Optical Vernier sensor based on a cascaded tapered thin-core microfiber for highly sensitive refractive index sensing

This study proposes a refractive index (RI) sensor using a cascaded tapered thin-core microfiber (TTCMF) based on the Vernier effect. The thin-core fiber was made into a TTCMF by arc discharging and flame heating and then sandwiched between two single-mode fibers (SMFs). The two structures with the same SMF–TTCMF–SMF but slightly different free spectral ranges (FSRs) were cascaded to generate the Vernier effect. The FSR varied with the taper parameters of TTCMF. The RI sensitivities of a single TTCMF sensor, series SMF–TTCMF–SMF sensor, and parallel SMF–TTCMF–SMF sensor were compared and analyzed. Using the Vernier effect in the RI measurement range from 1.3313 to 1.3392, a very high RI sensitivity of − 15 , 053.411 n m / R I U was obtained using the series SMF–TTCMF–SMF structure, and − 16 , 723.243 n m / R I U using the parallel structure, which were basically consistent with the simulation results. Compared with the RI sensitivity of the single TTCMF sensor, the RI sensitivities of series and parallel sensors were increased by 4.65 times and 5.16 times, respectively. In addition, in the temperature range from 35°C to 65°C, temperature sensitivities of − 0.196 n m / ∘ C and − 0.0489 n m / ∘ C were obtained using series and parallel structures, respectively; the corresponding temperature cross errors were 1.302 × 10 − 5 R I U / ∘ C and 2.92 × 10 − 6 R I U / ∘ C , respectively. Based on the advantages of high RI sensitivity, simple structure, low-temperature cross sensitivity, and convenient fabrication, the proposed sensors have great potential in biosensing fields.