Spectra from stimulated Raman scattering of short relativistically strong laser pulses in an underdense plasma

A set of equations describing large-angle stimulated Raman scattering (SRS) of a short, relativistically strong laser pulse propagating in an underdense plasma is derived and investigated numerically. It is shown that the SRS spectrum depends strongly on the pulse shape. If a pulse with a sharp leading edge excites a strongly nonlinear wake wave, the scattering occurs in relativistic electron flows and is accompanied by the Doppler frequency shift. When the electron flow is directed oppositely to the pulse propagation direction, the frequency upshift is maximum for the direct-backward SRS and decreases with decreasing scattering angle.