A WDM-PON architecture with selective-broadcast overlay

Abstract

We propose and demonstrate a novel WDM-PON architecture supporting both point-to-point service and selective broadcasting. For each WDM channel, the same light source is utilized to carry the two downstream services and the upstream re-modulated data. Introduction Wavelength division multiplexed passive optical network (WDM-PON) is an attractive approach for optical broadband access. To realize more flexible networking functions, some efforts have been paid to provide both point-to-point data service as well as broadcast video/data service [1-3]. They could be transmitted via time division multiplexing, but complicated timing control was needed and the downstream bandwidth had to be shared. Another common approach was to use one or more additional light sources [1-2], which led to increase in cost and complexity of the wavelength routing. In this paper, we propose a novel WDM-PON architecture which offers both kinds of services with the single light source. On the same wavelength channel, inverse-return-to-zero (IRZ) format is employed to carry the point-to-point data; while the differential-phase-shift keying (DPSK) broadcast data is superimposed onto it. No light source is needed at the ONU. The upstream data is re-modulated on the received downstream carrier, before being delivered back to the OLT, thus the ONU is colorless. Furthermore, by simple control in the transceivers at the OLT, the broadcast signal can be interrupted on any wavelength channel to achieve selective broadcast overlay. Network Architecture Fig. 1 depicts the WDM-PON architecture with selective-broadcast overlay. At the OLT, each pointto-point transceiver generates the downstream IRZ signals and receives the upstream re-modulated signals for the designated subscriber. The IRZ signal is generated by properly driving a Mach-Zehnder intensity modulator (IM) with a return-to-zero shaped data signal [3], which is generated by an electronic AND gate, driven by the point-to-point data and the clock signal, as shown in the inset in Fig. 1. The downstream IRZ signals from various transceivers are multiplexed, via an array waveguide grating (AWG) or other WDM multiplexers, before being opticallyamplified to boost up the power level. The amplified signals are then fed into an optical phase modulator (PM), driven by the differentially pre-coded digital broadcast data. Since an IRZ signal carries finite optical power at both one and zero levels, the DPSK broadcast data can be successfully superimposed to all wavelength channels simultaneously. At the ONU, a portion of the received downstream signal power is tapped off for reception. The IRZ data can be simply detected by a photodiode, followed by an electrical inverting post-amplifier; while the DPSK broadcast data can be detected after demodulation. Part of the received downstream power is fed into an optical intensity modulator for upstream data re-modulation. The finite optical power in each bit of the downstream IRZ signal provides the light source for the upstream data in every bit slot. As the upstream bit rate (say 2.5 Gb/s) is usually lower than the downstream bit rate (say 10 Gb/s), no bit synchronization is required at the ONU.