Showing papers in "Journal of Laser Applications in 1997"

Temperature‐dependent absorptivity and cutting capability of CO2, Nd:YAG and chemical oxygen–iodine lasers

Abstract: The most widely used high power industrial lasers are the Nd:YAG and CO2 lasers. The chemical oxygen iodine laser (COIL), whose wavelength (1.315 μm) is between that of the Nd:YAG (1.06 μm) and CO2 (10.6 μm) lasers, is another high power laser for industrial applications. The cutting capability of these lasers is investigated in this paper. The cut depth strongly depends on the absorptivity of the cut material, kerf width and cutting speed. The absorptivity is an unknown parameter for which experimental data at high temperatures are currently unavailable. Theoretical values of the absorptivities of various metals are obtained using the Hagen‐Ruben relationship. It is found that the absorptivity of a metal is linearly proportional to the square root of its resistivity and also inversely proportional to the square root of the wavelength. The absorptivities of the COIL and Nd:YAG lasers are 2.84 and 3.16 times larger than that of the CO2 laser, respectively. Based on these theoretical values of the absorptivity, the cut depths for several metals are analyzed at various laser powers and cutting speeds for these lasers. For identical cutting parameters, the cut depths for stainless steel and titanium are deeper than those of most other metals. Due to the wavelength dependence of the absorptivity, the cut depths for COIL and Nd:YAG lasers are expected to be 2.84 and 3.16 times deeper than that for the CO2 laser.

Continuous wave Nd:YAG laser cladding modeling: A physical study of track creation during low power processing

Abstract: This paper concerns the modeling of cladding using an Nd:YAG laser operating at low powers typically less than 800 W. Experimental observation of the evolution of the mass of the clads shows two power thresholds. The theoretical study relies on a calculation of the fluence provided to the substrate and on a model of heat transfer into the substrate. We suggest that the first threshold is the power required for substrate melting. The second power is the threshold when the powder is directly melted by the beam and is therefore a liquid when contacting the substrate.

High power laser welding in hyperbaric gas and water environments

Abstract: A hyperbaric laser welding facility has been constructed and the feasibility of high power CO2 and Nd:YAG laser welding in both high pressure gas and water environments, to simulated water depths of 500 m, has been established. From initial trials on welding through water at atmospheric pressure, it was found that the different absorption characteristics of water to 10.6 μm (CO2 laser) and 1.06 μm (Nd:YAG laser) radiation proved crucial. The Nd:YAG laser was totally unsuitable as the beam was largely diffused in the water, whereas the CO2 beam was readily absorbed and, using high speed video equipment, was found to form a high irradiance channel and a dry region around the weld area. Welding under a high pressure gas environment produced a highly energized plume which prevented keyhole welding at pressures over 1 × 106 Pa. An investigation carried out into the efficacy of a gas jet delivery system to alleviate the extent of the plume showed that argon blown horizontally across the weld was the optimum configuration, extending the welding range up to 5 × 106 Pa. A limited investigation into high pressure underwater welding showed porosity to be a problem although sound welds were produced at pressures up to 2 × 106 Pa.

Scaling laws for thick‐section cutting with a chemical oxygen–iodine laser

Abstract: Almost all laser‐assisted materials processing involves melting, vaporization and plasma formation which affect the utilization of laser energy for materials processing. To account for the effect of these phases, an effective absorptivity is defined, and a simple mathematical model is developed for the cutting of thick‐section stainless steel using a high power chemical oxygen—iodine laser (COIL). The model is based on an overall energy balance, and it relates the cutting depth with various process parameters that can be used to predictively scale the laser materials processing performance to very thick sections. The effects of various process parameters such as laser power, spot size, cutting speed and cutting gas velocity on the cutting depth are discussed. The results of the mathematical model are compared with experimental data. Such a comparison provides a means of determining the effective absorptivity during laser materials processing.

Laser through‐cutting and drilling models for ablating/decomposing materials

Abstract: A previously‐developed three‐dimensional conduction model for scribing of a thick solid has been extended to predict the transient temperature distribution inside a finite thickness slab that is irradiated by a moving laser source, and the cutting rate and profile carved by evaporation of material. The laser may operate in CW or in pulsed mode (with arbitrary temporal intensity distribution) and may have an arbitrary spatial intensity profile. The governing equations are solved using a finite‐difference method on a boundary‐fitted co‐ordinate system. Results for cutting rates and profiles are presented for materials that ablate or decompose upon laser irradiation (without significant formation of liquid), for different material thicknesses, traverse speeds, and pulsing conditions. For drilling (zero traverse speed), a numerically much more efficient two‐dimensional axisymmetric version of the model has also been implemented, and similar results for drilling behavior are also presented.

Ultrashort laser pulse bioeffects and safety.

Abstract: Recent studies of retinal damage due to ultrashort laser pulses have shown that less energy is required for retinal damage for pulses shorter than 1 ns than that for longer pulses. It has also been shown that more energy is required for near-infrared (NIR) wavelengths than in the visible because the light focuses behind the retina, requiring more energy to produce a damaging fluence on the retina. We review the progress made in determining the trends in retinal damage from laser pulses of 1 ns to 100 fs in the visible and NIR wavelength regimes. We have determined the most likely damage mechanism(s) operative in this pulse width regime.

Laser safety analysis of a retinal scanning display system

Abstract: The Virtual Retinal Display (VRD) is a visual display that scans modulated laser light on to the retina of the viewer's eye to create an image. Maximum permissible exposures (MPE) have been calculated for the VRD in both normal viewing and possible failure modes. The MPE power levels are compared to the measured power that enters the eye while viewing images with the VRD. The power levels indicate that the VRD is safe in both normal operating mode and in failure modes.

Effective laser ablation of enamel and dentine without thermal side effects

Abstract: We present a feasibility study into laser treating dental materials by using femtosecond pulses generated by a titanium:sapphire laser system which consisted of an oscillator and a regenerative amplifier. The pulse duration was varied between 200 fs and 2 ps. The observed energy thresholds for the ablation process of dentine and enamel were clearly smaller than those observed when longer pulse durations were used. The consequence of this observation is a lower thermal load within the vicinity of the radiated area. Thus no thermal damage or mechanical damage, such as cracks, were produced during the laser treatment. Commercially available femtosecond laser systems can produce ablation rates in healthy and in‐vitro demineralized dental material of 2 mm3 per min and 6–16 mm3 per min, respectively. These values are an order of magnitude larger than those produced by picosecond laser systems at the same pulse energy and pulse repetition rate. The brightness of the plasma spark generated by the laser treatment depended on the extent of the demineralization of the teeth. This may allow online control of the laser treatment.

Effect of nozzle orientation on the gas dynamics of inert-gas laser cutting of mild steel

Abstract: This investigation applies sonic and supersonic coaxial and off‐axis nozzles to the cutting of mild steel sheet of 1–4 mm thickness. A 1.5 kW CW CO2 laser is used with nitrogen as the assist gas. Sonic coaxial cutting is compared to that of off‐axis nozzles, which vary in orientation from 20–60° behind the laser axis and in exit Mach number from 1 to 2.4. Results show a 50% increase in maximum cutting speed at approximately a 40° off‐axis nozzle angle. In comparison, variations in off‐axis nozzle exit Mach number have little effect. A scale‐model kerf was used to visualize the kerf gas dynamics, revealing that nozzle angles of 0–20° cause a shock wave/boundary layer interaction with flow separation inside the kerf. Angles of 20–45° alleviate this interaction, producing a uniform supersonic flow throughout the kerf, which yields high cutting speeds due to uniformly high shear forces. For nozzle angles greater than 45°, the assist gas is diverted away from the kerf, reducing cutting speed. Cut edge quality observations were also conducted to verify the results of the model tests.

Experimental study of cutting thick aluminum and steel with a chemical oxygen–iodine laser using an N2 or O2 gas assist

Abstract: A chemical oxygen–iodine laser (COIL) was used to cut aluminum and carbon steel. Cut depths of 20 mm in aluminum and 41 mm in carbon steel were obtained using an N2 gas assist and 5–6 kW of power on target. The same laser at the same power level produced a cut depth of 65 mm in carbon steel with an O2 gas assist; a low quality cut to a depth of nearly 100 mm in carbon steel was also demonstrated. These data are compared with existing COIL and CO2 laser cutting data. COIL cuts carbon steel and stainless steel at approximately the same rate. For a given cut depth, power and spot size, COIL cuts steel approximately three times faster than a CO2 laser using an inert gas assist. COIL cutting speeds in carbon steel are improved by approximately a factor of three when an O2 assist is used in lieu of an N2 gas assist. With an N2 gas assist, COIL cuts aluminum at approximately the same rate as CO2 cuts steel. To improve the agreement between data and an existing theoretical cutting model, an empirical correction factor was added to the model; this modification provides excellent agreement with data.

Laser cladding with a pulsed Nd:YAG laser and optical fibers

Abstract: A new laser cladding technique for producing wear and corrosion resistant surfaces on engineering components is reported. The technique involves the combination of the pre‐placed and injected powder delivery methods for producing clad layers. The clad layers were formed with a pulsed Nd:YAG laser and optical fibers. The results using a nickel‐based alloy (Hastelloy C) indicate that uniform clad layers in excess of 1 mm in thickness can be achieved in a single pass with 240 W average power incident on the workpiece. The laser beam was delivered through a step‐index glass fiber of length 5 m and core diameter 600 μm. The clad layer obtained is homogeneous, without porosity or cracks, and has very low dilution (less than 2%) by the substrate, to which it is bonded. These results, combined with a relatively high powder utilization efficiency (about 60% when cladding a flat plate) and coverage rate (8 mm2 s−1), make the technique competitive with that using continuous wave lasers with powers greater than 1 kW, but at a quarter of the input power. The low heat input makes the technique particularly attractive for cladding thin substrates and engineering components that require both corrosion‐ or wear‐resistant surfaces and high dimensional stability. In addition, the use of fiber optics to deliver the laser beam to the workpiece offers great opportunities for surfacing components remote from the laser source.

Experimental development of a machining database for the CO2 laser cutting of ceramic tile

Abstract: This paper covers the cutting of commercially‐available ceramic tiles using a CO2 laser cutting machine, with the object of producing a laser beam machining database which contains the essential parameter information for their successful processing. Various laser cutting parameters were investigated that would generate a cut in ceramic tile which required minimal posttreatment. The effects of various shield gases, multipass cutting and underwater cutting were also examined.

Bioeffects of near-infrared lasers

Abstract: Retinal damage thresholds (ED50s) were determined in Rhesus monkey eyes for 100 ms exposures to collimated radiation from a tunable Ti:Sapphire laser at several wavelengths from 690 nm to 900 nm. Prior research using 15 ns duration laser pulses showed a strong variability of ED50 with wavelength for retinal exposure in Rhesus monkeys to laser radiation in the near infrared spectrum. Current studies with the Ti:Sapphire laser show similar variability of ED50 with wavelength for 100 ms retinal exposures. Previously measured light transmission and absorption properties of ocular tissues do not provide a complete or obvious explanation for the significant variations of threshold with small changes in wavelength. Similar wavelength dependencies of ED50 for the two exposure durations in the wavelength range of 750 nm to 830 nm suggest that linear absorption is a cause of the variability. However, differences in the ED50 curves at other wavelengths show that nonlinear mechanisms also contribute.Retinal damage thresholds (ED50s) were determined in Rhesus monkey eyes for 100 ms exposures to collimated radiation from a tunable Ti:Sapphire laser at several wavelengths from 690 nm to 900 nm. Prior research using 15 ns duration laser pulses showed a strong variability of ED50 with wavelength for retinal exposure in Rhesus monkeys to laser radiation in the near infrared spectrum. Current studies with the Ti:Sapphire laser show similar variability of ED50 with wavelength for 100 ms retinal exposures. Previously measured light transmission and absorption properties of ocular tissues do not provide a complete or obvious explanation for the significant variations of threshold with small changes in wavelength. Similar wavelength dependencies of ED50 for the two exposure durations in the wavelength range of 750 nm to 830 nm suggest that linear absorption is a cause of the variability. However, differences in the ED50 curves at other wavelengths show that nonlinear mechanisms also contribute.

Ophthalmoscopic and pathologic description of ocular damage induced by infrared laser radiation

Abstract: This study compares ocular damage effects induced by infrared (IR) lasers in the “eye-safe” wavelength range to those observed following visible wavlength laser exposures In addition to routine fundus camera and slit-lamp observations, exposed subjects were examined via confocal scanning laser ophthalmoscopy Histopathologic evaluation was conducted on eyes with both visible and IR laser induced focal lesions IR laser exposure parameters which resulted in corneal, lenticular and/or retinal damage will be detailed Unique aspects of the IR laser–tissue interaction will be discussed, including concurrent lesion formation in several tissues and secondary responses such as delayed inflammatory reactions This latter effect may involve tissue not directly irradiated during the laser insult Finally, implications for laser safety standards will be suggested

Air contaminants generated during laser processing of organic materials and protective measures

Abstract: As compared to other thermal processes, during laser material processing of organic materials gaseous and particulate emissions are generated.Characteristic for laser-generated air contaminants (LGACs) are the large number of components, the chemical complexity, and the varying emission rates. The characteristics and hazards of these LGACs must be well known, in order to take adequate protection measures to ensure a safe laser workplace and to achieve a high environmental compatibility. Within research projects and the framework of the European project Eurolaser: EU 643 “Safety in the industrial applications of lasers,” emissions have been characterized and medically assessed. Studies show that the quantity, the composition, and the chemical complexity of the LGACs are highly dependent on the laser application, the process parameters and the material processed. Generally, the particles generated are very small and therefore can be mainly respirated. For some plastics, like polyamides, the fume particles are viscous and can easily stick together. The gaseous hydrocarbon emissions usually consist of substances referring to the chemical structure of the material machined, often, the monomers. Aromatic hydrocarbons and polycyclic aromatic hydrocarbons are detected in all cases. Investigations on filter systems show that nonadhesive particles can be filtered efficiently with self-cleaning surface filters; sticky particles, however, cause high pressure loss in the filter and the capture efficiency will rapidly drop. For organic gaseous emissions, no standard filter solution is available. In general, adsorption techniques, catalytic combustion, or biological filtration can be used.

The drilling of slate tiles by a Nd:YAG laser

Abstract: We have undertaken a series of experiments to investigate the feasibility of using a Nd:YAG laser to drill slate tiles. Slate is used primarily as a roof building material after it is split into thin flat layers. The traditional fixation system of slate tiles in Europe consists of clamping the slate pieces by means of metallic clamps and overlapping the different tiles in order to form an impervious roof. An alternative to this clamping technique, is the use of nails to fix the slate tiles to a wooden frame. This method improves the fixation of the slate tiles and reduces the costs related to roof repair by extending the period between maintenance. However, the use of nails requires the production of holes in the slate pieces, which is currently achieved by a mechanical route that results in large holes and an extended manufacturing time. The aim of this work is to explore the capabilities of a pulsed Nd:YAG laser to drill tiny holes in slate tiles in order to produce a better quality drilled slate tile in a reduced time. Shallow holes were produced in slate samples using a 1 kW Nd:YAG pulsed laser focused onto the slate surface by an 80 mm fused silica lens. The results obtained in this study shows the enormous potential of Nd:YAG lasers in the production of slate tiles.

Improved hole drilling using a high peak power Nd:YAG laser at the second harmonic wavelength

Abstract: We have previously described the drilling of advanced materials such as intermetallics, superalloys, and composites using an Nd:YAG laser operating at the fundamental wavelength of 1.064 μm [1]. We have now extended these studies to the second harmonic wavelength of 532 nm of a high peak power Nd:YAG laser. The results show that drilling using the shorter wavelength further reduces undesired thermal effects such as recast layers and heat affected zones. A comparison of these results with those obtained at the fundamental wavelength is reported.

Relationship between airborne acoustic and optical emissions during laser welding

Abstract: This paper describes experimental work directed at understanding the relationship between acoustic and optical emissions from the laser welding process. Laser welds were performed and the emissions were measured with a microphone and photodiode and recorded with a data acquisition system. A time‐domain correlation analysis of the acoustic and optical signals revealed them to be highly related at a phase shift corresponding to the delay time for sound to propagate from the weld area to the microphone. A moving average model which predicted the acoustic signal from light signal measurements was fitted to experimental data using least squares regression. The acoustic signal predicted by the model was found to correspond fairly well to the experimental acoustic data. In addition, the least squares model indicated a very simple relationship between the two signals: the sound pressure predictions were proportional to the time derivative of the light signal samples. This relationship was explained by physical analysis of the mechanism for generation of sound and light signals by the plume.

Induced transmittance of eye-protective laser filters

Abstract: Different kinds of eye-protective laser filters were investigated with short laser pulses with respect to their transmittance and stability in dependence on the irradiance. The irradiance was varied between 3×106 and 3×1013 W/m2. The investigations were performed at the wavelengths of 1064 and 532 nm using a Nd:YAG laser with a pulse duration of 15 ns and at a wavelength of 798 nm generated by a Ti:Sa laser with a pulse duration of about 250 fs. For filters based on semiconductor-doped glasses, an increase in the transmittance was observed with increasing irradiance at 532 nm. It extends over six orders of magnitude due to the saturation of the absorption. A similar dramatic effect was measured also on polycarbonate filters at 1064 nm. The dependence of the transmittance on the irradiance is discussed in a phenomenological model to show the importance of the relaxation time, the concentration of the absorption centers, and the absorption cross section for the nonlinear behavior. Taking into account two ab...

Threshold laser beam irradiances for melting and welding

Abstract: A model based on the conservation of energy for a moving heat source incident on a flat plate is used to predict the threshold laser beam irradiance required to initiate melting on a metal surface. With the use of a non‐dimensional variable and its solution, the threshold irradiance can be predicted with the use of a simple equation. This equation is a function of the absorptivity of the surface, thermal conductivity of the metal, temperature increase for melting, diameter of the laser beam at the surface and the non‐dimensional variable value for the ratio of the thermal diffusivity of the metal to the product of the beam diameter and velocity used. Laser beam irradiances exceeding these predicted values are required for welding. The model predictions are validated with experimental results of beam irradiances required for welding different metals.

Integration of real time quality control systems in a welding process

Abstract: The automation of laser welding processes requires the control of the various process components as well as the control of the laser—material interaction. These systems are essential for ensuring the quality of the weld seam as they are able to react to dynamic fluctuations during the process. During the process various phenomena occur which are potential sources of diagnostic signals: these include thermal, electrical, optical, mechanical and acoustic events. This paper presents the development of an integrated real‐time system based on the monitoring of several of these phenomena that provide complementary information for the detection of different defect types. A system based on the use of infrared photodiodes has been validated and successfully applied to the detection of the appearance of geometric defects such as sagging or misalignment in the weld seam. A complementary system to monitor the airborne acoustic signals of the process is under development and the experimental results for defining the acoustic pattern of welding defects are presented.

Evolving issues in laser safety

Abstract: The approach to laser safety has come a long way since the 1960s when the first guidelines were issued by defense research organizations in the US and the UK, and then by the American Conference of Governmental Industrial Hygienists. Although the search for ‘eye‐safe’ numbers continues in a few laboratories, this work is almost exclusively centered on deriving retinal thresholds for ultra‐short (sub‐nanosecond) lasers. Setting limits in this temporal region has been difficult, since there are conflicting data sets and there is a limited amount of data to extrapolate to other spectral regions. In the standards arena, the concentrated efforts have been in terms of product classification and attempts to resolve the eternal question of ‘how safe is safe?’. Recent efforts to revise safety standards have not always taken into account the historical rationale for the maximum permissable exposures and forget that safety factors were already factored into the limits, and further safety factors are quite unnecessary. Finally, the study of accidents raises the question of whether our approach to eye protection and enclosures are adequate and whether separate standards and guidance is needed for different applications.

Neural motor ocular strategies associated with the development of a pseudofovea following laser induced macular damage and artificial macular occlusion. Is the fovea replaceable

Abstract: Loss of visual acuity following acute photic injury and subsequent recovery is well known. The mechanisms that underlie foveal neural plasticity were assessed in this study. In two patients that recovered to 20/15 Snellen visual acuity following accidental induced foveal damage, contrast sensitivity measurements made under ophthalmoscopic visualization of the retina revealed a preferred retinal location that was superior and temporal to the damaged fovea. This area was used especially for smaller contrast sensitivity targets requiring foveal resolution. Similar preferred retinal location sites were obtained in similar investigations using a Purkinje Eye-Tracker to simulate a small central scotoma. Measures of ocular accommodation obtained in these experiments suggest that a central scotoma may induce an accommodative shift as the oculomotor system shifts to a new PRL for smaller targets that require foveal resolution.

Experimental determination of retinal damages thresholds induced by multiple picosecond laser pulses

Abstract: Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. This effect is a reduction in the threshold energy per pulse relative to the single pulse threshold. The best fit to experimental data predicts that the threshold energy per pulse decreases in proportion to the fourth root of the number of pulses n. This model has been reported to be in agreement for exposures ranging from picoseconds to seconds. However, the very limited data reported in the sub-nanosecond domain of time were obtained with a low pulse ratio frequency. No data exists in the litterature concerning the effect on the retina of ultrashort pulses delivered with a very high repetition rate. Also, it has been hypothesized that one cannot extrapolate from longer pulses width because the ultrashort pulse (single or multiple) induce effects which may involve fundamentally different mechanisms of damage. Thus, some laser safety guidelines do not specifically provide exposure limits for a single-pulse duration less than 1 ns because of a lack of biological data1. If exposure limits are given, the limit values for these exposure times has been derived by maintening the irradiances applicable to nanosecond pulses2. With the goal in mind to obtain adequate data base for single and multiple ultrashort pulses, an experimental study has been directed toward determining the retinal damage thresholds induced by picosecond pulses emited in the visible spectrum.Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. This effect is a reduction in the threshold energy per pulse relative to the single pulse threshold. The best fit to experimental data predicts that the threshold energy per pulse decreases in proportion to the fourth root of the number of pulses n. This model has been reported to be in agreement for exposures ranging from picoseconds to seconds. However, the very limited data reported in the sub-nanosecond domain of time were obtained with a low pulse ratio frequency. No data exists in the litterature concerning the effect on the retina of ultrashort pulses delivered with a very high repetition rate. Also, it has been hypothesized that one cannot extrapolate ...

Intrapulpar temperature during continuous CO2 laser irradiation in human molars: An in vitro study

Abstract: To establish safety parameters, we in vitro studied the increase in intrapulpal temperature caused by the use of a cw CO2 laser. A thermistor was implanted in the inner part of the pulpal chamber of 25 human lower third molars to measure the intrapulpal temperature produced by laser powers between 2–10 W and exposure times of 0.5–25.0 s. The Pearson linear correlation factor applied to the measured values showed there is a direct relationship between the independent variable and the applied power. A variance analysis produced the linear regression equation: T = 1.10 + (0.127)E where T is the temperature and E the energy. The results showed that, with a power of 4 W and maximum exposure time of 2.5 s (10 J) and a power density of 12738.85 W cm−2, there will be no damaging reactions affecting the pulpal tissues.

In‐process monitoring of laser welding by the analysis of ripples in the plasma emission

Abstract: A novel in‐process monitoring system employing two detectors set above the workpiece at different aiming angles of 5° and 75° has been developed to detect whether or not CO2 laser welding fully penetrates through to the back surface of steel sheets. The acquired signal contained a.c. components of the emission of the laser‐induced plasma in the plume and in the keyhole with frequencies up to approximately 10 kHz. The mean square value of the a.c. signal obtained by using the 75° sensor during full penetration welding was much larger than that of the partial penetration welding, showing that full penetration welding can be monitored with a high accuracy by using the 75° sensor.

Epithelial damage thresholds for sequences of 80 ns pulses of 10.6 μm laser radiation

Abstract: Infrared radiation from a CO2 laser operating at 10.6 μm is almost totally absorbed by the tear film and corneal epithelium. Epithelial damage thresholds for single and multiple pulse exposures have been well characterized for exposures having pulse durations ⩾1 ms and a few damage thresholds have been reported for much shorter duration pulses. However until recently, no studies have been done to determine damage thresholds from sequences of very short pulses. The purpose of this study is to begin to fill this gap. New Zealand white rabbit corneas were exposed to sequences of 80 ns pulses from a CO2-TEA laser operating in the (TEM)00 mode (Gaussian beam profile). The damage threshold was defined as the energy density, EDth, that produced minimal superficial epithelial lesions. Thresholds have been determined for sequences of 2, 8, 32, 128, and 1024 pulses at a rate of 10 Hz. The damage data are shown to be correlated by empirical laws relating either the threshold energy density per pulse to the number of pulses, or the total threshold energy density to the duration of the pulse train. Research is underway to obtain data for pulse sequences at greater pulse rates and to examine the relative importance of thermal and acoustic damage mechanisms.Infrared radiation from a CO2 laser operating at 10.6 μm is almost totally absorbed by the tear film and corneal epithelium. Epithelial damage thresholds for single and multiple pulse exposures have been well characterized for exposures having pulse durations ⩾1 ms and a few damage thresholds have been reported for much shorter duration pulses. However until recently, no studies have been done to determine damage thresholds from sequences of very short pulses. The purpose of this study is to begin to fill this gap. New Zealand white rabbit corneas were exposed to sequences of 80 ns pulses from a CO2-TEA laser operating in the (TEM)00 mode (Gaussian beam profile). The damage threshold was defined as the energy density, EDth, that produced minimal superficial epithelial lesions. Thresholds have been determined for sequences of 2, 8, 32, 128, and 1024 pulses at a rate of 10 Hz. The damage data are shown to be correlated by empirical laws relating either the threshold energy density per pulse to the number of...

Laser generated air contaminants released during laser cutting of fabrics and polymers

Abstract: Environmental monitoring was conducted at an industrial facility to qualitatively identify the major contaminants generated while cutting fabrics and polymers with a 25 W CO2 continuous beam laser. Carbon monoxide, hydrogen cyanide, and particulates were also assessed, and a bulk sample of residue from the laser exhaust duct was analyzed for inorganic acids, pH, and volatile organic compounds. Samples were collected while cutting vinyl, acrylics, woven fabrics, felt, Formica®, and Plexiglass®. The laser parameters were standardized to allow for meaningful comparison of results for each target material. The volatile organic compound samples were collected in multibed sorbent tubes with subsequent analysis via thermal desorption and gas chromatography/mass spectroscopy. Depending on the material being cut, a wide variety of compounds were detected. The highest relative concentrations of volatile compounds were found during laser cutting of felt fabrics. The lowest concentrations and fewest number of compounds were from woven fabrics. The compounds detected included hydrochloric acid, aldehydes, benzene, vinyl chloride, various acrylates, acrylonitrile, acetonitrile, styrene, furans, phenol, and butyl cellosolve. Methyl methacrylate was a significant peak detected during the laser cutting of acrylic ester polymers, Plexiglass, and polyvinyl chloride with adhesive backing. Carbon monoxide was not detected above background (2 ppm) during any of the laser cutting trials. Hydrogen cyanide was detected during the laser cutting of felt (15 ppm) and Formica® (8–10 ppm). Particles ⩾ 0.3 μm in diameter (μmd) generated during the laser cutting exceeded background particle levels by a factor of ten or more. Most compounds detected in the thermal desorption air samples were also detected in the bulk sample, and the residue was acidic (pH = 3). Area samples collected outside the laser enclosure suggested the local exhaust ventilation system sufficiently contained the air contaminants.

Laser cladding of Al‐Sn alloy on a mild steel

Abstract: Direct laser cladding of Al alloy on mild steel produces continuous brittle intermetallic FexAly layers at the interface resulting in weak bonding of the clad and the substrate. To overcome this problem, both Ni and Al were investigated as intermediate coatings. Although a Ni coating with a thickness of 70 μm was successful in eliminating the formation of continuous Al‐Fe intermetallics, the bonding at the Al alloy/Ni interface was not strong enough to survive a bending test. A pure Al coating was found to be a better barrier layer. In both cases the optimization of processing parameters for Al alloy cladding was realized via microstructural screening and mechanical property testing including hardness, preliminary wearing and bending tests. Operating windows were established in terms of low dilution, freedom from cracks and porosity, coating-substrate bonding and chemical homogeneity. The results of a comparative study indicated that the laser clad layers showed superior wear resistance to the conventiona...

Histological study of retinal damages induced by multiple picosecond pulses

Abstract: The current laser safety standards do not define the exposure limit values for pulsewidth less than one nanosecond. It has been hypothesized that one cannot extrapolate from longer pulse widths because the ultrashort pulse contains high peak power and may induce non linear effects. These effects may involve fundamentally different mechanisms of damage particularly for repetitively ultrashort pulses1. Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. No data exists in the litterature concerning the histological effects on the retina of ultrashort pulses delivered with a high repetition rate.The current laser safety standards do not define the exposure limit values for pulsewidth less than one nanosecond. It has been hypothesized that one cannot extrapolate from longer pulse widths because the ultrashort pulse contains high peak power and may induce non linear effects. These effects may involve fundamentally different mechanisms of damage particularly for repetitively ultrashort pulses1. Most laser safety documents advise the laser user that caution must be used in the evaluation of exposure to repetitively pulsed radiation since they are only limited data on multiple pulse exposure criteria. The empirical multiple pulse formula is based on some data indicating that there is generally a cumulative effect in multiple-pulse exposures. No data exists in the litterature concerning the histological effects on the retina of ultrashort pulses delivered with a high repetition rate.