Optical Reflectometer Research Articles

Newest Methods and Approaches to Enhance the Performance of Optical Frequency-Domain Reflectometers.

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.

Enhancing Φ-OTDR Classification Performance through Event Augmentation

As one of the research focuses in the past decades, phase sensitive optical domain reflectometer (Φ-OTDR) has come to the tipping point of its wide application. With the expansion of its application, how to classify and identify Φ-OTDR events more effectively and efficiently in practical applications has become a key and urgent issue. Over the past several years, the incorporation of machine learning methodologies has garnered considerable attention in this area. Nevertheless, the performance of those machine learning models heavily relies on the quantity and quality of the collected data. That is, the challenge of collecting rare event signals in the sensing applications strongly limits the ability of the model to classify accurately. To overcome the above weakness and further boost the capability of Ф-OTDR, we propose an event augmentation method to enhance the diversity and generalization of raw data. Experimental results show that the proposed method improves the event classification accuracy of our Ф-OTDR from 76.4% to 91.0%. The proposed method represents a promising solution to tackle with the challenges of Ф-OTDR as well as other distributed fiber optic sensors.

Label-free optical fiber biosensor for the detection of CD44-expressing breast cancer cells

Increased level of CD44, transmembrane glycoprotein, is observed in several cancers such as breast, colorectal, head and neck, gastric, and ovarian cancer. Current methods for analyzing CD44 expressing cells are time-consuming, semi-quantitative, use labeled reagents for analysis or are not suitable for in situ analysis. Therefore exploring novel methods which are label-free and fast with the in situ detection capability for the quantification of these cells is of importance.Using optical fiber as a sensing element in biosensors offers low cost, high sensitivity, chemical inertness, and immunity to electromagnetic interference, and possibility of being used in in situ applications. This study reports the first label-free optical fiber biosensor for detection of CD44-expressing cancer cells. A fiber-optic ball resonator (BR) sensor was fabricated and then functionalized with CD44 antibodies. Different concentrations of the cells were incubated with the biosensor to measure the reflected light using optical backscatter reflectometer. The biosensor was able to detect cancer cells with a limit of detection of 335 cells/mL. The presence of the cells on the sensor surface was demonstrated by fluorescent and scanning electron microscope analysis. The work is the first proof-of-concept biosensor based on optical fibers for CD44 cancer cells and acts as a promising tool for in situ detection of cancer cells.

Investigation on the principle and performance of fiber optical reflectometer based on wave optics simulation

Fiber optical reflectometer (FOR) is a novel technique for dense droplet detection that combines conventional phase detection with non-invasive velocity measurement capabilities. In this study, the mechanism of FOR technique and its sensitivity to droplet trajectory are investigated by wave optics simulation and experiment. This paper first describes the simulation method and analyzes the dynamic response of probe during the droplet piercing process. Next, an exploration is conducted on the effective field of view for the probe and the detectable interface inclination. Afterwards, by precisely controlling the droplet size, eccentricity, and trajectory, the simulation reveals the sensitivity of FOR technique to droplet parameters and quantifies the accuracy of measurement on droplet velocity, which provides specific guidance for the design and application of FOR technique. Finally, in the static experiment, the measurement error of the droplet axial velocity is controlled within 13 %, which verifies the reliability of the simulation results.

Characterization of optical fibers doped with nanoparticles for distributed displacement sensing.

High-scattering optical fibers have emerged as a key component in distributed sensing systems, primarily due to their capacity to enhance signal-to-noise ratio. This paper presents an experimental characterization of optical fibers doped with oxide nanoparticles for displacement sensing. They were manufactured using the phase-separation technique and different doping compounds, including calcium, strontium, lanthanum and magnesium. The Rayleigh backscattering (RBS) signatures in time and frequency domains were acquired using an Optical Backscatter Reflectometer (OBR). The maximum representative length, backscattering gain and strain sensitivity were evaluated. The results indicate that the fiber co-doped with magnesium and erbium chlorides offered the best compromise between strain sensitivity (0.96 pm/μ ϵ) and maximum length (17 m). For conditions of single and multiple perturbations, strain saturation was reached at ≥7000 μm and <1500 μm, respectively. In addition, the results reveal that, under a condition of variable temperature (30-60 °C), the sensor response becomes significantly nonlinear over length, requiring a technique for temperature cross-sensitivity mitigation that accounts for nonlinearities in sensitivity and hysteresis.

Определение разновидностей оптических волокон и ранняя диагностика их физического состояния на основе анализа характеристик рассеяния Мандельштама–Бриллюэна

Research results into the automating the processing of measurement data obtained from the Brillouin optical reflectometer, light guides containing various types of optical fibers are presented in this paper. By analyzing the parameters of the Mandelstam — Brillouin scatter obtained from Brillouin reflectograms, we can classify optical fibers in the studied telecommunication optical cables. This makes it possible to evaluate the change of the Brillouin frequency shift and determine the longitudinal fiber tension. The initial values of the Brillouin frequency shift and the profile of the Mandelstam — Brillouin scatter spectrum are different for each fiber type. The programs for automated processing of Brillouin reflectogram data are discussed. Estimation of the level of the back-reflected signal allows you to identify the factor, that had a predominant effect on the parameters of the Mandelstam — Brillouin scatter signal in the studied sections of optical fibers, and to compensate for the influence of temperature changes in the longitudinal strain distribution graphs. After that, we can plot a graph of the distribution of longitudinal strain along the fiber, caused precisely by mechanical influences on optical fibers. Conclusions about the accuracy of the estimates obtained by various algorithms, based on the accumulated experience in working with the presented programs are drawn.

Droplet Size and Velocity Measurement in Annular Flow by Fiber Optical Reflectometer

Droplet Size and Velocity Measurement in Annular Flow by Fiber Optical Reflectometer

Distributed and polarimetric pressure sensitivity in spun highly birefringent optical fibers.

We examine experimentally the influence of the fiber inelastic twist on polarimetric sensitivity to hydrostatic pressure and pressure sensitivity in a Rayleigh-scattering-based optical frequency-domain reflectometer (OFDR) for highly birefringent side-hole fibers. The fibers were drawn from the same preform with different spin pitches varying from 5 mm to 200 mm. We also demonstrate that the sensitivities of spun fiber can be estimated analytically based on its spin pitch, measured birefringence, and the sensitivity of the corresponding non-twisted fiber, showing good agreement with the experimental results. We proved that polarimetric sensitivity to pressure decreases with the shorter spin pitches, while distributed pressure sensitivity decreases for the one polarization eigenmode and increases for the second polarization eigenmode. Therefore, the spun-fibers can operate well as the polarimetric sensors of other physical parameters inducing circular birefringence even under varying pressure. Moreover, they can be used to obtain the desired difference between sensitivities for both polarization eigenmodes.

An automation of the detection processes of the strain characteristics of optical fibers, classification of their varieties and identification the influence factor according to Brillouin reflectograms

Research results into the automating the processing of measurement data obtained from the Brillouin optical reflectometer, light guides containing various types of optical fibers are presented in this paper. By analyzing the parameters of the Mandelstam–Brillouin scatter obtained from Brillouin reflectograms, we can classify optical fibers in the studied telecommunication optical cables. This makes it possible to evaluate the change of the Brillouin frequency shift and determine the longitudinal fiber tension. The initial values of the Brillouin frequency shift and the profile of the Mandelstam–Brillouin scatter spectrum are different for each fiber type. The programs for automated processing of Brillouin reflectogram data are discussed. Estimation of the level of the back-reflected signal allows you to identify the factor, that had a predominant effect on the parameters of the Mandelstam–Brillouin scatter signal in the studied sections of optical fibers, and to compensate for the influence of temperature changes in the longitudinal strain distribution graphs. After that, we can plot a graph of the distribution of longitudinal strain along the fiber, caused precisely by mechanical influences on optical fibers. Conclusions about the accuracy of the estimates obtained by various algorithms, based on the accumulated experience in working with the presented programs are drawn.

Comparison of Time and Frequency Approaches to Simulation of Signals of Optical Rayleigh Reflectometers

Comparison of Time and Frequency Approaches to Simulation of Signals of Optical Rayleigh Reflectometers

Method for Increasing the Signal-to-Noise Ratio of Rayleigh Back-Scattered Radiation Registered by a Frequency Domain Optical Reflectometer Using Two-Stage Erbium Amplification

Method for Increasing the Signal-to-Noise Ratio of Rayleigh Back-Scattered Radiation Registered by a Frequency Domain Optical Reflectometer Using Two-Stage Erbium Amplification

Coherent Optical Frequency Reflectometer Based on a Self-sweeping Fiber Laser for Distributed Measurements

Coherent Optical Frequency Reflectometer Based on a Self-sweeping Fiber Laser for Distributed Measurements

Research on the Vibration Characteristics of Pumped Storage Unit Stator Core Based on Fiber Optic Sensing

Stator core vibration (SCV) response of pumped storage units (PSU) can effectively reflect its operation status and provide reference for excitation winding inter-turn short circuit (ITSC) fault diagnosis. Considering that traditional piezoelectric acceleration sensors (PAS) are difficult to be installed on the stator core surface, this study proposes an online monitoring method based on fiber optic vibration sensing (FOVS). Firstly, this study derives the SCV response equations for the normal and ITSC states of the excitation winding, and obtains the vibration characteristic harmonics corresponding to each state. Secondly, a three-dimensional finite element simulation model of PSU with a generating capacity of 278 MVA is built, and the acceleration amplitude and characteristic frequency harmonics of the stator core are calculated to provide a reference for fiber optic vibration detection. Thirdly, this study focuses on the principle of modulation and demodulation of optical phase, builds an optical frequency-domain reflectometer (OFDR) - based FOVS system, and calibrates its sensitivity, amplitude and frequency characteristics as well as the lower detection limit. Finally, FOVS probes are installed on the surface of synchronous generator stator core to capture the vibration signals under different degrees of ITSC, and compared with the collected signals of PAS. The experimental results prove that the proposed method has obvious advantages in vibration monitoring in the low frequency band below 100 Hz.

External Filtered Modes of a Quantum Dot Laser Under the Influence of Double-Filtered Optical Feedback

When semiconductor lasers SLs are exposed to filtered optical feedback (FOB), they behave in two different ways. Under weak or strong feedback, the first is a singularly stable longitudinal mode with a small linewidth. The second is the so-called coherence breakdown condition, which also includes low frequency fluctuations (LFF) and chaotic oscillations. The latter has received a lot of attention recently due to the possible uses of chaotic lasers in chaotic lidar, covert communications, and chaotic correlation time-domain optical reflectometers, while the former has received a lot of research in recent years. We investigate the behavior of a quantum dot (QD) semiconductor laser subject to FOF from two separate external cavities both, numerically and experimentally. Our findings show that the second FOF allows for rich adjustment of the laser frequency. Our analysis of double-filtered optical feedback (DFOF) lasers is on fundamental solutions, sometimes referred to as continuous waves (CW) or external filtering modes (EFM), which result in a QD laser output with constant amplitude and frequency. The time delay suppression is dependent on the spectral width of the filter and how far it is from the solitary laser frequency, according to numerical calculations.

Strain Transfer Mechanism in Surface-Bonded Distributed Fiber Optic Sensors under Different Strain Fields.

Mastering the strain transfer mechanism in distributed fiber optic (DFO) sensors holds the key to analyzing strain measurement errors from DFO sensing systems. However, the impact of the monitored structure's strain distribution on the strain transfer mechanism in DFO sensors has often been overlooked in the existing research. To address this issue, a strain transfer model of surface-bonded DFO sensors with multilayered structures was established based on the shear lag theory. The closed-form solutions of the strain transfer coefficient of DFO sensors subjected to uniform, parabolic, single-linear gradient, and bilinear gradient strains were obtained. With a high-accuracy optical frequency-domain reflectometer (OFDR), the theoretical model was validated by laboratory tests. Upon parametric analysis, suggestions were further offered about designing and installing DFO sensors.

Multichannel Classifier for Recognizing Acoustic Impacts Recorded with a phi-OTDR.

The purpose of this work is to increase the security of the perimeter of an area from unauthorized intrusions by creating an improved algorithm for classifying acoustic impacts recorded with a sensor system based on a phase-sensitive optical time reflectometer (phi-OTDR). The algorithm includes machine learning, so a dataset consisting of two classes was assembled. The dataset consists of two classes. The first class is the data of the steps, and the second class is other non-stepping influences (engine noise, a passing car, a passing cyclist, etc.). As an intrusion signal, a human walking signal is analyzed and recorded in frames of 5 s, which passed the threshold condition. Since, in most cases, the intruder moves on foot to overcome the perimeter, the analysis of the acoustic effects generated during the step will increase the efficiency of the perimeter detection tools. When walking quietly, step signals can be quite weak, and background signals can contain high energy and visually resemble the signals you are looking for. Therefore, an algorithm was created that processes space-time diagrams developed in real time, which are grayscale images. At the same time, during the processing of one image, two more images are calculated, which are the result of processing the denoised autoencoder and the created mathematical model of the adaptive correlation. Then, the three obtained images are fed to the input of the created three-channel neural network classifier, which includes convolutional layers for the automatic extraction of spatial features. The probability of correctly detecting steps is 98.3% and that of background actions is 97.93%.

Reflector-Less Shallow-Tapered Optical Fiber Biosensors for Rapid Detection of Cancer Biomarkers

Reflector-less biosensors based on optical fibers enable the detection of the refractive index (RI) variations, in reflection mode, without the need for any reflective element (such as a grating or an interferometer). The sensing system works by interrogating the wavelength shift occurring in the Rayleigh scattering signal at each location of a thinned fiber through an optical backscatter reflectometer (OBR); a high-scattering fiber is required in order to compensate for the propagation losses. The approaches reported so far make use of etched fibers, which are however fragile and require tens of minutes of fabrication, and U-bent fibers which are incompatible with in-situ devices. In this article, we present the working principle and main results of reflector-less biosensors fabricated through shallow tapering; this concept allows the fastest fabrication (&#x223C;20 seconds through a CO₂ automated laser splicer) reported so far, and allows maintaining a compact and robust form factor as the fiber maintains a diameter &gt;28 &#x00A0;&#x03BC;m (&gt;22&#x0025; of the initial thickness). The fabrication process allows achieving a peak sensitivity up to 1.33 nm&#x002F;RIU (1.05 nm&#x002F;RIU average over 5 tapers), using MgO-nanoparticle doped fibers. The tapered biosensors have been functionalized through the silanization method, in order to detect CD44 protein (from 100 fM to 10 nM), a breast cancer biomarker. The results show a successful detection with a low limit of detection (16.4 pM) and high specificity over two controls.

Experimental Study for Temperature Variation of Bridge Pier in Plateau Area Using Optical Frequency Domain Reflectometer Technology

The temperature variation in the plateau area is very important for the concrete curing of bridge pier. Due to the large temperature difference in the plateau area, the concrete temperature of the bridge pier varies unevenly, which may damage the quality of the bridge pier. Based on the large bridge project of Sichuan–Tibet railway, the advanced optical frequency-domain reflectometer (OFDR) technology is used to monitor the concrete temperature variations of the bridge pier during the 14 days curing period. The temporal and spatial evolution law of the bridge pier concrete temperature is analyzed. The monitoring result shows: the overall temperature variation of the concrete increases first and then decreases. The temperature variation of the bridge pier concrete in upper surface is obviously affected by the atmospheric fluctuation. The temperature variation on the 14th day of cross section center point is lower when it is farther from the upper surface of the pier. The temperature on the 14th day of the cross section edge point on the long axis increases first and then decreases when it is farther from the upper surface of the pier. On the same cross section, the temperature variation of the short-axis edge point is smaller than that of the long-axis edge point.

Temperature and Humidity Stability of Fibre Optic Sensor Cables for High Resolution Measurements.

Fibre optic sensors offer a means for the real-time continuous measurement of temperature or strain in concrete structures. Backscattered light along a fibre optic sensing (FOS) cable is interrogated to record a frequency shift and this shift is typically translated into a physical parameter such as strain or temperature using a calibration factor. However, when the measured frequency shift is a response to a combination of mechanical, thermal or hygral (humidity) loadings it is difficult to decouple individual influences. This presents a challenge in complex materials such as concrete where the strain, temperature and moisture levels change concurrently during the fresh and hardened states. Furthermore, depending on the application, both short- and longer-term measurements are required. As such, not only is the influence of these physical factors of interest but also the time and spatial stability of the measured frequency, which is highly dependent on the FOS cable composition. To investigate this aspect, fibre optic cables commonly used for strain (three tight-buffered cables) or temperature (two loose-buffered cables) measurement were considered. The cables were subjected to mechanical or environmental exposure and interrogated using a high-resolution optical backscatter reflectometer. The exposure regimes included three temperature cycles with sustained steps from 10 °C to 60 °C and back to 10 °C and an increasing and decreasing humidity cycle with steps between 30 to 90% relH. These ranges were selected to be indicative of typical environments for concrete. The results showed that the calibration factors back-calculated from increasing and decreasing temperature or humidity cycles differed. The third temperature cycle results were found to exhibit the smallest differences between heating and cooling suggesting that temperature pre-conditioning prior to installation could be advantageous. For all the cables, a drift in the readings was observed over the duration (2.5 h for temperature and 30 h for moisture) of the sustained steps. The magnitude of the drift depended on the cable type and exposure condition. In addition, local frequency fluctuations along the cable were observed which would need to be taken into account if only a single point along the cable length was used for analysis. The obtained results highlight the importance of the cable selection to maximise the FOS measurement fidelity for a given parameter of interest.

Investigation of the Optical Properties of Silicon-on-Insulator Microring Resonators Using Optical Backscatter Reflectometry

Introduction. Optical backscatter reflectometry is one of the most promising methods used to examine characteristic parameters relevant to the design of microring resonators. This method paves the way for experimental determination of the coupling coefficient and propagation loss. However, experimental verification of this technique by comparing the transmission characteristics obtained by reflectometry and those directly measured by an optical vector analyzer has not been carried out.Aim. To determine the parameters of microring resonators by optical reflectometry and to calculate on their basis the transmission characteristics of microring resonators. To compare the calculated transmission characteristics with those obtained experimentally using a high-resolution vector analyzer.Materials and methods. The characteristic parameters of silicon-on-insulator microring resonators were investigated using an ultra-high resolution reflectometer. An original algorithm was employed to derive the characteristic parameters of microring resonators from reflectograms. An optical vector analyzer was used to study the transmission characteristics of microring resonators. Numerical modeling of transmission characteristics considering the obtained parameters was carried out according an analytical approach based on partial wave analysis.Results. The obtained values of the power coupling coefficient κ = 0.167 and propagation losses α = 3.25 dB/cm were used for numerical simulation of the transmission characteristics of a microring resonator. These characteristics were found to agree well with those obtained experimentally. The free spectral range of 88.8 GHz and Q-factor of 45 000 were determined.Conclusion. An experimental study of the characteristic parameters of silicon-on-insulator microring resonators was conducted using an optical backscatter reflectometer. The performed comparison of the experimental and theoretical transmission characteristics showed good agreement, which indicates the high accuracy of the determined resonator parameters and, as a result, the relevance of the described method.