A 1-kHz femtosecond sub-terawatt Ti: sapphire laser system that has applications in many high-field experiments such as high-harmonic generation, optical-field ionization and plasma physics has been developed. The laser system, based on the chirped pulse amplification(CPA) technique, consists of a long-cavity femtosecond oscillator, a four-pass grating pulse stretcher, two multi-pass amplifiers and a double-pass grating pulse compressor. Thermal lensing in the amplifiers is reduced by cooling the Ti: sapphire crystals using Peltier coolers. A thermal-eigenmode amplifier is used as the main-amplifier for compensating for the thermal lensing effect. Gain narrowing and residual phase errors are compensated for by using an acoustooptic pulse shaper. The final laser output has an energy of 4.5 mJ per pulse and 26-fs pulse duration.
For the generation of few-cycle laser pulses, the laser pulses generated in ghe 1-kHz CPA system are launched into a differentially pumped hollow-fiber compressor. In a differentially pumped hollow fiber the gas pressure varies from minimum at the entrance of the hollow fiber, the peak power of the input laser pulse can be increased without the self-focusing effect. The self-phase modulation(SPM) induced by the optical Kerr effect in the gas causes spectral broadening and thus the pulse spectrum extends from 500 and 950 nm. The pulse energy out of the hollow fiber is about 1.1 mJ. After recollimation by a silver mirror, the output pulse are compressed to 5 fs with a pair of chirped mirrors. These 1-kHz 0.2-TW 5-fs pulses are well suited for attosecond pulse generation by high-order harmonic generation and for research on various nonlinear processes in which the carrier-envelope phase plays an important role.
1 kHz 펨토초 테라와트급 티타늄 사파이어 레이저 시스템을 개발하였다. 이 시스템은 처프 펄스 증폭 기술을 이용하고 있으며 긴 공진기형 펨토초 공진기, 에돌이발 펄스 확대기, 2개의 다중 통과 증폭기와 에돌이발 압축기로 구성되어 있으며 4.5 mJ의 에너지와 26 fs의 펄스폭을 가졌다. 수 주기만을 가지는 레이저 펄스를 생성하기 위해 차등 펌핑 Hollow fiber 압축기를 사용하였다. Fiber 입력 부분에서의 낮은 압력으로 자기 집속 효과를 억제할 수 있어 높은 첨두 출력을 가지는 입력 펄스를 사용할 수 있다. 기체에 의한 광학 커 효과의 자기 위상 변조에 의해 펄스의 스펙트럼은 500 nm에서 950 nm까지 넓어지고 펄스의 에너지는 1.1 mJ이였다. 출력 펄스는 처프 거울에 의해 5 fs로 압축이 되었다.
