A 2 μm single-mode laser based on a femtosecond-laser-modified fluoride microsphere resonator

In this Letter, we present a mode manipulation strategy based on femtosecond-laser-induced local modification of a fluoride microsphere resonator. Two symmetric modification regions are precisely inscribed on both sides of the equatorial plane of the resonator, introducing a permanent refractive index modification through nonlinear laser-material interactions. This engineered modification effectively suppresses high-order axial modes, enabling stable single-mode lasing at 1918.58 nm with a low threshold of 2 mW. The proposed approach simplifies the resonance spectrum and provides a built-in solution to the intrinsic multimode limitations of whispering-gallery-mode (WGM) lasers, offering great potential for integrated photonics and mid-infrared light sources.