Optical-density effect in heterodyne-detected accumulated photon echo

The effect of optical density on the heterodyne-detected accumulated photon echo is investigated both experimentally and theoretically. The cause of the distortion of the echo temporal behavior observed in the ${}_{}{}^4{}_{}{}^{}G_{\frac{5}{2}}^{}{}_{}{}^{}$ manifold of Nd:YAG (where YAG is yttrium aluminum garnet) is attributed to the optical-density effect, which is closely related to the change of the field correlation function as the excitation pulse propagates through the sample.