Thermo-Optical Tuning of Whispering Gallery Modes in Erbium Doped Microspheres

Spherical whispering gallery mode (WGM) resonators can be used in a broad range of applications from bio-sensing to laser engineering. Beyond the interest for applied studies, such resonators are also of interest for more fundamental studies, e. g. cavity QED. A key requirement for many applications is the ability to tune the resonator to an energy transition of the atomic species (or material) under investigation. Heretofore, heating the cavity with an external heater, or deforming the cavity mechanically, have been the two main approaches used to tune the cavity size. We demonstrate thermo-optical methods of tuning the WGM resonance frequencies of doped glass microspheres over a very large dynamic range. Er:Yb phosphate glass (IOG2) microspheres are pumped at 978 nm via a tapered optical fibre. This causes internal heating of the microsphere and the temperature of the mode volume can reach temperatures higher than 800 degrees C. With the heat concentrated in the optical mode volume, the resonance frequency has been tuned by similar to 700 GHz nonlinearly. Alternatively, we show that large linear tuning up to similar to 488 GHz is achievable if the microsphere is separately heated by coupling laser light into its support stem.