Ultrahigh-Range Evanescent Field Refractometer Based on a Fiber Bragg Grating Inscribed With Femtosecond-Laser Radiation

The paper reports on the results of an experimental study aimed at using fiber Bragg gratings inscribed following the point-by-point method with femtosecond laser radiation (FS-FBGs) as sensing elements of fiber refractometers. The study investigated the spectral properties of FS-FBGs associated with the features of cladding modes excitation which occurred via scattering the fundamental core mode by the structural elements of the grating. The work emphasizes on the geometry features of the FS-gratings’ structural elements, localized in a narrow region of the fiber core, which, by its physical nature, contributes to the efficient transfer of energy into the fiber cladding. The experimental data demonstrated an ultra-wide range of observed cladding mode cutoff corresponding to the refractive index range of the external media from 1 to at least 1.4 (at a wavelength of 1550 nm), with the dependence of the cutoff wavelength on the refractive index remaining linear over the entire range. In addition, the possibility of using such Bragg gratings to measure the refractive indices of gases was experimentally confirmed using nitrogen as an example at various pressures from 1 to 150 bar. At the same time the negligible contribution of the pressure-induced sensing element deformation to the readings was demonstrated. The conducted study convincingly expresses the potential of using femtosecond fiber Bragg gratings inscribed with femtosecond laser radiation as sensing elements of fiber sensors for a wide range of applications.