State-of-the-Art Methods for Determining the Frequency Shift of Brillouin Scattering in Fiber-Optic Metrology and Sensing (Review)

Nordin N.D., Abdullah F., Zan M.S., A Bakar A.A., Krivosheev A.I., Barkov F.L., Konstantinov Y.A.

In this paper, we studied the possibility of increasing the Brillouin frequency shift (BFS) detection accuracy in distributed fibre-optic sensors by the separate and joint use of different algorithms for finding the spectral maximum: Lorentzian curve fitting (LCF, including the Levenberg–Marquardt (LM) method), the backward correlation technique (BWC) and a machine learning algorithm, the generalized linear model (GLM). The study was carried out on real spectra subjected to the subsequent addition of extreme digital noise. The precision and accuracy of the LM and BWC methods were studied by varying the signal-to-noise ratios (SNRs) and by incorporating the GLM method into the processing steps. It was found that the use of methods in sequence gives a gain in the accuracy of determining the sensor temperature from tenths to several degrees Celsius (or MHz in BFS scale), which is manifested for signal-to-noise ratios within 0 to 20 dB. We have found out that the double processing (BWC + GLM) is more effective for positive SNR values (in dB): it gives a gain in BFS measurement precision near 0.4 °C (428 kHz or 9.3 με); for BWC + GLM, the difference of precisions between single and double processing for SNRs below 2.6 dB is about 1.5 °C (1.6 MHz or 35 με). In this case, double processing is more effective for all SNRs. The described technique’s potential application in structural health monitoring (SHM) of concrete objects and different areas in metrology and sensing were also discussed.

Gorshkov B.G., Yüksel K., Fotiadi A.A., Wuilpart M., Korobko D.A., Zhirnov A.A., Stepanov K.V., Turov A.T., Konstantinov Y.A., Lobach I.A.

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.

Wang Y., Chen L., Bao X.

Driven by the strong need for distributed high-frequency dynamic strain sensing, ultra-fast Brillouin optical time-domain analysis is rapidly becoming a vital technique. Thus, in this Letter, we propose and demonstrate a novel method by using a chirped pulse as a pump signal to extract the relative Brillouin frequency shift (BFS) changes through the real-time delays between adjacent Brillouin traces; this enables static and dynamic strain measurement without time-consuming frequency sweeping process. Benefiting from single-shot measurement based on Brillouin traces, the system has a high acquisition rate that is only limited by the sensor length without averaging, and is immune to the polarization fading problem, thanks to electrical delay time measurement. The pump source is a 1 MHz linewidth laser without a phase-locking loop; the laser frequency drifting noise could be compensated by the signal from the non-disturbed fiber section. In the experiments, BFS measurement resolution of 0.42 MHz with 4.5 m spatial resolution is demonstrated over a 5 km non-uniform fiber.

Krivosheev A., Konstantinov Y., Pervadchuk V., Barkov F.

Distributed fiber-optic sensors are becoming an increasingly common solution for monitoring and diagnosing extended information transmission lines, industrial devices, buildings and devices. One of the directions – Brillouin reflectometry, which allows diagnosing a fiber line for changes in ambient temperature or mechanical deformation, is becoming increasingly popular for engineers and researchers. However, modern standards impose increasingly strict requirements on diagnostic systems in terms of the accuracy of determined parameters. For Brillouin reflectometry, where the value of the environmental parameters is determined by the position of the maximum of the Brillouin gain spectrum, the task of more accurately determining this maximum becomes the main one. The paper considers modern computer-computational methods for detecting the maximum of the Brillouin gain spectrum in an optical fiber. The authors note that imperfections in the shape of the optical spectrum, such as the signal-to-noise ratio, as well as possible digital defects that occur during digitization, can significantly impair the accuracy of the system. The authors consider three approaches to detecting the maximum of the spectrum: the classical method of Lorentz curve fitting, the method of cross-correlation with the ideal Lorentzian function, as well as the method of inverse correlation developed by the authors earlier. To combine the results of the work of the three methods, a neural network model was developed that accepts the input data of each method, together with the parameters of noise and distortion of the spectrum. The model is presented in the form of a four-layer perceptron with two hidden layers. As a result, the authors achieved an increase in the accuracy of determining the position of the maximum of the spectrum by 10% on the model data.

Matveenko V.P., Kosheleva N.A., Serovaev G.S.

The article presents the experimental results of the measurement of strains with fiber-optic strain sensors based on Bragg gratings embedded into the material. he following experiments were conducted: measurement of strains in a sample made of a polymer composite material with a stress concentrator, measurement of process-induced strains during the formation of a polymer composite material, measurement of process-induced strains during the curing of a cement mixture in the manufacture of concrete, and measurement of creep strain in a polymer material. In addition to the experiments demonstrating the possibility of measuring strains with fiber-optic strain sensors based on Bragg gratings embedded into the material, the results of a numerical analysis of the problems arising from the use of these sensors are presented. The analysis of the redistribution of the stress-strain state resulting from the embedding of an optical fiber into the material and the reliability of the strain values calculated on the basis of physical quantities obtained using sensors are among these problems.

Choi B., Seo D., Kwon Y., Kwon I.

An optical-fiber-embedded composite cylinder was fabricated using the filament winding process with an interval of 12 mm in the longitudinal direction of the cylinder. The optical fiber was wound 160 turns around the cylinder, and the straight length was about 125 m. After a total of twelve impact events of 5, 10, 15, and 20 J, the residual strain in the cylinder was measured using the proposed time differential BOCDA sensor system. This method makes the traditionally used optical delay unnecessary while increasing the degrees of freedom of using the modulation rate, which determines the spatial resolution of this measurement system. The modulation rates of optical light in the system were applied up to 16 Gbps, which is an eight-fold increase compared to our previous experiments. Damage maps were obtained by mapping the measured residual strain onto the structure of the cylinder, and compared using three spatial resolutions of 20, 10, and 6.25 mm. In the measured damage map, expansion deformation due to impact was measured at all impact points, and the impact location on the map and the actual location on the cylinder were exactly the same. The map measured from the composite showed a clear point-symmetrical shape with an increase in sharpness as the measurement resolution increased. At the highest resolution, material expansion and compression were observed to alternate with respect to the center of impact, like the surface deformation of a liquid caused by a thrown object. Furthermore, considered together with our previous experiments, we confirmed that this phenomenon propagated from the surface of the composite material to the interior, where the optical fiber was embedded. The total amount of residual strain formed around each impact point was linearly proportional to the applied external impact energy.

Stepanov K.V., Zhirnov A.A., Koshelev K.I., Chernutsky A.O., Khan R.I., Pnev A.B.

We present a theoretical and experimental study in which we increased the sensitivity of a phase-sensitive optical time-domain reflectometer (phi-OTDR). This was achieved by constructing coils in the sensor cable, which increased the total amplitude of the impact on the fiber. We demonstrate this theoretically using the example of a phase-sensitive reflectometer model and practically in testing grounds with a buried nearby conventional sensor and a sensor with coils. The sensitivity increased 2.2 times. We detected 95% of events when using coils, versus 20% when using a straight cable. The suggested method does not require any modifications to the device.

Krivosheev A.I., Konstantinov Y.A., Barkov F.L., Pervadchuk V.P.

Using the same extremely noised data, two correlation methods for finding the maxima of Brillouin spectra are compared. The first method is a well-known method of correlating the received signal with the ideal Lorentzian function. In the second method, developed by the authors earlier, instead of the Lorentzian function, the same spectrum under study, but inverted along the frequency axis, is used (Backward correlation method, BWC). In addition to evaluating the accuracy of both methods, they are compared with the classical method of Lorentzian curve fitting. The accuracy of the considered methods is estimated depending on the probability of artefacts appearing in the Brillouin scattering spectra. It is shown that when the 9% probability of the artifact occurrence is exceeded, the BWC method shows better results than the other methods considered.

Matveenko V., Kosheleva N., Serovaev G., Fedorov A.

The results of strain measuring experiments, with the help of rosettes consisting of fiber Bragg grating sensors (FBG) embedded at the manufacturing stage in a polymer composite material are considered in this paper. The samples were made by the direct pressing method from fiberglass prepregs. A cross-shaped sample was tested under loading conditions corresponding to a complex stress state. A variant of strain calculations based on experimental data is discussed. The calculations were performed under the assumption of a uniaxial stress state in an optical fiber embedded in the material. The obtained results provide a reasonable explanation of the absence in the conducted experiment of two peaks in the reflected optical spectrum, the presence of which follows from the known theoretical principles. The experimental result with two peaks in the reflected optical spectrum was obtained for the same sample under a different loading scheme. The proposed variant of the numerical model of the experiment and the results of numerical simulation made for FBG rosettes embedded in the material allowed to estimate error in the strain values calculated on the assumption of the uniaxial stress state in the optical fiber and in the presence of two peaks in the reflected optical spectrum.

Elgaud M.M., Zan M.S., Abushagur A.A., Hamzah A.E., Mokhtar M.H., Arsad N., A. Bakar A.A.

For almost a half-decade, the unique autocorrelation properties of Golay complementary pairs (GCP) have added a significant value to the key performance of conventional time-domain multiplexed fiber Bragg grating sensors (TDM-FBGs). However, the employment of the unipolar form of Golay coded TDM-FBG has suffered from several performance flaws, such as limited improvement of the signal-to-noise ratio (SNIR), noisy backgrounds, and distorted signals. Therefore, we propose and experimentally implement several digital filtering techniques to mitigate such limitations. Moving averages (MA), Savitzky–Golay (SG), and moving median (MM) filters were deployed to process the signals from two low reflectance FBG sensors located after around 16 km of fiber. The first part of the experiment discussed the sole deployment of Golay codes from 4 bits to 256 bits in the TDM-FBG sensor. As a result, the total SNIR of around 8.8 dB was experimentally confirmed for the longest 256-bit code. Furthermore, the individual deployment of MA, MM, and SG filters within the mentioned decoded sequences secured a further significant increase in SNIR of around 4, 3.5, and 3 dB, respectively. Thus, the deployment of the filtering technique alone resulted in at least four times faster measurement time (equivalent to 3 dB SNIR). Overall, the experimental analysis confirmed that MM outperformed the other two techniques in better signal shape, fastest signal transition time, comparable SNIR, and capability to maintain high spatial resolution.

Wei H., Wang Y., Wang Q., Lu X., Wu H., Fan L., Li C., Xin X.

In this paper, Gaussian smoothing (GS), non-local means (NLM), and Quaternion Wavelet Transform (QWT) have been described in detail. Furthermore, a Brillouin optical time domain analysis (BOTDA) experimental system was built to verify the denoising algorithms. The principal and experimental analyses show that the QWT algorithm is a more efficient image denoising method. The results indicate that the GS algorithm can obtain the highest signal-to-noise ratio (SNR), frequency uncertainty, and Brillouin frequency shift (BFS) accuracy, and can be executed in an imperceptible time, but the GS algorithm has the lowest spatial resolution. After being denoised by using NLM algorithm, although SNR, frequency uncertainty, BFS accuracy, and spatial resolution significantly improved, it takes 40 min to implement the NLM denoising algorithm for a BGS image with 200 × 100,000 points. Processed by the QWT denoising algorithm, although SNR increases to 17.26 dB and frequency uncertainty decreases to 0.24 MHz, a BFS accuracy of only 0.2 MHz can be achieved. Moreover, the spatial resolution is 3 m, which is not affected by the QWT denoising algorithm. It takes less than 32 s to denoise the same raw BGS data. The QWT image denoising technique is suitable for BGS data processing in the BOTDA sensor system.

Zan M.S., Elgaud M.M., Zainuddin A.R., Kadhim A.S., Mokhtar M.H., Arsad N., Bakar A.A.

Abstract We report in this paper the simulation analysis on the simultaneous deployment of Brillouin gain and Brillouin loss in the Golay coded phase-shift pulse Brillouin optical time domain analysis (PSP-BOTDA) fiber optic sensor for improving the sensing distance and at the same time obtaining high spatial resolution measurement. In this technique, the Golay coded pump pulse is alternately frequency modulated for the purpose of generating Stokes and Anti-Stokes scattering. The technique also modulates the coded pump with the return-to-zero (RZ) formats. Simulation analysis has revealed that the simultaneous use of Brillouin gain- and loss and the pulse coding technique in the PSP-BOTDA has improved the dynamic sensing range and the signal-to-noise improvement ratio (SNIR) of the sensor. By performing simulation over 500 m of fiber, we have also successfully demonstrated 10-cm of high spatial resolution measurement with the use of 1ns of coded pulse duration.

Su L., Chen B., Liu X., Zhou Z., Song M., Wang Y., Yang J.

We propose a fast cross-correlation Brillouin frequency shift extraction algorithm for BOTDR sensors to realize temperature detection in real-time. Compared to conventional method, our method has much shorter processing time with comparable accuracy.

Stepanov K.V., Zhirnov A.A., Chernutsky A.O., Koshelev K.I., Pnev A.B., Lopunov A.I., Butov O.V.

Weak fiber Bragg gratings (WFBGs) in a phase-sensitive optical time-domain reflectometer (phi-OTDR) sensor offer opportunities to significantly improve the signal-to-noise ratio (SNR) and sensitivity of the device. Here, we demonstrate the process of the signal and noise components’ formation in the device reflectograms for a Rayleigh scattering phi-OTDR and a WFBG-based OTDR. We theoretically calculated the increase in SNR when using the same optical and electrical components under the same external impacts for both setups. The obtained values are confirmed on experimental installations, demonstrating an improvement in the SNR by about 19 dB at frequencies of 20, 100, and 400 Hz. In this way, the minimum recorded impact (at the threshold SNR = 10) can be reduced from 100 nm per 20 m of fiber to less than 5 nm per 20 m of fiber sensor.

Nordin N.D., Zan M.S., Abdullah F.

This paper demonstrates a comparative analysis of five machine learning (ML) algorithms for improving the signal processing time and temperature prediction accuracy in Brillouin optical time domain analysis (BOTDA) fiber sensor. The algorithms analyzed were generalized linear model (GLM), deep learning (DL), random forest (RF), gradient boosted trees (GBT), and support vector machine (SVM). In this proof-of-concept experiment, the performance of each algorithm was investigated by pairing Brillouin gain spectrum (BGS) with its corresponding temperature reading in the training dataset. It was found that all of the ML algorithms have significantly reduced the signal processing time to be between 3.5 and 655 times faster than the conventional Lorentzian curve fitting (LCF) method. Furthermore, the temperature prediction accuracy and temperature measurement precision made by some algorithms were comparable, and some were even better than the conventional LCF method. The results obtained from the experiments would provide some general idea in deploying ML algorithm for characterizing the Brillouin-based fiber sensor signals.

Konstantinov Y., Krivosheev A., Barkov F.

This paper demonstrates how the processing of Brillouin gain spectra (BGS) by two-dimensional correlation methods improves the accuracy of Brillouin frequency shift (BFS) extraction in distributed fiber optic sensor systems based on the BOTDA/BOTDR (Brillouin optical time domain analysis/reflectometry) principles. First, the spectra corresponding to different spatial coordinates of the fiber sensor are resampled. Subsequently, the resampled spectra are aligned by the position of the maximum by shifting in frequency relative to each other. The spectra aligned by the position of the maximum are then averaged, which effectively increases the signal-to-noise ratio (SNR). Finally, the Lorentzian curve fitting (LCF) method is applied to the spectrum with improved characteristics, including a reduced scanning step and an increased SNR. Simulations and experiments have demonstrated that the method is particularly efficacious when the signal-to-noise ratio does not exceed 8 dB and the frequency scanning step is coarser than 4 MHz. This is particularly relevant when designing high-speed sensors, as well as when using non-standard laser sources, such as a self-scanning frequency laser, for distributed fiber-optic sensing.

Nair S., Walsh T.F., Pickrell G., Semperlotti F.

This work presents a physics-driven machine learning framework for the simulation of acoustic scattering problems. The proposed framework relies on a physics-informed neural network (PINN) architecture that leverages prior knowledge based on the physics of the scattering problem as well as a tailored network structure that embodies the concept of the superposition principle of linear wave interaction. The framework can also simulate the scattered field due to rigid scatterers having arbitrary shape as well as high-frequency problems. Unlike conventional data-driven neural networks, the PINN is trained by directly enforcing the governing equations describing the underlying physics, hence without relying on any labeled training dataset. Remarkably, the network model has significantly lower discretization dependence and offers simulation capabilities akin to parallel computation. This feature is particularly beneficial to address computational challenges typically associated with conventional mesh-dependent simulation methods. The performance of the network is investigated via a comprehensive numerical study that explores different application scenarios based on acoustic scattering.

Choi B.

A time-differential (TD) Brillouin optical correlation domain analysis (BOCDA) sensor system was applied to measure the Brillouin gain spectrum of a 1 km long sensing optical fiber. The optical delay line used in all BOCDA measurement systems was eliminated in the TD-BOCDA system by using a bit-delayed modulation relationship between the probe and pump lightwaves. These lightwaves were phase modulated using 216-1 pseudo-random binary sequence codes at 5 Gbps. A 2 cm dispersion-shifted fiber placed at the end of the 1 km optical fiber was distinctly identified by the Brillouin frequency extracted from the Brillouin gain spectrum measurement. To investigate the measurement stability of the TD-BOCDA system, experiments were conducted under two different pumping conditions. A semiconductor optical amplifier (SOA) and an intensity modulator (MOD) were compared for the pump chopper used in the TD-BOCDA system to detect the extinction ratio of the pump and the resulting noise in the Brillouin gain measurement. The stability of the Brillouin frequency measurement from the Brillouin gain spectrum in the TD-BOCDA system was investigated by increasing the average value of the measurement using either the SOA or MOD. The repeated-measurement deviation of the system with the SOA was only half of the deviation observed in the system with the MOD. The performance of TD-BOCDA is equivalent to or better than that of conventional BOCDAs in terms of measurement reliability. Moreover, TD-BOCDA is free from the drawbacks of traditional BOCDA, which uses time-delayed fibers and varies the bit rates.

Cibira G., Glesk I., Dubovan J., Benedikovič D.

Many techniques have been studied for recovering information from shared media such as optical fiber that carries different types of communication, sensing, and data streaming. This article focuses on a simple method for retrieving the targeted information with the least necessary number of significant samples when using statistical population sampling. Here, the focus is on the statistical denoising and detection of the fiber Bragg grating (FBG) power spectra. The impact of the two-sided and one-sided sliding window technique is investigated. The size of the window is varied up to one-half of the symmetrical FBG power spectra bandwidth. Both, two- and one-sided small population sampling techniques were experimentally investigated. We found that the shorter sliding window delivered less processing latency, which would benefit real-time applications. The calculated detection thresholds were used for in-depth analysis of the data we obtained. It was found that the normality three-sigma rule does not need to be followed when a small population sampling is used. Experimental demonstrations and analyses also showed that novel denoising and statistical threshold detection do not depend on prior knowledge of the probability distribution functions that describe the FBG power spectra peaks and background noise. We have demonstrated that the detection thresholds’ adaptability strongly depends on the mean and standard deviation values of the small population sampling.

Zhang L., Li X., Wang J., Zhang L., Li Y.

Rayleigh Brillouin optical time domain analysis (BOTDA) uses the backscattered Rayleigh light generated in the fiber as the probe light, which has a lower detection light intensity compared to the BOTDA technique. As a result, its temperature-sensing technology suffers from a low signal-to-noise ratio (SNR) and severe sensing unreliability due to the influence of the low probe signal and high noise level. The pulse coding and LMD denoising method are applied to enhance the performance of the Brillouin frequency shift detection and temperature measurement. In this study, the mechanism of Rayleigh BOTDA based on a few-mode fiber (FMF) is investigated, the principles of the Golay code and local mean decomposition (LMD) algorithm are analyzed, and the experimental setup of the Rayleigh BOTDA system using an FMF is constructed to analyze the performance of the sensing system. Compared with a single pulse of 50 ns, the 32-bit Golay coding with a pulse width of 10 ns improves the spatial resolution to 1 m. Further enhanced by the LMD algorithm, the SNR and temperature measurement accuracy are increased by 5.5 dB and 1.05 °C, respectively. Finally, a spatial resolution of 1.12 m and a temperature measurement accuracy of 2.85 °C are achieved using a two-mode fiber with a length of 1 km.

Belokrylov M.E., Kambur D.A., Konstantinov Y.A., Claude D., Barkov F.L.

We describe a method for reducing the cost of optical frequency domain reflectometer (OFDR) hardware by replacing two reference channels, including an auxiliary interferometer and a gas cell, with a single channel. To extract useful information, digital signal processing methods were used: digital frequency filtering, as well as empirical mode decomposition. It is shown that the presented method helps to avoid the use of an unnecessary analog-to-digital converter and photodetector, while the OFDR trace is restored by the equal frequency resampling (EFR) algorithm without loss of high resolution and with good measurement repeatability.

Bogachkov I.V., Gorlov N.I.

Bogachkov I.V.

Early detection of potentially unreliable areas in optical fibers makes it possible to detect degradation of optical fibers of fiber-optic communication lines at an early stage. Early diagnostics of the physical state of optical fibers located in the laid optical cables of telecommunication systems is an important topical task. The paper presents adaptive algorithms that allow one to determine the maximum of the Mandelstam–Brillouin scattering spectrum (Brillouin frequency shift), and then the degree of tension of optical fibers. The process of determining the Brillouin frequency shift, whose values at wavelengths of laser radiation used in telecommunication systems belong to the microwave range, can be significantly accelerated if adaptive algorithms for obtaining and processing data are implemented due to a special choice of the step for the scanning frequency and the time of accumulation of measurement results. The adaptive algorithms considered in this paper make it possible to speed up the process of obtaining output results in Brillouin reflectometers by ignoring readings that do not significantly affect the final characteristics. The construction of approximate graphs of the distribution of the spectrum and tension along the length of the fiber allows the trained user of the Brillouin reflectometer to stop the analysis process in order to make corrections to the measuring process (selection of the scanning range in frequency, change in the scanning step in frequency, choice of the accuracy of the presentation of output results, change in spatial resolution, etc.), which also speeds up the testing of the selected fiber. The measurement process can also be accelerated by adaptively changing the number of averages. With a database of measured Mandelstam–Brillouin scattering characteristics of optical fibers of various types and manufacturers, the speed of obtaining tension plots can also be increased. Since the approximate value of the Brillouin frequency shift is calculated already at the initial steps of the measurement process, preliminary dependences of the tension distribution along the length of the fiber will be constructed rather quickly.

Taranov M.A., Gorshkov B.G., Alekseev A.E., Konstantinov Y.A., Turov A.T., Barkov F.L., Wang Z., Zhao Z., Zan M.S., Kolesnichenko E.V.

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.

Belokrylov M.E., Claude D., Konstantinov Y.A., Karnaushkin P.V., Ovchinnikov K.A., Krishtop V.V., Gilev D.G., Barkov F.L., Ponomarev R.S.

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.

Богачков И.В.

Белокрылов М.Е., Клод Д., Константинов Ю.А., Карнаушкин П.В., Овчинников К.А., Криштоп В.В., Гилев Д.Г., Барков Ф.Л., Пономарев Р.С.

Latkin K.

A description of the model of luminescence of active erbium ions in the core of an optical fiber with subsequent experimental verification is presented. The author suggested a verification technique based on local irradiation of the erbium-doped medium at the dopant absorption wavelength and subsequent registration of the power of induced luminescence. These data are necessary to establish the physical limit of the detection frequency of the system for analyzing the distribution of active erbium ions in the preform of the optical fiber. Calculations showed that the order of values of the luminescence steady-state duration is 10 ms, which agrees with the experimental results and other references. Thus, it was found that the fundamental limit of the acquisition frequency of any system investigating the luminescence properties of erbium cannot exceed 200 Hz. The phenomenon of parasitic back reflection from the preform was also simulated. In the future, this may help to further optimize the measurement process.

Karapanagiotis C., Krebber K.

This paper presents reported machine learning approaches in the field of Brillouin distributed fiber optic sensors (DFOSs). The increasing popularity of Brillouin DFOSs stems from their capability to continuously monitor temperature and strain along kilometer-long optical fibers, rendering them attractive for industrial applications, such as the structural health monitoring of large civil infrastructures and pipelines. In recent years, machine learning has been integrated into the Brillouin DFOS signal processing, resulting in fast and enhanced temperature, strain, and humidity measurements without increasing the system’s cost. Machine learning has also contributed to enhanced spatial resolution in Brillouin optical time domain analysis (BOTDA) systems and shorter measurement times in Brillouin optical frequency domain analysis (BOFDA) systems. This paper provides an overview of the applied machine learning methodologies in Brillouin DFOSs, as well as future perspectives in this area.

Shahbaz M., Butt M.A., Piramidowicz R.

Photonic sensing devices have become increasingly important in various fields such as agriculture, medicine, biochemical sensing, and manufacturing. They are highly sensitive and can classify minor changes in the physical and chemical properties of the ambient medium with high precision. This makes them practical in applications where accurate measurements are critical, such as medical diagnostics and environmental monitoring. In this review paper, recent advances in different types of photonic sensors are discussed, which include photonic crystal-based sensors, surface plasmon resonance-based sensors, optical fiber-based sensors, optical waveguide-based sensors, and wearable sensors. These highly fascinating sensing devices play a crucial role in countless applications and have several advantages over traditional sensors. As technology continues to advance, we can expect photonic sensors to become even more precise, versatile, and reliable.