Nano-colloids and nano-crystals doped with ions of rare-earth elements have recently attracted a lot of attention of scientific community. This attention is due to unique physical, chemical and optical properties attributed to nanometer size of the particles. They have great potential of being used in applications spanning from new types of lasers, especially blue and UV lasers, phosphorous display monitors, optical communications, and fluorescence imaging. In this paper we investigate the infrared-to-visible upconversion luminescence in bulk crystals doped with ytterbium and holmium co-doped and ytterbium and thulium co-doped NaYF4 upconversion phosphors. The phosphors were prepared by using simple co-precipitation synthetic method. The initially prepared phosphor has very weak upconversion fluorescence. The fluorescence significantly increased after the phosphor was annealed at a temperature of 600 0C. Nanocolloids of this phosphor were obtained using methanol as solvents and they were utilized as laser filling medium in photonic crystal fibers. Under 980 nm laser excitation very strong upconversion signals were obtained for ytterbium and holmium co-doped phosphor at 541 nm, 646 nm and 751 nm, and 376 nm, 476 nm, 646 nm, 696 nm and 803 nm for ytterbium and thulium co-doped phosphor. The particle sizes of the nanocolloids were analyzed using Atomic Force Microscope. The reported nanocolloids are good candidates for fluorescent biosensing applications and also as a new laser filling medium in fiber lasers.