Laser Profile Research Articles

High dynamic range, large temporal domain laser pulse measurement

Temporal pulse profile characterization is necessary to ensure and quantify the quality of short pulse laser systems. Yet it remains challenging to measure the temporal behavior of a pulse in all of its comprehensiveness. In this manuscript we present results which encourage to perform more ambitious pulse characterizations with optimized scanning cross-correlators. Several temporal laser pulse profile measurements in multiple nanosecond time scale with high dynamic range are shown. The measurements were taken by our in-house third-order cross-correlator EICHEL (Schanz et al. in Opt Express 25:9252, 2017), which is able to resolve the intensity dynamics down to the level of amplified spontaneous emission. With this device we show for the first time the onset of the plateau of the amplified spontaneous emission in the laser profile and investigate the origin of several side-pulses created early in the laser system.

Comparisons of laser-produced plasma in atmosphere between fiber-delivery and direct-focusing laser pulse

The evolution of the fiber-delivery laser-produced plasma induced by a single-lens laser probe and a double-lens laser probe were compared to the direct-focusing laser-produced plasma. It is found that the laser profile of the focused spot emitted by the fiber was uniform, showing a top-hat profile, with significant energy dispersion (standard deviation was increased to 1.2 from 0.76 and peak irradiance was decreased to 18.6 GW/cm2 from 28.2 GW/cm2). Using the method of photography with an effective 3-ns exposure time, the fiber-delivery laser-produced plasma displayed obvious changes in plasma morphology from a hemispherical form to a flat hemispherical form, even an oblate-cylinder form. Via optical emission spectroscopy, it was obvious the duration of the N II emission was shortened by 50–120 ns, and the axial diffusion distance was also reduced by 0.2 mm. By combining shadow diagnostics and fast photography, it appeared clear that the shock wave was slowed down to 1.9–2.1 km/s, and was separated from the plasma plume with a longer delay of 500–600 ns. In addition, the fringe shift profiles recovered from Mach–Zehnder interferograms showed a lower electron density inside the plasma plume and a weaker degree of air disturbance at the shock wave position, consistent with the other diagnosis methods. These different evolutionary characteristics in the case of the fiber-delivery mode clearly indicated that the peak irradiance would also have a pronounced effect on the plasma process in addition to the average irradiance. This phenomenon could be related to the reduction in energy deposition of laser absorption by plasma for the fiber-delivery mode, and this reduction was caused by the decreased peak irradiance in the focused spot.

A Simple Automated Laser Profile Meter

Core Ideas The proposed laser profile meter is simple yet offers good accuracy and precision. Data obtained by the profile meter compared well with those obtained by lidar. Profile meter provides a practical alternative to more expensive or operationally complex systems. An automated profile meter with vertical precision of ±0.2 mm was built to measure soil surface elevations. It consists of a laser distance meter mounted on an automated linear motion system. Operation and data logging is conducted wirelessly using a Bluetooth enabled mobile device. The profile meter performed well under various lighting conditions and on a variety of natural materials, with the exception of specular surfaces. It is inexpensive (<$1500), consists of readily available components, portable, and easy to setup in the field (<10 min). When measuring soil random roughness the profile meter was compared with a ground based terrestrial Light detection and ranging (lidar). The lidar systematically overestimated random roughness by approximately 15% compared with the profile meter.

Single-mode semiconductor lasers fabricated by standard photolithography for direct modulation.

The output characteristics, modal properties, far-field profiles, and dynamic modulation responses of semiconductor lasers with surface higher-order gratings fabricated by the standard photolithography are presented. Single-mode semiconductor lasers with 20th- and 37th-order gratings for the 1.55 µm wavelength range are realized. The single-mode semiconductor lasers with 20th-order gratings have lower threshold currents and higher slope efficiencies than those with 37th-order gratings. The surface higher-order grating placed closed to the output facet can deteriorate the vertical far-field profile of the semiconductor laser. However, the properties of the semiconductor laser's single-mode operation are not affected by the surface higher-order grating's position in the ridge waveguide. The -3 dB bandwidth of these single-mode semiconductor lasers can achieve 9 GHz at 100 mA, which is the highest, to the best of our knowledge, for such a kind of single-mode semiconductor laser with a surface higher-order grating.

Selectively mode-locked high-order-mode 2-[formula omitted]m thulium fiber laser

We report a high-power 2μm higher-order-mode thulium fiber laser passively mode-locked with a semiconductor saturable absorber mirror (SESAM). Based on both spatial-mode discrimination through tuning the SESAM’s feedback and spatial-mode-dependent spectral selection of a few-mode fiber Bragg grating (FBG), the fiber laser is stably mode-locked in the LP11 mode with average output power of 672 mW. The laser pulse has maximum pulse energy of 38.8 nJ with pulse duration of 3.93 ps, giving rise to peak power of 9.87 kW. In the LP11 mode-locked state, the laser wavelength has a single peak and is centered at 1939.5 nm with a spectral full width at half maximum (FWHM) of ∼2 nm. The laser beam quality M2 factor is measured to be 3.23 and 2.41 in the x and y directions, respectively. The LP11-mode-locked state is confirmed by both the spectral feature and beam profile of the output laser. This mode-locked higher-order-mode fiber laser thus provides a new and simple method to achieve mode-selective picosecond pulse generation from multimode fiber lasers.

Weld Bead Detection Based on 3D Geometric Features and Machine Learning Approaches

Weld bead detection is essential for automated welding inspection processes. The non-invasive passive techniques, such as photogrammetry, are quickly evolving to provide a 3D point cloud with submillimeter precision and spatial resolution. However, its application in weld visual inspection has not been extensively studied. The derived 3D point clouds, despite the lack of topological information, store significant information for the weld-plaque segmentation. Although the weld bead detection is being carried out over images or based on laser profiles, its characterization by means of 3D geometrical features has not been assessed. Moreover, it is possible to combine machine learning approaches and the 3D features in order to realize the full potential of the weld bead segmentation of 3D submillimeter point clouds. In this paper, the novelty is focused on the study of 3D features on real cases to identify the most relevant ones for weld bead detection on the basis of the information gain. For this novel contribution, the influence of neighborhood size for covariance matrix computation, decision tree algorithms, and split criteria are analyzed to assess the optimal results. The classification accuracy is evaluated by the degree of agreement of the classified data by the kappa index and the area under the receiver operating characteristic (ROC) curve. The experimental results show that the proposed novel methodology performs better than 0.85 for the kappa index and better than 0.95 for ROC area.

Investigation of the Profile of Laser Bends with Variable Scan Distance

Laser forming is a process capable of bending sheet metal only with the laser beam and without a physical interaction with the part. The part is formed by small and local deformations by the laser beam and it is done in increments with each pass of the laser. This incremental forming makes it difficult to ensure a smooth deformation field, which can result in non-smooth components and difficulties in achieving the required tolerance of the bends. This work presents an experimental investigation of the effect of varying the scan distance, number of passes per scan line and number of scan lines on the profile of a v-bend. The v-bends were measured on both sides by a laser range scanner to determine the surface profile along the bend. To evaluate the quality of the v-bend and define the profile as discrete or continuous, the bend roundness was evaluated manually and automatically by a Least Squares Circles method and by using second order differentiation method. The effect of varying the scan distance was tested experimentally on five samples with 16 mm and five samples with 20 mm bend length. The parts were subsequently evaluated on both the inside and outside of the bend and the bend angle was measured. The results show good correlation between the manual inspection and the automatic examination. The results show that the overlapping of the laser beam paths is required to create a continuous smooth bend.

Correlation between functional and structural properties of flexible pavement

The functional properties and structural properties are very important in order to evaluate and determine the pavements performance. In pavement condition assessment, monitoring and measurement on the functional and structural properties of the pavement are really needed in order to determine the remaining life of the pavement before any rehabilitation work can be done. This study had further looked on the correlations between these functional and structural properties on a flexible pavement. The study had collected 1198 numbers of functional properties data namely International Roughness Index (IRI), rut depth and texture depth. While for structural properties, 120 numbers of elastic modulus and 12 numbers of California Bearing Ratio (CBR) data has been measured. The functional properties were measured by using Multi Laser Profiler (MLP), while the structural properties were collected by using Falling Weight Deflectometer (FWD) and Dynamic Cone Penetrometer (DCP). The data collection was done along 60 km of asphalt pavement road. Regression analysis shows that the coefficient of determination, R2 obtained for all parameter is near to 0. These results generally conclude that there are no correlations between the functional properties and structural properties on a flexible pavement.

Influence of pavement condition towards accident number on Malaysian highway

Accidents in Malaysia are alarming and have been in the rise for the past decades. Increasing numbers of road users, condition of road surface, expanded road network and weather conditions are being carefully looked into determining the contributors towards fatality. Human factors has been widely known and established as one of the main contributor to road accidents. Therefore, this study had focused more on to pavement conditions, where its relation with accidents needs to be further explored. The purpose of this study is to determine the influence of pavement condition towards accident number on Malaysian Highway. Pavement condition data which consist of Mean Texture Depth (MTD), Rutting Depth (RD) and also International Roughness Index (IRI) were measured along 265 km southern stretch of Malaysia highway, by using Multi Laser Profiler (MLP). Accident data were also collected from the accident report prepared by Southern region of Malaysian Highway Authority. Based on the results, it was found that more than 70% of the accidents happened at a good condition of road which having high MTD, low RD and minimum IRI. MTD was found as the most significance parameters that relates to accident. Other factors such as human negligence and environments (weather and lighting), might be the major cause that contribute to the accidents that happened along Malaysian highway.

HIGH-RESOLUTION TSUNAMI-BEDLOAD COUPLED COMPUTATION IN AMR ENVIRONMENT

Conventional tsunami computations on coarser grids have employed Manning’s friction coefficients of subgrid equivalent roughness for buildings, vegetation and public facilities (roads, dikes and so on), depending on land-use at the grid location. This equivalent roughness macroscopically models to integrate all effects of resistances against the flow within the computational cells; that is, drag force and pressure reduction behind structures in addition to wall roughness defined in turbulent boundary layer theory. Recently high-resolution land elevation data (2-m resolution), measured by an aerial laser profiler, has been used for computing local inundation of tsunami flood. Since the high-resolution data resolves major buildings and facilities, the mechanical contributions of the structures, such as drag and pressure reduction, are included in the computed result. In this case, conventional equivalent friction may be unacceptable to use.

Analysis and Evaluation of Tunnel Contour Quality Index

Abstract Overbreak, which is inevitably caused by geological and operational conditions in a drill & blast tunnel and is one of elements consisting of tunnel contour quality, has been a major cause to increase construction time and costs. Thus, many attempts have been made to reduce it worldwide. Against this backdrop, this paper suggests Tunnel Contour Quality Index (TCI) established based upon laser profiling technique for the effective management of tunnel contour quality, and analyzes TCI as well as the factors on TCI through field investigations using laser profilers in Norway and Korea. TCI is defined as a fraction form consisting of three elements (overbreak depth, contour roughness, longitudinal overbreak variation) that are calculated from surveyed tunnel profiles, three weights for the elements and one constant for range adjustment. The result shows a large difference in TCI, i.e., TCI for the Korean tunnels is average 63.4 by overbreak depth of 21.4 cm, contour roughness of 1.066 (meaning a 6.6% increase than the planned contour area by blasting) and longitudinal overbreak variation of 5.7 cm, while TCI for the Norwegian tunnels is average 45.9 by 64.2 cm, 1.120 and 11.1 cm respectively. The main causes for the large difference are the different philosophy in tunnel design between the two countries, as well as the hard rock having distinct joints of Norway that makes it easier for blasting pressure to break off rock masses along existing joints than to develop new cracks in the hard rock. In addition, it is found that the major element affecting TCI is overbreak depth because large overbreak depth increases the possibility of high contour roughness. The results show that most overbreak primarily results from large look-out and poor rock masses, and major causes resulting in high contour roughness are heterogeneous geological conditions in the tunnel face and big deviation in drilling position of contour holes. Finally, it is thought that TCI established based upon laser profiling technique can be used for the more effective management of tunnel contour quality.

An upgraded LIBS system on linear plasma device PSI-2 for in situ diagnostics of plasma-facing materials

Laser induced breakdown spectroscopy (LIBS) is used to detect and monitor the D and H content on the W surface due to its capability of fast direct in situ measurement in extreme environment (e.g., vacuum, magnetic field, long distance, complex geometry). A new LIBS system has been set up on the linear plasma device PSI-2 to perform in situ, real-time and remote monitoring of deuterium retention on the target surface. The new system is an upgraded version of the previous LIBS system [1] and is improved in several ways. The laser profile is improved and better focused into the chamber, the emission light collecting optics are more efficient. In addition, the timing and triggering system is updated and two different spectrometers are optimised for blue and red regime on the spectrum respectively. One of the spectrometers with Littrow configuration is dedicated to the detection of hydrogen and deuterium Balmer-alpha lines. A number of key parameters including laser energy, the (de)focusing of the laser beam, the delay time of image intensifier (MCP) are investigated and improved for the new system. The measurement by the new LIBS system is then compared to thermal desorption spectroscopy (TDS) and nuclear reaction analysis (NRA) results. The overall LIBS sensitivity and measurement accuracy are greatly improved, the time evolution of D, H content on W samples is studied before, during and after plasma-off point in the scales of seconds, minutes and hours.

Role of the temporal profile of femtosecond lasers of two different colours in holography

The role of the temporal profile of femtosecond lasers in obtaining the holograms is uncovered when two lasers of different colours are used to produce the fringes and recording of holograms. It was shown that the contrast of the time average interference pattern of two short light pulses (pulse duration T) of different colours is remarkably high for frequency detuning (see Odoulov S. et al., Nat. Commun., 6 (2015) 5866). However, we prove that such a continuous range of frequency detuning is not possible but null regions develop and frequency bands are observed for realizing holograms if there is a deviation of the lasers from their Gaussian profiles. The increased pulse duration leads to the generation of additional frequency bands and their narrowing along with higher modulation depth. A shifting of the null regions is noticed for the lasers having different skewness and/or pulse duration.

Validation of Sea-Ice Topographic Heights Derived From TanDEM-X Interferometric SAR Data With Results From Laser Profiler and Photogrammetry

In this paper, the retrieval of sea-ice surface heights from the interferometric TanDEM-X data is investigated. The data were acquired over fast and drifting ice in Fram Strait located between Greenland and Svalbard. Additional measurements of the sea-ice surface topography were carried out using a stereo camera and a laser altimeter. The comparison of the surface elevation retrieved from TanDEM-X imagery with the results of the stereo camera measurements revealed that sea-ice ridges greater than 0.5 m can be estimated with a root-mean-square error of 0.3 m or less with the error decreasing as a function of ridge height. Although the helicopter-borne laser data are only available as 1-D profiles with a much higher across-track spatial resolution than the TanDEM-X data, they proved to be useful for the validation. The need for multilook averaging to reduce the phase noise is identified as the main challenge in achieving the spatial resolution necessary for retrieving sea-ice surface topography using synthetic aperture radar interferometry.

Real-Time Geometric Parameter Measurement of High-Speed Railway Fastener Based on Point Cloud from Structured Light Sensors.

With the increase in the number of service years for high-speed railways, the foundation of the rail track suffers from settlement, which causes rail track irregularity. To adjust the position of the track and meet track regularity demands, several components of the fastening system will be replaced by different sized components. It is important to measure the exact geometric parameters for the components of a fastening system before adjusting the track. Currently, the measurement process is conducted manually, which is laborious and error-prone. In this paper, a real-time geometric parameter measurement system for high-speed railway fastener based on 2-D laser profilers is presented. Dense and precise 3-D point clouds of high-speed railway fasteners are obtained from the system. A fastener extraction method is presented to extract fastener point cloud and a region-growing algorithm is used to locate key components of the fastener. Then, the geometric parameter of the fastener is worked out. An experiment was conducted on a high-speed railway near Wuhan, China to verify the accuracy and repeatability of the system. The maximum root-mean-square-error between the manual measurement and the system measurement is 0.3 mm, which demonstrates adequate accuracy. This system can replace manual measurements and greatly improve the efficiency of geometric parameter measurements for fasteners.

Automatic pavement defect detection using 3D laser profiling technology

Abstract Asphalt pavement defects e.g. cracks, potholes, rutting, often cause significant safety and economic problems, thus, to automatic detect these defects is vital for pavement maintaining and management. The fact that 3D defect detection methods is superior to traditional 2D methods and manual survey methods in terms of accuracy and comprehensiveness has been widely recognized. Based on 3D laser scanning pavement data, an automatic defect detection method is proposed to detect pavement cracks and pavement deformation defects information simultaneously in this paper. Specifically, a sparse processing algorithm for 3D pavement profiles is first designed to extract crack candidate points and deformations support points, these processing is based on the assumption that the cracks are microscopic local defects while deformations are macroscopic defects in profiles. Then, the crack information can be detected by combining the extracted candidate points and an improved minimum cost spanning tree algorithm. On the other hand, the deformation depth information is acquired based on the profile standard contours which are constructed by profile envelopes and deformation support points, the accurate location and classification information of deformation defects can be obtained by the regional depth property. Experimental tests were conducted using real measured 3D pavement data containing two categories of defects. The experimental results showed that, based on the 3D laser scanning data, the proposed method can effectively detect typical cracks under different road conditions and environments, with the detection accuracy above 98%. Furthermore, different types of deformation defects including potholes, rutting, shoving, subsidence, can also be accurately detected with location error less than 8.7%.

PUK20 - COST-UTILITY ANALYSIS OF EXTRACORPOREAL LITHOTRIPSY VERSUS URETERORENOSCOPY-HOLMIUM LASER FOR TREATMENT OF URINARY STONES

The objective od this study is to compare the cost-utility profile of ureterorenoscopy-holmium laser (URS) versus extracorporeal lithotripsy (ESWL) in the treatment of urinary stones smaller than 20mm. A total of 525 patients treated between 2012-2014 were included in the study. Of these, 229 (43.6%) were treated with URS and 296 (56.3%) with ESWL. For QALYs calculation, a sample of 78 patients (32 treated with ESWL and 46 with URS) were evaluated prospectively (January 2016-December 2016) using a quality of life survey (Euroqol 5QD-3L) before and after treatment. A Markov model (SWL + URS salvage vs primary URS) was constructed for the calculation of the quality of life in each possible state of the patient (lithiasis without double J stent, lithiasis with stent, free of lithiasis without stent and free of lithiasis with stent). A Monte Carlo simulation was performed as probabilistic sensitivity analysis to confirm the robustness of the results. Of the 525 patients, 345 (65.71%) were treated for renal lithiasis (41.7% by URS and 58.26% by SWL) and 180 for ureteral lithiasis (47.2% by URS and 52.7% by ESWL). Mean cost per patient treated with ESWL and URS 1.733,96 € and 2.337,31 € euros respectively. Renal stone-free rate was 91.7% for URS and 79.16% for ESWL and ureteral stone-free rate was 94% and 81%, respectively. The cost-utility analysis showed higher cost but incremental QALYS of 0.011 for URS, thus, the ICER was 50.829,82 €/QUALY, above the acceptability threshold. According to our results, ESWL showed to be a more efficient tool in cost-utility terms compared to URS in the treatment of reno-ureteral lithiasis smaller than 20 mm.

October 1993

Another manufacturing representative organisation; systems & commodity machines; NC program transfer developments; friction coating technology; aqueous degreasing on the up; laser profiling challenging punching; Vauxhall’s new V6 line

Applicability of Selective Laser Reflow for Thin Die Stacking

Abstract A 3-D packaging approach such as die stacking is an attractive way to package greater functionality and performance into a smaller footprint, often at a reduced overall product cost. Achieving such 3-D integration can however place significant demands on the manufacturing process, often requiring substantial production expense to bond multiple die in a sequential manner. One alternative, potentially offering significant throughput, is the use of selective laser reflow to bond stacked die. This process produces a localized heating of the stacked die sufficient to produce soldered interconnects at the bonding interface but with very short exposure times that minimize the heating of other parts of the assembly and decrease the overall duration of the bonding process. The use of a commercial infrared (IR) laser reflow instrument for the sequential attachment of thin die into a 3-D stack was considered. The bonding technology under study consists of an infrared laser coupled with a custom optics system that allows the laser beam to be shaped into a range of rectangular shapes and sizes. This allows for a single device or target area to be selectively heated and reflowed while other regions in the near vicinity are not. The target reflow area is exposed to a shaped laser beam for a combination of laser powers and exposure times according to the desired thermal profile. The top most materials absorb the IR energy of the incident laser and conduct that thermal energy down to the solder joints and into rest of the assembly. The rates of heat transfer realized depend significantly upon the sample geometry and material composition. Since this produces a highly accelerated reflow, typically 2–8 seconds, the entire process is highly transient in nature. This paper reports on the investigation into the utility of a selective area laser reflow process to sequentially bond thin die into a multi-chip stack. Thin silicon die (~60μm thick) were bonded to a 150μm thick laminate substrate using a selected area laser reflow process. The die interconnects were formed through the soldering of SnAg capped copper pedestals under the local laser heat. First level die were placed onto copper pads on the laminate and then bonded using the laser process. Subsequent die (2nd level) were placed on and bonded to gold plated nickel pad sites on the top of the first level die. At each step of the process the quality of the solder interconnects was examined using optical and electron microscopy. A range of laser profiles were utilized with several different thermal parameters: peak temperatures ranging from 235–255°C, time above liquidus between 1–3 seconds, and a fixed cooling rate of 40°C/s. Control samples of stacked die were assembled using conventional manufacturing techniques to enable direct comparisons of the interconnects and die bonding characteristics.

Laguerre-Gaussian and beamlet array as second generation laser heater profiles

The microbunching instability is known to be detrimental to x-ray free electron laser performance. At the Linear Coherent Light Source, the microbunching instability is suppressed with a laser heater, which increases the uncorrelated energy spread of e-beam in the injector. While the current system has been shown to improve x-ray brightness, other laser architectures could further enhance performance. In this study, we model the interaction between a laser and e-beam with arbitrary transverse profiles and examine the effect of various laser designs on the energy distribution of the electrons after the injector and laser heater, as well as their ability to suppress microbunching instability. This simulation incorporates random transverse jitter in order to reproduce physically representative operation of the Linac Coherent Light Source. We compare Gaussian and Laguerre-Gaussian modes, and explore composite beams in the form of an array of Gaussian beamlets. We conclude that the Gaussian laser profile is highly susceptible to e-beam ellipticity and random transverse jitter. The Laguerre-Gaussian profile, a mathematically ideal solution to suppressing microbunching, is less susceptible to these effects and can provide effective suppression even with a distorted e-beam, though performance can be improved by increasing stabilization. The array of beamlets presents a solution that produces consistent and smooth energy distributions with significantly less variance in heating than the Laguerre-Gaussian profile.