Showing papers on "Rayleigh length published in 2006"

Assessment of material damage in a nickel-base superalloy using nonlinear Rayleigh surface waves

Abstract: A reliable laser-based ultrasonic technique is developed to measure the second order harmonic amplitude of a Rayleigh surface wave propagating in metallic specimens. Rayleigh waves are experimentally generated with a wedge transducer and detected with a heterodyne laser interferometer. The capability of this system to measure the nonlinear contribution present in Rayleigh surface waves is demonstrated, and these results are interpreted in terms of a parameter developed for Rayleigh surface waves which corresponds to the nonlinear parameter of a longitudinal wave, β. The proposed measurement technique is used to assess damage in nickel-base high temperature alloy specimens, and the evolution of material nonlinearity under various loading conditions is quantitatively measured in terms of the increasing amplitude of the second order harmonic. These results show that there is a significant increase in the second order harmonic amplitude at monotonic tensile loads above the material’s yield stress, and that du...

Propagation of generalized vector Helmholtz-Gauss beams through paraxial optical systems.

Abstract: We introduce the generalized vector Helmholtz-Gauss (gVHzG) beams that constitute a general family of localized beam solutions of the Maxwell equations in the paraxial domain. The propagation of the electromagnetic components through axisymmetric ABCD optical systems is expressed elegantly in a coordinate-free and closed-form expression that is fully characterized by the transformation of two independent complex beam parameters. The transverse mathematical structure of the gVHzG beams is form-invariant under paraxial transformations. Any paraxial beam with the same waist size and transverse spatial frequency can be expressed as a superposition of gVHzG beams with the appropriate weight factors. This formalism can be straightforwardly applied to propagate vector Bessel-Gauss, Mathieu-Gauss, and Parabolic-Gauss beams, among others.

Enhanced angular domain imaging in turbid media using Gaussian line illumination

Abstract: Coherence or Time Domain Optical tomography within highly scattering media observes the shortest path photons over the dominant randomly scattered background light. Angular Domain Imaging employs micromachined collimators detecting photons within small angles of aligned laser light sources. These angular filters are micromachined silicon collimator channels 51 microns wide by 10 mm long on 102 micron spacing, giving an acceptance angle of 0.29 degrees at a CMOS detector array. Phantom test objects were observed in scattering media 5 cm thick at effective scattered to ballistic ratios from 1:1 to greater than 1E8:1. Line and space test objects detection limits are set by detector pixel size (5.5 microns) not collimator hole spacing. To maximize the ballistic/quasi-ballistic photons observed, a line of light aligned with the collimator holes increases detectability by reducing the amount of scattered background light. A Cylindrical Spherical Cylindrical beam expander/shrinker creates a 16 mm by 0.35 mm line of light. Best results occur when the scattering medium, collimator and detector are within 3X the Rayleigh Range of the beam's narrow vertical axis, allowing imaging of 51 micron lines/spaces at 3E8:1 scattering ratios. Restricting the light to a 1 mm line extends this to 8E9:1. Carbon coating the SMCA to reduce reflectivity shows that at high scattering levels absorbing walls will reduce background light, improving contrast. ADI has also been shown to work when the illumination is unaligned with the detector. This allows for side illumination with detection of structures at depths of 3mm with a scattering ratio of 1E6:1.

Nonspecular effects on reflection from absorbing media at and around Brewster's dip.

Abstract: We show that, for a TM (or p-state) Gaussian beam incident onto an absorbing medium at and around Brewster's dip, the reflected beam always remains Gaussian and undergoes a Goos-Hanchen-like (GH) shift, an angular shift, a focal shift, and a beam-waist modification, provided that the beam is sufficiently collimated that the third-order change of the (logarithmic) reflection coefficient can be ignored in the angular range of beam divergence. For weak absorption, not only are a large negative GH shift and an odd-functioned-like focal shift with greater magnitude found but also the angular shift, though small by itself, is shown to give an even larger lateral net shift at a distance beyond the Rayleigh range.

Generation and Detection of Higher Harmonics in Rayleigh Waves Using Laser Ultrasound

Abstract: This research studies higher harmonics of Rayleigh surface waves propagating in nickel base superalloys. Rayleigh waves are used because they carry most of the energy and travel along the surface of a specimen where fatigue damage is typically initiated. The energy concentration near the free surface leads to stronger nonlinear effects compared to bulk waves. An ultrasonic piezoelectric transducer together with a plastic wedge is used for the experimental generation of the Rayleigh wave. The detection system consists of a laser heterodyne interferometer. Measurements are performed to detect the fundamental wave as well as the second harmonic. The amplitude ratio is related to the nonlinearity parameter β which is typically used to describe changes in microstructure and investigate fatigue damage.

Narrow-band coherent Rayleigh scattering in a flame

Abstract: We report on the application of coherent Rayleigh scattering (CRS) to the measurement of temperature in a flame using narrow bandwidth pump and probe fields. High resolution measurements of the line shape were used to derive flame temperature based on fits to the line shape. An uncertainty in the temperature of 3% was determined utilizing a CRS model that includes scattering from a multicomponent gas for the first time. This model was validated at room temperature for a mixture of atomic and molecular species.

Interferometric Rayleigh Scattering Measurement System

Abstract: A method and apparatus for performing simultaneous multi-point measurements of multiple velocity components in a gas flow is described. Pulses of laser light are directed to a measurement region of unseeded gas to produce Rayleigh or Mie scattered light in a plurality of directions. The Rayleigh or Mie scattered light is collected from multiple directions and combined in a single collimated light beam. The Rayleigh or Mie scattered light is then mixed together with a reference laser light before it is passed through a single planar Fabry-Perot interferometer for spectral analysis. At the output of the interferometer, a high-sensitivity CCD camera images the interference fringe pattern. This pattern contains the spectral and spatial information from both the Rayleigh scattered light and the reference laser light. Interferogram processing software extracts and analyzes spectral profiles to determine the velocity components of the gas flow at multiple points in the measurement region. The Rayleigh light rejected by the interferometer is recirculated to increase the accuracy and the applicability of the method for measurements at high temperatures without requiring an increase in the laser energy.

A comparison of a first-order thermal lensing model to a closed aperture Z-scan for the propagation of light in ocular media

Abstract: The phenomenon of thermal lensing was investigated in water using a Z-scan method and corresponding first-order mathematical models. Data from first-order thermal lensing models and ABCD beam propagation methods were used to simulate the non-linear absorption of water held in a thin sample cuvette for a Z-scan optical set up of CW cases at 1313 nm. The single beam closed aperture Z-scan was then used to determine the non-linear absorption at 1313 nm for water in 10 mm and 2 mm cuvettes at 48.00, 16.80, 9.80 and 2.83 mW then compared to the first-order model data. The results from the closed aperture Z-scan were also used to back calculate the spot size in the far field for comparison to the model's prediction of the beam's temporal response. Experimental Z-scan data were found not to correlate strongly with our first-order model suggesting the need for higher order models to successfully predict spot size in absorbing media inside the Rayleigh range.

Micron-scale deep hole drilling for beryllium capsule fill applications

Abstract: A laser processing system has been developed to drill high aspect ratio holes through the impermeable beryllium capsules envisioned for ignition shots on NIF. The drilling system was designed to produce holes with an entrance and exit diameter of approximately 5 {micro}m through the full 175 {micro}m thickness of the capsule. To meet these requirements, a frequency doubled femtosecond-class Ti:Sapphire laser is directed through a high numerical aperture lens to provide the spot geometry needed to drill the hole. The laser pulse is confined by the metallic walls of the hole, thereby maintaining the diameter of the channel well beyond the Rayleigh range of the optical system. Presented is the current state of this work-in-progress, including descriptions of the device and the technique used to produce the holes. The various means of characterizing the laser-drilled channels are also discussed.

Short Rayleigh length free electron lasers

Abstract: Conventional free electron laser (FEL) oscillators minimize the optical mode volume around the electron beam in the undulator by making the resonator Rayleigh length about one third of the undulator length. This maximizes gain and beam-mode coupling. In compact configurations of high-power infrared FELs or moderate power UV FELs, the resulting optical intensity can damage the resonator mirrors. To increase the spot size and thereby reduce the optical intensity at the mirrors below the damage threshold, a shorter Rayleigh length can be used, but the FEL interaction is significantly altered. A new FEL interaction is described and analyzed with a Rayleigh length that is only one tenth of the undulator length, or less.

The LACARA Vacuum Laser Accelerator Experiment: Beam Positioning and Alignment in a Strong Magnetic Field

Abstract: LACARA (laser cyclotron auto‐resonance accelerator) is a vacuum laser accelerator of electrons that is under construction at the Accelerator Test Facility (ATF), Brookhaven National Laboratory It is expected that the experiment will be assembled by September 2006; this paper presents progress towards this goal According to numerical studies, as an electron bunch moves along the LACARA solenoidal magnetic field (∼52 T, length ∼1 m), it will be accelerated from 50 to ∼75 MeV by interacting with a 08 TW Gaussian‐mode circularly polarized optical pulse provided by the ATF CO2 106μm laser system The LACARA laser transport optics must handle 10 J and be capable of forming a Gaussian beam inside the solenoid with a 14 mm waist and a Rayleigh range of 60 cm The electron optics must transport a bunch having input emittance of 0015 mm‐mrad and 100 μm waist through the magnet Precision alignment between the electron beam and the solenoid magnetic axis is required, and a method to achieve this is described in detail Emittance‐ filtering may be necessary to yield an accelerated bunch having a narrow (∼1%) energy‐spread

Topological hysteresis as a model for Rayleigh damping

Abstract: To account for the large magnetomechanical damping of demagnetized specimens, we propose as a model Landau's arrangement of magnetic domains with no overall magnetic moment. To analyze the model, we use the nature of magnetostriction to transform the data from internal friction, Q −1 , and strain amplitude, ɛ , into energy loss and volume fraction reoriented by stress, both as functions of ɛ . The large peak in Q −1 ( ɛ ) above the upper end of the Rayleigh range is attributed to the onset of gross changes in the domain structure, the Rayleigh range to dynamics of closure domains on a finer scale, and the lower end of the range to the wall displacements becoming of the order of one atomic distance.

The observation study of infrared laser induced plasma plume

Abstract: The potential of laser-induced plasma is now becoming an important for fusion research in attempt to add the source of energy. Light energy can create plasma when high irradiance is focused. However, until today scientists and engineers still work hard to get the fusion energy into reality. Hence research is still going on to endeavor the knowledge and better understanding on laser plasma. In order to give some light on the problems, a fundamental study was carried to form the laser plasma. In this case, a high-power Q-switched Nd:YAG laser was employed to generate plasma. Camera lenses with various focal lengths were utilized to generate optical breakdown and initiate plasma plume on a cylindrical brass target. The plasma plume formation were visualized and recorded via CCD video camera. The recorded images were analyzed using Matrox Inspector version 2.1 and video test 5 softwares. The results obtained show that, the plasma area is linearly increased with focal length, which is in good agreement with the beam waist concept. Whereas, the length of plasma, related to the Rayleigh range was found quadratically change with focal length.

Compensation of laser thermal depolarization using free space

Abstract: Stress-induced birefringence in a single-transverse-mode YAG laser is well compensated simply by having the end mirror one Rayleigh range away from the gain.

Modeling Laser Wake Field Acceleration with the Quasi‐Static PIC Code QuickPIC

Abstract: We use the Quasi‐static Particle‐In‐Cell code QuickPIC to model laser wake field acceleration, in both uniform and parabolic plasma channels within current state of the art experimental laser and plasma parameters. QuickPIC uses the quasi‐static approximation, which allows the separation of the plasma and laser evolution, as they respond in different time scales. The laser is evolved with a larger time step, that correctly resolves distances of the order of the Rayleigh length, according to the ponderomotive guiding center approximation, while the plasma response is calculated through a quasi‐static field solver for each transverse 2d slice. We have performed simulations that show very good agreement between QuickPIC and three dimensional simulations using the full PIC code OSIRIS. We have scanned laser intensities from those for which linear plasma waves are excited to those for which the plasma response is highly nonlinear. For these simulations, QuickPIC was 2–3 orders of magnitude faster than OSIRIS.

Influences of the medium position and the beam-waist size on the performance of refractive-type optical limiters

Abstract: The operating properties of the refractive-type optical limiting devices based on the self-focusing and defocusing phenomena are stud- ied numerically, and the effects of both the installation position of the nonlinear refractive medium and the waist size of the incident Gaussian beam on the operating performance of the refractive-type optical limiters are examined. Our results show that the optical limiting curves of the refractive-type optical limiters exhibit a slowly attenuating oscillating waveform. The optical limiting threshold and the clamped output value of the optical limiters are dependant on the installation position of the sample and the convergence of the incident Gaussian beam to a great extent. When the sample position is given, the smaller the beam-waist radius, the better the optical limiting performance. When the beam-waist radius of the incident light is kept constant, an optimal position of the nonlinear sample can be found in the system. With fluctuating beam- waist size, however, the optimal location varies within a small range. For a self-focusing self-defocusing optical limiter, the optimal optical limiting effect can be obtained when the nonlinear medium is positioned at a certain location within half a Rayleigh range in the left right side of the focal plane.

Dependence of the optical limiting behavior on the medium position and the beam-waist radius in the refractive optical limiters

Abstract: We study numerically the optical limiting properties of the refractive optical limiting devices based on the self-focusing and defocusing phenomena, and examine the effects of the installation position of the nonlinear media and the waist size of the incident Gaussian beam on the operating performances of the refractive optical limiters. Our results show that the optical limiting curves of the refractive optical limiters assume a vibrating decaying waveform. The optical limiting threshold and the clamped output value of the optical limiters are dependant on the installation position of the sample and the convergence of the incident Gaussian beam to a great extent. When the sample position is given, the smaller the beam-waist radius, the better the optical limiting performance. When the waist radius of the incident light keeps constant, an optimum location of the nonlinear sample can be found in the system. With the fluctuation of the waist size, however, the optimum location changes within a small range. For a self-focusing (self-defocusing) optical limiter, the optimum optical limiting effect can be obtained when the nonlinear medium is positioned at a certain location within half a Rayleigh range in the left (right) side of the focal plane.

Laser processing of advanced materials with high brightness solid-state lasers at 532 nm

Abstract: Many materials e.g. diamond or silicon do not show sufficient absorption at one micron, the fundamental wavelength of Nd-based solid-state laser sources. Frequency conversion to the second and fourth harmonic allows the efficient processing of these materials. High beam quality is another important property for micro material processing with lasers. It facilitates small focus diameters and due to high Rayleigh length even at strong focusing drilling of holes with high aspect ratio. Together with high average output powers it allows fast processes with high quality. At least flexible pulse peak power and repetition rate is necessary to optimize the process. Two laser systems which fulfill these requirements are investigated. A pulse pumped Nd:YAP system which delivers an average output power of 124 W at the second harmonic and a Nd:YAG with an average output power up to 50 W in the second harmonic. Due to its active Q-switch the pulse peak power of this system is variable in a wide range. Efficient laser processing of different ceramics with these laser system are presented.

Emergent intensity from a plane-parallel medium exposed to a Gaussian laser beam: A comparison of Rayleigh and isotropic scattering

Abstract: The focus of this two-dimensional study is the radially varying intensity emergent from a plane-parallel scattering medium exposed to a collimated, Gaussian laser beam directed perpendicular to the upper surface. The method of analysis is the integral transform technique. Specifically, this work uses the generalized reflection and transmission functions from a previous study to construct the emergent intensity with the use of an inverse Hankel transform. Radially varying backscattered and transmitted intensities are calculated for media with isotropic and Rayleigh scattering phase functions and optical thicknesses that range from 0.125 to 8.0. The behavior of the emergent radiation inside and outside the beam is investigated for both narrow and wide beams. A new integration method is implemented to compute the emergent intensity at the beam center. The emergent intensity at the beam center is used to quantify when a one-dimensional model may be used. As expected, for small optical thicknesses and near the beam the phase function has significant influence, while far from the beam multiple scattering reduces the influence of the Rayleigh phase function. Results from this study will be useful in understanding and interpreting more complicated situations, such as those that include polarization.

Dissimilar Metal of K418 and 42CrMo Full Penetration Laser Welding

Abstract: The influences of laser welding velocity,flow rate of side-blow shielding gas,defocusing distance and other parameters on the full penetration laser weld seam of K418 and 42CrMo were experimentally investigated using continuous wave Nd∶YAG laser.Results show that the asymmetric cross-sections of weld seams exhibit X shape or T shape.With increasing welding velocity,the linear heat input and dimension of weld seam decrease,and the shape of it changes from X shape to T shape.Meanwhile,the rate of change in dimension of weld bottom region is larger than that of weld top region.The change of the top surface width of the weld seam is small under the condition that defocusing distance changes within the range of Rayleigh length;the top surface widths of the weld seam increase quickly under the condition that defocusing distance changes out of the range of Rayleigh length with enough density of laser power.The high-quality of weld seam with constant(3 kW) laser power and 35° angle of side-blow shielding gas can be obtained by optimizing welding velocity,flow rate of shielding gas,defocusing distance and other parameters.

Self-adaptive optical systems for long-distance flying optics.

Abstract: The focusing characteristics of long-distance flying optics were studied systemically for TEMmn Gaussian beams. The results show that the ABCD law of parameter q can be extended to Gaussian modes of any order when waist radius w in the imaginary part of parameter q is replaced by Rayleigh range Z(R) of a certain resonator in the equation. The difference between the real focal length and the geometric focal length, defined as Delta f, was calculated for laser applications. A novel self-adaptive optical system was demonstrated for precisely controlling the focusing characteristics of long-distance flying optics, Theoretical analyses and experimental results were consistent. (c) 2006 Optical Society of America.

Numerical Study on Rayleigh Scattering Property in High-Index-Core Bragg Fibers

Abstract: An analytical model for Rayleigh scattering in fibers is presented based on a full-vector finite element method(FEM) with edge/nodal hybrid elementBy using this model,the Rayleigh scattering property of F-doped and GeO_2-doped high-index-core bragg fibers(HICBF) is investigatedNumerical results show that the effects of fiber structure on Rayleigh scattering in F-doped HICBF are quite opposite to that in GeO_2-doped HICBFBy adjusting the cladding structure,minimized Rayleigh scattering loss(RSL) can be obtained in GeO_2-doped HICBF with a certain high-index core radiusConclusion in this paper is also suitable for other doped HICBF

Optical arrangement for pumping solid-state lasers

Abstract: In an optical arrangement for pumping solid-state lasers there is the object of producing an intensity distribution across the beam cross section of the pump radiation with a rectangular intensity profile, which intensity distribution is homogeneous at least in a region corresponding to the Rayleigh range in the direction of the beam propagation, without the beam quality being substantially impaired by the homogenization. The pump arrangement contains a rod-shaped homogenizer (1) with two opposed, polished end faces (2, 3), planar side limit faces (4), which are arranged parallel to the optical axis and with a cross-sectional area at right angles to the optical axis, which forms a regular polygon, with the regular polygon being restricted to those number of sides which permit a plurality of polygons to be positioned against one another on a surface in such a way that they fill the space.