Discharge-pumped Excimer Laser Research Articles

Discharge-Pumped Excimer Laser Technologies and Applications

Discharge-Pumped Excimer Laser Technologies and Applications

Numerical Analysis of Corona Pre-Ionization for High-Power Discharge-Pumped XeCl* Excimer Lasers

The working gas of discharge-pumped excimer lasers is generally a mixture of rare gases and monohalide. Typically, the working gas is preconized to generate reproducible uniform glow discharge. In high-power (more than 100 W) excimer lasers, this pre-ionization is achieved through spark discharge in the discharge chamber. In this study, a high-power discharge-pumped XeCl* excimer laser with corona preionisation was designed to investigate the feasibility of applying corona preionisation in high-power excimer lasers as well as analyse the correlation of corona discharges characteristics with laser discharge properties. The electron avalanche triggered by ultraviolet preionisation in the laser was numerically studied by creating 2D models. The simulation results revealed that the additional ionisation induced by the secondary electron emission of the cathode together with the dielectric constant of the corona bars significantly affect the discharge current of the principal electrodes. Under the condition of the design values, the peak preionisation electron density between electrodes was approximately 108 cm−3, and the laser discharge was quite uniform, which theoretically proved the feasibility of using corona preionisation instead of spark discharge preionisation in high-power practical devices.

High-power gas-discharge excimer ArF, KrCl, KrF, and XeCl lasers on buffer-gas free gas mixtures

Results of experimental studies of the gas mixture (laser active medium) effect on the lasing energy and overall efficiency of excimer discharge ArF (193 nm), KrCl (222 nm) KrF (248 nm), and XeCl (308 nm) lasers operating in buffer-free gas mixtures are presented. The optimal (in terms of maximum radiation energy) ratios of the gas components of the excimer laser active media are found, at which efficient operation is achieved with a sufficiently high power of the laser radiation. It is confirmed experimentally that for the rare gas halide discharge pumped excimer lasers the presence of a buffer gas in the active medium is not required for efficient laser operation. For example, in two-component excimer laser gas mixtures, containing working rare and halogen-containing gases, laser pulse energy of up to 170 mJ and high pulsed power of laser radiation of up to 24 MW have been attained for the first time for pulsed gas-discharge excimer lasers operating on electronic transitions at excimer ArF*, KrCl*, KrF*, and XeCl* molecules pumped by a transverse electric volume discharge of a low-pressure buffer-free gas mixture. A overall efficiency maximum of up to 0.8% was experimentally attained for binary gas mixture of KrF and XeCl lasers.

高效率放电抽运KrF准分子激光器

高效率放电抽运KrF准分子激光器

高效率放电抽运KrF准分子激光器

高效率放电抽运KrF准分子激光器

高效率放电抽运KrF准分子激光器

高效率放电抽运KrF准分子激光器

Photoelectric effects in x-ray preionization for excimer laser gases

We present detailed measurements on the x-ray preionization electron density in a discharge chamber filled with different gases relevant to discharge pumped high-pressure excimer lasers. By comparing experimental results with the theoretical electron densities, we conclude that the observed preionization is inconsistent with the standard picture of direct ionization through x-ray absorption in the gas. We conclude that depending on the gas pressure, type of gas, and the gap length between the discharge electrodes used, x-ray preionization in excimer gases is, to a significant extent or even dominantly, based on a different mechanism. We identify that this mechanism is based on fast photoelectrons emitted by the cathode into the discharge chamber.

Three-channel KrF laser with distributed magnetic-switch-based charging circuit

A novel magnetic-switch-controlled L−C inversion circuit is proposed for discharge pumped excimer lasers of three separate channels. The charging circuit is controlled by one thyratron and one common “distributed” magnetic switch. Stable synchronization of the channels and the capability for amplification or generation of three separate pulses of constant delay are unique features of this scheme.

Differential pumping scheme for discharge pumped excimer lasers

The beam homogeneity and stored energy of discharge pumped excimer amplifiers are critically dependent on the prepulse level of the electric excitation pulse. Here a scheme is presented to decrease significantly the prepulse level by differential charging of the two electrodes. The modified charging circuit also enables to increase the amplitude and improve the rise time of the main pulse, thus improving the discharge quality and the amplification properties of the excimer module. The circuit is especially well suited for short-pulse KrF amplifiers.

Kinetic modelling of a discharge-pumped ArF excimer laser and the effects of discharge filamentation

Results of kinetic modelling of an X-ray preionised, discharge-pumped ArF excimer laser, operating with a spiker-sustainer circuit are presented. The parallel-resistor model includes the complete ArF laser kinetics and calculations of the Boltzmann equation in each layer taking into account the effects of electron–electron and super-elastic collisions. The model further includes a detailed description of the electrical circuit and the formation of filaments with a realistic electrode profile. This model shows a good correspondence with the experimental results in predicting laser energy and optical behaviour. Neglecting the formation of filaments and the electron–electron and super-elastic collision processes yields remarkably poorer results. Parametric studies on the electrode profile, the formation of micro-arcs and on the electrical circuit parameters were performed numerically.

Measurement of temporal behavior of electron density in a discharge-pumped ArF excimer laser

Time-resolved number densities of electrons in a discharge-pumped ArF excimer laser are measured by an interferometric method. The peak electron density is 6.7/spl times/10/sup 15/ cm/sup -3/ at a total gas pressure of 2.5 atom, a gas mixture ratio of F/sub 2/-Ar-He=0.2-10.0-89.8, and a charging voltage of 24 kV for a 68-nF storage capacitor bank. The dependences of the electron density and laser output power on the Ar and F/sub 2/ fractions in Ar-F/sub 2/-He mixture and on the Ne-He mixing ratio in Ar-F/sub 2/-He-Ne mixture are investigated, and the effects of Ar-F/sub 2/-He-Ne mixing ratio on the ArF laser discharge are discussed. The experimental data of the peak electron density are also compared with the results of a computer simulation. A good agreement between them was obtained by considering the fact that the actual discharge volume occupied only part of the electrode width.

Effects of F/sup -/ ions and F/sub 2/ molecules on the oscillation process of a discharge-pumped ArF excimer laser

Time-resolved number densities of the fluorine negative ion in a discharge-pumped ArF excimer laser are measured by a dye laser absorption method. The peak density of F/sup -/ is 0.93 /spl times/10/sup 15/ cm/sup -3/ at a total gas pressure of 2.5 atm, a gas mixture ratio of F/sub 2/-Ar-He=0.2-10.0-89.8, and a charging voltage of 28 kV for a 68-nF storage capacitor bank. The dependences of the peak F/sup -/ density and the ArF laser output power on the F/sub 2/ gas fraction in F/sub 2/-Ar-He mixture are investigated. The effects of F/sup -/ ions and F/sub 2/ molecules on the ArF laser oscillation process are discussed by considering the F/sub 2/ mixture-ratio dependences of particle densities, laser output power, mean electron energy, and laser power extraction efficiency. With increasing F/sub 2/ mixing ratio, the ArF* excimer formation first increases as F/sup -/ increases, but in F/sub 2/-rich conditions the laser power decreases because of the laser photon absorption due to F/sup -/ ions and quenching of ArF* with F/sub 2/ molecules.

Numerical Study of Nonuniform Energy Deposition and Flowfield in Discharge-Pumped XeCl Excimer Laser

Inhomogeneous energy deposition in a discharge-pumped excimer laser rapidly heats the gas, resulting in a nonuniform distribution of the gas temperature. Shock waves and nonuniform flowfields generated by the relaxation of the heated gas can induce discharge instabilities and limit the repetition rate of the laser. The gas temperature profile, which is necessary for the numerical study of the shock waves and flowfields, is calculated using a self-consistent discharge model. The computational results show that the temperature profile is sensitive to the electric field distribution and gas mixture. The temperature profile tends to be steeper when the nonuniformity in the electric field increases and the xenon concentration increases. Furthermore, the shock waves and unsteady flowfields are studied using two-dimensional compressible gas dynamic equations and the total variation diminishing (TVD) numerical scheme. Finally, an analysis using a blast wave model shows that the shock waves from preionization pin arrays decay rapidly.

Two-Dimensional Computer Simulation of Discharge Pumped Excimer Lasers.

A two-dimensional simulation code of a discharge-pumped excimer laser was developed to study the effect of spatial discharge non-uniformity on laser output performance. The code consists of a circuit equation, a continuity equation to calculate two-dimensional electric field, a steady-state Boltzmann equation, and rate equations. The preionization distribution was found to affect the electron and the excited particle distributions and hence the laser beam profile. Even with uniform-field electrodes, the calculated electric field distribution was not uniform, being effected by the configuration of surrounding structures, and varying in time with the changing discharge plasma conditions. The electric field surrounding insulating structures were found to increase prior to gas breakdown and to decrease afterwards.

Continuous operation up to 3 kHz in a discharge-pumped XeCl excimer laser

Continuous operations at a high repetition rate up to 3 kHz for discharge-pumped XeCl excimer laser are presented. The dependence of the clearing ratio on laser-gas pressure and input energy densities is studied. It was found that suitable laser-gas pressures minimize the clearing ratio for various input-energy densities. It is also shown that few density disturbances in the discharge region are induced by gas heating in a discharge-pumped XeCl excimer laser with low input-energy density

One-dimensional computation of discharge-pumped excimer lasers under repetitive operations

Zero-dimensional numerical computation of electrical discharge-pumped excimer lasers is extended to a one-dimensional model that is used to study the effects of the density perturbations of the background neutral gas and the nonuniform predischarge (which means preionization in this paper) electron density on the transition of the uniform discharge to the nonuniform prestage state leading to the onset of arc formation (which is not included in this particular model). It was found that a local density depression of 1% or an enhancement of the local electric field of 1% can increase the local energy input by several hundred percent. The initial electron density perturbations, on the other hand, are found to modify the energy input by the same order of magnitude as the initial perturbations.

Optical Measurement of Excimer Laser Current Pulses

Abstract A high-bandwidth optical fibre current sensor has been developed at Salford to monitor the waveform of the driving current pulse feeding into a discharge-pumped excimer laser. The sensor is a conventional intrinsic Faraday effect device but overcomes the normal bandwidth-sensitivity limits because the very large electrical current pulse used to drive the excimer laser allows detectable polarization rotation over a 60 cm length of fibre. The performance of the intrinsic sensor is compared with that of a current-viewing resistor, and of a Be12GeO20 crystal sensor.

A New Nonstationary Boltzmann Solver in Self-Consistent Modelling of Discharge Pumped Plasmas for Excimer Lasers

A self-consistent spatially homogeneous numerical model for discharge pumped excimer laser plasmas is presented with the inclusion of the nonstationary Boltzmann equation of the electrons. The nonstationary treatment of this equation (including electron-electron interaction) is based on the conventional two-term approximation and the quasi-stationary description of the anisotropic part of the velocity distribution. A detailed presentation of the new and very efficient iterative solution technique of the nonstationary electron kinetic equation is given and its integration into the complete model is described. First results of model calculations for a Ne/Xe/HCI mixture are reported in order to illustrate the new technique.

放電励起形KrFレーザの出力エネルギーに与える混合ガス分圧率の影響

放電励起形KrFレーザの出力エネルギーに与える混合ガス分圧率の影響

超高速ガス流放電励起XeClエキシマレーザの流体特性

超高速ガス流放電励起XeClエキシマレーザの流体特性