High-$\mathrm{Q}$ Whispering-Gallery-Modes Microresonators for Laser Frequency Locking in the Near-Ultraviolet Spectral Range

This study presents a comprehensive analysis of a whispering gallery mode (WGM) microsphere optical properties in the near ultraviolet spectrum, and its practical implementation for laser linewidth reduction via frequency locking. The light coupling is achieved thanks to the utilization of a robust angle-polished fiber, enabling exploration of various coupling behaviors. The intrinsic $Q_{0}$-factor, measured at $2.2\times 10^{8}$, along with a finesse of $7.3 \times 10^{4}$, is reported at 420 nm. Physical mechanisms contributing to the $Q_{0}$-factor are discussed and routes to improve the performances are drawn. Through the implementation of frequency locking onto a high-Q resonance of the WGM microsphere, the reduction of the linewidth of an external cavity diode laser from 887 kHz to 91 kHz has been obtained. The study of these outcomes brings to performance assessment, enabling a thorough understanding of limitations and identifying potential pathways for enhancing noise reduction. Such high Q-factor and high finesse are key ingredients to ease the study of photonic devices based on WGM microresonators.