Repetitive petawatt-class laser with near-diffraction-limited focal spot and transform-limited pulse duration Cheng Liu, Sudeep Banerjee, Jun Zhang, Shouyuan Chen, Kevin Brown, Jared Mills, Nathan Powers, Baozhen Zhao, Gregory Golovin, Isaac Ghebregziabher and Donald Umstadter * Department of Physics and Astronomy, University of Nebraska, Behlen Laboratory, 500 Stadium Dr., Lincoln, NE USA 68588 ABSTRACT A repetitive petawatt-class Ti:sapphire laser system operating with high spatial and temporal beam quality is demonstrated. Maximum pulse energy of 30 J is obtained via five multi-pass amplification stages. Closed-loop feedback control systems in the temporal and spatial domains are used to yield Fourier-transform-limited pulse duration (33.7 fs), and diffraction-limited focal spot sizes (with several different tight focusing optics). The laser parameters have been fully characterized at high-power, and are monitored in real-time, to ensure that they meet the experimental requirements for laser-wakefield electron acceleration and x-ray generation. Keywords: petawatt, diffraction limited, transform limited, spatial and temporal closed loop, vacuum measurement 1. INTRODUCTION The application of the techniques of chirped-pulse amplification (CPA) 1 and Kerr-lens modelocking 2 to solid-state lasers has led to compact and robust systems capable of >1-petawatt peak power, and focused intensity of >10 21 W/cm 2 . 3‐5 This has created new research opportunities over a wide range of scientific disciplines. For instance, such high power lasers can now be used to accelerate electrons to near GeV energy over just a centimeter-scale distance, via the mechanism of laser-wakefield acceleration (LWFA). 6, 7 In order to optimize the focused intensity, which is critical for numerous applications, including LWFA, the quality of laser beam—both spatially as well as temporally—should be as high as possible. For high peak-power laser beams, the preservation of high beam quality requires, among other measures, avoidance of deleterious nonlinear effects, which can arise from propagation through atmospheric pressure air. For this reason, grating pulse compressors are generally operated only under vacuum conditions, as are high-field interaction experiments that make use of the compressed beam. However, optical systems necessary for measurement and assurance of beam quality at the interaction point are not designed to be compatible with operation under vacuum conditions. The spatial quality of the amplified beam comprises both the near- and far-field spatial distributions. The former is a concern for the safety of the optics, especially for the compressor gratings, which have a relatively low damage threshold. A homogenizer to smooth the spatial distribution of the laser pump beam is often implemented in order to obtain a flattop spatial distribution of the amplified beam. A flat and uniform wavefront is also required in order to obtain a diffraction-limited focal spot. 8 Optical aberration in the amplification chain and thermally induced distortion are the main sources for wavefront deterioration. An adaptive optical system can both measure and correct the wavefront distortion. Chromatic aberrations can also be minimized by implementation of all-reflective optics or an achromatic lens in the final beam expander. Because the temporal quality of the amplified beam is strongly affected by chromatic dispersion in CPA laser systems, it needs to be precisely controlled and minimized. Flat spectral phase over the entire spectral range is requisite for Fourier- transform-limited pulses. In a CPA laser system, the dispersion mismatch among the stretcher, any transmissive * donald.umstadter@unl.edu; phone +1 402 472-8115; fax +1 402 472-6148 Solid State Lasers XXII: Technology and Devices, edited by W. Andrew Clarkson, Ramesh K. Shori, Proc. of SPIE Vol. 8599, 859919 · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2005008 Proc. of SPIE Vol. 8599 859919-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/18/2014 Terms of Use: http://spiedl.org/terms