Laser Pulse Time Research Articles

Improving pulsed laser induced fluorescence distribution function analysis through matched filter signal processing.

Laser induced fluorescence is used to measure argon ion heating during magnetic reconnection in the PHase Space MApping experiment (PHASMA). Sufficient signal-to-noise ratio (SNR) of the processed signal with pulsed laser injection is a delicate balance between saturation of the absorption line and injecting enough laser power to overcome the spontaneous emission of the plasma at the fluorescence wavelength. Averaging over many laser pulses and integrating over the fluorescence lifetime improves the SNR of the processed signal (processed SNR) when the SNR of the laser pulse time series is small (pulse SNR), but for laser powers small enough to avoid saturation, averaging over hundreds of pulses is needed to obtain an appreciable processed SNR over the entire Doppler-broadened absorption line. Here, we describe a matched filter processing method that significantly improves the SNR of the final measurement with fewer shots averaged. Investigation of simulated measurements validated by experimental results suggests that the matched filter method provides up to a 20% improvement in the processed SNR, resulting in less uncertainty in distribution function fits.

Bio-thermal response during laser haemorrhoidoplasty: an exclusive analytical and numerical approach for theoretical investigation.

Haemorrhoidal disease is identified by declension of the inflamed and bleeding of vascular tissues of the anal canal. Traditionally, haemorrhoids are associated with chronicconstipation and the most common symptoms are irritation in anus region, pain and discomfort, swelling around anus, tender lumps around the anus and rectal bleeding (depending upon the grade of haemorrhoid). Among the several conventional treatment procedures (commonly mentioned as, rubber band litigation, sclerotherapy and electrotherapy), laser haemorrhoidoplasty is an out-patient and less-invasive laparoscopic procedure. From literature survey it has been observed that an exclusive theoretical model depicting the impact of 1064nm wavelength laser wave on living tissues subjected to haemorrhoid therapy is not available. This research work is a pioneering attempt to develop a theoretical study attributing specifically on laser therapy of haemorrhoid treatment based on Pennes' biological heat transfer model. The corresponding mathematical model has been solved by analytical method to establish thermal response of tissue during the treatment and also the same has been solved a numerical approach based on finite difference method to validate the feasibility of former method due to unavailability of any theoretical model. Impact of variation of blood perfusion term, laser pulse time and optical penetration depth on temperature response of skin tissue is captured. The tissue temperature decreases along with time of laser exposure with increasing the blood perfusion rate as it carries away large amount of heat. With the increase in laser pulse time, tissue temperature declines due to shorter pulse time resulting in higher energy consumed by electrons. The research outcome is successfully validated with less than 1% of error observed between the appointed analytical and numerical scheme.

Combined Compression of Stimulated Brillouin Scattering and Laser–Induced Breakdown Enhanced with Sic Nanowire

In this paper, the laser pulse time compression technique, based on stimulated Brillouin scattering (SBS) and passive laser–induced breakdown (LIB) series technology, is investigated. By doping a SiC nanowire in a CCl4 solution of an LIB breakdown medium, the LIB generation threshold is reduced, and the stability of the LIB compression output is improved. When OD is 0.2, the output pulse width is 254.4 ps, and the corresponding energy conversion efficiency and pulse compression rate are 34.2% and 50.2%, respectively. Our experiment proves the feasibility of this scheme.

On selection rules in two-dimensional terahertz-infrared-visible spectroscopy.

Two-dimensional terahertz-infrared-visible (2D TIRV) spectroscopy directly measures the coupling between quantum high-frequency vibrations and classical low-frequency modes of molecular motion. In addition to coupling strength, the signal intensity in 2D TIRV spectroscopy can also depend on the selection rules of the excited transitions. Here, we explore the selection rules in 2D TIRV spectroscopy by studying the coupling between the high-frequency CH3 stretching and low-frequency vibrations of liquid dimethyl sulfoxide (DMSO). Different excitation pathways are addressed using variations in laser pulse timing and different polarizations of exciting pulses and detected signals. The DMSO signals generated via different excitation pathways can be readily distinguished in the spectrum. The intensities of different excitation pathways vary unequally with changes in polarization. We explain how this difference stems from the intensities of polarized and depolarized Raman and hyper-Raman spectra of high-frequency modes. These results apply to various systems and will help design and interpret new 2D TIRV spectroscopy experiments.

First large capsule implosions in a frustum-shaped hohlraum

We report on the first indirect-drive implosions driven by a dual conical frustum-shaped hohlraum denoted “frustraum” and the experimental tuning campaigns leading up to two layered implosions. The campaign used 1.2 and 1.4 mm inner radius high density carbon (HDC) capsules and represented the largest HDC capsules to be imploded on the National Ignition Facility via indirect drive. Several techniques were successfully implemented to control the Legendre mode 2 capsule symmetry of the implosions, including changing the wall angle of the frustraum, which is not possible with cylindrical hohlraums. A mode 4 feature was observed and its implications for hotspot mix discussed. Two layered implosions were conducted with 1.2 mm inner radius capsules, the latter of which achieved the highest layered capsule absorbed energy on the National Ignition Facility using only 1.74 MJ of laser energy. The layered implosion results, along with generalized Lawson parameters, suggest that increasing the energy absorbed by the capsule at the expense of long coast times makes it more challenging to achieve ignition and that further reducing coast time (time between end of laser pulse and bang time) closer to the 1 ns level is warranted to improve the areal density and make it easier to achieve the hotspot temperature, alpha heating, and yield amplification required for ignition.

Modelling the spatial-temporal expansion of laser-produced tin (Sn) plasma with femtosecond laser

In this study, the 1-D hydrodynamic code (MED103) has been used to simulate the Tin (Sn) plasma in space and time with 400 cells as mesh and 10 ns simulation time that is produced by laser with the pulse of 500 fs and its wavelength of 1064 nm. Unique radiation sources of extreme ultraviolet and soft X-ray emissions can be generated from the high ions of laser-produced heavy element plasmas. The spatial-temporal distribution of Sn plasma parameters was studied when a laser pulse was focused on a target to 100 µm diameter with the intensity of 1.27 * 1015 w/cm2. There are two peaks of pressure, first inside the target after laser pulse time and the other close to the surface of the target at a time of 8 ns which is higher than 30 ≈ Mbar. The pressure is still high during simulation time. During and after the laser pulse time, the electron and ion temperature reaches maximum value but their values are not equal.

Structural organization and properties of surface layers of WC–Co hard alloys after pulsed laser processing

The article presents the metal-physical studies results of the structure formation effects in surface layers in the hard alloys of the WC–Co system under extreme thermal and deformation effects of pulsed laser radiation. It is shown that the structural organization and properties of hard alloys VK6, VK8, VK10 upon radiation treatment with a power density of 175 MW/m2 are determined by state of the zones which are formed around carbide inclusions due to the various kinds of stresses appearance at the “carbide-bond” composition boundaries, including thermostrictive and phase stresses. The result is dissolution of the carbides boundary zones due to contact melting, which is accompanied by mutual mass transfer of atoms at the boundaries in the “carbide-bond” system with the possible formation of a thin amorphous-like super hard shell. These processes make it possible to create compositions in hard alloys with a set of differentiated properties specified by varying the laser treatment process parameters and composition of the starting materials. After laser alloying with a radiation power density of 200 MW/m2, temperature gradients and thermal stresses appearing in the surface layers of hard alloys with coatings (cobalt, nickel) contribute to convective mixing of the molten coating components and their penetration into the hard alloy to a depth of more than 20 μm. Simultaneously, despite the extremely short laser pulse time (10–3 s), mass transfer of tungsten, carbon and titanium atoms from the melted boundary zones of carbides to the adjacent bond zones with their hardening is possible in the irradiated zones. It was established that after high-temperature laser heating, carbides, in contrast to the initial ones, achieve a globular shape of grains. They are dispersed, and stoichiometric characteristics change in the local zones bordering the bond (the complex type carbide CoxWyCz is formed). As a result, due to these processes, the surface layers’ viscosity of hard alloys and the irradiated products performance increase. Compared to non-irradiated samples of hard alloy, the ultimate strength increases by 15 %, strength and durability – by 30 – 40 %.

Mechanism analysis of space debris removal by nanosecond pulsed laser

In this study, a numerical model is used to study the mechanism of laser ablation for space debris removal. The model couples the physical processes involved in laser ablation, including the heating, melting and evaporation of target (i.e. Aluminum), the formation of plume and plasma, and plasma shielding. Validation of the model is performed by comparing the simulated impulse density and impulse coupling coefficient with experimental data, showing the availability of the model in accurately describing the processes involved in laser ablation for removing space debris. The effects of laser irradiance, wavelength and pulse width on laser ablation were systematically studied by the model. In addition, the continuous laser ablation is explored and the effect of the laser pulse time interval is demonstrated. The results show that laser irradiation and pulse width have a significant impact on laser ablation, while the impact of wavelength is much smaller compared to the former two. In addition, using two short duration laser ablation instead of a long duration can significantly improve the impulse density obtained by the target and the corresponding impulse coupling coefficient, but the time interval between two laser pulses should not be too long.

Influence of the laser pulse time profile on residual stress characteristics in laser shock peening.

In this paper, residual stress and plastic deformation of TC4 titanium alloys and AA7075 aluminum alloys after laser shock peening (LSP) with the laser pulses that have the same energy and peak intensity but different time profiles have been studied. The results show that the time profile of the laser pulse has a significant influence on LSP. The difference between the results of LSP with varying laser input mode has been contributed to the shock wave caused by different laser pulse. In LSP, the laser pulse with a positive-slope triangular time profile could induce a more intense and deeper residual stress distribution in metal targets. Residual stress distribution changing with laser time profiles suggests that shaping the laser time profile is a potential residual stress control strategy for LSP. This paper comprises the first step of this strategy.

Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of sim 170 ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.

Study on the mechanism of surface pressure of optical films formed by laser plasma shock wave

In a high-power laser system, when the surface pressure of the optical film caused by laser plasma shock wave is greater than the adhesion per unit area of the film layer, the film will produce mechanical damage, and in serious cases, the whole system may not work. Therefore, studying the formation mechanism of optical film surface pressure caused by laser plasma shock wave and calculating the pressure is the key to ensure the normal operation of high power laser system. In this paper, by studying the relaxation process of shock wave on optical film surface pressure, a theoretical calculation model of shock wave on optical film pressure is established, and the variation law of pressure with different parameters is obtained, which reveals the mechanism of forming the optical film surface pressure. The calculation and simulation results show that the maximum pressure is 108 N m−2 during the laser pulse, and the pressure decreases with the increase of laser pulse time after the pulse, and the total action time of laser plasma and shock wave on the film is in the order of microseconds. The pressure increases with the increase of incident laser energy, focal length of focusing lens and incident laser pulse width, which increases with the decrease of the distance between the film surface and the focal plane of the focusing lens. The pressure changes more obviously with the incident laser energy and the distance between the film surface and the focal plane of the focusing lens than with the focal length of the focusing lens and the incident laser pulse width.

Design of a High-Resolution Multifocal LIDAR: Enabling Higher Resolution Beyond the Laser Pulse Rise Time

3D imaging is increasingly impacting areas such as space, defense, automation, medical and automotive industries. The most well-known optical 3D imaging systems are LIDAR systems that rely on Time of Flight (ToF) measurement. The depth resolution of the LIDAR systems is limited by the rise time of the illumination signal, bandwidth of the detector systems, and the signal-to-noise ratio. The focal gradients can be utilized to obtain 3D images from 2D camera captures. In this study, we combine the basic LIDAR techniques with a physical measurement of focal gradient changes that are caused by the depth of field, to obtain a high-resolution 3D image using spatially separated single-pixel detectors. The system that we introduce here enables depth resolution that goes beyond the rise time of the incident laser pulse.

Arrival time diagnosis method of high refrequency hard X-ray free electron laser

X-ray free electron laser (XFEL) pulse time diagnosis technology is often used to detect the relative arrival time of XFEL pulse and auxiliary laser near the experimental station. It is an important auxiliary technology and provides a reference signal for the pump-probe pulse in the XFEL laser pump-probe experiment. With the development of XFEL towards high repetition frequency and short pulse, higher requirements are put forward for diagnostic frequency, pump sample and resolution in time diagnosis. The technology is realized by the pump-probe method and optical cross-correlation method. When the XFEL pulse is incident on the high-bandwidth semiconductor solid target instantaneously, the complex refractive index of the solid target will change, then the arrival time of XFEL will be encoded in the mutation space. In thiswork, we design an XFEL pulse arrival time diagnostic device based on two methods: spatial coding and spectral coding. In this framework, the interaction between X-ray and solid target is explored by Beer's absorption theory and atomic scattering theory. Therefore, the response to X-ray absorption and refractive index in this process are investigated, and the solid target selection model is developed. This model is used to analyze the influence of solid target type and thickness in diagnosis, while avoiding situations where the sample is too hot due to a lot X-ray absorption. Moreover, the influence of hard X-ray on sample temperature at high frequency is considered, and the samples suitable for different X-ray bands are given. The chirped pulse modulation in spectral coding is analyzed, and the influence of dispersion medium and pulse parameters on the diagnostic resolution of spectral coding are obtained. Finally, the error effects of X-ray, spatial coding and spectral coding on the results are analyzed, and the analysis methods and consideration factors of the two coding methods are given. This work is of great significance in using the XFEL pulse arrival time diagnostic device.

Enhancing Biomolecule Analysis and 2DMS Experiments by Implementation of (Activated Ion) 193 nm UVPD on a FT-ICR Mass Spectrometer.

Ultraviolet photodissociation is a fast, photon-mediated fragmentation method that yields high sequence coverage and informative cleavages of biomolecules. In this work, 193 nm UVPD was coupled with a 12 Tesla FT-ICR mass spectrometer and 10.6 μm infrared multi-photon dissociation to provide gentle slow-heating of UV-irradiated ions. No internal instrument hardware modifications were required. Adjusting the timing of laser pulses to the ion motion within the ICR cell provided consistent fragmentation yield shot-to-shot and may also be used to monitor ion positions within the ICR cell. Single-pulse UVPD of the native-like 5+ charge state of ubiquitin resulted in 86.6% cleavage coverage. Additionally, IR activation post UVPD doubled the overall fragmentation yield and boosted the intensity of UVPD-specific x-type fragments up to 4-fold. This increased yield effect was also observed for the 6+ charge state of ubiquitin, albeit less pronounced. This indicates that gentle slow-heating serves to sever tethered fragments originating from non-covalently linked compact structures and makes activation post UVPD an attractive option to boost fragmentation efficiency for top-down studies. Lastly, UVPD was implemented and optimized as a fragmentation method for 2DMS, a data-independent acquisition method. UVPD-2DMS was demonstrated to be a viable method using BSA digest peptides as a model system.

Theoretical investigation of electron dynamics driven by laser pulses in graphene nanoribbons

Control of electron dynamics in graphene nanoribbons (GNRs) is critically important for the future applications of graphene-based nanoelectronic devices. Based on real-time simulations, we investigate the manipulation of electronic transport by femtosecond laser pulses in armchair GNRs. The simulation results show that the multiphoton absorption process can drive the photon-excited electron flow along a GNR. The corresponding transferred charges are determined by the order of multiphoton absorption and depend on the central frequency and energy distribution of few-cycle laser pulses. Furthermore, the transferred charge can be controlled by the carrier-envelope phase of pulses; this dependence vanishes as the duration time of laser pulses increases. These results also provide insights into the manipulation of electron dynamics using lasers and the design of optoelectronic devices based on graphene materials.

Phase Change Analysis of Robot Laser Drilling with Accuracy Enhancement by Deep Learning

In this investigations, heat transfer mechanism of laser drilling process is analyzed by using Matlab program 8.5. Calculations of the heat impacted zone were also done. The moving boundary condition generates phase shift and influences heat transfer during the laser drilling process. The classical approach is used to study heat conduction in solids, with modifications made to account for the boundary condition transition from Stefan to continuous heat flow. According to the suggested model, for a given laser beam intensity and pulse time, the drilling hole profiles in a certain material will be quite near to one another. Robot placement errors are complicated since there are many different sources for them, programming in the python language examined. The positioning precision of the robot is improved, and its application capabilities are increased, to reinforced machine learning. The suggested methodology offers a simple and direct method for real-time robot position modification in industrial settings during production setup or readjustment situations. a deep learning approach used to increase the operational positioning precision of an articulated robot. Approximately 300 iterations later, the placement accuracy had significantly improved.

Space object identification via polarimetric satellite laser ranging

Low Earth orbits are becoming congested. The rapid identification and precise orbit determination of space objects is mandatory for space management. Satellite laser ranging (SLR) enables precise orbit determination by measuring the two-way photon travel time of laser pulses from a ground station to satellites equipped with retroreflectors. Here we propose polarization-modulated SLR, where specially designed retroreflectors positioned on a satellite switch the polarization state of received polarized photons and reflect them back to a ground station for analysis. Satellite identifiers can be coded into arrays of reflectors with different polarizing properties, while the orbit determining capability of conventional SLR is maintained. We design, build and test polarized light-switching retroreflector assemblies and investigate the feasibility of accurate signal measurement from SLR ground stations. The approach is passive, straightforward to integrate and requires no electricity. Polarization-modulated SLR could contribute to increasing demands of space object monitoring, for example of mega-constellations or during cluster launches.

Restoring light transmission of dusty glass surfaces on the Moon

Abstract The Lunar Laser Ranging Retro-reflectors (LRR) allow for the precise measurement of the travel time of laser pulses from the Earth to the lunar surface and back. Dust accumulation on the LRRs scatters the reflected light used for laser interferometry measurements leading to a reduction in the return signal amplitude by a factor of ≃ 10. We report on a series of experiments utilizing a method of an iterative application and removal of an acrylic adhesive kapton tape to remove lunar dust simulant from a glass substrate. The effectiveness of adhesive tape was determined by measuring the transmission of an expanded laser beam through a glass substrate with a dusted surface. Calibration data was obtained before the glass was exposed to any dust, and light transmission was measured after each iteration of adhesive removal using a power meter. The procedure was repeated using heavy gloves while keeping the glass substrate at 120 ° C to simulate the temperature conditions of a lunar day on the surface. A third test was conducted at room temperature in high-vacuum to ensure functionality in the lunar environment. These experiments show that glass substrates can be restored to 98 ± 2 % of their original transmission using the adhesive tape method. Therefore the light that travels into and out of the quartz surface of the LLRs can be restored up to 96 ± 4 % of their initial transmission.

An efficient predictive modeling for simulating part-scale residual stress in laser metal deposition process

Residual stress and deformation are common issues which prevent dimensional accuracy and lead early fatigue of products by laser metal deposition process. Finite element analysis is an efficient means to estimate the temperature history and thermal stress distribution. However, it is extremely challenging to predict thermal history and stress distribution of a practical large and complex geometry if each laser pulse is taken into account as by conventional laser pulse modeling. Therefore, in this study, an efficient predictive finite element model with assumptions on the laser heat source and loading is established to study the evolution of thermal history, residual stresses, and deformations of a test coupon. The efficient predictive model, which is also called as the 2-layer by 2-layer model, simulates two layers at each laser pulse time. This model is compared with conventional laser pulse model in terms of the evolution of thermal history of selected points and residual stresses. Results show that the 2-layer by 2-layer model considerably reduces the simulation time without much compromising the accuracy of the prediction of deformation and thermal stress. In addition, test coupon is designed and fabricated to capture temperature history and observe microstructure change. It is found that microstructure presents certain correlation with cooling rate. Microstructure and residual stress of the test coupon are evaluated and found to be consistent with the prediction by proposed model. This efficient predictive model can shed light on large-scale part fabrication by laser metal deposition process in industry.

The effect of the laser pulse shape on the wakefield generation in field-ionized plasma

In this paper, the effect of the laser pulse shape on the generation and evolution of the wakefield during the interaction of the intense laser pulse with the gas have been studied utilizing the parallel relativistic PIC simulation code. In order to reach this aim, three pulses with length 300 fs and different rise-times 30, 45, and 60 are typically selected. Our results show that, the amplitude of the laser wakefield produced in the gas in comparison with the plasma strongly depends on the laser pulse shape. The simulation results indicate that for the high-slope laser pulse time (here 30 fs), ionization and thus density fluctuations have no significant effect on the wakefield generation because of rapid increase of the laser electric field. While by increasing the laser pulse rise-time to 45 fs, the rapid wave breaking due to the change in the medium refractive index during the gas ionization, prevents from the wakefield amplitude growth, so that the wakefield with larger amplitude is emerged in the plasma. For a slow-sloping pulse (here 60 fs), the ratio of the wakefield generation in the gas to the plasma is altered for the different gas densities and laser intensities. Moreover, it is represented that the longer the laser pulse rise-time, the sooner difference between the wakefield produced in the gas and plasma is observed. In fact, the larger the rise-time, the greater the density fluctuations and, consequently, the larger the initial noise is generated to seed the Raman instability.