The Theory of Three-level Photon Echo Using of Rotating Wave   Approximation
  The three level photon echo has been described in different works by using rotating wave approximation but none of them did not get results which show the effects of field's frequencies on frequency of ground level of system. In this work, we studied a Lambda type system theoretically and numerically. By considering the Doppler effect in environment, we get different equation for polarization of echo signal and its intensity. 
Tara Ahmadi, Sergei.A Pulkin, Vladimir.A Sheftsov
quant-ph