Evaluation of laser drilling of holes in thermal barrier coated superalloys

Abstract

Laser drilling of precise holes in thermal barrier coated Ni based superalloys has been studied. The interplay between various hole geometrical features such as hole shape, taper, barrelling, undercut, etc. and laser parameters such as pulse energy, pulse width and pulse repetition rate have been examined. The hole diameters are seen to follow a linear dependence on the incoming laser power densities for pulse width up to 2·0 ms. However, such a linear dependence was not observed for a pulse width of 3·0 ms. It was found that high pulse energy and short pulse width (high power density) gave crack free recast layer, whereas low pulse energy and longer pulse width (low power density) gave microcracks in the heat affected layer of superalloy. The significant barrelling observed in IN718 material at low power density values is due to multiple reflection of the incident beam from the cavity in combination with plasma formation at the evaporation front and trapping of the incident radiation causing excessive heating in that region.     

Nonlinear scattering of light in nanodiamond hydrosol

The nonlinear scattering of light under the conditions of optical limiting of nanosecond pulsed laser radiation at a wavelength of 1064 nm in a nanodiamond (ND) hydrosol has been experimentally studied. Superstable hydrosols were obtained from detonation NDs with a modified surface. Using an improved scheme of z scanning, it is shown that a decrease in the optical transmission coefficient of an ND hydrosol under optical limiting conditions is due to enhanced nonlinear scattering. It is established that the energy of pulsed radiation scattered at a right angle obeys a power law in dependence on the energy density of incident radiation pulses. Hydrosols of detonation NDs with the modified surface exhibit high stability with respect to the periodic laser action at high power density.     

Effect of small profile deviations on the efficiency of metallic diffraction coatings

We analyze the present state of research on how small deviations of the profile shape of metallic diffraction gratings (MDGs) affect the conversion of the power (energy) of incident radiation at various orders of diffraction and discuss the possibility of using MDGs in laser photometry for metrological purposes. Translated from Izmeritel'naya Tekhnika, No. 3, pp. 32–36, March, 1994.     

Picosecond optical parametric chirped pulse amplifier as a preamplifier to generate high-energy seed pulses for contrast enhancement

We present the design, implementation, and testing of a novel picosecond optical parametric preamplifier system to generate high-energy seed pulses for the Vulcan laser facility. The preamplifier amplifies 100?fs pulses stretched to 3?ps pulses from 10?pJ to 70 μJ in a single stage of amplification before the pulses are further amplified in the Vulcan high-power Nd:glass laser facility to the petawatt power level. This increased seed energy has led to an improvement of the nanosecond amplified spontaneous emission contrast intensity to 10(-10) of the main pulse, without degrading the output of the laser system.     

Effect of laser-radiation polarization on the nonlinear scattering of light in nanodiamond suspensions

The effect of laser radiation polarization on the nonlinear scattering of light in aqueous suspensions of detonation nanodiamonds (DNDs) in a regime of optical power limiting (OPL) has been studied. It is established that the nonlinear transmission coefficient of DND suspension in the OPL regime in a field of nanosecond laser pulses with a wavelength of 532 nm is independent of the polarization of incident radiation. The nonlinear scattering of light observed at an angle of 90° in the plane perpendicular to the plane of polarization of the incident radiation depends on the polarization angle in accordance with a trigonometric law. It is shown that the ratio of the signals of scattered radiation for the vertical and horizontal polarizations exhibits nonmonotonic dependence on the laser-beam power density. The results are explained by the Rayleigh-Mie scattering and a change in the size of scattering centers as a result of the effect of a laser upon the DND suspension.     

Forward-scattered radiation from the compression paddle should be considered in glandular dose estimations

From major protocols on dosimetry in mammography, there is no doubt that the incident air kerma should be evaluated without backscattered radiation to the dosemeter. However, forward-scattered radiation from the compression paddle is neglected. The aim of this work was to analyse the contribution of forward-scattered radiation for typical air kerma measurements. Measurements of forward-scatter were performed with a plane-parallel ionisation chamber on four mammography units. The forward-scatter contribution to the air kerma was 2-10 % and increased with the compression paddle thickness, but also with the half-value layer value. For incident air kerma in mammography, it can be as important to consider forward-scattered as backscattered radiation. If an ionisation chamber is used, the compression paddle should be in contact with the chamber; otherwise the air kerma and absorbed dose will be underestimated. If a dosemeter based on semiconductors with much less sensitivity to scattered radiation is used, it is suggested that a forward-scatter factor (FSF) is applied. Based on the results of this work, FSF = 1.06 will lead to a maximum error of ~4 %.     

Laser remote-sensing system analysis for search and rescue

We develop a general model of a laser remote-sensing system for search and rescue using targets marked with fluorescent dye. The dye fluoresces at a longer peak wavelength than the incident radiation, enabling a dye-covered target to be distinguished from the unshifted ground echo by the search system. The principal result is a simple expression derived for the average laser power required to search at a particular rate given a required ground energy density. A similar expression is applicable to imaging lidar systems. The example system shown indicates that active probing for lost planes may be practical.     

Surface plasmon-mediated energy transfer in heterogap Au-Ag nanowires

We report the observation of energy transfer from a gold (Au) nanodisk pair to a silver (Ag) nanowire across a 120 nm gap via surface plasmon resonance (SPR) excitation. The enhanced electromagnetic (EM) fields generated by Au SPR excitation induce oscillation of the conduction electrons in the Ag segment, transferring energy to it even though the Ag segment has only weak resonant interactions with the incident EM radiation. The induced Ag SPR produces strong EM fields at the position of the Ag segment, leading to a Raman signal approximately 15 times greater than when the Ag segment is alone (not adjacent to the Au nanodisk pair). The Raman intensity is found to depend nonlinearly on the incident laser intensity for laser power densities of 10 kW/cm(2), which is consistent with the results of EM theory calculations which are not able to account for the factor of 15 enhancement based on a linear mechanism. This suggests that energy transfer from the Au disk pair to the Ag segment involves an enhanced nonlinear polarization mechanism such as can be produced by the electronic Kerr effect or stimulated Raman scattering.     

Speckle correlometry method for evaluating the transport scattering coefficient of a randomly inhomogeneous medium

We propose a method for evaluating the transport scattering coefficient of a randomly inhomogeneous medium, which is based on a correlation analysis of the intensity of laser radiation backscattered from the moving medium. The method employs a localized source of radiation (laser beam focused on the surface of medium) and makes use of the spatial filtration of detected scattered radiation in the image plane of the optical system with the aid of a programmed ring filter.     

Theoretical analysis of polarization patterns from incoherent backscattering of light

Anisotropy in the polarization of the backscattered light from a polarized laser beam incident upon a scattering medium has been observed experimentally. When the beam is viewed through an oriented polarizer, characteristic patterns in the backscattered light are observed. We present here a simple explanation of these patterns, using the theory of incoherent scattering of light by spheres. It appears that the major contribution to the observed patterns comes from the double scattering of light.     

Laser-diode-pumped passively Q-switched Nd:YVO4 green laser with periodically poled KTP and GaAs saturable absorber

A laser-diode-pumped passively Q-switched Nd:YVO₄ green laser with periodically poled KTP (PPKTP) and GaAs saturable absorber has been realized. The dependences of pulse repetition rate, pulse energy, pulse width and peak power on incident pump power for the generated-green-light pulses are measured. At the maximum incident pump power of 4.1 W, the maximum average output power of 113 mW is obtained, corresponding to an optical conversion efficiency of 2.8%. At the same pump power, stable green laser pulses of duration of 44.6 ns and energy of 0.28 µJ are generated at a repetition rate of 403.4 kHz. The coupling wave rate equations for a passively Q-switched laser are also given and the numerical solutions agree with the experimental results.     

Light-induced EMF in silver-palladium film resistors

We have studied the generation of nanosecond emf pulses in silver-palladium film resistors under the action of radiation of a Q-switched laser. The samples were fabricated using the technology of thick film resistors on dielectric substrates, based on fusing a resistive paste that contains palladium, silver oxide, glass, and an organic binder into the substrate at 880 K. The amplitude of detected pulses exhibits linear growth with the power of incident laser radiation, depends on the angle of light incidence (vanishes at the normal incidence) onto the film and the angle of film rotation about the normal to its surface, and changes its sign with that of the incidence angle. The duration of the generated emf pulses is several times that of the incident laser pulses. The signal is not of a thermoelectric nature and can be related to the current generation by means of the surface photogalvanic effect and the photon quasi-momentum transfer to charge carriers during light absorption by the film material.     

Statistics of intensity of backscattered semiconductor laser radiation in single-mode optic fiber

We have studied the statistics of the intensity of backscattered radiation of a semiconductor laser in single-mode optic fiber as a function of the probing laser pulse duration and coherence time. It is shown that, at a given radiation source coherence time, the intensity distribution function varies, depending on the pulse duration, from nearly exponential to nearly Gaussian. The exponential statistics provides a better sensitivity for a coherent reflectometer with direct detection. Using the calculated distribution function, it is possible to qualitatively determine the degree of deterioration of the reflectometer sensitivity with respect to external factors during increasing probing pulse duration or decreasing laser coherence time. These data provide criteria for the optimum choice of a radiation source for the coherent fiber-optic reflectometer.     

Spontaneous density grating formation in suspensions of dielectric nanoparticles

Experimental evidence for nonlinear optical behaviour due to the spontaneous formation of wavelength-scale density modulations or gratings in suspensions of dielectric particles is presented. A collection of dielectric particles pumped by a coherent radiation field may simultaneously form a density grating on the scale of the radiation wavelength and a coherently backscattered radiation field. The particle density grating is generated as a result of a periodic ponderomotive potential formed by the interference of the pump and backscattered fields. The experiment used a water suspension of latex microspheres (radius ≈ 56nm) pumped by a green CW laser (532nm, power ≤ 5W). A theoretical model of collective scattering of light from dielectric particles has been extended to include the effects of viscous and Brownian forces on the particles. This model predicts a small degree of particle bunching from which coherent backscattering of the pump occurs. The results of the theoretical model compare favourably with the experimental evidence. The relation between the results presented here and the phenomenon of Collective Rayleigh Scattering (CRS) is discussed.     

Chromatism compensation of the PETAL multipetawatt high-energy laser

High-energy petawatt lasers use series of spatial filters in their amplification section. The refractive lenses employed introduce longitudinal chromatism that can spatially and temporally distort the ultrafast laser beam after focusing. To ensure optimum performances of petawatt laser facilities, these distortions need to be corrected. Several solutions using reflective, refractive, or diffractive optical components can be addressed. We give herein a review of these various possibilities with their application to the PETAL (Petawatt Aquitaine Laser at the Laser Integration Line facility) laser beamline and show that diffractive-based corrections appear to be the most promising.     

Diode-Pumped Cr:forsterite Laser Mode Locked by a Semiconductor Saturable Absorber

We report the passive mode locking of a diode-pumped Cr:forsterite laser using a semiconductor saturable absorber. The laser generates 10-pJ pulses as short as 1.5 ps in duration with ~650 mW of absorbed pump power. We measured the transient reflectance of the saturable absorber mirror under different incident energy fluences by using a standard pump-probe method. The performance of this laser is also compared with a high-power fiber-laser-pumped femtosecond forsterite laser mode locked with the same absorber.     

New IBS coatings boost performance of petawatt laser mirrors

Simply put, the performance of a laser is just as high as the quality of its optics. Latest advances in petawatt laser technology have stimulated the development of large surface area, dielectrically coated mirrors to guide these demanding ultrashort pulse laser beams. We describe major achievements during the development and fabrication of ion beam sputtered 440 × 290 × 75 mm³ beam transport mirrors for the HAPLS laser at ELI Beam-lines. A laser damage threshold greater than 0.9 J/cm² was measured.     

Broadband flexible waveguides for free-electron laser radiation

We refined flexible waveguides previously developed for CO(2) and Er:YAG laser radiation to transmit free-electron-laser (FEL) radiation. One can tune this laser over several segments of the radiation spectrum. This laser has a high peak power of as much as 10 MW with pulse energy of as much as 100 mJ. We made the waveguides of either Teflon or fused-silica tubes internally coated with metal and dielectric layers. We optimized the internal coatings specifications for transmission of various radiation wavelengths in the mid-IR range and enabled transmission of high-peak radiation. We performed experiments in three major FEL sites in the United States over a more than 1-year period when we measured and examined various characteristics of transmission. We used the analysis of these experiments as feedback to further improve these waveguides. The good preliminary results encourage us to invest more effort to further develop these waveguides until a suitable waveguide is obtained for this type of laser and make possible its introduction to the medical field where its characteristics can be exploited in surgical applications.     

Laser-induced reaction in Cu---In---Se systems

Laser-induced synthesis of CuInSe₂ has been investigated at various laser fluences by using electron diffraction and Raman spectroscopy techniques. The analysis of the products of irradiation shows that different phases of Cu---In---Se are formed according to the incident power of the radiation. When the laser power is lower than the threshold value (164 W cm⁻²), several binary phases are obtained. At threshold power, however, almost all binary phases are absent and reasonably good quality CuInSe₂ thin films are obtained.     

Nd∶YAG激光辐照碳纤维复合材料的质量烧蚀

 通过实验研究了连续Nd：YAG激光辐照下碳纤维环氧树脂复合材料的质量烧蚀规律。结果表明：当激光功率密度大于 10 kW/cm2 时,材料烧蚀质量与激光辐照能量成正比关系;在同样激光辐照能量下,半径为0.65mm的激光辐照引起的复合材料的质量烧蚀率比半径为5.5mm的激光辐照的结果大一倍;激光功率在燃烧阈值附近时,燃烧现象引起质量烧蚀率的波动。