  We investigated the preservation of information encoded into the relative
phase and amplitudes of optical pulses during storage and retrieval in an
optical memory based on stimulated photon echo. By interfering photon echoes
produced in a Ti-indiffused single-mode Er-doped LiNbO$_{3}$ waveguiding
structure at telecom wavelength, we found that decoherence in the atomic medium
translates only as losses (and not as degradation) of information, as long as
the data pulse series is short compared to the atomic decoherence time. The
experimentally measured value of the visibility for interfering echoes is close
to 100 %. In addition to the expected three-pulse photon-echo interferences we
also observed interference due to a four-pulse photon echo. Our findings are of
particular interest for future long-distance quantum communication protocols,
which rely on the reversible transfer of quantum states between light and atoms
with high fidelity.
