Fig. 1. Schematic of tapered optical fiber coupler and basic principle of vernier effect

Fig. 2. Calculated results of mode properties of optical fiber coupler. (a) Modal-field distributions of odd mode and even mode in both x-polarization and y-polarization (λ=1000 nm,w=2.0 μm); (b) effective refractive index of even mode in optical fiber couplers with different widths; (c) effective refractive index of odd mode in optical fiber couplers with different widths; (d) birefringence coefficients of even and odd modes in optical fiber couplers with different widths

Fig. 3. Calculated results of sensing properties of optical fiber coupler. (a) Difference in group birefringence coefficient between even and odd modes for optical fiber couplers with different widths; (b) calculated refractive index sensitivities of optical fiber couplers with different widths at refractive index of 1.333; (c) simulated transmission-spectral response of optical fiber coupler varying with refractive index (width and length of coupling region are 1.6 μm and 6.01 mm, respectively)

Fig. 4. Fabrication and testing of optical fiber coupler. (a) Schematic of optical fiber tapering setup; (b) optical micrograph of tapered optical fiber coupler and scanning electron microscope image of waist region; (c) experimental setup for measurement

Fig. 5. Experimental results. (a) Transmission spectra of optical fiber coupler in x-polarization and y-polarization; (b) transmission spectrum of optical fiber coupler with vernier effect; (c) experimental results of optical fiber coupler assisted with vernier effect varying with surrounding refractive index; (d) calculated refractive index sensitivities of sensor