Laser polishing of silica slotted rods

Abstract

Lasers have been widely used in surface modification. In the present research, a CO₂ continuous wave laser has been used to polish slot surfaces of silica rods. Strong absorption of 10.6 μm CO₂ radiation by the silica surface promotes softening of a very thin layer of material that flows under the action of surface tension forces. A smooth surface forms, which decreases the surface roughness without substantial change in surface geometry. The effect of laser-surface inclination angle on the requisite power requirement has been assessed experimentally and theoretically. Using a computer controlled laser beam scanning system, reliability and reproducibility have been greatly improved compared with an alternative surface finishing method, manual fire polishing.     

Call for Papers

Abstract

A high-bandwidth optical fibre current sensor has been developed at Salford to monitor the waveform of the driving current pulse feeding into a discharge-pumped excimer laser. The sensor is a conventional intrinsic Faraday effect device but overcomes the normal bandwidth-sensitivity limits because the very large electrical current pulse used to drive the excimer laser allows detectable polarization rotation over a 60 cm length of fibre. The performance of the intrinsic sensor is compared with that of a current-viewing resistor, and of a Be₁₂GeO₂₀ crystal sensor.     

Stable 4 W CW solid-state green source pumped by fibre-coupled diode-laser arrays

Abstract

A high-power Nd:YAG laser end-pumped by a fibre-coupled diode-laser array system is described. When operated at 1 μm, the device produced 10 W of output power in the TEM₀₀ transverse mode with a slope efficiency of 43%. The system was operated using both 600 μm and 400 μm core, 0·37 N.A. multimode fibres for delivery of the pump radiation, and also with Nd:YVO₄ as the gain material. By intracavity doubling with KTP, 4 W of non-chaotic two-mode 0·5 μm output was obtained.     

Model for CO2 laser welding of stainless steel,titanium, and nickel: parametric study

Abstract

The present work has been carried out experimentally and theoretically with the aim of studying the effect of the operative pressure on the weld beads of different materials (AISI 304, Ti–6Al–4V, and Ni), welded using a CO₂ laser beam. At the same time, the reliability of a properly developed analytical model of the laser beam welding process has been confirmed. Such a model, taking into account the dynamics of the process itself, describes the laser induced thermal fields in terms of two heat sources, the first one representing the keyhole effect and the second one, the role played by the plume. A comparison between the experimental data and the beads predicted using the model gives satisfactory results, with average errors less than 5% for Ti–6Al–4V and ~10% for AISI 304 and Ni. The model allows the quantitative evaluation of the power distribution between the keyhole and the plume and a deeper understanding of the entire process.

MST/1591     

High-power HF laser pumped by an electron-beam-initiated chemical nonchain reaction

The development of a high-power HF laser pumped by a chemical nonchain reaction initiated by a radially converging electron beam is reported. A radiation energy of ∼ 115 J with an efficiency of ∼ 8% in terms of deposited energy has been achieved in a mixture with an active volume of ∼ 30 liters. It is shown that because of the high SF₆ density, the total pressure jump in SF₆–H₂(D₂) mixtures caused by the electron beam injection and the chemical reactions is several times smaller than that in the active mixtures of exciplex lasers for the same input energies. This factor considerably facilitates the development of wide-aperture HF and DF lasers with an SF₆ fluorine donor pumped by an electron-beam-initiated chemical nonchain reaction. Pis’ma Zh. Tekh. Fiz. 23, 58–64 (March 12, 997)     

Recording of long-period gratings in special single-mode fibers by a CO2 laser

The paper deals with the fabrication of long-period gratings (LPG) in special optical fibers with an inverted parabolic-index profile of the cladding by periodically exposing the fibers to radiation of a high-power CO₂ laser. Details on the fiber fabrication and preparation of LPGs with a period of about 1 mm in the fibers are presented. The fabricated LPGs were evaluated on the basis of their transmission spectra determined by measuring the output spectral density of the fiber with the LPGs. It is shown that the position of the band and its strength depend on the laser beam velocity, beam power and the interaction time of radiation with the fiber.     

Influence of laser array performance on spectrally combined beam

Abstract

Incoherent spectral beam combining (SBC) of multiple laser beams is accomplished along the emitters’ arraying direction. Considering that the output beams from a laser array (LA) usually have deflection angles, positional displacements and divergence angles even after being collimated, a propagation model of SBC systems based on multilayer dielectric gratings has been built up. On the basis, properties of the spectrally combined beam affected by parameters of the LA have been discussed in detail. Simulation results show that with the increase in the deflection angle, both the power and the beam quality of the combined beam degrade dramatically. The positional displacement has little impact on the intensity distribution and the beam quality of combined beam but change the wavelength composition of the combined beam. The divergence angle strongly affects the intensity distribution and the beam quality of the combined beam. Additionally, the effect of the deflection angle on the output beam quality is more obvious and may shift the beam spot when comparing with that of the divergence angle.     

Dual-beam CO2 laser cutting of thick metallic materials

A dual-beam technique involving two CO₂ gas lasers with a power capacity of 1.5 kW each, was used to cut steel and superalloy. A comparison with single-beam CO₂ laser cutting showed that dual beams were capable of enhancing the cutting thickness and speed without deteriorating the quality of cut. Heat-conduction models, assuming the laser beams as line sources, were used to estimate the cutting thickness and speed as a function of distance between the two laser beams. Experimental data, coupled with theoretical modelling, have provided a new concept, namely stretching the width of the laser beam in the direction of cutting to cut thicker section solids at moderate speeds.     

Processing aspects of laser surface alloying of titanium with aluminium

Abstract

An investigation has been made of the laser surface alloying of titanium using a continuous feed of aluminium powder. By means of a continuous wave CO₂ laser operating at 1·8 kW power, with a beam diameter of 3 mm, ranges of traverse speed (from 3 to 20 mm s^?1) and aluminium powder feed rate (from 0·03 to 0·11 g s^?1) were used to produce a series of alloyed zones with aluminium contents in the range ~20–80 at.-%. Conditions for obtaining reasonable homogeneity and reproducibility of composition were determined.

MST/1353     

Novel processing of NASICON and sodium carbonate/barium carbonate thin and thick films for a CO2 microsensor

The use of chemical microsensors can be advantageous for the monitoring of gas species in efforts to accurately monitor environmental conditions, detect fires, and determine emissions. The microsensor described herein shows promise for use as a carbon dioxide sensor. Carbon dioxide levels are of particular concern for passenger environments, as a chemical signature of a fire, and as a combustion product.This amperometric carbon dioxide microsensor was produced using microfabrication and micromachining techniques. Electron beam evaporation, sputtering, and thick film printing were used for deposition of the chemically active layers of the device—Na₃Zr₂Si₂PO₁₂ (NASICON) and Na₂CO₃/Ba₂CO₃. The deposition techniques used to deposit these materials were an important processing advance in this development endeavor. The films were incorporated into an amperometric, limiting current type sensor design with overall dimensions of 1.4 mm × 1.5 mm. The inclusion of this type of sensor in an array of differing sensors offers an advantage of high selectivity to analytes of interest.     

Development of a selective gas sensor utilizing a perm-selective zeolite membrane

Reported here is a novel sensor that utilizes a zeolite film to selectively limit gas exposure of the sensing surface. A unique amperometric sensor design based on a non-porous mixed conducting sensing electrode enables the formation of a continuous zeolite film covering the entire sensor surface. The sensor was tested in a variety of oxygen containing gases. The sensor without a zeolite film responded strongly to both oxygen and carbon dioxide at a bias of 1.8 V. In contrast, the sensor coated with a zeolite film showed a discernable, but diminished response to oxygen, and a more marked drop in response to CO₂ indicating that the diffusion of oxygen through the zeolite film is preferential to that of CO₂. The response of the zeolite coated sensor to a mixture of oxygen and carbon dioxide gases is attributed primarily to the oxygen content. Expanding this concept using a variety of different zeolite structures covering an array of sensors, complete analyses of complex gaseous mixtures could be performed in a very small device.     

Laser synthesis of nanopowders with yttrium aluminum garnet stoichiometry

A nanopowder of yttria-alumina mixture with the yttrium aluminum garnet (YAG) stoichiometry has been synthesized for the first time by the laser evaporation method. A high-power CO₂ laser with pulse duration above 200 μs, repetition frequency of 500 Hz, and pulse energy of about 1 J provided a high yield of powder at a rate of 24 g/h. The obtained nanopowder has been used to prepare YAG:Nd³⁺ ceramics with a cubic structure possessing an optical transmittance of about 77% at a wavelength of 1.06 μm. The successful synthesis of YAG nanopowder is based on the preliminary numerical simulation of the laser evaporation of a target using a three-dimensional model.     

A study of uv preionization pulsed tea CO2 laser

Abstract

A uniform volumetric discharge was obtained by means of an auxiliary UV preionization in a home‐made TEA CO₂ laser. The maximum output pulse energy of this laser system was about 12J per pulse with a pulse duration of 80 ns. The ratio of electric field to neutral particle density (E/N) in this laser was 7.6×10^‐16 V cm². The peak power and pulse shape of the laser were studied. The time delay between the predischarge and the maindischarge during the stable operation of this laser system has also been studied. It was observed that the laser was operating with uniform glow discharges when the time delayed between the predischarge and the maindischarge was in the range of 1.0 μs to 6.0 μs. The spark array used as a preionizer for producing the UV radiation in this system is new, simple, durable, and can be easily fabricated.     

Evaluating the effect of transmissive optic thermal lensing on laser beam quality with a shack-hartmann wave-front sensor

We examine wave-front distortion caused by high-power lasers on transmissive optics using a Shack-Hartmann wave-front sensor. The coupling coefficient for a thermally aberrated Gaussian beam to the TEM(00) mode of a cavity was determined as a function of magnitude of the thermally induced aberration. One wave of thermally induced phase aberration between the Gaussian intensity peak and the 1/e(2) radius of the intensity profile reduces the power-coupling coefficient to the TEM(00) mode of the cavity to 4.5% with no compensation. With optimal focus compensation the power coupling is increased to 79%. The theoretical shape of the thermally induced optical phase aberration is compared with measurements made in a neutral-density filter glass, Faraday glass, and lithium niobate. The agreement between the theoretical and the measured thermal aberration profiles is within the rms wave-front measurement sensitivity of the Shack-Hartmann wave-front sensor, which is a few nanometers.     

A high-performance CO2 laser heterodyne detector operating at 250 K

Abstract

The design, construction and assessment of a CO₂ laser heterodyne receiver based on a Hg_1-x Cd _x Te heterostructure is described. The structure has been designed to minimize the leakage currents at high temperatures while maintaining a high quantum efficiency. Measurements show that over 4dB of shot noise can be obtained with the application of less than 1 mW of laser power. This result can be combined with the measured low-frequency quantum efficiency of 42% to give a noise equivalent power of 7.2 × 10^?20 WHz^?1. The device is mounted on a single-stage thermoelectric cooler and coupled to an amplifier. It is hoped that the availability of high-performance detectors which do not need to be cooled to cryogenic temperatures will greatly improve the attractiveness of laser heterodyne systems.     

Life test of a sealed‐off CO2 laser with SnO2 coated cathodes

Abstract

A sealed‐off CO₂ laser with an intracavity structure and dual discharge sections has been built to test its lifetime under different treatments of cathodes. The output characteristics of this laser system, under different discharge conditions, have also been investigated. Several techniques, such as the sealing of optical components on a laser tube, the coating of tin‐oxide (SnO₂) on stainless steel cathodes, and the making of a discharge tube of three layers, are established to construct this tube. By using SnO₂ coated SUS 304 cathodes and a gas mixture of CO₂:N₂:He:Xe:H₂= 15:14:65:4:2 in this laser tube, an operational lifetime in excess of 1,800 hours and an output power up to 52W/m are obtained.     

Pulsed laser annealing of sodium super ionic conductor for carbon dioxide sensors

This paper discussed a way to improve solid electrolyte carbon dioxide (CO₂) sensor by excimer laser annealing of sodium super ionic conductors (NASICON). The CO₂ sensor used in this paper consists of a thin NASICON layer. We additionally annealed the NASICON to improve its electrical conductivity by pulsed excimer laser. The laser annealing results in re-crystallization of the NASICON thin film and changes the conductivity, grain sizes, and the structure of grain boundaries. From the scanning electron microscope pictures, we saw that NASICON grain sizes were enlarged after laser annealing. Grain sizes were also correlated to laser annealing energy and annealing times. After 2 times annealing of 420 mJ laser energy with 7 pulses each time at 1 Hz repetition rate, the conductivity of NASICON was increased by 90%. When the CO₂ concentration was changed from 1000 ppm to 5000 ppm, the sensor resolution was enhanced up to 66%. These results suggested that appropriate laser annealing treatment not only enlarges NASICON grain sizes but also reduces its resistance. Therefore, the NASICON CO₂ sensor resolution can be improved accordingly.     

Dynamics of the anode plasma in the magnetically insulated diode of the COBRA ion accelerator by modified laser deflectometry

A new type of optical system for measurements of laser beam deflection has been developed and implemented experimentally. The refractive index gradient in the anode plasma of a megavolt magnetically insulated diode in a high-power ion accelerator has been measured with submicron resolution. The refractive index distribution in the layer was measured during the pulse and the average electron density in the layer was determined. Pis’ma Zh. Tekh. Fiz. 23, 63–68 (May 26, 1997)     

CO2 laser deposition of diamond thin films on electronic materials

Laser-induced pyrolysis has been utilized to create gas-surface chemical reactions necessary for diamond deposition on electronic materials. A 1200 W CO₂ gas laser has been used as an energy source for depositing diamond thin films from a gas mixture of CH₄ and H₂ in a chemical vapour deposition chamber. The substrate temperature was about 500°C. The laser beam energy was largely absorbed by the gases that lead to their excitation and decomposition on contact with the nearby hot substrate. Raman spectroscopy and scanning electron microscope analysis revealed high quality, fine crystalline diamond structures.     

Densification and microstructural evaluation during laser sintering of M2 high speed steel powder

Abstract

In the present work, the densification and microstructure of M2 high speed steel powder processed by direct laser sintering method was studied. Test specimens were produced using a 200 W continuous wave CO₂ laser beam at different scan rates ranging from 50 to 175 mm s^?1. The building process was performed under argon and nitrogen atmospheres in order to evaluate the role of sintering atmosphere. It was found that the sintered density strongly depends on the laser scan rate and thus on the duration time of the laser beam on the surface of the powder particles. Generally, with a decrease in the scan rate higher densification was obtained. However, formation of large cracks and delamination of the sintered layers is feasible at low scan rates. The results also demonstrated that sintering under argon atmosphere yields better densification compared to a nitrogen atmosphere, in particular at higher scan rates. The microstructure of laser sintered parts consisted of large and elongated pores parallel to the building direction. The metal matrix structure was found to be heterogeneous, i.e. carbon rich austenite was formed due to carbon segregation. This structure consisted of fine cellulars or dendrites of martensite and retained austenite. This article describes the influence of manufacturing parameters on the densification of laser sintered M2 high speed steel powder. The microstructural features of the processed parts are also addressed.