Spectral contour length demodulation method for tilted fiber Bragg grating refractive index sensing

Tilted fiber Bragg gratings (TFBG) are often used for refractive index (RI) measurements. Such measurements consist in monitoring changes in grating spectrum transmission in the range of cladding modes. The most popular method of TFBG transmission spectrum demodulation is to determine the area of the cladding mode envelope. Recently, a much simpler method has been proposed for this purpose, i.e. determining the length of the spectral contour. The contour length can be used directly as a parameter indicating the value of the refractive index measured. The paper proposes to develop the method of the spectral contour length to establish the cutoff wavelength and next indirectly discovering the refractive index. The algorithm consists in determining the curve of changes in contour length along with the wavelength for each spectrum measured with a given refractive index. The cutoff wavelength is defined as the intersection of individual curves with a line defining a certain level. This method is very simple, while at the same time being characterized by very high sensitivity.