The up-conversion luminescence in the Er3+/Yb3+/Tm3+ tri-doped tellurite glass microsphere coupled by the tapered fiber

In this paper, transparent Er3+/Yb3+/Tm3+ tri-doped tellurite glass samples were prepared by high temperature calcination method and drawn into filaments when in molten state. The tip of each filament selected was melted by flame and subsequently fabricated into a microsphere via the surface tension of liquid. The microsphere resonators were effectively pumped by 976 nm/1550 nm continuous wave (CW) lasers respectively, using the same biconical tapered fiber which acted as a coupler, transmitting the pump light into the microspheres under uniform conditions and collecting the up-conversion (UC) fluorescence from the microspheres at the same time. We obtained blue, green and red UC emission upon both two types of excitation, but with different intensity distributions. The UC luminescence mechanism of the Er3+/Yb3+/Tm3+ tri-doped samples has been investigated in the perspective of the energy level structure of these rare earth (RE) ions. We find that different pump light sources may cause different energy transfer paths among ions, which thus results in the difference on spectral distributions. It is predicted that such kind of multi-color luminescence material has potential to be applied in the white lighting. In addition, we also did some theoretical analyses on the propagation constant matching that greatly influences the coupling between the microsphere and the tapered fiber, and discovered the fact that it may require a pretty small size of the microsphere for effective coupling when there exists a large refractive index difference between them. Besides, it seems that the waist of the fiber has to get thinner when the light has a shorter wavelength. The outcomes from our experiments were in correspondence with the results above. These results will pave the way for the research on further improvement of the coupling efficiency between the microsphere and the tapered fiber.