Wide Range Refractive Index Measurement Based on Off-Axis Tilted Fiber Bragg Gratings Fabricated Using Femtosecond Laser

In this paper, we propose an off-axis tilted fiber Bragg grating (OTFBG) in which the grating structure is limited to one side of the fiber core cross section. The tilted grating planes are fabricated using a femtosecond laser with a phase mask. The wavelength range of the OTFBG cladding mode resonance could be twice that of a conventional tilted fiber Bragg grating with the same tilt angle. The resonance intensity of the OTFBG core mode is significantly enhanced. We also fabricate a serial-double-angle OTFBG (SOTFBG) and a parallel-double- angle OTFBG (POTFBG). The wavelength ranges of their cladding mode resonances are effectively extended. The SOTFBG and POTFBG with tilt angles of 5° and 10° possess a wavelength range of cladding mode resonances of approximately 120 nm. They are capable of measuring the surrounding refractive index (SRI) with a refractive index (RI) sensible range from 1.24 to 1.45. A significantly wide range of cladding mode resonances is achieved by an SOTFBG with tilt angles of 10° and 15°, which is as wide as 215 nm. The RI sensible range of this SOTFBG is estimated to be from approximately 1.00 to 1.41, thereby making it suitable for SRI measurements in both gaseous media and aqueous solutions.