Laser spectroscopy is an important tool in condensed matter physics and life science. At present stage, our laser source is ultrafast laser with duration on the order of femtosecond (10 -15 s). Ultrafast lasers have two major unique properties. One is for its ultrashort pulse duration, which can be used to freeze the motion of the studied target on the order of picosecond (10-12 s) and nanosecond (10-9 s) time scale. It can thus enalbe us to study the ultrafast dynamics of physical process. Here's a fancy example to monitor the propagation of femtosecond light pulse. On the other hand, the light pulse has strong instantaneous electric field compared with a continuous-wave laser (such as a laser pointer) with the same average power (say 1mW). The instantaneous power can be up to 10GW if the energy of continuous-wave in one second is concentrated to 100fs pulse. Due to its strong optical field, the applications of pulse laser in medical and industrial fields are laser surgery and laser machining with high precision. Pulsed lasers are also tools for nonlinear-optics based applications such as two-photon microscopy and harmonic generation microscopy for bio-imaging.