Laser Beam Alignment Research Articles

Conserved scalar measurements in turbulent diffusion flames by a Raman and Rayleigh ribbon imaging method

A new method to obtain images of conserved scalars in turbulent flames is presented and implemented with simultaneous Rayleigh and fuel Raman measurements in a methane/air jet diffusion flame by the use of a single dye laser and two intensified CCD cameras. The laser beam is focused to a line and retroreflected with a slight offset to form a thin ribbon, sufficient to measure gradients in two dimensions. A robust, iterative data reduction technique is used to derive statistics of temperature, fuel mass fraction, mixture fraction ( f), and scalar dissipation (χ). Results for a flame of Reynolds number 20,600 show that the lower moments, pdfs, and scatter plots of the computed quantities do not differ markedly from published results of point measurements in similar flames, strengthening confidence in this new approach. The computed components of χ show behavior similar to that in nonreacting flows; there is some anisotropy, with the ratio of the radial to the axial component in the shear region around 2.0. The azimuthal component, measured by off-axis laser beam alignment, is roughly equal to the radial component. The correlation between f and χ is small on the flame axis, but the correlation coefficient R fχ rises to around 0.4 near the edge, which is largely consistent with other recent results for cold jets and jet flames.

Optical “friction wheels”: asymmetric seeding of the TEM 01 hybrid mode of a HeNe laser

It is shown that a helical wave laser injected by a helical wave emits the helicity of the injected wave for coaxial alignment of laser beam and injection beam; but the opposite helicity for a lateral displacement of laser- and injection beam. This corresponds to the motion of a mechanical rotating disk coupled to another disk by friction.

Fuzzy logic and control. Software and hardware applications

1. Introduction. 2. Set Theory - Fuzzy and Crisp Sets. 3. Propositional Calculus - Predicate Logic and Fuzzy Logic. 4. Fuzzy Rule-Based Expert Systems - I. 5. Fuzzy Rule-Based Expert Systems - II. 6. Fuzzy Logic Software and Hardware. 7. A Fuzzy Two-Axis Mirror Controller for Laser Beam Alignment. 8. Introduction of Fuzzy Sets in Manufacturing Planning. 9. A Comparison of Crisp and Fuzzy Logic Methods for Screening Enhanced Oil Recovery Techniques. 10. A Fuzzy Logic Rule-Based System for Personnel Detection. 11. Using Fuzzy Logic to Automatically Configure a Digital Filter. 12. Simulation of Traffic Flow and Control Using Fuzzy and Conventional Methods. 13. A Fuzzy Geometric Pattern Recognition Method with Learning Capability. 14. Fuzzy Control of Robotic Manipulator. 15. Use of Fuzzy Logic Control in Electrical Power Generation. 16. Fuzzy Logic Control of Resin Curing. 17. Fuzzy Logic Control in Flight Control Systems. 18. Tuning of Fuzzy Logic Controllers by Parameter Estimation Method. Subject Index. Author Index.

Development of a laser devolatilization gas chromatography/mass spectrometry technique for single coal particles

A CO 2 laser Py-GC/MS system capable of identifying substantial numbers of pyrolysis products from single coal particles in the 50-150-μm range is reported. Also, a specially designed two-wavelength radiation thermometer module with integral video microscope produces reliable temperature/time profiles of single coal particles during rapid laser heating. A novel microbeam footprint technique using thin quartz wafers in combination with videomicroscopy greatly facilitates laser beam focusing and alignment operations. Comparison of an EDB type particle levitator with an EM grid type particle support system reveals considerable advantages of the EM grid approach with regard to tar collection efficiency, particle position stability, particle visualization, and recoverability of residual char particles

Decentered ablation zones resulting from photorefractive keratectomy with an erodible mask

Four patients underwent photorefractive keratectomy for compound myopic astigmatism using the erodible mask. All achieved improvements in unaided visual acuities despite manifesting varying levels of decentration of the ablated zones. However, one patient had significant monocular diplopia because of the severity of the decentration. The problems with the present system include the use of a NON coaxial microscope; difficulty with obtaining and maintaining proper alignment of the excimer laser beam to the erodible mask and the central corneal zone; the inability to interrupt and restart the procedure due to the ablation of the reference marks on the mask; and patient discomfort. We suggest installation of the PMMA mask at the laser aperture in order to alleviate the above difficulties. In the meantime, use of a coaxial microscope, a limbal suction device for fixation, permanent reference marks on the mask and peribulbar anaesthesia may be helpful.

Laser beam welding of austenitic/ferritic dissimilar steel joints using nickel based filler wire

Laser beam welding of austenitic/ferritic dissimilar steel joints using a nickel based filler wire has been investigated. The feasibility of using the process to join these dissimilar steels was evaluated. Microstructural characterisation was performed using both optical and electron microscopy. The influence of the processing parameters, principally air gap width and beam alignment, on the microstructure, composition, and properties of the welds has been determined. In order to obtain a fully austenitic weld metal microstructure, an air gap 0·2 mm in width with a suitable wire feedrate is recommended, which avoids the unnecessary extra heat input and problems in controlling weld quality associated with a wider air gap. However, in some applications, the size of the air gap can be used to control the alloying effect of the filler wire and consequently to obtain the desired weld metal composition. Laser beam alignment plays an important role in processing. It can be aligned to a maximum of 0·25 mm on the stainless steel side without producing a significant change in the weld metal composition. However, beam alignment on the Cr–Mo steel side should not be used if martensite formation in the weld metals is to be avoided. Laser beam welding with nickel based filler wire shows potential for producing austenitic/ferritic dissimilar steel joints.MST/1731

Laser beam welding of austenitic/ferritic dissimilar steel joints using nickel based filler wire

Laser beam welding of austenitic/ferritic dissimilar steel joints using a nickel based filler wire has been investigated. The feasibility of using the process to join these dissimilar steels was evaluated. Microstructural characterisation was performed using both optical and electron microscopy. The influence of the processing parameters, principally air gap width and beam alignment, on the microstructure, composition, and properties of the welds has been determined. In order to obtain a fully austenitic weld metal microstructure, an air gap 0·2 mm in width with a suitable wire feedrate is recommended, which avoids the unnecessary extra heat input and problems in controlling weld quality associated with a wider air gap. However, in some applications, the size of the air gap can be used to control the alloying effect of the filler wire and consequently to obtain the desired weld metal composition. Laser beam alignment plays an important role in processing. It can be aligned to a maximum of 0·25 mm on the stainless steel side without producing a significant change in the weld metal composition. However, beam alignment on the Cr–Mo steel side should not be used if martensite formation in the weld metals is to be avoided. Laser beam welding with nickel based filler wire shows potential for producing austenitic/ferritic dissimilar steel joints.MST/1731

A fuzzy control approach to a two-axis mirror laser beam alignment system

Fuzzy logic and fuzzy expert control systems have been one of the most active research and development areas of artificial intelligence in recent years. Thanks to tremendous technical advances and industrial applications in Japan, fuzzy logic enjoys an unprecedented popularity. In this paper fuzzy logic is used to control a two-axis mirror gimbal for aligning a laser beam onto the center of a target (quadrant detector). A PC-based controller is implemented with a simple rule set to control and stabilize the device. A comparison between a non-fuzzy (full-state) proportional controller and the fuzzy controller was made. Experiments were performed to study the tracking characteristics and the effect of rule pruning on the system response. Considerable improvements on the overshoot and response time of the system under fuzzy control have been achieved. The paper presents the problem, hardware, software, and the experimental results.

Design and operation of the multipulse Thomson scattering diagnostic on DIII-D (invited)

This paper describes the design and operation of a 40 spatial channel Thomson scattering system that uses multiple 20-Hz Nd:YAG lasers to measure the electron temperature and density profiles periodically throughout an entire plasma discharge. As many as eight lasers may be fired alternately for an average measurement frequency of 160 Hz, or they may be fired in rapid succession (<10 kHz), producing a burst of pulses for measuring transient events. The high spatial resolution (1.3 cm) and wide dynamic range (10 eV–20 keV) enable this system to resolve large electron density and temperature gradients formed at the plasma edge and in the scrape-off layer during H-mode operation. These features provide a formidable tool for studying L–H transitions, edge localized modes (ELMs), beta limits, transport, and disruptions in an efficient manner suitable for large tokamak operation where shot-to-shot scans are impractical. The scattered light is dispersed by interference filter polychromators and detected by silicon avalanche photodiodes. Laser control and data acquisition are performed in real time by a VME-based microcomputer. Data analysis is performed by a MicroVAX 3400. Additional features of this system include real-time analysis capability, full statistical treatment of error bars based on the measured background light, and laser beam quality and alignment monitoring during plasma operation. Results of component testing, calibration, plasma operation, and error analysis are presented.

Laser beam alignment devices for ultrashort pulses

We describe two novel devices for optical beam alignment. One allows for a variable angle, fixed position of incidence upon a target. The other allows for a variable position, fixed angle of incidence upon a target. Cascaded together, these devices allow placement of a laser beam upon a sample with arbitrary angle and arbitrary position of incidence. These devices are particularly useful for experiments and applications involving ultrashort pulses since the time of flight through either of these devices is a constant independent of the angle of incidence and position of incidence.

Contributions to the technique and interpretation of the photothermal beam deflection experiment

Two aspects of the photothermal beam deflection experiment, which are essential for a quantitative interpretation and for an improved performance of the experiment are studied here: The alignment of the probe laser beam with respect to the pump beam and to the solid surface under investigation and, second, the smoothing of experimental data by convolution filtering.

Composition and Microstructure of Laser Beam Welds between Dissimilar Metals

An austenitic stainless steel AISI 304 and a low-carbon steel 1018 were laser welded in the vertical-up position to form a dissimilar metal joint, by varying the laser beam alignment, gap between the base metals, and filler metal (AISI 309). The welds were inspected visually and by optical microscopy, and composition profiles were measured using electron probe microanalysis. It was found that the laser welding produces a fairly inhomogeneous structure in the weld. This is revealed by both the microstructure and the composition profile. The average dilution was found to vary as a systematic function of laser beam alignment. Undesirable levels of dilution were identified as well as ranges of dilution which would provide acceptable weld metal properties.

Stabilization of laser beam alignment to an optical resonator by heterodyne detection of off-axis modes

We demonstrate a method for real time alignment of a Gaussian beam to an optical resonator. While thefrequency of a source laser is stabilized to a fundamental cavity mode resonance, phase modulation sidebands are applied at the off-axis mode frequencies. Asymmetrical transmission of the sideband at the frequency of each off-axis mode produces amplitude modulated optical signals and indicates the extent of the misalignments. Phase sensitive detection of these optical signals provides the error signals which are minimized by a control system that steers the input beam. In this way, optimum coupling of an injected source beam can be maintained to the fundamental mode of the resonator. This active alignment technique has demonstrated a sensitivity to tilts of 0.1 nrad/ radicalHz and to lateral beam displacements of 0.08 nm/ radicalHz in the ~1-Hz-1-kHz frequency range. These values correspond to 2 parts in 10(7) radicalHz for both the far-field divergence angle and the beam waist size. Such performance is within a factor of 2 of the shot noise limitation of the error signal measurement for a detected power of 160 microW.

Precision laser-ultrasonic velocity measurement and elastic constant determination

A laser-ultrasonic method to measure the acoustic velocities and the elastic constants of solid materials based on the crosscorrelation of successive echoes is presented. Measurements are performed in the thermoelastic regime or slight ablation regime, which permits simultaneous measurement of longitudinal shear velocities and an unlimited number of shots at the specimen. Diffraction corrections are calculated, and dispersion effects are evaluated by frequency analysis. The precision and accuracy of the technique are estimated in terms of sampling rate, thickness of the sample, signal-to-noise ratio, frequency bandwidth and laser-beam alignment. Examples of applications of the technique are given for PZT ceramic, metal-ceramic composites and single crystal germanium.

Rapid method for determination of isocenter of radiation gantry and alignment of laser beams for stereotactic radiosurgery.

A simple technique has been designed which identifies the gantry isocenter and aligns the laser beams to the isocenter of the gantry within 3-5 min with an error of less than 0.75 mm, depending upon the accuracy of gantry rotation.

Automatic laser beam alignment for a Thomson scattering system

A system for the automatic alignment of a pulsed ruby laser beam is under development for the Thomson scattering diagnostics of RFX, a large reversed-field pinch machine now under construction. In this experiment the laser will be 11 m from the machine and the beam alignment at the 750×4-mm scattering volume will be actively maintained to within 0.5 mm. The beam direction in space is measured in two reference planes fixed to the collection optics frame by means of two 80-mm2 fast quadrant photodiodes. A double-channel preamplifier is used for each quadrant in order to measure both the 30-ns FWHM ruby pulse and a low-power cw He–Ne beam propagating through the same optical path. Every time the main laser is fired, the relative direction of the two beams is determined so that the position of the He–Ne beam can be used for feedback control of the steering optics between plasma shots.

Photorefractive deflection technique using parallel and overlaping probe and pump beams

The present work provides an experimental study of a pulsed photorefractive technique to diagnose the interior of an irradiated region. Results obtained in a mixture of SF6 and Ar are presented to illustrate the experimental technique. Three distinct areas have been studied, (i) The two beams are parallel and their optical axes are coincident (collinear). At this particular position, the probe beam suffers a defocusing effect that is used to align collinearly two laser beams with a precision of ±3 μm. (ii) The two beams are parallel but slightly off axis. In this case, the probe beam suffers a deviation that is the sole result of a rarefaction in the irradiated region followed by the normal thermal equilibration. (iii) The two beams are parallel and slightly overlapping. The probe beam suffers a deviation similar to the preceding one. Moreover, it sees an initial outward-propagating acoustic (density) wave. It is in this particular region that the acoustic perturbation appears. Finaly, the gas mixture used under our experimental conditions is a good candidate for the collinear alignment of several laser beams. The only restrictions are that the pump laser must produce a beam with a symmetrical intensity profile, the other laser beams must be continuous-wave beams, and the gas mixture must be transparent to the latter.

Collinear alignment of different laser beams using the photoacoustic deflection technique

Collinear alignment of different laser beams using the photoacoustic deflection technique

An inexpensive, versatile sample illuminator for photo-CIDNP on any NMR spectrometer

Photochemically induced dynamic nuclear polarization (photo-CIDNP) of proteins requires the introduction of ca. l-7 W of 488 nm light directly into NMR samples (1). This has been previously accomplished by a number of configurations: directing the light beam via an optical bench and mirrors through the bottom of the supercon NMR probe to a flat-bottom NMR sample tube; or, alternatively, focusing the light through a cylindrical quartz rod with a 45” polished end to convey a diverged light beam to the side of a spinning sample tube in the rf coil (2). In both cases, costly probe modifications are necessary, including removal or alteration of the variable temperature control apparatus. Laser beam alignment (into a mirror at the magnet base) can also be awkward and time consuming. Lerman and Cohn (3) illuminated samples by inserting a coupled optical fiber directly into the NMR sample solution. Kaptein and Dykstra (personal communication) have fixed a prism at the end of an optical fiber and situated it between the rf coil windings of a modified Bruker 360 MHz proton probe. We report here an alternative modification utilizing fiber optics illumination. Coupling to the laser was accomplished with a Newport Corporation (Fountain Valley, Calif.) model F-LFI-CO-25 ($850.00) fiber optics coupler laser head mount (universally mounts to any laser bezel mount) with 25 ft of 200 p fiber in place of the standard 10 ft length and armored handpiece. The free output end was stripped free of cladding (l-2 cm length) and inserted into a 2 mm stem coaxial insert (Cat. No. 5203, Wilmad Glass Company, Inc., Buena, N.J.) which fits snuggly into a 5 mm sample tube (Fig. 1). Maximal illumination was observed when the fiber tip was positioned in the center of the rf coil region. The laser light probe was introduced by triggering an inexpensive solenoid mounted to the coupling head slide shutter under computer control. Light coupling was fine tuned with x, y, and z adjustments on the coupling head. For maximal efficiency the fiber must be cut perfectly flat and well polished. Coupling efficiencies of up to 90% were typically measured. In a nonspinning sample of 1 mA4 tyrosine (pH 7.0) and 0.4 r&4 ‘?N-carboxyethyllumiflavin, the tyrosyl 3,5 protons were enhanced (light intensity-dark intensity)/ (dark intensity) 1Zfold at 500 MHz on an NTC spectrometer with a Spectra Physics 165-9 argon ion laser operating at 7.0 W (all lines). Figure 2 displays the light-dark

Accurate (0.25 mrad) perpendicular alignment of a continuous-wave single-mode dye laser beam and an atomic beam

Two simple methods are described for an accurate perpendicular alignment of a laser beam and an atomic beam, which is required for a velocity independent, i.e., Doppler-free interaction, of the laser beam with the atomic beam. With the first method a free running single-mode cw dye laser beam and an atomic beam are aligned perpendicular with an accuracy of 0.25 mrad. This alignment takes typically 2 h. The second method needs a laser beam that is absolutely stabilized to the investigated atomic transition. With this method a 0.2-mrad accurate perpendicular alignment is obtained within only 5 min.