## 1 Introduction  
Optical sensors are used in many fields of science and technology. They enable online measurements at a distance by using optical fibre as a transmission medium. One of the most important physical quantities measured using fibre optic sensors is the refractive index [1]. The surrounding refractive index (SRI) is the ratio of light velocity in the air to the velocity in the liquid that surrounds the optical fibre. Determining the relationship between the value of the desired quantity and the sensor's response is performed using the calibration process. On its basis, the sensor's properties, such as linearity or resolution, can be determined. One of the most popular optical sensors is the fibre optic Bragg gratings. Such sensors are based on changing a specific parameter of the optical spectrum under the influence of an interacting physical quantity. The optical spectrum of the sensor can be measured using the reflection or transmission method. Bragg gratings are periodic structures created in the fibre core. Classic FBGs have a Gaussian shape spectrum. Information about the measured quantity is encoded in the position of this shape on the wavelength axis. Therefore, it is an encoding in the wavelength domain, as opposed to the classic analysis of spectra, in which the information is contained in the intensity of the transmission spectrum. Full use of the information contained in the spectrum requires the use of an appropriate demodulation method. A periodic structure with a tilted refractive index modulation in the core, called TFBG (Tilted fibre Bragg grating), is a universal optical fibre measurement element that can be used to determine temperature, stress, fibrecurvature, polarisation angle, and, above all, the refractive index of the fluid surrounding the fibre. In terms of measurement properties, FBGs and TFBGs differ significantly [2]. The measurement capabilities are the result of the excitation of several dozen cladding modes. The transmission spectrum parameters can be converted into the refractive index value using various mathematical operations. It is mainly SRI measurements that lead to the creation and research of new structures and applications of TFBGs. The development of measurement methods for TFBGs also uses special types of optical fibres [3]. Unlike other fibre optic methods for determining SRI, TFBGs do not require removing the fibre cladding or implementing a special sensor head [4]. Changes in SRI values determined using TFBGs are used in measurements in many areas. They include, for example, biomedical measurements such as the detection of rheumatoid arthritis biomarkers [5] or solution concentration measurement [6]. By applying an appropriate coating, a TFBG can be used to monitor transformer oils [7]. In such practical applications, a data processing method must be developed, selected or modified. This is done in order to improve the metrological parameters obtained from the measurements. Many methods using the transmission amplitude changes of the TFBG optical spectrum have been developed. Most of them use the envelope created by the cladding modes. The entire spectrum is then represented only as an envelope. It changes under the influence of interacting factors. Such algorithms do not use the possibilities offered by advanced methods such as pre-processing, signal enhancement and feature extraction. Methods other than the mode envelope are much less frequently used. Algorithms calculating the amplitude parameters of the spectrum are susceptible to the unfavourable influence of changes in the background spectrum as well as measurement noise. The determined spectrum parameters are often limited to identifying the graphical relationship between a specific spectrum parameter and an influencing quantity. Taking into account the literature reports, it can be concluded that there is a lack of studies obtaining information from local spectrum parameters. Most quantitative measurements of SRI values use the envelope parameter of the cladding modes [8].