Analysis of experimental data obtained in study of cryogenic CO laser with glow discharge and cryogenic slab CO laser with RF discharge showed that oxygen concentration in laser mixture is much less than in initial gas mixture. With low oxygen content, ozone plays role of supplier of electrons in cryogenic discharge. Since ionization potential of ozone is somewhat higher than that of oxygen, and ozone quickly condenses on cold walls of cryogenic discharge, conversion of O2 to O3 is accelerated. In addition, in plasma-chemical reactions, ozone regenerates carbon monoxide after dissociation. Acceleration of ozone generation in cryogenic barrier discharge was experimentally demonstrated.
Different frequency conversion processes in the mid-IR were experimentally studied with slab RF discharge CO and CO2 lasers and various nonlinear crystals ZnGeP2, GaSe, BaGa2GeSe6, AgGaSe2 and PbIn6Te10 : second and third harmonic generation of CO laser; sum and difference frequency generation of CO and CO2 lasers. Mid-IR emission on hundreds of narrow spectral lines from 1.7 μm to 19.3 μm was obtained
Super-broadband mid-IR laser systems are under development in the Gas Lasers Lab of the Lebedev Physical Institute. These systems consist of different sorts of molecular gas lasers and various nonlinear crystals converting multi-line emission of those lasers into broadband radiation on sum and difference frequencies. As frequency converters, both main mid-IR nonlinear crystals such as ZnGeP2, GaSe, AgGaSe2 and new ones such as BaGa2GeSe6, PbIn6Te10 are applied. As radiation sources, repetitively-pulsed Q-switched lasers pumped by both longitudinal DC discharge and transverse slab RF discharge emitting microsecond pulses, and also actively mode-locked e-beam sustained discharge lasers emitting nanosecond pulses are applied. It was experimentally demonstrated for the developed laser systems to emit broadband radiation on hundreds spectral lines covering mid-IR range from ~1.7 to ~17 micron due to sum and difference frequency generation including third harmonic generation.
Experimental and theoretical study of the post-filamentation stage of focused high-power Ti:Sa laser pulses in air is presented. Angular divergence of the laser beam, as well as angular and spatial characteristics of specific spatially localized light structures, the post-filament channels (PFCs), under different initial focusing conditions and laser beam energy are investigated. We show that PFC angular divergence is always less than that of the whole laser beam and tends to decrease with laser pulse energy increase and beam focal length elongation.
Remote monitoring of water pollution, namely thin films of oil or oil products on water surface, can be carried out by laser fluorimetry. The pollutants fluorescence during its interaction with ultrashort UV laser pulses was experimentally studied in this paper. The laser pulses power was considered in a wide range of values including the filamentation regime. We compared fluorescence stimulated by femtosecond UV laser pulses with two central wavelengths (248 and 372 nm) for detection of crude oil and the following oil products: oil VM-5, oil 5W-40 and solvent WhiteSpirit. It was shown that shorter UV wavelengths are more suitable for fluorescence excitation. The spatial resolution of the fluorescence localization was no worse than 30 cm. We discuss techniques of high intensity emission delivery to the remote target as post-filamentation channels and multifilamentation beam propagation regime as well experimentally and numerically.
The filamentation of focused beams at wavelength of 800 and 248 nm in air is studied experimentally and numerically. The results indicate that relatively tight focusing can lead to the coalescence of individual regions of high fluence and high plasma density that result from multiple refocusing, whereas in the case of weak focusing such regions are separated in the pulse propagation direction. The lower multiphoton ionization order in the case of UV radiation leads to a stronger effect of geometric focusing on filament formation. We show the possibility to control the parameters of femtosecond laser plasma filaments by introducing astigmatism in laser beam wavefront. Strong astigmatism can lead to the splitting of the channel into two separate regions. We demonstrate that the self-phase modulation in the thin passthrough dielectric plate decreases the distance to the filament start in air and increases the length of plasma channel.
Electric breakdown and non-self-sustained electric discharge were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire-KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. UV filaments of ~100 m length were observed when transporting the laser radiation over the long distance.
Non-self-sustained electric discharge and electric breakdown were triggered and guided by a train of ultrashort sub-TW ultraviolet (UV) pulses overlapped with a long free-running UV pulse of a hybrid Ti:Sapphire - KrF laser facility. Photocurrent sustained by this train is two orders of magnitude higher, and electric breakdown distance is twice longer than those for the discharge triggered by the long UV pulse only. When transporting the laser radiation over the long distance, UV filaments of ~ 100 m length were observed.
Theoretical modeling of photo-induced electron-hole plasma and bandgap dynamics in GaAs at high femtosecond laser intensities ( ∼ TW/cm2) employing a quantum kinetic formalism based on a generalized Boltzmann-type equation, predicts for the first time against expectations, the saturation of plasma densities despite the strong direct bandgap narrowing. Though the transient electronic bandgap renormalization provides a significant positive feedback for all relevant single-photon and impact ionization mechanisms, which is clearly observable at moderate (sub-TW/cm2) laser intensities, the counterintuitive plasma density saturation at higher laser intensities and high plasma densities ( ∼ 1022 cm−3) is dictated by much stronger negative feedback, originating from a highly-nonlinear transient enhancement of the corresponding Auger recombination coefficient for the shrinking bandgap. These theoretical predictions are in semi-quantitative agreement with the results of our time-resolved reflectivity infrared (IR)-pump experiments, which support this newly predicted process of self-limiting ionization dynamics in strongly photo-excited semiconductors, such as GaAs, with induced bandgap shrinkage.
Compact slab repetitively-pulsed RF discharge overtone CO laser is developed. Results of parametric study of the laser
with cryogenically cooled electrode system operating in sealed-off mode are reported. The average output power of the
laser was up to 0.5 W with maximal efficiency of ~1.5 %. More than ~80 ro-vibrational overtone CO laser lines
(summarizing two spectral ranges 2.5-3.1 μm and 3.1-4.2 μm corresponding to two sets of laser resonator mirrors used in
the experiments) were observed. Start-up experiments on development the laser head of new design with enlarged active
medium length (40 cm) were carried out. Preliminarily the output power of the overtone CO laser with the new laser
head reached 0.75 W at the efficiency up to 1 %.
In this work femtosecond laser photo-excitation of GaAs is studied numerically. The transient plasma densities photogenerated
during the pumping IR fs-laser pulses were evaluated having in mind experimental data of time-resolved
reflectivity measurements of transient bandgap shifts.
Theoretical modeling employing quantum kinetic formalism based on a generalized Boltzmann-type equation, including
one/multi-photon photo-excitation, Joule heating and free-carrier absorption, interband excitation, impact ionization,
Auger recombination of electron-hole plasma, thermal exchange with the lattice, etc. is performed.
For the first time the effect of enhancement of ionization by transient bandgap renormalization (BGR) is considered both
experimentally and theoretically. The energy spectra of the electron distribution function and the time dependence of the
electron density are calculated and the key role of BGR in the transient electron-hole plasma dynamics is pointed out.
Gradual evolution of silicon surface topology from one-dimensional to two-dimensional nanogratings and then to isotropic sets of nanospikes was observed by increasing IR and UV femtosecond laser irradiation dose (the variable number of incident laser pulses at the constant laser fluence). The fundamental mechanisms of these topological transformations are discussed.
One-dimensional quasi-periodic structures whose period is much smaller than the wavelength of exciting optical
radiation have been obtained on a titanium surface under the multi-shot action of linearly polarized femtosecond laser
radiation at various surface energy densities. As the radiation energy density increases, the one-dimensional surface
nanogratings oriented perpendicularly to the radiation polarization evolve from quasi-periodic ablative nano-grooves to
regular lattices with sub-wavelength periods (90-400 nm). In contrast to the preceding works for various metals, the
period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface
nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface
electromagnetic wave excited by this radiation, as shown by our transient reflectivity measurements and modeling,
because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases
versus increasing electron excitation of the material.
Results of experimental and theoretical study of singlet delta oxygen (SDO) production in transverse gas flow RF slab
discharge for an electric discharge oxygen-iodine laser are presented. The electric discharge facility operating in both
pulse-periodic and CW mode was manufactured: gas flow duct including multi-path cryogenic heat exchanger, dielectric
slab channel, and slab electrode system incorporated in the channel for RF discharge ignition. Experiments on SDO
production in transverse gas flow RF discharge were carried out. SDO production depending on gas mixture content and
pressure, gas flow velocity, and RF discharge power was experimentally studied. It was shown that SDO yield increased
with gas pressure decrease, gas flow deceleration and helium dilution of oxygen at the same input power. CW RF
discharge was demonstrated to be the most efficient for SDO production as compared to pulse-periodic RF discharge
with the same averaged input power. SDO yield was demonstrated to be not less than 10 percent. The model developed
was further modified to do simulations of CW and pulse periodic RF discharges. A reasonable agreement between
experimental and theoretical data on SDO production in CW and pulse-periodic RF discharges in oxygen is observed.
Experimental and theoretical study of influence of nitrogen oxides NO and NO2 admixtures in oxygen containing gas
mixture excited by pulsed electron-beam sustained discharge on input energy and time behavior of singlet delta oxygen
(SDO) luminescence was carried out. Temperature dependence of the constant of SDO relaxation by unexcited
molecular oxygen was estimated.
Overtone lasing and fundamental band tuning was for the first time obtained in a slab carbon monoxide laser. The
compact slab CO laser with active volume 3×30×250 mm3 was excited by a repetitively pulsed capacitive RF discharge
(81.36 MHz) with pulse repetition rate 100-500 Hz. The laser electrodes were cooled down to 120 K. Gas mixture
CO:Air:He at gas pressures 15-22 Torr was used. An optical scheme "frequency selective master oscillator - laser
amplifier" was applied for getting fundamental band tuning. Single line lasing with average power up to several tens of
mW was observed on ~100 rotational-vibrational transitions of CO molecule within the spectral range ~5.0 - 6.5
micron. Multiline overtone lasing was observed on ~80 spectral lines within the spectral range ~2.5 -4.0 micron, with
maximum single line average output power 12 mW. Total output power of the slab overtone CO laser came up to 0.3 W,
with maximum laser efficiency 0.5%. Results of parametric studies of the overtone CO laser including complicated time
behavior for laser pulses on different overtone vibrational-rotational transitions are discussed.
Mechanisms of nonlinear absorption and ionization of pure atomic argon and molecular nitrogen gases by UV femtosecond
laser pulses were studied using photogalvanic and photoacoustic techniques. The effect of the intermediate Rydberg
resonance, its dynamic Stark perturbation and ponderomotive upshift of the first ionization potential of argon
atoms and nitrogen molecules by the intense laser pulses has been revealed by observing an increase of a power slope of
ion yield from three to four at increasing laser intensity.
Actively mode-locked electron-beam-sustained-discharge CO-laser producing a train of ~5-15 ns (FWHM) spikes
following with repetition rate 10 MHz for both single-line and multiline mode of operation in the mid-IR range of
~5 micron was experimentally studied. Total laser pulse duration was ~0.5 ms for both mode-locked and free running
laser. Specific output energy in multiline CO-laser mode of operation was up to 20 J l-1 Amagat-1 and the laser
efficiency up to 3.5%. The active mode-locking was achieved for single-line CO-laser mode of operation in spectral
range 5.2-5.3 micron. This sort of radiation can be used for pumping an optical parametric amplifier for optical
stochastic cooling in Relativistic Heavy Ion Collider, for laser ablation, and for studying vibrational and rotational
relaxation of CO and NO molecules.
Parametric study of a slab first-overtone carbon monoxide laser was performed. The compact slab first-overtone CO
laser with active volume 3x30x250 mm3 was excited by a repetitively pulsed capacitive RF discharge (81 MHz or
60 MHz) with pulse repetition rate 100-500 Hz. The laser electrodes were cooled down to 120 K. Gas mixtures
CO:O2:N2:He with different component contents at gas pressures 15-22 Torr were used. Two laser resonator mirrors
sets were used in the experiments on multiline lasing. More than 100 spectral lines within the spectral range ~2.5-4.0
μm with maximum single line average output power 12 mW were observed. Total output power of the slab firstovertone
CO laser came up to 0.3 W, with maximum laser efficiency 0.5%. Special details of long time output laser
power behavior are discussed.
Actively mode-locked electron-beam-sustained-discharge CO-laser producing ~10 ns (FWHM) pulses repetition rate
10 MHz for both single-line and multiline mode of operation was experimentally studied. The total laser pulse duration
was ~0.5 ms for both mode-locked and free running laser conditions. The specific output energy in multiline CO-laser
mode of operation was up to 20 J l-1 Amagat-1 and the laser efficiency up to 3.5%. The active mode-locking was
achieved for single-line CO-laser mode of operation in the spectral range 5.2-5.3 micron. The radiation can be used for
laser ablation, for pumping an optical parametric amplifier in optical stochastic cooling of relativistic heavy ions, and
for studying vibrational and rotational relaxation of NO molecules.
Influence of nitrogen oxides NO and NO2 on specific input energy (SIE) and time behavior of singlet delta oxygen
(SDO) luminescence excited by pulsed e-beam sustained discharge in oxygen was experimentally and theoretically
studied. NO and NO2 addition into oxygen results in small increase and decrease of SIE, respectively, the latter being
connected with large energy of electron affinity to NO2. The addition of 0.1-0.3% nitrogen oxides was experimentally
and theoretically demonstrated to result in notable enhancement of SDO lifetime, which is related to a decrease of
atomic oxygen concentration in afterglow. There was experimentally demonstrated that for getting high SDO
concentration at gas pressure 30-60 Torr for the time interval less than ~0.5 s one needs to add not less than 0.2%
nitrogen oxides into oxygen. Temperature dependence of relaxation constant for SDO quenching by unexcited oxygen
was estimated by using experimental data on time behavior of SDO luminescence.
Singlet delta oxygen O2a1&Dgr;g (SDO) production in a slab discharges ignited in oxygen gas mixtures was experimentally studied. An influence of gas mixture content and input electric power on SDO yield was analyzed. In self-sustained RF slab discharge SDO yield of 10±5 % was measured by comparison luminescence intensity of SDO going from a chemical generator and SDO generated in electric discharge. SDO yield of 7.2 % was measured by intracavity laser spectroscopy method. It was demonstrated that the choice of electrodes material is very important. Experiments on SDO production in slab non-self-sustained discharge with external ionization by repeating high-voltage pulses were carried out. SDO concentration was measured by the method of intracavity laser spectroscopy. The measured concentration of SDO was about 1.5×1016 cm-3, with SDO yield of ~10.6 %. A development of electric discharge oxygen-iodine laser with SDO production in long electrodes slab discharge is discussed.
First experiments on fundamental band CO lasing in sealed-off cryogenically cooled slab facility with RF discharge
excitation were carried out. Repetitively pulsed and CW modes of RF discharge excitation were studied. Average output
power achieved 12 W. Lasing efficiency exceeded 10%. The output laser spectrum was observed within wavelengths
range 5.08-5.34 &mgr;m. Stable lasing was obtained for tens minutes.
Admixing of iodide to the effluent of MW discharge in oxygen was used to measure yield of singlet oxygen generated.
Singlet oxygen yield was evaluated using the equilibrium condition. The method was absolutely calibrated using a
traditional chemical singlet oxygen generator.
The multi-stage hybrid laser system producing ultrashort pulses of radiation with peak power ~1014 - 1015 W now under
developing at the Lebedev Physical Institute of the Russian Academy of Sciences is discussed. The distinctive feature
of the laser system is direct amplification of ultrashort pulses produced by solid state laser system, first going through a
prism stretcher with negative dispersion, in gas active medium without using a rather expensive and complicated grating
compressor of laser pulses. Two hybrid schemes are being developed now based on the amplification of femtosecond
pulses of the third harmonic of Ti:Sapphire laser at the wavelength 248 nm in the active medium of KrF laser amplifier,
and on the amplification of the second harmonic of Ti:Sa laser at the wavelength 480 nm in the active medium of
photochemical XeF(C-A)-laser excited by VUV radiation of an e-beam pumped Xe2 lamp. The final stage of the laser
system is supposed to be an e-beam pumped facility with a laser chamber of 60 cm in diameter and 200 cm long in the
case of KrF laser, and with another laser chamber of 30-40 cm in diameter put into the former one in the case of XeF(CA)
laser. The parameters of such e-beam facility are close to those of previously developed at the Institute of High-
Current Electronics: electron energy ~600 keV, specific input power ~ 300-500 kW/cm3, e-beam pulse duration ~ 100-
200 ns. A possibility of using Kr2F as an active medium with saturation energy 0.2 J/cm2 for amplification of ultrashort
laser pulses is also under consideration. There was theoretically demonstrated that the energy of a laser pulse at the exit
of the final stage of the laser system could come up to ~ 17 J with pulse duration ~50 fs in the case of KrF laser, and
~75 J with pulse duration of 25 fs in the case of XeF laser. Two Ti:Sa laser systems producing ~50 fs pulses with
energy ~0.5 mJ at the wavelength 248 nm and ~5 mJ at the wavelength 480 nm have been already developed and are
being now installed at the Lebedev Institute. Preliminary
First experiments on fundamental band CO lasing in sealed-off cryogenically cooled slab facility with RF discharge
excitation were carried out. Repetitively pulsed and CW modes of RF discharge excitation were studied. The laser
output characteristics for different slab geometries were compared. Average output power achieved 12 W. Lasing
efficiency came up to ~14 %. The output laser spectrum was observed within wavelengths range 5.08-5.34 &mgr;m. Stable
lasing was obtained for more than one hour.
The process of laser ablation of fused and crystal silica and natural silicates induced by pulsed CO2 laser irradiation (total pulse duration of 35 μs, laser pulse rise time of 0.1 μs, pulse energy of 10 J) has been studied. By action of focused laser radiation at the surface of samples the appearance of erosion plume at the irradiated surface take place. For study of ablation regimes the frame image of laser induced plume by use of high speed photography method with 2 microsecond expose of each frame have been made. It has been observed the formation of two different type of laser plume during laser pulse action. First type plume represents a long narrow plume with the onset close to start of laser pulse. The maximal length of this plume is about of 20 millimeter. The second type of plume represents a conical plume with the onset, which is late from laser pulse start for a few microsecond. The first type plume expands in to laser beam direction. The second type plume expands normal to the surface. The velocity of laser plume particles by means of measurements of moment transferred into samples during laser pulse action has been measured. It has been obtained, that value of this velocity varies in region of (l÷3)103 m/s in dependence of laser fluency value. The appearance of first time plume have a threshold which equal to 0.30 kJ/cm2 whereas the appearance of second type plume have not a such threshold and may take place at low laser fluency. We believe, that the first type plume connects with the selective ablation of SimOn complexes whereas second type plume connects with the thermal heating caused by dissipation of laser energy in to system of low frequency acoustics phonons.
Singlet delta oxygen O2(a1Δg) (SDO) production in a slab discharges ignited in oxygen gas mixtures was experimentally
studied. An influence of gas mixture content and input electric power on SDO yield was analyzed. In self-sustained RF
slab discharge SDO yield of 10±5% was measured by comparison luminescence intensity of SDO going from a
chemical generator and SDO generated in electric discharge. SDO yield of 7.2% was measured by intracavity laser
spectroscopy method. It was demonstrated that the choice of electrodes material is very important. Experiments on SDO
production in slab non-self-sustained discharge with external ionization by repeating high-voltage pulses were carried
out. SDO concentration was measured by the method of intracavity laser spectroscopy. The measured concentration of
SDO was about 1.5 1016 cm-3, with SDO yield of ~10.6%. A development of electric discharge oxygen-iodine laser with
SDO production in long electrodes slab discharge is discussed.
Characteristics of CW CO laser with supersonic gas flow are discussed. Radio frequency discharge CO overtone (V+2→V) lasing with output power 50 W was demonstrated with gas cooling due to supersonic expansion. The overtone lasing was observed on 9→7, 10→8 and 11→9 vibrational transitions within spectral range 2.6-2.7 μm. The laser active medium length was 10 cm. The small signal gain on the overtone transitions was estimated to be 0.1 m-1. Fundamental band (V+1→V) lasing was observed within 4.9 - 5.7 μm spectral range. In fundamental band output power 2.1 kW with efficiency 21% was obtained, with typical small signal gain being about 1 m-1. Modeling experiments on electron-beam sustained discharge facility were carried out at experimental conditions (gas pressure, temperature) corresponding to those of supersonic gas flow. Possibilities of supersonic overtone CO laser design improvement to obtain overtone lasing on highly excited vibrational transitions of CO molecule corresponding to the spectral range ~3-4 micron are discussed.
Singlet delta oxygen O2(a1Δg) (SDO) production in a slab discharges ignited in oxygen gas mixtures was experimentally studied. An influence of gas mixture content and input electric power on SDO yield was analyzed. In self-sustained RF slab discharge SDO yield of 10±5% was measured by comparison luminescence intensity of SDO going from a chemical generator and SDO generated in electric discharge. SDO yield of 7.2% was measured by intracavity laser spectroscopy method. It was demonstrated that the choice of electrodes material is very important. Experiments on SDO production in slab non-self-sustained discharge with external ionization by repeating high-voltage pulses were carried out. SDO concentration was measured by the method of intracavity laser spectroscopy. The measured concentration of SDO was about 1.5 1016 cm-3, with SDO yield of ~10.6%. A development of electric discharge oxygen-iodine laser with
SDO production in long electrodes slab discharge is discussed.
Small-signal gain time behavior for cryogenic pulsed e-beam sustained discharge CO laser amplifier on high (V>15) fundamental band vibrational transitions was studied experimentally for different CO containing gas mixtures including ones typical for CO lasers and amplifiers (CO:He and CO:N2), and CO:O2 used for singlet delta oxygen production.
Last years, CO laser physics has been advanced by researches of CO overtone laser operated on high vibrational levels. An extension of kinetic model to multi-quantum vibration-vibration exchange and development of fully self-consistent model of CO laser are described. The theoretical model developed is verified by comparison with experimental data on overtone CO laser characteristics and laser gain dynamics. Current status of experimental achievements in CO laser
characteristics in both fundamental and overtone bands are reported.
Singlet delta oxygen (SDO) production in a pulsed e-beam sustained discharge (EBSD) ignited in molecular oxygen with carbon monoxide stabilizing this discharge is theoretically and experimentally studied. Temporal behavior of SDO concentration and yield in the EBSD afterglow is analyzed. Experimentally measured SDO yield for oxygen mixture O2:Ar:CO=1:1:0.05 at total gas pressure 30 Torr comes up to 7% at specific input energy (SIE) of ~3.0 kJ/(1 atm(O2+CO)), whereas its theoretical value riches ~ 17.5%. The efficiency of SDO production is theoretically analyzed as function of the SIE.
Time behavior of gas temperature in CO containing gas mixture excited in pulsed electron-beam sustained discharge (EBSD) was experimentally studied under different experimental conditions. To study time behavior of gas temperature, the fact that the gas temperature is equivalent to the rotational temperature of gas molecules was used. Rotational distribution of the excited states of CO molecule was reconstructed from measured small-signal gain dynamics on different ro-vibrational transitions. The time behavior of small-signal gain was obtained with probe low-pressure CW CO laser for ten ro-vibrational spectral lines. Gas mixtures CO:He=1 :4 and CO:N2=1:9 typical for a CO laser were used. It was demonstrated that gas temperature grew from 1 10 K (initial temperature) up ~15O K for the first hundred microsecond after EBSD beginning and was staying at this value for a long time (more than 1 ms) for both gas mixtures. EBSD pulse duration was -3Oµs. The method of reconstruction of gas temperature time history was also applied for oxygen gas mixture CO:O2=1:20 at gas pressure 0.04 atm, which was used for obtaining singlet delta oxygen in EBSD. The method can be used for diagnostic of non-equilibrium gas mixtures containing CO molecule.
The problem of development of a singlet delta oxygen O2(1Δg) (SDO) generators alternative to chemical one needs application of the accurate methods of measuring the SDO concentration. A chemical SDO generator providing efficient operation of a chemical oxygen-iodine laser (COIL) is proposed to be used as a reference source for absolute calibration of the system measuring the SDO concentration. The principle of the COIL operation results in the threshold and output COIL parameters make it possible to evaluate the SDO yield with a satisfactory accuracy. A convenient sparger chemical SDO generator was applied as a reference source for absolute calibration of detectors of dimole (λ=634nm) and b→X (λ=762 nm) radiations. The values of b-state concentration formed in a longitudinal electric discharge were evaluated. The intracavity laser spectroscopy (ICLS) was absolutely calibrated for measuring the SDO concentration. ICLS method has a very high sensitivity and makes it possible to monitor the absorption corresponding to the O2(1Δg)→O2(1Σg+) (λ = 1.91 μm) transition. The cross-sections of lines of the Q - branch of the vibrational 0-0 band of the a1Δg → b1Σg+ transition of molecular oxygen were measured. The method developed was applied to measure the concentration of singlet oxygen produced in the microwave discharge. He - Ne laser (λ = 633 nm) was used for absolute calibration of a system monitoring the dimole radiation. The rate constant of the process responsible for dimole emission was measured. The value obtained kd=7.34•10-23 cm3/s is in agreement with literature.
The laser ablation of fused and crystal silica and natural silicates induced by pulsed CO2 laser irradiation (total pulse time of 35 μs, pulse increase time of 0,1 μs, pulse energy of 10 J) has been studied. By action of focused laser radiation at the surface of samples the appearance of erosion plume at the irradiated surface take place. For study of ablation regimes the frame image of laser induced plume by use of high speed photography method with 2 microsecond expose of each frame have been made. It has been observed the formation of two different type of laser plume during laser pulse action. First type plume represents a long narrow plume with the onset close to start of laser pulse. The maximal length of this plume is about of 20 millimeter. The second type of plume represents a conical plume with the onset, which is late from laser pulse start for a few microsecond. The first type plume expands in to laser beam direction. The second type plume expands normal to the surface. The velocity of laser plume particles by means of measurements of moment transferred into samples during laser pulse action has been measured. It has been obtained, that value of this velocity varies in region of 1-3•103 m/s in dependence of laser fluency value. The appearance of first time plume have a threshold which equal to 0,15 kJ/cm2 whereas the appearance of second type plume have not a such threshold and may take place at low laser fluency. We believe, that the first type plume connects with the selective ablation of SimOn complexes whereas second type plume connects with the thermal heating caused by dissipation of laser energy in to system of low frequency acoustics phonons.
Electric properties and spectroscopy of an e-beam sustained discharge (EBSD) in oxygen and oxygen gas mixtures at gas pressure up to 100 Torr were experimentally studied. The pulsed discharge in pure oxygen and its mixtures with noble gases was shown to be very unstable and characterized by low input energy. When adding small amount of carbon monoxide or hydrogen, the electric stability of the discharge increases, specific input energy (SIE) per molecular component being more than order of magnitude higher and coming up to 6.5 kJ/(l atm) for gas mixture O2:Ar:CO = 1:1:0.1. The results of experiments on spectroscopy of the singlet delta oxygen O2(a1Δg)(SDO) and O2(b1Σg+) states in the EBSD are presented. The calibration of the optical scheme for measuring the SDO absolute concentration and yield using the detection of luminescence of the SDO going from a chemical SDO generator was done. The preliminary measurement of the SDO yield demonstrated that it was ~3% for the SIE of ~1 kJ/(l atm), which is close to the results of theoretical calculations for such a SIE. Theoretical calculations demonstrated that for the SIE of 6.5 kJ/(l atm) the SDO yield may reach ~20% exceeding its threshold value needed for oxygen-iodine laser operation at room temperature, although a part of the energy loaded into the EBSD goes into the vibrational energy of the molecular admixture, (which was experimentally demonstrated by launching a CO laser operating on an oxygen-rich mixture O2:Ar:CO = 1:1:0.1 and measuring its small-signal gain).
An idea to replace singlet delta oxygen (SDO) generator working with wet chemistry by electric discharge generator has got much attention last years. Different kinds of discharge were examined for this purpose, but without a great success. The existing theoretical models are not validated by well-characterized experimental data. To describe complicated kinetics in gas discharge with oxygen one needs to know in detail processes involving numerous electronic excited oxygen molecules and atoms. To gain new knowledge about these processes experimental studies were made on electric discharge properties in gas mixture flow with independent control of inlet SDO concentration. The theoretical model extended to include minor additives like oxygen atoms, water molecules, ozone was developed. Comparison with careful experimental measurements of electric characteristics along with gas composition allows us to verify the model and make theoretical predictions more reliable. Results of numerical simulations using this model for an electron-beam sustained discharge are reported and compared with the experimental data.
Small-signal gain time behavior for a pulsed e-beam sustained discharge CO laser amplifier operating on fundamental band vibrational transitions V → V-1 from 6 → 5 (λ ~ 5.0 μm) up to 32 → 31 (λ~7.5 μm) was studied both experimentally and theoretically at various parameters of active medium. Special attention was paid to the small-signal gain time behavior on high vibrational transitions (V > 15). As was previously shown by us, multi-quantum theoretical model of vibrational exchange has to be applied for a correct description of a CO laser operating on vibrational transitions V → V-1 with V higher than 15 instead of a single-quantum one. To make easier theoretical interpretation of the experimental results, the binary nitrogen free gas mixture CO:He = 1:4 was used in our experiments. Total gas density and initial gas temperature was 0.12 Amagat and ~100 K, respectively. The complete kinetic model of a CO laser taking into account multi-qauntum vibrational exchange was employed for theoretical description of the small-signal gain time behavior. The theoretical results were compared with the experimental data.
Electric properties and spectroscopy of an e-beam sustained discharge (EBSD) in oxygen and oxygen gas mixtures at gas pressures up to 100 Torr was experimentally studied in large excitation volume (~18 liter). The discharge in pure oxygen and its mixtures with noble gases was shown to be very unstable and characterized by low input energy. When adding small amount of carbon monoxide or hydrogen, the electric stability of the discharge increases, specific input energy per molecular component being higher more than order of magnitude and coming up to 6.5 kJ/(l atm). Theoretical calculations demonstrated that for the experimental conditions the singlet delta oxygen yield may reach ~20% exceeding its threshold value needed for oxygen-iodine laser operation at room temperature. The results of experiments on spectroscopy of the singlet delta and singlet sigma oxygen states in the EBSD are presented.
Laser annealing, laser surface processing and laser lift-off procedure are reviewed as applied to semiconductor nitride- based structures (GaN films and InGaN/GaN optoelectronic device structures grown on sapphire substrates). Data on laser ablation of composite GaN/sapphire material are reviewed with more detailed consideration of the ablation rate under subpicosecond laser pulses and under long-pulse irradiation in the IR range (wavelengths of 5.0-5.8 and 9.6 micrometers ).
Temporal behavior of small signal gain (SSG) for a frequency tunable first-overtone CO laser has been studied both experimentally and theoretically. The laser operates on highly excited vibrational transitions from 20→18 up to 38→36 that correspond to laser wavelength range between ~3 and 4 microns coinciding to atmospheric 'transparency window'. Maximum SSG comes up to 0.4 m-1. It is shown that multiquamtum and asymmetric VV exchange has to be taken into account when analyzing processes of population formation on high vibrational transition.
Studies of vibration-vibration exchange in CO molecules excited up to vibration quantum numbers v=20 are performed both theoretically and expethnentally. New kinetic model taking into account muhi-quantum VV exchange in temperature range T=100Ö300K is described for the first time. Description of the experimental methodology allowing for studies of effects of relaxation of the vibrational distribution after a sudden disturbance is given. The disturbance of the vibrational distribution is produced by Q-switched short pulse of single line radiation in fundamental band The relaxation is studied by measuiing laser pulse energy of the second pulse initiated by resonator Q-switching produced with variable time delay relative to the first pulse. A set of kinetic rate constants accepted in the model for various gas temperatures, vibration level numbers and number of exehanged vibration quanta from 1 to 4 is presented. Good agreement between experiniental data and results of the advanced theory is the first direct evidence in support of multi-quantum exchange model.
Experiments on laser-rock-fluid interaction have been carried out by using pulsed CO and CO2 lasers which irradiated rocks typical for oil field: sandstone, limestone, shale and granite. Energy fluence and laser intensity on rock surface were up to 1.0 kJ/cm2 and 107W/cm2, respectively. The dependencies of specific energy consumption (i.e. energy per volume needed for rock excavation) on energy fluence, the number of pulses, saturated fluid, rock material and irradiation conditions have been obtained for various rock samples. The dependencies of momentum transferred to the rock on energy fluence for dry rocks and rocks with surface saturated by water or mineral oil have been measured. High-speed photography procedure has been used for analyzing laser plasma plume formation on a rock surface. Infrared spectra of reflectivity and absorption of rocks before and after irradiation have been measured.
Experiments on laser-rock-fluid interactions have been carried out by using pulsed CO and CO2 lasers which irradiated rocks typical for oil field: sandstone, limestone, shale and granite. Energy fluence and laser intensity on rock surface were up to 1.0 kJ/cm2 and 107 W/cm2, respectively. The dependencies of specific energy consumption (i.e. energy per volume needed for rock excavation) on energy fluence, the number of pulses, saturated fluid, rock material and irradiation conditions have been obtained for various rock samples. The dependencies of momentum transferred to the rock on energy fluence for dry rocks and rocks with surface saturated by water or mineral oil have been measured. High-speed photography procedure has been used for analyzing laser plasma plume formation on a rock surface. Infrared spectra of reflectivity and absorption of rocks before and after irradiation have been measured.
Frequency selected Q-switched e-beam controlled-discharge CO- laser has been researched and developed for surface heat treatment of polymeric materials [poly(ethyleneterephthalate) and nylon] having strong absorption bands near wavelength of approximately 6 micrometer. The laser generates pulses (including short ones with duration (tau) 0.1 approximately 1 - 10 microsecond(s) ) having different spectral content within 4.9 - 6.5 micrometer spectral range. Different geometry and methods of irradiation were used to process samples with the laser radiation of different temporal, spectral and energy density characteristics. Different types of microstructure were formed on the surfaces of the samples. Experimental conditions corresponding to each type of microstructure were defined. Visual (macro) changes of polymeric material properties (if any) and their correlation with formed microstructures were analyzed.
Frequency selected Q-switched e-beam controlled-discharge CO-laser has been researched and developed for surface heat treatment of polymeric materials (poly(ethyleneterephthalate) and nylon) having strong absorption bands near wavelength of approximately 6 micrometers . The laser generates pulses (including short ones with duration (tau) 0.1 approximately 1 - 10 microsecond(s) ) having different spectral content within 4.9 - 6.5 micrometers spectral range. Different geometry and methods of irradiation were used to process samples with the laser radiation of different temporal, spectral and energy density characteristics. Different types of microstructure were formed on the surfaces of the sample. Experimental conditions corresponding to each type of microstructure were defined. Visual (macro) changes of polymeric material properties (if any) and their correlation with formed microstructures were analyzed.
Frequency selected Q-switched e-beam controlled-discharge CO-laser has been researched and developed for surface heat treatment of polymeric materials (poly(ethyleneterephthalate) and nylon) having strong absorption bands near wavelength of approximately 6 micrometers . The laser generates pulses (including short ones with duration of (tau) 0.1 approximately 1 - 10 microsecond(s) ) having different spectral content within 4.9 - 6.5 micrometers spectral range. Different geometry and methods of irradiation were used to process samples with the laser radiation of different temporal, spectral and energy density characteristics. Various types of microstructure were formed on the surfaces of the samples. Experimental conditions corresponding to each type of microstructure were defined. Visual (macro) changes of polymeric material properties (if any) and their correlation with formed microstructures were analyzed.
A frequency selected (FS) Q-switched (QS) e-beam controlled- discharge (EBCD) CO-laser has been researched and developed. The laser generates short pulses (tau0.1 approximately equals 1 - 10 microseconds) having different spectral content including single line one within 4.92 - 6.54 micrometer spectral range. An influence of spectral content of CO laser radiation 'locked' or 'blocked' inside a laser resonator on output energy and efficiency of lasing on selected wavelengths has been studied.
The results are presented of computer modeling jointly with preliminary results of experimental investigations of sequentially Q-switched frequency selected CO laser. Calculations show the essential difference in pulse form and inversion restoration time between the models of single- and multi-quantum vibrational exchange kinetics.
An influence of different parameters such as gas pressure, specific input energy, laser mixture content, intensity ratio for probe and co-propagating pumping wave, coherency and geometry, etc., upon characteristics of phase conjugation (PC) signal and PC reflectivity (PCR) at intracavity degenerate four-wave mixing (DFWM) of long pulse carbon-dioxide and carbon lasers radiation in their inverted medium has been studied.
Frequency selected pulsed CO laser was used for surface heat treatment of polymeric materials. Nylon and poly(ethyleneterephthalate) samples having strong absorption bands near wavelength of approximately 6 micrometers were processed by the laser radiation of different temporal, spectral and energy density characteristics by using different geometry and methods of irradiation. Different types of microstructure were formed on the surfaces of the samples. Experimental conditions corresponding to each type of microstructure were defined. Visual (macro) changes of polymeric material properties (if any) and their correlation with formed microstructures were analyzed.
A frequency selected (FS) Q-switched (QS) e-beam controlled- discharge (EBCD) CO-laser has been researched and developed. The laser generates short pulses (tauout approximately 1 - 10 microseconds) having different spectral contents including single line one with 4.95 - 6.50 micrometer spectral range. A special optical scheme has been chosen, which has enabled us to study an influence of spectral contents of CO laser radiation 'locked' or 'blocked' inside a laser resonator on output energy and efficiency of lasing on selected wavelengths. The influence of laser mixture contents, gas pressure and temperature, laser pulse length and spectral width upon the laser characteristics also has been studied. A use of frequency selection and Q-switching decreases the laser efficiency dramatically from approximately 30% for non-selected (NS) free-running (FR) mode of operation down to approximately 0.5% (tauout equals 5 microseconds) for optimal wavelength (lambda approximately 5.3 micrometer) and to 0.1% at the long wave edge of laser spectrum (lambda approximately 6 micrometer). Output energy and laser efficiency strongly depend on the number of short pulses in a train for a single electrical pumping pulse. For instance, eight short pulses lasing (tauout approximately 5 microseconds) has increased the output energy up to approximately 0.5 J (0.6 J/l Amagat) and efficiency up to 0.6%. For some wavelengths the output energy and laser efficiency of FS QS CO laser does not strongly depend on temperature within 100 - 150 K interval. Comparative analysis of NS FR, FS FR, NS QS and FS QS mode of operation of carbon monoxide laser has been done.
A frequency selected (FS) Q-switched (QS) e-beam controlled-discharge (EBCD) CO-laser has been researched and developed. The laser generates short pulses ((tau) out approximately 1 - 10 microsecond(s) ) having different spectral contents including single line one within 4.95 - 6.50 micrometers spectral range. A special optical scheme has been chosen, which has enabled us to study an influence of spectral contents of CO laser radiation `locked' or `blocked' inside a laser resonator on output energy and efficiency of lasing on selected wavelengths. The influence of laser mixture contents, gas pressure and temperature, laser pulse length and spectral width upon the laser characteristics also has been studied. A use of frequency selection and Q-switching decreases the laser efficiency dramatically from approximately 30% for non-selected (NS) free-running (FR) mode of operation down to approximately 0.5% ((tau) out equals 5 microsecond(s) ) for optimal wavelength ((lambda) equals 5.3 micrometers ) and to 0.1% at the long wave edge of laser spectrum ((lambda) approximately 6 micrometers ). Output energy and laser efficiency strongly depend on the number of short pulses in a train for a single electrical pumping pulse. For instance, eight short pulses lasing ((tau) out approximately 5 microsecond(s) ) has increased the output energy up to approximately 0.5 J (0.6 J/l Amagat) and efficiency up to 0.6%. For some wavelengths the output energy and laser efficiency of FS QS CO laser do not strongly depend on temperature within 100 - 150 K interval. Comparative analysis of NS FR, FS FR, NS QS and FS QS mode of operation of carbon monoxide laser has been done.
The results of experiments on four-wave interaction of electron-beam-controlled discharge long pulsed CO2-lasers' (t-20 microsecond(s) ) and CO- lasers' (t approximately 200 microsecond(s) ) radiation inside their active medium are presented. Linearly polarized probe CO2 (CO-) laser beam intersects strong electromagnetic waves counterpropagating through an inverted medium inside the laser resonator. The laser beam reflected from the active medium has been registered both in near-field zone and in far field one. The experiments on recovering of optical images in near-field zone and recovering of angular divergency of laser radiation in far-field zone demonstrated that reflected beam was phase-conjugated one. The reflectivity of phase-conjugated beam was up to 2% for CO2-laser and up to 0.2% for CO- laser. The time-history of phase-conjugation reflectivity and comparison with theoretical results are discussed.
The results of experiments on four-wave interaction of electron-beam-controlled discharge (EBCD) pulsed CO2- and CO- lasers' radiation inside their active medium are presented. Linearly polarized probe CO2 (CO-) laser beam intersects strong electromagnetic waves counterpropagating through an inverted medium inside the laser resonator. The laser beam reflected from the active medium has been registered both in near-field zone and in far field. The experiments on recovering of optical images in near-field zone and recovering of angular divergency of laser radiation in far-field zone demonstrate that the reflected beam was a phase- conjugated one. The reflectivity of the phase-conjugated beam was up to 2% for CO2- laser and up to 0.2% for CO- laser. The time-history of phase-conjugation reflectivity and comparison with theoretical results are discussed.
KEYWORDS: Gas lasers, Pulsed laser operation, Carbon dioxide lasers, Molecules, Molecular lasers, Modes of laser operation, Energy efficiency, NOx, High power lasers, Carbon monoxide
The characteristics and potential applications of high power pulsed electron beam controlled discharge (EBCD) N2O laser are discussed. The influence of pressure, temperature, composition, and components' ratio of laser mixture on the discharge properties, laser gain, and lasing parameters have been studied experimentally. The small signal gain measurements and N2O laser wavelength tuning were carried out for P- and R- branches of N2O molecule from P(2) to P(46) and from R(47) to R(2), respectively. The maximum specific output energies were 40 J/l atm (efficiency approximately 10%, free running laser mode), 16 J/l atm (P-branch line P(18), single line selective laser mode), and 12 J/l atm (R-branch line R(20), single line selective laser mode). The relaxation constant of N2O laser levels and the excitation efficiency in EBCD are estimated.
High power pulsed electron beam controlled discharge (EBCD) N2O laser (?, ~ 10.9 (?m) has been created. The influence of the molecular (N2 ,CO) and atomic ( He, Ar, Xe ) gases addition to the laser mixture on the energy
characteristics has been studied experimentally. The laser energy (more than 102 J) was two orders of magnitude higher than that known for pulsed electric discharge N2O lasers.
KEYWORDS: Gas lasers, Electrodes, Distributed interactive simulations, Carbon monoxide, Electron beams, Energy efficiency, Chemical lasers, Picosecond phenomena, Mirrors, Argon
An electron-beam controlled discharge (EBCD) supersonic CO-laser operating
at transient conditions of excitation (?dis /?tr~1) has been investigated. Mach
number equaled 3.0. The e-beam current density was 1 mA/cm2 . The discharge has been excited between two electrodes spaced 41 mm. The axis of the main resonator has been shifted from the discharge region centre by 120 mm downstream. Small additions of H2 (1-2%) into CO:N2: Ar gas mixture resulted in
the increase of a specific input energy, but the efficiency has been reduced. Maximum efficiency reached 20 % . The output peak power has been gradually growing with the growth of the gas density,and reached 90 kW ( the gas density being 0.5 Amagat ). Five additional cavities have been placed downstream. The usage of the cavities has allowed us to measure the lifetime of the inverse population in CO:N2: Ar mixtures.
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