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The presented literature review was prepared by a team of authors united by the Program and Organizing Committees of the "Optical Reflectometry, Metrology, and Sensing" (ORMS) conference in 2023. It is intended to assess the state and prospects in this area for the coming years. The review covers the following topics: distributed acoustic sensors, fiber-optic measurement systems based on Brillouin scattering, research methods based on the principles of optical reflectometry in the frequency domain, and low-coherence approaches to distributed temperature and strain monitoring. [ABSTRACT FROM AUTHOR]

An experiment on seismic survey of the geological structure of the Black Sea seabed using a fiber-optic cable laid on the sea bottom (in shallow water) and a distributed acoustic sensor (DAS) is described. Based on the experimental results, it is shown that the proposed technology is promising. [ABSTRACT FROM AUTHOR]

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A new architecture of a fiber phase-sensitive optical time-domain reflectometer (φ-OTDR, i.e., a distributed acoustic sensor) suitable for engineering geology application is proposed. The sensor is based on a double-pulse scheme in which a pair of pulses is formed using an unbalanced Michelson interferometer. A symmetrical 3 × 3 coupler built into the Michelson interferometer is used to obtain the phase delay needed for the demodulation of the backscattered light. Using the unbalanced Michelson interferometer in the circuit for dual-pulse probe signal generation, it is possible to reduce the requirements for the degree of coherence of the light source, since the delay line introduced between the dual-pulse parts is compensated in the φ‑OTDR fiber under test. As a result, it is possible to use a laser with a wide spectral line (~1 GHz) and generate short (7-ns-wide) laser pulses by directly modulating the laser-diode injection current. In order to reduce the signal fading in the φ-OTDR and to improve the linearity of its response, responses are averaged over 16 optical frequencies. The efficiency of the proposed distributed acoustic sensor has been demonstrated by detecting a strong impact on a cable that was horizontally buried in the ground as well as by detecting seismic waves using a cable inserted in a well at the sea bottom. [ABSTRACT FROM AUTHOR]

The possibility of measuring strain (temperature) over distances up to 85 km in a single-ended optical fiber using the technique of low coherence Rayleigh reflectometry has been demonstrated. The indicated range is provided by employing Raman amplification of radiation in combination with amplification by short erbium doped fiber segments built into the fiber under test. The possibility of tuning a low coherence source in a wide range of wavelengths made it possible to measure strain in the range of 500 με, which is equivalent to 56°C in units of temperature. At a unit measurement time of 10 min and a spatial resolution of 2.6 m, the standard uncertainty of the strain measurements was 3.8 με and of the temperature ones was 0.42°C. [ABSTRACT FROM AUTHOR]

A study has been made of the accuracy of measuring the attenuation coefficient in a single-mode optical fiber using Rayleigh reflectometry. It has been theoretically established that the minimum achievable uncertainty of such measurements is determined only by the spectral properties of the probe radiation and decreases with an increase in the length of the measured fiber to the power of 1.5. A statistical analysis of the results of measuring the attenuation coefficient in different segments of a real single-mode fiber by their reflectograms demonstrates results that are consistent with theoretical conclusions. The results set limits on the minimum fiber length for which the attenuation coefficient can be measured with the required accuracy. [ABSTRACT FROM AUTHOR]