# Portable Optical Fiber Bragg Grating Sensor for Monitoring Traffic Density
## 2 Methods
### Design and Realization of Portable FBG Sensor  
In the Introduction, we summarize the current approaches to using fiber Bragg grating sensors for road traffic monitoring. Against the presented research [25; 26; 27; 28; 29], where an invasive installation of sensors into the road is necessary, we focused on the design of a portable sensor that can be installed non-invasively on the road at any place for any time with a focus on urban car traffic. From the results of research, polymer materials were selected for the portable sensor's design. A polymer in liquid form is a simple solution for encasing optical fibers. In selecting a suitable mold, any desired shape of the resulting sensor can be obtained. The polymer then cures either at either room temperature or raised temperature. Based on experimental measurements with several types of polymers, ZA 50 LT two-component additive rubber (ELCHEMCo, Zruc nad Sazavou, Czech Republic) was selected. This material cures at room temperature. The selection criteria for a suitable polymer included polymer hardness, resistance to passing cars, sensor lifespan and curing temperature. Table 2 summarizes the specification of ZA 50 LT rubber. For creating the portable sensor, we used an apodized Bragg grating that was made using the phase mask method at the producer Network group (Brno, Czech Republic). For the production process, we used a standard single-mode optical fiber (ITU-T recommendation G.652) with primary acrylic protection of 250 \(\upmu\)m. This type of material and size of protection was used for the recounting at the place of the Bragg grating. The length of Bragg grating itself was 1.6 mm. The portable sensor strip consisted of an optical fiber Bragg grating with a Bragg wavelength of 1573.158 nm, reflectivity of 94.04% and spectrum width (full width at half maximum, FWHM) of 211 pm. The first prototype of the portable sensor was completed by casting a bare optical fiber with FBG. Manual handling of the portable strip and passing cars created a large deformation of more than 10,000 \(\upmu\)strain. This large deformation exceeded the maximum deformation (5000 \(\upmu\)strain) defined by the manufacturer of the used Bragg gratings (Network group, Brno, Czech Republic). Different types of optical fiber protection tubes (polymer, metal, fiberglass and carbon) were therefore tested to compensate. Testing showed that the most suitable form of protection was a carbon tube with an internal diameter of 0.3 mm and external diameter of 0.7 mm. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Value** \\ \hline Hardness & 50 Shore A \\ Initial viscosity & 20,000 mPas \\ Specific weight after curing & 1.22 g/cm\({}^{3}\) \\ Tensile strength & 4 N/mm\({}^{2}\) \\ Tear resistance & 12 N/mm \\ Extension & 320\% \\ Mixing ratio & 1:1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Properties of ZA 50 LT rubber. Figure 2 shows a scheme of a portable fiber optic sensor with a Bragg grating in a carbon tube, which was subsequently encased in a ZA 50 LT polymer. The length of the sensor was selected according to the width of the car lanes. The criterion for monitoring traffic density was a vehicle passing over the portable strip with at least one wheel in the lane. The minimum width of the local road in the Czech Republic is 2.75 m, and a length of 2.5 m was therefore selected. The width of the active area of the measuring tape is given by the length of the carbon tube, which was 2.35 m long. Outside the active area, sensitivity to a passing car is significantly reduced. The height of the strip was 2 cm, and its width was 6 cm. These values represent a compromise in order to sufficiently protect the FBG sensor while also minimizing the axle load of a passing vehicle. The optical fiber was pre-stressed by 500 pm in the carbon tube and glued at both ends with a two-component adhesive.