Sensitivity enhancement of a dual-tapered fiber refractive index sensor assisted by a tilted fiber Bragg grating

To avoid further reducing the fiber diameter to several micrometers and to maintain the mechanical strength of fiber tapers, a sensitivity enhancement scheme of refractive index (RI) sensing based on dual-tapered fiber is reported. A tilted fiber Bragg grating (TFBG) is cascaded with the fiber tapers, and the superimposed envelope spectrum is extracted via the Hilbert transform. Within the RI range 1.3325–1.3382, experimental results demonstrate that the sensitivity of TFBG-assisted dual-taper structure is improved to 84.245 nm/RIU from 37.511 nm/RIU with the same waist diameter of 20.11 µm. For comparison, the RI sensitivity of the single dual-taper is increased by only 12.12% to 42.131 nm/RIU when the spatial frequency concentrated algorithm is adopted for data processing. A sensitivity amplification factor 2.25 is achieved for the TFBG-assisted dual-tapers sensor, offering a low-cost and effective solution for trace detection in biological sensing and environmental monitoring, etc.