Quasi-passive reconfigurable nodes for optical access networks [electronic resource]
Keywords: Electrical Engineering.

Access networks connect subscribers to their service providers. Passive optical networks (PONs) are an important class of optical access solutions to offer high bandwidth to endusers. Time Division Multiplexing PONs (TDM-PONs) are already widely deployed while Wavelength Division Multiplexing PONs (WDM-PONs) are seen as the future solution to meet the ever-rising bandwidth demand. However, due to the use of rigid passive components, such as fixed power splitters and WDM couplers, current PON systems are facing several limitations. TDM-PONs are constrained by their limited average bandwidth per user, inflexible power distribution and weak privacy protection. WDM-PONs do not fully exploit the available bandwidth because of the static wavelength allocation. In addition, although the increasing bandwidth demand will motivate an upgrade from legacy TDM-PONs to future WDM-PONs, one user may not fully utilize the bandwidth of one dedicated wavelength with most present multimedia applications. Thus, for economical reasons, graceful upgrade capability is required. Next-generation optical access networks should have sufficient reconfigurability to manage different power and bandwidth requirements without sacrificing energy efficiency and cost effectiveness. To achieve these goals, we propose QPAR, a Quasi-PAssive Reconfigurable node, which offers adaptive power distribution for different geographical users' distributions, and provides flexible wavelength allocation for user specific requirements in future PONs. Importantly, QPARs are quasipassive and only consume power during reconfiguration. Once the state of a QPAR is reconfigured, it is maintained without requiring any power; thus, the passive nature of PONs is preserved, since the power needed during reconfiguration can be provided remotely and optically. In TDM-PONs, QPARs can act as optical power splitters with controllable and uneven power splitting ratios. Simulation studies show that, compared to passive power splitters, QPARs can improve the optical power budget by redistributing power from near users to distant users, and the saved power can be used to increase the number of supported users by about 100% and extend network reach by about 30%. In WDM-PONs, QPARs can act as dynamic wavelength routers, which can route any incoming wavelength to any output port. According to our simulation results, compared to fixed wavelength routers, QPARs reduce traffic latency in the presence of unbalanced traffic as well as reduces the number of active channels by about 50% under low traffic load. QPARs can accommodate TDM-PON and WDM-PON users simultaneously: one or more wavelengths may be shared by a set of ONUs, while others are assigned a dedicated wavelength. With QPARs at the remote node, pay-as-you-grow bandwidth upgrade can be achieved without major modifications at the user premises such as the equipment of costly tunable transceivers. We designed several architectures of QPARs to realize the proposed functions and features. QPARs can be implemented using either discrete or integrated components. Discrete-component-based QPARs utilize WDM couplers for wavelength multiplexing/ demultiplexing, optical latching switches (OLSs) for quasi-passive space routing, and cascaded 3dB couplers for power splitting. We proposed and analyzed generalized QPAR architectures with arbitrary number of wavelengths, power levels, and outputs. With the designed architectures, any and all the combinations of wavelengths and power levels can be achieved at any output port. We experimentally demonstrated the functions of several QPAR nodes and investigated their performances. The performance of QPARs using two different optical latching switches: Micro-Electro-Mechanical Systems (MEMS) and Magneto-Optic (MO) optical latching switches have also been compared. To avoid having any power supply at the remote node as well as on-site reconfiguration and maintenance, we designed remote power system for QPARs. During reconfiguration, the needed power is transmitted from the central office along the feeder fiber. Two design options for QPAR remote power were proposed and analyzed. A QPAR control circuit was designed, and a remotely powered and reconfigured QPAR node with a 0.1F/5V supercapacitor (SC) remotely charged by a 1×8 photodiode array was successfully experimentally demonstrated. The charged SC can power the QPAR for at least 6s with 24 consecutive reconfigurations (200ms each), or two reconfigurations within a maximum period of 40 hours, before the SC needs to be recharged. The applications of QPARs in the Stanford UltraFlow access network were also investigated, where QPARs assure the coexistence and high utilization of wavebands adopted by different optical access services. We show, both through simulations and experiments, the feasibility and advantages of QPARs and how they can be implemented and used to provide flexible resource management as well as smooth and cost-efficient upgrade in next generation optical access networks.