## 1 Introduction  
Fiber lasers are one of human breakthroughs that have changed the modern world. They can be found practically everywhere in daily life. Besides their traditional role in telecommunications as amplifiers in optical networks [1; 2], they are a key component of sophisticated photonic devices which are used in data processing systems, time metrology, mechanical engineering, etc. [3; 4]. An increase in fiber laser power from 30 W to 100 kW in the last two decades [5; 6] should be considered as a great success of modern photonics. However, high laser powers have brought demanding requirements on fiber laser materials, such as high thermal stability, improved luminescence, etc. [7]. Recent progress in nanotechnologies has brought a completely novel powerful tool to this field.  
Rare-earth doped glass has been used as basic material in active fiber core. [PERSON] [8] suggested to replace glass in the fiber core with glass-ceramic material to improve the active fiber properties. This innovative approach started a new research field of glass-ceramic and nanoparticle doped fibers [9; 10]. However, the prepared fibers exhibited quite high background losses in comparison with conventional glass fibers [6; 11; 12; 13; 14]. The nanoparticle doping of alumina nanoparticles, for the first time presented at 20 th Annual Meeting of the IEEE Lasers and Electro-Optics Society in 2007 [15], has brought the major breakthrough in the field of nanoparticle doped active optical fibers. It was demonstrated that the nanoparticle doping is fully competitive with the conventional solution doping method. This novel method started a glamour career; it has attracted huge attention of the researchers [6; 16; 17] and has rapidly found its place in industry [18; 19]. It has been generally accepted that the nanoparticle doping significantly improves properties of active optical fibers [16; 20; 21] in comparison with the traditional solution doping method [11]. A homogeneous distribution of nanoparticles inside the fiber core suppresses optical losses. The nanoparticle structure and their composition can improve luminescence properties of the incorporated rare-earth element which determines the operating wavelength of the fiber laser [1; 6; 17].  
A number of advanced methods and materials have been studied to produce active fibers suitable for fiber lasers. Besides the nanoparticle doping, several concepts of fiber preparation like phase nanoseparation [21; 22; 23], in-situ thermally-induced crystallization [24; 25], and direct nanoparticle deposition [26], etc. have been elaborated. However, the nanoparticle doping approach is still very popular because it allows us to simply extend the common MCVD process.  
Here we present the preparation of fiber lasers doped with Er-doped yttrium aluminum garnet nanocrystals. The conventional MCVD method was combined with a solution-doping deposition of Er:YAG nanocrystals which were prepared by a hydrothermal reaction. Effects of the nanocrystal size and silica frit's porosity on the final structural and optical properties of the prepared preforms and optical fibers were studied and optical and wave-guiding properties of prepared fibers were determined. Selected optical fibers were tested as an active medium in a ring laser setup and their laser characteristics were examined.