A new approach is presented to reveal the temporal structure of femtosecond laser pulses by recording the correspond- ing time-resolved shadowgraphs of the laser-induced air plasma. It is shown that the temporal structures of femtosecond laser pulses, normally not observable by the ordinary intensity autocorrelator, can be detected through intuitively analyz- ing the ultrafast evolution process of the air plasma induced by the femtosecond laser pulses under examination. With this method, existence of pre- and post-pulses has been clearly unveiled within the time window of 4-150 fs in reference with the main 50-fs laser pulses output from a commercial 1-kHz femtosecond laser amplifier. The unique advantage of the proposed method is that it can directly provide valuable information about the pulse temporal structures＇ effect on the laser-induced ionization or material ablation.
A peculiar and regular diffraction pattern is recorded while using either a color or a monochrome charge-coupled device (CCD) camera to capture the image of the micro air plasma produced by femtosecond laser pulses. The diffraction pattern strongly disturbs the observation of the air plasma, so the origin and eliminating method of these diffraction patterns must be investigated. It is found that the Fourier transform of the periodic surface structure of either the mask mosaic of the color CCD or the pixel array of the monochrome CCD is responsible for the formation of the observed pattern. The residual surface reflection from the protection window of a CCD camera plays the essential role in forming the interesting two- dimensional diffraction spots on the same CCD sensor. Both experimental data and theoretical analyses confirm our understanding of this phenomenon. Therefore remov-ing the protection window of the CCD camera can eliminate these diffraction patterns.
Kuanhong XUXiaonong ZHUPeng HUANGZhiqiang YuNan ZHANG
