We report on the development of a preamplifier module for temporal contrast enhancement and control at petawatt-class lasers. The module is based on an ultrafast optical parametric amplifier (uOPA), which produces temporally clean pulses at the 60 lJ level for seeding a chirped pulse amplification (CPA) system, namely the petawatt facility PHELIX. The amplifier module allows for gain reduction in the following amplifiers, resulting in an attenuation of amplified spontaneous emission (ASE) by more than 4 orders of magnitude. Since the ASE of a CPA system linearly depends on the seeding energy, we were able to demonstrate a continuous variation of the temporal contrast by tuning the gain of the uOPA.

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Magnesium aluminosilicate glasses doped with 0.2 mol% Sm 2 O 3 (1 Â 10 20 Sm 3+ cm À3 ) have been prepared in a very broad compositional range. The effect of the MgO, Al 2 O 3 and SiO 2 concentrations as well as the effect of partial substitution of MgO by CaO, SrO, BaO, ZnO or MgF 2 have been studied. Increasing the network modifier concentration results in decreasing the glass transformation temperature and increasing the coefficient of thermal expansion due to the formation of non-bridging oxygen sites and decreasing glass network connectivity. Although the network connectivity is changed substantially by the addition of network modifier oxides, the maximum phonon energy and the fluorescence lifetime of Sm 3+ are not affected. Equimolar replacement of up to 9 mol% MgO by MgF 2 results in increasing Sm 3+ fluorescence lifetimes without increasing the coefficient of thermal expansion or decreasing the glass forming ability. Glasses with fairly small thermal expansion coefficients ($3.2 Â 10 À6 K À1 ), low thermal stress values ($0.5 MPa K À1 ), broad fluorescence emission peaks and fluorescence lifetimes in the range from 2.4 to 2.8 ms are obtained. Such glasses are interesting candidates for laser host materials in ultrahigh peak power laser systems.

An analytical model is presented describing the temporal intensity contrast determined by amplified spontaneous emission in high‐intensity laser systems which are based on the principle of chirped pulse amplification. The model describes both the generation and the amplification of the amplified spontaneous emission for each type of laser amplifier. This model is applied to different solid state laser materials which can support the amplification of pulse durations ≤350 fs . The results are compared to intensity and fluence thresholds, e.g. determined by damage thresholds of a certain target material to be used in high‐intensity applications. This allows determining if additional means for contrast improvement, e.g. plasma mirrors, are required for a certain type of laser system and application. Using this model, the requirements for an optimized high‐contrast front‐end design are derived regarding the necessary contrast improvement and the amplified “clean” output energy for a desired focussed peak intensity. Finally, the model is compared to measurements at three different high‐intensity laser systems based on Ti:Sapphire and Yb:glass. These measurements show an excellent agreement with the model.

We report on results from the fully diode-pumped chirped-pulse amplification laser system Polaris. Pulses were amplified to a maximum energy of 54.2 J before compression. These pulses have a full width at half-maximum spectral bandwidth of 18 nm centered at 1033 nm and are generated at a repetition rate of 0.02 Hz. To the best of our knowledge, these are the most energetic broadband laser pulses generated by a diode-pumped laser system so far. Due to the limited size of our vacuum compressor, only attenuated pulses could be compressed to a duration of 98 fs containing an energy of 16.7 J, which leads to a peak power of 170 TW. These pulses could be focused to a peak intensity of 1.3×10²¹ W/cm². Having an ultra-high temporal contrast of 10¹² with respect to amplified spontaneous emission these laser pulses are well suited for high-intensity laser-matter experiments.