A novel method is suggested for temporal and spatial cleaning of high-brightness laser pulses, which seems more energy-scalable than that based on crossed polarizers and offers better contrast improvement compared to the plasma mirror technique. The suggested arrangement utilizes nonlinear modulation of the beam in the Fourier-plane leading both to directional and to temporal modulation. By the use of a ‘conjugate’ aperture arrangement before and after the nonlinear spatial selector, intensity dependent transmission is obtained; simultaneous temporal and spatial filtering can be realized both for amplitude and phase modulation. In the case of phase modulation introduced by plasma generation in noble gases the experimental observations are in good agreement with the theory; demonstrating  >103 improvement in the temporal contrast, ~40% throughput, associated with effective spatial filtering. Due to the broad spectral and power durability of the optical arrangement used here, the method is widely applicable for energetic beams even of UV wavelengths, where most of the former techniques have limited throughput.

Recently a novel method called nonlinear Fourier-filtering was suggested for temporal and spatial cleaning of high-brightness laser pulses. In this paper experimental demonstration of the associated spatial filtering of this method and significant improvement of the temporal filtering feature are presented. The formerly found limit of ~10 for the temporal contrast improvement is identified as diffraction effects caused by the limited numerical aperture of imaging. It is shown by numerical simulation that proper apodization of the object can lead to sufficiently higher limit (>10). Using an advanced experimental arrangement the improvement of >2 orders of magnitude is experimentally verified in the ultraviolet and an indirect proof is presented that the background caused by the optical arrangement is reduced below 10.

Abstract. Important figure of merit of high-intensity laser systems is the temporal and spatial quality of their pulses. Spatial filtering is a well known technique to improve the spatial quality by modulating the spatial components at the Fourier-plane, using a pinhole of appropriate size or recently by a nonlinear process. Modulation of the beam in the Fourier-plane allows however a simultaneous spatial and temporal filtering. By the use of a "conjugate" pinhole arrangement before and after the nonlinear spatial selector, intensity dependent transmission is obtained: the low intensity part is efficiently suppressed. Numerical calculations predict practical operation for both amplitude and phase modulation at the Fourier-plane. In the preferred latter case the experimental observations are in good agreement with the theory, demonstrating >40% throughput.

In high-brightness excimer systems, the direct amplification of short pulses allows temporal filters to be integral parts of the ultraviolet (UV) amplifier chain, where the only origin of the noise is the amplified spontaneous emission (ASE), generated by the amplifier(s) following the filter. The ASE, however, develops faster than the short main pulse; in this paper, the dynamic short- and long-pulse amplification properties of KrF, XeCl and XeF excimers are studied, with special emphasis on the temporal contrast. It was found that, beyond the saturation of amplification, the relaxation of the B state in KrF, together with the contribution of the absorption of the transiently populated X state in XeCl and XeF, are the main limitations for both the extraction efficiency and the contrast. For all excimers, the stimulated transition rates and the dependence of the achievable contrast on the level of saturation were derived. Local quantities were introduced to characterize the deterioration of the contrast for a unit gain length of KrF amplifiers. A KrF power amplifier of limited gain (G ≈ 3), following the newly introduced nonlinear Fourier filter, is capable of reaching contrast levels beyond the previously reported 1011–1012.

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