High energy and high repetition rate femtosecond Ti:sapphire lasers are widely used in isolated attosecond pulses and high-order harmonic generation. Enhancing the driving laser energy is a convenient and effective way to improve attosecond pulse energy. A 1 kHz or higher repetition rate millijoule level femtosecond Ti:sapphire amplifier is generally used to generate isolated attosecond. However, due to the limitation of its green pump laser energy, the energy of femtosecond Ti:sapphire laser is limited to several millijoules. Appropriately reducing the requirements for repetition rate, realizing high energy driving laser will significantly improve attosecond pulse energy and extend its application scope. Meanwhile, a 532 nm pump laser from frequency doubled 1064 nm Nd:YAG flash lamp pumped laser at 100 Hz repetition rate can achieve high pump energy with lower cost. Accordingly, we develope a 100 Hz repetition rate high energy amplifier based on Ti:sapphire crystal. The femtosecond amplifier system consists of oscillator, stretcher, ring cavity regenerative amplifier, four-pass amplifier and grating compressor. The ring cavity regenerative amplifier is the first amplifier as pre-amplifier, and the four-pass amplifier is the booster amplified-stage. 80 MHz seed pulse from the oscillator has a full width at half maximum bandwidth of 61 nm with a 20 fs duration. Then the seed pulses are stretched to 200 ps with a Martinez grating stretcher, rotated to vertical polarization and injected into the regenerative amplifier. The amplified uncompressed 1 kHz repetition rate laser pulses with 3 mJ pulse energy are selected to be 100 Hz and input into the four-pass amplifier. With a pulse energy of 75.1 mJ, wavelength at 532 nm flash lamp pumped pump laser at 100 Hz repetition rate, single pulse energy up to 25.4 mJ is obtained from a Ti:sapphire crystal, corresponding to a high energy conversion efficiency of 33.8%. We believe that higher energy should be possible if the pump energy can be further increased. After expanding the beam to 10 mm in diameter, the amplified chirped pulse is compressed using a four-pass, single grating compressor, with an overall efficiency of 72%. The highest pulse energy after compression is 18.3 mJ. For a fluctuation of the 100 Hz pump laser is as high as 3.4% for over 10000 shots, the 3.6% energy stability of the amplifier has a room to be improved. The typical spectrum bandwidth after the compressor is 39 nm, which can support transform-limited pulse duration of 32.8 fs. After fine dispersion compensation by the compressor, A pulse duration of 37.8 fs is measured using a single shot autocorrelator (Minioptic Technology, Inc). In addition, the spatial profile of the output beam from the compressor is measured using a commercial laser beam analyzer (Spiricon, Inc). The beam quality M2 factor are 1.8 and 1.6 in X and Y directions, respectively. In summary, a peak power of 0.48 TW compact 100 Hz femtosecond laser with pulse duration of 37.8 fs, pulse energy of 18.3 mJ is achieved from a two-stage amplifier system based on Ti:sapphire crystal. We believe that with a more stable and better spatial profile pump source, even better performance can be obtained by this scheme. Nevertheless, the current results show that this system should be favorable for high energy attosecond pulse generation and further amplification such as Terawatt system.

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