## 4 The Experiment and Its Results  
A commercial Brillouin analyzer and standard telecommunication optical fiber SMF-28e were used for the experiments. The optical fiber on a split spool (free winding) was placed in a thermal chamber, where the temperature was maintained at 25 \({}^{\circ}\)C during the experiment to eliminate the influence of temperature variations in the laboratory on the experimental data. Figure 8 shows the experimental setup, including a simplified installation of BOTDA. In this experiment, the DiTeSt 202 STA-R instrument from Omnisens (Morges, Switzerland) was employed. Furthermore, the results obtained with this system have allowed us to compare the outcomes of our previous studies, thereby facilitating the assessment of the efficacy of the proposed methodologies under identical conditions. A typical instrument of this type comprises a pair of main modules: a pump and probe modules. The initial radiation for both modules is typically generated by a narrow-bandwidth laser operating at a wavelength of 1550 nm in continuous mode. The pump module transforms continuous radiation into pulsed radiation through modulation and subsequently amplifies it through a two-stage erbium amplification process. As a consequence of this configuration, the pump module generates rectangular pulses with a duration of approximately tens of nanoseconds, which are then fed to the sensor fiber. The duration of the pause between pulses is typically characterized by microseconds, which is due to the pulse's time travel through the length of the sensor. In this case, we used pulses with a duration of 10 nanoseconds, which enabled the system to obtain traces with a resolution of 1 m. Additionally, the system shifts this radiation in frequency. We used measurements with a fixed frequency shift of 1 MHz and then performed frequency downsampling to emulate a coarse scanning step. Pigtails equipped with E2000/APC connectors (NTSS, Kostroma, Russia) were welded to the ends of the optical fiber under study using an Ilsintech KF4 arc splicing machine (Ilsintech, Daejeon City, Republic of Korea). Subsequent to this, the optical fiber under study was placed on a split spool in an Espec MC71, (Espec, Osaka, Japan) thermal chamber for thermostatting. The pressure of individual fibers on each other, axial twisting, and, consequently, undesirable changes in signal polarization and attenuation coefficient were prevented by using a split spool and careful laying of the fiber in the chamber. A temperature close to room temperature of 25 \({}^{\circ}\)C was chosen for thermostatting. Once the temperature of the coil and fiber stabilized, cyclic measurements were initiated. The rationale behind the deliberate disabling of the automatic frequency scanning step selection for this study is the need to calculate a discrete correlation function. Figure 7: Dependence of the BFS extraction precision on the number of averaged spectra for spectra with SNR = 2 dBm. Green arrow shows gain in BFS extraction accuracy. Inset: BFS extraction precision gain as a function of the scan step. The frequency variation range of 10,700 to 10,900 MHz was selected based on the characteristic BFS variations of this fiber type under normal conditions. Over the 10-h experimental period, 105 measurements were obtained and subsequently transformed into images with varying signal-to-noise ratios by modifying the BGS sample size for averaging. The signal-to-noise ratio was calculated using the LCF method with subsequent subtraction of the Lorentzian fitted curve from the signal. The spectrum intensity at the maximum point of the Lorentzian curve was taken as the signal level, and the average signal value after its subtraction was taken as the noise level. Based on these two obtained values, the signal-to-noise ratio was calculated. The number of spectra accumulated for averaging by the BGS device varied from \(10^{3}\) to \(10^{7}\), allowing for the acquisition of spectra with varying SNRs.