Grating Technique Research Articles

Density of states within the bandgap of perovskite thin films studied using the moving grating technique.

In this work, we further study the moving grating technique applied to halide perovskite thin-film materials. First, we show some problems that emerge when analyzing the experimental data with the classical formulation, which does not distinguish between free and trapped carriers and hence only gives average quantities for the transport parameters. We show that using a more general framework, taking into account the multiple trapping of carriers within a density of localized states, allows for an accurate description. Since it includes the density of states (DOS) of the material, it enables the possibility to test different DOS models proposed in the past for halide perovskite thin films. We check whether these models give rise to the type of curves we have measured under different experimental conditions. Finally, we propose a new model for the DOS in the forbidden gap, which results in the best fit found for the measurements performed. This allows us to give ranges of values for the parameters that define the DOS, which, as far as we know, are given for the first time.

Aquifer Characterization Using Fiber Bragg Grating Multi-Level Monitoring System.

Groundwater responses measured from multiple wells at different depths are essential for delineating the aquifer heterogeneity using hydraulic tomography (HT). In general, conducting HT requires many wells because traditional well monitoring is usually partially open at a specific depth interval or is fully penetrating. Accordingly, conducting an HT survey is typically costly and time-consuming. To tackle these issues, a new multi-level monitoring system (MLMS) for the HT survey was developed using the fiber Bragg grating (FBG) technique. This FBG MLMS could collect the depth-discrete groundwater observations from a fully penetrated 2-inch well. Three field campaigns were conducted to validate the capability of the FBG MLMS for HT surveys. The results show that the accuracy and stability of this MLMS are reliable and that FBG MLMS is beneficial for conducting an HT survey. Specifically, compared to the traditional monitoring well in an injection event, this FBG MLMS can concurrently cause an increase in the number of cross-hole tests several times and collect many more head observations than the standard methods, resulting in the observed flow fields efficiently reaching ergodic conditions and effectively improving the accuracy of the estimated hydraulic heterogeneity. Therefore, the FBG MLMS could be an alternative MLMS for efficiently and economically conducting an HT survey.

Unsteady ballistic heat transport in two-dimensional harmonic graphene lattice

We study the evolution of initial temperature profiles in a two-dimensional isolated harmonic graphene lattice. Two heat transfer problems are solved analytically and numerically. In the first problem, the evolution of a spatially sinusoidal initial temperature profile is considered. This profile is usually generated in real experiments based on the transient thermal grating technique. It is shown that at short times the amplitude of the profile decreases by an order magnitude and then it performs small decaying oscillations. A closed-form solution, describing the decay of the amplitude at short times is derived. It shows that due to symmetry of the lattice, the anisotropy of the ballistic heat transfer is negligible at short times, while at large times it is significant. In the second problem, a uniform spatial distribution of the initial temperature in a circle is specified. The profile is the simplest model of graphene heating by an ultrashort localized laser pulse. The corresponding solution has the symmetry of the lattice and many local maxima. Additionally, we show that each atom has two distinct temperatures corresponding to motions in zigzag and armchair directions. Presented results may serve for proper statement and interpretation of laboratory experiments and molecular dynamics simulations of unsteady heat transfer in graphene.

Development of a FBG Stress Sensor for Geostress Measurement Using RSR Method in Deep Soft Fractured Rock Mass

The rheological stress recovery (RSR) method was proposed to obtain measurements of in-situ stress. Rock stress can be evaluated by monitoring the recovery process of stress sensors embedded in rock mass. In order to achieve this application, a novel stress sensor employing the fiber Bragg grating (FBG) technique was designed and manufactured. This stress sensor consisted of three parts: A sensing spherical head, connecting rod, and coupler box. In the sensing spherical head, six independent pressure sensing units were assembled together with a temperature compensation unit. In addition, wavelength division multiplexing (WDM) technology was adopted to ensure that only one fiber splice for each stress sensor is output. The fiber splicing of the sensing units was assembled in the coupler box. The transformation equations from the six pressure sensing units to the stress sensor were established. Furthermore, a calibrating device for the stress sensor was designed, and the general calibration and long-term stability tests were carried out to investigate the characteristic indexes (maximum range, full-range output, and sensitivity) and measurement error (zero drift index, hysteresis index, and repeatability index). Measurement errors showed that the degrees of linearity, zero drift, hysteresis, and repeatability were all below 1.5%. The stability test indicated that the creep of the stress sensor can gradually stabilize in 24 days, and the errors were less than 1.5%. As a result, the stress sensor developed here satisfies the requirements for the RSR method and can be used in field.

Observation of second sound in graphite over 200\u2009K

Second sound refers to the phenomenon of heat propagation as temperature waves in the phonon hydrodynamic transport regime. We directly observe second sound in graphite at temperatures of over 200 K using a sub-picosecond transient grating technique. The experimentally determined dispersion relation of the thermal-wave velocity increases with decreasing grating period, consistent with first-principles-based solution of the Peierls-Boltzmann transport equation. Through simulation, we reveal this increase as a result of thermal zero sound—the thermal waves due to ballistic phonons. Our experimental findings are well explained with the interplay among three groups of phonons: ballistic, diffusive, and hydrodynamic phonons. Our ab initio calculations further predict a large isotope effect on the properties of thermal waves and the existence of second sound at room temperature in isotopically pure graphite.

Performance Assessment of Dispersion Compensation Using Fiber Bragg Grating and Dispersion Compensation Fiber Technique

Dispersion should be flattering to achieve the most optimal output of the contact device. Since dispersion causes pulse spreading, which causes the output signal pulses to overlap, dispersion compensation is the most important attribute needed in an optical fiber communication device. The 8*1 Wavelength Division Multiplexing (WDM) Multiple Input Multiple Output (MIMO) channel with compensation using Fiber Bragg Grating (FBG) and Dispersion Compensation Fiber (DCF) has been designed with a 5Gbps data rate for each channel. The optical fiber's length can range from 50 to 200 kilometers. In this, primarily the proposed system is implemented without applying the grating technique and the outcomes are simulated and analyzed. Subsequently, the system is designed and implemented with FBG and DCF and the outcomes are simulated with opti-system-16.0 software. The parameters like Q-factor, Bit Error Rate (BER) and SNR were simulated and calculated by exploiting the 8-channel device.

Radiative contribution to thermal grating decay

The decay of a spatially sinusoidal temperature perturbation (thermal grating) via thermal transport by both conduction and radiation is analyzed theoretically. The intrinsic non-radiative conduction is described by the heat diffusion equation while radiative transport is described by the radiation transport equation for a non-scattering absorbing medium. We solve the coupled equations analytically and obtain the thermal grating decay rate, which leads to an explicit expression for the radiative contribution to the effective thermal conductivity measured with the laser-induced transient thermal grating technique. We find that at a given thermal grating period, the radiative contribution to thermal transport is maximized when the absorption length of thermal radiation is about 1/4 of the period. We present the expression for an upper bound for the radiative contribution and discuss the results for representative materials. We conclude that in a typical transient grating measurement with the grating period in the micrometer range, the radiative contribution is negligible, but it may be significant if the grating period is in the millimeter or centimeter range. Our analysis also provides a Fourier-domain Green's function that can be used to find the temperature field produced by an arbitrary spatiotemporal distribution of heat sources.

Time-resolved detection of association/dissociation reactions and conformation changes in photosensor proteins for application in optogenetics.

Photosensor proteins are important not only because of their biological functions but also because of their applications in optogenetics. To understand the molecular mechanism behind their biological functions and consequently seek possible applications to optogenetics, the dynamics of their intermolecular interaction (for example, association/dissociation reaction and conformational changes) upon photoexcitation need to be elucidated. Although it has been difficult to trace such reactions in the time domain using traditional spectroscopic techniques, the time-resolved diffusion method based on the transient grating technique has been demonstrated to possess a significant advantage in detecting such spectrally silent dynamics in a time-resolved manner. In this paper, the principle and studies on blue light sensor proteins, phototropins, are described. Reaction kinetics of dimerization, dissociation reactions, and conformational changes were measured, and reaction schemes were determined. This method can be employed to study protein reactions from the viewpoint of diffusion and to elucidate the reaction schemes and kinetics that cannot be detected by other spectroscopic methods.

Investigation of the photo-excited charge carrier dynamics in Bismuth vanadate (BiVO4) photoanode by the heterodyne transient grating technique

We measured the heterodyne transient grating (HD-TG) responses of the BiVO4 photoanode for the first time. We demonstrated that the responses observed in acetonitrile, an inactive solvent, consisted of the recombination arising from the surface trapped holes and the thermal diffusion component. In addition, we found two components in the responses observed in aqueous solution under the applied bias condition, which were assigned to the recombination arising from the surface trapped holes and mixed component corresponding to the competition between back electron/hole recombination and water oxidation, respectively. The observed responses showed the strong dependence of the signal intensities on the applied voltage and the retardation of the recombination, resulting from the band bending.

Quasi-distributed fiber-optic in-situ monitoring technology for large-scale measurement of soil water content and its application

Accurate acquisition of in-situ soil water content is of great significance for mastering the water content distribution of unsaturated soil and understanding soil engineering properties. The actively heated fiber-optic (AHFO) method has recently become a research focus because of the capability to realize distributed and real-time measurement of soil water content. However, most reported AHFO systems have been coupled to distributed temperature sensing (DTS) which does not allow for remote measurements with high spatial resolution. In this paper, a new AHFO method based on the quasi-distributed fiber Bragg grating (AHFO-FBG) technique is proposed for the large-scale in-situ measurement of soil water content. An AHFO-FBG in-situ monitoring system is developed which consists of a sensor module, heating module, data acquisition and transmission module, and data processing module. The established system is applied to a loess slope in Yan'an City, Shaanxi Province, China to monitor its water content distribution in real time. The effectiveness of the monitoring system is verified and the water content distribution of the monitored loess slope are analysed. The in-situ measurement results indicate that in the spatial dimension, the water content in the top and bottom of the slope is low, and that near the scarp is high; in the time dimension, the water content in winter is low while that in autumn and spring is high. The monitored results also suggest that soil water evaporation in winter can significantly enhance the spatial distribution difference of soil water content.

Performance evaluation of buried pipe under loading using fiber Bragg grating and particle image velocimetry techniques

Accurate and real-time estimation of underground pipelines’ response to external loads is essential, in which the fiber Bragg grating (FBG) technique has shown great potential. A series of 1-g plane strain model tests were conducted to investigate the feasibility of using FBG sensors for monitoring the behavior of the soil-pipe system under surcharge loadings, while particle image velocimetry (PIV) was employed to measure the soil shear strain fields. The test results show three types of soil failure surfaces, i.e., logarithmic spiral, linear, and logarithmic. Regarding various failure modes, two theoretical methods to estimate the ultimate bearing capacity of the soil-pipe system are introduced. Detailed discussions are presented regarding the evolution of pipe strain measurement under different ground settlements. Finally, a method is proposed to calculate the radial displacements based on strain measurements for a shallowly buried pipe and a relative error below ±10% is obtained in this study.

Application of Fiber Bragg Grating Sensing in Bidirectional Tests of Pile Foundations

Abstract The fiber Bragg grating (FBG) technique has emerged as a new strain monitoring technique in civil engineering applications. In this paper, based on the principle of the FBG technique and the bidirectional test method, FBG sensors and conventional vibrating wire strain gauges (VWSGs) were employed to monitor the strain variation law of a pile under different loads and analyze the relationship between the side resistance and pile-soil relative displacement of soil layers in bidirectional field testing of a large-diameter cast in situ pile. Four field bidirectional test cases of cast in situ piles, prestressed high-strength concrete (PHC) and steel pipe pile, and pile-base post-grouting pile are presented. The applicability and stability of the FBG sensing technique were verified by analyzing the side resistance distribution and bearing capacity. The results showed that FBG sensors can remove the influence because of residual force and the effect of temperature change and concrete expansion photosensitive material provides an improved sensitivity to the external environment compared with a VWSG. The strain and side resistance variation tendency of the cast in situ pile were consistent between the two kinds of sensors. The FBG sensing technique had high test accuracy in the four bidirectional O-cell field test cases of the cast in situ pile, PHC and steel pipe pile, and pile-base post-grouting pile. The pile-base post-grouting technique is an effective procedure that was implemented to increase the ultimate bearing capacity. The research results of this paper reveal a new strain monitoring technique for the bidirectional test method of pile foundations and represent a new test method for measuring the bearing capacity of PHC and steel pipe piles that has a remarkable application value in practical engineering.

Investigating temperature and moisture profiles of seasonally frozen soil under different land covers using actively heated fiber Bragg grating sensors

In this study, the actively heated fiber Bragg grating (AH-FBG) technique is developed to measure temperature and moisture profiles of partially frozen soil. The working principle of the AH-FBG sensing system is introduced, which employs an aluminum oxide tube sensor (AOTS). The unfrozen water contents and ice contents are estimated by combining AH-FBG with the frequency-domain reflection (FDR) technique. The feasibility of AH-FBG for soil moisture measurement is evaluated by laboratory and field tests under four different land covers, i.e., bare soil, concrete slab, plastic mulch (PM), and grass. It is found that AH-FBG can be used to capture unfrozen water contents and ice contents with high accuracy. The relationships between the thermal conductivity measured by AOTSs and the total water contents of frozen and unfrozen soils can be well fitted by the Côté and Konrad model. Both PM and concrete slab have a warming effect on the ground soil, which makes their depths of soil freezing shallower than that of bare soil, while the grassy land has a deeper freezing depth due to the inherent plant root-to-soil structure. The field monitoring results indicate that the PM can effectively block water migration channels at the ground surface and prevent water vapor exchange between the atmosphere and the soil, resulting in a “pot effect” that commonly exists under an impermeable layer.

High data rates in optic fiber systems based on the gain optimization techniques

Abstract The signal gain is improved by nearly about 15 dB by using the hybrid optical amplifiers technique. The gain flattened filter technique has outlined a larger value of signal gain than the hybrid optical amplifiers technique by about 17 dB whereas the Glass Composition method in the hybrid modified model has a better increase in the optimum value of signal gain that reached to about 70 dB. Fiber Bragg Grating (FBG) technique has outlined optimum gain value that is reached to about 50 dB. The combination of these techniques are used for the best optimized value of signal gain.

Further insight into the oscillating photocarrier grating technique: influence of the oscillation amplitude

Allowing for the simultaneous determination of the photocarriers drift mobilities and their small-signal recombination lifetime, the Moving photocarrier Grating Technique (MGT) is a useful characterization tool for photoconductive semiconductors. This technique is based on measuring the steady-state direct current induced by an interference pattern (IP) moving at a constant velocity between two coplanar ohmic contacts deposited on the semiconductor. The main drawback of the technique is the low level of the signal to be measured, which can be masked by noise. The Oscillating Photocarrier Grating technique (OPG), where the IP oscillates at a constant speed, has been proposed as an alternating current version of MGT, providing a higher signal-to-noise ratio. The IP oscillation is produced by the phase modulation of one of the interfering beams. Using the multiple trapping model we deduce the expression of the current density generated by OPG in a photoconductor. We observed theoretically and experimentally that OPG is not equivalent to MGT for the previously used amplitude of oscillation, especially when the IP moves at high speeds. However, we show that the desired equivalence between both techniques could be recovered by increasing the amplitude of oscillation. A phase modulator capable of achieving such amplitudes is required for the correct implementation of OPG.

Fibre optic sensors for the monitoring of rotating electric machines: a review

Accurate and efficient monitoring of electrical machine (EM) operating parameters, including temperature, mechanical vibration, torque and rotating speed and others that can indicate the EM health conditions is becoming ever more important in the world of electrical drives. The traditional methodology of one sensor per parameter can be theoretically replaced by a “one sensor measures all” technology, which can be achieved through the use of fibre-optic sensors (FOS). In this paper, several FOSs, which use different optical sensing principles for multiple physical parameter measurements of EMs, are reviewed. This paper also provides an insight into the major developments, and discusses the engineering challenges of FOS used for EM monitoring over the last few decades, and compares the advanced features of FOS with those of conventional sensors in use. Finally, a novel FOS-EM observer system scheme employing the Fibre Bragg Grating technique for multi-parameter monitoring of EM health is proposed, after discussion of the preceding industrial and academic FOS cases for EM applications.

Polarization control and mode optimization of 850\xa0nm multi-mode VCSELs using surface grating

To control the polarization of 850 nm multi-mode oxide-confined vertical-cavity surface-emitting lasers (VCSELs), monolithically integrated surface gratings are investigated. VCSELs with different grating parameters are simulated by the rigorous coupled-wave analysis (RCWA) method. Optical field intensity of two polarization orientations with different grating periods and duty cycles is obtained. The mode characteristics of VCSELs with different grating parameters are achieved by FDTD method. 850 nm surface grating VCSELs are fabricated and characterized, a maximum orthogonal polarization suppression ratio (OPSR) of 20.7 dB is observed. Meanwhile, VCSEL’s output mode characteristics are improved by integrating the surface grating. Application of integrated surface grating technique to achieve dynamically polarization-stable multi-mode VCSELs which enable polarization division multiplexed data transmission, the data throughput over a given link can be doubled.

Time-resolved detection of SDS-induced conformational changes in α-synuclein by a micro-stopped-flow system.

An intrinsically disordered protein, α-synuclein (αSyn), binds to negatively charged phospholipid membranes and adopts an α-helical structure. This conformational change is also induced by interaction with sodium dodecyl sulfate (SDS), which is an anionic surfactant used in previous studies to mimic membrane binding. However, while the structure of the αSyn and SDS complex has been studied widely by various static measurements, the process of structural change from the denatured state to the folded state remains unclear. In this study, the interaction dynamics between αSyn and SDS micelles was investigated using time-resolved measurements with a micro-stopped-flow system, which has been recently developed. In particular, the time-resolved diffusion based on the transient grating technique in combination with a micro-stopped-flow system revealed the gradual change in diffusion triggered by the presence of SDS micelles. This change is induced not only by binding to SDS micelles, but also by an intramolecular conformational change. It was interesting to find that the diffusion coefficient decreased in an intermediate state and then increased to the final state in the binding reaction. We also carried out stopped-flow-kinetic measurements of circular dichroism and intramolecular fluorescence resonance energy transfer, and the D change was assigned to the formation of a compact structure derived from the helix bending on the micelle.

Time-resolved spectroscopic study of photo-excited charge carrier dynamics in hematite (α-Fe2O3): Effect of re-growth treatment

In order to figure out the origin of the improvement of the photo-electrochemical performance of the re-growth treated hematite, in which the deposition process was repeatedly performed, resulting in the reduction of the amount of the defect states, we investigated the photo-excited charge carrier dynamics in both bare and re-growth treated hematite films under the bias condition using heterodyne transient grating technique. In this study, we were able to observe the strong dependence of the signal intensities of the components appearing in the responses of the re-growth treated hematite film on the applied bias voltage, which was attributed to the Fermi level unpinning, caused by the reduction of the surface defect states.

Optical Grating Techniques for MEMS-Based Spectrometer—A Review

This paper examined the innovations of the spectrometers for the measurement of consistency based parameters of the handheld Micro Electronic Mechanical System (MEMS) Spectrometer. Fast, highly sensitive, miniature spectroscopy techniques empowered quick, savvy and effective measures for various applications. In the light spectroscopy-based identification and quantification, advances in the field of wavelength discrimination are significant and essential. The identification of grid parameters and limiting conditions are necessary for the design and fabrication of diffraction gratings, for the spectrometer. This work evaluates the emerging trends in Micro-Spectrometer's Grating Techniques, focusing on the aspects of grating parameters and the recent developments of grating. The main parameters for evaluating the performance of a grating have been reviewed and found that grating efficiency, groove density, free spectral range and resolving power play a significant part in the grating performance. The fabrication technique employed as well as the materials used in the fabrication process, play a significant role in the efficiency of the grating. Silicon, Silicon dioxide (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), Glass (Silica glass), Poly methyl methacrylate (PMMA), Chromium and Silicon nitride(Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) are the most used materials. The integration of new materials that are ideal for the state-of-the-art semiconductor industry techniques for MEMS fabrication along with a new blazing structure would increase the efficiency of the grating.