Optical pulse position modulation processing: Architecture and demonstration in an optical code division multiple access system

We present a novel architecture for an optical pulse position modulation (PPM) scalable modulator and demodulator with a simple all-optical design based on ultrafast sampling. The use of PPM for two-dimensional time-wavelength incoherent optical code division multiple access (OCDMA) systems is investigated. The shift PPM-OCDMA scheme, where PPM slot overlaps as well as frame overlaps are allowed, is analyzed and compared to the more traditional on-off keying (OOK) and PPM-OCDMA schemes. For our design's parameters, we show that OOK performs better when the network operates at low data-rate transmissions (2.5 GHz or lower) or when the network is small in size. However, our proposed shift PPM-OCDMA outperforms both OOK and PPM-OCDMA when the user's transmission rate increases, relaxing both coding and hardware constraints. An experimental demonstration of shift PPM-OCDMA using our novel PPM modulator and demodulator with four levels was performed. Up to six simultaneous OCDMA users, each operating at 10 Gbits/s, were multiplexed on a star network while operating at a bit error rate of 10^-9 or better.