Single-pass Laser Research Articles

The possibility of demonstration of gamma-ray lasing in nuclei

Estimates are made of the parameters of a possibledemonstration experiment on the generation of gamma raysin a single-pass laser with hidden population inversion in afree-nuclei amplifying medium.

The significance of orbital anatomy and periocular wrinkling when performing laser skin resurfacing.

Knowledge of orbital anatomy and the interaction of muscle contractions, gravitational forces and photoagingis fundamental in understanding the limitations of carbon dioxide (CO2) laser skin resurfacing when rejuvenating the skin of the periocular area. Laser resurfacing does not change the mimetic behavior of the facial muscles nor does it influence gravitational forces. When resurfacing periocular tissue, the creation of scleral show and ectropion are a potential consequence when there is an over zealous attempt at improving the sagging malar fat pad and eyelid laxity by performing an excess amount of laser passes at the lateral portion of the lower eyelid. This results in an inadvertent widening of the palpebral fissure due to the lateral pull of the Orbicularis oculi. Retrospectively, 85 patients were studied, who had undergone periorbital resurfacing with a CO2 laser using anew treatment approach. The Sharplan 40C CO2 Feather Touchlaser was programmed with a circular scanning pattern and used just for the shoulders of the wrinkles. A final laser pass was performed with the same program over the entire lower eyelid skin surface, excluding the outer lateral portion (e.g. a truncated triangle-like area),corresponding to the lateral canthus. Only a single laser pass was delivered to the lateral canthal triangle to avoid widening the lateral opening of the eyelid, which might lead to the potential complications of scleral show and ectropion. When the area of the crows' feet is to be treated, three passes on the skin of this entire lateral orbital surface are completed by moving laterally and upward toward the hairline. Patients examined on days 1, 7, 15, 30, 60, and one year after laser resurfacing showed good results. At two months after treatment, the clinical improvement was rated by the patient and physician as being "very good" in 81 of the 85 patients reviewed. These patients underwent laser resurfacing without complications. The proposed technique of periocular resurfacing prevents complications of scleral show and laxity in the lateral eyelid opening and even ectropion, because treatment conforms to the osseo-muscular anatomical relationship of eyelid structures.

Hardfacing of stainless steel with laser melted colmonoy

Stainless steel substrates were hardfaced with laser melted Colmonoy applying a continuous wire injection technique using a CO2 laser. The microstructures of single pass laser clad deposits were examined and related to workpiece traverse velocity at constant laser output. Particular attention was paid to the effect of dilution of the cladding alloy due to convectional mixing with the partially melted substrate material. The solidification microstructure of the rapidly cooled cladded layer consisted predominantly of nickel rich cellular dendrites and interdendritic lamellar eutectic borides.

Laser machining of glass forming ceramics

Analyses and results from the laser machining of a cordierite-based (2MgO⋅2Al2O3⋅5SiO2) glass forming ceramics are presented. Three separate pulsed Nd:yttrium–aluminum–garnet laser systems were investigated, including a 2 kW Lumonics Multiwave laser. Efforts were focused on determining the relationship between the laser machining parameters and the resulting depth of cut, residual surface quality, and material removal rate, such that a reasonable level of prediction and optimization could be established. A figure of merit for predicting laser induced microcracking was also resolved. Single pass laser machining rates as high as 1.69 cm3/min and depths of cut as large as 1.27 mm were achieved. Chemical analysis energy dispersive analysis by x ray and microhardness measurements were performed to evaluate sample integrity after machining.

Investigation and optimization of generation characteristics for an XeCl excimer laser–Compressed-hydrogen IRS-converter system

A comprehensive experimental investigation of the power and spatial characteristics of XeCl electricdischarge excimer laser radiation and the power characteristics of a single-pass compressed-hydrogen IRS laser excited by the focused flux of this radiation is performed. The existence of optimum laser-radiation pulse energies and divergences which ensure the maximum IRS-conversion to different Stokes components is established. It is shown that, to estimate the optimum conditions of IRS-conversion to the first Stokes component, use can be made of a simple model of the description of a multimode focused pumping flux.

Nonlinear saturation and efficiency enhancement in a Cherenkov laser using a dielectric grating

With the aid of particle simulation, this paper discusses nonlinear saturation and efficiency enhancement in a single-pass Cherenkov laser using a dielectric grating, taking into account the nonlinear properties of a relativistic electron beam. For the analysis of the problem, a two-dimensional model for the Cherenkov laser is considered which is composed of a planar relativistic electron beam and a parallel plate waveguide, one plate of which is loaded with a dielectric grating. First, the nonlinear characteristics for the growth of the electromagnetic wave are considered for a dielectric grating with constant parameters. The variation of the groove depth or the slot width in a dielectric grating is found to have the same effect on the growth characteristics as the variation of the permittivity of a dielectric sheet with constant thickness. Then, in order to enhance the efficiency in energy transfer from the electron beam to the electromagnetic wave in the Cherenkov laser, one of the grating parameters or the groove depth is adiabatically decreased in accordance with the decrease in the average velocity of the electron beam. The result of numerical simulation shows that sufficient efficiency enhancement in energy transfer is achieved with the use of a dielectric grating with a variable parameter.

Formation of sharply peaked sidelobes in large aperture single-pass semiconductor laser amplifiers

Single-pass semiconductor laser amplifiers are shown theoretically to exhibit sharply peaked lobes ('bat ears') at the edges of the near-field output. We discuss the mechanism for their formation and show that it is qualitatively different from spontaneous filamentation in homogeneous media.

Short wavelength FELs using the SLAC linac

Recent technological developments have opened the possibility to construct a device which we call a linac coherent light source (LCLS) (C. Pellegrini et al., Nucl. Instr. and Meth. A 331 (1993) 223; H. Winick et al., Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, May 1993; C. Pellegrini, Nucl. Instr. and Meth. A 341 (1994) 326; J. Seeman, SPIE Meet. on Electron Beam Sources of High Brightness Radiation, San Diego, CA, July 1993 [1–4]); it would be a fourth-generation light source, with brightness, coherence, and peak power far exceeding other sources. Operating on the principle of the free electron laser (FEL), the LCLS would extend the range of FEL operation to much shorter wavelength than the 240 nm that has so far been reached. We report the results of studies of the use of the SLAC linac to drive an LCLS at wavelengths from about 3 to 100 nm initially and possibly even shorter wavelengths in the future. Lasing would be achieved in a single pass of a low emittance, high peak current, high-energy electron beam through a long undulator. Most present FELs use an optical cavity to build up the intensity of the light to achieve lasing action in a low-gain oscillator configuration. By eliminating the optical cavity, which is difficult to make at short wavelengths, laser action can be extended to shorter wavelengths by self-amplified-spontaneous-emission (SASE), or by harmonic generation from a longer wavelength seed laser. Short wavelength, single pass lasers have been extensively studied at several laboratories and at recent workshops (M. Cornacchia and H. Winick (eds.), SLAC Report 92/02; I. Ben-Zvi and H. Winick (eds.), BNL report 49651 [5,6]). The required low-emittance electron beam can be achieved with recently-developed rf photocathode electron guns (B.E. Carlsten, Nucl. Instr. and Meth. A 285 (1989) 313; J. Rosenzweig and L. Serafini, Proc. IEEE 1993 Particle Accelerator Conf., Washington, DC, 1993 [7,8]). The peak current is increased by about an order of magnitude by compressing the bunch to a lenght of about 0.2 ps (rms). Techniques for beam transport, acceleration, and compression without emittance dilution have been developed at SLAC as part of the linear-collider project (J. Seeman, Advances of Accelerator Physics and Technologies, ed. H. Schopper (World Scientific, Singapore, 1993 [9]). The undulator length required to saturate the laser varies from about 15 m for a 100 nm FEL to about 60 m at 3 nm. Initial experiments, at wavelengths down to about 50 nm are planned using the 25-m long Paladin undulator now located at LLNL. In a proposed future LCLS R&D facility the short wavelength light pulses are distributed to multiple end stations using grazing-incidence mirrors. About 10 14 photons per pulse can be produced at a 120 Hz rate, corresponding to average brightness levels of about 10 21 photons/s/mm 2/mrad 2 within 0.1% BW and peak brightness levels of about 10 31 photons/s/mm 2/mrad 2 within 0.1% BW. Peak power levels are several hundred megawatts to several gigawatts. Electron energies required range from about 500 MeV for the 100 nm FEL to about 7 GeV for 3 nm.

Traveling-wave excitation of the Ar ii shake-up laser at 4765 nm

We describe the construction and operation of a single-pass laser in Ar ii at 476.5 nm pumped by soft x rays from a laser-produced plasma. Temporal and spatial coherence properties have been characterized by means of a variety of experimental techniques. We use a 5-J, 100-ps, 1053-nm laser pump pulse in a traveling-wave geometry to measure a maximum output energy of 550 μJ. The aspect ratio of the unsaturated gain region directly affects the divergence of the output laser beam, resulting in a half-angle divergence of 10 mrad for 10 cm of excitation length. A small-signal gain coefficient of 2.1 cm−1 was observed, with the onset of saturation occurring after just 2 cm of excitation.

Single Pass Laser Cutting of Polymers

Since the early 1980's discovery of the ability of excimer lasers to etch polymers precisely and cleanly with minimal thermal damage, excimer lasers have been used actively in the semiconductor industry for various manufacturing processes. In the present study a process for laser cutting of polymers has been investigated. A laser beam of controlled cross-sectional shape is imaged onto a polymer surface. The geometry of the cutting path is controlled by precise movement of a sample mounted on a 2-axis computer controlled work stage. This type of a process is useful for applications such as selective laser cutting or laser marking of polymers. A parametric study of laser cutting of polymers using a 30 watt average power, 308 nm wavelength, XeCl pulsed laser was carried out. The effect of incident laser fluence and laser frequency were studied to determine maximum single pass cutting speed for a given thickness of polymer. Three different types of polymers were studied. Polymer films of different thickness were spin-coated on a silicon wafer and were cured to their respective full cure temperature. Polymer thicknesses from 7 microns to 38 microns were studied. Incident laser fluence was varied from 200 mj/cm2 to 1.0 J/cm2. Laser frequency was varied from 1 Hz to 50 Hz. An analytical formula for optimum single pass cutting speed as a function of the etch rate of a polymer, laser frequency, laser beam length in cutting direction, and polymer thickness was developed. Experimental results obtained indicate that cutting speed is directly proportional to the incident laser fluence and laser frequency, and is inversely proportional to polymer thickness. Predictions of optimum cutting speed obtained from the analytical formula for different polymers were in good agreement with the experimentally measured values.

Residual stresses in laser surface hardening of large areas

Abstract Laser surface hardening is a method of surface treatment in which high density energy is employed as a heat source to harden the substrate surface. Owing to the small spot size of the focused laser beam only a small surface area can be hardened in a single laser pass. To accomplish surface hardening for a larger workpiece, therefore, multiple lasing passes must be used. In this study, the residual stress in the multipass laser surface-hardening process was analysed by using a finite element method. The simulation results revealed that a region of residual tensile stress remains on the hardened surface, if the distance between the neighbouring passes is larger than the width of the hardened zone of each pass. The residual tensile stress on the surface disappears, however, when the distance between the passes is so short that the hardened zone of each pass is partially overlapped. In multipass laser surface hardening, the region of residual tensile stress occurs below the hardened zone of the workpiece, while the maximum residual tensile stress occurs along the centre of each lasing path. In comparison with a gaussian distribution of the beam power, the square beam mode produces a wider and thinner hardened zone, which results in a wider and thinner region of residual tensile stress below the surface.

Mössbauer measurements backscattering technique (CXMS) of laser irradiated cold forming tool steel (X210CR12)

Conversion x-ray Mossbauer measurements (CXMS) in backscattering technique are performed on a commercial cold forming tool steel (X210Cr12), subjected to single-pass laser irradiation. The structure of the affected layer is examined for laser powers ranging from 2700 W to 3050 W at different depths. The results of the spectra analysis are discussed in connection with Vickers hardness and metallographic data. If a complete molten layer is quenched, a high carbon content austenite is retained down to room temperature. This phase is mixed with some low carbon martensite and (Cr 0.5 Fe 0.5)> C3 carbide. The effects of carbon diffusion from the coating medium into the bulk are discussed. It is demonstrated that Mossbauer spectroscopy is a sensitive tool for phase analysis, including information about phase formation, and for delimiting the range of the quenched molten layer after laser treatment. It is thus a promising technique in this special kind of steel engineering.

Saturation of the Xe iii 109-nm laser using traveling-wave laser-produced-plasma excitation

We describe the construction and operation of a 109-nm, photoionization-pumped, single-pass laser in Xe III. The laser is pumped by soft x rays emitted from a laser-produced plasma in a traveling-wave geometry. Using a 3.5-J, 300-psec, 1064-nm laser pump pulse, we measure a small-signal gain coefficient of 4.4 cm(-1) and a total small-signal gain of exp(40). The laser is fully saturated and produces an output energy of 20 microJ in a beam with 10-mrad divergence.

Effects of single-pass laser heat treatment on erosion behavior of cast irons

A single-pass laser heat treatment was performed on the surfaces of pearlitic gray and ductile cast irons using a 5 kW continuous wave CO 2 gas laser. Surface-hardened layers affect solid particle impingement erosion in that, for given abrasive flow conditions, the erosion rate of laser-treated gray iron was five times greater than that of untreated gray iron while the erosion rate of laser-treated ductile iron was 25 times greater than that of untreated ductile iron. The erosion mechanisms of gray iron included micromachining and ploughing in the untreated condition and intergranular cracking in the laser-treated condition. In contrast, ductile iron was eroded by severe plastic deformation and tearing in the untreated condition and by the fatigue mechanism in the laser-treated condition.

Multiphoton ionization spectra and tunable fifth-harmonic production near five-photon resonances in Xe and Ar.

Through multiphoton ionization measurements, we have observed the interference of five-photon and internally generated fifth-harmonic excitation processes in ..delta..J = 1 even-odd parity resonant transitions in Xe and Ar. In agreement with theory, these five-photon resonances are not observable in single-pass laser beams, but are convincingly observed in counterpropagating beams. In the negatively dispersive regions of the 5d(3/2)/sup 0/J = 1 level of Xe, tunable fifth-harmonic light is generated by phase matching with various buffer-gas mixtures. The fifth-harmonic profiles are in good agreement with calculated phase-matching curves.

Formation of austenite in laser-processed Fe-0.2%C-20%Cr alloy

Laser surface alloying of chromium on a 0.2% C steel was performed using a 1200 W continuous wave CO 2 gas transport laser to produce an alloy containing 20% Cr. The effects of rapid melting and subsequent quenching on the microstructure and microhardness of this alloy were then investigated. A single-pass laser melting was employed to produce a penetration depth of 600 μm. The microstructure, characterized by scanning and thin foil transmission electron microscopy, consisted of austenite needles embedded in a ferritic matrix. The factors governing the formation of austenite and its substructure are discussed and some evidence for the mechanism is presented.

Threshold and dispersion effects in the anti-Stokes Raman laser

Threshold characteristics for an anti-Stokes Raman laser based on inverted, metastable TI * (6p(2)P degrees (3/2)) atoms are studied. Raman upconversion of a tunable dye laser at 535 to 378 nm is observed with an overall tuning range of 1125 cm(-1) for a single-pass laser device. The anti-Stokes pump-laser threshold varies inversely as the square of the detuning from the intermediate Raman level, except in a region ~50 cm(-1) to either side of the resonance, where the threshold is constant. These observations are explained in terms of group-velocity dispersion effects and are in good agreement with a simple theoretical model.

Bidirectional amplification with nonsaturable absorption and amplified spontaneous emission

A theory of bidirectional amplification is developed and applied to single-pass and double-pass laser amplifier configurations. The effects of nonsaturable absorption, longitudinal amplified spontaneous emission (ASE) gain saturation, and ASE line narrowing are included in a self-consistent formulation. An estimate of aperture scaling limitations due to transverse ASE is included. Laser performance is discussed for some conditions typical of large aperture, multikilojoule, rare gas halide (RGH) fusion amplifiers. This formulation permits tradeoff studies to be performed rapidly by altering variables spanning an eight-parameter space.

High Power Laser Welding - 1978

Current high power laser welding technology is reviewed. The nature of the beam-material interaction, the influence of laser, process and material parameters on welding response and the unique characteristics of the process are discussed. Examples are given of representative laser welding performance in ferrous and nonferrous alloys and selected illustrations are presented of weld mechanical properties. A relationship is noted for the dependence of maximum single-pass laser weld penetration on power and a range of laser welding applicability is delineated. Laser welding efficiency is identified and process advantages and disadvantages are compared. The potential for future utilization of lasers in welding is discussed.

Gain Narrowing in a Laser Amplifier

The phenomenon of gain narrowing of an inhomogeneously broadened line in stimulated emission has been observed and was found to give good agreement with theory. The effect was observed by measuring the variation in gain across the amplifying line using a fixed frequency laser oscillator and a tunable single-pass laser amplifier.