Abstract: The present invention provides optical fiber devices, which emit optical radiation at pre-selected multiple wavelengths. The fiber devices incorporate semiconductor nanocrystals into the fiber core or cladding which fluoresce when irradiated by light of greater energy than the energy gap of the nanocrystal. The nanocrystals are chosen so that, when irradiated by an excitation source, they fluoresce thereby emiting optical radiation and, thus, act as a light source with tunable wavelength of emission depending on the size of nanocrystal, which is incorporated within the fiber itself. In one embodiment the fiber optic device is a fiber optic diffuser, which emits at multiple wavelengths by incorporating semiconductor nanocrystals having a preselected distribution of sizes, and such multi-wavelength diffusers offer an additional capability of tuning the spectral content of diffused radiation for specific medical or fiber optic communication applications.