## 3 Experimental Setup  
Initially, we describe the probe radiation source used during the experiment. An Er-doped CW self-sweeping laser with ring cavity was used as the radiation source. The dynamics of an identical laser were described in detail in [20]. The laser provided wavelength tuning in the 40-50 pm (5-6 GHz) range near the wavelength of 1565 nm. The laser radiation was a sequence of long weakly overlapping pulses with a rectangular intensity profile corresponding to the successive generation of neighboring longitudinal modes. Figure 4a,b shows the evolution of the optical frequency of the laser and the intensity dynamics of sequentially generated longitudinal modes, respectively [20]. Different colors in Figure 4b corresponded to different laser cavity modes. As a result, the laser integral (over simultaneously generating modes) intensity dynamics, which was measured at the output of the laser and presented in Figure 4c, is a CW signal with short intensity bursts occurring with a period of T\(\sim\)1 ms. The bursts corresponded to the rapid change in mode composition with simultaneous generation of two neighboring modes, when there is a rapid (\(\sim\)100 \(\upmu\)s) increase in one mode and a fading in the other. The frequency tuning step between two neighboring modes was \(\delta\upsilon\) = 9.7 MHz. During a single mode pulse, the radiation is highly coherent with linewidth not exceeding 40 kHz [20]. Thus, this laser used in OFDR systems provided a maximum sensing line length of \(L_{max}\)\(\sim\)5.35 m and spatial resolution of \(\delta\)\(\sim\)2 cm, according to Expressions (2) and (3), respectively. Considering the preliminary data processing analysis of the interferograms, it was essential to conclude that the laser generation consisted of a sequence of long (T\(\sim\)1 ms) sections with fixed optical frequency separated by short transitions with a fixed value of optical frequency change (\(\delta\upsilon\) = 9.7 MHz). The scheme of the experimental OFDR system is shown in Figure 5. The OFDR system was based on polarization-maintaining (PM) fibers and components. The radiation of the probe self-sweeping Er-doped fiber laser described above was divided into two parts using an additional fiber coupler. The first part was used as a reference signal for normalization in order to eliminate the influence of the laser output power fluctuations. The other part entered the MZI formed using the input and output couplers with coupling coefficients of 10% and 5%, respectively, and a circulator connected to a LUT. In our experiments, a piece of 1.6 m long PM fiber with short (0.2 m long) coreless fiber spliced at the far end was used as the LUT. The coreless fiber minimized the influence of the Fresnel back reflection coming from the far end of the LUT, since the radiation propagating inside the coreless fiber  
Figure 3: Model sine function (**a**) and corresponding FFT (**b**) with matching and non-matching number of points. Black and red lines correspond to non-matching and matching number of points, respectively, while solid and dashed lines correspond to rectangular and Hann window, respectively. diffracted rapidly (experiencing huge losses) and did not actually reach the end of the LUT. The signal scattered/reflected in the LUT returned back to the circulator and was redirected to the output coupler, where it interfered with the other portion of radiation coming from the reference arm of the MZI. The laser power after passing the MZI reduced to a few tens of microwatts. It was noted that the MZI was vibro-isolated from the environment fluctuations using foam rubber. A digital oscilloscope (LeCroy Wavesurfer 3034R, USA) was used for data recording. A fast photodetector (PD) Thorlabs DET08 CFC was used to measure the reference signal at the output of the probe laser (input of the MZI). The interference signal was measured at the MZI output with an InGaAs switchable gain amplified PD (Thorlabs PDA20 CS2, USA) used in 10 dB gain mode.