  We propose a new scheme of microwave frequency standards based on pulsed
coherent optical information storage. Unlike the usual frequency reference
where the Ramsey fringe is printed on the population of a certain state, we
print the Ramsey fringe on the coherence. Then the coherence is detected in the
form of a retrieval light. The central line of the Ramsey fringe can be used as
a frequency reference in an absorption-cell-based atomic frequency standard.
This scheme is free of light shifts as the interrogating process is separated
from the optical pumping processes, and the cavity pulling effect is negligible
due to the low Q requirement. Encoding the Ramsey interference into the
retrieval light pulse has the merit of high signal to noise ratio and the
estimated frequency stability of shot noise limit is about $2\times10^{-14}$ in
1 second, this scheme is promising for building small, compact and stable
atomic frequency standards.
