%, and P2O5 2.0 mol%. High background attenuation of 6 dB/m at 1 053 nm limites the slope efficiency and maximum output power.

50 μm core diameter Yb³⁺Al³⁺/F⁻ codoped silica fiber with M²<1.1 beam quality.

This paper reports, for the first time to our best knowledge, a nearly diffraction-limited output in a Yb(3+)/Al(3+)/F(-) codoped double cladding silica fiber with a 50 μm core diameter and 0.02 core NA. The core glass with a diameter >6  mm was fabricated through solgel process combined with high temperature sintering. Laser performances of this fiber at different bend diameters were studied. The mean M2<1.1 in this 50 μm core diameter fiber was achieved at a bend diameter of 0.35 m. The core glass with the refractive index nearly equal to pure silica glass is suitable for the fiber design, such as large-mode-area photonic crystal fiber.

2 μm laser properties of Tm 3+ -doped large core sol-gel silica fiber

Tm3+-doped 0.1Tm2O3-1Al2O3-98.9SiO2 (mol%) silica glass with good uniformity was prepared by sol-gel method combining with high temperature sintering. The core glass sized Φ3.2 × 50 mm with Δn of 5 × 10−4 was obtained after gelation, heat treatment, melting and polishing. Its spectroscopic properties were evaluated according to the detected absorption and fluorescence spectra. The maximum emission cross-section of Tm3+ ion in this glass is 6.2 × 10−21 cm2 and tested fluorescence lifetime is 836 μs at 1806 nm. Large core double cladding fiber with core NA of 0.102 was prepared by rod-in-tube and high temperature drawing. Its tested optical loss is 1.1 dB/m at 1333 nm. 1.11 W fiber laser output centered at 1969 nm with M2 factor of 1.99 was obtained from a 140 cm length double-cladding fiber with core diameter of 38 μm. The quasi-single mode laser with M2 factor of 1.33 was achieved in the fiber with core diameter of 19 μm.

Phosphate ytterbium-doped single-mode all-solid photonic crystal fiber with output power of 13.8 W

Single-mode ytterbium-doped phosphate all-solid photonic crystal fiber (AS-PCF) with 13.8 W output power and 32% slope efficiency was reported. By altering the diameter of the rods around the doped core and thus breaking the symmetry of the fiber, a polarization-maintaining AS-PCF with degree of polarization of >85% was also achieved, for the first time to knowledge, in a phosphate PCF.

Yb/Er co-doped phosphate all-solid single-mode photonic crystal fiber

An all-solid Yb3+/Er3+ co-doped single-mode phosphate photonic crystal fiber (PCF) with Watt-level output power and 20 μm core diameter is demonstrated for the first time. A PCF whose refractivity of the active core is lower than that of the background glass is suggested and theoretically confirmed to be in single-mode operation at 40 μm core diameter.

Ytterbium-doped silica photonic crystal fiber laser fabricated by the nanoporous glass sintering technique

Based on the sintering of ytterbium-doped nanoporous glass, a nonchemical vapor deposition process aimed at fabricating a large-mode area of photonic crystal fibers has been proposed for the preparation of active silica glass. With the relatively low doping concentration and boron in the silica glass as benefits, a close refractive index between the core and the cladding was obtained. The refractive index variation indicated good homogeneity of the doped fiber core. Up to 34.8 W laser output was demonstrated with the PCF laser and the slope efficiency was 71.3%. The output laser beam quality had slightly multimode behavior, with a mean M 2 of 2.42.

Optical and laser properties of Yb 3C - doped Al 2 O 3 -P 2 O 5 -SiO 2 large-mode-area photonic crystal fiber prepared by the sol-gel method

Yb 3C , Al 3C and P 5C co-doped silica glass with the composition of 0:05Yb2O3‐2:0Al2O3‐ 1:0P2O5‐96:95SiO2 (mol%) was prepared by the sol‐gel method. Using this glass as the fiber core, the corresponding large-mode-area (LMA) photonic crystal fiber (PCF) was drawn by the stack-capillary-draw technique. Spectral properties of the silica glass chip and LMA PCF are discussed. Pumped with a 976 nm laser diode, the PCF fiber achieved the highest laser output power of 34.6 W at 1040 nm, with a slope efficiency of 62%. To the best of our knowledge, this is the highest laser output power report on Yb 3C -doped silica LMA PCF prepared by the sol‐gel method. (Some figures may appear in colour only in the online journal)

Spectroscopic and laser properties of Al-P co-doped Yb silica fiber core-glass rod and large mode area fiber prepared by sol-gel method

A large-size (∅5 mm) Yb3+-doped silica fiber core-glass rod with an Al-P co-doped composition: 0.035Yb2O3-1.0Al2O3-1.0P2O5-97.965SiO2 (mol%) was prepared by the sol-gel method combined with high-temperature melting technology. We successfully solved the doping homogeneity problem caused by the volatility of P2O5. The doping homogeneity of the glass rod was very high with the maximum refractive index fluctuation Δn < 2 × 10−4. The refractive index of the glass rod was very low because of the equimolar amounts of Al and P co-doping, which yielded an AlPO4 structure. A large-mode-area single cladding fiber (LMA SCF) and photonic crystal fiber (LMA PCF) with core diameters of 35 and 50 µm, respectively, were drawn from this glass rod. Owing to the low refractive index of the core-glass rod and the corresponding low numerical aperture (NA, 0.033) of the core, the LMA SCF exhibited a high laser beam brightness with M2 = 1.3-1.4. The LMA PCF exhibited the maximum output power, which was limited by the available pumping power, of 46 W and the slope efficiency of 61%.

Seven-core neodymium-doped phosphate all-solid photonic crystal fibers

We demonstrate a single-mode seven-core Nd-doped phosphate photonic crystal fiber with all-solid structure with an effective mode field diameter of 108 μm. The multicore fiber is first theoretically investigated through the finite-difference time-domain method. Then the in-phase mode is selected experimentally by a far-field mode-filtering method. The obtained in-phase mode has 7 mrad mode field divergences, which approximately agrees with the predicted 5.6 mrad in seven-core fiber. Output power of 15.5 W was extracted from a 25 cm fiber with slope efficiency of 57%.