Journal of Laser Applications

Published by Laser Institute of America
Online ISSN: 1938-1387
Print ISSN: 1042-346X
Low energy laser therapy has gained varying acceptance as a treatment for a broad range of soft tissue, musculoskeletal and neurological conditions. A controversial, but surprisingly large body of research with cell cultures suggests that laser irradiation can nondestructively alter cellular processes. Unfortunately, animal and human studies are often contradictory and difficult to evaluate due to differing study designs. As a result, the clinical effectiveness of low energy laser therapy remains debatable. Nevertheless, the findings are intriguing and deserve further investigation.
Past trials with soft and calcified tissues have demonstrated that long pulse train (2.5 mus) Er:YAG (2.94 mum) laser may be used to ablate tooth structure of human teeth. Determination of physical and thermal damage to surrounding tissue during removal of enamel and dentin is a primary objective of this study. Extracted human teeth with thermal probes imbedded in the pulp chambers were submitted to cavity preparation using an Erbium YAG laser with water mist. Wavelength selection as well as use of a water mist during the procedure resulted in efficient tissue removal without significant surrounding damage. Ground sections and SEM sections of teeth showed little or no melting or ash formation in adjacent dentin and enamel and no visible change in the pulp chamber. The surfaces produced by laser ablation were rough and irregular with craters and grooves. Average temperature change in the pulp chamber monitored during tooth preparation was 2.2 degrees Centigrade. These findings suggest that constantly available water aids vaporization and microexplosions, increasing the efficiency of tooth structure removal, and aids in cooling of the tooth structures. The long pulse Er:YAG (2.94 mum) laser may be an effective method for tooth reduction applications when used with a water mist.
Limited steerability and injury to the normal vessel wall are major drawbacks of laser coronary angioplasty. To overcome these limitations a new generation of laser systems has been developed which allows not only to eliminate the atherosclerotic plaque but to guide the laser beam by analyzing the laser induced tissue fluorescence (= spectroscopy) for the treatment of the atherosclerotic vessel. An excimer laser (MAX 10 LP, 308 nm, Technolas, Munich, Germany) was used with an emitting (phi 1070 microns) and a detecting (phi 130 microns) optical fiber to induce tissue fluorescence which was analyzed quantitatively by a computerized system. Specimens from the descending (thoracic) aorta were obtained from 24 patients (mean age 68.1 years, range 44-92). Tissue fluorescence was induced with ablating (26-30 mJ/mm2) and nonablating (3 mJ/cm2) laser activations. The emitted fluorescence (range 380-575 nm) was normalized to a wavelength of 380 nm; as a measure of tissue fluorescence the intensity ratio at 500 nm divided by 400 nm was calculated in normal (n = 78), mildly atherosclerotic (n = 40), and severely atherosclerotic (n = 48) tissue samples. Repeated laser activations were carried out and tissue fluorescence was checked until the fluorescence spectrum was normalized. All tissue samples were analyzed histologically by a semiquantitative score. Normal tissue samples showed the highest intensity ratios (5.9 +/- 3.4), whereas mildly (2.9 +/- 1.3) and severely atherosclerotic (2.1 +/- 1.0) samples elicited a significantly reduced fluorescence. Repeated tissue ablations were associated with a normalization of fluorescence intensity ratios in the mildly (7.0) as well as in the severely diseased (4.9) vessels. A curvilinear relationship between intensity ratio and the semiquantitative score was observed (r = 0.66) as well as between intensity ratio and intimal wall thickness (r = 0.62). No gender related differences were found but there was an inverse relationship between fluorescence intensity ratio and age (r = 0.56) as well as between intimal thickness and age (r = 0.41). Excimer laser spectroscopy allows reliable detection of atherosclerotic vessel alterations. Fluorescence intensity ratio is inversely proportional to the intimal wall thickness and the severity of the histologic alterations. There is an age dependency of fluorescence intensity ratio which can be explained by an increase in intimal wall thickness. Successful tissue ablation can be obtained by laser angioplasty and allows determination of the optimal point where complete tissue ablation is achieved by laser activation. Thus, excimer laser spectroscopy is an effective method for selective tissue ablation by laser angioplasty.
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 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 time pulse energy and pulse repetition rate. The brightness of the plasma spark generated by the laser treatment depended on the dimineralization of the teeth. This may allow online control of the laser treatment.
Federal law requires that all laser products that are imported into or introduced into commerce in the United States comply with the performance standard published in the Code of Federal Regulations (CRF), Title 21, Parts 1040.10 and 1040.11, administered by the Center for Devices and Radiological Health (CDRH), US Food and Drug Administration. Although it contains somewhat different requirements for hazard classification, engineering controls and labeling, the ANSI Z136.1 standard defers to the CDRH standard. The CDRH standard became effective in August, 1976 and was amended, in 1978 and also in 1985. In the early 1990s, US experts met to formulate an approach to bring the requirements of the CDRH standard and those of the International Electrotechnical Commission (IEC) standard, IEC 825, into closer agreement in order to lower barriers to international trade and to remove any excessive compliance burdens on manufacturers. In 1993, the CDRH published, formally in the Federal Register and informally, a Notice of Intent to amend the CDRH standard. Responses to those notices have now been analyzed and informal draft amendments were distributed in 1996. This draft is now being prepared for formal issuance as a Notice of Proposed Rulemaking. Meanwhile, the IEC standard was amended in 1993 and republished as IEC 825-1; these amendments created considerable controversy since they resulted in over classification of the hazard of many products, especially light emitting diodes (LEDs) that have a large divergence and increased source dimensions. Additional amendments are now being developed to correct this problem. The CDRH has carefully monitored developments in the IEC and actively participated in its proceedings as a guide in developing its own proposal. This paper describes the major changes that are being proposed for the CDRH standard and presents some rationale for the major changes. The more significant changes include expansion of applicability to include LEDs, reduced emission durations for classification, revised measurement for hazard classification, reduced performance requirements for lower power visible radiation products, and revised requirements for medical products.
Since the introduction of the first dedicated dental laser system in 1988, the increased utilization of lasers within the dental profession has resulted in the expression of numerous concerns by national and state regulatory boards, experts within the field, and other authorities. The present article attempts to identify specific problem areas which currently confront laser use in dentistry, and proposes some possible solutions to this dilemma.
At the 279 bed Marymount community hospital, Cleveland, Ohio, the importance of providing a safe laser environment for patients as well as staff was identified. The decision was made to cross-train all full-time RNs, LPNs, and SAs to allow flexibility in staffing all laser procedures with a qualified operator. A competency based training program was implemented, utilizing a combination of techniques to assist the adult learner. Selected video tapes were used to teach laser physics and applications. Comprehension was assessed through written and oral quizzing. Individual time was spent familiarizing trainees with the equipment and accessories. Practice setups were performed until that individual reached a comfort level satisfactory to him/her and the laser coordinator. Actual experience in the various surgical specialties was dependent upon our case load. Initially very close supervision was provided. Each trainee was gradually given more autonomy as his/her skills improved and permitted to operate the laser independently after passing our competency assessment. During the first year, each operator was given competency assessment reviews at regular intervals. This approach was found to have both strengths and weaknesses and the program was modified accordingly.
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.
AT PRESENT there is some discussion to consider the conversion of military project technology to civilian applications. There are a number of opportunities where we can consider, in a rather superficial manner, some of the practical features of such conversion, which could be of benefit not only for biomedical uses in the military but also for biomedical applications in civilian medicine and surgery. Many military laser systems have been studied in detail and it appears at least some of these could be converted to important civilian uses.
The description of special features of laser light interaction with biotissues, such as the skin, eye and dental tissues etc., with respect to laser diagnostics and therapy methods development is done. Optical models of transparent and turbid biotissues are analyzed. The role of static and dynamic light scattering in the light dosimetry, tissue heating, and receiving information of biotissue optical parameters, its structure, movements, and vibrations is considered.
Photodynamic therapy of transplantable N-[4-(5-nitro-2-furyl)-2-thiazolyl] formamide-induced tumors engrafted onto Fischer CDF (F-344)/CrlBR rats that had been sensitized with the photosensitizer tin (ll) etiopurpurin dichloride was performed in combination with visible light (approximately equal to 660 nm) emitted by either a continuous wave argon-dye laser or a pulsed, frequency-doubled Nd:Yag laser. Tumor control was assessed either by tumor dry-weight 12 days after treatment or by the palpatory absence of tumor at 60 days after treatment. Both laser sources were effective in creating the desired photodynamic effect. This study demonstrates the potential for the use of a solid-state pulsed laser for photodynamic therapy when used in combination with the tumor sensitizer tin (ll) etiopurpurin dichloride.
Complications are often produced with the removal of bone cement from the femoral cavity in the treatment of a failed hip prosthesis. Apart from being slow and difficult the conventional process runs the risk of producing damage to the femur. Ultrasonic techniques have been suggested to achieve these ends but removal of the cement by this approach is not entirely easy. The alternative laser-based approach would seem to have significant advantages over conventional techniques. The laser is capable of delivering energy to a specific region or surface under close control. The choice of laser is determined by its ability to ablate the cement and the ease with which it can be delivered to the base of the femur cavity. This paper examines several laser wavelengths: CO2 (10.6 microns), excimer (248 nm), Hol:YAG (2.12 microns), and presents polymethylmethacrylate (PMMA) vaporization thresholds for each laser.
The five years survival rate of deep-seated malignant brain tumors after surgery/radiotherapy is virtually 100% mortality. Special problems include: (1) lesions often present late; (2) position: lesions overlies vital structures, so complete surgical/radiotherapy lesion destruction can damage vital brain-stem functions; and (3) difficulty in differentiating normal brain from malignant lesions. This study aimed to use the unique properties of the laser: (a) to minimize damage during surgical removal of deep-seated brain lesions by operating via fine optic fibers; and (b) to employ the propensity of certain lasers for absorption of (nontoxic) dyes and absorption and induction of fluorescence in some brain substances, to differentiate borders of malignant and normal brain, for more complete tumor removal. A fine laser endoscopic technique was devised for removal of brain lesions, which minimized thermal damage and shock waves. A compatible endoscopic fluoroscopic laser technique was developed to differentiate brain tumor from normal brain.
This paper presents pragmatic particulars pertaining to plume protection problems with special emphasis on the health care environment. Practical approaches to optimizing protection are discussed with an eye toward facilitating the safety of personnel and the general health care environment from particulates, parasites, viruses and other undesirable entities, elements and compounds.
Laser products that are sold in the United States are required to comply with the regulations published by the Center for Devices and Radiological Health (CDRH) within the US Food and Drug Administration [1]. The need to certify products is straightforward in most instances, however, the applicability of the regulations is not as clear for: lasers sold only to Original Equipment Manufacturers (OEMs) as components for incorporation into end-use products, end-use products that contain lasers which have already been certified by their manufacturers, and end-use products that are being imported into the US and that already comply with international laser safety standards. This article will discuss these applications in an attempt to clarify the need for certification by laser product manufacturers and importers. The discussion applies equally for lasers and laser systems.
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 12,738.85 W cm-2, there will be no damaging reactions affecting the pulpal tissues.
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.
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 permissible 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.
The basic characteristics of fibers that are appropriate for surgical use in the infrared are reviewed. New fiber materials, such as sapphire, fluorozirconate and chalcogenide glasses, and polycrystalline fibers are discussed as well as their applicability for surgical procedures.
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
Hazard evaluation methods for lasers, with wavelengths greater than 1.4 microns (mostly in the middle infrared), have changed significantly in the current version of the American National Standard for the Safe Use of Lasers, ANSI Z136.1-1993. A correct evaluation involves comparing the hazard potential based on two evaluation models; one based on individual pulses and the other based on an equivalent continuous-wave exposure. An example of the hazard evaluation method within this spectral region is provided.
Laser safety standards are detailed technical documents arising from the complexity of the different mechanisms, depending on the parameters of the laser beam, by which lasers interact with living tissue. As a result, their interpretation by non-expert laser users, who are concerned about safety issues, can cause many difficulties. Based on experience in advising industrial and medical laser users, we have developed a primer which (i) provides step-by-step guidance in the interpretation and use of the laser safety standards, and (ii) explains how to calculate the laser irradiance (on the retina or the skin) to which the user may be subjected in typical situations. A computer program has also been developed which, through a series of questions and answers, guides the user through the calculation of the maximum permissible exposure level and the nominal optical hazard distance for the application of interest. The overall package should be particularly useful to laser safety officers and users of lasers for research applications, as an independent check on safety calculations. It also provides a useful complement to the many worked examples in the laser safety standards.
Dr. David H. Sliney participated in the Laser Therapy Session of the meeting, held in the Pallazzo Le Stelline, Milan, Italy on Friday–Sunday, 18–20 June 1999. At the conclusion of his Report Dr. Sliney notes: “Attendance at this meeting was quite interesting. Remarkable benefits continue to be claimed for laser, and even incoherent, light for treating wounds. Despite the fact that those who employ these techniques are impressed with their results, a sound understanding of the underlying photobiological mechanisms of LLLT remain elusive. Highly convincing, objective studies of clinical efficacy which meet all of the demands of the skeptics are required.”
General surgery represents a speciality where, while any procedure can be performed with lasers, there are no procedures for which the laser is the sine quo non. The general surgeon may perform a variety of procedures with a multitude of laser wavelengths and technologies. Laser safety in general surgery requires a multidisciplinary approach. Effective laser safety requires the oversight of the hospital's "laser usage committee" and "laser safety officer" while providing a workable framework for daily laser use in a variety of clinical scenarios simultaneously. This framework must be user-friendly rather than oppressive. This presentation will describe laser safety at the Rochester General Hospital, a tertiary care, community-based teaching hospital. The safety program incorporates the following components: input to physician credentialing and training, education and in-servicing of nursing and technical personnel, equipment purchase and maintenance, quality assurance, and safety monitoring. The University of Rochester general surgery residency training program mandates laser training during the PGY-2 year. This program stresses the safe use of lasers and provides the basis for graded hands-on experience during the surgical residency. The greatest challenge for laser safety in general surgery centers on the burgeoning field of minimally invasive surgery. Safety assurance must be balanced so as to maintain a safe operating-room environment while ensuring patient safety and the ability to permit the surgery to proceed efficiently. Safety measures for laparoscopic procedures must be sensitive to the needs of the surgical team while not providing confusing signals for the "gallery" observers. This task is critical for the safe operation of lasers in general surgery. Effective laser safety in general surgery requires constant vigilance tempered with sensitivity to the needs of the surgeon and the patient as laser technology and its applications continue to evolve.
The use of laser diode pointers that operate in the visible radiation region (400-760 nm) is becoming widespread. These pointers are intended for use by educators while presenting talks in the classroom or at conventions and meetings. They are also useful in any situation where one needs to point out special items during any instructive situation. The pointers can be purchased in novelty stores, mail-order magazines, office supply stores, common electronic stores, and over the internet. The power omitted by these laser pointers ranges from 1 to 5 mW. The potential for hazard with laser pointers is generally considered to be limited to the unprotected eyes of individuals who might be exposed by a direct beam (intrabeam viewing). No skin hazard usually exists. There are, however, even more powerful laser pointers now appearing. The units are imported into the U.S. often without proper manufacturer certification or labeling. The potential for hazards with these devices is not well understood by the general public and workers, and numerous exposure incidents have been recorded by the authors. Users of these products need to be alerted to the potential hazards and be encouraged to follow appropriate safety recommendations. These factors are discussed and safety recommendations for laser pointers are presented.
Photodynamic therapy (PDT) is a new treatment for solid tumors utilizing the combined action of light and a photosensitizing drug. Laser-fiber optic delivery systems make it practical to treat superficial and interstitial cancers, including malignancies of the skin, head and neck, esophagus, endobronchial tract, stomach, urinary bladder, female genital tract, and other sites. The putative action mechanism in PDT involves photochemical destruction of tumor tissue membranes mediated by singlet molecular oxygen. Light dosimetry modeling based on tissue optics is applicable for treatment planning. Related research areas include non-invasive optical diagnosis and imaging, development of new PDT drugs and light sources, and the extension of PDT methods to treatment of atherosclerotic vascular disease and blood detoxification.
In the laser scribing of glass a thermal stress is introduced into a glass plate by means of CO2 laser irradiation. The glass plate is rapidly cooled down by water jet immediately after the irradiation. For the purpose of theoretical clarification of the factors ruling the scribable condition and the crack depth, scribable conditions were acquired in laser irradiation experiments using a soda-lime glass plate having a thickness of 0.7 mm. Furthermore, the crack depth and the crack profile were observed for various values of the distance between the heating area and the cooling area. On the basis of the scribable conditions obtained from the experiments, a three-dimensional thermal stress analysis was conducted by a finite element method, allowing the following findings to be obtained. The scribable condition can be estimated from the maximum surface tensile stress in the cooling area and the maximum surface temperature. The crack depth in laser scribing depends on the surface tensile stress in the cooling area and the compressive stress field immediately under that area.
Taking laser as the heat source for ceramic sintering, 0.94K0.5Na0.5NbO3-0.06LiTaO3 ceramics with unique physical properties were prepared. Fine ferroelectric properties with remnant polarization of 18 μC/cm2 and coercive field of 18.8 kV/cm were obtained. The piezoelectric properties and density of the ceramics were d33∼120 pC/N, kp∼36.6%, TC∼420 °C, and ρ∼4.125 g/cm3. The shift of the two phase transition temperatures (tetragonal-orthorhombic phase transition temperature TO-T shifting to 100 °C from 190 °C while Curie temperature Tc shifting to 420 from 394 °C) in laser-sintered ceramics reflects large structure distortion of phase transition and Li+ substitution of high temperature phase. Orientation degree of the texture structure based on anisometric grains of submicron diameter in the ceramics was 23%. The texture structure corresponding to the fine ferroelectric properties was resulted from the mass transform in liquid phase and strictly heating directivity of laser irradiation. O2 discharge through coaxial nozzle takes some actions like O2 annealing and effectively suppression on the volatilization of Na2O during laser sintering. Lager substitution of Li+ leads to the presence of small amount of K3Li2Nb5O15 phase damaging in d33. By analysis of the sintering effects on the properties and structure evolution, laser sintering was shown as a promising method in preparation of functional ceramics with excellent and applicable physical properties, compared to a traditional furnace sintering method.
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
This study involved the corneal effects of the laser beam in infrared at 1573 nm. Investigations were directed toward verifying the limit values by defining the thresholds of corneal damage in experiments carried out on the rabbit, and using biochemical techniques, to specify a damage threshold at the cellular level and a better understanding of the cellular steps of the damaging process. Corneal damage thresholds were determined for exposures to 3 ns single pulses using clinical observations and histology. The ED50 obtained with a corneal beam diameter of 400 μm was 26.6 J cm−2. The corresponding radiant exposure, calculated with the 1 mm aperture diameter recommended by standards, was 4.3 J cm−2. This value was still higher than the present 1 J cm−2 exposure limit. In vitro experiments were also carried out on primary keratocytes and HT1080 epithelial cell line. An expanded beam diameter of 3.5 mm was delivered on plates for study of cell proliferation, senescence, and cytotoxicity. The laser beam was expanded to 6 mm on chamber slide systems used for morphological analysis and apoptosis detection. Cells were irradiated with a pulse duration of 3 ns, a repetition rate of 10 Hz, a radiant exposure of 0.395 J cm−2 per pulse on plate, and 0.131 J cm−2 per pulse on chamber slide systems. A number of pulses varying from 13 to 40 on plates, and varying from 10 to 90 on chamber slide systems, led to total doses of 5.1 to 15.9 J cm−2 and 3.9 to 11.8 J cm−2, respectively. The frequency of apoptotic bodies seemed the most suitable criterion capable of revealing the cellular stress induced by laser radiation. The threshold for the appearance of apoptotic bodies in chamber slide systems was the total dose of 3.9 J cm−2.
The effect of a 200 μm wavelength free electron laser (FEL) beam on vertebrate tissue culture cells in two study series is described. Cell cultures exhibited no morphological alterations. However, a statistically significant proportion of the cells exhibited a reduction in tritiated thymidine incorporation. The results suggest that this wavelength might affect DNA synthesis, and the studies demonstrate the feasibility of biological investigations with the FEL.
Microstructure and microhardness profile of laser bent AA 2024-T3 sheets were obtained to determine the effects of such a process on this material. A fast-axial flow CO2 laser unit (TEM00) was used at 250, 500, and 800 W continuous wave to bend the samples; scanning velocities ranged from 10 to 80 mm/s while laser spot diameters used were 1, 5, and 10 mm. Characterization techniques included light microscopy, scanning electron microscopy, and Vickers microhardness. Laser bending process variables were arranged into a parameter called accumulated energy density (AED) in J/mm2. Light microscopy observations and microhardness results revealed that there was no damage to the microstructure for AED values less than 25 J/mm2. At these AED values, the microhardness level across the depth is closer to the average value of the as-received material (150.9+/−2.7 Hv). However, some AED values resulted in samples with microhardness above and below that number. Higher microhardness levels were attributed to a small grain size and a localized T6 temper while the lower values were associated with grain growth. For AED values between 25 and 125 J/mm2 subgrain structures and dispersoids at grain boundaries were observed under scanning electron microscopy. Partial melting at recrystallized grain boundaries was the principal form of damage. Remelting of the upper region of the sheet underneath the alclad layer took place leaving a cast structure. For AED values of 125 and 250 J/mm2, the alclad layer fused to the substrate. For AED values that caused partial melting and a considerable fusion drop in the microhardness value was observed.
Calibration and validation of two temperature measurement techniques both using optical pyrometry, usable in the framework of the study of the heated metals in highly oxidizing environments and more generally during laser processing of materials in the range of 2000–4000 K have been done. The 2D single-band pyrometry technique using a fast camera provides 2D temperature measurement, whereas spectral pyrometry uses a spectrometer analyzing the spectra emitted by a spot on the observed surface, with uncertainties calculated to be, respectively, within ±3% and 6% of the temperature. Both techniques have been used simultaneously for temperature measurement of laser heated V, Nb, Ta, and W rods under argon and to measure the temperature of steel and iron rods during combustion under oxygen. Results obtained with both techniques are very similar and within the error bars of each other when emissivity remains constant. Moreover, spectral pyrometry has proved to be able to provide correct measurement of temperature, even with unexpected variations of the emissivity during the observed process, and to give a relevant value of this emissivity. A validation of a comsol numerical model of the heating cycle of W, Ta, Nb, V rods has been obtained by comparison with the measurement.
The laser bending of near-α alloy Ti–6Al–2Sn–4Zr–2Mo sheets was investigated with a 1200 W Nd-YAG laser. The effect of scanning a material with a laser beam is to produce complex thermal cycles, which result in thermal stresses and angular deflections (bending angles). The conditions to promote large bending angles are first studied using an existing model of a moving Gaussian heat source. Analysis showed that bending could be explained from a characteristic temperature defined from material properties: the yield temperature. The morphology associated with its isotherm, which defines both the depth and the width of the yielded zone, adequately described bending. Optimal bending angles were observed when asymmetry of this yielded zone was maximum with respect to the neutral axis; i.e., before the yielded depth penetrated through the sheet thickness. The effects of repeated scans, such as the increases in bending angles, are also discussed together with the heat flow. With a three-dimensional heat flow, significant thickening occurred and sheet bendability decreased rapidly with repeated scans. The forming response was different when heat flow was more two dimensional. Although the asymmetry of the yielded zone was significantly reduced, bending angles were as large as earlier ones because of wider yielded zones. Also, thickening was barely observed, whereas bending rate continued to decrease. Causes of reduced bendability other than thickening are thus discussed. The expected contribution from transformations in microstructure was studied, but was not found to be significant.
Fundamental field distributions of a spherical nanoparticle (a), a nanorod (b), and a nanocone (c) calculated with the MMP method (Refs. 15 and 16). The particles are illuminated by a plane wave (E 0 ; k x ) incident from the left (k ¼ 800 nm), and they are embedded in a medium with n ¼ 1.5. The secondharmonic dipole moments p 2x are marked by white arrows.
SHG field intensities for a nanorod circle in the z ¼ 150 nm plane parallel to the xy plane, and in the xz plane for different angles a of incidence. The green vector symbols show the emitting dipoles. The solid green lines mark the a-dependent shift of the SHG field maxima.
A possible application of the SHG field maximum: SHG emitters are placed in a transparent polymer matrix (brown), whose surface lies 100 nm above the emitters. The SHG field maximum A from the emitters can yield photochemical reactions in a photoreactive layer (green) deposited on that surface.
Second-harmonic generation (SHG) of light at nanoparticles provides the possibility to generate light (of a desired frequency) in-situ instead of introducing it by focusing an external light beam. Our theoretical study provides steering SHG light through the superposition of the radiation from a number of nanoparticles which are arranged in a circle. The authors assume cone-shaped or rod-shaped nanoparticles. Their radiation can be modeled as radiating dipoles. The superposition of their fields yields a “hot spot” with a full width at half-maximum of around 100 nm. Even more important, the position of the hot spot within the circular arrangement of nanoantennas can be adjusted in the xy plane simply by changing the incident angle of the exciting beam.
Investigations of two-photon polymerization (TPP) with sub-100 nm in the structuring resolution are presented by using photosensitive sol-gel material. The high photosensitivity of this material allows for TPP using a large variety in laser pulse durations covering a range between sub-10 fs and ≈140 fs. In this study, the authors demonstrate TPP structuring to obtain sub-100 nm in resolution by different approaches, namely, by adding a cross-linker to the material and polymerization with sub-10 fs short pulses. Additionally, a simulation and model based characterization method for periodic sub-100 nm structures was implemented and applied in an experimental white light interference Fourier-Scatterometry setup.
The effect of the excimer laser on the surface of Ti–6Al–4V is reported. Particular concentration is given to surface modification for potential materials processing applications. Results showed that: (i) there is an optimum energy for smoothing titanium; (ii) at this energy density increasing the number of pulses has some, but not a significant, effect on the smoothing process; and (iii) relatively smooth surfaces could be achieved at higher energy densities if the sample was processed in a helium atmosphere. Other typical surface modfications and features are also reported, including the effect of different gases on the process.
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
Radiation power and image size of the source on the viewer's retina (apparent source) determines the hazard for the eye. Following IEC 60825-1, the effective emission limits are dependent on the tabulated accessible emission limits and the assigned measurement conditions—both depending on the source size. In the case of a light emitting diode (LED), this emitting area is not only determined by the chip size but also by the housing—due to built-in lenses, reflectors, and scattering materials. Also the virtual source position will be distorted by the integrated lenses. Since the measurement distance is related to the apparent source, the source's location must be known for the proper classification and hazard assessments of LEDs. As the accompanying measurement instructions of the standard are developed for coherent sources, they do not fully match the peculiarities of LEDs and lead to different interpretations. A comparison of several approaches will be presented. In this context, the sometimes surprising implication of the new “Amendment 2” on the safety limits of LEDs will also be discussed.
There are two important trends that are currently taking place in the forming of sheet metal into automotive body components. One of these trends is the substitution of aluminum sheet alloys for steel. The other trend is the use of tailor welded blanks. Currently, there exists strong motivation for research to combine these technologies in order to produce tailor welded aluminum blanks. The focus of the current study is to develop welding procedures for autogenous CO2 (continuous mode) and Nd:YAG (continuous and pulsed mode) laser beam welding of 6111-T4-aluminum alloy. The mechanical and microstructural characteristics of the welded joints were evaluated using tensile tests, microhardess tests, optical microscopy, and chemical analysis. Results indicate that this alloy can be autogenously laser welded with full penetration, minimum surface discontinuities and little if any loss of magnesium through vaporization from the fusion zone. The total elongation (all weld metal) in the longitudinal direction for the laser welds made using 3 kW CO2 and 2 kW pulsed Nd:YAG show a decreasing trend with increasing travel speed. Studies indicate that the decreasing trend is probably due to the combination of two factors: orientation of the grains with respect to the loading direction and solidification cracking. The total elongation (all weld metal) of the laser welds made using 5 kW CO2 and 3 kW cw Nd:YAG did not show a consistent trend with travel speed. The main reason for this is solidification cracking. The welds made with 2 kW pulsed Nd:YAG with travel speed between 42 and 63 mm/s displayed the highest total longitudinal elongation (20.78% to 16.45%), compared to the base metal values of 27.8%. A very interesting observation was that the weld surface condition did not have any effect on the ductility of the 6111-T4-aluminum alloy studied in this investigation.
A series of experiments were performed to investigate the one-step laser cladding of Inconel 625 powder, injected off-axially onto mild steel substrates. The experiments were carried out using a 6 kW high power diode laser mounted to a six axis robot system. The rectangular shape of the delivering beam was focused to a spot size of 22 mm×5 mm on the work piece. The powder feeding head, which consisted of a cyclone and flat nozzle, spread the powder stream to a spot size used. The coating samples were produced using different levels of powder feed rate (77–113 g/min), constant traveling speed of 400 mm/min, and laser power of 6 kW. Powder and laser-clad coating microstructures were studied by x-ray diffraction, scanning electron, and optical microscopy. The coating microstructure was found to consist of a directionally solidified single phase (highly alloyed Ni), face-centered cubic structure with a lattice parameter of 0.3596 nm. The corrosion resistance of the one-step laser-clad coatings was tested in 3.5 wt % NaCl using electrochemical methods. Laser-clad coating was compared with wrought Inconel 625 alloy, high-velocity oxy-fuel sprayed, and plasma transferred arc welded coatings. The corrosion resistance of laser-clad coatings was found to be equivalent to corresponding bulk material and superior to sprayed and welded coatings.
Laser metal deposition (LMD) is used in industry to coat, additive manufacture, and/or repair high value metal components through deposition and laser induced melting of powder delivered in a gas stream. This study relates to the production of three-dimensional Inconel 625 components by LMD. After LMD, a dense cellular – dendritic structure containing carbides of the type MC, M23C6, and M6C has been detected by x-ray diffraction. Parts with tensile yield strengths, ultimate strengths, and elongations in the range of, respectively, 480–656 MPa, 882–1000 MPa, and 24%–36% have been obtained. Compression testing along and perpendicular to the build direction reveals a slight anisotropy in fracture strength. This is attributed to the preferential orientation of the dendrites parallel to the build direction. Tensile test samples have been fabricated in “lying” and “standing” orientations. The tensile yield and ultimate strength are considerably lower and the elongation is larger for the samples built in standing orientation compared to those built in lying orientation. The tensile properties are affected both by the tensile loading orientation relative to the build orientation and the difference in cooling rate for the two build geometries. The former effect is related to the anisotropic microstructure after LMD. The impact of build geometry on the other hand results in a coarser microstructure and different phase constitution—including larger amount of carbides—in the standing oriented samples due to the lower cooling rate during LMD compared to the lying oriented samples.
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.
We have developed a new laser ablation process using a water-microdrop with a diameter of 70 μm. The shapes of the debris were investigated at various delay times. A Q-switched Nd:yttrium-aluminium-garnet laser of 25 ns pulse width and 532 nm wavelength was used. The spattering of debris was remarkably reduced by using the water microdrop. The debris pattern was dependent on the shape of the microdrop on the wafer, which in turn was dependent on the surface condition of the wafer and the delay time. The use of a smaller microdrop would result in laser dicing with a small ridge at the rim of the ablation point and no debris. The flexural stress of 720 MPa was obtained near the blade dicing.
This article describes the measurement of the fractional ionization in an ablated plume produced by excimer laser, and the application of the plume as an ion source. Laser ablation of iron, aluminum, and tantalum with a KrF excimer laser was performed in a vacuum chamber. The ablated plume was caught by a cup type electrode used to measure the amount of charged particles as current wave forms. Applying electrostatic potential larger than 40 V, the current wave forms were separated into electropositive and electronegative peaks. Each peak indicates ions and electrons, respectively. The amount of ions, 6×1013 ions/pulse, was estimated by integrating the electropositive peak. The ionization degree of 1.7% was also estimated by a series of analyses. For an aluminum target, the ion current reached 2 A at the peak and 1 mA on average at 60 kJ/m2, 500 mJ, and 50 Hz.
A self-induced mechanism leading to the formation of periodic microstructures has been observed during femtosecond pulsed laser ablation with a high repetition laser source. The bottom of an ablated area shows two different kinds of microstructures. A periodic ripple or a chaotic columnar structure emerges. It has been observed that the second morphology grows out of the first one dependent mainly on the amount of fired laser pulses. This transition and the influence of several parameters on it are described. Both structures have several variations in morphology. Regarding the possible applications of such structures, this is of special interest. The presented experiments give a first glimpse of the possible microstructures emerging after ultrafast laser machining. Concerning the physical background of the found phenomena, it has been observed that a liquid phase is present in the ablation process. The formation of ripples may be closely linked to that fact. Existing models on ripple formation could not be linked to the ripple spacing found in the experiments.
Top-cited authors
Reinhart Poprawe
  • Fraunhofer Institute for Laser Technology ILT
Wilhelm Meiners
  • Fraunhofer Institute for Laser Technology ILT
Seiji Katayama
  • Osaka University
Yosuke Kawahito
  • Japan Agency for Marine-Earth Science Technology
Damien Buchbinder
  • Fraunhofer Institute for Laser Technology ILT