Design of rare-earth doped microspheres lasers

In this paper, the design of a lasing system constituted by a tapered fiber coupled to Er³⁺-doped microsphere and operating in Mid-IR wavelength range is numerically investigated. The device performance has been evaluated by means of a dedicated 3D numerical model based on the coupled mode theory and solving the rate equations. In particular, a detailed rate equations model including the main Er³⁺ energy level transitions, the amplified spontaneous emission (ASE) and the most relevant upconversion and cross relaxation mechanism among the Er³⁺ ions is considered. The obtained results highlight that the developed 3D numerical code is an useful tool to evaluate the lasing performance and to calculate the temporal evolution of the optical signals inside the microsphere, correctly working for several device configurations.