Overview of the coupling methods used in whispering gallery mode resonator systems for sensing

Whispering gallery mode (WGM) microcavities have a small mode volume, a high quality (Q) factor and great application potential in the field of integrated optics. The coupling of light into microcavities has attracted wide attention in the development of optical filters, sensors, and low-threshold lasers. WGM microcavities based on different coupling methods have promising applications for lasing and high-precision sensing. In a WGM coupling system, the coupling method determines the coupling efficiency, Q factor and sensor integration and even helps to determine the WGMs that are excited in the microcavity. This paper briefly reviews the coupling methods used for WGMs, summarizes their characteristics, and introduces the coupling methods of recent advances in the field, all of which are instructive for improving the coupling methods in the future.