Experimental study on the propagation of high intense laser pulse in an underdense plasma with pulse length comparable with the background plasma wavelength

Optical investigation are reported of 2TW, 45fs Ti:sapphire laser pulses in an optically ionized helium gas. The stimulated Raman backscattering (BSRS ) spectrum, together with ion energy spectrum, are measured as a function o f the electron density from 4.8x10(18)/cm(3) to 1.4x10(19)/cm(3). The SRS r eflectivity and the phenomena of transition from strongly coupled BSRS to w eakly coupled BSRS were studied and compared with theory. Second harmonic e mission in forward direction was also observed, and its origin is due to th e radial electron density gradient caused by the ionization process. A good agreement is obtained between theory and experimental results.