Optically pumped rare gas lasers have the potential to provide high-power output with excellent beam quality. They lase at wavelengths that are readily transmitted by the atmosphere, and their optical characteristics are closely similar to those of diode-pumped alkali vapor lasers (DPAL’s). As compared to DPAL’s they present two significant advantages. The first is that they use entirely inert reagents that are gases at ambient temperature (Helium plus a heavier rare gas). As there is no chemistry involved, these devices can be operated in a completely closed-cycle mode. A second advantage is derived from the energy level structures of metastable rare gas atoms. While the sp transitions of alkali metals will provide just one laser wavelength for a given element, each rare gas atom can offer a range of different output wavelengths from the first manifold of sp transitions. The primary technical challenge for the optically pumped rare gas laser is the requirement to generate the lasing medium (metastable Rg(3P2) where Rg=Ne, Ar, Kr or Xe) at a density of approximately 1e13 cm-3 in the presence of helium at total pressures in the range 0.5 – 1.0 atm. For this application we have developed a quasi-CW discharge driven by a high repetition rate power supply. The frequency and time duration of the pulses are tailored to exploit the non-steady high field characteristics of the pulsed breakdown, while sustaining a temporally steady argon metastable concentration in the gain medium. This system has been scaled to a 912.3 nm output of ~4 W when pumped by a 20 W diode laser. Recently, the pulsed discharge system has been improved by increasing the upper limit for the voltage available from the power supply. The present system operates at voltages up to 2200 V with a nominal pulse duration of 50 ns and pulse repetition frequency of 100 kHz. This has been used to sustain a discharge in 1 atm of a He/Ar mixture that produces an Ar* metastable density of 2e13 cm-3, a path length of 3 cm and a total volume of 1.2 cm3 (0.64x0.64 cm2 cross section). A diode pumped Ar* laser that employed this discharge has been operated for extended periods with no sign of performance degradation. As expected for a system that uses only inert gases, there was no indication of window damage or chemical activity. Computational models indicate that scaling to the 100 kW level is feasible with a discharge volume of 10 cm3 and discharge power of 200 W.
In this communication, we present the results of experiments with optically pumped rare gas laser (OPRGL) with a dielectric barrier discharge (DBD) as the source of metastable Ar atoms and a pulsed OPO system as the optical pump. In a longitudinal pumping scheme, the threshold input intensity to achieve lasing was 3.9 kW cm-2. Lasing was observed in the pressure range from 100 to 750 Torr. However, lasing was possible only during the limited time of the DBD applied voltage cycle.
The mandatory condition for efficient operation of an optically-pumped all-rare-gas laser (OPRGL) is the presence of rare gas metastable atoms in the discharge plasma with number density of the order of 1012-1013 cm-3. This requirement mainly depends on the choice of a discharge system. In this study the number density values of argon metastable atoms were obtained in the condition of the dielectric-barrier discharge (DBD) at an atmospheric pressure.
Rate constants for the removal of O2 b1∑+g by collisions with O2, N2, CO2 and H2O have been determined at temperature 297 K. O2(b1 ∑+g) was excited by pulses from a tunable dye laser, and the deactivation kinetics were followed by observing the temporal behavior of the b1∑+g - X3∑-g fluorescence. The removal rate constants for CO2, N2 and H2O were not strongly dependent on temperature, and could be represented by the expressions kCO2=(1.8±0.05)×10-16; kN2=(2.2 ± 0.2)×10-15, and kH2O=(6.12±0.67)×10-12 cm3 molecule-1 s-1. Rate constant for O2(b1∑+ ) removal by O2(X), being orders of magnitude lower, represented by the fitted expression kO2=(3.67 ± 0.06)×10-17 cm3 molecule-1 s-1. All of the rate constants measured at room temperature were found to be in good agreement with previously reported values.
This paper describes systematic measurements of pressure broadening coefficients for argon and krypton lines in an RF (radio-frequency) discharge plasma sustained in a mixture of inert gases. Using tunable diode laser spectroscopy we obtained experimental data for pressure broadening of argon and krypton lines. Pressure broadening coefficients were determined for Ar+Ne and Kr+Ne and Kr+Ar. For krypton, the isotopic structure of the line was taken into account and an appropriate fitting function was used to determine pressure broadening coefficients for the natural mixture of isotopes. These data may be used for diagnostics of the active medium of optically pumped all-rare-gas lasers.
The results of experiments with a dielectric barrier discharge (DBD) are presented, where the production of metastable argon atoms was studied. The recently proposed optically pumped all-rare-gas laser (OPRGL) utilizes metastable atoms of heavier rare gases as lasing species. The required number density of metastables for efficient laser operation is 1012÷1013 cm-3 in an atmospheric pressure of He buffer gas. Recent experiments had shown that such densities are easily produced in a nanosecond pulsed discharge, even at pressures larger than atmospheric, but problems appear when one is trying to produce them in a CW regime. The reason for difficulties in the CW production of metastables at an atmospheric pressure seems to be the low value of the E/N parameter (<5-6 Td). In our experiments a 20 KHz DBD in 2-5% Ar mixture with He at an atmospheric pressure was studied. [Ar(1s5)] number density of the order of 1012 cm-3 was readily achieved. Temporal behavior of [Ar(1s5)] throughout the DBD cycle was obtained. The results demonstrate the feasibility of DBDs for OPRGL development.
Absorption spectroscopy measurements of long-lived metastable argon atoms Ar* in a low-pressure RF-discharge were carried out to measure gas leaks in a vacuum chamber. Argon as a carrier gas was flowing through the test chamber and the discharge cell at a rate of 55 μmol/s. If a leak occurs, the ambient air is admixed to the carrier gas flowing through the test chamber. The presence of air in the carrier gas flowing through the discharge plasma produced a decrease in the number density of Ar*, which was measured by means of diode laser absorption spectroscopy. This is because the lifetime of atoms is limited by losses due to collisions with air molecules. The leak-rate of the ambient air ranged from 0.14 to 0.95 μmol/s was measured by a mass flow meter and compared with the amplitude of the absorption signal.
Optically pumped all-rare-gas laser (OPRGL) with unique properties was recently proposed. To study this promising laser system it is necessary to have reliable diagnostics for the active medium. A set of pressure broadening coefficients, for self- and foreign- gas collision partners, is needed for measurements of the number density of metastable atoms and temperature in a rare gas discharge plasma by means of spectroscopy. However, literature analysis had shown that pressure broadening coefficients for rare gas lines in mixtures that are of interest for OPRGL’s are surprisingly hard to find, or were not yet measured. Diode laser absorption spectroscopy was employed for measurements of pressure broadening coefficients for the Krypton 811.3 nm line in an RF discharge. A multi-quantum well diode laser (L808P030, Thorlabs) with an original short external cavity was used as a source of probe radiation. The natural isotopic distribution of Kr was taken into account, and an appropriate fit function was constructed. This permitted the determination of pressure broadening coefficients using the natural mixture of isotopes. The coefficients for the Kr 811.3 nm line at 300 K, measured for the first time, were ξKr-Ne = (1.50 ± 0.05) ×10-10 s-1cm3 for broadening by Neon, and ξKr-Ar = (3.5 ± 0.3) ×10-10 s-1cm3 for broadening by Argon.
Experiments and modeling of CH3I dissociation in the plasma generated by a 40 MHz RF discharge were performed. A discharge chamber of an original design, consisting of quartz tubes between two planar electrodes, permitted the production of iodine atoms with number densities up to 2×1016 cm-3. In this discharge chamber, contamination of the walls of the tubes did not hinder discharge stability, providing a good iodine production rate. Addition of oxygen into Ar:CH3I mixture resulted in a substantial increase in iodine extraction efficiency. When the discharge power reached 200 W, complete CH3I dissociation in a Ar:CH3I:O2 mixture was observed. The fraction of discharge power spent on iodine atom production at a 0.17 mmol/s CH3I flow rate was 16%.
Results of experiments and modeling of CH3I dissociation in a 40 MHz RF discharge in a discharge chamber of original design to produce iodine atoms for cw oxygen-iodine laser are presented. In experiments a substantial increase in CH3I dissociation efficiency due to addition of oxygen into Ar:CH3I mixture was observed. Complete CH3I dissociation in Ar:CH3I:O2 mixture occurred at 200 W discharge power. Fraction of discharge power spent on iodine atoms production was equal to 16% at 0.17 mmol/s CH3I flow rate. The rate of carbon atoms production as a function of molecular oxygen and water contents in CH3I:Ar mixtures was studied with the help of numerical modeling. It was found that addition of water vapor resulted in increase while addition of molecular oxygen and HI in decrease of the rate of carbon atoms production. Due to diffusion most of carbon atoms had enough time to deposit on the walls of the discharge chamber. However, contrary to the situation in a DC discharge, in the RF discharge accumulation of carbon on the walls of the discharge chamber did not hamper discharge stability and iodine production, as it was observed in our experiments.
The development of a discharge oxygen iodine laser (DOIL) requires efficient production of singlet delta oxygen O2(α1 Δ) in electric discharge. It is important to understand the mechanisms of of O2α1 Δ) quenching in these devices. To gain understanding of this mechanisms quenching of O2(α]1 Δ)in O/O2/O3/CO2/He mixtures has been investigated. Oxygen atoms and singlet oxygen molecules were produced by the 248 nm laser photolysis of ozone. The kinetics of O2(α1 Δ) quenching were followed by observing the 1268 nm fluorescence of O2α1 Δ → X3 Σ transition. It is shown that vibrationally excited ozone O3(υ;) formed in the three-body recombination O + O2 + M →O3(υ) + M is an important O/O2/O3 quenching agent in O/O2/O3 systems. The process O3(υ ≥2) + O2(a1 Δ)→ 2O2 + O is the main O2(α1 Δ) deactivation channel in the post-discharge zone. If no measures are taken to decrease oxygen atom concentration, the contribution of this process into overall O2(α1Δ) removal is significant even in the discharge zone. It was found in experiment that addition of species that are good quenchers of O3(υ;) decrease O2(a1 Δ) deactivation rate in the O/O2/O3 mixtures.
A hardware and a computative technique for tunable laser spectroscopy was developed for simultaneous measurement of Gaussian and Lorentian components of line broadening by fitting Voigt line profile. The technique was tested in measurements of pressure broadening coefficient for 811.5 nm Ar absorption line in a 40 MHz discharge in the pressure range 15-75 Torr with the help of tunable diode laser with a short external resonator. The obtained values for this coefficients reduced to 300 K are: ξAr-Ar = (2.85±0.1)×10-10 s-1cm3 for broadening in the parent gas and ξAr-He = (3.3±0.1)×10-10 s-1cm3 for broadening in helium. A good agreement with published results is observed. Measured Ar(1s5) number density amounted to 10-11cm-3 for the discharge power density ~10 W cm-3.
Usage of iodine atoms instead of molecules in oxygen-iodine laser permits to expand its range of operation parameters
and improve the weight per power ratio. Results of experiments and modeling of plasma chemistry processes resulting in
CH3I dissociation in a planar 40 MHz discharge are presented, showing that addition of oxygen or water into
Ar:He:CH3I mixtures can lead to a noticeable increase in CH3I dissociation rate and inhibit iodine recombination during
transport.
Usage of an external iodine atom generator can improve energy efficiency of the oxygen-iodine laser (OIL) and expand
its range of operation parameters. However, a noticeable part of iodine atoms may recombine or undergo chemical
bonding during transportation from the generator to the injection point. Experimental results reported in this paper
showed that uncoated aluminum surfaces readily bounded iodine atoms, while nickel, stainless steel, Teflon or Plexiglas
did not. Estimations based on experimental results had shown that the upper bound of probability of surface iodine atom
recombination for materials Teflon, Plexiglas, nickel or stainless steel is γrec ≤ 10-5.
KEYWORDS: Oxygen, Carbon dioxide, Molecules, Chemical analysis, Chlorine, Temperature metrology, Chemical lasers, Absorption, Spectroscopy, Chlorine gas
Measurements of the absolute spectral irradiance from O2(a)-O2-H2O gas, produced by a chemical singlet oxygen
generator, were performed. FWHM of singlet oxygen collision induced emission (CIE) at 634 and 703 nm have been
measured in the temperature range 150-400 K. The measured rate constant of CIE at 634 nm - (6.72±0.8)×10-23 cm3/s is
in agreement with the value of the band intensity of the collision induced absorption. The rate constant of CIE at 703 nm
relates to the rate constant of CIE at 634 nm as 1.06. The efficient rate constant of 8×10-17cm3/s for the reaction
O2(a)+O2(a)→"products" at about 360 K and the rate constant of (4.4±1)×10-17 cm3/s for the reaction
O2(a)+O2(a)→O2(b)+O2 at about 330 K have been measured. These rates are larger than listed in the standard chemical
oxygen-iodine laser (COIL) package. Nonequilibrium fraction of O2(b,v=1) was measured against water fraction. It was
deduced that the maximum number of oxygen vibrational quanta, generated in the sequences of the reactions
O2(a)+O2(a)→O2(b)+O2, O2(b)+H2O→O2+H2O, is less than 0.05. The analysis predicts that the fraction of
vibrationally excited oxygen molecules at the exit of most-used chemical SOG most likely corresponds to the thermal
equilibrium. Analysis of the data obtained from the tests of jet type SOG predicts O2(a) nascent yield of about 90%.
The development of a discharge oxygen iodine laser (DOIL) requires efficient production of singlet delta oxygen (O2(a))
in electric discharge. It is important to understand the mechanisms by which O2(a) is quenched in these devices. To gain
understanding of this mechanisms quenching of O2(a) in O(3P)/O2/O3/CO2/He/Ar mixtures has been investigated.
Oxygen atoms and singlet oxygen molecules were produced by the 248 nm laser photolysis of ozone. The kinetics of
O2(a) quenching were followed by observing the 1268 nm fluorescence of the O2a → X transition. Fast quenching of
O2(a) in the presence of oxygen atoms and molecules was observed. The mechanism of the process has been examined
using kinetic models, which indicate that quenching by vibrationally excited ozone is the dominant reaction.
Experiments were carried out with a subsonic chemical oxygen-iodine laser (COIL), equipped with an electric discharge
generator of iodine atoms. CH3I entrained in a carrier flow of Ar was used as atomic iodine precursor. About 50% of
iodine contained in CH3I molecules was extracted in the generator. Up to 3.5% of electric power loaded into the
discharge was spent on CH3I dissociation. A straightforward comparison of COIL performance for two cases -
conventional, when I2 was injected in the singlet oxygen flow and when iodine atoms produced externally together with
other discharge products were injected - was made. In the latter case nearly four times increase in output power was
observed.
The development of discharge singlet oxygen generators (DSOG's) that can operate at high pressures is required
for the power scaling of the discharge oxygen iodine laser. In order to achieve efficient high-pressure DSOG
operation it is important to understand the mechanisms by which singlet oxygen (O2(a1Δ)) is quenched in these
devices. It has been proposed that three-body deactivation processes of the type O2(a1Δ))+O+M→2O2+M
provide significant energy loss channels. To further explore these reactions the physical and reactive quenching
of O2(a1Δ)) in O(3P)/O2/O3/CO2/He/Ar mixtures has been investigated. Oxygen atoms and singlet oxygen
molecules were produced by the 248 nm laser photolysis of ozone. The kinetics of O2(a1Δ)) quenching were
followed by observing the 1268 nm fluorescence of the O2 a1Δ-X3Ε transition. Fast quenching of O2(a1Δ)) in the
presence of oxygen atoms and molecules was observed. The mechanism of the process has been examined using
kinetic models, which indicate that quenching by vibrationally excited ozone is the dominant reaction.
Concentration of iodine molecules at the outlet of an electric discharge iodine atoms generator was measured using
laser-induced fluorescence. Methyl iodine was used as an iodine atom precursor. Fraction of iodine extracted from
CH3I in the discharge generator was about 50%. Optimal mode of operation at which 80-90% of total extracted iodine
was in the form of iodine atoms was found. Iodine atom content in the gas flow decreased during transportation down to
20-30% at the point of iodine injection into the oxygen flow. Fraction of power load spent on CH3I dissociation
amounted to ≈3%.
Experiments with a flow cell apparatus imitating conditions of oxygen-iodine laser, equipped with a chemical jet singlet oxygen generator and an electric discharge iodine generator have been performed. I2 and CH3I in the flow of Ar were used as atomic iodine precursors.
The distributions of the electronically excited species along the flow were examined detecting their optical emissions. A straightforward comparison of two methods of oxygen-iodine medium production - conventional, by means of I2 dissociation in the singlet oxygen flow and with iodine atoms produced externally in the electric discharge - was performed.
It was found that stored electron energy lifetime had been about 30% longer, when iodine was produced from CH3I in the discharge, compared to the conventional I2 dissociation in the singlet oxygen flow. It was observed that maximums of the I(2P1/2) and I2(B) concentrations had shifted to the nozzle plane, when I2 in Ar carrier was subjected to the glow discharge, pointing to a nearly twofold increase in the I2 dissociation rate. Contrary to the known results for low iodine and singlet oxygen concentrations, squared dependence of the amplitude of the I2(B) luminescence maximum with I(2P1/2) concentration was observed in the dissociation region for both methods of iodine production.
The mechanism by which I2(B) is excited in the chemical oxygen-iodine laser was studied by means of emission
spectroscopy. Using the intensity of the O2(b1&Sgr;,&ngr;'=0) → O2(X3&Sgr;,&ngr;''=0) band as a reference, I2(B) relative number densities were assessed by measuring the I2(B,&ngr;')→ I2(X,&ngr;") emission intensities. Vibrationally excited singlet
oxygen molecules O2(a1&Dgr;, &ngr;'=1) were detected using IR emission spectroscopy. The measured relative density of O2(a1&Dgr;,&ngr;'=1) for the conditions of a typical oxygen-iodine laser medium amounted to ~15% of the total O2 content. Mechanisms for I2(B) formation were proposed for both the I2 dissociation zone and the region downstream of the dissociation zone. Both pumping mechanisms involved electronically excited molecular iodine I2(A', A) as an intermediate. It has been suggested that, in the dissociation zone, the I2 A', and A states are populated in collisions with vibrationally excited singlet oxygen molecules O2(a1&Dgr;,&ngr;'). In the region downstream of the dissociation zone the intermediate states are populated by iodine atom recombination process. I2(B) is subsequently formed in collisions of I2(A',A) with singlet oxygen. We also conclude that I2(B) does not participate measurably in the I2 dissociation process and that energy transfer from O2(b1&Sgr;) does not excite I2(B) to a significant degree.
The possibility to produce atomic iodine decomposing methyl iodide with the help of the vortex-flow DC glow discharge for use in oxygen-iodine laser has been investigated experimentally. Number density of iodine atoms had been measured via absorption of single frequency tunable semiconductor laser radiation at 1.315 μm. Two cases were studied: (1) the products of discharge in oxygen decomposed methyl iodide in the downstream afterglow; (2) methyl iodide was decomposed in the discharge plasma. Atomic iodine concentrations enough to operate oxygen iodine laser were obtained. In the first case iodine atoms number density up to 1.3•1015 cm-3 was achieved for oxygen pressure 24 Torr. A kinetic model showing good agreement with experiment had been developed. In the second, vortex flow discharge stabilization permitted to sustain glow discharge in highly negative gas mixture that contained halogen, using Ar as carrier gas at pressures up to 20 Torr. Iodine atoms number density of 3.6•1015 cm-3 was achieved at 15 Torr pressure of Ar.
The possibility to produce high concentrations of singlet delta oxygen, enough to operate oxygen-iodine laser, using discharge techniques without dangerous chemicals was investigated. The results of study of singlet oxygen yield in the vortex-flow glow discharge in pure oxygen are presented. The discharge in the vortex flow is used because it permits to have extremely stable CW discharge with very high power load at high pressures.
Pulsed Nd-glass lasers usually have low beam quality (200 - 300 mm-mrad), and are used only for surface hardening of metals. However, high pulse energy make them feasible for deep penetration welding if their beam quality could be improved. We investigated beam properties of Nd-glass laser with unstable resonator with semitransparent output coupler (URSOC). We had found that beam divergence of the laser with URSOC was an order of magnitude smaller than that of the laser with stable resonator. The achieved beam quality (40 - 50 mm-mrad) permitted to perform deep penetration welding with the aspect ratio of approximately 8. For beam divergence of 3 mrad melt depth of 6.3 mm was achieved with the ratio of depth to pulse energy of 0.27 mm/J.
Since COIL was invented, an interest exists to produce singlet oxygen using techniques without dangerous chemicals. One possible solution is to employ a discharge in pure oxygen or in oxygen mixtures. Up to date, no discharge system has produced enough singlet oxygen to operate a laser, as it is necessary to provide a very high energy load in the discharge —up to 100 kJ/mol. However, several recent papers claim up to 15-20% of singlet oxygen yield. Glow discharge in a vortex flow is known to be extremely stable at high pressures and currents and, therefore, permits very high power load. Pure oxygen was exited in a vortex tube of 1 .5 cm diameter at the pressures of 2—15 ton in the DC glow discharge with the gap between electrodes of 1—8 cm. Exited oxygen concentrations of several percents in afterglow of a1? and b1? states were measured detecting emission at 1 .27 and 0.762 microns. Current density up to 1 .5 A/cm2 was achieved, that is more than order of magnitude larger that typical values for stationary discharge. The dependencies of exited molecules concentration on current, pressure for a number of different discharge geometry are represented.
The possibility and conditions of single mode laser operation for lasers with resonator Fresnel number up to 10 and substantial intracavity astigmatism employing unstable resonator with uniform semitransparent output coupler (URUSC) have been investigated experimentally. The influence of astigmatism may be significant in multipass resonators with spherical folding mirrors. The experimental condition of single mode operation in URUSC was for the resonator to be in an unstable region if only for one plane of symmetry determined by astigmatism. At the same time in the other plane the resonator might be stable. Comparison of the far field measurements with numerical simulations was performed. It revealed that even if equivalent Fresnel number in `unstable plane' was greater than unit the laser operation was still single mode.
An unstable resonator with a semitransparent output coupler is feasible for lasers with moderate gain and a large cross section of active medium. The resonator fundamental mode can be obtained up to Fresnel numbers of 10 and more. Output beam quality does not differ much from the Gaussian beam, but at the same time intensity distribution is rather flat and mode-medium coupling is better. This approach does not require the mirrors with tapered reflectivity profile. In practice the design of this type of resonator often requires the developer to place a spherical mirror inside to fold the optical path and that inevitably causes astigmatism. We describe the results of the investigation of resonator sensitivity to intracavity astigmatisms. The requirements for the resonator setup to obtain nearly unperturbed fundamental mode operation and a convenient resonator design to meet these requirements are discussed.
An unstable resonator with semitransparent output coupler (NRPOZ) and low magnification is investigated theoretically and experimentally for Fresnel numbers 5 - 15 and small signal gain 0.5 - 2.4. With the help of numerical simulation the condition of minimum diffraction losses is found. The beam divergence in the dc-discharge transverse-flow kilowatt-power CO2 laser was approximately two times less in comparison with telescopic unstable resonator without substantial loss of mode volume and output power.
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