Decreasing Pulse Duration Research Articles

Measurement accuracy and spatial resolution of a distributed temperature sensor based on a two-pulse differential coherent reflectometer

We present a model and numerical simulation of a distributed temperature sensor based on a two-pulse differential coherent optical time-domain reflectometer (COTDR). The differential phase measured using a phase-sensitive Rayleigh reflectometer is shown to have a regular component, which is a linear function of temperature, and a random component, which is related to a random distribution of scattering centres in the fibre and restricts the accuracy of measurements of variations in temperature. Measurement accuracy can be improved by reducing the relative contribution of the random component via a decrease in pulse duration and/or an increase in the time delay between pulses. The spatial resolution of a differential two-pulse phase-sensitive reflectometer is shown to be determined by the time delay between pulses and to vary little with pulse duration. At a typical pulse duration (200 ns) and delay time (300 ns), the accuracy in measurements of variations in temperature in the 0.1-K range is 2 % and the spatial resolution is about 30 m.

Bandwidth analysis of optical parametric amplifier pumped by broadband pulses

We present analytical solutions to nonlinear coupling equations for parametric down-conversion of a broadband pump at large gain. The feasibility of analytical expressions is demonstrated for the case of a collinear type II parametric interaction in beta-barium borate crystal and group-velocity matching of signal and pump waves. The correlation function of amplified signal wave is obtained, and it is revealed that the bandwidth of a noise-seeded optical parametric amplifier increases with a decrease in pump pulse duration, especially for femtosecond pump pulses.

Short-Pulse Properties of the Gunn Diode

The pulsed ${I}$ – ${V}$ characteristics of a Gunn diode are measured under the test pulse durations from 18 ns to 50 $\mu \text{s}$ . For the test, a Gunn diode fabricated at the Scientific-Research Institute of Semiconductor Devices (Russia, Tomsk) on an ${n}^{+}$ - ${n}$ - ${n}^{+}$ GaAs epitaxial structure was chosen. We found that it is important to provide low internal resistance of the measurement system to prevent voltage skipping in the suprathreshold section of the ${I}$ – ${V}$ characteristic. The measurement system with an internal resistance of about $0.5~\Omega $ and linear calibration of the measurement circuit has been developed. An increase in the threshold voltage with a decrease in the test pulse duration was observed. This effect is explained by the influence of the contacts’ resistance in combination with the temperature dependence of the active area resistance. It is established that this effect also leads to a delay of the domain mode start. We found that the duration of the test pulse maximally affects the threshold current at the durations approximately equal to the thermal relaxation time of the active area (73 ns). Two components can be singled out in this dependence: the first component is associated with the self-heating of the crystal lattice of active area and the second one (the fastest process, 15 ns) is associated with the transfer of some electrons to the second valley in the hottest part of the active area. Registration of the last process probably can be used for diagnostics of the quality of the Gunn diode contacts.

Planar multifrequency mid-IR microwave-pumped lasers

The characteristics of planar repetitively pulsed diffusion-cooled HF, DF, HF – DF, Xe, and HF – DF – Xe(Kr) lasers pumped by a microwave discharge (2.45 GHz) are studied depending on the pump pulse duration (10 – 40 μs) and repletion rate (50 – 400 Hz), as well as on the composition and pressure of the working gas mixture (30 – 200 Torr) at low (to 0.6 L s−1) gas flow rates. It is demonstrated for the first time that a HF – DF – Xe gas-discharge laser may simultaneously operate as a HF – DF chemical molecular laser and a recombination laser based on xenon atom transitions and generate broadband (octave) radiation in the range 2.0 – 4.2 μm with an average output power of 43 mW and an efficiency of 0.9 %. With substitution of xenon for krypton, lasing in a HF – DF – Kr laser is obtained at wavelengths of 2.52 – 4.15 μm. Lasing in a HF – DF laser was achieved simultaneously in two spectral regions (2.7 – 2.9 and 3.6 – 4.2 μm) with an output power of ∼50 mW. It is found that a decrease in the pump pulse duration with a simultaneous increase in the pulse repletion rate for retaining the average energy deposition leads to an increase in the average output power and efficiency of the laser. At low pulse repetition rates (50 – 100 Hz) and a low gas mixture flow rate, helium buffer gas can be effectively substituted for neon. Operation of the Xe laser was achieved in the spectral range 2.03 – 3.65 μm with an average output power of 580 Mw at a pump pulse duration of 20 μs, a pulse repetition rate to 10 kHz, and a maximum efficiency of 55 %. The obtained experimental results demonstrate the possibility of creating broadband HF – DF – Xe lasers emitting in the frequency range 2 – 4 μm with a desired ratio of intensities in different spectral ranges.

Mode transitions of a helium dielectric barrier discharge from Townsend, normal glow, to abnormal glow with varying voltage rising time

Transition from a Townsend mode to a normal glow mode has been reported in the literature for uniform dielectric barrier discharge (DBD) at atmospheric pressure. In this paper, through a one-dimensional fluid model, more modes of uniform DBD in helium and transitions between them are found with varying rising time of a saw-tooth voltage. The results indicate that a positive discharge initiates at the positive-slope voltage phase, whose pulse duration decreases, while the peak value increases with decreasing rising time. During this process, a negative discharge initiating at the negative-slope voltage phase keeps weakening to almost zero current. The predominant positive discharge is then investigated through analyzing spatial distributions of electron density, ion density, and electric field at the peak current moment. In combination with the voltage-current curve, discharge modes of DBD are revealed to transit from a Townsend, a normal glow, to an abnormal glow with decreasing voltage rising time. These mode transitions are qualitatively explained by analyzing the gap voltage and electron density averaged in the gap just before discharge initiation. The results also suggest that by reducing the rising time or increasing voltage amplitude, DBD is prone to operate in the abnormal glow mode. Moreover, DBD in the abnormal glow mode has an increasing peak current and a decreasing pulse duration with increasing voltage amplitude. Finally, the critical voltage amplitude is given as a function of voltage rising time for the mode transitions from the Townsend to the normal glow and the normal glow to the abnormal glow.

Effects of Nd:YAG Pulsed Laser Welding Parameters on the Electrochemical Corrosion Properties of Carbon-Coated 316L Foils in a Simulated PEMFC Environment

Recently, PEM fuel cell bipolar plates made of carbon-coated AISI 316L austenitic stainless steel have attracted a lot of attention. Parameters suitable for Nd:YAG pulsed laser welding on the lap joint were evaluated in a simulated fuel cell environment for the first time in this study. To this end, the parameters: peak power, pulse duration, and frequency, were optimized using the response surface methodology, and a second-order polynomial was developed to predict the trend of corrosion current density. The potentiodynamic corrosion evaluation proved that with a decrease in pulse duration from 3.5 to 1.5 ms, a decrease in peak power from 650 to 130 W, and a decrease in frequency from 20 to 12 Hz occurred. Corrosion current density decreased by 76%, 34%, and 18%. According to the potentiostatic test, the optimum conditions, i.e., a peak power of 130 W (at 2.5 ms) and pulse duration of 1.5 ms (at 390 W), were achieved at anodic current densities of 0.77 µA/cm2 and 0.61 µA/cm2, respectively.

Investigation of climate change impacts on flow regime in the Lucas Creek catchment using multiple CMIP5 ensembles

ABSTRACTClimate change is projected to affect the flow regime in urban waterways adversely. This study investigates the climate change impacts on the flow regime in the Lucas Creek catchment under three Representative Concentration Pathways (RCPs): RCP 2.6, RCP 4.5 and RCP 8.5. Flow Duration Curves (FDCs) and Indicators of Hydrological Alteration (IHA) methods are used to assess flow regime variations. Results of FDCs show a maximum rise of 370% in peak flow under RCP 4.5 however, low flow increases by 40%. Monthly streamflow increases up to 380% except for October and December. Similarly, the majority of the IHA parameters also show at least a 10% increase in their magnitudes under RCP 8.5. However, some parameters such as high pulse duration and rise rate decrease by 50% under RCP 4.5. The catchment would mainly suffer severe impact variations under RCP 4.5 and RCP 8.5 in the future.

Simulation of saturable absorber material length effect on characteristics of Passive Q-switching and Stokes pulses

The effect of saturable absorber material length on characteristics of passive Q-switching and Stokes pulses has been simulated. A software computer program has been based for solving a rate equations model numerically by Rung –Kutta- Fehalberg method. (Nd:GdVO4), (Cr+4:YAG), and Ba(NO3)2 are used as a active medium, saturable absorber material, and Raman medium respectively. The study shows that an decrease in pulse duration for both pulses (passive Q- switching and Stocks) with saturable absorber material length increases. While the energy and the power are increase with saturable absorber material length increases.

Experimental investigation of effective factors of pulsed electric field in osmotic dehydration of apple

The effects of various factors of the pulsed electric field (PEF) as an apple osmotic dehydration (OD) preprocess by using response surface methodology (RSM) are investigated and optimized in this paper. Electric field strength, pulse numbers and pulse duration were considered as PEF pretreatment effective factors in constant osmotic dehydration conditions such as concentration and temperature. After 4 h of osmosis, the results indicate decrease in amounts of water loss (WL) and solid gain (SG) by increasing field strength from 1 to 2 kV/cm and decreasing pulse duration from 0.5 to 0.02 s, respectively. The effect of increasing the pulse numbers from 8 to 16 give rise to the increase in amounts of water loss and solid gain at first and then by increasing to 24 pulses, the decrease in these amount has been evident. The ratio of water loss to solid gain (WL/SG) and response surface methodology (RSM) were used for optimization effects of PEF factors on the OD process. In optimal values of electric field strength, pulse numbers and pulse duration, the WL/SG ratio provided highest reduction sample mass.

Shock Initiation of Nano‐TATB Explosives under Short‐Duration Pulses

AbstractBecause of the potential of nano‐TATB as a charge in slapper detonator, the shock initiation threshold of nano‐TATB explosive was studied over short pulse durations (down to 0.017 μs) and high pressure ranges (up to 17 GPa). The pulses were produced by the impact of thin plastic flyer plates, which were accelerated by electrically exploded metal foils. Nano‐TATB powders with a mean particle size of 60 nm were prepared and pressed into cylindrical sample charges with three densities (1.56, 1.64 and 1.74 g/cm3). The flyer impact velocity data versus charging voltages was measured with different flyer thicknesses. Together with the Hugoniot relationships of flyer and samples, the impact pressures and pulse durations were calculated. By using Langlie method, the threshold pressures for shock initiation were determined. In case of the identical flyer plate, the threshold pressure increases with increasing density. In case of the identical explosive density, the threshold pressure increases as the thickness of the flyer decreases (i. e. the pulse duration decreases) except for the highest tested density. These data are represented well by a constant P2τ initiation criterion for the short, high‐pressure pulses and deviate from the P2τ behavior at lower pressures. Finally, nano‐TATB is proved to be a little more sensitive to short‐duration pulses than superfine TATB, but much more sensitive than production grade TATB.

Electrochemical Micromachining with Sinusoidal Signals

In ultra-short pulse electrochemical micro-machining, step voltage signals are used with pulse duration as the only effective control parameter of machining resolution. Decreasing pulse duration increases the machining resolution, but requires an expensive ultra-short pulse power source. A novel electrochemical micromachining technique using sinusoidal signals is proposed in this paper, and a differential equation of the equivalent circuit under sinusoidal signals is developed for the method. From this, the equation of the relationship between machining accuracy and frequency of the sinusoidal signals is provided. Using the equation, machining resolution as a function of the frequency of sinusoidal signals is simulated and compared with the experimental findings. Results show a good correspondence is obtained, and that machining resolution significantly increases with the frequency of the sinusoidal signals. Using the technique, micro structures are fabricated, and good machining resolution is achieved. The proposed technique does not require an expensive ultra-short pulse power source, and as sinusoidal signals are the most readily available electrical signals, this technology will be more convenient in industrial applications.

Numerical simulation for pulsed laser–gas tungsten arc hybrid welding of magnesium alloy

Based on the extended application of COMSOL multiphysics, a novel dual heat source model for pulsed laser–gas tungsten arc (GTA) hybrid welding was established. This model successfully solved the problem of simulation inaccuracy caused by energy superposition effect between laser and arc due to their different physical characteristics. Numerical simulation for pulsed laser–GTA hybrid welding of magnesium alloy process was conducted, and the simulation indicated good agreements with the measured thermal cycle curve and the shape of weld beads. Effects of pulse laser parameters (laser-excited current, pulse duration, and pulse frequency) on the temperature field and weld pool morphology were investigated. The experimental and simulation results suggest that when the laser pulse energy keeps constant, welding efficiency of the hybrid heat source is increased by increasing laser current or decreasing pulse duration due to the increased ratio of the weld bead depth to width. With large laser currents, severe spatters tend to occur. For optimized welding process, the laser current should be controlled in the range of 150–175 A, the pulse duration should be longer than 1 ms, and the pulse frequency should be equal to or slightly greater than 20 Hz.

Effect of Spark Discharge Plasma on Water, Physiological Saline, and Hanks’ Solution

The effect of the duration of a current pulse of spark discharge in air on the composition of products formed in liquid both by the action of plasma radiation and with the participation of species formed in the discharge itself has been studied. The products formed in water, 0.9% NaCl, and in Hanks’ solution have been determined. It has been that in all the cases, nitrous acid is one of the primary products. The yield of nitrous acid is the same in all the three solutions. With a decrease in the current pulse duration, the influence of the species formed in the discharge itself on the yield of nitrous acid increases. The products formed in water decompose within up to 13 days. Peroxynitrite and N2O3 were identified as degradation products.

Chirp control of femtosecond-pulse scattering from drag-reducing surface-relief gratings

The role of chirp on the light–matter interaction of femto- and pico-second laser pulses with functional structured surfaces is studied using drag-reducing riblets as an example. The three-dimensional, periodic microstructure naturally gives rise to a mutual interplay of (i) reflection, (ii) scattering, and (iii) diffraction phenomena of incident coherent light. Furthermore, for femtosecond pulses, the structure induces (iv) an optical delay equivalent to a consecutive temporal delay of 230 fs in places of the pulse. These features enable studying experimentally and numerically the effect of tuning both pulse duration τ and spectral bandwidth Δω on the features of the wide-angle scattering pattern from the riblet structure. As a result, we discovered a significant breakdown of fringes in the scattering pattern with decreasing pulse duration and/or increasing spectral bandwidth. This unique type of chirp control is straightforwardly explained and verified by numerical modeling considering the spectral and temporal interaction between different segments within the scattered, linearly chirped pulse and the particular geometric features of the riblet structure. The visibility of the fringe pattern can be precisely adjusted, and the off-state is achieved using τ 2.85×1013 rad/s.

Decreasing Pulse Duration at Receiver Output under Emitter Excitation with Complex Waveforms

The pulsed operation mode of an emission–reception system consisting of two immersion piezoelectric transducers separated by a liquid (glycerine) layer has been studied. Identical piezoceramic plates were used as active elements in the emitter and receiver. The rear sides of the plates were loaded on air. The emitting transducer was excited by composite-waveform electric signals consisting of two sine-wave half-periods with different amplitudes (with their sign taken into account). The length of each half-period corresponded to the half-period of emitting plate vibration at the antiresonance frequency. The first half-period was the exciting one, with the second, supplied at the appropriate moment, being the compensating one. The amplitudes of the compensating half-periods were precalculated based on a mathematical algorithm developed by the authors in their previous works. It has been shown that applying composite-waveform signals allows one to considerably reduce the duration of a signal at the receiver output as compared with the case where the emitter is excited by an electric pulse in the form of one half-period of vibration at the antiresonance frequency. Experimental data are in fair agreement with computation results.

In vitro investigations of propulsion during laser lithotripsy using video tracking.

Ureteroscopic laser lithotripsy is an important and widely used method for destroying ureter stones. It represents an alternative to ultrasonic and pneumatic lithotripsy techniques. Although these techniques have been thoroughly investigated, the influence of some physical parameters that may be relevant to further improve the treatment results is not fully understood. One crucial topic is the propulsive stone movement induced by the applied laser pulses. To simplify and speed up the optimization of laser parameters in this regard, a video tracking method was developed in connection with a vertical column setup that allows recording and subsequently analyzing the propulsive stone movement in dependence of different laser parameters in a particularly convenient and fast manner. Pulsed laser light was applied from below to a cubic BegoStone phantom loosely guided within a vertical column setup. The video tracking method uses an algorithm to determine the vertical stone position in each frame of the recorded scene. The time-dependence of the vertical stone position is characterized by an irregular series of peaks. By analyzing the slopes of the peaks in this signal it was possible to determine the mean upward stone velocity for a whole pulse train and to compare it for different laser settings. For a proof of principle of the video tracking method, a specific pulse energy setting (1 J/pulse) was used in combination with three different pulse durations: short pulse (0.3 ms), medium pulse (0.6 ms), and long pulse (1.0 ms). The three pulse durations were compared in terms of their influence on the propulsive stone movement in terms of upward velocity. Furthermore, the propulsions induced by two different pulse energy settings (0.8 J/pulse and 1.2 J/pulse) for a fixed pulse duration (0.3 ms) were compared. A pulse repetition rate of 10 Hz was chosen for all experiments, and for each laser setting, the experiment was repeated on 15 different freshly prepared stones. The latter set of experiments was compared with the results of previous propulsion measurements performed with a pendulum setup. For a fixed pulse energy (1 J/pulse), the mean upward propulsion velocity increased (from 120.0 to 154.9 mm · s-1 ) with decreasing pulse duration. For fixed pulse duration (0.3 ms), the mean upward propulsion velocity increased (from 91.9 to 123.3 mm · s-1 ) with increasing pulse energy (0.8 J/pulse and 1.2 J/pulse). The latter result corresponds roughly to the one obtained with the pendulum setup (increase from 61 to 105 mm · s-1 ). While the mean propulsion velocities for the two different pulse energies were found to differ significantly (P < 0.001) for the two experimental and analysis methods, the standard deviations of the measured mean propulsion velocities were considerably smaller in case of the vertical column method with video tracking (12% and 15% for n = 15 freshly prepared stones) than in case of the pendulum method (26% and 41% for n = 50 freshly prepared stones), in spite of the considerably smaller number of experiment repetitions ("sample size") in the first case. The proposed vertical column method with video tracking appears advantageous compared to the pendulum method in terms of the statistical significance of the obtained results. This may partly be understood by the fact that the entire motion of the stones contributes to the data analysis, rather than just their maximum distance from the initial position. The key difference is, however, that the pendulum method involves only one single laser pulse in each experiment run, which renders this method rather tedious to perform. Furthermore, the video tracking method appears much better suited to model a clinical lithotripsy intervention that utilizes longer series of laser pulses at higher repetition rates. The proposed video tracking method can conveniently and quickly deliver results for a large number of laser pulses that can easily be averaged. An optimization of laser settings to achieve minimal propulsive stone movement should thus be more easily feasible with the video tracking method in connection with the vertical column setup. Lasers Surg. Med. 50:333-339, 2018. © 2017 Wiley Periodicals, Inc.

Effects of nanomaterial saturable absorption on gain-guide soliton in a positive group-dispersion fiber laser: Simulations and experiments

In recent years, several kinds of nanomaterials have been discovered, and successfully used as saturable absorbers (SAs) for passively mode-locked fiber lasers. However, it is found that most of nanomaterials-based SAs cannot stably generate gain-guide solitons in positive group-dispersion fiber lasers, which is urgently expected to fully understand the inherent reasons. In this paper, we numerically and experimentally investigate the effects of nanomaterial saturable absorption (e.g. modulation depth and saturation optical power) on gain-guide soliton in positive group-dispersion Er3+-doped fiber laser (PGD-EDFL). By numerically solving the Ginzburg–Landau equation, the evolutions of both the mode-locked optical spectrum and pulse duration as a function of modulation depth and saturation optical power are analyzed, respectively. In experiment, we firstly prepare five nanomaterial SAs with the similar insertion loss, which have the different modulation depth from 1.80% to 23.36%, and the different saturation optical power from 8.8 to 536 W. We then perform the experimental comparison by incorporating the five SAs in a same PGD-EDFL cavity, respectively. The experimental results are in good agreement with the numerical ones. Our result reveals that: (1) a low modulation depth cannot support the formation of gain-guide soliton, (2) as the modulation depth increases, the spectral bandwidth of gain-guide soliton increases, the pulse duration decreases and the pulse chirp becomes large, (3) the saturation optical power has the weak influences on the gain-guide soliton performances.

Pulsed photothermal interferometry for spectroscopic gas detection with hollow-core optical fibre.

Gas detection with hollow-core photonic bandgap fibre (HC-PBF) and pulsed photothermal (PT) interferometry spectroscopy are studied theoretically and experimentally. A theoretical model is developed and used to compute the gas-absorption-induced temperature and phase modulation in a HC-PBF filled with low-concentration of C2H2 in nitrogen. The PT phase modulation dynamics for different pulse duration, peak power and energy of pump beam are numerically modelled, which are supported by the experimental results obtained around the P(9) absorption line of C2H2 at 1530.371 nm. Thermal conduction is identified as the main process responsible for the phase modulation dynamics. For a constant peak pump power level, the phase modulation is found to increase with pulse duration up to ~1.2 μs, while it increases with decreasing pulse duration for a constant pulse energy. It is theoretically possible to achieve ppb level detection of C2H2 with ~1 m length HC-PBF and a pump beam with ~10 ns pulse duration and ~100 nJ pulse energy.

Changes of intensity distribution of a tightly focused plane-wave pulse induced by lens dispersion

The tightly focusing properties of a linearly polarized plane-wave pulse through a high numerical aperture (NA) lens with dispersion are formulated. The effects of lens dispersion on the tightly focused intensity distribution in the focal region are studied in detail. It is found that, because of the lens dispersion, the intensity of the side lobe of the focused light spot strengthens in the transverse direction and the size of the main lobe of the focused light spot increases in the longitudinal direction as the pulse duration decreases. In addition, we compare the effects of dispersion of lens made by different types of glass on tightly focusing properties of a pulse beam. The results will be helpful to choose suitable high NA objective lens in experiment.

Diffraction of short-pulse Laguerre–Gaussian beams

Based on the numerical solution of the parabolic wave equation for the complex spectral amplitude of the wave field, the propagation of short-pulse vortex optical beams in a homogenous medium is studied by an example of a Laguerre–Gaussian femtosecond beam. It has been shown that the diffraction spreading of the Laguerre–Gaussian beam decreases with decreasing pulse duration as compared to continuous wave beams.