## 6 Discussion  
A basic limitation in fiber-optic interferometry is associated with the design of the reference channel in the fiber-optic interferometer. The basis of interferometry is measuring the differences between signals in the measurement (sensor) and reference channels. The reference channel of an interferometer must be designed in a way that allows all unwanted signals to be eliminated. Background noise (unwanted signals) originate from thermal phenomena, mechanical changes, and other factors. Precise positioning of the reference channel is also highly critical to a sensor's sensitivity, which must be carefully taken into account in fiber-optic interferometer design [52; 53; 54; 55]. The error signal caused by the instability of light polarization and external influences (temperature) can be described as a slow change in phase with a frequency that is one order of magnitude less than the lowest frequency of the desired signal. Because of this difference, the first-order high-pass (HP) \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Measuring** & \multirow{2}{*}{**Year Period**} & **Number of Passes** & **Detection Success,** & **Detection Success,** \\
**Day** & & **(Direction/Opposite)** & **Interferometric Sensor (\%)** & **Seismic Station (\%)** \\ \hline
1 & winter & 40/39 & 100 & 100 \\
2 & winter & 36/38 & 100 & 100 \\
3 & winter & 37/36 & 100 & 100 \\
4 & winter & 38/37 & 100 & 100 \\
5 & winter & 36/38 & 100 & 100 \\
6 & summer & 41/39 & 100 & 100 \\
7 & summer & 38/40 & 100 & 100 \\
8 & summer & 39/38 & 100 & 100 \\
9 & summer & 37/39 & 100 & 100 \\
10 & summer & 42/41 & 100 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of tram vehicle detection measurements. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Tram Type**} & \multirow{2}{*}{**Number of Passes**} & \multicolumn{2}{c}{**Bandwidth (Hz)**} & \multicolumn{2}{c}{**Dominant Component (Hz)**} \\ \cline{3-5}  & & **Seismic Station** & **Interferometric Sensor** & **Seismic Station** & **Interferometric Sensor** \\ \hline Vario LFR & 176 & 20–45 & 24–46 & 35–38 & 32–36 \\ CKD T3 & 162 & 22–48 & 20–48 & 35–36 & 36–38 \\ Ineokon LTM 10.08 & 158 & 25-45 & 24–48 & 34–36 & 32–38 \\ Ineokon 2001 TRIO & 145 & 25–55 & 20–50 & 32–36 & 33–38 \\ CKD T6A5 & 128 & 20–45 & 22–50 & 34–35 & 34–36 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the results of measured frequencies from both devices with regard to the type of tram. filter with a corner frequency of 2 Hz can be used. For the compensation of unwanted signals like snow, rain, wind, and other environmental influences, we can use a software filtration method. As our sensor system uses G.652 optical fibers, which are widely used in current telecommunications systems, they can be readily connected to existing telecommunications networks (dark cables around tracks) without the need for any additional transducers. The low attenuation of single-mode fibers (0.2 dB/km) allows the evaluation unit to be placed several km from the sensor (the evaluation unit is much more expensive than the sensor; therefore, remote evaluation or placing the evaluation unit at a safe location is a clear advantage). Optical fibers are currently being increasingly dedicated to communications applications. However, there is also a global trend in which many research and industrial corporations are applying optical fiber sensors in the monitoring or recording of various physical phenomena in transportation. For an example of the application of optical sensors in monitoring underground traffic, please see [56]. An essential difference between a standard seismic station and the alternative in the form of a fiber-optic interferometric sensor is primarily the price of the equipment, which is an order higher for seismic stations by thousands of U.S. dollars [8; 9; 10]. The price of an interferometric sensor based on classic telecommunications fibers and affordable materials is only hundreds of U.S. dollars for a prototype.