  The influence of sinusoidal density modulation on the stimulated Raman
scattering (SRS) reflectivity in inhomogeneous plasmas is studied by three-wave
coupling equations, fully kinetic Vlasov simulations and particle in cell (PIC)
simulations. Through the numerical solution of three-wave coupling equations,
we find that the sinusoidal density modulation is capable of inducing absolute
SRS even though the Rosenbluth gain is smaller than {\pi}, and we give a region
of modulational wavelength and amplitude that the absolute SRS can be induced,
which agrees with early studies. The average reflectivity obtained by Vlasov
simulations has the same trend with the growth rate of absolute SRS obtained by
three-wave equations. Instead of causing absolute instability, modulational
wavelength shorter than a basic gain length is able to suppress the inflation
of SRS through harmonic waves. And, the PIC simulations qualitatively agree
with our Vlasov simulations. Our results offer an alternative explanation of
high reflectivity at underdense plasma in experiments, which is due to
long-wavelength modulation, and a potential method to suppress SRS by using the
short-wavelength modulation.
