Fig. 1. (a) Proposed refractive index sensor structure. The input single-mode fiber (SMF) was fusion spliced to V-groove fiber (VGF) where MMI occurred. The MMI spectrum is transmitted through the output SMF. The liquid droplet was dispensed over the VGF, and the spectral shift caused by the RI change in the surrounding medium was measured. (b) Cross-sectional microphotograph of the fabricated VGF used in the experiments; (c) schematic process to make the VGF.

Fig. 2. (a) Schematic cross-sectional view of the VGF in simulations; (b)–(h) guide modes along the V-groove fiber and their intensity distribution at λ=1550  nm. Here we assume the refractive index of liquid to be 1.35.

Fig. 3. (a) Impacts of nliq on the effective index of the fundamental mode neff. (b) Impact of the liquid outer boundary radius r on the effective index of the fundamental mode neff.

Fig. 4. Schematic view of the RI sensing based on an SVS structure. Here ASE is the amplified spontaneous emission light source; OSA is the optical spectrum analyzer. SVS stands for SMF-VGF-SMF structure.

Fig. 5. (a) Transmission spectra through the proposed sensor for various ethanol concentrations in the aqueous solution. (b) Shift of the spectral dip for various ethanol concentrations. (c) Spectral shift versus the RI of the solution.

Table 1. Effective RI and the Corresponding Coefficient of the Excited Modes in VGF