Due to their ultra-high quality (Q) factor and extremely small mode volume, optical microcavities supported whispering gallery modes are becoming increasingly popular in photonic devices such as micro-lasers, optical filters, nonlinear effects, and optical sensors. However, the external environment, such as changes in environmental pressure, mechanical vibration, dust, and temperature fluctuations, has a considerable impact on the resonance characteristics of this type of microcavity system. The material loss and surface roughness are two important factors that affect the Q value of the microcavity and should not be overlooked. If the silica microcavity is exposed to normal environment, it will adsorb water molecules in the air, reducing the Q factor; therefore, the external environment's interference with the microcavity must be eliminated. To make it suitable for outside applications, the packaging method based on the microcavity must be examined further. Furthermore, the packaging technology must be updated to improve the stability of the coupled microcavity system.
The silica microsphere cavity is fabricated by using arc discharge to melt the tip of a single-mode optical fiber. When the end of a standard single-mode fiber is melted and solidified, it forms a sphere with an ultra-smooth surface. The tapered fiber, created using the heat-and-pull technique, serves as a waveguide to excite the microsphere's whispering gallery modes. Subsequently, we propose a whispering gallery mode microcavity and fiber tapered waveguide coupling system that is indirectly packaged. The adjusted coupling state before packaging is not changed during the packaging process, and the silica microcavity is fixed in a sealed and clean transparent glass box. The packaging method has the advantages of ensuring a good coupling state, being isolated from the external environment, portability, and long-term stability of the ultra-high-Q factor. The stability of the transmittance, resonant wavelength point, and full width at half maximum are calibrated and analyzed using the Allan variance. Finally, the proposed packaged devices are compared to other packaging methods.
The resonance spectrum of the packaged microsphere cavity is measured using an experimental setup. The Lorentz function is used to fit the resonant peak at 9.29 GHz. A typical Q factor after packaging is still approximately 5.1×107 (Fig. 4). The stability of the resonance performance is evaluated to test the stability of the packaged microcavity. The Allan variance is used to describe the variation of the transmittance, resonant wavelength point, and full width at half maximum(Δf) with time. The results indicate that the transmittance is basically constant (0.4 nm), indicating that the resonant state is very stable for up to 1000 s. (Fig. 6). Furthermore, the resonant wavelength point shows a rising trend, implying that the resonant point appears in a red-shifted state (Fig. 7). The minimum value of Allan variance of the resonant point is near 1 s and the minimum value of Allan variance of Δf is near 23 s. In this experiment, the Q factor of the packaged microcavity device reaches 5.1×107, which is five times higher than the Q factor of the spot-packaged silica microsphere, and one to two orders of magnitude higher than the Q factors of wholly-packaged silica microsphere, microbottle, microbubble, and As2S3 microsphere (Table 1). The Q factor of our packaged silica microcavity is four orders of magnitude greater than that of the indirectly packaged polymer microcylindrical cavity. The ultra-high Q factor contributes to the high application value for micro-lasers, optical filters, optical gyroscopes, and high sensitivity sensors.
In this study, we propose a novel indirectly packaged whispering gallery mode optical microcavity device. A photodetection system is used to analyzed the resonance characteristics of the packaged device, and the Q value reaches 5.1×107, which is higher than the Q values of other spot-packaged and wholly-packaged devices. The packaging method not only has excellent characteristics such as strong robustness, stability, interference immunity, and portability, but also guarantees an ultra-high Q factor. The stability of the packaged microcavity is thoroughly examined, and Allan variance is used to assess the stability of the resonant mode. The result demonstrates that the resonant state is stable for up to 1000 s, but the resonant point drifts due to the thermal effect. In the following work, a temperature control system and a frequency locking system are being considered to solve this problem. Our research provides a solid foundation for the practical use of microcavity devices and contributes to the advancement of microcavity technology. The evaluation method introduces a novel idea for analyzing the stability of microcavity devices. The packaged devices are highly promising in many applications including micro-lasers, optical filters, optical gyroscopes, and high sensitivity sensors.
