## 1 Introduction
The plasma mirror is a simpler, more robust, and more effective technique that can be applied at the full compressed pulse energy focused to high fluence levels for which there are consequently no additional downstream contributions to ASE. Therefore the plasma mirror is used in many laboratories, and significant improvement in the contrast to levels much greater than 10\({}^{10}\) have been reported [37]. However, in contrast to the XPW or saturable absorber methods, this also means that its efficiency, which is typically 50%-70% for the single stage implementation, is not recoverable downstream. It is challenge to implement this technique at high repetition rates because each targeted area is used once for each laser shot. The spatial beam qualities such as focusability and stability are also very sensitive to variation of the fluence on the plasma mirror [40, 41]. More recently, self-diffraction in a bulk Kerr medium has been investigated and demonstrated contrast improvement by four orders of magnitude. However, the diffusive light from the Kerr medium and the angular dispersion are obstacles to this new contrast enhancement technique. If an OPA front end is utilized, the achieved contrast is at the 10\({}^{10}\) level. Optical parametric chirped-pulse amplification (OPCPA) [42, 43] is becoming increasingly popular as a front-end preamplifier system that can significantly improve contrast, but is demonstrated only for the front-end of PW-class Nd:glass laser system for fusion research [44, 45, 46, 47] upstream of an amplifier chain and where the pulse energy is much lower. The use of an OPCPA preamplifier in a petawatt-class high-intensity Ti:sapphire laser has not been demonstrated. Homogeneous flat-top circular profiles with steep edges are needed to pump a large-aperture Ti:sapphire crystal, to enable high beam quality, CPA systems at the petawatt power level. In PW-class systems the flat-top transverse beam profile is required for both amplification efficiency and to guarantee operation below the damage threshold on large aperture Ti:sapphire crystals and large gratings which are typically very expensive optical components in the system. Frequency-doubled large-aperture high-energy Nd:glass green lasers are widely used to pump PW-class laser systems [4, 5, 7, 8, 9, 10, 11]. The flat-top beam profile is required from the pump lasers to maintain acceptable beam quality, reliable damage-free operation and to increase amplification efficiency. However, the direct output beam of the Nd:glass laser system typically exhibits a poor (unacceptable) spatial beam profile which can lead to local hot spots (high intensity). These hot spots can induce intensity modulations and enhance parasitic transverse lasing. It is necessary to control the green pump profile to achieve a mandatory flat-top distribution. Some methods for maintaining the flat-top profile of Nd:glass green pump lasers have been investigated such as image relaying (with spatial filtering in some cases) [4, 5, 7] and the use of a microlens array [48]. The shaping the glass pump beam profile with a microlens array homogenizing system has been demonstrated [48]. The working principle of this technique is to divide the incident beam into many sub-apertures which are all expanded onto the same area in the so-called target plane which lies inside the laser crystal. The resulting profile in the target plane, however, can exhibit the Talbot effect, which might result in low amplification efficiency due to low beam overlap efficiency. The utilization of diffractive optical elements (DOE) has been proposed for profile homogenization. However, to date, there have been only a limited number of reports on beam homogenization of high-energy lasers using a DOE [49]. The development of energetic, intense laser systems with adequately high temporal contrast and high spatial quality remains an important challenge.