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Multicore fiber (MCF) which contains more than one core in a single fiber cladding has attracted ever increasing attention for application in optical sensing systems owing to its unique capability of independent light transmission in multiple spatial channels. Different from the situation in standard single mode fiber (SMF), the fiber bending gives rise to tangential strain in off-center cores, and this unique feature has been employed for directional bending and shape sensing, where strain measurement is achieved by using either fiber Bragg gratings (FBGs), optical frequency-domain reflectometry (OFDR) or Brillouin distributed sensing technique. On the other hand, the parallel spatial cores enable space-division multiplexed (SDM) system configuration that allows for the multiplexing of multiple distributed sensing techniques. As a result, multi-parameter sensing or performance enhanced sensing can be achieved by using MCF. In this paper, we review the research progress in MCF based distributed fiber sensors. Brief introductions of MCF and the multiplexing/de-multiplexing methods are presented. The bending sensitivity of off-center cores is analyzed. Curvature and shape sensing, as well as various SDM distributed sensing using MCF are summarized, and the working principles of diverse MCF sensors are discussed. Finally, we present the challenges and prospects of MCF for distributed sensing applications.

We demonstrate distributed measurement of the polarization beat length along single-mode optical fibers (SMFs) using slope-assisted Brillouin optical correlation-domain reflectometry (BOCDR) with high spatial resolution, long measurement range, high sampling rate, and single-end accessibility. The beat lengths of SMFs wound on mandrels with radii of 10.5, 7.5, and 3.4 cm are measured to be 36.0, 18.2, and 3.7 m, respectively, which agree well with theoretical calculations. This indicates that a unique beyond-nominal-resolution effect of slope-assisted BOCDR does not influence the measurement. Subsequently, distributed beat length measurement is demonstrated using an SMF comprising two sections wound on mandrels with different radii (10.5 and 3.4 cm). [ABSTRACT FROM AUTHOR]

Copyright of Journal of Mine Automation is the property of Industry & Mine Automation Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Optical-pulse-coding (OPC) technique can break the trade-offs between key parameters of distributed fiber-optic sensing systems, therefore it has received great attention in recent years. OPC Φ-OTDR has achieved significant performance improvements over conventional Φ-OTDR; however, scan-rate, interference fading and frequency drift remain critical issues for this type of sensing systems. In this article, for the first time bipolar Golay coding Φ-OTDR with heterodyne detection is proposed and realized to address the scan-rate issue, the laser frequency drift is compensated by a novel real-time compensation method, and the interference fading is eliminated by incorporating the spectrum extraction and remix method. Theoretical analysis is described in detail, and the benefits provided by bipolar code are experimentally demonstrated in 10 km sensing range with 0.92 m spatial resolution, achieving 7.1 dB SNR improvement and halving measurement time compared with unipolar code case. By addressing frequency drift and fading issues, the distributed sensing capability of OPC Φ-OTDR becomes available, at a reasonable expense of enlarged spatial resolution (6 m). It should be noted that, such a bipolar scheme can also be applied in other coding schemes, opening up more possibilities for OPC in Φ-OTDR. [ABSTRACT FROM AUTHOR]

Monitoring of drying-induced volume shrinkage and desiccation cracking in clayey soils is of great importance in geological and geotechnical engineering. Compared with conventional strain monitoring methods providing discrete measurements, the Brilliouin optical time domain reflectometer (BOTDR) technique enables continuous measurement of the distributed strain generated along optical fibers. In this study, a physical model test is conducted to investigate the feasibility of monitoring drying-induced soil shrinkage and desiccation cracking using BOTDR. Three optical fibers with different surface protections (thermoplastic polyester elastomer (TPEE) jacket, nylon jacket, and acrylate coating) are tested and compared. Experimental results validate that BOTDR is applicable for the direct strain monitoring of desiccation cracking soils. Monitored strain values are strongly influenced by water content, soil cracking, and fiber types. The strain measured by the optical fiber reaches only several micro strains when the soil is over-saturated, gradually increases with the decreasing water content and the increasing soil-fiber interfacial shear stresses, and drops rapidly after the occurrence of decoupling between fiber and soil resulting from the mature development of desiccation cracks. The optical fiber with acrylate coating is not suitable because of its fragility and the poor interfacial coupling with the soil. Optical fibers covered with TPEE jacket or nylon jacket are both applicable for soil strain monitoring, with the former one more sensitive to water content variations. The study is the first attempt to apply the BOTDR technique for the direct and continuous monitoring of drying-induced soil shrinkage and desiccation cracking process. It is expected bring new insights into the fundamental understanding of volumetric shrinkage and desiccation cracking in clayey soils. [ABSTRACT FROM AUTHOR]

An integrated design of ultra-high-speed Fiber Bragg Grating (FBG) interrogation system is illustrated and experimentally demonstrated. A Fourier Domain Mode Locked (FDML) laser is employed as a high-speed wavelength-swept light source. The use of the Mach-Zehnder interferometer (MZI) and the single-peak filter (SPF) overcomes the nonlinear effect and the scanning wavelength drift problem. A signal acquisition and peak-searching method based on the hardware is designed, allowing a real-time and multichannel interrogation. The controlled experiment has verified the feasibilities of the MZI and the SPF. Dynamic and static FBG experiments show that the integrated system has an accuracy of 0.02 nm, a stability of 0.006 nm, a linearity of 0.9994 and a 40 kHz demodulation frequency with 4 simultaneous interrogation channels. [ABSTRACT FROM AUTHOR]

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Mode excitations in D-shaped multimode fiber are analyzed in detail in terms of the effect of launch offset, polishing depth, and length of the D-shaped section. Based on the analysis, specklegrams from D-shaped multimode fiber structures are proposed to be applied in refractive index sensing. Glycerol solutions with different concentrations were used to study the relationship between the zero-mean normalized cross-correlation coefficient (ZNCC) of fiber output specklegrams and the surrounding refractive index. Responses of different D-shaped fiber structures are investigated and compared. For all of the structures, regular gradual changes of the ZNCC for each specklegram with the refractive index are found in a wide refractive index range of 1.352-1.444, which shows the feasibility of refractive index sensing. A high sensitivity being 10.47 RIU−1 is obtained by the structure with the polishing depth being $18~\mu \text{m}$ and length being 5cm. The proposed refractive sensing scheme is easy to realize with wide measurement range and high flexibility. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Present a ultraviolet (UV) detection method based on the all-fiber structure of zinc oxide (ZnO) composite graphene, and applied to the field of partial discharge detection. • The sensitive materials are tightly and uniformly combined with the single-mode conical, multi-mode single-mode (STMS) optical fibers by the surface functionalization method. • The response performance of the fabricated interference fiber sensor were explored and analyzed. • The UV sensitivity of STMS coated flower-shaped ZnO composite graphene is 857.41 pm/nW·cm−2, and the minimum detection voltage was 21.55 kV. The occurrence of partial discharge in high-voltage transmission equipment can lead to insulation breakdown, emphasizing the critical need for early fault detection. Traditional sensors are highly vulnerable to electromagnetic interference, which significantly impacts the accuracy and reliability of the obtained results. This paper presents an all-fiber ultraviolet (UV) sensor based on the STMS structure coated with zinc oxide (ZnO) composite graphene, realizing partial discharge detection based on optical theory. Firstly, using first principles calculation method, we conducted a comparative study on the ultraviolet-sensitive materials ZnO and ZnO composite graphene. The results demonstrate that the composite material exhibits optical response characteristics that are more suitable for partial discharge detection. Subsequently, sensitive materials with different surface morphology were prepared by hydrothermal method. These materials were uniformly coated on the waist cone region of the STMS optical fiber sensing unit using surface functionalization techniques. The UV test results showed that the flower-shaped ZnO composite graphene coated STMS optical fiber sensing has a higher sensitivity of 857.41 pm/nW cm−2, reaching 1.62 times that of rod-shaped sensitive materials. Finally, the test results of the partial discharge environment indicated that the sensor showed a good linear response when the partial discharge voltage was above 21.55 kV. The UV optical fiber sensor demonstrates promising potential for applications in the field of partial discharge detection. [ABSTRACT FROM AUTHOR]

• The equipment in-situ measurement of the optical loss of optical fibers under hydrothermal conditions has been developed. • The deterioration behavior of single-mode polyimide-coated optical fibers was characterized. • The polyimide layer and silica cladding easily dissolved in water in that environment. When distributed fiber optic sensing (DFOS) devices are deployed in a geothermal borehole, the optical fiber as the key element, must be able to withstand the harsh environment because the temperature of the geothermal boreholes is higher than 250 °C in most cases. Understanding the deterioration behavior of optical fibers in hydrothermal conditions higher than 250 °C is important for predicting the lifetimes of DFOSs in operation. Geophysical equipment using optical fiber sensing was developed for in-situ measurement of the optical loss of optical fibers under hydrothermal conditions, and a survey method was established. Using this optical fiber sensing, we characterized the deterioration behavior of single-mode polyimide-coated optical fibers. Our experiments with carbon / polyimide-coated fibers under subcritical water from 250 to 350 °C revealed that the polyimide layer and silica cladding easily dissolved in water in that environment. [ABSTRACT FROM AUTHOR]

Fiber-optic hydrophones based on optical fiber sensing and communication technology have developed rapidly in recent years. With the advantages of high sensitivity, small size, and large dynamic range, they are easy to detect underwater acoustic signals, showing great potential in the field of marine ecological protection. Researchers can monitor the marine ecological environment by recognizing underwater optical fiber sensing signals collected by fiber-optic hydrophones in real-time. Deep learning models can achieve high recognition accuracy, but they usually have large amounts of parameters that makes them difficult to be deployed in mobile terminals of marine survey ships. To solve the key problem, this paper discussed the design of the Deep Light-Weight Attention Residual Network (DLA-ResNet). It introduces the depth-wise separable convolution (DSC) and the convolutional block attention module (CBAM) into ResNet18 to make its performance better. ResNet18 is a two-dimensional convolutional network (CNN) model applying residual blocks. Experiments verify that the accuracy of DLA-ResNet improved from ResNet18 is increased by about 2.04% and its size is reduced by 85.67%. The proposed scheme in this paper realizes an average recognition rate of 96.50% for eight types of underwater optical fiber sensing signals in the application of marine ecological environment monitoring. Moreover, its model size is only 6.12 MB and it only takes 27.90 ms, which is available to meet the needs of reliability and effectiveness. [ABSTRACT FROM AUTHOR]

We report the experimental application of distributed optical fiber sensors, based on stimulated Brillouin scattering (SBS), to the monitoring of a small-scale granular slope reconstituted in an instrumented flume and subjected to artificial rainfall until failure, and to the monitoring of a volcanic rock slope. The experiments demonstrate the sensors’ ability to reveal the sudden increase in soil strain that foreruns the failure in a debris flow phenomenon, as well as to monitor the fractures in the tuff rocks. This study offers an important perspective on the use of distributed optical fiber sensors in the setting up of early warning systems for landslides in both rock and unconsolidated materials.

This paper presents a study on trailing edge deflection estimation for the SmartX camber morphing wing demonstrator. This demonstrator integrates the technologies of smart sensing, smart actuation and smart controls using a six module distributed morphing concept. The morphing sequence is brought about by two actuators present at both ends of each of the morphing modules. The deflection estimation is carried out by interrogating optical fibers that are bonded on to the wing’s inner surface. A novel application is demonstrated using this method that utilizes the least amount of sensors for load monitoring purposes. The fiber optic sensor data is used to measure the deflections of the modules in the wind tunnel using a multi-modal fiber optic sensing approach and is compared to the deflections estimated by the actuators. Each module is probed by single-mode optical fibers that contain just four grating sensors and consider both bending and torsional deformations. The fiber optic method in this work combines the principles of hybrid interferometry and FBG spectral sensing. The analysis involves an initial calibration procedure outside the wind tunnel followed by experimental testing in the wind tunnel. This method is shown to experimentally achieve an accuracy of 2.8 mm deflection with an error of 9%. The error sources, including actuator dynamics, random errors, and nonlinear mechanical backlash, are identified and discussed.

The phase-sensitive optical time-domain reflectometry ($\Phi$ -OTDR), which could locate and demodulate perturbation along tens of kilometers sensing fiber with high spatial resolution and high sensitivity, has been applied in many fields, such as seismic wave detection, structure health monitoring, and so on. The phase retrieval of the Rayleigh backscattering light is the most critical process of $\Phi$ -OTDR; therefore, the research on the phase retrieval of $\Phi$ -OTDR has attracted lots of attentions. It should be noted that in fiber-optic communications, the phase retrieval is also a crucial process for phase-encoded schemes. Particularly, as an effective phase retrieval scheme, the Kramers-Kronig (KK) receiver proposed recently has become a hot topic in fiber-optic communications for its high spectral efficiency and low requirement of continuous-wave-to-signal power ratio. Therefore, this paper discusses the feasibility of applying the KK receiver into $\Phi$ -OTDR and analyzes the signal retrieval error with KK relation in $\Phi$ -OTDR; then, its performance in phase retrieval is verified through numerical simulations and experiments. All the results show that generally the KK receiver is a good phase retrieval scheme for heterodyne $\Phi$ -OTDR. [ABSTRACT FROM AUTHOR]

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The interference fading, which is detrimental for signal retrieval in phase-sensitive optical time-domain reflectometry (${\Phi }$ -OTDR), is analyzed in frequency domain, and a novel spectrum extraction and remix method is proposed to achieve fading-free operation for ${\Phi }$ -OTDR with heterodyne detection. The operation principle is theoretically analyzed, and it is well confirmed by both simulation and experimental results. This method utilizes single rectangular probe pulse and makes full use of its spectral contents. Therefore, no complicated pulse modulation is required and the signal processing executed in digital domain is simple. [ABSTRACT FROM AUTHOR]

Optical fiber sensing (OFS) systems can be used to accurately measure temperature, strain, pressure, and other physical parameters by detecting the change in the field of lightwave. As they are used in more application areas, a new type of OFS with the ability of providing multi-dimensional, highly sensitive, ultra-fine, and real-time measurement is necessary. This is possible with the application of digital coherent optical technologies in the OFS systems. In this paper, we show that many aspects of OFS systems can be replaced or enhanced by digital coherent optical technologies. For example, digital optical frequency comb (DOFC) can replace the sweeping laser source, and the digital coherent optical receiver can replace the direct detection receiver to enable simultaneous detection of all the dimensions of lightwave. Based on these techniques, we present our research progresses on OFS with multi-dimensional, real-time, ultra-fine spectral analysis, and single-shot Brillouin optical time-domain analysis based DOFC for distributed OFS. We show that the OFS systems can be bought to a whole new level with the capabilities of real-time time–frequency analysis, ultra-fine spectral resolution, and ultra-low sensitivity. Opportunities and challenges of using digital coherent optical technologies in OFS systems are also discussed. [ABSTRACT FROM AUTHOR]

• Strain coefficient of ordinary single-mode optic fiber polyurethane packaged with 2 mm diameter is 0.0497 MHz/με. • The law of the deformation of overlying strata caused by coal mining and the strain of fiber monitoring is obtained. • The main technical problems of distributed optical fiber in the test process are analyzed in detail. The common utilization research methods in mining engineering include theoretical analysis, mechanical modeling, similarity model test, numerical simulation, and field test. And physical similarity model test is one of the main research methods for mining engineering problems. Underground engineering is often in the complex three-dimensional stress state. Compared with plane model, three-dimensional model can actually reflect the stress state of surrounding rock. Nevertheless, traditional measurement methods can't achieve internal deformation of the model with multi-scale distributed monitoring. Optical fiber sensing technology provides a solution to these problems. The sensing fibers are arranged in the plane model with the size of 3000 × 200 × 1280 mm and the three-dimensional model with the size of 3600 × 2000 × 2000 mm, and the strain distribution of the model has been analyzed with the consideration of different positions relationship between the working face and the optic fiber. The results show: the strain coefficient of the test optic fiber with 2 mm diameter calibrated by the uniform strength beam experiment is 0.0497 MHz/με. The frequency shift of the optic fiber is positive when the fiber is under tension state, and the result is opposite when under compression; In physical model tests, when the working face is close to the sensing fiber, the strain curve shows a negative step change caused by the abutment pressure. When the working face cross the fiber, the strain curve shows a positive step change due to the downward movement of broken rock layer; after the working face away from the fiber, the re-compaction of the broken rock block induced by gravity load causes the optical fiber to be under pressure, and the sensing fiber generates compressive strain. The three-segment distribution area of the strain curve corresponds to the "three-zone" height range of the overburden caused by mining, respectively. The test results could provide theoretical guidance for the application of distributed optical fiber in the determination of the caving zone and fractured zone range of overburden induced by coal mining. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In recent years, fiber optic sensors, primarily based on fiber Bragg gratings (FBGs), have been gradually applied in the monitoring of electrical equipment. This article provides an overview of the sensing principles and classifications of fiber Bragg gratings and related sensors in the monitoring of electrical equipment. It also discusses their applications in various types of electrical equipment. Addressing the current issues, such as packaging, cross-sensitivity, multiplexing, and lifespan, this article analyzes the advantages, disadvantages, and future prospects of these sensors. Additionally, a brief introduction is provided for some rapidly emerging technologies like WFBG array. This article aims to provide technical references for researchers in the field of fiber optic sensing for electrical equipment. [ABSTRACT FROM AUTHOR]

This paper presents a systematic review concentrating on the design and extensive applications of optical fiber sensors based on embedded long-period fiber gratings (LPFGs). First of all, the preparation procedures and the working principle is introduced in detail. Then, the applications of different types of sensors based on the embedded LPFG are investigated, which is explored to detect physical parameters such as temperature, strain, refractive index (RI), bending, torsion, liquid level. Also, the method for enhancing sensing performance is presented. Compared with LPFGs proposed in these years, the embedded LPFGs sensors with high selectivity and flexibility have proved to behave excellent sensing characteristic, especially in the recognition in bending and RI measurement. Furthermore, the sensors are briefly described according to the design scheme and sensing performance. Finally, the drawbacks of the sensors are analyzed and the future prospect is discussed. [ABSTRACT FROM AUTHOR]

Owing to advances in optical fiber sensing, sensing techniques based on surface plasmon resonance (SPR) have received much attention. In this work, an SPR temperature sensor based on the anti-resonant fiber (ARF) is designed and analyzed. By filling the core with a thermosensitive liquid, the ambient temperature in the range of 5–55 °C can be detected. The proposed ARF was simulated utilizing the finite element analysis software COMSOL, followed by systematic and comprehensive structural parameter optimization, and the basic characteristics of the ARF-SPR temperature sensor were analyzed. Filling with a gold medium instead of the traditional coating process reduces the difficulty in manufacturing the ARF-SPR temperature sensor. The average temperature sensitivity of the ARF-SPR sensor is 16.055 nm/°C, the minimum structural parameter sensitivity is 1.0 nm/μm, and the maximum FOM is 360.8 °C−1 at 5 °C. The ARF sensor with excellent properties has large commercial potential in biochemical analysis, temperature monitoring, as well as long-distance sensing. [ABSTRACT FROM AUTHOR]

A fishhook vector bending sensor (FV) is demonstrated based on the Mach-Zehnder interference (MZI) principle. The sensor is composite of a dual up-taper optical sphere array (DUOS) and an axial offset splice region (AO), which is fabricated on a polarization maintaining fiber (PMF) using two special arc fusion processes. Due to the overall non-uniform symmetry of the combined structure, the regions in the mutually symmetric direction are subjected to forces in opposite directions when bending. The feasibility of DUOS-FV-MZI for space vector curvature (horizontal and vertical curvature) is confirmed by finite element analysis (FEA) and experiments. In particular, a spatial light polarization system can equivalently replace this vector-type sensor with a tiny size, and several specific incident polarization angles (0°, 45°, 90°) are chosen to develop a detailed vector bending sensing. The experimental results show that maximum bending sensing sensitivity can reach 8.681 ± 0.012 nm/m−1. Besides, the structure shows a significant change in light intensity when the incident polarization angle is 45°, consistent with the theoretical analysis. Combining with a differential integration demodulation method can enhance the light intensity-modulation response, the max curve sensitivity can reach 285.04 ± 0.41 dB/m−1. Consequently, this polarization related type device integrates wavelength-sensitive and light-intensity-sensitive sensing functions. In addition, the advantages of low-temperature crosstalk and high structural stability make it suitable for flexible wearable, micro-deformation civil engineering measurements. [Display omitted] • Design of a space vector bending sensing structure FV-MZI. • This vector bending sensor has a good polarization correlation. • Polarized space vector bending sensor with optimized strain control. • Selective curvature testing using wavelength and intensity. • Enhanced response via differential integration. [ABSTRACT FROM AUTHOR]

• ITO and PVA doped GO synergistically constitute multi-layer humidity sensitive film. • Microporous composite polymer films improve humidity sensitivity parameters. • Surface functionalized PMF exhibits high sensitivity and rapid response to humidity. Real-time monitoring of environmental relative humidity (RH) plays a crucial role in ensuring both the quality of industrial production and the quality of life. In this work, polarization maintaining fiber (PMF) was used as carriers to creatively constructed multi-layer film surface functionalization on its cladding surface. The indium tin oxide (ITO) film and polyvinyl alcohol (PVA) doped graphite oxide (GO) composite polymer constitute multi-layer humidity sensitive film, which has the effect of dual absorption of water and improves the humidity sensitivity. Microporous composite polymer films created by laser processing increase the area for water evaporation and absorption, which improve response speed, and reduce recovery time. The surface functionalized PMF sensor exhibits high sensitivity and rapid response to humidity, achieving bidirectional repeated measurement of 0.28 nm/% within 10 s in the measurement range of 30%-90%, rendering it a reliable means for real-time monitoring of humidity. [ABSTRACT FROM AUTHOR]

• A transverse pressure sensor based on Sagnac cascaded FBG is proposed. • Based on PMF birefringence effect. • FBG structure is introduced to improve sensitivity. • Finite element method combined with experiment. In the field of engineering, the deformation and lateral stress in structural health detection have attracted extensive attention to obtain the structural health information of building objects in time. Based on the birefringence effect of polarization maintaining fiber (PMF), a transverse pressure sensor based on Sagnac cascade fiber bragg grating (FBG) was proposed and prepared. The performance of the sensor is analyzed by the combination of the finite element method and experiment. The simulation results show that the cross section of PMF is deformed under the pressure along the fast axis and the slow axis and then modulates the refractive index. The experimental results show that by adding FBG structure, the lateral pressure sensitivity of the sensor is 408.7 pm/N, and the linear fit is 98.3%. The lateral pressure sensitivity of the structure without FBG is 281.7 pm/N, and the linear fit is 99.3%. That is to say, the FBG structure increases the sensitivity of the sensor by 45.1%. The experimental results are in full agreement with the theoretical derivation. The optical fiber lateral pressure sensor proposed in this paper has the characteristics of simple fabrication, low cost, and high repeatability. It plays an important role in the field of structural stress monitoring. [ABSTRACT FROM AUTHOR]

Abstract The acoustic emission signal of laser plasma shock wave, which comes into being when femtosecond laser ablates pure Cu, Fe, and Al target material, has been detected by using the fiber Fabry-Perot (F-P) acoustic emission sensing probe. The spectrum characters of the acoustic emission signals for three kinds of materials have been analyzed and studied by using Fourier transform. The results show that the frequencies of the acoustic emission signals detected from the three kinds of materials are different. Meanwhile, the frequencies are almost identical for the same materials under different ablation energies and detection ranges. Certainly, the amplitudes of the spectral character of the three materials show a fixed pattern. The experimental results and methods suggest a potential application of the plasma shock wave on-line measurement based on the femtosecond laser ablating target by using the fiber F-P acoustic emission sensor probe.

Abstract In order to monitor the process of surface subsidence caused by mining in real time, we reported two types of fiber Bragg grating (FBG) based sensors. The principles of the FBG-based displacement sensor and the FBG-based micro-seismic sensor were described. The surface subsidence monitoring system based on the FBG sensing technology was designed. Some factual application of using these FBG-based sensors for subsidence monitoring in iron mines was presented.

Compared with conventional laser, random laser (RL) has no resonant cavity, reducing the requirement of cavity design. In recent years, the random fiber laser (RFL), a novel kind of RL, has made great progress in theories and experiments. The RFL has a simpler structure, a more flexible design, and higher reliability. It has valuable applications for earth sciences, biological life sciences, and national defense security, due to these unique properties. This paper reviews the development of RFLs in the last decade, including their configurations based on various optical fibers and their output properties, especially the method of control. Moreover, we also introduce their applications in the optical fiber sensing system, which is a very important and practical orientation to study. Finally, this paper presents the prospects of RFLs.

An ultrahigh resolution thickness measurement sensor was proposed based on a single mode–hollow core–single mode (SMF–HCF–SMF) fiber structure by coating a thin layer of material on the HCF surface. Theoretical analysis shows that the SMF–HCF–SMF fiber structure can measure coating thickness down to sub-nanometers. An experimental study was carried out by coating a thin layer of graphene oxide (GO) on the HCF surface of the fabricated SMF–HCF–SMF fiber structure. The experimental results show that the fiber sensor structure can detect a thin layer with a thickness down to 0.21 nanometers, which agrees well with the simulation results. The proposed sensing technology has the advantages of simple configuration, ease of fabrication, low cost, high resolution, and good repeatability, which offer great potential for practical thickness measurement applications.

A simple and reliable ultraviolet sensing method with high sensitivity is proposed. ZnO and ZnO composite graphene are successfully prepared by the hydrothermal method. The optical fiber sensor is fabricated by coating the single-mode-taper multimode-single-mode (STMS) with different shapes of ZnO. The effects of the sensitivity of ultraviolet sensors are further investigated. The results show that the sensor with ZnO nanosheets exhibits a higher sensitivity of 357.85 pm/nW·cm−2 for ultraviolet sensing ranging from 0 to 4 nW/cm2. The ultraviolet characteristic of STMS coated flake ZnO composite graphene has been demonstrated with a sensitivity of 427.76 pm/nW·cm−2. The combination of sensitive materials and optical fiber sensing technology provides a novel and convenient platform for ultraviolet detection technology.

Copyright of Journal of Harbin University of Science & Technology is the property of Journal of Harbin University of Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Highlights • Multi-peak detection algorithm based on the Hilbert Transform introduces the thought of Divide and Conquer, including splitting the reflection spectrum and single peak detection, which can overcome the shortage of traditional single peak detection algorithms (DP, centroid detection, polynomial fitting, Gaussian nonlinear fitting, and so on) to extract the Bragg wavelengths from reflection spectrum in WDM systems. • Theoretical analysis and experimental results show that this algorithm using zero as threshold of splitter can greatly reduce the computation to improve demodulation speed. Abstract A multi-peak detection algorithm based on the Hilbert Transform is proposed to overcome the shortage of traditional single peak detection algorithms, which can extract the Bragg wavelengths from the reflection spectrum in WDM systems. This algorithm introduces the thought of Divide and Conquer, including splitting the reflection spectrum and single peak detection. The multi-peaks can be pre-positioned by splitting the input reflection spectrum after the Hilbert Transform and derivation. Besides, theoretical analysis and the experimental results show that this algorithm using zero as threshold of splitter can greatly enhance the algorithmic portability under different dimensions as well as improve demodulation speed and accuracy. This implies that the proposed algorithm provides a precise demodulation algorithm for the distributed FBG sensor network. [ABSTRACT FROM AUTHOR]

Featured Application: Structural health monitoring. Distributed fiber-optic sensing based on Brillouin scattering has been extensively studied and many configurations have been developed so far. In this paper, we review the recent advances in Brillouin optical correlation-domain reflectometry (BOCDR), which is known as a unique technique with intrinsic single-end accessibility, high spatial resolution, and cost efficiency. We briefly discuss the advantages and disadvantages of BOCDR over other Brillouin-based distributed sensing techniques, and present the fundamental principle and properties of BOCDR with some special schemes for enhancing the performance. We also describe the recent development of a high-speed configuration of BOCDR (slope-assisted BOCDR), which offers a beyond-nominal-resolution detectability. The paper is summarized with some future prospects. [ABSTRACT FROM AUTHOR]

The spatial resolution of the conventional Brillouin Optical Time Domain Reflectometry system is limited because of the limitation of probe pulse’s pulse-width. To improve the spatial resolution, a novel Brillouin Optical Time Domain Reflectometry based on differential pulse pair is proposed, which is proved theoretically and experimentally to achieve a sub-meter spatial resolution. Additionally, we discuss the influence on spatial resolution with different pulse-width difference and different rise/fall time. [ABSTRACT FROM AUTHOR]

Brillouin optical time domain reflectometry (BOTDR), a fully-distributed optical fiber sensing technology, was applied to monitoring the deformation behavior of overlying strata during underground mining. Three types of optical cables, Metal kieso sensing optical cable (MKS), Glass fiber reinforced sensing optical cable (GFRS) and 10 m interval fixed-point sensing optical cable (10 m IFS), were embedded vertically into two boreholes that are located 582 m and 1746 m from the open-off cut, respectively. The strain distribution characteristics of the cables and variations of the mechanical and hydraulic properties in the overburden were analyzed with consideration of the lithology and mining positions. The results show that MKS cable has better performance in strength and accuracy than the GFRS and 10 m IFS cables. The strata in front of the panel provided front bearing loads and were compressed in the vertical direction. As the panel passed the cables to approximately 90 m, the tensile stress increased, and the peak value moved up gradually. With the occurrence and development of the caved zone, the strains of the lower optical drill were negative and in the compression state. Throughout the monitoring process, the upper cable showed a process from compression to tension, while the lower cable showed a process from compression through tension to compression. In addition, the strain distribution of the cables corresponded well with the strata. The vertical compression of the strata appears to be inversely proportional to their Young's modulus. Tensile failure dominates in the overlying strata during the mining process. With the advancing of the panel, the tensile failure zone moved upward episodically, and the height of the fractured zone reached the maximum where the distance between the face and cable was approximately 90 or 100 m. Compared with the traditional monitoring methods, BOTDR monitoring provided more accurate data on the dynamic height of the fractured zone. The research results are of practical significance for monitoring overburden deformation under mining, and they are helpful to prevent or mitigate water inrush and surface ecological geological disasters. [ABSTRACT FROM AUTHOR]

Taking the end facet of a multicore optical fiber as a platform, an optical system is designed and fabricated by using three dimensional two-photon lithography technology, which is consists of micro-pillars, prisms, tapers, micro- and nano-waveguide, and connects two cores of the multicore optical fiber. The response properties of this system to the vapor of ethanol are investigated experimentally. With the optimized geometrical parameters of the micro- and nano-waveguide, the time response to the vapor of ethanol is 1. 54 nm/s, the lowest detectable concentration is 27.3 g/m³, and the response sensitivity of the system to the change of the vapor concentration is 3.7 x 10-13 m/(g ⋅ m-3), under standard atmospheric pressure and room temperature of 22 °C. This device with advantages of miniaturization, remote sensing and easy-to-use, could be applied in the situation with limited space. [ABSTRACT FROM AUTHOR]

We propose a novel BOTDA that can measure a BFS change appearing on a time axis with a single-shot measurement by using a frequency-swept probe pulse. The proposed method employs a very simple laser control and detection system, without precise test light frequency control, and with an oscilloscope. We demonstrate a fastBFSdistribution measurement that is completed within 3.4 s with ±1.2 MHz BFS accuracy over a 40 km range with a 10 m spatial resolution and without averaging. And only 10 times averaging achieved a 1.0 MHz BFS accuracy for an 80 km measurement. [ABSTRACT FROM AUTHOR]

• Distributed optical fiber Brillouin optical time-domain reflection sensing system is used as passive railway monitoring system, which has advantages of anti-electromagnetic interference, high sensitivity, strong stability and low maintenance cost. • Monitoring Brillouin scattering signal of whole optical cable can be completed by using a redundant cable of railway communication optical fiber, which greatly saves construction cost and capital cost. • The railway communication optical fiber Brillouin scattering signal is used to complete position determination of track area/slopes/tunnels, and complex manual calibration work is completed by technological ways, which reduces time cost and capital cost of calibration and fault resolution. • Accumulated Brillouin scattering signal data of railway communication optical fiber for a long time has potential to assist in the analysis and judgment of problems such as abnormal structural state of railway track area/slopes/tunnels. Wide coverage and complex environment along the railway make a huge demand for calibration in critical areas such as tunnels or slopes. In this paper, an on-line identification method of railway tunnels and slopes based on Brillouin scattering signal is proposed. By monitoring Brillouin scattering signal of a redundant communication optical fiber, tunnels and slopes can be recognized by feature analysis. Experimental results indicate that the on-line identification method of railway tunnels and slopes based on Brillouin scattering signal is effective, invulnerable and accurate. [ABSTRACT FROM AUTHOR]

Fiber-optics sensors using interrogation based on incoherent optical frequency-domain reflectometry (I-OFDR) offer benefits such as the high stability of interference in the radio-frequency (RF) domain and the high SNR due to narrowband RF detection. One of the main impairments of the technique, however, is the necessity of high-frequency detectors and vector network analyzers (VNA) in systems requiring high resolution. In this paper, we report on two C-band implementations of an I-OFDR architecture based on homodyne electro-optic downconversion enabling detection without VNA and using only low-bandwidth, high-sensitivity receivers, therefore alleviating the requirements of conventional I-OFDR approaches. The systems are based on a pair of modulators that are synchronized to perform modulation and homodyne downconversion at a reference frequency of 25.5 kHz. In the first system, we attain centimeter resolution with a sensitivity down to −90 dB using the modulation frequency range comprised between 3.2 and 14.2 GHz. In the second, we measured, for the first time using this approach, Rayleigh backscattering traces in standard single mode fiber with resolution of 6 m and a sensitivity of −83 dB by use of the 10.1−30.1 MHz range. These results show the feasibility of these simple, homodyne downconversion I-OFDR systems as compact interrogators for distributed or quasi-distributed optical fiber sensors.

Crack generation starts at the crack tip, which bears the highest stress concentration. Under further stress, the crack propagates and leads to severe structural damage. To avoid such damage, the identification of the crack tips, and monitoring of the surrounding stress and strain fields, are very important. In this work, the location of, and strain distribution monitoring around, the crack tip are achieved using a dense ultra-short (DUS) fiber Bragg grating (FBG) array together with an improved optical frequency domain reflectometry (OFDR) interrogator. The adjacent grating interference correlation algorithm helps overcome the limitation on the demodulation precision, which is imposed by the inherently broad reflection spectra of individual ultra-short gratings. High spatial resolution measurement of the strain profile around the crack tip is performed at different levels of induced strain. Furthermore, the vertical-crossed layout is adopted to avoid the omission of cracks, which usually occurs in the case of the one direction layout. We achieve 1 mm spatial resolution and 7.5 m detection distance. Location of a single crack, multiple cracks, and an oblique crack was realized experimentally by locating the crack tips. The experimental results are consistent with the theoretical analysis, verifying the feasibility of the DUS-FBG system for high-density distributed crack tip sensing.