Brillouin gain spectrum characterization in an acoustic anti-guided delivery fiber for high power narrow linewidth laser.

The Brillouin gain spectrum (BGS) provides key information for stimulated Brillouin scattering (SBS), such as the Brillouin frequency shift (BFS), Brillouin spontaneous linewidth, and Brillouin gain coefficient. In this study, we theoretically investigate the field distributions and BGS characterization of Ge-doped, Al-doped, and Al/Ge co-doped fibers. Additionally, we analyzed and compared the relationship between the BGS and acoustic refractive index. In particular, we demonstrate the crucial role played by acoustic modes in anti-waveguide structures. The simulation results show that the Brillouin gain coefficient decreases with a decreasing acoustic index in the fiber core region. Furthermore, we experimentally measure the SBS threshold and BGS of the Al/Ge co-doped fiber to examine the validity of the numerical model. The simulated and experimental results are consistent.

Nanocrystalline Yb:YAG-Doped Silica Glass with Good Transmittance and Significant Spectral Performance Enhancements.

In this study, Yb:YAG-nanocrystal-doped silica glass with high transmission and excellent spectral properties was successfully prepared using a modified sol−gel method. The X-ray diffraction (XRD), micro-Raman spectroscopy, electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), and high-resolution TEM (HR-TEM) analyses confirmed that the Yb:YAG nanocrystals, with their low content, homogeneous distribution, and small crystal size, directly crystallized into the silica glass network without annealing treatment. In contrast with conventional microcrystalline glass having large particles (>0.1 µm) and a large particle content, nanocrystalline glass with a homogeneous distribution and sizes of ~22 nm had higher optical transmittance and better spectral properties. Compared with Yb3+ doped silica glass without nanocrystals, the Yb:YAG-nanocrystal-doped silica glass had a 28% increase in absorption cross-section at 975 nm and a 172% enhanced emission cross-section at 1030 nm without any changes in the spectral pattern of the Yb3+ ions in the silica glass. Meanwhile, the Yb:YAG-doped silica glass with large size and high optical quality was easily prepared. Therefore, the Yb:YAG-nanocrystal-doped silica glass is expected to be a promising near-infrared laser material.

Photodarkening-resistance improvement of Yb3+/Al3+ co-doped silica fibers fabricated via sol-gel method.

Yb3+/Al3+ co-doped silica fibers (YDFs) with almost identical core glass compositions were prepared using the sol-gel and modified chemical vapor deposition (MCVD) methods. The photodarkening (PD) and laser performance before and after the PD process were tested under 974 nm pumping. The doping homogeneity of Yb3+ ions and clusters of Yb3+ ions in preform slices of these two fibers were investigated via optical absorption spectroscopy, photoluminescence emission spectra, electron probe microanalysis (EPMA), and low-temperature (4 K) electron paramagnetic resonance (EPR). It is known that the PD resistance of YDFs prepared via the sol-gel method is significantly better than that of YDFs prepared via MCVD under the same test conditions. EPMA mapping reveals that the doping homogeneity of Yb3+ ions in the sol-gel fiber core glass is better than that in the MCVD fiber. The low-temperature (4 K) EPR and cooperative luminescence spectra of Yb3+ ions indicate that the clustering degree of Yb3+ ions in the sol-gel fiber is lower than that in the MCVD fiber. In the absorption and emission spectra, small amounts of Yb2+ ions are observed in the preform slice from the sol-gel method. A model of the color-center generation in the PD process was proposed to explain the mechanism of PD resistance improvement for the YDFs fabricated via the sol-gel method.

Centimeter-scale Yb-free heavily Er-doped silica fiber laser.

The laser behavior of a centimeter-scale Er3+/Al3+ codoped silica fiber with core numerical aperture and core diameter of 0.15 and 8 µm, respectively, is reported. The core glass was prepared by the sol-gel method combined with high-temperature sintering; it contained Er3+ ion concentration as high as 1.32×1020 ions/cm3 and an Al/Er mole ratio of 10. The high doping homogeneity of Er3+ ions in the fiber core was confirmed by an electron probe microanalyzer element scanning, long Er3+: I13/24 emission lifetime of 11.4 ms, and low refractive index fluctuation of fiber core (±1×10-4). The signal gain of fibers with 4.6 cm, 10 cm, and 16 cm lengths was tested in the 1500-1620 nm range. A gain of 7 dB is achieved at 1560 nm in a 16-cm-long EDF under 230 mW pump power. Aimed for CO2 sensor application, the laser behavior of Er/Al codoped fiber was tested at 1572 nm. A slope efficiency of 12% and an output power of 15 mW were achieved in a 10-cm-long fiber under 166 mW absorbed pump power. The newly developed silica fiber is promising for use in high-repetition-rate lasers.

Preparation and characterizations of Nd:YAG ceramic derived silica fibers drawn by post-feeding molten core approach.

This paper presents a novel molten core approach - post-feeding molten core approach - to draw Nd:YAG ceramic derived silica fibers. This technique can effectively mitigate the diffusion of silica from cladding. The diffused silica concentrations decrease from 73.76 wt.% to 45.08 wt.% at the center of cores, by using the post-feeding method. Micro-Raman spectra indicate that the core materials of those fibers are amorphous and maintain an environment similar to YAG glass. The output laser power and slope efficiency are greatly improved. The enhanced performance of this approach shows that it has considerable potential in fabricating hybrid fibers.

50 µm core diameter Yb³âºAl³âº/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.

Neodymium-doped phosphate fiber lasers with an all-solid microstructured inner cladding.

We report on high-power fiber lasers based on index-guiding, all-solid neodymium-doped (Nd-doped) phosphate photonic crystal fiber (PCF) with a hexagonal-shaped inner cladding. The optimum fiber laser with a 36 cm length active fiber, generated up to 7.92 W output power at 1053 nm, which benefited from a high absorption coefficient for pump power due to its noncircular inner cladding. The guiding properties of the all-solid PCF were also investigated. A stable mode with a donut-shaped profile and a power-dependent laser beam quality have been observed experimentally and analyzed.