Near 2 µm Singlemode Laser Emission from a Fluoride Microbottle Resonator by Loss Engineering

The manipulation of laser modes in whispering gallery mode (WGM) lasers has been extensively explored, particularly at near-infrared wavelengths. Extending these efforts into the mid-infrared region has substantial value, given the range of applications possible in that region. Here, we demonstrate singlemode lasing emission at 2 µm in a thulium-doped fluoride microbottle resonator achieved through loss engineering. Leveraging the well-defined spatial distribution of WGMs along the longitudinal axis of microbottle resonators, laser mode selection and suppression are realized by introducing a probe microfiber. Four distinct strategies for singlemode emission are demonstrated: adjusting the axial position of the probe microfiber along the microbottle, modifying the contact angle between the probe microfiber and the microbottle, increasing the diameter of the probe microfiber, and employing an add-drop configuration. Our work establishes a systematic approach to laser mode manipulation in fluoride microbottle resonators, advancing the practical applicability of mid-infrared WGM microlasers.