Single-mode selection in a non-strongly coupled whispering gallery mode cavity

Mode selection through coupling multiple cavities has proven to be an effective method for constructing single-mode lasers. In coupled Fabry–Perot cavities, it is well accepted that mode selection relies on the strong coupling effect of closely contacted component structures. However, in coupled whispering gallery mode (WGM) cavities, the mode-selection mechanism remains under debate due to the inconsistent characteristics of the selected mode. Herein, we elucidate that the inconsistency can be attributed to varied selection mechanisms led by the trade-off between coupling strength and field distortion. Particularly, it is demonstrated that except for a traditional strong-coupling configuration, single-mode selection can also be achieved in an intermediate-coupling WGM configuration, where two component cavities depart from each other. This study addresses the gap in understanding mode selection in WGM cavity structures and explains the varied characteristics of single-mode lasing reported in the literature, thereby offering new insights into the development of miniaturized, low-threshold, single-mode laser devices.