The paper proposed and studied a Mach-Zehnder mode interferometric refractive index sensor, which is based on splicing points tapered SMF-PCF-SMF (SMF, single-mode fiber; PCF, photonic crystal fiber) structure. For the reason that the effective refractive index of photonic crystal fiber cladding high-order modes near fiber core are more sensitive to surrounding refractive index changes, the refractive index measurement sensitivity of splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer can be enhanced further through tapering the splicing points. Relations between refractive index measurement sensitivity and photonic crystal fiber length and taper waist diameter are studied through numerical simulations and experiments. Simulation and experimental results show that sensitivity will be increased with the increase of photonic crystal fiber length and the decrease of taper waist diameter. In the refractive range of 1.3333-1.3737, splicing points tapered SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4cm and taper waist diameter of 60.4μm has refractive index measurement sensitivity of 260.8nm/RIU, compared with sensitivity of 224.2nm/RIU of direct splicing SMF-PCF-SMF Mach-Zehnder mode interferometer with PCF length of 4cm, the sensitivity increased by 16.3%. The research shows that the sensing structure is with good linearity and repeatability.

We designed a cylinder-type fiber-optic Vernier probe based on cascaded Fabry-Perot interferometers (FPIs) in this paper. It is fabricated by inserting a short single-mode fiber (SMF) column into a large-aperture hollow-core fiber (LA-HCF) with an internal diameter of 150 µm, which structures a length adjusted air microcavity with the lead-in SMF inserted into the LA-HCF from the other end. The length of the SMF column is 537.9 µm. By adjusting the distance between the SMF column and the lead-in SMF, the spectral change is displayed intuitively, and the Vernier spectra are recorded and analyzed. In sensitivity analysis, the probe is encapsulated in the medical needle by ultraviolet glue as a small body thermometer when the length of the air microcavity is 715.5 µm. The experiment shows that the sensitivity of the Vernier envelope is 12.55 times higher than that of the high-frequency comb. This design can effectively reduce the preparation difficulty of the optical fiber Vernier sensor based on cascaded FPIs, and can expand the applied fields by using different fibers and materials.

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