It is shown that mode matching, aberration compensation, control of beam overlap, and feedback are critical issues in designing multipass amplifiers for ultrashort pulse amplification. We have developed a Ti:sapphire amplifier system capable of producing aberration-free millijoule-level 25-fs pulses at 1 kHz. This system has a unique five-mirror ring design, which is capable of compensating second-order aberrations, and the flexibility to mode match the input beam and the pump laser. A unique compressor, which is a part of the amplification system, was designed to eliminate feedback.
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