Fiber coupled erbium doped microsphere: NIR and mid-IR wavelength ranges

The paper is a review on modelling and experimental results regarding microsphere resonators. The applications of the whispering-gallery-modes (WGMs) resonances in conventional glass microspheres to perform chemical spectroscopy, biomedical sensing, low threshold lasing are recalled. The behaviour of silica microsphere coated via silica hafnia 70SiO₂-30HfO₂ and activated with 0.3% mol Er³⁺ for NIR operation is described. The feasibility of amplifying systems, operating in Mid-IR wavelength and based on Er³⁺-doped Ga₅Ge₂₀Sb₁₀S₆₅ chalcogenide microspheres coupled to tapered a fiber is discussed. The investigation has been performed via a dedicated 3D numerical model, based on the coupled mode theory and solving the rate equations. The code includes the transitions among the erbium energy levels, the amplified spontaneous emission and the most important secondary transitions pertaining to the ion-ion interactions. The model follows the particle swarm optimization (PSO) approach, allowing both the amplifying system optimization and to perform the microsphere glass spectroscopy.