Fabrication of an all-silica microsphere-lens on optical fiber for free-space light coupling and sensing in extreme environments

In this paper, an all-silica microsphere-lens was designed and fabricated on the fiber end face, which can effectively improve the coupling efficiency of free-space light. In the production process, a coreless silica fiber with specific length was spliced on the end face of the fiber and melted by a CO₂ laser fusion splicer. Due to the effect of surface tension, the coreless silica fiber would form a microsphere-lens on the fiber end face and the diameter of the microsphere-lens could be adjusted by controlling the light-passing time of the CO₂ laser fusion splicer. Through experiments, it can be found that the 3 dB bandwidth optical coupling distance of the microsphere-lens with a diameter of 270 µm is about 200 µm, and the focus depth is about 450 µm. In order to verify the feasibility of using the microsphere-lens in the fiber-optic Fabry-Perot sensors, a Fabry-Perot interferometer was constructed by using the microsphere-lens and the single-mode fiber end face. The experimental results showed that the interference spectrum of the Fabry-Perot interferometer has a good contrast ratio. Integrating the advantages of all-silica structure, simple manufacturing process, low cost, small size, and sturdy construction, the proposed microsphere-lens is expected to be a potential candidate for free-space light coupling and fiber-optic sensors in extreme environments.