Absorption Coefficient Of Laser Radiation Research Articles

Features of heat/mass transfer and explosive water boiling at the laser fiber tip

Experimentally, as well as using numerical simulation methods, the features of the processes of heat and mass transfer and explosive boiling of water, initiated by continuous laser radiation with λ = 1.94 μm, emerging from the tip of the laser fiber, are studied. The trajectories of the microvolume with the maximum temperature of the superheated liquid under laser heating, in which the probability of explosive boiling up is maximum, are determined. The calculations took into account changes in the density of superheated water and the dependence of the absorption coefficient of laser radiation on temperature. Using acoustic methods and high-speed video recording, it is shown that during explosive boiling up, the duration of the leading front of the acoustic signal is ∼300 ns. Such a long duration of pressure rise at the receiving point is explained by the fact that explosive boiling occurs in a wide area of superheated liquid near fiber tip, when the nuclei located in it are stimulated by an initially arising pressure pulse.

Calculation of intrinsic absorption coefficient in high power laser material processing

A method of analytical and numerical calculation of the absorption coefficient of laser radiation was proposed and tested based on the results of a simplified experimental cutting setup. The results are compared with available theoretical predictions and experimental data the literature forecasts in a wide range of angles of inclination. A significant difference between the absorption coefficients for mild steel used in the theory and reconstructed from experimental data. The obtained absorption coefficient can be used for further modeling of laser material processing technologies.

Technology ability of laser bonding of compacted graphite iron

Laser welding features of compacted graphite iron are described in this paper. An approach that allows to reduce residual stresses and the probability of occurrence of “hot” cracks in the weld seam as well as to increase the absorption coefficient of laser radiation due to the use of metal powder Ni-Cr-B-Si-Fe, BoroTec-Eutalloy® 10009-chromium-nickel alloy are proposed. Recommendations are given when conducting a welding with compacted graphite iron.

Generation of acoustic waves by cw laser radiation at the tip of an optical fiber in water

We investigate the specific features of acoustic signals generated in water under the action of cw laser radiation with a power of 3 W at wavelengths of 0.97, 1.56, and 1.9 μm, emerging from an optical fiber. It is established that when a fiber tip without an absorbing coating is used, quasi-periodic pulse signals are generated according to the thermocavitation mechanism due to the formation and collapse of vapor–gas bubbles of millimeter size. In this case, the maximum energy of a broadband (up to 10 MHz) acoustic signal generated only at wavelengths of 1.56 and 1.9 μm is concentrated in the range of 4–20 kHz. It is shown that when there is no absorbing coating, an increase in the laser-radiation absorption coefficient in water leads to an increase in the frequency of generated acoustic pulses, while the maximum pressure amplitudes in them remain virtually constant. If there is an absorbing coating on the laser-fiber tip, a large number of small vapor–gas bubbles are generated at all laser-radiation wavelengths used. This leads to the appearance of a continuous amplitude-modulated acoustic signal, whose main energy is concentrated in the range of 8–15 kHz. It is shown that in this case, increasing the absorption coefficient of laser radiation in water leads to an increase in the power of an acoustic emission signal. The results can be used to explain the high therapeutic efficiency of moderate-power laser-fiber apparatus.

Powerful laser pulse absorption in partly homogenized foam plasma

The internal volume structure of a porous medium of light elements determines unique features of the absorption mechanism of laser radiation; the characteristics of relaxation and transport processes in the produced plasma are affected as well. Porous materials with an average density larger than the critical density have a central role in enhancing the pressure produced during the ablation by the laser pulse; this pressure can exceed the one produced by target direct irradiation. The problem of the absorption of powerful laser radiation in a porous material is examined both analytically and numerically. The behavior of the medium during the process of pore filling in the heated region is described by a model of viscous homogenization. An expression describing the time and space dependence of the absorption coefficient of laser radiation is therefore obtained from the model. A numerical investigation of the absorption of a nanosecond laser pulse is performed within the present model. In the context of numerical calculations, porous media with an average density larger than the critical density of the laser-produced plasma are considered. Preliminary results about the inclusion of the developed absorption model into an hydrodynamic code are presented.

Interaction of near-IR laser radiation with plasma of a continuous optical discharge

The interaction of 1.07-μm laser radiation with plasma of a continuous optical discharge (COD) in xenon and argon at a pressure of p = 3–25 bar and temperature of T = 15 kK has been studied. The threshold power required to sustain COD is found to decrease with increasing gas pressure to P t 20 bar and to P t 15 bar. This effect is explained by an increase in the coefficient of laser radiation absorption to 20−25 cm–1 in Xe and 1−2 cm–1 in Ar due to electronic transitions between the broadened excited atomic levels. The COD characteristics also depend on the laser beam refraction in plasma. This effect can be partially compensated by a tighter focusing of the laser beam. COD is applied as a broadband light source with a high spectral brightness.

Absorption coefficient for nanosecond laser pulse in porous material

The absorption of powerful laser radiation in a porous material is investigated theoretically and numerically. The behavior of the medium during the process of pores filling in the heated region is described by a model of viscous homogenization. The porous material is described as a partially homogenized plasma where the density within the pores increases with time from zero to an average density of the porous substance depending on the ratio of laser pulse duration and homogenization time. An expression describing the time and space dependence of the absorption coefficient of laser radiation in such a material is derived. The initial pore’s sizes, the average density of the material and the laser intensity are the parameters of the model. The absorption of a nanosecond laser pulse in totally ionized plasma of porous material of light elements is investigated numerically within the present model. The calculations are performed for porous media with an average density larger or smaller than the critical density of the laser-produced plasma. For the latter, the results are compared with those for a homogeneous plasma with the same density.

Experimental comparison of laser energy losses in high-quality laser-oxygen cutting of low-carbon steel using radiation from fibre and lasers

We report a comparative experimental study of laser-oxygen cutting of low-carbon steel using a fibre laser with a wavelength of and a laser with a wavelength of at the sheet thickness of . For the two lasers we have measured the dependence of the cutting speed on the radiation power and determined the cutting speed at which the surface roughness is minimal. The coefficient of laser radiation absorption in the laser cutting process is measured for these lasers at different values of the cutting speed and radiation power. It is found that the minimal roughness of the cut surface is reached at the absorbed laser energy per unit volume of the removed material, equal to ; this value is the same for the two lasers and does not depend on the sheet thickness.

Mass spectrometry determination of the properties of the fullerite consisting of a C60-C70 mixture

The transformations of the crystal structure of the fullerite consisting of a mixture of C60 and C70 that occur in the course of heating during pulsed laser irradiation are described using experimental results, the fullerite surface temperature is determined during secondary synthesis, and the absorption coefficient of laser radiation is calculated.

High-quality laser cutting of stainless steel in inert gas atmosphere by ytterbium fibre and lasers

Processes of cutting stainless steel by ytterbium fibre and lasers have been experimentally compared. The cut surface roughnesses for 3- and -thick stainless steel sheets are determined. The absorption coefficient of laser radiation during cutting is measured. It is established that the power absorbed by metal during cutting by the laser exceeds that for the ytterbium laser (provided that the cutting speed remains the same). The fact that the maximum cutting speed of the laser is lower than that of the ytterbium fibre laser is explained.

Optical breakdown in supersonic air jet

The characteristics of a free supersonic air jet with a diameter of 10 mm and Mach numbers within 1.7–3.7 have been studied in the presence of energy supply via pulsed optical discharge generated by a periodic-pulsed mechanically-Q-switched CO2 laser. The coefficient of laser radiation absorption by optical discharge plasma in supersonic air jet has been studied and it is established that this parameter can reach 60%. It is demonstrated for the first time that a threshold density of air corresponding to sharp growth in the absorption efficiency amounts to 1.8–2 kg/m3 and is independent of the Mach number in airflow. Distributions of the gasdynamic parameters (including dynamic pressure and temperature) in supersonic air jet with energy supply via optical discharge have been determined.

K α radiation of foil under interaction with laser pulse of relativistic intensity

An analytical model of Kα radiation of thin laser targets has been developed. It has been shown that, for such targets, the motion of fast electrons is significant not only in the target itself but also in vacuum. The considered dependences for the free path length of a fast electron and for the absorption coefficient of laser radiation on the laser intensity with allowance for the electron motion in vacuum make it possible to match the results of the proposed model with the experimental data on generation of Kα radiation in wide ranges of laser intensities (1018–1021 W/cm2) and thicknesses (1–100 μm) of targets.

Theory of the dynamical Casimir effect in nonideal cavities with time-dependent parameters

This is a brief review of the recent progress in the theory of dynamical (non-stationary) Casimir effect in non-ideal cavities and of existing proposals to observe this effect in a laboratory, with an emphasis on the experiment which is under preparation in the University of Padua. The main idea of this experiment is to simulate periodical displacements of the cavity wall by periodical strong changes of conductivity of a thin semiconductor slab illuminated by picosecond laser pulses. In this connection the theory of quantum damped oscillator with arbitrary time dependence of the frequency and damping coefficient has been developed in order to take into account intrinsic losses in the semiconductor slab due to a finite conductivity during the intermediate part of the excitation-recombination cycle. The influence of different parameters, such as the diffusion and mobility coefficients of carriers, surface recombination velocity, absorption coefficient of laser radiation, thickness of the slab and geometry of the cavity, is analysed. Analytical and numerical evaluations show that under realistic experimental conditions, several thousand of quanta of EM field (‘Casimir photons’) could be produced from the initial vacuum state in a high-quality cavity at the lowest resonance frequency 2.5 GHz.

QED effects in a cavity with a time-dependent thin semiconductor slab excited by laser pulses

We consider the problem of the creation of quanta of an electromagnetic field from the initial vacuum (or thermal) state in a closed high-Q cavity due to periodical variations of conductivity of a thin semiconductor boundary layer excited by short laser pulses. Fast changes of conductivity from practically a zero value to a high one and then again to zero simulate periodical displacements of the cavity wall. This scheme has been chosen to model the non-stationary Casimir effect in the experiment which is under preparation at Padua University. We provide analytical and numerical evaluations for the number of photons which could be created under realistic experimental conditions. We show the importance of taking into account intrinsic losses in the semiconductor slab caused by the finite conductivity during the intermediate part of the excitation–recombination cycle. We analyse the influence of different parameters, such as the diffusion and mobility coefficients of carriers, surface recombination velocity, absorption coefficient of laser radiation, thickness of the slab and geometry of the cavity. We conclude that a significant amount (>103) of ‘Casimir photons’ with a frequency of 2.5 GHz can be produced from vacuum in a cavity with dimensions of the order of 10 cm, if one can arrange several thousand strongly periodical laser pulses with a duration of the order of 1 ps, periodicity close to 200 ps and energy ∼10−3 J, illuminating the semiconductor slab of thickness ∼1 mm and the mobility ∼1 m2 V−1 s−1, provided the recombination time can be reduced below the critical value ∼30 ps.

Plasma behind the front of a damage wave and the mechanism of laser-induced production of a chain of caverns in an optical fibre

The properties of the plasma behind the front of a damage wave generated by laser radiation in an optical fibre are considered. A plasma with a low degree of ionisation but a relatively high electron density is shown to emerge. However, the high absorption coefficient of laser radiation at a temperature of the order of 2000 K cannot be attributed to the presence of bremsstrahlung. The production of a chain of uniformly spaced caverns during the laser damage of the optical fibre is qualitatively explained. It is shown that this effect cannot be explained by the capillary Rayleigh instability because of the high viscosity of the glass. It is found that the fibre core deformation by a high pressure leads to an increase of the fibre volume sufficient to account for the emergence of the caverns after cooling. It is assumed that the periodicity of caverns is caused by the instability of a new type. A high-density double electrical-charge layer is produced at the plasma—liquid interface. Due to the repulsion of similar charges, the surface tends to increase, resulting in the instability development responsible for the production of the chain of caverns.

Opto-acoustic sounding of the atmospheric parameters

Experimental investigations of sound pulse generation accompanying the propagation of high-power pulsed laser radiation in the atmosphere were started at the IAO SB RAS in 1986. Based on these investigations, new opto-acoustic methods of sounding of the atmospheric temperature, wind velocity, relative humidity, and number density of micron fraction of atmospheric aerosols were suggested. A source of the acoustic signal in an opto-acoustic sounding system is a discrete laser spark made by focusing a CO2 laser beam into the atmosphere at distances up to 500 m. The number density of coarsely dispersed aerosols was determined from the number of pulses in the acoustic signal generated by the discrete laser spark. Results of opto-acoustic measurements of the number density of coarsely dispersed aerosols agree well with the microphysical model of aerosol atmosphere for Western Siberia. A threshold laser energy density resulting in the origin of local plasma formations is about 6−10 J /cm2. For lower laser energy densities, the acoustic radiation is generated due to laser-induced thermal expansion of the propagation medium. The measurable parameters are the effective laser beam radius, the coefficient of laser radiation absorption, and the total laser beam energy.

Overheated metastable states in pulsed laser deposition versus laser radiation wavelength

The effect of absorption coefficient of laser radiation on temperature distribution in the material and phase front dynamics is studied by numerical simulation. Overheating of both solid and liquid phases is shown and discussed

Stimulated scattering and filamentation of laser radiation in a plasma

A state of the art review is given of experimental and theoretical investigations of nonlinear processes that appear when laser radiation acts on a plasma and solid targets. It is shown that the absorption coefficient of laser radiation and the absorption quality improve greatly when submicron wavelengths are used. However, an increase in the ratio of the plasma corona thickness to the radiation wavelength enhances the role of stimulated Brillouin scattering, filamentation, self-focusing, and stimulated Raman scattering. Experimental results of studies of stimulated Brillouin and Raman scattering, of self-focusing, and filamentation of laser radiation in a plasma demonstrate that these processes should play an important role under conditions typical of pulsed thermonuclear reactions characterized by a positive yield. The experimental results also show that the stimulated scattering and filamentation in a laser plasma are related and mutually interdependent: filamentation lowers the stimulated Brillouin scattering threshold, whereas such scattering prevents the growth of stimulated Raman scattering, and so on. Observations which can be explained theoretically are pointed out. Future trends in the theory are discussed and methods for controlling nonlinear interaction of waves in a laser plasma are considered.

Water vapor cooling when radiation of wavelength ?=2.8 ?m is absorbed

Investigations of the processes arising under the influence of electromagnetic radiation on resonantly absorbing gaseous media have now been widely developed. Particular interest is shown in the penetration of a pulse of laser radiation through the atmosphere. The main component absorbing the radiation of both CO2 and HF lasers (wavelengths, respectively, 10.6 and 2.8 μm) in the earth's atmosphere is water vapor [1]. Numerous experimental investigations show that the integrated coefficient of laser radiation absorption by water vapor is fairly large [1–3], while at the same time the energy absorption leads to the heating of the medium in a channel around the beam and, as a consequence, to its defocusing. However, all these investigations were carried out with continuous sources of laser radiation or with pulses of fairly great duration. It will be shown below that gas cooling in the channel around the beam is possible when a pulse of radiation with wavelength λ 2.8 μm whose duration is less than the vibrational-translational (V-T) relaxation time of the energy absorbed by the H2O molecules passes through a stationary medium containing water vapor.

Non‐Linear Optical Properties of the Degenerate Band Charge Carrier Gas in the Magnetic Field

AbstractA theory of non‐linear optical properties of a band charge carrier gas of arbitrary concentration and temperature due to the influence of electromagnetic radiation on the probability of the carrier scattering by lattice vibrations is developed. A general expression for the polarization created by the carrier response in a laser field in the presence of other electromagnetic fields and a constant magnetic field is obtained. The generalized dielectric susceptibility tensor is calculated for various scattering mechanisms and arbitrary laser field intensities. The non‐linearity of the laser radiation absorption coefficient and the dependence of the probe wave absorption coefficient on the laser field intensity are studied.