Study on spectral and refractive index sensing characteristics of etched excessively tilted fiber gratings.

We investigated the spectral and refractive index (RI) sensing characteristics of the excessively tilted fiber grating (Ex-TFG) with different cladding diameters. The Ex-TFG is inscribed in standard single-mode fiber, and the cladding reduces from 125 μm to around 15 μm by the chemical etching method. Experimental results show that the number of cladding modes decreases, and the spacing of adjacent resonance peaks becomes larger and larger with the reduction of the cladding diameter in the observed wavelength range of 1250-1650 nm. The average RI sensitivity in the index region of 1.33-1.38, the one near 1.33, and the one at around 1.38 of the etched Ex-TFG with a diameter of 15 μm is ∼6.3, ∼5.3, and ∼6.67 fold compared to those of the no-etched Ex-TFG, respectively. Also, the RI sensing performances of the etched Ex-TFG with a diameter smaller than 30 μm are better than those of the Ex-TFG inscribed in SM1500 (4.2 μm/80 μm) fiber in the index region of 1.33. The proposed micronano Ex-TFG has higher RI sensitivity and a more compact structure in biosensing applications, compared to the standard Ex-TFGs and Ex-TFGs inscribed in SM1500 fiber.