A femtosecond terawatt Ti:sapphire laser system was developed that can be applied for many research fields such as high-harmonic generation, X-ray laser development, and ultrafast extreme ultra-violet pulse generation. The laser system, based on the technique of chirped-pulse amplification, was composed of a femtosecond Ti:sapphire oscillator, a pulse stretcher, a preamplifier, a power amplifier, and a pulse compressor. The femtosecond Ti:sapphire oscillator was a typical Kerr-lens mode-locked laser of an X-folded-cavity type, and 17-fs pulses could be generated at a 90-MHz repetition rate. Before amplification the laser pulses from the oscillator were temporally stretched to about 240 ps by an all-reflective, Offner-triplet-type pulse stretcher. The preamplifier was an eight-pass, bow-tie-type one pumped by a frequency-doubled Nd:YAG laser, and the amplified pulse energy was about 3.5 mJ at a 50-mJ pumping energy. The output pulse from the preamplifier was upcollimated to 6 mm in diameter and further amplified to 130 mJ at a five-pass power amplifier pumped on both sides by a total energy of 540 mJ. The amplified pulses were upcolliamted to about 3 cm in diameter and re-compressed to a femtosecond regime by a pulse compressor composed of two 1200 grooves/mm gratings and flat mirrors. The pulse energy after the pulse compressor was 60 mJ.
For the relaxation of the gain-narrowing problem and the generation of a broad amplified spectrum, a long-wavelength injection (LWI) method was developed in which an input spectrum is prepared on the red side of the gain peak and the pulse is amplified to a gain-saturated regime. For confirmation of the LWI method the simulation of the gain-narrowing and gain-saturation effects was performed using a modified Frantz-Nodvick model, and it was found that the LWI method is effective in generating a broad amplified spectrum for an input spectrum of a super-Gaussian shape which has rapidly falling edges. Experimentally, a broad amplified spectrum of a 52-nm width could be generated at the preamplifier using the LWI method. Although the spectral width was slightly narrowed to 49 nm after the power amplifier, it was still broad enough to support sub-20-fs pulses. After pulse compression with dispersion compensation up to the fourth order using two SF10 prisms installed before the pulse stretcher, 20-fs, 3-TW pulses could be obtained.
A cavity-dumped Ti:sapphire laser was incorporated as a front-end oscillator in the terawatt Ti:sapphire laser to suppress amplified spontaneous emission (ASE) and preceding femtosecond pulses. The basic configuration of the cavity-dumped laser was the same as the normal Kerr-lens mode-locked laser except that a fused silica Bragg cell was installed as a cavity-dumper between two concave mirrors at the end of the cavity. When the Bragg cell was driven by 9-ns, 6-W RF signal the dumping efficiency was about 50% and the dumped pulse energy of more than 30 nJ could be obtained without any degradation of pulse duration or spectral width. The contrast ratio between the preceding and dumped pulses was better than 100:1, and the total contrast ratio in combination with the Pockels cell was improved to better than 50000:1. Due to the high energy of the dumped pulses the total gain at the preamplifier could be lowered by reducing the number of passes from eight to seven with the same amplified energy, and the ASE level without the injection of seed pulses was significantly reduced from 0.5 mJ to 0.06 mJ. The amplified spectral width was also improved to 65 nm and 57 nm at the preamplifier and the power amplifier, respectively. After pulse compression 20-fs, 3-TW pulses could be demonstrated. This laser system has been successfully applied to investigation on high-harmonic generation and intense X-ray source development using gas jet targets.
