This paper presents a method for measuring two components of bending force and temperature using one sensor head. Indirect inference based on the spectra of two fiber Bragg gratings (FBGs) placed on a cantilever beam is used. The method was developed during work on the inverse problem of determining a nonuniform stress distribution based on FBG spectra. A gradient in the FBG stress profile results in a characteristic shape of its reflective spectrum. The simultaneous measurements of force and temperature were possible through the use of an appropriate layout of the sensor head. The spectral characteristics of the sensor’s gratings do not retain full symmetry, which is due to the geometry of the sensor’s head and the related difference in the distribution of the axial stress of the gratings. In the proposed approach, the change in width of the sum of the normalized transmission spectra was used to determine the value of the applied force. In the presented method, an increase in the sensitivity of this change to the force is obtained relative to the other known systems. A change in the spectral width was observed for an increase in bending forces from 0 to 150 N. The sensitivity coefficient of the spectral width to force, defined as the ratio of the change of the spectral half-width to the change in force was $2.6\mathrm{e}-3\mathrm{nm}/\mathrm{N}$ for the first grating and $1.2\mathrm{e}-3\mathrm{nm}/\mathrm{N}$ for the second grating. However, the sensitivity of the whole sensor system was $5.8\mathrm{e}-3\mathrm{nm}/\mathrm{N}$, which is greater than the sum of the sensitivities of the individual gratings. For the purpose of this work, a station with a thermal chamber has been designed with a bracket on which fiber optic transducers have been mounted for use in further measurements. The sensor head in this experiment is considered to be a universal device with potential applications in other types of optical sensors, and it can be treated as a module for development through its multiplication on a single optical fiber.

This letter presents a novel method for the analysis of tilted fiber Bragg grating (TFBG) transmission spectra. The processing method is based on the transmission spectrum contour length, which is calculated in the wavelength range where cladding modes appear. The proposed algorithm is based on a modified analysis method that is used in spectrometric surveys of gas spectra in cases with unknown or fluctuating baselines. Experimental results obtained using the proposed method are compared with results from the most commonly used TFBG spectral analysis methods that utilize global monitoring of cladding modes.

We propose and experimentally demonstrate a method for the detection of steel material defects utilizing a fiber Bragg grating sensor. The considered defects are periodic grooves along the length of the tested steel profile. Direct measurement of the spectral reflectance characteristics of the fiber is performed, and the related inverse problem of indirect defect shape determination is solved. It has been demonstrated that the defect periodicity estimation is 2.5 mm, with an error of less than 0.1. Furthermore, it has been shown that for periodic intervals of the order of 5 mm, the difference between the strain amplitude calculated using our method and the amplitude obtained via the finite element method was 1.4 mϵ.

In this Letter, it has been shown that the twisting of TFBGs (tilted fiber Bragg gratings) decreases their sensitivity to the polarization of the input light. It has been proved by a theoretical simulation study that twisting the TFBG by 180° eliminates the sensitivity of the grating to input light polarization changes. We have experimentally proved that our structure produced has an 18 times lower coefficient of variation of transmission for polarization changes than the untwisted TFBG. This Letter also presents a new method for manufacturing the twisted tilted fiber Bragg grating (TTFBG). We have demonstrated the possibility of writing the TTFBG structure twisted by 90° on a fiber with a length of 10 mm. The structure has been further twisted in the other direction by 90°, which is consistent with a structure twisted by 180°. The properties of both structures were determined. Due to the fiber’s strength, the TTFBG twisted by 180° has to be 2 cm long. An advantage of the proposed method of producing the TTFBG is that there is a twist of the structure itself after fabrication. This creates new possibilities of using the structure in many applications. This type of grating can be used to reduce the sensitivity to polarization in various sensing applications, in particular, to measure the refractive index. In addition, a 180° twisted TTFBG has properties of direction discrimination in twist sensing applications.

This article presents the use of a sensor with fiber Bragg grating along with an interrogation system used for monitoring the overhead lines’ wire elongation. The possible interrogation methods based on adjusted filters were considered. In the experimental part, three types of fiber Bragg grating pairs, characterized by a small shift in spectra in pairs and gratings with exact matching, were examined. The study showed that, by choosing the appropriate mechanical parameters of the elongation transformer with the optical parameters of the sensor and dedicated filter, the optomechanical system can be adjusted to the required range of overhead line wire sag observation. The range of sag depends on the distance between the poles, the wire type, and its real length in the span, which effectively determines the sag.