Fig. 1. Experimental configuration of the proposed RI sensor based on cascaded in-line interferometer and MWP interrogation system. SMF, single-mode fiber; NCF, no-core fiber; TCF, thin-core fiber; BOS, broadband optical source; ISO, isolator; TS, translation stage; PC, polarization controller; EDFA, Er-doped fiber amplifier; EOM, electro-optic modulator; DCF, dispersion compensation fiber; PD, photodiode; VNA, vector network analyzer.

Fig. 2. (a) Interference spectra of the STNTS structure with different tapers’ lengths and the same length of TCF. (b), (c) The microscope images of the fabricated MZI near two ends, respectively.

Fig. 3. (a) Transmission spectra and (b) the upper envelope changes of the cascaded interferometer under different SRIs with 20 mm TCF. (c) The quadratic fitting curve in the wavelength domain. (d), (e) The frequency responses of the sensor and the corresponding fitting curve, respectively. The blue-shaded region represents the linear region of the quadratic fitted curve within $1.397-1.412\text{RIU}$.

Fig. 4. (a), (d) Transmission spectra evolution of the proposed STNTS structure with different SRIs when TCF = 25 and 30 mm, respectively. (b), (e) The corresponding frequency responses. (c), (f) The quadratic fitting curves in the frequency domain. (g) The linear relationship between sensitivity and SRI. The blue-shaded region represents the linear region of the quadratic fitted curve within $1.397-1.412\text{RIU}$.

Fig. 5. (a) Frequency response of the proposed sensor with no taper and TCF = 30 mm. (b) The data points of central frequency change with different SRIs.

Fig. 6. (a) Frequency response of the MPF based on cascaded interferometer under different temperatures with 30 mm TCF. (b) The relationship between central frequency and temperature. (c), (d) The characteristic of stability within 1 h at $n=1$.

Table 1. Comparison of the parameters of different RI sensing schemes.