Experimental Demonstration of a SAC-OCDMA PON With Burst-Mode Reception: Local Versus Centralized Sources

In this paper, we demonstrate experimentally the uplink of a 7 times 622 Mb/s incoherent spectral amplitude coded optical code-division multiple access (SAC-OCDMA) passive optical network (PON) with burst-mode reception. We consider two network architectures: local sources (LS) at each optical network unit (ONU) versus a single source located at the central office. We examine both architectures over a 20-km optical link, as well as a reference back-to-back configuration. Our architectures can adopt two-feeder and single-feeder topologies; however, we only test the two-feeder topology and therefore the effect of Rayleigh backscattering is neglected. We also study the relative merits (cost and performance) of local sources versus centralized architectures. A penalty of less than 2 dB between the LS and the centralized light sources (CLS) architectures was measured at a bit error rate (BER) of 10^-9 under certain assumptions on the relative power of the sources. The power budget in the CLS architectures is more critical than in the LS architectures; extra splitting and propagation losses exist as the uplink travels through the network back and forth. Doubling the number of users while maintaining the same distance and source power in LS architectures imposes 3-dB additional losses, whereas for CLS architectures, there are 6-dB extra losses. CLS architectures can overcome these penalties using amplification at the central office. Alternately, central office amplification can be used to more than double the number of users in LS SAC-OCDMA PONs. A standalone (no global clock) burst-mode receiver with clock and data recovery (CDR), clock and phase alignment (CPA), and Reed-Solomon RS(255,239) forward-error correction (FEC) decoder is demonstrated. A penalty of less than 0.25 dB due to the nonideal sampling of the CDR is reported. The receiver also provides an instantaneous phase acquisition time for any phase step between consecutive packets, and a good immunity - - to silence periods. A coding gain of more than 2.5 dB was reported for a single-user system, and BER floors were completely eliminated. Error-free transmission (BER < 10^-9 ) for a fully loaded PON was achieved for the LS architecture as well as the CLS architecture. Continuous and bursty upstream traffic were tested. Due to the CPA algorithm, even with zero preamble bits we report a zero packet loss ratio (PLR) for up to four simultaneous users in case of bursty traffic, and more than two orders of magnitude improvement in the PLR for fully loaded PON systems.