Temperature field distribution in multi-layered solid media heated with multi-modes laser beam

In the present paper we want to develop an analytical model to study the thermal behavior of multi-layered solid media in interaction with a laser beam. We suppose that each layer has a constant thermal conductivity. We consider that the laser beam is formed by decoupled [Phys Rev A 1996; 53: 4435] multi-modes Hermite–Gauss beams. The model is applied using the classical heat equation which was solved using the integral transform method [Infrared Phys Technol 2002; 42: 31; Optics Laser Technol 2004; 36: 219–22; Optics Laser Technol 2004; 36: 677–9; Proc SPIE 2003; 5227: 323].     

Mathematical modeling of tunable TEA CO2 lasers

A mathematical model, based on the Landau–Teller equations of six-temperature model for the CO₂–N₂–He–CO system, to describe the process of dynamic emission in tunable TEA CO₂ lasers is introduced. In this model, the Landau–Teller equations are rewritten with regard to fine longitudinal mode frequencies in the laser resonator. These revised equations can be utilized to estimate the laser output spectra as well as other laser output pulse parameters. Examples are given to show the modeling results of non-tunable, grating tuned or injection-locking TEA CO₂ lasers.     

Low-fluence CO2 laser irradiation decreases enamel solubility

This study investigated whether subablative-pulsed CO₂ laser (10.6 μm) irradiation, using fluences lower than 1 J/cm², was capable of reducing enamel acid solubility. Fifty-one samples of bovine dental enamel were divided into three groups: control group, which was not irradiated (CG); group laser A (LA) irradiated with 0.3 J/cm²; and group laser B (LB) irradiated with 0.7 J/cm². After irradiation, the samples were subjected to demineralization in an acetate buffer solution and were then analyzed by SEM. A finite-element model was used to calculate the temperature increase. The calcium and phosphorous content in the demineralization solution were measured with an ICP-OES. ANOVA and the t-test pairwise comparison (p < 0.016) revealed that LB showed significantly lower mean Ca and P content values in the demineralization solution than other groups. A reduction in the enamel solubility can be obtained with pulsed CO₂ laser irradiation (0.7 J/cm², 135 mJ/pulse, 74 Hz, 100 μs) without any surface photomodification and a less than 2°C temperature increase at a 3-mm depth from the surface.     

Tritium isotope separation by CO2-laser irradiation at low temperatures

Tritium isotope separation by CO₂-laser induced multiphoton dissociation of CTF₃ is investigated. For the optimization of the performance of this working substance, trifluoromethane, the conditions to yield high-selectivity at high-operating pressure and low-critical fluence for complete dissociation are studied using our deconvolution procedure. The irradiation conditions are varied over the following ranges; wavenumber: 1052–1087 cm^–1, gas temperature: 25°C to –78°C, CHF₃ pressure: 5–205 Torr. The selectivities exceeding 10⁴ are observed for 85–205 Torr CHF₃ at –78°C by the irradiation at 1057 cm^–1.     

Temperature dependence of resistance to micro- and macrodeformation in precipitation-hardening Co-Ni-Ti and Co-Ni-Ti-Al alloys. II

The temperature dependence of mechanical properties (T) in the region of micro- and macroformations ( - 10^–6-10^–3) of the Co-Ni-Ti, and Co-Ni-Ti-Al alloys was studied in various structural states. It was shown that precipitation of Co³Ti phase in the alloy being investigated does not determine the anomalous temperature dependence (T) similar to that observed in various alloys with precipitation of Ni₃Al phase. Experimental data obtained indicated that temperature dependence of macroscopic yield point is determined by the dependence of the resistance to the onset of plastic flow on temperature.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 52–57, September, 1972.     

The effect of irradiation angle on laser ignition of cellulose sheet in microgravity

The objective of this work was to investigate the effect of external radiation angle on radiative ignition of solid materials. A laser ignition experiment was performed in microgravity to investigate events occurring in the ignition process in a quiescent atmosphere. Filter paper was used as the test material, and it was heated by infrared radiation (CO₂ laser 10.6 μm) or near-infrared radiation (diode laser, 800.1 nm). The ignition time was determined for various irradiation angles, and the gas phase density change before ignition was observed by a Mach–Zehnder interferometer for each test condition. The results showed that the ignition by CO₂ laser occurred on the laser beam line depending on the irradiation angle, while diode laser caused a similar ignition position independent of the irradiation angle. The period from gasification to ignition with CO₂ laser was almost the same for different irradiation angles, while it varied with the irradiation angle for diode laser, and the ignition time was much shorter than that with diode laser. According to these results, it is considered that solid ignition with inclined external radiation is characterized based on (1) solid surface heating and (2) gas phase heating, and the second factor, gas phase heating, causes the different dependence of solid ignition on irradiation angle with different radiation wavelengths.     

Isotopically selective CVD of silicon by IRMPD of Si2F6

The IRMPD of Si₂F₆ by a CO₂ TEA laser was applied to isotopically selective CVD of silicon. A white film, probably consisting of polymers of SiF₂, was deposited on a metal foil during the irradiation of natural Si₂F₆ with the laser radiation at 951.19 cm^–1 and about 1.5 J cm^–2. Upon heating, the film became dark brown, evolving SiF₄. The³⁰Si content was found to be as high as about 20%.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.     

New CW FIR laser lines obtained in ammonia pumped by a CO2 laser, using the Stark tuning method

We report fifty seven CW FIR emissions observed in NH₃, by resonant pumping with a CO₂ laser. Exact coincidences between IR absorption lines of the gas and emission lines of the CO₂ laser have been carried out by Stark tuning. IR frequency shifts, up to 30 GHz, have allowed the pumping of forty three NH₃ transitions.These FIR emissions correspond to thirty one different wavelengths in the 50–400 m range; eighteen ones of them are new emitted wavelengths by pumping with the CO₂ laser.     

Lasers effects on enamel for caries prevention

The aim of this study was to ascertain whether laser irradiation is able to reduce caries incidence. For this purpose, the effects of laser on enamel and on fluoride uptake were discussed. Current literature regarding the preventive effect of laser irradiation on dental hard tissue has been reviewed. An evaluation of the results of the available in vitro and in vivo studies on the efficacy of anticaries and induced changes on enamel by laser irradiation were also performed. Articles were selected using the Medline, Web of Science, Embase, and Cochrane databases, and the results of these studies were described. The most common lasers employed for caries prevention on enamel are Nd:YAG; CO₂; Er:YAG; Er,Cr:YSGG; and argon. The percentage of inhibition of dental caries varied from 30 to 97.2%, and the association with fluoride has demonstrated the best results on inhibition of caries development. Laser irradiation under specific conditions can change the crystallographic properties of apatite crystals, increasing the acid resistance of lased enamel. The combined treatment of laser irradiation with fluoride propitiates an expressive fluoride uptake, reducing the progression of carieslike lesions, and this treatment is more effective than laser or fluoride alone. Available data suggest that lasers combined with fluoride is a promising treatment in caries prevention.     

A gas cluster ion beam accelerator

An assembled CO₂ gas cluster ion beam system was assessed using a retarding field analyzer and a time-of-flight mass spectrometer. The CO₂ gas was expanded to form gas clusters at the input pressure of 1–5 bar through a quartz Laval nozzle. At 4 bar, it is confirmed that the clusters consisted of about 500 molecules. Also the dependence of the mean cluster size distribution on source temperature was examined. At the low fluence of ion beam, an isolated gas cluster ion impact on solid surfaces was investigated. CO₂ gas cluster ions were irradiated at the acceleration voltage of 40–60 kV on highly oriented pyrolytic graphite. Si with native oxide layers, and Cu film deposited on Si wafer. After very short exposure of cluster ions, induced hillocks with about 0.8–1 nm in height and 20 nm in width were outgrown from the impacted surfaces. After prolonged irradiation on Si and Cu/Si, humping was more developed and consequently the surface morphology seemed to be saturated because of gradual filling the gap between the hillocks.     

Effect of CO2 laser irradiation on the transport properties of La0.67Ba0.33MnO3 thin film

The effect of CO₂ laser irradiation on La_0.67Ba_0.33MnO₃ epitaxial thin film was investigated. Epitaxial thin films grown by pulsed laser deposition were irradiated by a CO₂ laser in air for 10-60 s. It is shown that after CO₂ laser irradiation treatment, the crystallinity of the film is strongly enhanced. It is found that a dramatic decrease in the resistivity of the CO₂-laser-irradiated film is accompanied by a remarkable increase in its insulator-metal transition temperature and the temperature coefficient of resistance. This significant improvement of its structure and properties is achieved in several dozens of seconds and surpasses that observed in films annealed in an oxygen atmosphere at 900 ^°C for 12 h, suggesting that CO₂ laser irradiation is a new and effective tool to optimize CMR manganites for bolometric applications.     

Assignments of optically pumped CD3OH laser lines

Six FIR laser lines from CD₃OH pumped by the 10R(36) and the 10R(18) CO₂ laser lines are assigned to specific rotational energy levels in the excited C–0 stretch state. It is found that their upper laser levels are shifted by a Fermi resonance between the C–0 stretch vibration and the third and forth harmonics of the torsional mode. The Fermi resonance shifts are +0.332 cm^–1 and +2.251 cm^–1 for the upper laser levels pumped by the 10R(36) and the 10R(18) CO₂ laser lines, respectively. Calculated frequencies of the pump and the laser transitions agree with those of the pump CO₂ laser lines and the observed FIR laser lines within estimated accuracy.     

Amplification and saturation in a CO2 waveguide amplifier

An ir CO₂, dc current pumped, optical waveguide (WG) amplifier has been built, and its active medium optical parameters measured for several CO₂ emission lines, and their dependence from active medium total pressure, discharge current and temperature was investigated.High gain is found which, coupled with relatively high saturation power in the WG fundamental mode and ease of fabrication with this technology in long (up to 1.5 m) lengths, indicates promising use to efficiently amplify high spectral and spatial purity output of a short, highly tunable WG laser up to power levels suited for nonlinear spectroscopy and optical pumping. The dependence of the small signal gain coefficient and of the saturation parameter for individual rotational lines on the radiation intensity was computed using experimentally known parameters of the discharge plasma. The computation was carried out using the two mode rate equation approach for CO₂–N₂–He gas mixtures. A satisfactory agreement between theoretical and experimental results was obtained.Work supported by G.N.S.M.-C.N.R. and M.P.I.     

Spectroscopic investigation of the plasma of the capillary discharge of a waveguide CO2 laser

The temperature conditions and the chemical composition of the gas in the plasma of a capillary discharge of a waveguide CO₂ laser are investigated It is shown that in helium-rich CO₂–N₂–He mixtures the gas temperature is independent, in a wide range of its variation, of the discharge-tube diameter and of the working-mixture pressure. It is established by the optical actinometry method that capillary discharges are characterized by a stronger decomposition of the working CO₂ molecules and by a higher density of the reaction products than in reduced-pressure discharges in wide tubes.Translated from FIAN Preprint No. 119, Lebedev Physics Institute, Moscow, 1990.     

Temperature profiles modeling in IR optical components during high power laser irradiation

In this paper the temperature profiles of heat propagation in homogeneous IR optical components during irradiation with a high power laser (e.g. CO₂ laser) having different cross-section power distributions are examined. A mathematical model to calculate the temperature variation was developed, and the analytical solution for the heat equation was found. The method is based on the integral transform techniques applied to the partial differential equation of heat conduction in solids. Thermal profiles are plotted as functions of the sample dimensions and exposure time, for laser beams operating in TEM₀₁ and TEM₃₀ modes. Our results indicate that: (i) temperature profile has an angular dependence; (ii) high temperatures can be achieved in the sample center, even in the situations when the laser beam has zero power densities in this point; and (iii) the temperature profile depends strongly on the heat transfer coefficient.     

Development of Transmission Optics for High-Power TEA CO2 Lasers and Methods of Increasing Laser-Radiation Resistance of Materials for High-Power IR Optics

The results of analytical and experimental investigations aimed at increasing laser-radiation resistance of materials for IR optics and developing high-power optics for microsecond TEA CO₂ lasers with energy per pulse up to 12–25 kJ and gas-dynamic CO₂ lasers with energy per pulse up to 130 kJ are presented. It is demonstrated that the integrated approach that combines the improvement of already existing technological methods and the development of novel technological methods for refining the parameters of materials for transmission IR optics (including techniques of growth of single crystals, strain hardening, and laser, ionic, and chemical treatment), the design and optimization of optical units (including the development of segmented transmission optics, the improvement of optical schemes for spatial formation of laser beams, the use of fast-response physical effects to screen optical elements from high-power fluxes of laser radiation) is necessary to solve this problem.     

Infrared laser stark shift spectroscopy in ammonia

The Stark effect in ammonia has been theoretically and experimentally analyzed using lead salt tunable diode laser absorption spectroscopy and CO₂ laser absorption spectroscopy of several absorption lines around 1050 cm^–1 applied to an all-optical sensor for measuring of electric field strength. Measurements of the Stark splitting effect of theaR(5,K) ammonia lines forK=1–5 as well as for the sR(3,K) lines forK=0–3 have been made at Doppler broadening pressures and for several different electric field strengths. Theoretical electric field dependent energy levels have been evaluated by diagonalization of a 6×6 energy matrix constructed using both electric field independent and dependent terms. From the theoretical analysis the resolution can be predicted and optimized both in the Doppler broadened and in the pressure broadened regimes. The predicted resolution is 0.5% at an electric field strength of 20 kV/cm. The theoretical calculations and the experimental data recorded with the tunable diode laser system were compared with independent measurements made with a CO₂ laser system. The agreement between experimentally recorded and theoretically calculated spectra is good which indicates that the theoretical model is satisfactory for our purposes. The contribution from the normally forbidden ssR(5, 3) ammonia line to the absorption at theP(12) CO₂ laser line in the 9 m band is briefly discussed.     

Enrichment of 13C by IRMPD of CBr2F2

The CO₂-laser-induced infrared multiple photon decomposition of natural CBr₂F₂ in the presence of oxygen has been examined as a function of pulse number (30–1500), reactant pressures (CBr₂F₂, 10–150 Torr and O₂, 5–90 Torr), laser line [9P(8)–9P(32)], and laser fluence (1–3 J cm^–2) to optimize irradiation conditions for ¹³C-enrichment. CF₂O was the main carbon containing product and afterwards was converted into CO₂ via hydrolysis. A small amount of C₂Br₂F₄ was detected only under extreme conditions, for example, at high laser fluences or wavenumbers close to an absorption band. The ¹³C-atom fraction of the final product CO₂ was found to be 20–80%, depending on experimental conditions. The two-stage IRMPD process proposed previously has been examined in further detail in the present study. First, CBr₂F₂ containing about 30% of ¹³C was prepared in the ¹³C-selective IRMPD of natural CHClF₂ in the presence of Br₂. The second-stage IRMPD of the CBr₂F₂ in the presence of oxygen under selected conditions resulted in the high enrichment of ¹³C beyond 90%.     

Absorption of CO 2 laser emission by freon-12

The infrared (IR) absorption of freon-12 (CF₂Cl₂) was studied in the emission range of a 3-W tunable CW CO₂ laser by using a brass cell with KBr windows that was located outside the laser resonator. The results show that CF₂Cl₂ absorbs all CO₂ laser emission lines in the ranges of 1073–1083 cm^-1 and 937–943 cm^-1. The most strongly absorbed laser line was 10P (28) ( 937.21 cm^-1). Absorption coefficient values were obtained for all available wavelengths of the CO₂ laser as the CF₂Cl₂ pressure was varied from 5 to 1000 mbar. By using the HITRAN database for freon-12, the absorption coefficients were calculated at the 10P (28) and 9R (28) lines as functions of the gas pressure and compared with the experimental values. The calculated results are in reasonable agreement with the experiment. PACS 33.20.Ea; 42.55.-f; 42.55.Lt