Hybrid WDM and optical CDMA implemented over waveguide-grating-based fiber-to-the-home networks

This paper presents a hybrid wavelength-division multiplexing (WDM) and optical code-division multiple-access (OCDMA) scheme over fiber-to-the-home (FTTH) networks. Exploiting the periodic free-spectral-range (FSR) properties of arrayed-waveguide grating (AWG) routers, the total ONU (optical network unit) capacity in the network is partitioned into different groups based on the subscriber's geographical locations. Maximal-length sequence (M-sequence) codes are taken as signature addresses to take advantage of the cyclic properties of AWG routers. The proposed scheme is suitable for networks in which the distance between the OLT (optical line terminal) and the individual ONUs varies. The influence of the degree of polarization (DOP) on the signal to phase-induced intensity-noise ratio is evaluated. By introducing a depolarizer in front of the balanced photo-detector, the maximum permissible number of simultaneous active ONUs can be improved by approximately 20% for a 10−9 error probability. By exploiting the inherent cyclic FSR characteristics of AWG routers, the OLT enables a significant reduction in the number of AGW-based network codecs compared to the previous coded WDM (i.e., OCDMA in spectral domain) scheme. Furthermore, while the number of AWGs in the proposed scheme is comparable to that of conventional WDM-EPON schemes, the current scheme is more flexible, confidential, and scalable.