###### Abstract  
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 (\(G\approx 3\)), following the newly introduced nonlinear Fourier filter, is capable of reaching contrast levels beyond the previously reported \(10^{11}\)-\(10^{12}\). high-brightness laser systems; intensity contrast; ultraviolet; excimer lasers; short-pulse amplification +
Footnote †: journal: _Scienced Reviews of Applied Physics_  
In the UV part of the spectrum--due to the frequency scaling of the spontaneous emission and the short pumping time--the gradual evolution of the temporal, spatial, and spectral qualities of the pulse, as a result of many roundtrips in the laser cavity, are generally not allowed. The straightforward way is to use a dual-wavelength system, where the generation of the high spatial and temporal quality pulse is carried out at longer wavelengths. After frequency conversion (usually frequency doubling or tripling) the short-pulse is amplified in UV gain modules [9; 10; 11; 12; 13; 14; 15; 16], as shown in Figure 1. This approach provides a great ability to precisely control the pulse parameters and, therefore, the characteristics of the system output. Moreover, the frequency converter, as a nonlinear optical device, can efficiently decouple the temporal noise of the long wavelength front end, and using a novel technique (called as active spatial filtering [17]), both the directional and spectral properties of the main pulse can efficiently be controlled [17; 18]. The basic idea behind this method is that the frequency converter is positioned at the Fourier plane of the beam. It is the most intense zeroth order that is efficiently converted with the simultaneous removal of the high-order spatial frequencies from the beam. In this way, both the temporal and the spatial properties of the main pulse are reset in the \"middle\" of the system, prior to the amplification in the excimer amplifier chain. Eximer denotes a molecule that exists only in the excited state [19]. A typical potential diagram of such a molecule for _KrF_ is shown in Figure 2, offering an ideal four-level system for efficient amplification of long pulses, even in case of amplifying media of short storage times. Figure 1: Hybrid strategy involving the use of a UV seed-beam produced with a femtosecond IR or visible source that subsequently undergoes amplification in excimer amplifier modules (based on [9]). Figure 2: Typical potential diagram of an excimer (_KrF_) molecule (based on [19]). _KrF_ excimer is a high gain medium and best suited for short-pulse amplification in the UV [1]. It has a relatively narrow spectral bandwidth (\(\Delta\lambda/\lambda\approx 1/300\)) and low saturation energy density (\(\varepsilon_{sat}\approx 2\) mJ/cm\({}^{2}\)) compared to solid-state media [1]. This allows the use of direct amplification of the short-pulse. Moreover, the low density gaseous medium provides minimum phase front distortions [1,20].