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A high-sensitivity and high-resolution refractive index (RI) sensor with ultrawide measurement range based on no core fiber (NCF) with large offset splicing and microwave photonic filter (MPF) interrogation is demonstrated theoretically and experimentally for the first time. In the sensor, an inline Mach–Zehnder interferometer (MZI) based on NCF with large offset splicing is used as the sensing head, and a single-passband MPF is designed to interrogate the sensing signal. Compared with the traditional interrogation in optical domain, the MPF interrogation converts the periodic interference pattern in optical spectrum into the nonperiodic bandpass frequency response in radio frequency (RF) domain, so the measurement range is unlimited in principle. Owing to the large offset splicing in the inline MZI structure, the sensitivity of the sensor as high as −1.1054 GHz/RIU is obtained in the experiment. More importantly, MPF interrogation improves the resolution by an order of magnitude to $9\times10$ −7 RIU. Meanwhile, the sensitivity and the resolution of the sensor are easily adjusted by changing the length of dispersive media. The high sensitivity and high resolution of the sensor have potential applications in ultralow concentration biochemical detection field. [ABSTRACT FROM AUTHOR]

This article presents a novel and cost-effective vector network analyzer (VNA)-based phase-compensated channel sounder operating in the frequency range of 10–50 GHz using radio-over-fiber (RoF) techniques, which can support multilink/channel long-range phase-coherent measurements. The optical cable enables long-range channel measurements with a dynamic range of 115.7 dB at 30 GHz (for the back-to-back connection). The phase compensation scheme is utilized for stabilizing the inherent phase variations introduced by the optical fiber of the channel sounder to enable its application in multichannel/antenna measurements. A novel optical delay line and combiner scheme is proposed and implemented to separate the signals, thereby saving the port resource on the VNA for multilink/channel measurements. The proposed channel sounder is validated in back-to-back measurements under two optical cable conditions, i.e., with the presence of thermal changes and mechanical stress. The phase change could be maintained within 3° at 10–30 GHz and 7° in 30–50 GHz compared to the over 80° phase variation introduced by the cable effects at 10–50 GHz, demonstrating the robustness and effectiveness of the developed channel sounder in practice. With the proposed optical delay line and combiner scheme, multiple channels can be measured simultaneously with minimal VNA ports, significantly reducing the measurement cost and time for multilink channel measurements. Two multilink indoor channel measurements are conducted and analyzed using a virtual uniform rectangular array (URA) at 28-GHz bands. The results demonstrate the capability of the proposed channel sounder to perform high-fidelity long-range ultrawideband multilink channel measurements. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) detection from protrusion defects is performed in an actual gas-insulated switchgears (GISs) based on a built-in sensor integrated with fluorescent fiber and ultrahigh frequency (UHF) antenna. The characteristics of both optical and UHF signals are analyzed in terms of inception voltage, pulse distribution, and statistical characteristics including the phase-resolved partial discharge (PRPD) pattern and PD number. In the experiment, protrusions with different curvatures are adopted to investigate the reason for the differences in the characteristics of UHF and optical signals. The results show that UHF and optical methods have different detection sensitivity for different discharge types owing to the different energies of electromagnetic waves and optical signals radiated in different discharge types. The optical method can easily detect avalanche discharge, while the UHF method can easily detect streamer discharge. This difference leads to the difference of the PD characteristics obtained by the two partial methods. For example, for the PD of small curvature protrusions, the PD inception voltage (PDIV) of the optical method is lower than that of the UHF method, because the defect triggers avalanche discharge. Moreover, the optical and UHF signal pulses are not always simultaneous in the time domain because they detect different discharge types. The PRPD patterns obtained by the optical method have a wider phase distribution than the UHF method because the optical method can detect avalanche discharge under low voltage. [ABSTRACT FROM AUTHOR]

We propose a new scheme for polarization-sensitive distributed fiber-optic sensing, called polarization optical correlation-domain reflectometry (POCDR), and experimentally demonstrate its fundamental operations. In this scheme, the states of polarization (SOPs) of reference lights are controlled so that they are uniformly distributed on the Poincaré sphere using a calibration method, which cancels the effect of unknown birefringence in the fibers used in the system. Then the SOP of spontaneous Brillouin scattering in a sensing fiber is estimated using a least-squares method, by measuring the interference powers for the multiple reference lights with different SOPs. As a pilot demonstration, the SOP transitions along a fiber including a three-paddle polarization controller are measured under different conditions. We also measure the SOP transition of a fiber wound on a bobbin and calculate its polarization beat length, which agrees well with the theoretical value. By exploiting its unique nature called random accessibility, high-speed measurement of the roundtrip birefringence between two arbitrary points in a sensing fiber will be potentially achieved. [ABSTRACT FROM AUTHOR]

Simultaneously increasing the sensitivity and dynamic range of an optical fiber sensor is desired and yet challenging. In this article, we demonstrate an optical fiber curvature sensor based on a no-core fiber (NCF) cascaded with a hollow-core fiber (HCF), realizing simultaneously high sensitivity and a large dynamic range with the assistance of machine learning analysis. The sensor is fabricated by simply fusion splicing a section of NCF and HCF to two single-mode fibers (SMFs), forming the SMF-NCF-HCF-SMF hybrid structure. It is shown that the multimode interference in the NCF can increase the sensitivity of the device for curvature measurements, compared to the conventional SMF-HCF-SMF structure. However, the enhanced sensitivity poses a limitation on the dynamic range of the proposed curvature sensor. We propose the use of machine learning to analyze the measured spectra of the sensor device, achieving one-to-one mapping between the measured raw spectrum and the exerted curvature on the sensor, and thereby the issue of the limited dynamic range is resolved. The proposed strategy, enabling the co-existence of high sensitivity and a large dynamic range, is a highly generalizable technique and can be extended to other optical fiber sensors for measuring other physical, chemical, and biological quantities in different applications. [ABSTRACT FROM AUTHOR]

The use of an active measurement system to localize contamination in concrete structures of nuclear facilities offers advantages such as an easy on-site analysis within a short measurement time. A measuring system based on a scintillation crystal connected to a long optical fiber with a large diameter could satisfy the demands of a measuring system for gamma-ray spectroscopy in boreholes. This work describes the development of a gamma-ray spectroscopy system using a fiber optic radiation sensor. A prototype was built with a GAGG scintillation crystal, which could reach an energy resolution of 33% at the 662-keV gamma-ray emission energy of 137Cs. A minimal time for the detection of a contamination of a 137Cs source with an activity of 21 kBq and a layer of 6-cm concrete between sensor and radiation source was determined to be under 1 min with three different evaluation methods. They were based on the decision threshold and the Kolmogorov–Smirnov test corresponding to the energy distribution and the distribution of time differences between detector events. In addition, it was possible to estimate the distance of a point-like source from the sensor by establishing a further analysis parameter comparing different energy regions of a spectrum. [ABSTRACT FROM AUTHOR]

In this paper, a differential micro-opto-electro-mechanical system (MOEMS) accelerometer based on the Fabry-Pérot (FP) micro-cavities is presented. The optical system of the device consists of two FP cavities and the mechanical system is composed of a proof mass that is suspended by four springs. The applied acceleration tends to move the PM from its resting position. This mechanical displacement can be measured by the FP interferometer formed between the proof mass cross-section and the optical fiber end-face. The proposed sensor is fabricated on a silicon-on-insulator wafer using the bulk micromachining method. The results of the sensor characterization show that the accelerometer has a linear response in the range of $\boldsymbol {\pm }1\text{g}$. Also, the optical sensitivity and resolution of the sensor in the static characterization are 6.52 nm/g and $153 \boldsymbol {\mu }\text{g}$. The sensor sensitivity in the power measurement is 49.6 mV/g and its resonant frequency is at 1372 Hz. Using the differential measurement method increases the sensitivity of the accelerometer. Based on the experimental data, the optical sensitivity in static mode is two times as high as that of a similar MOEMS accelerometer with one FP cavity. [ABSTRACT FROM AUTHOR]

For low fineness extrinsic Fabry-Perot inter-ferometer (EFPI) sensors, the discrete gap transform (DGT) algorithm can be usually adopted to acquire the cavity length, and the magnitude of its demodulation error is often larger than several hundred picometer. In this paper, three-parameter cosine fitting (TPCF) method is proposed to compensate for the EFPI cavity length error caused by traditional spectrum truncation, which can improve the demodulation accuracy by 3800 times of that of DGT. This method has a large cavity length demodulation range of 20 μm ∼30000 μm with a maximum error of 65 pm, which was proved by numerical simulation. The experiment shows that when the cavity length is 31 μm and 282.3 μm, the demodulation resolutions with a confidence interval of 68.27% are 15.142 pm and 27.582 pm, respectively, and its actual demodulation time is no more than 0.021 s. Compared with the traditional cavity length demodulation algorithms, the proposed algorithm has broader demodulation range, higher demodulation accuracy and resolution. Moreover, it will be unrestricted by wavelength shift for more than one period and can realize real-time fast demodulation. [ABSTRACT FROM AUTHOR]

We present ultra-broadband flat-topped supercontinuum laser sources with programmable repetition rate, spectral bandwidth, and wavelength regime. Supercontinuum sources with multiband coverage can be achieved by merging high-repetition-rate electro-optic frequency comb techniques with optical line-by-line pulse shaping. Because nonlinear broadening in the optical wave-breaking regime does not provide the required flatness and spectral bandwidth to the comb-based sources, we iteratively manipulate the spectral amplitude and phase of the optical pulse shaper. We implemented here a smooth flat-topped supercontinuum with a 97 nm bandwidth at 10 dB and programmable repetition rate up to 50 GHz by applying a bandwidth-limited quasi-super-Gaussian apodised pulse train to a highly nonlinear medium with programmable super-Gaussian and phase coefficients. This work opens new possibilities for generating robust and coherent multiple optical carriers with further C-bands in optical networks. We believe that the supercontinuum sources based on electro-optic frequency comb and pulse shaping techniques enable the provision of coherent flat-topped optical sources, at least from the L- to S-band with a programmable repetition rate and spectral bandwidth. [ABSTRACT FROM AUTHOR]

Near-infrared optical wireless communications (OWCs) have been widely studied in indoor environments, and the beam steering has been widely used to achieve both high-speed and wide coverage. Compared with active steering, the passive method eliminates the need for local power supply at OWC access points. However, current passive beam steering is based on wavelength tuning, which requires precisely controlled and costly tunable lasers. In this letter, we propose a beam-steered indoor OWC system without the need for local power through remote powering, providing the benefits of low power consumption, passive OWC access points, and low-cost. The proposed remote-powered indoor OWC system is experimentally demonstrated with 10 Gb/s OWC provided to users. An indirect remote powering scheme is also proposed and realized to support large-scale OWC access points. [ABSTRACT FROM AUTHOR]

A high sensitivity human chorionic gonado-tropin (hCG) detection was conducted by a tapered side-polished (TSP) optical fiber sensor. Experimentally, the TSP fiber sensor was made by side polishing a short section of single mode fiber to a D shape structure and tapering the D shape section to a small diameter ($ < 10~\mu \text{m}$ in the experiments). By functionalizing the primary antibody of hCG onto the TSP fiber surface, the sensor was used for detecting hCG concentration. Experimental results show that when the hCG concentration is 0.1 mIU/mL, the sensor has an average wavelength shift of 0.82 nm. The limit of detection (LoD) of the hCG is estimated 0.058 mIU/mL, assuming three times of maximum wavelength variation (${3}\times {0}.{15}=0.45$ nm) in Phosphate buffer saline (PBS) to the measurement limit. The specificity has also been tested by immersing the sensor into a mixed biomaterial solution (hCG - 1 mIU/mL, pig-IgG - 1 $\mu \text{g}$ /mL, Staphylococcus aureus $-{6}\times {10}^{{5}}$ CFU/mL and Escherichia coli - ${2.5}\times {10}^{{5}}$ CFU/mL). The result showed that the TSP optical fiber sensor has excellent specificity. The biosensor has potential application in clinical/medical diagnostics, human health, environmental quality and food safety monitoring. [ABSTRACT FROM AUTHOR]

To figure out the energy attenuation of micro/nanofibers (MNFs) more flexibly and conveniently, a backpropagation neural network (BPNN) is proposed to forecast the output intensity of rhodamine B (RhB) doped polymer microfibers (PMFs). According to the diameter, doping concentration, and propagation distance (L), we realize the L-dependence of output energy predictions for the excitation light (IE) and fluorescence (IF) of the doped PMFs. Hundreds of propagation distance-intensity data pairs acquired from dozens of RhB doped PMFs are used for the BPNN training. The prediction ability of the model is evaluated by the root-mean-square error (RMSE), the mean absolute percentage error (MAPE), and R2. The output intensity prediction performance of BPNN is compared with the traditional exponential-fitting (EF) method. The prediction results indicate that the two-hidden-layer network with one and seventeen neurons respectively provides the best performance. After training, BPNN gives a good intensity prediction for both the IE (RMSE = 3.16×10−2, MAPE = 7.3%, and R2 = 0.9802) and the IF (RMSE = 0.91×10−2, MAPE = 0.89%, and R2 = 0.9696) from the output end of the PMF with different diameters and doping concentrations. The energy losses of the two kinds of light from different doped PMFs are also calculated based on the predicted values, which are similar to the ones obtained from the EF method. The approach based on the BPNN prediction for the energy attenuation of the PMFs shows superiority in flexibility and applicability toward the traditional methods, which could promote the optimal design of the MNF devices and the practical application. [ABSTRACT FROM AUTHOR]

This paper is an examination of the effect of an axially varying fluid temperature on the measurement of $\Delta {T}({z})$ , the difference in the temperature between the optical fiber within the thermal mass and the optical fiber within the capillary tube that is attached to the outer sheath of an Optical Fiber Based Gamma Calorimeter (OFBGC). Our analysis indicates that for an OFBGC with the general design of the OFBGC that was built and tested in the Ohio State University Research Reactor (OSURR), a sinusoidal variation with spatial frequency ${k}_{B}$ in the bulk fluid temperature ${T}_{B}$ along the length of the OFBGC will create a sinusoidal variation in $\Delta {T}({z})$ , which could be misinterpreted as a sinusoidal variation in the linear energy deposition rate within the OFBGC’s thermal mass. However, for the specific design of the OFBGC that was built and tested in the OSURR, for the slowly varying spatial variation of ${T}_{B}({z})$ along the length of the OSURR’s Water Irradiation Facility, within which the OFBGC was tested, the perturbation in $\Delta {T}({z})$ is negligibly small. [ABSTRACT FROM AUTHOR]

We present an electroabsorption modulated laser based on an identical epitaxial scheme, side-wall grating, on- chip microwave probe interface, and a new low-permittivity planarization method. The modulation speed is significantly increased by reducing the electrode capacitance by planarizing with a 5- $\mu \text{m}$ -thick HSQ layer. Furthermore, implementing the electrode with a direct ground-signal-ground probe interface provides a straightforward interconnection that obviates the need for an external circuit and bonding wires. The device operates at 1565 nm wavelength with stable single-mode lasing, no mode-hopping, and a side mode suppression ratio above 35 dB. An extinction ratio of 19.5 dB was recorded at the maximum modulator bias of −4 V. The electrical to optical power response of the modulated signal at–3-dBo demonstrated a 19 GHz bandwidth at an extinction ratio of 7 dB, which supports error-free data transmission up to 27 Gbit/s. [ABSTRACT FROM AUTHOR]

The use of in-fiber core-to-cladding coupling components for thermal heating purposes has been well assessed in the last decades within the development of fiber optic devices for flow measurements and water thermal conductivity calculation. In these devices, light travelling in the fiber core is transferred into the cladding and absorbed by a metallic layer surrounding the fiber, with the consequent resistive heating generation. Here we demonstrate the use of a Turn-Around-Point (TAP) Long Period Grating (LPG) as resonant core-to-cladding light coupling mechanism for the fabrication of a highly efficient heating device based on metallic coated Fiber Bragg Grating (FBG). A properly designed TAP LPG was fabricated by means of point-to-point UV laser and spliced to a 150 nm thick Au-coated FBG. The heating efficiency characterization of the final device was analyzed, in both air and water, by evaluating the temperature increase in the gold layer surrounding the FBG at incremental values of the injected power. Collected results confirm that the use of LPGs involving the excitation of higher order cladding modes provides an excellent transferring mechanism of the fiber core light into the cladding, which in turn guarantees very high thermal heating efficiency to the final device. Moreover, by comparing such results with the performance of other in-fiber core-to-cladding coupling components already presented in literature, it was found that the TAP LPG-based device exhibits an actuation efficiency 2.5 times greater, thus resulting the most effective and highly performing solution for energy transfer to the metallic overcoat. [ABSTRACT FROM AUTHOR]

A two-dimensional (2-D) fibre Bragg grating (FBG) inclinometer with the ability to measure the inclination and azimuth angle was investigated herein. Its design involved pasting eight FBGs on an elastic cylinder cantilever (ECC). Based on derivations of formulae and finite element simulations, the dependence of the wavelength shift of FBGs on inclination and azimuth angles was established using the bending and tensile strains of the ECC. A series of sensing tests were conducted using a homemade inclination–azimuth test system to ascertain the sensor performance of the 2-D FBG inclinometer; good consistency was observed between theoretical and experimental data. The average sensitivity of the inclination angle was 6.73 pm/° – in the range of 0° to 90° – and the average sensitivity of the azimuth angle was 9.22 pm/° in the angle range of 0 to 360°. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a fiber optic multi-point bending measurement system that uses Bragg gratings inscribed along a multi-core fiber (MCF) and a silicon avalanche photodiode (Si-APD) that produces two-photon absorption (TPA) photocurrent for 1.55- $\mu \text{m}$ light. Because the TPA photocurrent from the Si-APD is proportional to the mean square of the optical intensity, the intensity correlation between 1.55- $\mu \text{m}$ probe and reference beams is measured from it without using complex electrical circuits. The proposed system has different lengths of optical fibers between each core of MCF and input/output ends of a single mode fiber optic system, which makes different the correlation signal between the reference beam and the probe beam reflected from different Bragg gratings in MCF. That enables the reflection spectra of all the Bragg gratings in MCF to be separately measured even when they are overlapped in the wavelength domain. As a result, the bending radius and its direction are measured without using multiple detectors or optical switching devices. We conducted proof-of-concept experiments by using a 7-core MCF with almost the same Bragg gratings inscribed in each core at three points along the fiber. We also demonstrated an example of application to shape measurement. [ABSTRACT FROM AUTHOR]

Distributed vibration sensing (DVS) utilizes an optical fiber as an array of virtual strain sensors. It can reveal dynamic strain distributions such as seismic events and car traffic, thus giving a new function to existing optical communication fibers. In this work, we use optical frequency domain reflectometry (OFDR) to investigate the dynamic strain distribution along an optical fiber implemented in a deployed aerial optical fiber cable that vibrates with the wind. The OFDR setup is customized for distant location interrogation with a high repetition rate of the probe light and overcomes the OFDR drawbacks of short measurement range and low repetition rate, thus enabling application to the investigation of dynamic strain along a range of telecommunications outside a plant. Sub-1-m spatial resolution measurements revealed that dynamic strain along the optical fiber in an aerial cable depends on the deployment condition. It also provides the deployed optical telecommunication fiber with a function for visualizing the deployment conditions of the telecommunications outside plants. [ABSTRACT FROM AUTHOR]

This paper describes a novel cantilever type accelerometer using a mirror-terminated hetero-core optical fiber for fault diagnosis of machinery. Hetero-core optical fiber sensors are suitable for long-term measurement in on-site fields with attractive advantages of robustness to environmental disturbances such as electromagnetic interference, chemical corrosion, and temperature fluctuation. Moreover, the proposed sensor contained a mirror termination on the hetero-core fiber in order to form a compact and cantilever-shaped structure. It was experimentally found that the mirror-terminated hetero-core fiber worked as Mach-Zehnder interferometry having interference fringes in transmission spectrum, in which linear changes in peak intensities enabled to detect the deflection of fiber beam using a cost-effective LED/PD measurement instrument. For the vibration response characteristics, the sensor reproduced the waveforms of input vibrations to optical loss changes when sinusoidal vibration was applied. Furthermore, the sensitivities were stable in the range of $5.4\times10^{3}$ to $2.3\times10^{2}$ dB/g for the frequency ranges less than 620–130 Hz, when setting a fiber beam length by 6–13 mm. Additionally, finite element simulations indicated that the frequency response characteristics were well correlated with the vibration characteristics of the cantilever-shaped fiber beam, which implied that the sensor performance can be optimized with respects to the sensitivity and the measurable frequency range by changing the fiber beam length. [ABSTRACT FROM AUTHOR]

Interface pressure between cable and accessory is the key to ensure the long-term reliable operation of cable accessories. The traditional measurement method of interface pressure is off-line measurement through a built-in piezoelectric sensor. However, this method is difficult to operate and damages the interface structure. In addition, the offline measurement cannot reflect the real-time dynamic change of interface pressure during the operation of cable accessories. Therefore, this paper proposes a new method for the on-line measurement of interface pressure between cable and accessory by means of an exterior optical fibre curvature sensor, whose design mechanism is based on the pure bending theory of optical fibre. Then the relationship model involving the exterior curvature of the cable accessory, the radial displacement and the inner interface pressure is established. Finally, interface pressures of the 8.7/10 kV cable accessory under different expansion ratios are measured by the calibrated optical fibre curvature sensor. The measurement results show that the optical fibre curvature sensor measures the exterior curvature of the accessory accurately; the average sensitivity reaches 7.679 nm/mm−1, and the measurable curvature ranges from 0.03 to 0.05 mm−1. The conversion results of interface pressure are basically consistent with the results measured directly by a traditional piezoelectric sensor or simulated mechanically by finite element software. Therefore, the exterior optical fibre sensing technology proposed in this paper is expected to be used for the on-line measurement of the interface pressure between cable and accessory. [ABSTRACT FROM AUTHOR]

Distributed optical fiber sensors using Brillouin scattering have already demonstrated their ability to measure changes in strain and temperature over long distance. However, the simultaneous sensitivity of the Brillouin scattering to these two parameters is a real disadvantage during the instrumentation and monitoring of structures, such as for civil engineering, oil industry applications (i.e. instrumentation of pipelines or wells) or electrical power cables monitoring. In this article, we present a new instrument enabling independently measurement of temperature or strain using a unique fiber in a single measurement. This method, which we will call B-OTDR DSTS for “Distributed Strain and Temperature Sensing”, is based on the measurement of frequency (as a standard BOTDR) but also intensity of Brillouin scattering. However, as the measurement of the latter requires an accurate measurement of power levels along the fiber, it is necessary to correct the losses by a Rayleigh OTDR measurement. The architecture of the BOTDR-DSTS allows simultaneous measurement of the Brillouin and Rayleigh backscatters. We will detail the validation results of this new device and show the interest of such a measurement on the instrumentation of a cable into a 2 km deep observation well. [ABSTRACT FROM AUTHOR]

A novel technique was designed to measure the meniscus height (MH) of liquids on a standard single mode fiber (SMF) using fiber optics. Once the MH is known, the contact angle could be calculated, and the surface tension of liquids can be determined. In this paper, the MH is measured using a cleaved and side polished optical fiber attached to a SMF with a known separation distance between their fiber tips. When the liquid reaches the SMF, the liquid meniscus rises on the vertical wall of the SMF, which also causes a variation in mass (measured by a $\mu \text{g}$ scale). At this point, the liquid moves upwards until the liquid meniscus touches the side polished fiber, which registers a variation of the reflected optical power. The MH seems to increase as the surface tension decreases, and the variation of the mass measured by the scale increase with the increase of the surface tension. This sensor was also used to measure the interfacial surface tension of the air-liquid and liquid-liquid interface since the contact angle could be measured. In addition, the small dimension of the sensor is adequate for applications where using small amounts of liquid is critical, i.e., biomedical applications. [ABSTRACT FROM AUTHOR]

The nondispersive per-sample channel model for the optical fiber channel is considered. Under certain smoothness assumptions, the problem of finding the minimum amount of noise energy that can render two different input points indistinguishable is formulated. This minimum noise energy is then taken as a measure of distance between the points in the input alphabet. Using the machinery of optimal control theory, necessary conditions that describe the minimum-energy noise trajectories are stated as a system of nonlinear differential equations. It is shown how to find the distance between two input points by solving this system of differential equations. The problem of designing signal constellations with the largest minimum distance subject to a peak power constraint is formulated as a clique-finding problem. As an example, a 16-point constellation is designed and compared with conventional quadrature amplitude modulation. A computationally efficient approximation for the proposed distance measure is provided. It is shown how to use this approximation to design large constellations with large minimum distances. Based on the control-theoretic viewpoint of this paper, a new decoding scheme for such nonlinear channels is proposed. [ABSTRACT FROM AUTHOR]

Water leakages in water distribution systems due aging of pipes is a problem affecting most countries around the world. Official reports from government-commissioned surveys indicate that water losses are estimated to amount to an average of around 30% in most countries, with peaks up to 60% [1]–[3]. Such amounts increase in urban areas, reaching 70 to 80% in some cities. From an economic point of view, these losses have a cost that has been estimated for the European region to an overall of 8 billion a year [1]. [ABSTRACT FROM AUTHOR]

A refractometric sensor was applied to measure in real-time the concentration of Active Pharmaceutical Ingredients (APIs) in crystallization experiments. Paracetamol was used as a model system due to the extensive literature available for this API. The refractometric sensor was fabricated by a simple and inexpensive method that consisted in splicing a short section of a multimode fiber to a single mode fiber. The compact geometry of this sensor, with an external diameter of just 125 μm, allowed it to measure the concentration of paracetamol, both in a stirred tank crystallizer operating in batch and in an oscillatory flow crystallizer operating continuously. The proposed technique shows the potential to monitor the concentration of APIs in crystallizers of different sizes and geometries as an alternative to more expensive and complex analysis equipment. [ABSTRACT FROM AUTHOR]

For the purpose of monitoring seawater temperature, a novel Mach–Zehnder (MZ) temperature-sensing structure with a wedge-shaped open cavity is presented and experimentally studied in this article. The open-cavity structure realizes the direct contact between the sensing optical path and the external environment, and ensures the high sensitivity of the structure. At the same time, it achieves not only low loss by direct fusion splicing instead of dislocation fusion splicing, but also low cost by hydrofluoric (HF) etching instead of femtosecond processing. In addition, the temperature sensitivity of the sensor structure proposed in this article can reach 1.5 nm/°C without temperature-sensitive material filled, and 6 nm/°C with silicone oil filled. Moreover, the resolution of the sensing structure is 0.24 °C based on the criterion of $3\sigma $. [ABSTRACT FROM AUTHOR]

A broadband optoelectronic (O/E) frequency response measurement method utilizing photonics-based frequency conversion is proposed and experimentally demonstrated. It is characterized by a sub-kilohertz frequency resolution and a doubled measurement bandwidth compared with the RF frequency sweeping range and the working bandwidth. A carrier-frequency-shifted optical double-sideband (ODSB) signal is produced by employing a dual-drive Mach-Zehnder modulator (DD-MZM) and stimulated Brillouin scattering (SBS). Then, a photodetector (PD) under test receives and converts the optical signal into a photocurrent. By detecting the frequency up- and down-conversion components generated by the two first-order sidebands and the optical carrier, the O/E frequency responses in the low- and high-frequency regions are achieved. After stitching the two measured responses together, an O/E frequency response with a frequency range that is twice the bandwidth of the input microwave signal is obtained. In an experiment, the O/E frequency response of a commercial high-speed PD is precisely characterized with a frequency resolution up to 5.55 MHz. A frequency bandwidth of 66.8 GHz (0.1–66.9 GHz) is achieved by using a 25-GHz DD-MZM. The measured O/E frequency response is coincident with that measured by a commercial instrument. [ABSTRACT FROM AUTHOR]

Real-time identification of threats has been the focus of attention in the security field in recent years. Researchers have aimed to build a fast, efficient, and accurate model to address risks in threat event identification. This study proposes a fast, effective deep learning model—the squeeze and excitation WaveNet (SE-WaveNet). WaveNet is a 1-D convolutional neural network (CNN) model with a large receptive field. It is small, fast, and highly accurate. Thus, it has a great advantage in processing time series data. On this basis, the squeeze and excitation (SE) structure of the attention mechanism is introduced to adapt to matrix data with complete information. To ensure the integrity of information in the experimental verification, the distributed optical fiber vibration sensing (DVS) system is used to collect the data near the vibration point and form the time–space matrix as the input of the model. SE-WaveNet is compared with WaveNet and related models. Results show that the accuracy of SE-WaveNet based on WaveNet is increased by approximately 3.5%, the model size is reduced by 39%, and the speed is increased by 32%. Compared with the five other related models, the average performance is 3%–9% higher, and the final test accuracy rate can reach approximately 97.73%. [ABSTRACT FROM AUTHOR]

Measurement of optical transfer delay (OTD) is crucial to applications such as fiber-distributed multiantenna systems, fiber-optic sensors, and high-capacity optical fiber communications. However, present OTD measurement techniques are inadequate for the demands of high accuracy, high speed, and large measurement range, simultaneously. Here, we propose a novel method based on nonlinear frequency sweeping and phase derived ranging to achieve all the above-mentioned performance. A continuous-wave light modulated by a microwave signal propagates in a device under test. Then, the OTD is mapped into the phase variation of the microwave signal by photodetection. A microwave phase discriminator is used to extract the phase variation from the microwave signal, while the nonlinear frequency sweeping and a novel phase unwrapping algorithm are proposed to resolve $2\pi $ phase ambiguity caused by phase detection. Frequencies of the microwave swept signals are set at four selected points in a range of 10 MHz, which ensures high speed and large measurement range. Our experiment results verify an accuracy of ±0.05 ps in measuring an ultrahigh-accuracy optical delay line. In addition, long fiber is also tested, which proves that a measurement range of at least 37 km (theoretically 100 km) can be achieved. Moreover, the measurement speed reaches milliseconds per measurement. [ABSTRACT FROM AUTHOR]

In this study, a portable optical instrument was designed and developed for real-time nondestructive determination of fruit ripeness. A prototype of the instrument using inexpensive parts was developed and calibrated for apple ripeness detection based on moisture content, soluble solids content, pH, and firmness. An array of six optical point sources (light-emitting diodes (LEDs) that emit light at 520-, 670-, 920-, 970-, 1050-, and 1320-nm wavelengths) were selected using visible/near infrarred (Vis/NIR) spectral analysis on apple samples. The instrument was equipped with optical fibers, optical sensors, and an Arduino board for sending and receiving the waves and processing the received signals, as well as controlling the operation of the instrument. The decision tree method was utilized to create a classification model for training the instrument to classify ripe and unripe apples. Test results showed acceptable performance of the developed instrument for apple ripeness estimation and proved useful in classifying fruits into ripe and unripe groups based on their internal quality. [ABSTRACT FROM AUTHOR]

High-resolution and high-sensitivity optical vector analysis (OVA) is highly desired by many applications, such as optical sensors and photonic integrated circuits (PICs). However, it is difficult to achieve high resolution and high sensitivity simultaneously. Here, we propose and experimentally demonstrate a high sensitivity OVA using coherent detection to achieve the complex frequency responses (including magnitude and phase responses) of a device under test (DUT). Carrier phase noise (CPN) cancellation is employed to eliminate the carrier phase fluctuation induced by the coherent detection. In an experiment, the frequency responses of a fiber Bragg grating (FBG) is accurately measured by the proposed method when the probe light is suppressed by ~18 dB. As a comparison, the method using coherent detection without CPN cancellation can only achieve the magnitude response, and the method using direct detection cannot obtain an effective measurement. The influence of the linewidth and time delay on the measurement is discussed. The proposed OVA provides a stable, high resolution, high sensitivity and low cost approach for characterizing optical devices with arbitrary response, especially for optical devices with large loss or bandpass response. [ABSTRACT FROM AUTHOR]

An absolutely consistent fiber-optic phase synchronization scheme based on fixed-phase-reference optical active compensation is proposed. By measuring the time interval of time pulse and the phase difference of frequency signal, and then using the optical delay line to compensate the phase change in real time according to a fixed phase reference, the phase period ambiguity and the phase-lock control initial randomness are eliminated, realizing a stable and repeatable fixed phase difference between different sites. Then the phase synchronization performances of stability and absolute consistence have been demonstrated through experiments in different scenarios. The phase difference fluctuation within 10000 s is only ±0.0006 rad, showing an ultra-high stability. Under the total of 5 different operations, including the signal source restart, system restart, adding 20 m additional fiber, adding 1 km additional fiber, and adding 2 km additional fiber, the maximum inconsistency of the mean phase difference is about 1% of one full cycle. The proposed and verified absolutely consistent phase synchronization method can well maintain the coherence in transferring frequency signal, having important application prospects in radar network and other time-frequency-phase synchronization-based coherent detection. [ABSTRACT FROM AUTHOR]

This study investigates a thermal detection method using the classification of a support-vector machine and the application to an optical fiber sensor. Since the conventional simple threshold determination requires to adjust the threshold level according to the environments but does not consider features of the temperature-time relation, it sometimes causes false positives. False positives of an abnormal temperature rise can be avoided when the threshold level is set enough high, however it still remains a problem not to be optimal for early detection. To solve such problems, the proposed method in this work takes into accounts features of the temperature-time relation. Its features exclude unnecessary fluctuations by calculating the differences, emphasize the effect of temperature rise by using a rectified linear unit function, express the similarity by the dynamic time warping distance, and are used for the support-vector machine classification of machine learning. Proof-of-principle simulations are conducted and experiments using an optical fiber sensor are also successfully performed. The abnormal temperature rise is early detected as the abnormal state before reaching to the maximum temperature. Nevertheless, the rise-and-fall temperature profile is effectively detected as the normal state. These results show that the proposed method considering temporal factors improves the classification quality. [ABSTRACT FROM AUTHOR]

The chirp parameter of a silicon optical modulator (SOM) is difficult to measure precisely because the plasma dispersion effect is due to electro-refraction and electro-absorption occurring simultaneously. A novel method for measuring the chirp parameter of a SOM is presented, which is adopted from the beat-frequency method. Firstly, based on the model of the differential driver silicon Mach–Zehnder modulator(DDSMZM), the chirp parameter of the DDSMZM is presented by mathematical derivations, and we prove that achieving the multi-order coefficients of the radio frequency phase shift is a key point for characterizing the chirp accurately. In the proposed method, a wavelength tunable laser, as a local source, is applied to mix with the chirped modulated optical signal of the DDSMZM. In this way, the multi-order electric signals of beat frequency are mapped from high-frequency to low-frequency. Meanwhile, a balanced detector with a low-speed trans-impedance amplifier(TIA) is employed for realizing photo-electric conversion, which can improve the signal-to-noise ratio of the measurement. Finally, we have examined the chirp parameter of the DDSMZM in the case of low-frequency and high-frequency modulation signal condition and the results show the beat-frequency method has a higher frequency resolution and measurement accuracy for measuring the chirp parameter of the DDSMZM compared with the optical spectrum analysis(OSA) method and the optical heterodyne method. [ABSTRACT FROM AUTHOR]

Identification of white matter fiber tracts of the brain is crucial for delineating the tumor border during neurosurgery. A custom-built Mueller polarimeter was used in reflection configuration for the wide-field imaging of thick sections of fixed human brain and fresh calf brain. The maps of the azimuth of the fast optical axis of linear birefringent medium reconstructed from the experimental Mueller matrix images of the specimen by applying a non-linear data compression algorithm showed a strong correlation with the silver-stained sample histology image, which is the gold standard for ex-vivo brain fiber tract visualization. The polarimetric maps of fresh calf brain tissue demonstrated the same trends in the depolarization, the scalar retardance and the azimuth of the fast optical axis as seen in fixed human brain tissue. Thus, label-free imaging Mueller polarimetry shows promise as an efficient intra-operative modality for the visualization of healthy brain white matter fiber tracts, which could improve the accuracy of tumor border detection and, ultimately, patient outcomes. [ABSTRACT FROM AUTHOR]

A novel sensing system based on single mode optical fiber in reflective configuration has been developed to measure the critical meniscus height (CMH) of low volumes of liquids, which is then used to calculate the contact angle. The sensing system has been designed especially for very low volumes of liquids (e.g. bio-liquids) and the work has demonstrated that measurements are possible with a minimum liquid volume of $5~\mu \text{L}$. The sensing system is based on monitoring the spectral variation induced by the difference in the refractive index regions surrounding the fiber tip, at the air-liquid or liquid-liquid interfaces. From the experiments performed in water, (by immersing and extracting the fiber sensor in the liquid sample), it can be concluded that the CMH forming on the fiber decreases as the temperature increases. The change of temperature (in this experiment from 22 to 60 °) does not influence the CMH of the sample used in the evaluation (P3 mineral oil), giving an indication of its thermal stability. In addition, a fixed fiber was used to measure the variation in the liquid level when another fiber is immersed in the liquid. The error in the liquid level obtained in the work was small, at 0.34 ± 0.04 %. Such a sensor, allowing accurate measurements with very small quantities is especially useful where liquid sample volumes are limited e.g. biologically sourced liquids or specialized, expensive industrial material in the liquid phase. [ABSTRACT FROM AUTHOR]

The benefits and limitations inherent to the 2D post-processing of measurements from Brillouin optical time-domain analyzers are investigated from a fundamental point of view. In a preliminary step, the impact of curve fitting on the precision of the estimated Brillouin frequency shift is analyzed, enabling a fair comparison between the representative noise-reduction algorithms studied in this article. The performances in terms of signal-to-noise ratio, experimental uncertainty $\sigma _B$ on the Brillouin frequency shift and spatial resolution delivered by advanced image processing methods—such as wavelet transform and non-local means algorithms—are then compared with the impact of a 2D Gaussian filter. The major discrepancies observed when comparing the gain in signal-to-noise ratio to the $\sigma _B$ reduction are then determined by exploiting the separability of the Gaussian filter, which reveals that noise reduction is only effective along 1-D of the 2D array of measurements and originates from a digital reduction of the system analog bandwidth. The signal-to-noise ratio improvement obtained from filtering in the spectral dimension is only illusory, since its action is redundant with the curve fitting procedure to estimate the Brillouin frequency shift. Finally, the maximum $\sigma _B$ reduction achievable by digital post-processing is theoretically given, hence setting a fundamental limit to the improvement brought by data processing. [ABSTRACT FROM AUTHOR]

Cerium doped silica glasses are attractive materials for radiation dosimetry. In this article, Ce-doped air-clad optical fiber has been fabricated for real-time X-ray dose rate measurements through the radioluminescence (RL) signal. The structural properties of the obtained material were studied using Raman spectroscopy. The presence of Ce3+ ions inside the sol-gel-derived silica core was confirmed using photoluminescence (PL) measurements. This optical fiber, with a high numerical aperture, was tested as active and guiding material in an all-fibered remote X-ray dosimeter configuration. We demonstrated that the response dependence of RL versus dose rate is linear from 52 mGy(SiO2)/s up to at least 1.5 Gy(SiO2)/s, allowing to monitor the dose rate evolution during an irradiation run. The presented results confirm the potentialities of this microstructured optical fiber to monitor ionizing radiations in harsh environments. They also pave the way toward an alternative to the widely used approach that uses scintillator material at the end of a transport’s optical fiber. [ABSTRACT FROM AUTHOR]

Gas detection is of great importance for the characterization of gas-liquid two-phase flow structures. Gas holdup distributions in gas-liquid slug and churn flows are experimentally investigated with a distributed multi-fiber optical probes (MFOP). The MFOP composed by 16 optical fiber probes is designed for a vertical upward pipe with 20 mm inner diameter. The cone angle of optical probe was determined by the energy of reflected light. Gas holdup on different positions at pipe cross section are acquired by processing the fluctuating signals of the MFOP. Thereafter, we verified the accuracy of the MFOP in gas holdup measurement using quick closing valves (QCV). Furthermore, gas holdup distribution at the whole pipe cross section is obtained by cubic spline interpolation. The research results indicate that local gas holdup distribution of liquid slug in slug flow is dependent on the length of gas slug. For gas holdup distribution of churn flow without large bubbles, the maximum of gas holdup sometimes occurs deviates from the pipe center, due to the bubble groups randomly flow in the pipe. [ABSTRACT FROM AUTHOR]

Quantum-entangled photons have drawn considerable attention for application in quantum cryptography and quantum computers. The optical systems used for measuring polarization-entangled states mostly consist of free-space optical components, which are bulky and sensitive to environmental changes and requires persistent optical alignment. In this study, we propose a solution for the stable measurement of the quantum entanglement of photons based on polymeric waveguide integrated optics. The proposed system consists of two quarter-wave plates inserted in a polymeric optical waveguide, an asymmetric X-junction optical waveguide, four birefringence modulators, and two polarizing beam splitters. The basic performance of the integrated optic device has been confirmed using a 1550-nm distributed feedback laser. High polarization conversion and splitting efficiencies are demonstrated, which are important for the measurement of entangled photon states. [ABSTRACT FROM AUTHOR]

Facing increasingly large and discontinuous structural evolution under the influence of extensive mining activities, some traditional geophysical technical methods of monitoring overburden deformation have become of restricted use. As an advanced sensing technology, the distributed optical fiber sensor has been introduced into rock deformation monitoring by virtue of its good adaptability and high precision. In this paper, an overburden stratum deformation monitoring experiment was performed within a physical model test to explore the deformation characteristics of mining-induced overburden stratum based on distributed optical fiber sensing. The results indicate that distributed optical fiber monitoring is effective, and the Brillouin frequency shift (BFS) variation distribution reflects every aspect of the space-time and movement laws of the overburden stratum. The comparison of measurements of the distributed optical fiber, fiber Bragg gratings, and dial gauges proved the feasibility and reliability and revealed the ability of optical fiber measurement to monitor large-scale macroscopic deformation, and capture subtle deformation along the entire overburden stratum. More importantly, the BFS variation distribution presents a three-step form, which explicitly identifies the zoning development of overburden stratum. In turn, a conceptual model for describing the overburden deformation process is provided; it illustrates the inversion mechanism of the BFS variation distribution in reaction to the typical overburden deformation structure. This study aims to provide a theoretical and methodological foundation for the application of distributed optical fiber sensing (DOFS) measurement in mining ground control research. [ABSTRACT FROM AUTHOR]

We describe the implementation of a versatile system and the corresponding method to perform fast postmortem optical attenuation measurements on irradiated optical-fibers (OFs) for dosimetry purposes. The measurement is based on the radiation-induced attenuation (RIA) phenomenon. We use a dual-wavelength optical time-domain reflectometer (OTDR) coupled with a suitable radiation-sensitive OF. We demonstrate that radiation dose levels higher than 100 Gy(SiO2) can be measured efficiently with 1-m-long samples. The accuracy of the measurement increases with the increasing radiation dose if the length of the sample is kept fixed. We also highlight the possible limitations of our approach and, more generally, limitations related to performing accurate RIA measurement (online or postmortem) on short single-mode (SM) OF samples. The technological solution we propose can be tailored to address different radiation environments. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a wavelength switchable mode-locked Erbium-doped fiber laser with carbon nanotubes for mode-locking and a tapered two-mode fiber (TTMF) based filter for wavelength selection. The TTMF based filter constructed with single-mode fiber-TTMF-single-mode fiber configuration, features smooth interference spectrum. It is attributed to that only two modes are involved into the interference in the specially designed TTMF, which is confirmed by both simulations and experiments. Moreover, the free spectral range (FSR) of the TTMF based filter can be arbitrarily chosen by controlling the taper diameter. Depending on the FSR of the TTMF based filter, the central wavelengths of the mode-locking operation of the Erbium-doped fiber laser based on carbon nanotubes can be switched between two wavelengths with FSR of 8.5 nm and three wavelengths with FSR of 3.1 nm, respectively. This fiber laser is flexible source for various applications. [ABSTRACT FROM AUTHOR]

This paper extends the capabilities of chirped-pulse phase-sensitive optical time-domain reflectometry to the measurement of large dynamic strains over hundreds of meters of standard single-mode fiber. Benefitting from single-shot strain measurements, this technique has already demonstrated dynamic strains of the order of submicrostrains with a sensitivity of picostrains-per-root-Hertz. Yet, for large dynamic strains, it relies on the accumulation of incremental measurements, where each trace is cross correlated with its predecessor to determine the relative change of strain. However, practical time records of measured high slew-rate applied perturbations contain disturbing outliers. We then detail and analyze a post-processing strategy to mitigate this limitation. Through this strategy, we are able to achieve for the first time (to our knowledge) high signal-to-noise Rayleigh-backscattering-based distributed measurements of large and fast dynamic strains of a longitudinally vibrating 4 m section at the end of 210 m of a single-mode fiber: from peak to peak 150–1190 μϵ at vibration frequency of 400 Hz and 50 Hz, respectively. [ABSTRACT FROM AUTHOR]

We propose a method of variable mode control that generates an equilibrium modal distribution (EMD) in step-index multimode fibers. The proposed method can control the modal power distribution variably from a low-order mode distribution to a high-order mode distribution (including EMD) by twisting fiber loops. The attenuation of the controller for EMD generation is five times smaller than those of conventional EMD generation methods. As a reference light source with high measurement reproducibility, the EMD light source obtained by the variable mode control method can be used to evaluate hard polymer cladding fiber link systems. The proposed method is applicable to all types of step-index multimode optical fibers, including all-glass fibers, hard polymer cladding fibers, and plastic optical fibers. [ABSTRACT FROM AUTHOR]

A full-duplex analog radio-over-fiber (RoF) system using an ultra-compact integrated III-V-on-silicon transceiver is proposed. The downstream link transmitter is realized by a 15-GHz C-band silicon ring modulator, and the upstream link receiver is constructed based on transfer printing an 11.5-GHz O-band III-V PD with a responsivity of ~0.5 A/W. Since the transfer-printed III-V PD is transparent to the C-band downstream signal, the III-V PD is directly placed on top of the grating coupler without affecting the downstream signal, which reduces the footprint of the transceiver and also enables bidirectional transmission of the C-band and O-band signals in the same optical fiber. With the integrated transceiver, an error vector magnitude (EVM) of less than 6% is achieved for the transmission of X-band 16-QAM downstream and upstream signals with 1-Gbps data rate through a 5-km RoF link. [ABSTRACT FROM AUTHOR]

In the diagnosis of respiratory diseases, it is necessary to have a wearable device capable of measuring the breathing rate. We propose a sensor based on the integration of a smartphone with a plastic optical fiber (POF); the system behaves as an all-fiber sensor, using the flashlight as the source and the camera as a photodetector, and a 3D-printed connector to perform light coupling. The method of intensity modulation is used to detect the breathing rate. The ubiquitousness of smartphones and the low cost of POF make the proposed sensor a viable alternative to standard devices. The proposed configuration has been calibrated and then experimentally validated. The results are displayed in both the time and frequency domains to extract the information about the periodicity of breathing. [ABSTRACT FROM AUTHOR]

Mechanical stability of no-insulation (NI) high-temperature superconducting (HTS) coils will be greatly enhanced after using a solder impregnation (SIM) method. For superconducting high-field magnet applications, besides the mechanical stability, the thermal stability of SIM coils is also a crucial parameter which needs to be investigated. Due to its high insulation, convenient installation, distributed temperature measurement, and insensitive to stress, a Raman-based optical distributed temperature sensor (RmDTS) system is an ideal option for monitoring the temperature of SIM coils. In this study, an NI HTS coil and an optical fiber were successfully impregnated together by solder. The critical current of SIM coil determined by the central magnetic field is 69 A. To create quench process, a dc current of 240 A was injected into an SIM coil, and the temperature of the coil was monitored by the RmDTS simultaneously. The test results show that the temperature of the SIM coil was unchanged although a four times Ic was applied. [ABSTRACT FROM AUTHOR]

A digital optical current transducer designed for the measurement of small alternating currents is applied to current measurements for an HVDC earthing line. A novel scanning band pass filter was used to make experimental measurements of small high frequency currents. Currents from 1 kHz to 5 kHz and 0.2 A to 2 A in the presence of 20 ADC have been experimentally made. With accuracies of less than 0.4% for the 2 A measurement. The optical current transducers can measure currents remotely up to 20 km from the converter station. This new optical current sensor measurement method can allow the location of faults on HVDC earthing lines. [ABSTRACT FROM AUTHOR]

The goal of this work was to measure the directivity of a reflectance-based fiber-optic hydrophone at multiple frequencies and to compare it to four theoretical models: rigid baffle (RB), rigid piston (RP), unbaffled (UB), and soft baffle (SB). The fiber had a nominal 105- $\mu \text{m}$ diameter core and a 125- $\mu \text{m}$ overall diameter (core + cladding). Directivity measurements were performed at 2.25, 3.5, 5, 7.5, 10, and 15 MHz from ±90° in two orthogonal planes. Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to the directivity measurements using the sensitive element radius as an adjustable parameter. Over the range from 2.25 to 15 MHz, the average magnitudes of differences between the effective and nominal sensitive element radii were 59% ± 49% (RB), 10% ± 5% (RP), 46% ± 38% (UB), and 71% ± 19% (SB). Therefore, the directivity of a reflectance-based fiber-optic hydrophone may be best estimated by the RP model. [ABSTRACT FROM AUTHOR]