Status of short-pulse KrF amplifier research and development at HILL, Szeged

Abstract

3 , I. B. Földes 2,4 I. IntroductionIn the view of the recent progress of IR solid-state laser systems high-brightness ultraviolet (UV) excimer lasers can be regarded as complementary sources. Their main advantage occurs mainly in those experiments where high photon energy, optimum spatial concentration and/or efficient conversion of the pulse energy to radiation of even shorter wavelength are needed [1,2]. At present the maximum peak power of short-pulse excimer systems is limited to the TW level [1,2,4,5] by the difficulties associated with the construction of short-pulse UV amplifiers [1,2] and by the inherently limited energy extraction from excimer amplifiers of short energy storage time [1,3].Excimers are ideal four-level systems allowing very efficient operation even in the UV for pulses longer than the storage (or pumping) time. However, they exhibit moderate extraction efficiency for shorter pulses, because of the relatively short (several ns) storage time compared to the accessible pumping times (several times 10 ns, or more).The saturation energy density of excimers is very low compared to solid-state systems; typically is in the range of several mJ/cm 2 . In KrF power amplifiers the optimum operation both for efficiency and contrast is a critical function of the energy density [1,6], which can only be maintained when the energy density is set to εopt ≈ 2.2 x εsat ≈ 4.5 mJ/cm 2 [1,2,6,7]. This condition requires large amplifier cross-sections already for moderate output energies. Due to these requirements, pumping of excimer gain modules can only be realized by an efficient and temporally short pumping mechanism capable of homogeneously excite a large volume of large crosssection in a short time comparable to the energy storage time. Discharge pumping of excimers is more