No 5 (2023)

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.

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The paper presents the results and analysis of a radiophotonic device for instantaneous frequency measurements of microwave signals, including the situation with simultaneous measurements of multitude instantaneous frequencies. The device’s principle of operation is to combine the “frequency–amplitude” measurement conversion for the determined frequency with carrier suppression and the formation of equidistant channels based on the frequency comb to estimate its magnitude. An efficient method for unequal symmetric optical frequency comb generation based on the phase switching of an optical carrier with its suppression in a phase modulator is proposed. The comb allows for the formation of up to ten channels with a width of 2 GHz that can be adjusted. The amplitudes of the boundary frequencies of the channels are not equal, which makes it possible to differentiate the measured frequencies in relation to the powers of their beats. Separately, the features of the instantaneous frequency measurement in the zero channel of the device are studied. The use of information signals with a suppressed carrier makes it possible to reduce the requirements for the laser frequency stability. The bandwidth of the photodetector is equal to the channel width, thus making it possible to use it as a channel filter. The device is first simulated in the Optiwave System software environment, and the factors affecting the system performance are then studied on a test bench. The design simplicity of the device, built on only two modulators, is noted.

The authors propose to use the K-nearest neighbor (K-NN) algorithm to process the probe signals from the previously proposed Brillouin optical time-domain reflectometry (BOTDR) fiber sensor called the differential cross-spectrum BOTDR (DCS-BOTDR), which features a high spatial resolution. Widening of the Brillouin gain spectrum (BGS) when the pulse duration TL is set shorter than the phonon lifetime (~10 ns) is a key problem associated with the DCS-BOTDR. Although the spectrum distortion is reduced also, deterioration in the Brillouin frequency shift (BFS) resolution is observed, which is caused by the broadening of the Brillouin spectrum. On the other hand, while the TL duration exceeding the steady state of the Brillouin signal narrows the spectrum, sidelobes are produced within the steady-state range, which result in the deterioration of the BFS resolution. This limitation is observed only in the DCS-BOTDR, although it is capable of measuring with a high spatial resolution. The experimental data obtained for a fiber with a length of ~400 m and with optimized TL duration in the temperature range of 40–80°C were used in our model to extract the Brillouin temperature coefficient CT. Ideal BGSs were then constructed in the training phase by simulation with different linewidths of 50–70 MHz in order to train the K-NN model with due account for the linewidth variation caused by the difference in the conditions between the training and testing phases. The model was thereby made flexible for various fiber conditions. Experimental data for the sensing of a ~3.6-km-long fiber with TL = 60 ns were used in the testing phase to obtain the temperature distribution. By employing K-NN, the accuracy in determining the temperature for TL = 60 ns has been improved to approximately 2.77°C. Therefore, the K-NN model can be an excellent alternative tool for processing BGSs measured by the DCS-BOTDR and obtaining the temperature distribution along the fiber.

Simple measures to improve the signal-to-noise ratio of optical frequency domain reflectometer (OFDR) readings are described. After applying a two-stage optical amplification of the backscattered signal, as well as eliminating the source of spurious reflections, it was possible to increase the signal-to-noise ratio of the frequency domain trace from 8 to 19 dB. This technique can be applied in fiber optic sensors and metrology of fiber optic and integrated optical elements.

The results of experimental studies into the Mandelstam–Brillouin scattering characteristics are presented for single-mode optical fibers of various types and manufacturers. The experimental dependences of optical fibers obtained using a Brillouin optical reflectometer (graphs of the distribution of the Brillouin scattering spectrum along the length of the fiber and multireflectograms) are presented. For each type of optical fibers considered, an estimate of the Brillouin frequency shift is given, the value of which, at wavelengths of laser radiation used in telecommunication systems, refers to the microwave range. The frequency dependences of the Mandelstam–Brillouin scattering characteristics of some varieties of single-mode optical fibers with different cutoff wavelengths are presented. A comparative analysis of their characteristics with the characteristics of previously studied single-mode optical fibers is carried out. Optical fibers of similar varieties (but different manufacturers) can have noticeable differences in the frequency response of Mandelstam–Brillouin scattering. A table with the main characteristics of Mandelstam–Brillouin scattering is presented for all experimentally studied single-mode optical fibers.

The sensitivity of the Sagnac interferometer for various acoustic influence coordinates was studied. The principles of the formation of a dead zone in an acoustic distributed fiber-optic sensor based on the Sagnac interferometer have been obtained and experimentally confirmed. The response of the interferometer was studied for different types of acoustic impact on the circuit: in the form of a rectangular pulse, sinusoidal, and in the form of a periodic triangular function. The nature of the change in the phase difference at the output of the Sagnac interferometer for each of them was studied. With the found value of the typical frequency ft,hit=10.8 kHz and a loop length of 20 km, numerical simulation and experimental study of the amplitude of the phase difference under acoustic impact through each 1 km of the loop in the range from 0 to 10 km were carried out. A dead zone elimination method is proposed for integrating the Sagnac interferometer into a complex monitoring system using a phase-sensitive optical time domain reflectometer.

This paper analyses the spectral, spatial and temporal probing pulse characteristics of the fiber-optic distributed acoustic sensor prototype. The signal spectral distribution and mean value, the interference pattern contrast dependencies on the light source pulse duration and the optical amplifier pump current are obtained. Directions for further research in this area are proposed. The collected data made it possible to draw conclusions about such devices cost reduction possibilities and the proposed design operation suitability.

The paper presents the results of a study of the effect of signal amplitude fluctuations on the measurement error of the attenuation coefficient of optical fibers of small length made by Rayleigh incoherent reflectometers. Based on the analysis of signal processing in a reflectometer, using methods of statistical radio engineering, and involving experimental data and empirical techniques, it is shown that the value of measurement error of attenuation coefficient is determined by the number of uncorrelated counts of the reflectogram used for averaging, taking into account the nature of fluctuations, with restrictions imposed on the signal-to-noise ratio at the receiver input. A method of consideration for the effect of fading during birefringence fluctuations is proposed based on an analogy with the radar task of estimating the parameters of detecting a fluctuating target.

A study of the optimal parameters of a distributed fiber microphone circuit based on a laboratory setup of a phase-sensitive optical reflectometer (φ-OTDR), which reproduces the work of φ-OTDR at one point, has been carried out. Various schemes for constructing a microphone based on φ-OTDR and various sampling rates of the analog-to-digital converter are investigated. The optimal configuration of the φ-OTDR circuit is substantiated, which made it possible to ensure high quality of human speech recognition during voice recording by the proposed method with a sampling frequency of 40 kHz. In a standard phrase based on Harvard sentences containing 80 words, 71 words were recognized, i.e., 88.75% of their total number.

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.

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A brief review of the applications of distributed acoustic sensors for solving geophysical and seismometric problems is given. Theoretical estimates and experimental measurements of the level of self-noise in distributed acoustic sensors, including those at subhertz frequencies, are obtained. Numerical modeling has been carried out, which makes it possible to quantify the power of low-frequency noise (associated with a slow temperature change of the probed fiber) in the signal of distributed acoustic sensors. The obtained results are supplemented by theoretical estimates of the power spectrum of a signal from a distant earthquake; they demonstrate the importance of taking into account temperature effects in the fiber when planning experiments related to the registration of weak seismic events.

To expand the possibilities of solving geophysical problems using fiber-optic distributed systems for recording acoustic waves, a comparison was made of the signals obtained by traditional hydrophones and a distributed fiber-optic system using a cable containing straight and helical fibers. The studies were carried out by the method of cross-well seismic survey. The possibility of separating direct and refracted head waves recorded by a distributed fiber-optic system and using them to obtain geological and geophysical information about the state of the massif is considered. It is shown that, when using a helically wound fiber, the first arrivals of a direct longitudinal wave can be traced to conduct cross-well seismic of the rock massif and evaluate the velocity distribution by the method of cross-well time-travel tomography on direct waves. For both straight and helical fibers, the stacking of the head waves makes it possible to obtain sufficiently clear arrivals of the head wave even in the dry part of the well and use it to determine the velocities of compressional waves of the near-borehole massif. The composition of the cross-well wave field depends on the radiation patterns of both the source and the receiver of acoustic waves. The use of multiple overlap systems makes it possible to vary the composition of the recorded wave field due to the mutual arrangement of the receiving and exciting lines depending on the tasks being solved.

Thin-film coatings of tin oxide on the surface of a chemically thinned section of a single-mode quartz fiber have been obtained and experimentally characterized. The materials were synthesized on the fiber surface by metal-organic chemical vapor deposition (MOCVD). To change the surface morphology, we used different amounts of tetramethyltin (SnMe4) supplied by a carrier gas (dried air) to the deposition zone by varying the temperature of the bubbler with the reagent. During deposition, the transmission spectrum of the optical path was recorded in real time, and the temperature of the bubbler in the experiments varied from –20 to +20°С. After studying the surface on a scanning electron microscope, the deposited films were tested for chemical resistance to an aqueous solution of sulfuric acid and the sensitivity of the lossy mode resonance (LMR) to changes in the refractive index of the environment in the range from 1.35 to 1.41 was evaluated. Samples produced at higher reagent flow rates exhibited a greater resonance sensitivity of 3800 nm/refractive index unit (RIU) for the first-order TM component of the resonance, but such coatings dissolve noticeably in concentrated sulfuric acid solutions, in contrast to coatings obtained with low reagent consumption.

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