Measurement of $^{85}Rb$ excited state lifetime using two-pulse photon echo

We have observed two-pulse photon echoes in a Doppler broadened rubidium vapor. The system interacts with traveling wave optical pulses that are $\sim10 ns$ in duration. The pulses are on resonance with the $F=3 \rightarrow F'=4$ transition in $^ {85}Rb$ and they are generated from a cw laser using an acousto- optic modulators. The first pulse, occurring at $t=0$, induces a macroscopic dipole moment that dephases due to atomic motion. The second pulse, occurring at $t=T$, reverses the direction of the dephasing process so that the echo is formed at $t=2T$. The echo is detected using a heterodyne tehnique and its intensity decays exponentially as a function of $2T$. Our results suggest that the excited state lifetime can be determined to a precision of $\sim1$ $\%$ from the decay time constant. We present a detailed analysis of the systematic effects that contribute to the decay.