Absorption Of Laser Beam Research Articles

Investigation into absorption of the incident laser beam during Nd:YAG laser processing of metals

Laser materials processing is highly affected by the existence of surface plasma. The absorption of surface plasma during drilling alters the power intensity distribution of the incident laser beam across the irradiated spot. The present study is carried out to measure the electron number density and temperature using a Langmuir probe while a mathematical formulation is conducted for the absorption coefficients due to electron-ion, electron-neutral atom collisions, inverse Bremsstrahlung, and photoionization processes. Consequently, a computer program is developed to compute the relevant absorption coefficients as well as the overall absorption coefficient. The laser power intensity distribution before and after the plasma absorption is computed at a plane 2.6 mm above the workpiece surface. It is found that 13% of the reduction occurs in the incident laser output power intensity at this plane in the plasma.

Energy-pooling collisions in cesium: 6PJ+6PJ-->6S+(nl=7P,6D,8S,4F).

We report experimental rate coefficients for the energy-pooling collisions Cs(6${\mathit{P}}_{1/2}$)+Cs(6${\mathit{P}}_{1/2}$)\ensuremath{\rightarrow}Cs(6${\mathit{S}}_{1/2}$)+Cs(${\mathit{nl}}_{\mathit{J}\ensuremath{'}}$) and Cs(6${\mathit{P}}_{3/2}$)+Cs(6${\mathit{P}}_{3/2}$)\ensuremath{\rightarrow}Cs(6${\mathit{S}}_{1/2}$)+Cs(${\mathit{nl}}_{\mathit{J}\ensuremath{'}}$) where ${\mathit{nl}}_{\mathit{J}\ensuremath{'}}$=7${\mathit{P}}_{1/2}$, 7${\mathit{P}}_{3/2}$, 6${\mathit{D}}_{3/2}$, 6${\mathit{D}}_{5/2}$, 8${\mathit{S}}_{1/2}$, 4${\mathit{F}}_{5/2}$, or 4${\mathit{F}}_{7/2}$. Atoms were excited to either the 6${\mathit{P}}_{1/2}$ or 6${\mathit{P}}_{3/2}$ state using a single-mode Ti:sapphire laser. The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode ring dye laser beam, tuned to either the 6${\mathit{P}}_{1/2}$\ensuremath{\rightarrow}8${\mathit{S}}_{1/2}$ or 6${\mathit{P}}_{3/2}$\ensuremath{\rightarrow}7${\mathit{D}}_{3/2,5/2}$ transitions, which could be translated parallel to the pump beam. Transmission factors, which describe the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed, were calculated for all relevant transitions. Effective lifetimes of levels populated by energy-pooling collisions are modified by radiation trapping, and these factors were calculated using the Molisch theory. These calculated quantities have been combined with the measured excited-atom densities and fluorescence ratios to yield absolute energy-pooling rate coefficients. It was found that the rate for production, in all cases, is greatest for 6D, but that 1/2-1/2 collisions are significantly more efficient than 3/2-3/2 collisions for populating 7P. It was also found that 7${\mathit{P}}_{1/2}$ is populated two to three times more efficiently than 7${\mathit{P}}_{3/2}$ in 1/2-1/2 collisions, but that the 7P fine-structure levels are approximately equally populated in 3/2-3/2 collisions. \textcopyright{} 1996 The American Physical Society.

Dynamics of optical caviton generation in a CW-seeded frequency shifted feedback cavity

We analyze the dynamics of soliton creation in a frequency-shifted feedback ring laser with injected continuous wave seed. We show that for an active cavity the soliton buildup time may be estimated analytically. The soliton emission from the CW-pumped laser is analogous to the generation of cavitons by the resonant absorption of laser beams in inhomogeneous plasmas. We study the dynamics of optical cavitons by means of perturbation theory and numerical simulations.

Calculations and experiments of material removal and kinetic energy during pulsed laser ablation of metals

Numerical calculations are presented, which describe the processes of target heating and ablation of pure metals (for instance Fe) during irradiation by 30 ns laser pulses at 248 nm. The following effects are taken into account: the absorption of the laser radiation and the heat conduction within the target, the evaporation of material from the target surface, the cooling of the target surface by the heat of evaporation and the partial absorption of the incident laser beam in the evaporated material. As results of the calculations, the temperature profile in the target and the ablated material can be obtained. Furthermore, by energy balance, an energy of the ablated material of up to some 100 eV is obtained. Experiments performed concerning the threshold energy of ablation, the removed mass from the target and the kinetic energy of the deposited ions (by time-of-flight measurements) are in good agreement with the performed calculations.

ORION: Clearing near-Earth space debris using a 20-kW, 530-nm, Earth-based, repetitively pulsed laser

When a large piece of space debris forced a change of flight plan for arecent U.S. Space Shuttle mission, the concept that we are trashing space as well as Earth finally attained broad public awareness. Almost a million pieces of debris have been generated by 35 years of spaceflight, and now threaten long-term space missions. The most economical solution to this problem is to cause space debris items to reenter and burn up in the atmosphere. For safe handling of large objects, it is desired to do this on a precomputed trajectory. Due to the number, speed, and spacial distribution of the objects, a highly agile source of mechanical impulse, as well as a quantum leap in detection capability are required. For reasons we will discuss, we believe that the best means of accomplishing these goals is the system we propose here, which uses a ground-based laser system and active beam phase error correcting beam director to provide the impulse, together with a new, computer-intensive, very high-resolution optical detection system to locate objects as small as 1 cm at 500-km range. Illumination of the objects by the repetitively pulsed laser produces a laser-ablation jet that gives the impulse to de-orbit the object. A laser of just 20-kW average power and state-of-the-art detection capabilities could clear near-Earth space below 100-km altitude of all space debris larger than 1 cm but less massive than 100 kg in about 4 years, and all debris in the threatening 1–20-cm size range in about 2 years of continuous operation. The ORION laser would be sited near the Equator at a high altitude location (e.g., the Uhuru site on Kilimanjaro), minimizing turbulence correction, conversion by stimulated Raman scattering, and absorption of the 530-nm wavelength laser beam. ORION is a special case of Laser Impulse Space Propulsion (LISP), studied extensively by Los Alamos and others over the past 4 years.

The effect of plasma formation on beam focusing in deep penetration welding with CO2 lasers

This paper deals with the absorption and defocusing of a CO2 laser beam by the laser-induced plasma plume in deep penetration welding. To derive the `effective` intensity distribution in the focal plane theoretically, the laser beam propagation through the plasma plume is calculated by solving the paraxial wave equation with a finite-difference scheme. Corresponding to experimental results, documented in the literature, the properties of the plasma plume (spatial temperature distribution and shielding gas content) are pre-set within the calculation. Parametric studies demonstrate that the intensity at the focus is reduced due to the defocusing effect of the plasma plume, mainly, and only to a minor extent due to absorption within the plume. Because of refraction within the plume, the intensity distribution in the focal plane is dependent on the plasma`s size, position and temperature. On studying the dependency of the optical properties on plasma temperature and shielding gas composition, it is found that, by applying a shielding gas mixture of He and Ar in the ratio 3:1, the variation of the focal diameter with plasma temperature can be significantly reduced. This shielding gas mixture, therefore, is recommended for enhancing process stability when welding with high-power CO2 lasers.

A real-time, nonintrusive trace gas detector based on laser photothermal deflection

We present the layout and technical details of a trace gas monitor based on photothermal deflection. The operating principle of this instrument, i.e., the deflection of a (weak) probe laser beam by the thermal refractive index gradient induced by trace gas absorption of an intense pump laser beam, allows nonintrusive measurements with good space and time resolution. An intra-cavity CO2 laser is used as the pump beam and a red HeNe laser as the probe. The latter runs perpendicular to the pump beam to optimize spatial resolution. To increase sensitivity, the probe laser is incorporated in a multipass setup. The instrument is demonstrated by the localization of ethylene emission sites on a cherry tomato and by monitoring ammonia production due to nitrogen fixation by cyanobacteria. Both C2H4 and NH3 can be detected at the 1–3 ppb level, at a spatial resolution of 2 mm (along the pump laser)×0.6 mm (perpendicular to it), and a response time of 0.1 s (without background correction) or 15 s (including background correction). Sensitivity can be increased at the expense of spatial resolution, and vice versa. In principle, this instrument is applicable to all those gases possessing a characteristic (‘‘fingerprint’’) spectrum in the CO2 laser range. The great advantage of the photothermal deflection technique with respect to other trace gas detection schemes lies in the nonintrusive character of the measurements. There is no need to enclose the sample in a vessel or to suck large volumes of air into the detector; measurements can be performed in open air and in real time. This should prove especially useful where sticky (polar) gases, like H2O, NH3, CH3OH, etc., are to be detected quantitatively. Main applications include air quality monitoring, especially concerning dry deposition rate measurements using the eddy correlation technique, and the study of volatile metabolite emission of biological samples.

Catastrophic degradation lines at the facet of InGaAsP/InP lasers investigated by transmission electron microscopy

Material transformations occurring at the facets of optically ‘‘stressed’’ planar InGaAsP/InP diode lasers have been investigated by transmission electron microscopy and energy dispersive x-ray spectroscopy. Catastrophic degradation lines (CDLs) which are characteristic of catastrophic optical damage are observed for optical power densities ∼107 W/cm2. Analysis of the microstructure reveals a series of 150 nm wide GaAs-rich tracks and the formation of unique void/InGa-rich precipitate pairs within the InGaAsP active layer. These observations suggest that the formation of local group III-rich regions is the first stage in the formation of CDLs. Subsequently, the strong absorption of the impinging laser beam leads to propagation of an InGa-rich melt, thereby producing the GaAs-rich tracks through a process similar to liquid phase epitaxy. These results are discussed in the context of standard physical models for CDLs.

Calculation of CO2 laser beam absorptance as a function of temperature for steels by the numerical method

Temperature distributions were measured during the irradiation of a CO2 laser beam at one end of a rod-shaped specimen and at the centre of a thin plate-shaped specimen. Regarding the measured temperature distributions as one-dimensional and two-dimensional unsteady state heat transfer solutions, CO2 laser beam absorptances were calculated using a modified finite difference method. Temperature dependence of thermal properties, heat loss due to convection and latent heat during melting of the specimen were taken into account in this numerical calculation. Increasing the specimen temperature from room temperature to melting point, absorptances of STS304 stainless steel and SM45C steel were calculated as 8 ∼ 40% and 6 ∼ 41% for the one-dimensional calculation, and as 9.3 ∼ 41% and 5 ∼ 41% for the two-dimensional calculation, respectively. These calculated absortances were very close to theoretical values at relatively low temperature, which were calculated from the electrical resistivities of the specimens. Increasing the temperature of the specimens, absorptances increased considerably due to oxidation of the specimens. Regardless of specimen composition and specimen dimension, both absorptances showed nearly the same value of 41% at their melting points, in which the structures of both metals became amorphous phases.

High-powered CO2 laser cystoscope for endoscopic bladder surgery.

The CO2 laser is potentially useful for the surgical treatment of superficial bladder tumors. High water absorption of the CO2 laser beam and the lack of a flexible fiber delivery system has hindered development of cystoscopes designed to evaporate bladder tumors. This report describes a cystoscope capable of delivering a high-power CO2 laser beam to all parts of the urinary bladder. A rigid 24 Fr. cystoscope transmitting 80 Watts of power via an articulated arm and infrared waveguide was built. Beam manipulation is by reflection using a movable mirror. Smoke and urine are evacuated by continuous flow of CO2 purge gas. An isotopic laser delivering an 11.1 micron wavelength beam prevents resonant absorption. Conventional 30-70 degrees telescopes are used. Scope performance and lesion morphology were tested in four female pigs. A total of 80-85% of the incident energy is delivered to the mucosa. All areas of the bladder mucosa can be evaporated. All components tolerate wetting. Mucosa and lamina propria are evaporated with 30 Joules of delivered energy. Perforation occurs after 120-160 Joules are delivered to a single spot. The limitations of power, beam transmission, beam manipulation within a small cavity, and resistance to wetting have been overcome.

Relaxation oscillation theory for the Nd 3+: YAB self-frequency-doubling laser

The single-mode rate equations are solved to determine the relaxation oscillation dynamics in the self-frequency-doubling Nd 3+:YAB laser. Based on the solution of a cubic characteristic equation, it is found that spiking of the fundamental laser beam is suppressed significantly by the self-frequency-doubling effect, although the suppression is reduced to a certain extent by self absorption of the second-harmonic laser beam. The relations between relaxation oscillation parameters and the nonlinear optical coefficient X (2) eff and the second-harmonic absorption coefficient k 2, are obtained. It is shown that in a certain range of X (2) eff and k 2 values, spiking is suppressed completely. The conclusions are applicable to other laser systems with intracavity second-harmonic generation.

IR absorption in crossed jets: relative rotational energy transfer efficiencies for CO(J=0,1) with He, N2, and Ar

A seeded free jet of CO was crossed perpendicularly with a second free jet of collider molecules. Direct absorption of a tunable IR diode laser beam was used to measure relative cross sections for translational to rotational energy transfer (RET) in low rotational states of CO. Multiquantum rotational transitions populating a broad range of final J states were demonstrated to be important. The relative, net depletion rates of CO(J=O) and CO(J=1) were measured and used to estimate the total RET cross sections. Subject to inaccuracies of ±10% in the measured flow rates, the relative, total, RET cross sections σ He :σ N2 :σ Ar , were found to be 1:1.7:2.0 (±20%) for CO(J=O)

Three-photon resonance-enhanced ionization spectroscopy of CO(A 1Π−X 1Σ+). I. Influence of phase matching on the appearance and adverse pressure effect of the spectra

Three-photon resonance-enhanced ionization spectra of pure CO via the A 1Π state have been obtained under a pressure range of 0.1–13 torr. It is found that the spectra can only be observed by using tightly focused laser beams, and its intensity manifests adverse pressure effect, i.e., the signal decreases with the increase in pressure for gas pressures above 1.0 torr, and almost disappears at 13 torr. Convincing experimental evidence is provided to show that the appearance of the spectra is not due to direct three-photon absorption of the fundamental laser beam, but to reabsorption of the laser-induced third harmonic generation (THG) in the CO medium. In other words, under tightly focused conditions, THG reabsorption is much stronger than direct three-photon absorption. Hence, contrary to the assertions given by previous investigators in Xe that the anomalous phenomena in three-photon excitation are caused by destructive interference, we have shown that phase matching, which greatly increases the THG, plays a major role in the three-photon resonance-enhanced ionization spectra of CO(A 1Π−X 1Σ+), and accounts for both the appearance of the spectra and the adverse pressure effect of the spectral intensity. In addition, we have given an analytical treatment of the adverse pressure effect in terms of the phase-matching condition.

新しいレーザシェイピング光学系の開発とその応用（第２報） 直線偏光レーザを用いる変態硬化

Transformation hardening of cabon steel (S45C) by CO2 laser beam was analyzed by using a novel beam shaping optics, which provides linear-polarized beam spot with uniform intensity distribution and variable aspect ratios. At normal incidence, the absorptivity of CO2 laser beam was 20% in the bare specimen (in Ar), and 45% in graphite coated specimen (in air). The absorptivity in p-polarization, AP, increased significantly with increasing incident angle, θ, reaching 50% at θ=80 degrees with bare specimen in Ar, which exceeds the value of graphite coated specimen at θ=0. In the graphite coated specimen, AP also increased with increasing incident angle, reaching 80% at θ=80 degrees. In the bare metal, the case depth was approximately 0.4 mm at normal incidence, and increased with increasing incident angle, reaching 2.5 mm at θ=80 degrees. The depth of laser hardening was estimated by a simple approximation equation, showing satisfactory agreement with the experimental values. It was also shown that the hardened depth can be changed independently of the surface temperature by using the shaping optics, which can provide laser beam spot with variable aspect ratios.

Low power lasers in physical therapy: measurement of optical output power of devices.

This report outlines the most essential characteristics of low power lasers commonly used in physical medicine. The optical output power of two Helium-Neon (HeNe) lasers (CW scanner) and two collimated Infrared (IR) laser diodes was measured with a CoherentR Model 212 meter designed to measure output power and its power density. The energy of two collimated IR lasers was measured with a Laser Precision Energy Meter equipped with an RJP-735 probe. The difficulties arising from the large divergence of the laser beam in diode lasers and the importance of the scattering and absorption of laser beams are discussed. The criteria for the evaluation of the lasers as well as the essential parameters for an adequate characterization of laser therapy are given.

Two-photon excitation and bleaching of the 400 nm luminescence band in germanium-doped-silica optical fibres

The 400 nm luminescence band of germanium-doped-silica optical fibres was excited via two-photon absorption of an argon ion laser beam. It was observed that the photoluminescence decreased by as much as 25% during the exposure of the fibres to several hundred milliwatts of optical power fora few hours. Surprisingly, the amount of bleaching is not a given proportion of the initial intensity of the luminescence band. These luminescence measurements are qualitatively related to the photosensitivity of the optical fibres. The saturation of two-photon absorption at relatively low power is also observed.

A Study on the Relationship between Laser Beam Polarization and Absorptivity. 1st Report. Effect of Surface Roughness.

The relationship between the incident angle of a laser beam with linear polarization and absorptivity on S40C steel at a wavelength of 10.6μm was investigated by both laser calorimetry and theoretical calculation based on the Fresnel equations. A simplified triangulate surface model and a highly polished specimen were used to determine the effect of surface roughness on absorptivity. A comparison was also made with a normally machined specimen to confirm the effect. With regard to the absorption characteristics, it was found that the calculated and the measured absorptivities for an ideally flat or highly polished surface were nearly the same over the entire range of incident angles examined. Absorptivity was strongly dependent on incident angle. A pronounced absorptivity peak was observed near the Brewster's angle which was approximately 86° from the normal. When the surface was asymmetrically triangulated, the angle at which peak absorptivity occurred was either larger or smaller depending on the incident direction of the laser beam. The peak absorptivity value on triangulated specimen was lower than that on the polished specimen. These tendencies agree closely with the predictions made by theoretical calculation and with the results for the normally machined specimen.

A Study on the Relationship Between Laser Beam Polarization and Absorptivity. 2nd Report. Absorptivity of Polarized Beam during Laser Hardening.

Transformation hardening was performed on S40C steel by a linear polarized CO2 laser beam without the use of any additional absorbent coating. The depth and width of the hardened zone were measured at different incident angles of the beam. In addition, the relationship between the heat input of the laser beam and the characteristics of hardened zone, such as depth and width, was investigated by finite-element analysis. Absorptivities at different incident angles during laser hardening were deduced by comparing the relationship between the laser power and hardened depth, which was determined experimentally, with the relationship obtained by calculation. A pronounced absorptivity peak of approximately 58% was observed during laser hardening in air when the incident angle was approximately 74°. Absorption characteristics were also deduced in a different atmosphere. The results were compared with absorption characteristics calculated theoretically or measured calorimetrically to investigate the influence of surface temperature and oxidization on absorptivity. It was found that the peak absorptivity value increased when surface temperature rose or oxidization occurred.

Fokker-Planck simulations of ultrashort-pulse laser-plasma interactions

The interaction of ultrashort laser pulses with a fully ionized plasma is investigated in the plane geometry by means of numerical simulation. The impact of the space oscillations in the amplitude of the laser electric field on the shape of the electron distribution function, on laser beam absorption, and on electron heat transport is demonstrated. Oscillations in the absorption rate of laser radiation with the minima coincident to the maxima of the laser electric field lead to a further decrease in the absorption of laser radiation. Heat flux in the direction of increasing temperature in the underdense region is caused by the modification of the electron distribution function and by the density gradient. A limitation of heat flux to the overdense plasma isobserved with the flux limiter in range 0.03–0.08, growing moderately with the intensity 1014–1016 W/cm2 of the incident 1.2-ps laser pulse.

Contactless nondestructive measurement of bulk and surface recombination using frequency-modulated free carrier absorption

A measurement procedure is described which allows the contactless measurement of bulk lifetime and surface recombination. The procedure uses the the free-carrier absorption of a long-wavelength laser beam by a modulated free-carrier wave to measure and separate the bulk recombination from the surface recombination. The dependence of the absorption on the modulation frequency is used to accomplish the separation. Limitations of the technique are also discussed.