Small volume coaxial discharge as precision testbed for 0D-models of XeCl lasers

In order to check the predictions of 0D-models experimentally, a small coaxial discharge configuration for the generation of homogeneous high pressure glow discharges (diameter 11 mm, length 20 mm) in rare gas halogen excimer laser gas mixtures under accurately controlled conditions has been developed. It uses X-ray preionization and a special pulse-forming network (PFN) delivering fast rising (8 ns) single square pulses (U ₀=25 kV; I=300 A; =100 ns). Discharge current and voltage are measured precisely by a capacitive voltage divider and a shunt integrated into the discharge chamber. All circuit data needed for the model calculations have been evaluated. Interferometric and spectroscopic diagnostics of the bulk of the discharge and of the cathode sheath have been performed. First results for Ne/Xe/HCl mixtures are compared with model calculations.     

Design and operation characteristics of a high power transverse flow pulser sustained cw CO2 laser

A transverse flow, transverse discharge cw CO₂ laser in which de discharge is sustained by employing high repetition rate high voltage pulses has been developed. Pulser sustained discharge through electrodes of innovative design provided uniform excitation at electrical input power densities more than 10 W/cc. Laser output power more than 2.5 kW was obtained in a laser gas mixture consisting of 0.5 mbar of CO₂, 16 mbar of N₂ and 38.5 mbar of He. Design details and operational characteristics of this laser are presented.     

New cw far-infrared D2O, 12CH3F and 14NH3 laser lines

The operation of a continuous CO₂ waveguide laser at nearly atmospheric pressure is reported. A plasma injection technique is used to produce a uniform discharge at atmospheric pressure. Continuous output power of 0.6 W was obtained at the inlet pressure of 800 torr. Corresponding outlet pressure was 340 torr. While the maximum output power was 1.4 W at the inlet pressure of 600 torr. The gas mix ratio was CO₂ : N₂ : He = 1 : 1 : 6.     

The mechanism of the CW sodium-vapor/hydrogen laser

Measurements have been carried out to gain information on the mechanism of the cw 9 m sodium-vapor/hydrogen laser [2]. Observations were carried out on the laser excited with both cw and pulsed discharges and the low-power gain of various gas mixtures was measured using tunable diode lasers. The laser was found to oscillate with mixtures of sodium vapor with H₂, D₂ and HD, but with no other gas. On substituting D₂ for H₂ a more than three-fold increase in laser gain was observed. The results showed that most of the processes used to explain lasing action in other metal-vapor lasers were not applicable to the present case. It was concluded that the primary process which leads to the production of the population inversion involves differential quenching of the laser levels by molecular hydrogen, with the lower 3D level being quenched more efficiently than the upper 4P level. Measurements carried out on the calcium/hydrogen laser [21] with D₂ supported these conclusions.     

Investigation of Gas Degradation in CO2 ? N2 Pulse Discharges I. Long Term Behaviour of Pulsed Laser Discharges in CO2 — N2 Gas Mixtures

The correlation between the plasma resistance and the laser output parameters has been investigated in pulsed CO₂ — N₂ discharges. The strong increase of the discharge resistance during gas degradation can be explained on the basis of strong attaching gas components. In degraded mixtures a supression of discharge instabilities together with the restoration of the laser pulse energy is possible by increasing the reduced electric field E/N temporarily.     

The stability of the active medium of RF-exited CO2 lasers with gold as catalyst

Using mass-spectrometric investigations the gas composition of the active medium of sealed-off cw RF-excited CO₂ waveguide lasers have been studied. It has been found that a low degree of CO₂ dissociation and a laser power improvement can be achieved by means of a gold catalyst in the laser discharge volume. The conditions for long operational lifetimes of these lasers are described. Received: 21 December 1999 / Revised version: 1 May 2000 / Published online: 6 September 2000     

Ultraviolet laser emission at 337 and 358 nm by collisional energy transfer in fast electron-beampumped,high-pressure Ar-N2 mixtures

Successive laser emission at 337.1 nm and 357.7 nm in the second positive band of N₂ have been observed in high-pressure Ar-N₂ gas mixtures excited by a high current (15 kA) and fast rise time (8 ns) relativistic electron beam. Laser action at 357.7 nm was confirmed in the pressure range 450–6000 torr where output intensity was approximately independent of pressure, while strong pressure dependence of the emission at 337.1 nm was observed in a rather limited pressure range of 760–3000 torr. From the results of spectrally resolved pressure dependences of the laser lines, the processes of successive laser oscillation at these two wavelengths are discussed.     

Ultra-violet ion lasers

Cw laser action has been extended down to 220nm. Moreover, cw oscillation has been obtained on twenty additional transitions in the spectral range between 220 and 320 nm. Ultra-violet laser thresholds as low as 2 A have been observed. In comparison, rare gas ion lasers require 20 to 50 A to reach threshold. Metals of interest are sputtered into the discharge giving metal densities of 10¹⁴ atoms cm^-3 without using external ovens or discharge heating. Recent attempts to scale the output power levels to 1 W cw are presented with particular emphasis on the 780 nm transitions of Cu II.     

Design considerations and scaling laws for high power convective cooled CW CO2 lasers

Various criteria for designing high power convective cooled CO₂ lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO₂ lasers have been established. In transverse flow CO₂ lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 10⁴ p ^1/2 V ^-1 cms andW = 0.04P ^1/2 cms. The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO₂ lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.     

Temporal extension of stable glow discharges in fluorine-based excimer laser gas mixtures by the addition of xenon

The effect of addition of xenon on the long term homogeneity of discharges in F₂and ArF excimer laser gas mixtures was investigated in a small-volume discharge chamber. The gas mixture in the discharge chamber was preionized by X-rays. A special electrical excitation circuit containing a pulse forming line provided a long, square-shaped current pumping pulse of a predetermined duration to the discharge electrodes. The initiation and the development of the discharge was monitored via its fluorescence signal with an intensified CCD camera. We found that adding Xe up to partial pressures of 0.53 mbar extended the homogeneous phase of the discharge from 80 ns to approximately 200 ns in He/F₂as well as in He/Ar/F₂and Ne/Ar/F₂excimer laser gas mixtures. Monitoring of the ArF and XeF spontaneous emission signals showed that the formation of ArF excimers remained unaffected by the addition of xenon (up to 1.3 mbar) to the laser gas mixture.     

CW laser oscillation on transitions of As+ in He-AsH3 gas discharges

Operation of a cw arsenic ion (As⁺) laser using the gaseous arsenic trihydride, AsH₃, as a free arsenic donor in hollow-cathode He-AsH₃ discharges is reported. Optimum conditions of the gas mixture are set by simple gas handling techniques at room temperature. Laser performance compares favourably with that obtained with the He-As laser.     

Laser gain measurements at 193 nm in a small discharge cell containing ArF excimer laser gas mixtures

Spatial and temporal gain profiles as well as the peak net gain at 193 nm have been measured in X-ray preionized discharges excited by a single pulse electrical system working in the charge transfer mode. Ar- and F₂-containing laser gas mixtures with He or Ne as a buffer gas have been used. With a pumping pulse duration of ∼ 100 ns (FWHM) and a specific peak power deposition of ∼ 1 MW cm^-3 bar^-1 in a gas mixture containing F₂ : Ar : He (0.1%:5%:94.9%), at 2 bar total pressure, a very high peak net gain coefficient of ∼ 30% cm^-1 was measured in the gas discharge. The FWHM of the gain waveform was ∼ 60 ns. PACS 42.55.Lt; 42.60.Lh; 52.80.-s     

Numerical solution of Boltzmann tranport equation for TEA CO2 laser having nitrogen-lean gas mixtures to predict laser characteristics and gas lifetime

Selective laser isotope separation by TEA CO₂ laser often needs short tail-free pulses. Using laser mixtures having very little nitrogen almost tail free laser pulses can be generated. The laser pulse characteristics and its gas lifetime is an important issue for long-term laser operation. Boltzmann transport equation is therefore solved numerically for TEA CO₂ laser gas mixtures having very little nitrogen to predict electron energy distribution function (EEDF). The distribution function is used to calculate various excitation and dissociation rate of CO₂ to predict laser pulse characteristics and laser gas lifetime, respectively.Laser rate equations have been solved with the calculated excitation rates for numerically evaluated discharge current and voltage profiles to calculate laser pulse shape. The calculated laser pulse shape and duration are in good agreement with the measured laser characteristics. The gas lifetime is estimated by integrating the equation governing the dissociation of CO₂. An experimental study of gas lifetime was carried out using quadrapole mass analyzer for such mixtures to estimate the O₂ being produced due to dissociation of CO₂ in the pulse discharge. The theoretically calculated O₂ concentration in the laser gas mixture matches with experimentally observed value. In the present TEA CO₂ laser system, for stable discharge the O₂ concentration should be below 0.2%.     

Performance studies of a pulsed HF laser with a sliding discharge plasma cathode

The plasma electrode design concept is applied for the first time to an HF laser. The discharge along the surface of a dielectric (sliding discharge) is used as a plasma cathode for the main laser discharge. The laser operates at atmospheric pressure with a gas mixture of He/SF₆/C₃H₈. Details are presented on the efficiency of energy transfer, the dependence of laser performance on circuit parameters, gas mixture, relative energy loading and time delay between the plasma electrode and main discharges. The F atom production rate is estimated from the linear dependence of the output energy on the electric charge passed through the discharge. Output energies of 600 mJ were obtained at 1.6% efficiency from a small active discharge of 108 cm₃ volume and 38 cm length, while the maximum specific input and output energies were 370 J/1 and 5.7 J/1, respectively. These values compare favourably with those reported in the literature for non-chain-reaction-type gas mixtures at 1 atm pressure and demonstrate that the plasma electrode design is a powerful scheme for developing gas-discharge lasers.     

Optimization of a compact photoacoustic quantum cascade laser spectrometer for atmospheric flux measurements: application to the detection of methane and nitrous oxide

Room temperature (RT) quantum cascade lasers (QCL) are now available even in continuous wave (cw) mode, which is very promising for in situ gas detectors. Ambient air monitoring requires high sensitivity with robust and simple apparatus. For that purpose, a compact photoacoustic setup was combined with two cw QCLs to measure ambient methane and nitrous oxide in the 8 μm range. The first laser had already been used to calibrate the sensitivity of the photoacoustic cell and a detection limit of 3 ppb of CH₄ with a 1s integration time per point was demonstrated. In situ monitoring with this laser was difficult because of liquid nitrogen cooling. The second laser is a new RT cw QCL with lower power, which enabled one to reach a detection limit of 34 ppb of methane in flow. The loss in sensitivity is mainly due to the weaker power as photoacoustic signal is proportional to light power. The calibration for methane detection leads to an estimated detection limit of 14 ppb for N₂O flux measurements. Various ways of modulation have been tested. The possibility to monitor ambient air CH₄ and N₂O at ground level with this PA spectrometer was demonstrated in flux with these QCLs. PACS 07.88; 92.60.Sz     

Operational characteristics of a MAGPIE coaxial CO2 discharge system

The operational characteristics of a convectively cooled Magnetically stabilized, Photo-initiated, Impulse-enhanced, Electrically-excited (MAGPIE) coaxial discharge system are described. terminal behavior is examined as a function of several parameters, such as gas flow, pulser ionization, and magnetic field strength. In-situ plasma potential measurements are also presented, which indicate that CO₂ attachment effects have considerable influence on the spatial electrical characteristics of the gas discharge.     

CO2-laser gas discharges in narrow gaps

We have studied RF discharges as excitation mechanisms for distributed feedback (DFB) CO₂ lasers. For CO₂ laser plasmas the reduced electric fieldE/N has to be in a well-defined range. The reduced electric fieldsE/N of gas discharges in the narrow gaps with widths of the order of 100 m required for DFB are considerably above this range. In order to study the feasibility of these RF-excited discharges for DFB CO₂ lasers we have measured the electron temperatureT _e in their plasmas. From helium-line-intensity ratios we have deduced a lower limit of the electron temperatureT _e of 4eV. The observed high intensities of bands of singly ionized nitrogen indicate an even higher electron temperature, but an efficient pumping of the upper laser level is not possible with an electron temperature above 2.5 eV.We have estimated the electron densityn _e and the current densityj _e from ratios of the intesities of forbidden and allowed helium lines. The high current densityj _e is in the range of abnormal glow discharges.In the gas discharges between narrow gaps the electron oscillation amplitudex _e is large than the electrode separationd. In order to replace the resulting high electron losses a high electron temperatureT _e is necessary to sustain the gas discharge. Because of this high electron temperatureT _e an efficient pumping of the upper laser level is not possible.     

Effect of leakage transformer on laser power and mode pattern of CO2 laser excited by 60 Hz AC discharges

The 60 Hz AC discharges generated by a leakage transformer instead of an isolation one, which is usually used in the power supply of DC CO₂ laser, are employed as a pumping source in this experiment. The laser performance characteristics as functions of pressure and discharge current have been investigated. The maximum laser power is about 40 W at conditions of 18 Torr and 35 mA. The output power is not less than that of DC discharge at the same condition. In addition, the laser outputs of 60 Hz AC discharges obtained by different transformers such as a leakage type and an isolation one have been measured. It was found that the laser power obtained by the leakage transformer is saturated at the higher current compared with the isolation transformer.     

Study of shock waves interacting with Ar and N<Subscript>2</Subscript> low pressure dc discharges

Acoustical shock waves (Mach number <2) generated in situ by spark gap are propagated in weakly ionized dc discharges working at low pressure (399 Pa) and containing either Ar or N₂ gas. The electrical characterization and the laser deflection technique are used to measure the characteristics of dc discharge (such as voltage, resistance and power of discharge) and the structure and velocity of shock wave, respectively. The results stress the importance of atomic and molecular nature of the gases in affecting the power deposition and the shock wave properties.     

Chemical oxygen-iodine laser with external production of iodine atoms in CH3I/Ar dc glow discharge

A cw chemical oxygen-iodine laser (COIL) operating in a subsonic mode with a high water content of ～15% and external production of iodine atoms in CH₃I/Ar dc glow discharge has been demonstrated. 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, suggesting that the relaxation of the energy stored in the singlet oxygen slowed down substantially, when the laser operated using CH₃I/Ar discharge products instead of iodine molecules.