Photon echoes are generated on the 6S/sub 1/2/-6P/sub 1/2/ transition 
in Cs vapor with excitation pulses short enough to excite all hyperfine 
states. As the excitation pulse separation is varied, the temporal 
profile of the echo reshapes and the time-integrated echo intensity 
modulates. These effects are typical of all the alkali vapors and 
provide the simplest, nontrivial display of quantum beating in a 
four-level system. The present paper describes a detailed experimental 
study of both kinds of modulation effects and utilizes the billiard 
ball model to analyze them.