The whispering-gallery-mode (WGM) microcavity sensor has the advantages of a small mode volume and a high quality (Q) factor, and thus it can be applied in high-sensitivity sensing of various physical quantities. Now, common coupling methods for exciting WGMs include prism coupling, tapered fiber coupling, and fiber end coupling. The main disadvantage of prism coupling is that the system is bulky and not easy to be applied to sensing. Tapered fiber coupling is the most common method, whose coupling efficiency can reach 99%. However, the waist diameter of the tapered fiber is too small, and the effective waist diameter should be less than 2 μm to effectively excite WGMs, which makes the overall structure fragile. The fiber end coupling features low efficiency and poor stability, and the control of the coupling angle is difficult. In this paper, an in-fiber WGM microsphere resonator is proposed, which is composed of single-mode fiber (SMF) and hollow-core fiber (HCF). The inner diameter of HCF is small, and the light intensity reflected by the fiber end after corrosion is relatively large, which can effectively improve the stability of the reflection spectrum and play a role in temperature and refractive index sensing.

First, we use simulations to analyze the phase matching of the coupling between microsphere cavities of different sizes and fiber structure and obtain the influencing factors of the spectral shape. It is concluded that the phase difference δ can be changed by the control over the distance between HCF etching end and coupling region to obtain a better Fano profile and increase the slope. Second, in device preparation, the phases of SMF and HCF are fused, and the HCF is cut into a segment of about 2 mm by a fixed-length cutting device. The segmented HCF is then vertically immersed in a hydrofluoric acid (HF) solution with a volume fraction of 40% for etching. Third, a tapered fiber is used as a probe to pick up and move the barium titanate microspheres, which are embedded in the HCF to form a fiber-type resonator structure. In the experiment, it is found that the WGM excited in the microsphere cavity interacts with the reflected light at the HCF end, which results in Fano resonance. The resonator has both temperature and refractive index sensing capabilities. The conclusions obtained by calculation and simulation are consistent with the experimental results.

The optical fiber simulation model is built by the beam propagation method. When the fiber length is fixed, a smaller inner diameter of HCF means stronger light intensity reflected by the fiber end (Fig. 2). In addition, the appropriate size of microspheres is selected by simulation to excite WGMs (Fig. 3). The simulation shows that the phase difference δ is the main factor affecting the spectral shape, and δ can be changed by the control over the distance between HCF etching end and coupling region to obtain a better Fano profile and increase the slope (Fig. 4). During the sensing experiment, the WGM excited in the microsphere cavity participates in the Fano resonance with a slope of -99.3 dB/nm (Fig. 9), and the cavity can sense the temperature and refractive index. In the temperature sensing experiment, the temperature sensitivity of Fano line of the resonator is 26.8 pm/℃ (Fig. 10), which is consistent with the simulation results obtained in the previous section (Fig. 5) and is higher than the sensitivity of the Lorentz line (Fig. 11). In the refractive index sensing experiment, the Fano line is degraded to the Lorentz line, and the refractive index sensitivity is -244.97 dB/RIU (Fig. 12). The calculation method of the optical path difference can be used to confirm that WGM is excited inside the microsphere cavity (Fig. 13).

In this paper, an in-fiber WGM microsphere resonator is fabricated and investigated, and the temperature and refractive index sensing characteristics are studied. The influence of different parameters on the shape of the Fano resonance spectrum is explored. Through simulation, the formation of the Fano profile is researched by the matching of the fiber structure and microsphere diameter with the help of the propagation constant. Moreover, the interval of the theoretical value L that can lead to a better Fano profile is calculated, which is of guiding significance for subsequent experimental operations. The experiments demonstrate the temperature and refractive index sensing characteristics of the designed structure, with temperature sensitivity of 26.8 pm/℃ and reflective index sensibility of -244.97 dB/RIU. The resonator is stable, compact, and simple to process, and this in-fiber structure is expected to be applied in complex sensing environments.