This paper presents an implementation of a modulation technique which is effective in obscuring spectral phase encoded time-spreading (SPECTS) optical code division multiple access (O-CDMA) data streams from eavesdroppers tapping into single-user uplinks. This data modulation technique employs a finite-state Markov chain following a four-state trellis to encode user data in the electronic domain. The encoding redistributes the SPECTS O-CDMA user data bits across four different waveforms to defeat the eavesdropper attacks on upstream links via power detectors or differential phase-shift keying (DPSK) receivers. The four-state encoder-decoder is implemented in a field-programmable gate array (FPGA) with high-speed serial transceivers. A SPECTS O-CDMA testbed with four-state encoded data modulation at up to 2.5 Gb/s/user is demonstrated and its single user link security is tested using a DPSK demodulator to emulate the eavesdropping detection. The security test verifies that this modulation technique effectively prevented interception by DPSK detection. The four-state coding can be extended to be time variable through switching among several trellis state definitions to achieve more rigorous security.
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