Spatial Profile Of Laser Beam Research Articles

Neuro fuzzy prediction of laser fluence based on photoacoustic signal analysis in different gas mixtures

Photoacoustic spectroscopy achieves high sensitivity and selectivity using high-power lasers. Variations of parameters such as laser beam spatial profile and fluence Φ can alter precision of photoacoustic measurements. Numerous commercial instruments aren’t usable for high Φ values measurement, due to possible harmful effects. To estimate high Φ values from photoacoustic signals in time domain we applied computational intelligence method: adaptive-network-based fuzzy inference system (ANFIS). Experimental photoacoustic signals are generated in two different gas mixtures: SF6 + Ar and C2H4 + Ar, for Φ values ranging from (0.2–1.4) J∙cm-2. Obtained results indicate that different absorption characteristics of examined molecules and various signal intensities don’t influence ANFIS prediction, due to its adaptation ability and error tolerance in dealing with imprecise and noisy data. Furthermore, robustness, high learning capability and self-correction, make this technique computational effective and recommendable for in situ photoacoustic measurements. Aside from many advantages, limitations of the proposed method are also discussed.

Analysis of laser focusing effect on quantification of LII images

Laser-induced incandescence (LII) is a widely used technique for measuring soot concentrations. For flame applications LII is frequently deployed as a planar diagnostic to measure the two-dimensional soot field. However, when the laser sheet is focused, as is typical to reach the requisite laser fluence level and achieve good spatial resolution, the complex laser power dependence of the LII signal generation process can introduce a large variation in LII signal sensitivity across an LII image. In this work, this effect is quantified for the first time as a function of laser pulse fluence, using a typical planar LII excitation scheme with a clipped Gaussian YAG laser beam focused with a 1 m focal length lens. Furthermore, the cross-sectional energy distribution in the laser sheet was measured across the image plane, to relate the details of the laser sheet focal properties with the resultant LII behavior. The results show that a unique laser fluence level (referenced to the focal plane) exists whereby there is essentially no dependence of LII signal on position relative to the focal plane. However, at lower or higher fluences, the radial signals either decrease (low fluence) or increase (high fluence) rapidly with increasing distance away from the focal point. For measurements using an LII ‘plateau’ laser fluence level, as is usual in environments with significant optical depth (i.e. sufficiently strong soot levels), the LII signals are found to be 2.5X larger 40 mm away from the focal point. An analysis conducted by combining a previously measured LII fluence dependence for a top-hat laser profile with the laser sheet cross-sections measured in this work shows general agreement with the measured results for LII signal variation. Further, the sensitivity of LII signals at high fluences to the laser beam spatial profile, particularly away from the sheet focus, is highlighted.

Measurement of laser beam spatial profile by laser scanning

In several undergraduate courses related to laser physics and optics it is crucial that students should be able to understand basic concepts of laser beam propagation. In addition, experimental activities focusing on capability building in photogrammetry and remote sensing technologies are considered essential for geoinformatics undergraduate studies and for sustainable development of geospatial sciences. The understanding of laser beam scanning technologies and the fundamental physics of laser beam propagation are key factors in order to awaken students’ motivation to study remote sensing technologies. Recent advances in CCD sensors and beam profiler systems have made the measurement of laser beam spatial profile easy and accurate. However, an experiment based solely on a beam profiler could have lower pedagogical impact than expected, especially in the case of courses where scanning technology understanding is considered fundamental. With this in mind, we propose a straightforward laboratory experiment based on a modified pinhole technique where instead of using a spinning blade or a pinhole to cut the beam, a galvanometer scanner is employed to drive the laser beam through a small pinhole and record its intensity passing with a photodetector. Students are able to visualize the spatial profile of a laser beam, calculate the beam width and divergence and compare their results with direct measurement with a CCD sensor. The capability to efficiently control the scanning mechanism and take simple measurements of the spatial profile of the beam can significantly help the educational process toward the examination of more complex issues of remote sensing technologies.

Development and validation of an inverse method for identification of thermal characteristics of a short laser pulse

The paper presents development and validation of an inverse method for the identification of thermal characteristics of a short super-Gaussian laser pulse interacting with a metal sample. The method was applied to find unknown power of the laser pulse, dimensionless shape parameter of the super-Gaussian function describing the spatial energy distribution of the beam as well as beginning and end times of its interaction with the heated body. The proposed inverse method is based on the Levenberg-Marquardt technique and utilizes temporal and spatial distributions of temperature on the rear surface of the sample, i.e., the opposite to the irradiated one. The temperature profiles were registered by a high-speed IR camera. During the experiments some of the laser beam parameters, i.e., the power, the laser beam spatial profile, beginning and end times of the exposition as well as thermophysical and optical parameters of the aluminum sample were known. Therefore, the measured data were used for both validation of the numerical model which described thermal interaction of a laser pulse with the sample as well as the assessment of correctness and accuracy of the inverse method.At the first step, numerical model of the forward problem, i.e., heat transfer in the aluminum sample irritated by the laser pulse, was developed and validated based on experimentally-determined temperature distributions. The validation was performed for both single and multiple laser pulses. The numerical model of the forward problem was implemented in the commercial software ANSYS Fluent. Then an inverse algorithm was developed and implemented with the aid of the GNU Octave environment. Subsequently, series of numerical tests were carried out. During these numerical simulations, sensitivity analysis as well as initial calibration and verification of the developed algorithm were performed. Parallel to the modeling tasks, the experimental stand was built and series of experiments were performed which allowed to assess the performance of the method using physical temperature data. Performed investigations showed that the problem is ill-conditioned. Nevertheless, relatively good accuracy of the retrieval has been obtained. It was revealed that the sensitivity of the objective function to the end time of the laser pulse is relatively low and that two of the parameters affect measured temperatures in a similar way. These two properties contributed to ill-conditioning of the inverse problem. Dependence of inverse problem solutions on the initial guess has been observed and methods allowing to minimize its influence have been identified. The accuracy of the method was affected by relatively low temporal resolution of the IR camera (500 Hz, with the exposure time approximately from 0.2 to 1 ms). Despite aforementioned problems, the method was abled to retrieve unknown laser pulse parameters with 20–25% accuracy.

Laser Fluence Recognition Using Computationally Intelligent Pulsed Photoacoustics Within the Trace Gases Analysis

In this paper, the possibilities of computational intelligence applications for trace gas monitoring are discussed. For this, pulsed infrared photoacoustics is used to investigate $$\hbox {SF}_{6}$$ –Ar mixtures in a multiphoton regime, assisted by artificial neural networks. Feedforward multilayer perceptron networks are applied in order to recognize both the spatial characteristics of the laser beam and the values of laser fluence $$\Phi $$ from the given photoacoustic signal and prevent changes. Neural networks are trained in an offline batch training regime to simultaneously estimate four parameters from theoretical or experimental photoacoustic signals: the laser beam spatial profile R(r), vibrational-to-translational relaxation time $$\tau _{V-T} $$ , distance from the laser beam to the absorption molecules in the photoacoustic cell r* and laser fluence $$\Phi $$ . The results presented in this paper show that neural networks can estimate an unknown laser beam spatial profile and the parameters of photoacoustic signals in real time and with high precision. Real-time operation, high accuracy and the possibility of application for higher intensities of radiation for a wide range of laser fluencies are factors that classify the computational intelligence approach as efficient and powerful for the in situ measurement of atmospheric pollutants.

Performance characteristics of an excimer laser (XeCl) with single-stage magnetic pulse compression

Performance characteristics of an excimer laser (XeCl) with single-stage magnetic pulse compression suitable for material processing applications are presented here. The laser incorporates in-built compact gas circulation and gas cooling to ensure fresh gas mixture between the electrodes for repetitive operation. A magnetically coupled tangential blower is used for gas circulation inside the laser chamber for repetitive operation. The exciter consists of C–C energy transfer circuit and thyratron is used as a high-voltage main switch with single-stage magnetic pulse compression (MPC) between thyratron and the laser electrodes. Low inductance of the laser head and uniform and intense pre-ionization are the main features of the electric circuit used in the laser. A 250 ns rise time voltage pulse was compressed to 100 ns duration with a single-stage magnetic pulse compressor using Ni–Zn ferrite cores. The laser can generate about 150 mJ at ∼100 Hz rep-rate reliably from a discharge volume of 100 cm 3. 2D spatial laser beam profile generated is presented here. The profile shows that the laser beam is completely filled with flat-top which is suitable for material processing applications. The SEM image of the microhole generated on copper target is presented here.

Spatially modulated laser pulses for printing electronics

The use of a digital micromirror device (DMD) in laser-induced forward transfer (LIFT) is reviewed. Combining this technique with high-viscosity donor ink (silver nanopaste) results in laser-printed features that are highly congruent in shape and size to the incident laser beam spatial profile. The DMD empowers LIFT to become a highly parallel, rapidly reconfigurable direct-write technology. By adapting half-toning techniques to the DMD bitmap image, the laser transfer threshold fluence for 10 μm features can be reduced using an edge-enhanced beam profile. The integration of LIFT with this beam-shaping technique allows the printing of complex large-area patterns with a single laser pulse.

Formation of a spatial laser-beam profile in a channel of high-power neodymium facilities

A system for one-dimensional spatial profiling of a laser beam is suggested, capable of compensating for the spatial laser-beam distortions that arise due to a nonuniform gain distribution over the aperture in the amplifying channel of high-power Nd : glass lasers with wide-aperture stages on disk active elements. The principle of operation, the approach to calculation of the key element parameters, and calculation and experimental results of studying the formation of spatial profiles of the laser beam intensity at the output from the system in question are described. Possible applications of the system both in single-beam and multi-beam optical schemes are considered.

Computationally intelligent pulsed photoacoustics

In this paper, the application of computational intelligence in pulsed photoacoustics is discussed. Feedforward multilayer perception networks are applied for real-time simultaneous determination of the laser beam spatial profile and vibrational-to-translational relaxation time of the polyatomic molecules in gases. Networks are trained and tested with theoretical data adjusted for a given experimental set-up. Genetic optimization has been used for calculation of the same parameters, fitting the photoacoustic signals with a different number of generations. Observed benefits from the application of computational intelligence in pulsed photoacoustics and advantages over previously developed methods are discussed, such as real-time operation, high precision and the possibility of finding solutions in a wide range of parameters, similar to in experimental conditions. In addition, the applicability for practical uses, such as the real-time in situ measurements of atmospheric pollutants, along with possible further developments of obtained results, is argued.

Characterization of laser ablation of copper in the irradiance regime of laser-induced breakdown spectroscopy analysis

The LIBS signal depends both on the ablated mass and on the plasma excitation temperature. These fundamental parameters depend in a complex manner on laser ablation and on laser–plasma coupling. As several works in the literature suggest that laser ablation processes play a predominant role compared to plasma heating phenomena in the LIBS signal variations, this paper focuses on the study of laser ablation. The objective was to determine an interaction regime enabling to maximally control the laser ablation. Nanosecond laser ablation of copper at 266nm was characterized by scanning electron microscopy and optical profilometry analysis, in air at 1bar and in the vacuum. The laser beam spatial profile at the sample surface was characterized in order to give realistic values of the irradiance. The effect of the number of accumulated laser shots on the crater volume was studied. Then, the ablation crater morphology, volume, depth and diameter were measured as a function of irradiance between 0.35 and 96GW/cm². Results show that in the vacuum, a regular trend is observed over the whole irradiance range. In air at 1bar, below a certain irradiance, laser ablation is very similar to the vacuum case, and the ablation efficiency of copper was estimated at 0.15±0.03atom/photon. Beyond this irradiance, the laser beam propagation is strongly disrupted by the expansion of the dense plasma, and plasma shielding appears. The fraction of laser energy used for laser ablation and for plasma heating is estimated in the different irradiance regimes.

Spatial laser beam determination by pulsed photoacoustics: detection radius/signal wavelength approximation

Utilizing a mathematical algorithm developed for photoacoustic tomography, it is possible to deduce the laser beam spatial profile within pulsed photoacoustic measurements. This method is based on the temporal shape of the photoacoustic signal analysis. The exact solution for the laser beam spatial profile obtained with this method involves summation of a series and may take much time to compute. Sometimes the simplification of this solution can be done if the detection radius is much larger than the photoacoustic signal wavelength. This simplification is the so-called detection radius/signal wavelength approximation. Our numerical results obtained with this approximation are presented here. These results are compared with the one obtained using a different method and an exact solution. Experimental conditions for photoacoustic signal generation used in our numerical analysis correspond to the multiphoton absorption in SF6–Ar mixtures. Together with the laser beam spatial profile, the molecular vibrational to relaxational time of the excited molecules is calculated simultaneously. Application of neural computation and genetic optimization in pulsed photoacoustics is also discussed.

Genetic Algorithms Application for the Photoacoustic Signal Temporal Shape Analysis and Energy Density Spatial Distribution Calculation

Recently, a few numerical methods based on the photoacoustic (PA) signal temporal shape analysis and energy density spatial distribution calculation, which is directly related to the laser beam spatial profile, have been presented. It has been shown that these methods allow a precise reproduction of the spatial profile and the radius of the laser beam, determining the vibrational-to-translational (V–T) relaxation time with good accuracy. Their applicability has been shown and confirmed for the analysis of an arbitrary symmetric laser beam spatial profile in cylindrical geometry. Here, the application of genetic optimization for solving the problem of a simultaneous laser beam spatial profile and V–T relaxation time determination by pulsed PAs is presented. Real-coded genetic algorithms are used to calculate the mentioned relaxation time by fitting the experimental signal \({\delta }{p}(\mathbf{\textit{r} }, t)\) with the theoretical one. The aim is to find combinations of PA signal parameters, namely, the radius of the laser beam and the V–T relaxation time that provide the best match with the given signal. A calculated PA signal with a known profile is used to simulate an experimental signal, and the sum of the square deviations representing deviations of the given and fitted signals is minimized by means of genetic optimization. In that way, the genetic algorithms are used to simultaneously estimate the radius of the laser beam and the V–T relaxation time efficiently and with high accuracy. Compared to previous methods, the presented method is much simpler and requires less time to compute.

Neural Networks-Based Real-Time Determination of the Laser Beam Spatial Profile and Vibrational-to-Translational Relaxation Time Within Pulsed Photoacoustics

This paper concerns with the possibilities of computational intelligence application for simultaneous determination of the laser beam spatial profile and vibrational-to-translational relaxation time of the polyatomic molecules in gases by pulsed photoacoustics. Results regarding the application of neural computing through the use of feed-forward multilayer perception networks are presented. Feed-forward multilayer perception networks are trained in an offline batch training regime to estimate simultaneously, and in real-time, the laser beam spatial profile (profile shape class) and the vibrational-to-translational relaxation time from given (theoretical) photoacoustic signals. The proposed method significantly shortens the time required for the simultaneous determination of the laser beam spatial profile and relaxation time and has the advantage of accurately calculating the aforementioned quantities.

Initial Second Harmonic Generation in Narrowband Surface Waves by Multi-Line Laser Beams for Two Kinds of Spatial Energy Profile Models: Gaussian and Square-Like

Abstract Acoustic nonlinearity of surface waves is an effective method to evaluate the micro damage on the surface of materials. In this method, the   (magnitude of the fundamental wave) and   (magnitude of the second-order harmonic wave) are measured for evaluation of acoustic nonlinearity. However, if there is another source of second-order harmonic wave other than the material itself, the linear relationship between   and   will not be guaranteed. Therefore, the second-order harmonic generation by another source should be fully suppressed. In this paper, we investigated the initial second-order harmonic generation in narrowband surface waves by multi-line laser beams. The spatial profile of laser beam was considered in the cases of Gaussian and square-like. The temporal profile was assumed to be Gaussian. In case of Gaussian spatial profile, the generation of the initial second-order harmonic wave was inevitable. However, when the spatial profile was square-like, the generation of the initial second-order harmonic wave was able to be fully suppressed at specific duty ratio. These results mean that the multi-line laser beams of square-like profile with a proper duty ratio are useful to evaluate the acoustic nonlinearity of the generated surface waves.

Computational intelligence based simultaneous determination of the spatial profile of the laser beam and vibrational-to-translational relaxation time by pulsed photoacoustics

This paper is concerned with the possibilities of computational intelligence application for simultaneous determination of the laser beam spatial profile and vibrational-to-translational relaxation time of the polyatomic molecules in gases by pulsed photoacoustics. Results regarding the application of neural computing and genetic optimization are presented through the use of feed forward multilayer perception networks and real-coded genetic algorithms. Feed forward multilayer perception networks are trained in an offline batch training regime to estimate simultaneously, and in real-time, laser beam spatial profile R(r) (profile shape class) and vibrational-to-translational relaxation time ?V?T from a given (theoretical) photoacoustic signals ?p(r,t). The proposed method significantly shortens the time required for the simultaneous determination of the laser beam spatial profile and relaxation time and has the advantage of accurately calculating the aforementioned quantities. Real coded genetic algorithms are used to calculate ?V?T by fitting the ?p(r,t) with the theoretical one. The previously developed methods determine the laser beam profile and relaxation time with sufficient precision, but the methods based on the application of artificial intelligence are more suitable for practical applications, such as the real-time in-situ measurements of atmospheric pollutants.

Optimization of the pumping laser beam spatial profile in the MEOP experiment performed at elevated3He pressures

The efficiency of the metastability exchange optical pumping (MEOP) performed at elevated 3 He gas pressures in high magnetic field was improved by modifying the spatial profile of the pumping laser beam in such a way that it matched the nonuniform distribution of plasma in the optical pumping cell. Comparing to a conventional Gaussian profile, at the 3 He gas pressure equal to 267 mbar, both the achieved nuclear polarization (equal to 26%) and the total magnetization in the cell (equal to 1.37 sccfp) are 60% higher when the annular laser beam profile is applied.

Laser beam spatial profile determination by pulsed photoacoustics: exact solution

The pulsed photoacoustic spectroscopy technique is one of the oldest but respectable techniques which offer some unique features that are relevant to trace gas monitoring. Despite obvious advantages there are some drawbacks that have so far alienated this spectroscopy technique from routine air monitoring. One of them is the additional instrumentation necessity to fulfill the requirement for an accurate knowledge of the spatial and temporal profiles of the excitation light source. Here we present one method for a laser beam spatial profile determination and simultaneous vibrational–translational relaxation time calculation. It is based on the temporal shape analysis of the photoacoustic signals. Symmetric laser beam spatial profiles are obtained with a high level of accuracy. Even though this method does not work in real time, it gives a good basis for future beam profile investigations. Experimental signals used in our analysis are obtained after infrared multiphoton absorption in the SF6–Ar mixture.

Determination of the laser beam spatial profile by pulsed photoacoustics

Pulsed photoacoustics should fulfil some of the requirements to be a powerful and precise trace gas detection technique. Beside the high sensitivity and selectivity, large dynamic range and good temporal resolution, pulsed photoacoustics has the potential to measure numerous parameters of the laser beam spatial and temporal characteristics with one or no additional instruments. The different numerical methods can be distinguished for this purpose with respect to the measuring procedure and result analysis. In the following article the numerical method for the laser beam spatial profile determination will be presented. Simultaneously this method allows calculation of the molecular vibrational to translational relaxation time in different gas mixtures.

Maximizing the brilliance of high-order harmonics in a gas jet

We have measured the wave front of high-order harmonics generated in a gas jet with a Hartmann sensor. With this setup we have investigated the influence of typical adjustable parameters such as gas pressure and focal position as well as the spatial laser beam profile on the conversion efficiency and the extreme ultraviolet (XUV) beam profile. Independent of the control parameter we always observed the highest conversion efficiency in connection with the best beam profile, i.e. the highest flux corresponds also to the highest brilliance. Furthermore, we have shown that aberrations of the fundamental laser beam are not simply imprinted onto the XUV beam.

Controlling quantum dynamics regardless of laser beam spatial profile and molecular orientation

In a typical experiment aiming to control quantum dynamics phenomena, each molecule experiences the same temporal laser field, but with an amplitude that depends on the spatial location and orientation of the molecule in the laser beam. It is proved under commonly arising conditions that at least one optimal laser field exists which will control all molecules in the sample, regardless of their orientation or spatial location. The optimal laser field may consist of a multipolarization control containing up to three orthogonal, independently shaped components. The analysis also includes the prospect of multipartite control where the field couples distinct groupings of states (e.g., multiple vibronic states), but without direct coupling within a group of states. This conclusion shows that achieving quantum control is not a matter of striking a compromise over the sample diversity, but rather a task subject to optimization to reach the highest possible level of control for all molecules in the sample.