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This paper presents a simple and effective security enhancement mechanism for spectral phase-encoded time spread (SPECTS) optical code-division multiple-access (O-CDMA) users against eavesdroppers tapping at the isolated single-user uplink stream. Traditional SPECTS O-CDMA users apply pseudorandom spectral phase codes to spread ON-OFF keyed ultrashort optical pulses in time. Isolated single-user upstream transmission is prone to energy detection by eavesdroppers, even those without code knowledge. In addition, it has been shown that the use of a pair of (bright/dark) codes to represent binary data transmission is still susceptible to differential phase shift keying (DPSK) eavesdroppers without code knowledge. To remedy this security weakness, we introduce a new type of O-CDMA data modulation with memory. This new method not only effectively defeats DPSK eavesdropping but also can provide good detection performance for authorized users. We successfully demonstrate the concealment of information from DPSK eavesdroppers with different levels of modulation information.