## 1 Introduction  
Optical reflectometry (OR) in optical fibers is one of the main techniques for distributed sensing/measurements [1]. Approaches based on OR cover a wide range of temperature, strain, and vibration measurement tasks. The lengths of sensing fibers in such tasks can exceed 100 km [2], while the spatial resolution can reach the sub-millimeter scale [3]. OR is based on the measurements of the local light scattering that occurs as the probe radiation propagates along the fiber. One of the main types of OR is based on Rayleigh light scattering (RLS) [4, 5]. It is elastic light scattering at microscopic inhomogeneities of refractive index presenting in cores of all commonly used optical fibers. This hype of OR can be divided into two main branches depending on properties of probe radiation. Pulsed probe radiation with fixed optical frequency and continuous-wave (CW) tunable probe radiation are used in the time and frequency domain in branches of OR, respectively. Optical time domain reflectometry (OTDR) is based on measurements of scattered/ reflected signals produced through short (nanosecond) pulses. As a pulse propagates along the fiber, the scattered/reflected signal returns back to the laser source with a time delay proportional to the distance between the laser and the local scatterer/reflector [6]. The result of OTDR is a reflectogram demonstrating a dependence of the reflection amplitude/scatterer density on the fiber longitudinal co-ordinate. In this case, the maximum sensing line length is limited only by the fiber losses and reaches tens of kilometers [7]. Atthe same time, the spatial resolution is limited by the pulse duration and is typically in the order of units of meters [7]. In turn, in the case of optical frequency domain reflectometry (OFDR), the local response can be also connected with scattered signal delay. However, analyzed signals are measured in interferometric two-beam configuration, in which coherent probe radiation with optical frequency tuning is used to construct an interferogram. Relative delay between scattered and reference signals passed through the test and reference arms of the interferometer, respectively, is contained in frequency of spectral oscillations of the interferogram [8; 9]. Spectral analysis of such interferogram with Fast Fourier transform (FFT) makes it possible to construct a reflectogram and achieve high spatial resolution (units or even fractions of mm) at the expense of the sensing fiber line reduction [10]. The sensing lines usually have lengths of several tens of meters. Such sensing lines are useful for distributed testing various systems and components, the detection of cracks, and construction health monitoring [11; 12]. OFDR requires use of highly coherent laser sources with frequency tuning capability. The use of Fourier analysis imposes strict requirements on the linearity of frequency tuning. As a rule, semiconductor tunable lasers are used as sources of probe radiation sources. Such lasers typically have non-linearity in the optical frequency tuning rate [10]. In the vast majority of OFDR articles and patents, this problem is solved either via using additional methods to linearize the laser frequency tuning [13; 14] or measuring the laser frequency and further compensating the non-linearity [15; 16; 17]. An alternative to this approach is the use of lasers with frequency changing in stepwise manner with fixed frequency steps. In case of stepwise frequency tuning, the linearity is achieved a priori without the need for correction via external means. One implementation of such a source is a fiber self-sweeping laser [18]. An important feature of such lasers is that the optical frequency tuning occurs due to internal processes related to the formation of dynamic filters based on population inversion gratings [18], which are found directly in the active medium during laser generation, without the need for any additional external filters.