Nd:YAG nanocrystals incorporated into silica fiber via dip-coating: Effect of the drawing process on their structure and emission

We studied the dip-coating method for developing active fibers with a volumetric distribution of nanocrystals doped with rare-earth ions in the optical fiber core. The influence of the two-step fiber drawing process on the structure and luminescence properties of nanocrystals was investigated. Neodymium-doped yttrium aluminum garnet (Nd:YAG, 1 mol. %) nanocrystals were incorporated on a set of Ge-doped silica rods, which were stacked together in a hexagonal lattice, forming a preform of the fiber core. After the first drawing of the preform into the fiber subpreform, we observed the extended emission lifetime of Nd3+ cations at 1064 nm (τ = 457 μs), broadened luminescence bands compared to the crystalline Nd:YAG, and the lack of Raman shifts in the range of 100–900 cm−1 characteristic for the crystals. These results indicate the dissolution of Nd:YAG crystals in the glass during thermal treatment. After the second thermal drawing process, the final active fiber was developed. It had attenuation below 1 dB/m for 750–2050 nm, a cutoff wavelength at 1670 nm, and a numerical aperture NA = 0.159. Moreover, its photoluminescence spectra and lifetime (τ = 468 μs) were examined. These results show that although the Nd:YAG nanocrystals dissolved in the glass, the dip-coating method can easily introduce active ions into the optical fiber, with a uniform volumetric distribution in the core and without clustering of active cations.