For many applications it is desirable to extend the emission wavelengths of fiber laser systems to the visible spectral range. Most state-of-the-art fiber lasers for the visible range use either a nonlinear process to convert infrared light generated in the fiber, complex host materials like ZBLAN [1], or laser dyes as dopant for polymer fiber exhibiting amplification factors of around 10 dB in a 25 cm amplifier [2]. All of these possibilities have disadvantages like the need for fluoride glasses or the degradation of laser dyes [3]. Core/shell nanoparticles (NP) consisting of crystalline Pr:YLF or Pr:LiLuF could be used as an active material for optical fibers which directly emits in the visible spectral range. This configuration preserves the crystalline character of the gain medium and reduces quenching effects to the host material as discussed in a corresponding contribution submitted by us [4]. In this contribution, we demonstrate that it is feasible to utilize these NPs as doping material for polymer fiber lasers providing amplification factors comparable to laser dyes. It is important to mention that the numerical model used is not restricted to polymer fiber doped, but adaptable to any host material.