Output power scaling of single mode large mode area (LMA) photonic crystal fiber (PCF) amplifiers urgently requires the low refractive index of Yb³⁺-doped silica glasses whilst maintaining high optical homogeneity. In this paper, we report on a promising alternative Yb³⁺/Al³⁺/F^–/P⁵⁺-co-doped silica core-glass (YAFP), which is prepared by modified sol-gel method developed by our group and highly suitable for fabricating high power LMA PCF amplifiers. By controlling the doping combinations of Al³⁺/F^–/P⁵⁺ in Yb³⁺-doped silica glass,it not only ensures low refractive index (RI) but also maintains the excellent optical homogeneity and spectroscopic properties of Yb³⁺. The spectroscopic properties of Yb³⁺ ions have not deteriorated by the co-doping of F^– and P⁵⁺ in YAFP glass compared with that of Yb³⁺/Al³⁺ co-doped silica glass. A large-size (⌀5 mm × 90 mm) YAFP silica-core glass rod with low average RI difference of 2.6 × 10⁻⁴ (with respect to pure silica glass), and low radial and axial RI fluctuations of ~2 × 10⁻⁴, was prepared. A LMA PCF with 50 µm core diameter was obtained by stack-capillary-draw techniques using YAFP core glass. Its core NA is 0.027. An average amplified power of 97 W peaking at 1030 nm and light-light efficiency of 54% are achieved from a 6.5 m long PCF in the pulse amplification laser experiment. Meanwhile, quasi-single-mode transmission is obtained with laser beam quality factor M² of 1.4.

A series of Yb-doped silica glasses containing equimolar amounts of P₂O₅ and Al₂O₃ were prepared using a sol–gel method combined with high-temperature sintering. Raman spectroscopy confirmed the formation of AlPO₄ join with a P/Al ratio of 1. However, for a P/Al ratio slightly larger than 1, both the AlPO₄ join and P=O bond are formed. Based on the absorption, emission spectra, and electron paramagnetic resonance measurements, it was determined that the increase of the AlPO₄ join concentration in Yb-doped silica glass leads to (i) a decrease in the refractive index with a factor of 1.1005 × 10⁻⁴ (which allows for a low numerical aperture facilitating single-mode laser), (ii) the sustained spectroscopic properties of Yb³⁺, (iii) a decrease in Yb cluster concentration, and (iv) an improved radiation hardening performance (beneficial to high-power space lasers). These results suggest that Yb-doped Al₂O₃-P₂O₅-SiO₂ glass with an appropriate concentration of AlPO₄ join has potential applications in high-power space fiber lasers.

This paper reports, for the first time to our best knowledge, a nearly diffraction-limited output in a ${\mathrm{Yb}}^{3+}/{\mathrm{Al}}^{3+}/{\mathrm{F}}^{-}$ codoped double cladding silica fiber with a 50 μm core diameter and 0.02 core NA. The core glass with a diameter $>6\mathrm{mm}$ was fabricated through solgel process combined with high temperature sintering. Laser performances of this fiber at different bend diameters were studied. The mean ${\mathrm{M}}^2<1.1$ 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.

In this work, a heavily Er-doped fiber with an 8 µm core diameter and a numerical aperture of 0.13 was prepared by the modified chemical vapor deposition (MCVD) technique combined with the sol-gel method. The background loss and absorption coefficient at 1530 nm were measured to be 20 dB/km and 128 dB/m, respectively. Thanks to the sol-gel method, the fiber showed a good doping homogeneity, which was confirmed through unsaturable absorption measurement. The net gains of three 25, 45, and 75-cm-long fibers were measured in the range of 1520 to 1600 nm, and the highest gain reached above 23 dB at both 1530 and 1560 nm in 25 and 75-cm-long fibers, respectively. The short-cavity laser performance was measured using centimeter-scale fibers. The maximum output power of 12 mW was demonstrated in a 6.5-cm-long active fiber with a slope efficiency of 20.4%. Overall, the prepared heavily Er-doped silica fiber is a promising item to be applied in a high-repetition-rate or single-frequency fiber laser.

Owing to their excellent performance in terms of short pulse duration and high peak power, ultrashort pulse lasers have gained much attention in the fields of fiber-optics communication, biophotonics, and industrial processing.^{1 – 8} Recently, because of the advantages of compactness, robustness, and freedom from maintenance over conventional solid-state lasers,^{9 – 13} fiberized ultrashort pulses operating in the wavelength range from 900 to 950 nm produced by passive mode-locking techniques have become highly desirable for use in two-photon bioimaging (TPM) systems for the efficient excitation of green fluorescent proteins and their variants.^{14 – 18} Combined with nonlinear frequency conversion, $1.0\text{-}\mu \mathrm{m}$ ${\mathrm{Yb}}^{3+}$-doped,¹² $1.5\text{-}\mu \mathrm{m}$ ${\mathrm{Er}}^{3+}$-doped,¹⁹ and $1.9\text{-}\mu \mathrm{m}$ ${\mathrm{Tm}}^{3+}$-doped^10,13 ultrashort fiber lasers have been utilized to achieve 900- $\sim 950\text{-}\mathrm{nm}$ ultrashort pulses, which have propelled the advances of TPM systems, with the only drawback being the reduced all-fiber configuration as a result of using these additional nonlinear media. As a comparison, the direct generation of ultrashort pulses at 920 nm using ${\mathrm{Nd}}^{3+}$-doped fibers is a more straightforward method^{9,20 – 24} and has attracted considerable research interest because the emission of its three-level (${{}^4\mathrm{F}}_{3/2}\to {{}^4\mathrm{I}}_{9/2}$) transition falls into the 900 to 950 nm wavelength band.