Some design limitations for large-aperture high-energy per pulse excimer lasers

Summary Some limitation problems for gas discharge excimer lasers, when scaled to a high pulsed energy output with high repetition rate are discussed. As an example, we present some experimental results obtained with an X-ray preionized (10×10×100) cm³ active volume, low-repetition-rate-operated gas discharge XeCl laser system. ENEA student. ENEA guest.     

Miniature high-repetition-rate TEA CO2 laser with surface-wire-corona preionization

In this article, an experimental study of a miniature, sealed-off, high-repetition-rate transversely excited atmospheric-pressure (TEA) CO₂ laser with a kind of surface-wire-corona preionization (SWCP) is described. We have utilized an SWCP consisting of SiO₂ dielectric tube and a fine wire strained and attached to the dielectric surface. A BN ceramic material, which has an extremely low coefficient of thermal expansion of about 5 × 10⁻⁷/°C was employed as a supporter of the resonator. A measurement on emission spectra of SWCP has been reported. By applying SWCP to the TEA CO₂ laser, efficient laser operation at an overall efficiency of 9.8% with an output energy of 150 mJ has been achieved from a small discharge volume of 25 cm³ with an active length of 230 mm. At the pulse repetition frequency of 60 Hz, the TEM₀₀ mode of laser beam with pulse width of 60 ns was obtained.     

Fluctuation of femtosecond X-ray pulses generated by a laser-Compton scheme

A short-pulse X-ray-generation experiment was performed by Compton scattering through interaction between a 3-ps electron beam and 100-fs laser photons in a 90° scattering configuration. The observed X-ray intensity was typically 3×10⁴ photons/pulse and roughly matched the theoretically expected intensity. The X-ray energy and pulse duration were estimated theoretically to be 2.3 keV and 280 fs from the observed electron- and laser-beam parameters. The fluctuation of the X-ray output was measured as 25% (rms) during a 30-min operation. The fluctuation was expressed as a function of the fluctuation of the timing between the electron and laser beams. The measured fluctuation of the X-rays was approximately consistent with that caused by the fluctuation of the timing between the beams. Received: 19 November 2001 / Revised version: 23 January 2002 / Published online: 14 March 2002     

Rapid discharge-pumped wide aperture X-ray preionized KrF laser

A wide aperture X-ray preionized discharge-pumped KrF excimer laser has been constructed. A flat plate pulse-forming line (36 nF, 340 kV) charges a peaking capacitor (6 nF) through a rail-gap to facilitate a rapid discharge in the laser head. Collimated X-ray preionization is employed to obtain a wide and uniform discharge. The laser is intended to be used as a short pulse amplifier and results are presented when characterized as an oscillator. The active cross-section of the laser beam is 10×8 cm² with 50 cm effective electrode length. The laser pulse energy exceeds 4.7 J in a 28 ns pulse (FWHM).     

Hundred mJ, sub-picoseconds, high temporal contrast OPCPA/Yb:YAG ceramic thin disk hybrid laser system

The authors have demonstrated an optical parametric chirped-pulse amplification (OPCPA)/Yb:YAG ceramic thin disk hybrid laser system having hundred mJ level pulse energy sub-picosecond pulse duration with high temporal contrast. At an input chirped-pulse energy of 3.8?mJ from an OPCPA preamplifier an output energy of 130?mJ has been generated from multipass diode-pumped Yb:YAG ceramic thin disk amplifier. A recompressed pulse duration of 450?fs with a contrast level of less than 7.2×10^?9 at ?150?ps before the main pulse has been obtained. The contrast level is the highest value achieved in Yb:YAG chirped-pulse amplification (CPA) laser system at 100?mJ level.     

Photo-emission studies from Zn cathodes under plasma phase

In this paper, we report investigations of the electron emission from pure Zn cathodes irradiated by UV laser pulses of 23 ns (full-width at half-maximum) at a wavelength of 248 nm (5 eV). The metal cathodes were tested in a vacuum photodiode chamber at 10^?5 Pa. They were irradiated at normal incidence and the anode–cathode distance was set at 3 mm. The maximum applied accelerating voltage was 18 kV, limited by the electrical breakdown of the photodiode gap. Under the above experimental conditions, a maximum applied electric field of 6 MV/m resulted. In the saturation regime, the measured quantum efficiency value increased with the accelerating voltage due to the plasma formation. The highest output current was achieved with 14 mJ laser energy, 18 kV accelerating voltage and its value was 12 A, corresponding to a global quantum efficiency (GQE) approximately of 1×10^?4. The temporal quantum efficiency was 1.0×10^?4 at the laser pulse onset time and 1.4×10^?4 at the pulse tail. We calculated the target temperature at the maximum laser energy. Its value allowed us to obtain output pulses of the same laser temporal profile. Tests performed with a lower laser photon energy (4.02 eV) demonstrated a GQE of two orders of magnitude lower.     

Optimisation of drive pulse configuration for a Ni-like Sn X-ray laser at 12 nm

The current saturated operation of X-ray lasers at wavelengths > 15 nm requires at least kilojoule drive energy, which is only available at the largest laser installations in the world. Using a specially designed drive pulse configuration, saturated operation of a Ni-like Sn X-ray laser at 12 nm has been achieved with only 75 J drive energy. An efficiency as high as 9 × 10⁶ in converting laser energy from the 1 eV optical spectral range to the 100 eV soft X-ray range has been reached. This paves the way for applications of saturated X-ray lasers at 12 nm at many other smaller laboratories.     

Optically pumped molecular-iodine laser on the 342-nm band

Lasing of molecular iodine has been observed on 342-nm band system. The laser was produced with optical pumping by the radiation of high-current discharge mixtures of iodine vapour, sulfur hexafluoride and argon. The highest energy output 0.4 J in a 5 μs pulse corresponding to the specific laser energy of ～10^-3 J/cm³ has been obtained.     

Diode-side-pumped AO Q-switched Tm, Ho:LuLiF laser

Diode side pumped and acousto-optical (AO) Q-switched Tm, Ho, LuLiF laser at 2 μm wavelength obtained in a ring resonator is reported in this paper. The three LDAs were placed 120° apart around the laser rod of ?4 × 20 mm dimension. A ring resonator has been used for its At free run mode, four different output loss ouplers has been tried. When the output loss is 20%, 103 mJ per pulse at 1 Hz pulse repetition frequency has been obtained with the optical to optical energy conversion efficiency of 3.17%. At the Q-switched mode, the laser pulse energy is 30 mJ with the pulse width of 417 ns. The optical to optical energy conversion efficiency is 0.93%. The slope efficiency is 6.36 and 2.90% under the two running mode, respectively.     

Laser on nitrogen-electronegative gas mixtures, pumped by inductive energy storage generator: Experiment and theoretical model

The advantages of inductive energy storage (IES) generators for increasing the pulse energy, power, and duration for nitrogen laser pumped by self-sustained transverse discharge have been experimentally demonstrated. A theoretical model is developed and the operation of IES-pumped laser on nitrogen-electronegative gas mixtures is numerically simulated. It is shown experimentally and theoretically that, adding electronegative gases, one can control the pulse shape of lasing on the C³II _u -B³II _g transition in nitrogen. The increase in the electric field strength in the laser gap in N₂-NF₃ and N₂-SF₆ mixtures produced 337.1-nm laser pulses consisting of two spaced peaks and 40–50-ns pulses close to rectangular. The increase in the laser active volume to 6 l (discharge cross section to 6×10 cm²) in N₂–SF₆ mixtures made it possible to obtain the maximum output energy (Q=110 mJ) and UV power (P _las =6 MW). In N₂-NF₃ mixtures, the laser pulse duration was up to ∼100 ns with an energy up to Q=30 mJ.     

激光柱形腔靶的X射线温度和X射线转换效率

本文根据实验和数值模拟给出的信息,解析研究激光加热柱形腔靶(简称“腔靶”)X射线温度与激光转换的X射线能之间的定标规律,推断了1989年在神光激光器上做的系列腔靶实验,对于每束激光能量为300—500J,脉冲宽度为0.7—1.0ns,波长λ为1.06μm的高斯型激光源,双束靶的X射线转换效率约为(50—55)%,X射线温度为(1.5—1.7)×10⁶K。 关键词：     

Parametric measurements on a doped CO2TEA laser

Parametric measurements have been performed on a CO₂TEA laser with a discharge volume of 6.5 × 6.5 × 45 cm³. The effect of a low ionization seed gas, tri-n-propylamine, upon amplification, power output and voltage-current characteristics has been measured. A small-signal gain of 4.8%/cm has been measured in a 1:1:3 mixture and a power output of 60J/? in a 3:11:21 mixture.     

High slope efficiency and low threshold in a laser-diode-pumped passively Q-switched ion-implanted Nd:YVO<Subscript>4</Subscript> planar waveguide laser with GaAs saturable absorber

A passively Q-switched waveguide laser, to our knowledge, has been firstly demonstrated in Nd: YVO₄ crystal formed by 3 MeV Si⁺ ion implantation at a dose of 1 × 10¹⁵ ions/cm² at room temperature, in which GaAs was used as saturable absorber. The dependences of the average output power, pulse width, pulse repetition rate on absorbed pump power have been measured at different output plane mirror transmissions. At an absorbed pump power of 78.8 mW and output transmission of 20%, the shortest pulse width of 3.88 ns was obtained, corresponding to the peak power and single pulse energy of 212 W and 0.82 μJ, respectively. The threshold pump power was as low as 40 mW, and the slope efficiency was about 64.5% when the absorbed pump power was lower than 70 mW.     

Reflection measurements in CO2 laser solid deuterium target interaction

Using a 60 nsec, 300 MW CO₂ TEA laser reflection measurements from solid deuterium targets have been investigated. Energy, reflected and scattered pulse shape are recorded at various angles: 0°, 45°, 90°, 135°. Reflection, X-ray measurements and ion mean kinetic energy are correlated at the focussing lens position with respect to the target position. The maximum plasma temperature varies from 20 to 35 eV for incident laser fluxes ranging from 5 × 10¹⁰ up to 5 × 10¹¹ W/cm². The cut-off density inside the deuterium ice has been observed and located. In each case reflection has been found to be weak less than 5% for each direction. For the maximum fluxes X-ray energies greater than 0.5 keV have been observed.     

Enhanced soft X-ray emission from carbon nanofibers irradiated with ultra-short laser pulses

A comparative experimental study of the X-ray emission in the water-window spectral region has been performed using carbon nanofibers (CNFs) of different sizes and graphite plate targets, irradiated with ultra-short (Ti:sapphire) laser pulses. More than an order of magnitude enhancement in the X-ray yield is observed from CNFs of 60-nm diameter with respect to graphite targets. The X-ray emission from CNFs of 160-nm diameter was also high. The integrated X-ray yield of these carbon-based targets scales with the laser intensity (I _L) as I_L ^{~ 1.3-1.4}I_{\mathrm{L}}^{\sim 1.3-1.4} in the intensity range of 4×10¹⁶–4×10¹⁷ W/cm². The effect of the laser pulse duration on the X-ray emission from the CNFs was also studied by varying the pulse duration from 45 fs up to 3 ps at a constant fluence of 2×10⁴ J/cm². The optimum laser pulse duration for maximum X-ray emission increases with the diameter of the CNFs used. The results are explained from physical considerations of heating and hydrodynamic expansion of the CNF plasma in which resonance field enhancement takes place while passing through two times the critical density. The results add to the efforts towards achieving an efficient low-cost water-window X-ray source for microscopy.     

Temporal evolution of the xenon laser pulse

An experimental investigation was made of the xenon excimer laser. Several laser cavities have been employed with excitation by electron beams of cross section (15×2) cm² and (55×4) cm² and current density 100–150 A cm^-2 injected transverse to the cavity axis. A numerical model of the xenon laser is compared with experimental results. The laser pulse width was found to be dependent on laser intensity and cavity mirrors. Early termination of the laser pulse was observed consistent with changing reflectivity of the cavity mirrors. An uncoated MgF₂ retroreflector produced a laser output of significantly longer pulse width. Quasi-cw laser action was observed.     

On the performance of an ArF and a KrF laser as a function of the preionisation timing and the excitation mode

An X-ray preionised ArF and KrF excimer laser has been studied with three different spiker-sustainer excitation circuits. We observed large differences in the laser performance, when the preionisation delay timing was varied on a nanosecond timescale. The behaviour of both lasers was found to be equivalent. The observations can be understood by considering the effect of the discharge excitation technique on the preionisation process. An excitation mode with a prepulse well above the steady-state voltage V_SS with a subsequent reversed overshoot voltage for initiating the discharge, in combination with a well-timed preionisation pulse is found to give the best results. Optimum output energies of 50 mJ with ArF and 175 mJ with KrF were obtained from an active volume of 60×1.5×1.2 cm in the so-called swing mode, with the preionisation applied 60 ns before the discharge breakdown. Received: 23 February 1999 / Revised version: 4 June 1999 / Published online: 16 September 1999     

Performance improvement of an x-ray preionized XeCl laser of very small discharge volume

Increasing the preionization electron density is found to be an effective method for improving the output performance of discharge-pumped excimer lasers at high gas pressures when the power-supply voltage becomes a limiting factor. In a small volume (2 cm³), x-ray preionized discharge, orders of magnitude improvement in XeCl laser power output (up to 400 kW) and pulse energy (up to 26 mJ) over previously reported results have been found possible using this method. Modifications to the low energy x-ray source and discharge system leading to these improvements are described. Various laser output characteristics are also presented and discussed.     

The effect of pulse forming line impedance on the performance of an X-ray preionised XeCl discharge laser

The construction and performance of an X-ray preionised XeCl discharge laser is described. The laser gives an output energy of up to 4.65 J with a pulse duration ?140 ns and a beam cross-section of 4.2 x 3.0 cm². Electrical efficiencies of up to 3.2% have been obtained without the use of an external prepulse circuit. This is 90% of the efficiency which is predicted to be possible using such a circuit in this laser system.     

The dependence of the Fe <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> yield on the chirp of the femtosecond exciting laser pulse

The hard X-ray yield generated with femtosecond laser pulses is studied for differently chirped irradiating laser pulses. The radiation of a Ti:sapphire CPA laser system (29 fs, 750 μJ, 1 kHz) is focused onto an iron containing solid state target producing incoherent hard X-ray radiation, Bremsstrahlung as well as target-specific K_α and K_β lines. The hard X-ray yield has been optimized by introducing negative and positive group delay dispersion (GDD) and third order dispersion (TOD) to the femtosecond laser pulse. The K_α yield could be enhanced by a factor of 1.7 and reached 1.9×10⁸ Fe K_α photons/s in 4π with the laser pulse positively chirped, and 1.5×10⁸ Fe K_α photons/s with the pulse negatively chirped. When the pulse energy is lowered to about 400 μJ the yield maximum at negative chirp vanishes and only the maximum at positive chirp remains. We explain this behavior with an increased electron temperature caused by the induced GDD and TOD in the pulse. PACS 42.65.Re; 52.38.Ph; 52.50.Jm