  By Polnarev's method we analytically calculate the polarization spectra of
the cosmic microwave background radiation (CMB) generated by cosmic relic
gravitational waves (RGW). In this analytic approach the physics involved in
this generating process is more transparent. Consequently, the effects due to
various elements of physics can be isolated easily. In solving the equation for
evolution of RGW in the expanding universe, both the sudden transition and the
WKB approximation for the scale factor during the radiation-matter transition
have been taken. To describe more precisely the decoupling process, we have
introduced an analytic expression for the visibility function, consisting of
two pieces of half-Gaussian curves. We also include the damping on
polarizations due to the photon diffusion up to the second order of the tight
coupling. Analytic polarization spectra $C_l^{XX}$ have been obtained with the
following several improvements over the previous results. 1. The approximate
analytic result is quite close to the numerical one evaluated from the cmbfast
code, especially, for the first three peaks of the spectrum that are observable
(see the text).
