Pulsed Sensor Research Articles

Pulsed interferometric optical fibre sensor detecting wiretapping in long transmission lines

A modified optical fibre based Mach-Zehnder interferometer was applied as a sensor to detect wiretapping in long transmission optical fibre lines. The signal consisting of short pulses (around 1 ns) was launched to the input of the interferometer based on the polarization maintaining fibres and polarization elements. When the sensing line was undisturbed, detectors registered only a single pulse. The additional two side pulses appear, if the wiretapping attempt took place. For robust detection of any alarm situation we proposed two-criteria algorithm to minimize false alarm rate. Moreover, slow environmental fluctuations were continuously monitored and compensated by polarization controllers. We measured frequency characteristics of the sensor and performed a hundred wiretapping attempts, which proved high performance of the sensor.

Passively Q-Switched and Mode-Locked Fiber Laser Based on an ReS2 Saturable Absorber

Transition metal dichalcogenides, a family of two-dimensional material with unusual electronic, optical, mechanical, and electrochemical properties, have received much research attention in recent years. Here we demonstrate that, another type of few-layer transition metal dichalcogenides, rhenium disulfide (ReS2) nanosheets display saturable absorption property at 1.55 μm. By incorporating the ReS2 nanosheets with the polyvinyl alcohol (PVA), a film-type ReS2-PVA saturable absorber is fabricated to realize Q-switching and mode locking of erbium-doped fiber lasers. The repetition rate of the Q-switched laser pulses varies from 12.6 to 19 KHz while the duration changes from 23 to 5.496 μs by tuning the pump from 45 to 120 mW. By optimizing the polarization state, the mode-locked operation is also obtained, emitting a train of pulses centered at 1558.6 nm with the duration of 1.6 ps and the fundamental repetition rate of 5.48 MHz. It is demonstrated that ReS2 nanosheets have the similar saturable absorption property as that of MoS2 and WS2, and may find potential applications in pulsed laser, optical modulators, and sensors.

Novel <italic>In Situ</italic> Boundary Detection Algorithm for Horizon Control in Longwall Mining

Real-time horizon control of the cutting head is very important in longwall mining for maximizing production, minimizing wear, and reducing postimpurity processing cost. If the horizon control sensor has to be mounted directly onto the cutting drum, then it needs to withstand the impacts from mining debris as well as shock and vibration at an average level of 26 Gs and a peak level of 100-G force. A single-frequency boundary detection sensor has been developed for this purpose for its simple design and extremely high measurement rate compared to other more sophisticated pulsed or swept-frequency radar sensors, and thus is more suitable for fast rotating cutting drums. However, the accuracy and effectiveness of this method in practice are severely limited by: 1) the interference of the much stronger reflection arising from the air–ground interface and 2) the uncertainty of the permittivity and conductivity of the ground. These two issues are alleviated by the proposed practical in situ calibration procedure discussed in this letter. This procedure only requires more than four calibration control cuts prior to the normal longwall mining cuts. The effectiveness of this method is demonstrated in this letter via both simulated and experimental data.

Development of optoelectronic-based pulsed current sensor to detect the poor laser discharge condition

A current transformer (CT)-based sensor has been developed to detect poor discharge conditions in copper vapour laser. The optoelectronic-based pulsed current sensor architecture involves the optical transmitter–receiver HFBR, high-frequency current transformer, and fiber optic link. The CT has been designed and calibrated to ensure generation of an optical signal at the current threshold crossover. Bandwidth analysis of the CT is carried out using the bode plot. The optoelectronic inter-conversion of the pulsed voltage of the CT and transmission via fiber optic link provides the non-contact current sensing and remote signal processing of the signal. This study discusses the details of the sensor.

Pulsed Polarization-Based NOx Sensors of YSZ Films Produced by the Aerosol Deposition Method and by Screen-Printing.

The pulsed polarization technique on solid electrolytes is based on alternating potential pulses interrupted by self-discharge pauses. Since even small concentrations of nitrogen oxides (NOx) in the ppm range significantly change the polarization and discharge behavior, pulsed polarization sensors are well suited to measure low amounts of NOx. In contrast to all previous investigations, planar pulsed polarization sensors were built using an electrolyte thick film and platinum interdigital electrodes on alumina substrates. Two different sensor layouts were investigated, the first with buried Pt electrodes under the electrolyte and the second one with conventional overlying Pt electrodes. Electrolyte thick films were either formed by aerosol deposition or by screen-printing, therefore exhibiting a dense or porous microstructure, respectively. For screen-printed electrolytes, the influence of the electrolyte resistance on the NOx sensing ability was investigated as well. Sensors with buried electrodes showed little to no response even at higher NOx concentrations, in good agreement with the intended sensor mechanism. Electrolyte films with overlying electrodes, however, allowed the quantitative detection of NOx. In particular, aerosol deposited electrolytes exhibited high sensitivities with a sensor output signal ΔU of 50 mV and 75 mV for 3 ppm of NO and NO2, respectively. For screen-printed electrolytes, a clear trend indicated a decrease in sensitivity with increased electrolyte resistance.

Peltier Supercooling with Isosceles Current Pulses: Cooling an Object with Internal Heat Generation

Applying a current pulse enables a short-term transitory state where the cold junction of a Peltier couple reaches temperatures below that obtainable via maximum temperature delta steady-state current. Short-term cooling applications like on-chip hot spot and pulsed laser sensor cooling have been studied using pulsed cooling. Some studies have proposed applications that utilize consecutive repeating pulses for longer term cooling applications. These studies have found or theorized increased cooling and coefficient of performance (COP). Considering these studies, it is desirable to have a more detailed analysis of how the additional cooling and COP are achieved. The objective herein is to provide a detailed analysis of cooling rate and COP during pulses using a realistically modeled system simulated in SPICE. It was found that cooling rate for long term consecutive pulse cooling applications can be increased over steady-state but COP in most cases is reduced during current pulses. The reasons why this happens are studied in depth.

Range imaging pulsed laser sensor with two-dimensional scanning of transmitted beam and scanless receiver using high-aspect avalanche photodiode array for eye-safe wavelength

We demonstrate a range imaging pulsed laser sensor with two-dimensional scanning of a transmitted beam and a scanless receiver using a high-aspect avalanche photodiode (APD) array for the eye-safe wavelength. The system achieves a high frame rate and long-range imaging with a relatively simple sensor configuration. We developed a high-aspect APD array for the wavelength of 1.5 μm, a receiver integrated circuit, and a range and intensity detector. By combining these devices, we realized 160×120 pixels range imaging with a frame rate of 8 Hz at a distance of about 50 m.

Technologies and experience with monitoring sediments for protecting turbines from abrasion

Abrasion of turbines by sediments is a constant threat in high head and high sediment load situations. It is widely recognized that larger grains cause abrasion, although no consensus on a critical size exists. Grain hardness plays a second key role. Thus monitoring of sediment concentration is highly desirable, particularly with attention paid to the large grains. This has recently become possible with LISST instruments that use laser diffraction (LD) technology. These in-line instruments measure multi-angle laser light scattering, which is converted to a particle size distribution in a pre-defined size range. In order to reach high concentrations, the instruments incorporate auto-dilution capability. The data are transmitted to the control room. Provided software displays concentration history in up to 4 size classes, and the software is capable of generating alarms when sufficiently high concentrations occur. Since no definition exists for this sufficiently high concentration, in this paper we propose an objective criterion based on the rate of revenue generation contrasted with rate of cost of turbine repair. This simple idea helps guide the plant operator to set shut-down thresholds during sediment transport events. We also introduce a lower cost, high-frequency pulsed acoustic sensor for sediment monitoring. The rather lower accuracy of this device is offset by its lower cost that is suitable for small plants.

Detection of microparticles in dynamic processes

When a metal plate is subjected to a strong shock impact, its free surface emits a flow of particles of different sizes (shock-wave “dusting”). Traditionally, the process of dusting is investigated by the methods of pulsed x-ray or piezoelectric sensor or via an optical technique. The particle size ranges from a few microns to hundreds of microns. The flow is assumed to include also finer particles, which cannot be detected with the existing methods yet. On the accelerator complex VEPP-3-VEPP-4 at the BINP there are two experiment stations for research on fast processes, including explosion ones. The stations enable measurement of both passed radiation (absorption) and small-angle x-ray scattering on synchrotron radiation (SR). Radiation is detected with a precision high-speed detector DIMEX. The detector has an internal memory of 32 frames, which enables recording of the dynamics of the process (shooting of movies) with intervals of 250 ns to 2 μs. Flows of nano- and microparticles from free surfaces of various materials (copper and tin) have been examined. Microparticle flows were emitted from grooves of 50-200 μs in size and joints (gaps) between metal parts. With the soft x-ray spectrum of SR one can explore the dynamics of a single microjet of micron size. The dynamics of density distribution along micro jets were determined. Under a shock wave (∼ 60 GPa) acting on tin disks, flows of microparticles from a smooth surface were recorded.

Improved Energy Management System for Low-Voltage, Low-Power Energy Harvesting Sources

This paper focuses on improving the energy conversion process for low-voltage energy harvester powered wireless sensors by optimising the conversion stages for pulsed sensor operation. The proposed circuit has been designed to operate efficiently with both a low-voltage low-power energy harvester source and a low-power pulsed load. This ensures that continuous conversion losses are kept to a minimum and power is only delivered to the sensor when required. This has shown an increase in energy delivered to a sensor of up to 10% versus that of the best existing solution.

Influence of metallic shields on pulsed eddy current sensor for ferromagnetic materials defect detection

Pulsed eddy current (PEC) with transient analysis is a significant advance over conventional eddy current testing using impedance analysis at a particular frequency. The fabrication and optimization of PEC sensor are important issues in PEC applications in nondestructive testing (NDT) field especially for ferromagnetic materials. In this work, the influence of metallic shields on circular PEC sensor for defect detectability in ferromagnetic metals has been investigated through finite element models (FEM) and experimental studies by comparing iron and aluminum shields with no-shield. Both simulations and experiments indicate that PEC probe with an aluminium shield can effectively enhance the amplitude of PEC response in both surface and subsurface defect detection. However, the sensitivity is decreased. In contrast, iron shield has better shielded performance and can effectively improve the sensitivity of both surface and subsurface defects detection in ferromagnetic materials.

Nonlinear Saturable Absorption of Liquid-Exfoliated Molybdenum/Tungsten Ditelluride Nanosheets.

Molybdenum disulfide (MoS2 ) and tungsten disulfide (WS2 ), two representative transition metal dichalcogenide materials, have captured tremendous interest for their unique electronic, optical, and chemical properties. Compared with MoS2 and WS2 , molybdenum ditelluride (MoTe2 ) and tungsten ditelluride (WTe2 ) possess similar lattice structures while having smaller bandgaps (less than 1 eV), which is particularly interesting for applications in the near-infrared wavelength regime. Here, few-layer MoTe2 /WTe2 nanosheets are fabricated by a liquid exfoliation method using sodium deoxycholate bile salt as surfactant, and the nonlinear optical properties of the nanosheets are investigated. The results demonstrate that MoTe2 /WTe2 nanosheets exhibit nonlinear saturable absorption property at 1.55 μm. Soliton mode-locking operations are realized separately in erbium-doped fiber lasers utilizing two types of MoTe2 /WTe2 -based saturable absorbers, one of which is prepared by depositing the nanosheets on side polished fibers, while the other is fabricated by mixing the nanosheets with polyvinyl alcohol and then evaporating them on substrates. Numerous applications may benefit from the nonlinear saturable absorption features of MoTe2 /WTe2 nanosheets, such as visible/near-infrared pulsed laser, materials processing, optical sensors, and modulators.

Signal processing of the optical fiber impulse sensor acceleration

Introduction. Feature of the pulsed fiber-optic sensor is a mutual influence of the components of the acceleration vector measured. When processing the signals of the pulse sensor should provide adequate correction value counts for correct measurement results. Main part. It consists of four sections. The first section outlines the method of determining the undistorted values of horizontal components of the acceleration vector acting on the sensor, in the absence of the vertical component of the acceleration vector. The second section outlines the method of determining the undistorted value of vertical component of the acceleration vector. Based on the results obtained in the first two chapters, the third chapter described a method of determining undistorted measured quantities of all components of the acceleration vector. In the fourth chapter provides a quantitative assessment of the effectiveness of the proposed information processing in the case of precision measurements of acceleration. Conclusions. It is pointed to the principal possibility of determining the undistorted values of samples of the vector acceleration components along all axes of sensitivity of the pulse sensor. The method of determination is proposed and a general algorithm is formulated for measuring randomly oriented components of the acceleration vector. It is evaluated the efficiency of correction of pulse sensor signals for example acceler-ometer of micro-accelerations.

Fast Resistance Relaxation in Nanostructured La–Ca–Mn–O Films in Pulsed Magnetic Fields

The results of fast resistance relaxation of nanostructured La1− x Ca x MnO3 (LCMO) films with different compositions $x$ (0.218, 0.285, 0.296, 0.41) grown by the metal–organic chemical vapor deposition technique are presented. The fast ( $\sim 200\mu \text{s}$ ) resistance relaxation process observed after the magnetic field pulse was switched off was studied in pulsed fields up to 14 T in the temperature range of 80–170 K. It was found that the remnant resistivity of the fast relaxation process is proportional to the magnetoresistance of the films up to 10 T. The smallest remnant relaxation values were found for the films with highest insulator–metal transition temperature $T_{m}$ . It was shown that the fast process could be analyzed using the Kolmogorov–Avrami–Fatuzzo model, taking into account the reorientation of magnetic domains into their equilibrium state. It was concluded that LCMO films with $x=0.218$ should be favored for the development of fast pulsed magnetic field sensors operating at cryogenic temperatures.

Calibrations of Resistance Welding Equipment With High Pulsed Currents

Rogowski coils are widely used as high pulsed current sensors in resistance welding applications. The operating conditions of the coils determine the requirements for their calibration. A new calibration system has been developed at National Research Council (NRC) Canada for calibrating current sensing coils and weld current monitors (WCMs) used in AC resistance welding. The calibration system, which includes a high-current source and a coaxial copper cage, has been evaluated at peak pulsed currents of up to 80 kA at 60 Hz. The expanded measurement uncertainties (k = 2) have been estimated to be less than 350 μA/A for magnitude. A method for the calibration of WCMs separately from their Rogowski coils at conditions that correspond to those obtained at high pulsed currents but without the need to use actual high pulsed currents has also been developed.

Magnetoresistance and Resistance Relaxation of Nanostructured La-Ca-MnO Films in Pulsed Magnetic Fields

The results of magnetoresistance (MR) and resistance relaxation of nanostructured La1− x Ca x MnO3 films, with different composition x grown by metal–organic chemical vapor deposition technique, are presented and compared with the La0.83Sr0.17MnO3 films. The MR was investigated in pulsed magnetic fields up to 60 T in the temperature range 1.5–294 K while the relaxation processes were studied in pulsed fields up to 10 T and temperatures in the range of 80–300 K. It was demonstrated that at low temperatures the MR has higher values in the LCMO films in comparison with the LSMO ones, while at room temperatures, the highest MR values are obtained for the LSMO films. The fast ( $\boldsymbol \sim 100~\boldsymbol {\mu } $ s) and slow ( $\boldsymbol \sim $ ms) resistance relaxation processes were observed after the magnetic field pulse was switched off. It was shown that the fast process could be analyzed using the Kolmogorov–Avrami–Fatuzzo model, considering the reorientation of magnetic domains into their equilibrium state, while the slow process—by the Kohlrausch–Williams–Watts model considering the interaction of the magnetic moments in disordered grain boundaries having spin-glass properties. It was concluded that La1− x Ca x MnO3 films having a higher sensitivity and lower memory effects and should be favored for the development of fast pulsed magnetic field sensors operating at low temperatures.

Comparative diel oxygen cycles preceding and during a Karenia bloom in Sarasota Bay, Florida, USA

The diel change in dissolved oxygen concentrations were recorded with an automated incubator containing a pulsed oxygen sensor in Sarasota Bay, Florida. The deployments occurred during a ‘pre-bloom’ period in May to June 2006, and during a harmful algal bloom dominated by Karenia brevis in September 2006. The diurnal (daylight) increase in dissolved oxygen concentrations varied from 16 to 104μmolO2l−1 with the corresponding nocturnal decrease in oxygen varying from 16 to 77μmolO2l−1. Nocturnal respiration consumed 42–113% of the diurnal net oxygen production with the minimum and maximum during the pre-bloom period. Hourly production rates closely followed fluctuations in irradiance with maximum rates in the late morning. Hourly oxygen utilization rates (community respiration) at night were highest during the first few hours after sunset.

Assessment of stream ecosystem health based on oxygen metabolism: Which sensor to use?

Here, we compared sensor precision and drift among four commonly used dissolved oxygen (DO) sensor types under laboratory conditions, and evaluated their accuracy of measurement of stream ecosystem metabolism (EM) and subsequent assessment of stream ecosystem health in the field. Of all tested sensor types, optical sensors had the highest precision, followed by miniaturized polarographic sensors. Traditional polarographic and pulsed polarographic sensors had a lower precision. The traditional polarographic sensors were the only ones exhibiting detectable sensor drift that had to be corrected for, prior to calculations of EM. In parallel EM measurements in two rural and two pristine tropical streams, the four compared DO sensor types estimated relatively similar gross primary production (GPP), ecosystem respiration (R) and GPP:R ratios in each stream, and classified all streams as highly heterotrophic. Nonetheless, parallel GPP and R estimates with different sensors had relative standard deviations of the mean between 6.2% and 34.9%, and between 7.9% and 16.3%, respectively. Thus, sensor accuracy can be a major source of variability in EM estimates. There were only minor differences in stream ecosystem health assessments among different DO sensors, compared to differences between different assessment methods. While all tested sensor types yielded useful ecosystem health classifications, the optical sensor type was the most accurate, and thus, the most suitable for functional impact assessment and monitoring.

Thermally Pulsed Metal Oxide Gas Sensor Combined with a Colorimetric Gas Sensor for the Detection of Trace Gases

A low-cost gas sensor array with low-power consumption consisting of two chemical sensors based on different principles and a smart pattern recognition technique is presented. The sensor array consists of a thermally pulsed metal oxide gas sensor combined with an optical sensor based on the colorimetric principle. Metal oxide gas sensors suffer from inadequate selectivity and colorimetric sensors from slow response times. The aim of the presented work is to compensate these drawbacks through intelligent data fusion using an artificial neural network. It can be shown that the disadvantages of the two types of sensors can be compensated by the data fusion. The proof of principle is demonstrated by trace gas measurements with carbon monoxide, nitrogen dioxide and ammonia.

Slow relaxation of resistance in nanostructured La<sub>0.83</sub>Sr<sub>0.17</sub>MnO<sub>3</sub> films induced by pulsed magnetic fields

The results of colossal magnetoresistance (MR) relaxation investigations in nanostructured and epitaxial La-Sr-Mn-O films grown by the MOCVD technique are presented. The films were studied in a pulsed magnetic field ranged from 2 to 10 T at the temperature of 80 K. The slow relaxation of resistance which takes place during milliseconds when the magnetic field is switched off is analysed using the Kohlrausch-Williams-Watts model. It was found that this relaxation is typical of spin-glass materials and is related with properties of disordered grain boundaries of nanostructured films. The MR relaxation of epitaxial films was not observed. The influence of film preparation conditions on MR relaxation was analysed in order to develop high pulsed magnetic field sensors exhibiting small dynamic “memory” effect and operating at low temperatures.