Direction Of Laser Beam Research Articles

Precision absolute measurement and alignment of laser beam direction and position.

For the construction of high-precision optical assemblies, direction and position measurement and control of the involved laser beams are essential. While optical components such as beamsplitters and mirrors can be positioned and oriented accurately using coordinate measuring machines (CMMs), the position and direction control of laser beams is a much more intriguing task since the beams cannot be physically contacted. We present an easy-to-implement method to both align and measure the direction and position of a laser beam using a CMM in conjunction with a position-sensitive quadrant photodiode. By comparing our results to calibrated angular and positional measurements we can conclude that with the proposed method, a laser beam can be both measured and aligned to the desired direction and position with 10 μrad angular and 3 μm positional accuracy.

Observation and theoretical simulation of electromagnetically induced transparency and enhanced velocity selective optical pumping in cesium vapour in a micrometric thickness optical cell

Observation of electromagnetically induced transparency (EIT) and enhanced velocity selective optical pumping (VSOP) signals in a micrometric cell with cesium is reported. The line shape and non-linear features observed in the case of fluorescence in the direction parallel to the cell windows and the transmission spectra observed along the propagation direction of the probe beam show considerable differences in the spectral profile. A theoretical model based on five level optical Bloch equations is used to simulate the spectra. The Doppler convolution includes all possible orientations of atomic velocities with respect to the laser beam direction. Atoms moving nearly parallel to the windows and perpendicular to the collinear pump and probe beams have much lower Doppler shift and hence produce considerable narrowing of the Doppler background in the fluorescence spectra. The coherence decay rate is also low for such atoms as they do not meet with the cell walls. The simulated curves reproduce the observed sharp EIT peaks and enhanced broad VSOP signals for the closed probe transition in the fluorescence and absorption spectra. The observed effect of detuning of the pump frequency on the non-linear features is also reproduced by the simulation.

Understanding the deposition mechanism of pulsed laser deposited B-C films using dual-targets

Boron carbide thin films with stoichiometry (boron-carbon atomic ratio) range of 0.1 ∼ 8.9 were fabricated via pulsed laser deposition by using boron-carbon dual-targets. However, this experimental data on stoichiometry were smaller than the computer simulation values. The discrepancy was investigated by studies on composition and microstructure of the thin films and targets by scanning electron microscopy, excitation laser Raman spectroscopy, and X-ray photoelectron spectroscopy. The results indicate that the boron liquid droplets were formed by phase explosion after laser irradiation on boron sector. Part of the boron droplets would be lost via ejection in the direction of laser beam, which is tilted 45° to the surface of substrate.

A simplified procedure of metrological testing of the terrestrial laser scanners

This paper presents a simplified test procedure of the terrestrial laser scanners (TLS) accuracy investigation by the general criteria of the ISO17123 standard. The proposed procedure covers investigation, analysis and accuracy estimation of the TLS system in the controlled environmental conditions. This procedure is mainly developed based on the model of a system calibration and network design procedures. Metrological Laboratory of the Faculty of Civil Engineering in Belgrade is customized for TLS testing. The accuracy investigation of a commercial TLS has been performed. The accuracy of the resulted point cloud, which is indirectly represented by the coordinates of the acquired control points, is determined. Beside the analysis of the residuals of the 3D spatial transformation, the analysis of the true and the measured distance differences is also performed. The results show that these two approaches give the statistically same uncertainty indicators. Precision of the target recognition and influence of the unlevelled point cloud to the TLS centering error is also analyzed. Normality tests are used to determine whether a data set is well modeled by a normal distribution or not before testing of hypothesis for the variances. By varying the number of the control and check points it is shown that increasing the number of the control points more than three well distributed is not relevant for significantly higher transformation accuracy. On the basis of the conducted spherical orientation analyzes it can be concluded that the global orientation of the error vectors follows the laser beam direction due to relatively short distances. The most important result related to investigated TLS is that this particular sensor must be used only by indirect georeferencing experiment design.

Mid-IR beam direction stabilization scheme for vibrational spectroscopy, including dual-frequency 2DIR

A compact laser beam direction stabilization scheme is developed that provides the angular stability of better than 50 μrad over a wide range of frequencies from 800 to 4000 cm-1. The schematic is fully automated and features a single MCT quadrant detector. The schematic was tested to stabilize directions of the two IR beams used for dual-frequency two-dimensional infrared (2DIR) measurements and showed excellent results: automatic tuning of the beam direction allowed achieving the alignment quality within 10% of the optimal alignment obtained manually. The schematic can be easily implemented to any nonlinear spectroscopic measurements in the mid-IR spectral region.

‘Open-loop’ tracking interferometer for machine tool volumetric error measurement—Two-dimensional case

Abstract The tracking interferometer, or the laser tracker, is a laser interferometer with a steering mechanism to regulate the laser beam direction to follow a retroreflector (“target”). Applying the multilateration principle, it measures the target's three-dimensional position at an arbitrary location in the workspace. Its application to the volumetric accuracy measurement for coordinate measurement machines or machine tools has been long studied. In this paper, we propose the ‘open-loop’ tracking interferometer, where the laser beam is regulated toward the command target position. This eliminates the automated tracking mechanism and thus may significantly reduce the manufacturing cost of conventional tracking interferometers. The objective of this paper is to validate this ‘open-loop’ tracking interferometer concept by investigating its measurement uncertainty both experimentally and analytically. To simplify the problem, this paper focuses on the measurement of the target's two-dimensional position by using a single-axis ‘open-loop’ tracking interferometer prototype.

Neutronic investigations of a laser fusion driven lithium cooled thorium breeder

The paper investigates the main parameters of a Laser Inertial Confinement Fusion Fission Energy (LIFE) driven thorium breeder. A similar blanket to the (LIFE) engine design in Lawrence Livermore National Laboratory is chosen in order to allow mutual feedback between two geographically separated teams towards a more advanced and improved design under consideration of totally independent views. In the basic design, frozen (D,T) fusion fuel ice is shot to the center of 5 m diameter spherical fusion reactor chamber cavity in pulsed mode (10–30 Hz). Fusion fuel burns through direct or indirect laser beam irradiation. The first wall surrounds the fusion chamber and is made of S-304 steel (2 cm). The fusion reactor cavity is kept in high vacuum. It is followed by a natural lithium coolant zone. A 2nd S-304 layer (2 cm) separates the lithium zone on the right side from the graphite reflector (30 cm). The outer boundary of the graphite reflector is also covered with a 3rd S-304 layer (2 cm).The calculations have been performed for a fusion driver power of 500 MWth with the last available version of MCNP, namely with MCNPX-2.7.0. In the first calculation phase, the thickness of the natural lithium coolant-tritium breeder zone (ΔRLi) has been varied as 50, 60, 70, 80, 90 and 100 cm to select the coolant thickness ΔRLi to have a satisfactory tritium breeding ratio (TBR) for continuous fusion reactor operation. For a pure fusion blanket without any fissionable elements in the coolant, TBR values are calculated as 1.237, 1.312, 1.370, 1.415, 1.449 and 1.476, respectively, for corresponding coolant thicknesses. A ΔRLi value of 50 cm would keep TBR > 1.05 for self-sustaining tritium supply. These ΔRLi values lead to blanket energy multiplication values of M = 1.209, 1.216, 1.219, 1.222, 1.223 and 1.224, respectively, and have been calculated, as a result of exoenergetic neutron absorption in 6Li. For coolant thickness values >50 cm, the increase of “M” would remain minor.In the second phase, ThO2 has been suspended in the form of micro-size tristructural-isotropic (TRISO) particles in the lithium coolant for 233U breeding. TRISO fuel has the great advantage of high mechanical stability. Furthermore, fission products will be separated from the coolant. TRISO particles have been dispersed homogenously in the lithium coolant with volume fractions Vtr = 1, 2, 3, 4, 5 and 10 vol-%. Calculations with ΔRLi = 50 cm and by variable Vtr result with TBR = 1.229, 1.222, 1.214, 1.206, 1.1997 and 1.1622, respectively. Parasitic neutron absorption in Thorium decreases the TBR values. For Vtr < 5 vol-% TRISO in the coolant, the increase of the neutron absorption in thorium will be compensated to a great degree through neutron multiplications via 232Th(n,f) and 232Th(n,2n) reactions so that the sacrifice on TBR remains acceptable. However, for Vtr > 5 TRISO vol-%, neutron absorption in thorium reduces TBR drastically. On the other hand, the blanket energy multiplication M increases with thorium volume fraction, namely as M = 1.2206, 1.2322, 1.2426, 1.2536, 1.2636, 1.3112 for respective TRISO volume fractions due to the contribution of fission energy. Fissile fuel productions in the blanket are calculated as 17.23, 33.09, 48.66, 64.21, 79.77 and 159.71 233U (kg/year), respectively.

Direct on-strip analysis of size- and time-resolved aerosol impactor samples using laser induced fluorescence spectra excited at 263 and 351 nm

We report a novel atmospheric aerosol characterization technique, in which dual wavelength UV laser induced fluorescence (LIF) spectrometry marries an eight-stage rotating drum impactor (RDI), namely UV-LIF-RDI, to achieve size- and time-resolved analysis of aerosol particles on-strip. The UV-LIF-RDI technique measured LIF spectra via direct laser beam illumination onto the particles that were impacted on a RDI strip with a spatial resolution of 1.2mm, equivalent to an averaged time resolution in the aerosol sampling of 3.6h. Excited by a 263nm or 351nm laser, more than 2000 LIF spectra within a 3-week aerosol collection time period were obtained from the eight individual RDI strips that collected particles in eight different sizes ranging from 0.09 to 10μm in Djibouti. Based on the known fluorescence database from atmospheric aerosols in the US, the LIF spectra obtained from the Djibouti aerosol samples were found to be dominated by fluorescence clusters 2, 5, and 8 (peaked at 330, 370, and 475nm) when excited at 263nm and by fluorescence clusters 1, 2, 5, and 6 (peaked at 390 and 460nm) when excited at 351nm. Size- and time-dependent variations of the fluorescence spectra revealed some size and time evolution behavior of organic and biological aerosols from the atmosphere in Djibouti. Moreover, this analytical technique could locate the possible sources and chemical compositions contributing to these fluorescence clusters. Advantages, limitations, and future developments of this new aerosol analysis technique are also discussed.

Proposal of “open-loop” tracking interferometer for machine tool volumetric error measurement

The tracking interferometer, or the laser tracker, is a laser interferometer with a mechanism to control the laser beam direction to follow a retroreflector (“target”). Applying the multilateration principle, the target's three-dimensional position is measured. This paper proposes a novel concept of “open-loop” tracking interferometer, where the laser beam is controlled towards the command target position. Its advantage is in the elimination of the automated tracking mechanism, which may significantly reduce its manufacturing cost. The paper's emphasis is on analytical evaluation of its measurement uncertainty, introduced by the elimination of automated tracking mechanism. A prototype “open-loop” tracking interferometer is developed, and its measuring performance is experimentally investigated.

Efficiency improvement in MgO:PPLN based optical parametric generation using soft-edge top-hat transverse intensity profile for the pump beam

Pulsed nanosecond optical parametric generators (OPGs), in general, are pumped by a direct laser beams having Gaussian intensity profiles in temporal and spatial domains. This limits the conversion efficiency due to long build-up time of signal and idler intensities, and back-conversion effects. Efficiency optimization in an OPG involves reducing these effects; however, spatial and temporal distribution of pulse intensity across the pump beam limits the achievable conversion efficiency. We show that by using a spatially uniform intensity profile for the input pump beam, one can achieve significantly higher optimized efficiency because of uniform conversion over larger area of cross section of the pump beam. We present simulation results for MgO:PPLN based OPG with soft-edge top-hat (SETH) and Gaussian profiles which shows an improvement of about 27% in the optimized pump conversion efficiency, in the case of a pump beam with SETH transverse intensity profile. We also discuss the requirements for efficient conversion in an OPG using a SETH profile for the pump beam. Further, we present results of our study on the effect of pump pulse duration on the conversion efficiency for a given set of pump power and OPG parameters.

Inside Front Cover: LED pumped polymer laser sensor for explosives (Laser Photonics Rev. 7(6)/2013)

Schematic drawing of an organic semiconductor laser sensor for explosive vapors. The divergent incoherent light from an InGaN lightemitting diode is converted into a coherent, directional laser beam by a nanostructured polymer laser. This compact and low‐cost laser can be used to detect few‐parts‐per‐billion concentrations of nitroaromatic explosivemolecules, exploiting a photophysical interaction between the gainmedium and surrounding vapor.(Picture: YueWang et al.10.1002/lpor.201300072, pp. L71–L76, in this issue)

Effect of readout direction in the edge profile on the modulation transfer function of computed radiographic systems by use of the edge method

We investigated the effect of the readout direction of the edge profile obtained by the edge method on the presampled modulation transfer function (MTF) in various computed radiographic (CR) systems. There were no differences in the MTFs derived from two edge profiles in the sub-scanning direction of four CR systems used in this study. On the other hand, the MTFs measured at a readout direction from the low (edge) to the high (direct exposure) exposure region were higher than those measured at a readout direction from the high to the low exposure region in the laser-beam scanning direction for three of the four CR systems. Although this phenomenon depends on the CR system, it is important to understand and indicate both MTFs at the two edge profiles in the laser-beam scanning direction for accurate assessment of the resolution property.

Spatial emission profiles at different interface orientations in third harmonic generation microscopy

We present experimental investigations, accompanied by numerical simulations on third harmonic generation microscopy at interfaces. In particular we study the variation of the emitted third harmonic intensity profile with the interface orientation. Our data confirm previous theoretical predications that only at interfaces perpendicular to the direction of the fundamental laser beam can the generated third harmonic profile exhibit a single spot in the forward direction. At interfaces parallel with the direction of the fundamental beam, the third harmonic intensity profile moves outside the forward direction and develops into a double-spot beam with a large opening angle. As an important consequence for implementations of harmonic generation microscopy, the numerical aperture of the double-spot third harmonic beam exceeds the numerical aperture of the fundamental beam.

Built-Up Alignment System by Four-Quadrant Detector in High Power Laser System

Spatial stability is an important property of a laser beam for a laser system. Because four-quadrant detector has more high damage threshold than CCD, it can be applied in a high laser system to monitor the direction of laser beam. In this article, the four-quadrant detector as pointing monitor device is used for a 532nm semiconductor laser, which can obtain the center position of laser spot quickly. In the experiment, the amount of shift and jitter for laser beam is measured at first. Next step is combining rotating stage and lens to measure the alignment error for three angles. The results show that the top amount of shift and jitter is 3.6″ and the average of combined pointing error is around 6″. The data can support the spatial beam pointing stability system which needs feedback system with fast steering mirrors.

Precision absolute positional measurement of laser beams

We describe an instrument which, coupled with a suitable coordinate measuring machine, facilitates the absolute measurement within the machine frame of the propagation direction of a millimeter-scale laser beam to an accuracy of around ±4 μm in position and ±20 μrad in angle.

Fast dynamic interferometric lithography for large submicrometric period diffraction gratings production

This paper describes a successful technological transfer, from a state laboratory to an industrial company, for writing long and large submicron period gratings (potentially square-meter sized), implemented in an industrial direct laser beam writing equipment at 355 nm wavelength (Dilase 750 from KLOE SA company). The writing head, which has been inserted in the machine, consists of a phase mask which enables it to project a set of fringes pattern of small area onto a photoresist-coated substrate. As the substrate is continuously moving, one-dimensional or two-dimensional gratings can be fabricated over large areas limited only by the machine’s stage displacement range. The optical scheme is described from the beam shaping to the printing processes and the phase mask optimization. In order to demonstrate the technology, 600 nm period gratings of over 20 cm in length have been fabricated. Such submicron structures can be used in solar cell modules for photon trapping and as antireflection coatings.

Performance improvement of long-range scanning Mie lidar for the retrieval of atmospheric extinction

To improve the detection range and signal to noise ratio in the remote sensing of atmospheric processes using long-range scanning Mie lidar, simultaneously obtained analog (A) and photon-counting (P) data traces were merged using a novel gluing method based on the modification of the A trace so that it follows Poisson statistics. In comparison with the traditional method based on the minimization of the distance between A and P, the advantage of this new approach is an improved uniformity of the merged data trace as well as a reduced influence of atmospheric conditions on the obtained transfer coefficients. The atmospheric extinction was subsequently retrieved using the two- and multi-angle method. The offset of the laser beam direction was calibrated against a self-leveling laser, as an offset in elevation angle was found to produce considerable error in the atmospheric extinction profiles obtained under the assumption of horizontal atmospheric homogeneity.

Investigation on the Spatial Evolution of the Emission Spectra in Laser-Induced Ni Plasmas

The spatialresolvedemission spectrumof Ni atominlaserinduced Niplasma is measured in the wavelength region from 350 nm to 600 nm. The spatial evolution of the relative intensities and the Stark broadening of the 385.83 nm emission spectrum lines are also obtained. It is shown that Stark broadening and intensity of the spec- trum lines increases firstly to its maximum and then decreases along the direction of laser beam when the distance from the target surface is in the range from 0 to 2.5 mm. The maximum value of Stark broadening and relative intensity of the spectrum lines appear at 1.5 mm from the target surface.

Formation and applications of periodic structures in transparent materials induced by single fs laser beam

In this paper, we present our recent research development on single fs laser beam induced periodic structures, i.e. nanovoid array, tilted grating structures, nanograting etc, and discuss the mechanisms and promising applications of the observed periodic structures. Periodic structures can be used as grating and lens etc. Usually they are prepared by point-by-point or line by line laser scanning and using multi-beam laser interference technique. However, periodic nanovoid array was observed in transparent materials along the propagation direction of single fs laser beam. (1) By adopting a physical model which combines the nonlinear propagation of fs laser pulses with the spherical aberration (SA) effect at the interface of two mediums of different refractive indices, reasonable agreements between the simulation results and the experimental results were obtained. By comparing the fluence distributions of the case with both SA and nonlinear effects included and the case with only consideration of SA, we suggest that SA, which results from the refractive index mismatch between air and fused silica glass, is the main reason for the formation of the self-organized void array. (2) We observed self-organized microgratings in the bulk SrTiO3 crystal by scanning the laser focus in the direction perpendicular to the laser propagation axis. The groove orientations of those gratings could be controlled by changing the irradiation pulse number per unit scanning length, which could be implemented either through adjusting the scanning velocity at a fixed pulse repetition rate or through varying the pulse repetition rate at a fixed scanning velocity. (3)

2A1-J06 高速ビジョンを用いたレーザー描画システム(ロボットビジョン(1))

In this study, we developed a 1000-fps stereo laser tracking system that can simultaneously track a fast moving object and draw an arbitrary laser pattern on the surface of the tracked object. Our system can always control the direction of laser beam with a 2-DOF galvano-mirror along a given trajectory on the surface of the tracked object by processing stereo 512X512 images in real time at 1000 fps. The performance was demonstrated by showing an experimental result when a triangle pattern was laser-drawn on the surface of the tracked object moving back-and-forth in three dimensional space.