Tunable single-frequency lasing in whispering gallery resonators

Whispering gallery resonators (WGRs) are ideally suited for the realization of miniaturized lasers. Due to their high quality factor and small mode volume, they allow for low-threshold and narrow-linewidth emission from (sub)millimeter-sized cavities made of laser-active materials. However, so far the majority of experimental realizations relies on expensive pump light sources like narrow-linewidth or pulsed laser systems, impeding most applications. We demonstrate two whispering-gallery-based single-frequency lasers pumped by compact spectrally multimode low-cost laser diodes. The spheroidally-shaped millemeter-sized WGRs are made of Pr:LiLuF4 and Nd:YVO4. They provide quality factors beyond 107 at the lasing wavelengths (640 nm and 1064 nm, respectively). The pump light is focused onto the rim of the WGR. We observe single frequency emission at milliwatt output powers. The temporal stability of the output power and of the output frequency are determined to be ±1:5 % and ±30 MHz within 30 min, respectively. By changing the temperature of the cavity, we achieve mode-hop-free tuning exceeding 11 GHz.