This work presents a detailed review of the development of distributed acoustic sensors (DAS) and their newest scientific applications. It covers most areas of human activities, such as the engineering, material, and humanitarian sciences, geophysics, culture, biology, and applied mechanics. It also provides the theoretical basis for most well-known DAS techniques and unveils the features that characterize each particular group of applications. After providing a summary of research achievements, the paper develops an initial perspective of the future work and determines the most promising DAS technologies that should be improved.

Long dynamic population gratings (DPGs) formed in rare-earth-doped fibers have unique spectral characteristics compared to other types of fiber gratings, making them suitable for controlling the spectral composition of lasers. Depending on the type, length, and position of the DPGs in the cavities of lasers, they can be used for various purposes, ranging from the stabilization of single-frequency radiation to regular wavelength self-sweeping (WLSS) operation. Lasers based on DPGs are sources of narrow-band radiation with a fixed or sweeping generation spectrum. One of the main advantages of such lasers is the simplicity of their design, since they do not require special spectral elements or drivers for spectrum control. In this paper, we review the research progress on fiber lasers based on DPGs. The basic working principles of different types of DPGs will be introduced in the theoretical section. The operation of lasers based on absorption and gain DPGs and their practical applications will be discussed and summarized in experimental section. Finally, the main challenges for the development of such lasers will be presented.

We experimentally demonstrate a linearly polarized Tm-doped fiber laser with sweeping range up to 26 nm in the region of 1.92 μm. The main feature of the laser is generation of periodic microsecond pulses in which each contains practically only single longitudinal mode radiation. The laser frequency changes from pulse to pulse with high linearity by one intermode beating frequency of the laser $\sim 8\mathrm{MHz}$. The developed source is applied to measure spectrum of water absorption lines in air.

In this review, we summarize the latest advances in the design of optical frequency-domain reflectometers (OFDRs), digital signal processing, and sensors based on special optical fibers. We discuss state-of-the-art approaches to improving metrological characteristics, such as spatial resolution, SNR, dynamic range, and the accuracy of determining back reflection coefficients. We also analyze the latest achievements in the OFDR-based sensors: the accuracy of spatial localization of the impact, the error in detecting temperatures, deformation, and other quantities, and the features of separate measurement of various physical quantities. We also pay attention to the trend of mutual integration of frequency-domain optical reflectometry methods with time-domain optical reflectometry, which provides completely new sensing possibilities. We believe that this review may be useful to engineers and scientists focused on developing a lab setup, complete measurement instrument, or sensing system with specific requirements.

We experimentally demonstrate for the first time, to the best of our knowledge, a Nd-doped fiber laser with wavelength self-sweeping. The main feature of the laser is the generation of periodic microsecond pulses, where each of them contains practically only single longitudinal mode radiation with a linewidth of about 1 MHz. The laser frequency changes from pulse to pulse with high linearity by one intermode beating frequency of the laser $\sim 7.1\mathrm{MHz}$. The laser generates a linearly polarized radiation near wavelength of 1.06 μm with a self-sweeping range of up to 1.8 nm.

We report on the development of an optical frequency domain reflectometry (OFDR) system based on a continuous-wave Er-doped self-sweeping fiber laser. In this work, we investigate the influence of the input data processing procedure in an OFDR system on the resulting reflectograms and noise level. In particular, several types of signal averaging (in time and frequency domain) and Fourier analysis are applied. We demonstrate that the averaging in the frequency domain can be applied to evaluate absolute values of the local scattering amplitudes related to the Rayleigh light scattering (RLS), which is associated with the interference of scattering signals on microscopic inhomogeneities in optical fibers. We found that the RLS signal remains unchanged in the case of signal averaging in time domain, while the noise floor level decreases by 30 dB with an increasing number of points from 1 to ~450. At the same time, it becomes possible to detect the spectral composition of the scattering at each point of the fiber using windowed Fourier transform. As a result, the sensitivity of the developed system allows us to measure the RLS signal at a level of about 20 dB above the noise floor. The described analysis methods can be useful in the development of distributed sensors based on Rayleigh OFDR systems.