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.